This manual covers the troubleshooting and overhaul of the systems and assemblies of the drive train, to include the transmission, transfer case, power takeoff, propeller shaft assembly, and differential. The information is intended as a general guide since there are a number of different models for all the assemblies presented, but it will provide you enough background information for troubleshooting and overhaul for each assembly.
The first topic is troubleshooting the standard or manual transmission. Although standard transmissions are not as common as they once were, even in military vehicles, the construction mechanic still must be accomplished in the troubleshooting and overhaul of the standard transmission.
The next topic covers the transfer case, which is found on a great number of military type vehicles. It can be said it is as common as the transmission since there are so many 4x4 or 6x6 vehicles. Troubleshooting the power takeoff is another topic. The power takeoff is what allows a dump truck to dump, a fuel truck to dispense fuel, a transit mixer to mix cement, and a wrecker to lift its load.
The topic of troubleshooting propeller shafts is an extensive one, covering driveline geometry, inspection, lubrication, U-joint replacement and lubrication analysis, yoke inspection, U-joint reassembly, driveshaft installation, chassis vibration diagnosis and PTO drive shafts.
The last topic covered is troubleshooting differentials, which provides a wide range of exposure to the components of the differential. The drive train is the last bastion for a mechanic to be a mechanic, with its gears, yokes, bushings, universal joints, splines, etc.
When you have completed this manual, you will:
To isolate the root cause of any transmission problem, you have to follow a troubleshooting procedure. Begin by discussing the problem and its symptoms with the driver/operator. Check any information you can get from the driver/operator with the vehicle history jacket.
Visually inspect the transmission. Look for obvious problems such as broken transmission mounts, fittings, or brackets, and signs of leakage. Check the air system for leaks. Road test the vehicle whenever possible. Mechanics usually get second- or third-hand reports of trouble experienced with the unit, and these reports do not always accurately describe the actual conditions. Sometimes symptoms may point toward a transmission problem but the true problem is actually elsewhere in the drivetrain, such as the axle, propeller shaft, U-joint, engine, or clutch. This can be especially true of complaints of noise and vibration. Road testing the unit yourself reduces the importance of secondhand reports on performance from the drivers. Riding with the driver can also be informative, but if the mechanic drives, the road test can be more effective.
Troubleshooting should always begin with information gathering. Next, you will have to analyze the information and attempt to identify a root cause. Nothing is more helpful than experience with troubleshooting and knowledge.
This complaint is common and can be caused by so many different things that you should begin with a road test. Noise has a way of reverberating through the drivetrain, so let us begin with some non-transmission noise.
It is your responsibility as a mechanic to determine the cause, so it is really important that you do not jump to conclusions based on the driver's/ operator's complaint. First determine the source of the noise. Next, if you are sure it is coming from the transmission, attempt to define the nature of the noise. Be aware of the exact conditions that produce the noise.
Growling. Low frequency noises such as growling, humming, or a grinding sound are usually caused by worn, chipped, rough, or cracked gears. Once the symptoms are first noted, as gears continue to wear, a growling/grinding noise becomes more noticeable, usually in gear positions that throw the most load on the worn or damaged gears.
Growling can also be caused by improper timing of the transmission during reassembly, or improper timing due to a gear turning on the counter shaft.
Thumping or knocking. A thumping or knocking noise can begin as growling. As bearings wear and retainers start to break up, a thumping or knocking noise can be produced. This noise is heard at a low shaft speeds in any gear position.
Irregularities on gear teeth such as cracks, bumps, and swells can also cause thumping or knocking, Bumps or swells can be identified as highly polished spots on the face of the gear tooth, and these are often caused by rough handling during assembly (Figure 1). In most housings, the noise produced will be more prominent when the gear is loaded, making it easier to identify the problem gear. At high speeds a knocking or thumping noise can turn into a howl.
Figure 1 - Bumps or swells on the gear teeth caused by improper handling.
Rattles. Metallic rattles within the transmission result from a variety of conditions. Engine tensional vibrations are transmitted to the transmission through the clutch. In heavy-duty equipment featuring clutch discs without vibration dampers, rattles may result that are most noticeable in neutral. In general, engine speeds should be 600 rpm or above to eliminate rattle and vibrations noticeable during idle. A cylinder misfire could cause an irregular or lower idle speed, producing a rattle in the transmission. Another cause of rattles is excessive backlash in the PTO unit mounting.
Vibration. Drivetrain vibrations can be very difficult to source. Never forget that they can be produced by any component in the drivetrain, so your troubleshooting must involve everything from the engine to the drive wheel assemblies. Most driveline vibrations are produced only at a specific road speed or in a specific gear; these clues can help to determine whether the cause is in the transmission or elsewhere.
If you have isolated the cause of the vibration to the transmission, first note the speeds at which it is most prominent. Use what you know of the transmission to make your diagnosis. Remember that the input shaft rotates at engine speed when the clutch is engaged, and the output shaft rotates at driveshaft speed. Make a note of the gear the vibration occurs at, and note whether it occurs only in high range or only in low. Make sure that you have some evidence that the cause is in the transmission before removing it from the chassis.
Whining. Gear whine is usually caused by insufficient backlash between mating gears, such as improper shimming of a PTO. A high-pitched whine or squeal can also be caused by mismatched gear sets. Such gear sets are identified by an uneven wear pattern on the face of the gear teeth. Pinched bearings with insufficient axial or radial clearance can also produce a whine.
Noise in neutral. Possible causes of noise while the transmission is in neutral include:
Noise in gear. Possible causes of noise while the transmission is in gear include:
Difficult to shift. Difficult to shift is a condition that occurs when the shift lever is difficult to move from gear to gear. In some instances, only one gear is hard to select; in others, all shifting may be difficult.
The usual causes of hard shifting are bent or worn shift linkage and linkage in need of lubrication. Hard shifting may also be caused by binding of the shift rails or forks. Occasionally, hard shifting may be caused by a misaligned transmission case or a problem in the clutch.
Hard shifting can be diagnosed easily. If the gearshift lever is hard to move even when the engine is stopped, the shift linkage is probably the source of the problem. If the gearshift lever moves easily when the engine is stopped but shifting becomes hard with the engine running, the problem is probably in the vehicle clutch, or else the case is misaligned.
In transmissions having an external shift linkage, hard shifting may be corrected by adjusting or lubricating shift linkages. In contrast, if internal parts of a shift linkage are bent or sticking, the transmission must be removed and disassembled to make repairs. Bad synchronizers, which can be responsible for hard shifting, or clutch problems wil require transmission removal. Misalignment problems may require transmission removal; however, if the case is misaligned because of dirt or other foreign material, it may be possible to clean and realign it without removal.
Jumps out of gear. When a transmission jumps out of gear, the shift lever "pops," or moves into neutral during operation.
First, check the transmission linkage and shift lever arms. If the shift assembly is badly worn, it should be rebuilt or replaced.
A worn clutch pilot bearing may also cause the transmission to jump out of gear. Severe vibration, caused by the wobbling transmission input shaft, can wiggle and move the shift forks and synchronizers.
Other problems inside the transmission can cause it to jump out of gear. They include worn synchronizer inserts and springs, worn shift fork assemblies or shift rails, and wear and excessive play in the countershaft and output shaft assemblies.
Locked in gear. When the shifter is locked in one gear, check the transmission shifter assembly and linkage. Look for bent shift rods, worn linkage, bushings, and shifter arms. Also check linkage adjustment. With a shift rail mechanism, worn or damaged rails, detents, or forks could be the cause.
A transmission can also become locked in gear when drive gear teeth are broken. The teeth can jam together and be locked by bits of metal from chipped teeth.
Diagnosis chart. Refer to the quick reference diagnosis chart (Figure 2).
Figure 2 - Transmission quick reference diagnosis chart.
A good preventive maintenance program can help avoid failures, minimize vehicle down time, and reduce the cost of repairs. Often the transmission failure can be traced directly or indirectly to poor maintenance.
Daily Inspections. Many of the practices listed here are part of the driver/operator pre- trip inspection:
A - Inspection. The following should be checked at each A or lube inspection.
B - Inspection. Figure 3 identifies key areas of a transmission that should be routinely checked during B inspection.
Figure 3 - Inspection points on standard transmission.
C - Inspection.
Oil leakage in standard transmissions is not common, but when leaks occur the rear transmission seal is a common offender (Figure 4). A leaking rear main seal can lead to complete transmission failure, so it must be repaired. It can be difficult to source oil leaks on transmissions, however, so be sure to identify the leak path before disassembling any components. You can use a checklist such as the following to help you identify the source of oil leaks, but it is most important before beginning to pressure wash the transmission, removing all evidence of oil leaks. Take care not to directly aim the pressure hose into the rear main seal.
Figure 4 - Transmission yoke seal.
Rather, use a wiper to clean up this area. Next, run the vehicle. It can really help to run a vehicle on a chassis dynamomter when attempting to locate hard-to-find leaks, because leaking oil is not smeared all over the transmission housing by the air flow.
Transmission oil checklist:
Overtorquing a transmission output yoke retaining nut can cause output shaft bearing failure. Using a 1-inch air gun to tighten yoke retaining nuts should be avoided, although it is okay to remove the nut using this tool.
Many fasteners used to couple flanges and manifolds to the transmission are bored completely through the housing. These fasteners must have thread sealant applied to the threads. If oil leakage is observed at the PTO covers, replace thee PTO cover gaskets and thoroughly clean the fasteners and their mating threads before reinstalling.
Always check for a plugged transmission breather when identifying the cause of a transmission oil leak. When transmission oil is raised from cold to operating temperatures, it expands to occupy a greater volume. If the breather is plugged, this can cause seal failure.
With the power plant in the vehicle, you can inspect all seals except the input shaft retainer seal. If this seal is leaking, oil will drip out through the plughole in the bottom of the pan under the flywheel housing when the plug is removed.
If oil does drip out at the flywheel housing drain plug, examine the oil closely. It may be engine oil leaking from the engine crankshaft rear oil seal. The engine oil is much thinner (has less viscosity) than the transmission oil, so you should be able to tell which seal is leaking.
An oil leak, either from the engine or transmission input shaft seals, is serious, because the oil can ruin the clutch. An oil-soaked clutch disk will almost always slip or grab.
In addition to the leakage problems, there are other problems that can develop in the standard transmissions used in almost all trucks. We can classify these as mechanical problems.
The best way to locate mechanical problems in the transmission is to road test the vehicle. Before road testing, however, check for missing or loose bolts and be sure the oil is at the proper level in the transmission case. Check the parking brake mechanism for proper mounting and correct adjustment. Check all moisture seals or boots. Check the action of the gearshift levers.
The transmission is often blamed for problems that are elsewhere. For example, with the engine running and the vehicle standing still, disengage the clutch and move the gearshift lever into first or reverse. You should be able to shift into either of these gear positions without any gear clashing or without the vehicle moving. If the gears clash or the vehicle attempts to move with the clutch disengaged, the trouble is in the clutch and not the transmission.
Check the clutch pedal free travel and adjust it if necessary. The clutch must be correctly adjusted before the transmission can operate properly. The clutch must fully disengage every time the clutch pedal is pushed all the way down, and it must fully engage every time the pedal is released.
With the transmission in neutral, the engine running, and the clutch engaged, all of the constant-mesh gears in the transmission will be turning. There should be very little gear or bearing noise.
If the transmission is quiet in neutral with the clutch engaged, disengage the clutch. If a noise is now heard, the trouble is with the clutch and not the transmission. Usually, the clutch release bearing or the clutch shaft pilot bearing is at fault if a noise is heard only when the clutch is disengaged.
Sometimes, noises in other parts of the power train, such as U-points, propeller shafts, and differential, sound as if they are in the transmission. The misalignment of power train components usually produces a noise that may sound as if it is coming from the transmission. So be sure to check all mounting bolts on the engine, transmission, and differentials before road testing the vehicle. Also, check the propeller shafts and U-joints for evidence of wear or looseness.
Loose, bent, or shifted suspension system components will cause misalignment of the power train components that can produce a noise that may sound like a defective transmission.
Noises that may originate in the transmission are difficult to describe. A noise that may sound like a howl to you may sound like a squeal to someone else. Other terms often used to describe gear or bearing noises may include such words as "hum," "knock," "grind," "whine," and "thump."
If a tooth is broken off of one of the gears, a distinct thumping noise will be heard once during a complete revolution of the gear. The thump will be more pronounced if torque is being delivered through that gear.
Gears with worn, rough teeth will usually produce a grinding noise, especially when torque is being transmitted through them. Bearing noise is usually described as a howl, whine, or squeal. Actually, the type of noise made by a defective bearing will vary, depending on the type of defect and the load the bearing is supporting. In any event, loud noises coming from inside the transmission mean trouble.
Some whining or grinding noise can be expected, especially when the vehicle is being driven in first or reverse gear. The first-and-reverse sliding gear together with its mating countershaft gear and reverse idler gear are spur gears. Spur gears are always noisy, but they are frequently used because they are cheaper and do not produce thrust.
In the second-, third-, and fourth-speed ranges, the transmission should be much quieter than in first or reverse.
If, after a road test, you think the transmission is too noisy, be sure and report it to the maintenance supervisor. Be sure to describe the conditions under which the noise occurs.
Another common mechanical problem with transmissions of this type is slipping or jumping out of gear. Actually, the transmission is much less likely to slip or jump out of first or reverse than out of second-, third-, or fourth-speed gear. Second-, third-, and fourth-speed gears are all helical gears which, you recall, produce thrust.
The most likely causes of the transmission slipping out of gear are worn detent balls or springs in the shifter shaft cover. These spring-loaded balls hold the shifter shaft in position. If the spring does not have enough tension or if the balls are worn, the transmission will almost certainly slip or jump out of gear. Synchronizer damage will also cause the transmission to jump out of gear.
Slipping out of any gear is most likely to occur when the driver suddenly takes his or her foot off the accelerator pedal, especially when descending a steep hill. The thrust produced by the helical gears will tend to move all rotating gears and shafts to the rear of the transmission, as long as the torque provided by the engine is being delivered to the rear wheels by the transmission. However, when the driver takes his or her foot off of the accelerator pedal, the situation is changes. The rear wheels now try to drive the engine through the transmission. This reverses the direction of the torque being delivered through the transmission gears, and the thrust is now toward the front of the transmission. If this thrust is not controlled by the thrust washers and bearing retainers, it is likely to force the shifter shaft to move in spite of the spring-loaded ball that holds it. When this happens, the transmission slips out of gear.
Occasionally, a transmission slips out of gear because the driver does not fully engage the gear when moving the lever. However, when a transmission slips out of gear fairly often, it should be replaced.
When repairing a manual transmission, you must be able to identify the exact type of transmission that you will be working on. Usually, there will be an ID tag or stamped set of numbers on the transmission (Figure 5). Many tend to look alike from the outside, but the tag will identify the characteristics of your transmission. DO NOT REMOVE OR DESTROY THE TRANSMISSION IDENTIFICATION TAG!
Figure 5 - Sample transmission identification tag and location.
When removing a transmission, use a transmission jack. A transmission jack has a special saddle and chains or straps for securing the transmission to keep from falling during removal and installation (Figure 6). If you do not have a transmission jack, you can use a floor jack.
Figure 6 - Transmission jack.
When using a floor jack, place a piece of wood between the jack pad and the transmission case as illustrated in Figure 7, View A, or a transmission adapter for the floor jack as shown in Figure 7, View B. Move the transmission to the workbench with the jack lowered.
Figure 7 - Floor jacks converted as transmission jacks.
Use the following procedure to remove a manual transmission:
Figure 8 - Removing the transmission.
Teardown procedures will vary from one transmission to another. Always consult the service manual for detailed procedures.
Basically, remove the shift fork assembly and cover. With a shift rail type, remove the shift lever assembly (Figure 9).
Figure 9 - Removing the shift fork assembly.
If the transmission has an inspection cover, observe transmission action with the cover removed. Shift the transmission into each gear by moving the small levers on the shift forks. At the same time, rotate the input shaft while inspecting the condition of the gears and synchronizers.
Unbolt the rear extension housing. Tap the extension housing off with a brass hammer.
Going to the front of the transmission, remove the front bearing cover and any snap rings. Carefully pry the input shaft and gear forward far enough to free the main output shaft.
Next, use a dummy shaft or arbor shaft (shaft tool designed for driving) to drive out the counter shaft and reverse idler shaft.
Now you can remove the input shaft and the output shaft assemblies. Slide the output shaft and gears out of the back or top of the transmission as a unit. Be careful not to nick the gears on the case (Figure 10).
Figure 10 - Removing the main shaft
With all of the parts removed from the case for metal shavings, inspect them closely. First, check inside the case for metal shavings. If brass-colored particles are found, one or more of the synchronizers or thrust washers are damaged. These are normally the only parts in the transmission made of this material.
If iron chips are found, the output drive gears are probably damaged. After checking the case, clean the inside with solvent. Then blow it dry with compressed air while wearing eye protection. Also clean the transmission mission bearings.
Next, inspect all of the output gears. Look for wear patterns or chips on the gear teeth. The gears are usually case-hardened. If wear is more than a few thousandths of an inch, the hard outer layer will be worn through, and the gear must be replaced.
Transmission shaft runout is the amount of wobble produced when a bent or worn shaft is rotated. If gear tooth wear is uneven, check the shaft bearings and shafts. They may be worn or bent. A dial indicator can be used to check the transmission shafts for straightness. Refer to specifications for the amount of allowable runout.
Inspect the synchronizer assemblies, especially if the transmission had gear shift problems (Figure 11). Check the teeth, splines, and grooves on the synchronizers. Replace parts as needed. View A shows all the components of the assembly. View B shows basic synchronizer components. View C shows checking shift forks and synchronizers for wear. View D illustrates inspect ridges inside the blocking ring.
Figure 11 - Inspecting synchronizer assembly.
Any worn or damaged part in the transmission must be replaced. This is why your inspection is very important. If any trouble is not corrected, the transmission rebuild may fail. You would have to complete the job a second time.
It is generally recommended to always replace all gaskets and seals in the transmission. Even though a seal or gasket might not leak before teardown, it could start to leak after assembly. Figure 12 shows a typical way of replacing a rear seal. The rear seal can be removed and installed with the transmission still in the vehicle. View A shows the seal being removed and View B shows driving in a new seal. Coat the outside of the new seal with non-hardening sealer before installing.
Figure 12 - Replacing the oil seal.
When replacing a gear on the output shaft, you should also replace the matching gearset on the countershaft. If a new gear is meshed with an old worn gear, gear noise can result.
Frequently, you will need to replace input shaft bearings. These bearings are prone to wear because they support a great amount of load. You can turn by hand to feel signs of wear or unevenness. A special puller may be needed to remove some bearings (Figure 13).
Figure 13 - Removing bearings with puller.
After obtaining new parts to replace the old worn ones, you are ready for reassembly. Typically, the transmission is assembled in reverse order of disassembly. Again, refer to the service manual for detailed procedures for reassembly.
The service manual usually has exploded views of the transmission (Figure 14) and assemblies (Figure 15). They will show how each part is located in relation to the others. Step-by-step instructions will accompany the illustrations.
Figure 14 - Exploded view of a 5-speed transmission.
Figure 15 - Exploded view of a mainshaft assembly.
To hold the needle bearings in countershaft gears, coat the bearings with heavy grease. Then fit each bearing into position. The grease will hold the bearings as you slide the countershaft into the gear (Figure 16).
Figure 16 - Installing needle bearings.
Figure 17 - Using a feeler flat gauge to measure clearance between mating parts.
Also, following the manufacturer's instructions, measure the end play or clearance of the gears and synchronizers (Figure 17).
Assemble the shift fork mechanism and with the synchronizers and shift forks in neutral, fit the shift fork assembly on or in the case. Check the action of the shift forks.
Make sure the transmission shifts properly before installing it. This will definitely save you from having to remove the transmission later when problems are discovered.
While the transmission is removed from the vehicle, it would be recommended to disassemble and inspect the clutch.
Before transmission installation, place a small amount of grease in the pilot bearing and on the inner surface of the throw-out bearing. Do not place lubricant on the end of the clutch shaft, input shaft splines, or pressure plate release levers. Grease in these locations can spray onto the clutch friction disk, causing clutch slippage and failure.
Shift the transmission into high gear. This will help position the input shaft into the clutch disk during transmission installation.
Place the transmission on the transmission jack. Position it behind the engine. Double check that the throw-out bearing is in place on the clutch fork. Carefully align the transmission with the engine.
The input and output shaft must line up perfectly with the centerline of the engine crankshaft. If the transmission is tilted, even slightly, it will not fit into place.
With the transmission in high gear to hamper input shaft rotation, slowly push the transmission into the clutch housing. You may need to raise or lower the transmission slightly to keep it in alignment. When the transmission is almost in place, wiggle the extension housing in a circular pattern while pushing toward the engine. This should help start the input in the crankshaft pilot bearing. If the clutch and pilot bearing are installed correctly, the transmission should slide fully into place by hand.
Do not use the transmission bolts to draw the transmission into the clutch housing. The transmission input shaft could be smashed into the crankshaft pilot bearing. Serious component damage may result.
With the transmission bolted to the clutch cover, install the rear cross member and motor mount.
Reinstall the clutch linkage and transmission linkage.
Reconnect electrical connectors. Install parking brake hardware and exhaust pipes (if removed). Install the speedometer cable assembly.
Install the driveshaft assembly, making sure to line up the marks you made during removal.
Fill the transmission to the proper level using specified lubricant. Install the fill plug and tighten.
To adjust many types of transmission linkage, place the gearshift lever and transmission levers in neutral. Then insert an alignment pin (special diameter tool or rod) through the linkage arms. The pin must fit through the holes in the shifter levers (Figure 18).
Figure 18 - Adjust external linkage.
If the pin will not fit through the hole, lengthen or shorten the linkage rods. Adjust the rods so that the alignment pin fits easily through the hole in the shifter assembly and into the corresponding hole in the housing.
This basic procedure will vary with different types of gear shift mechanisms. When in doubt, refer to the service manual for specific instructions for your particular condition.
After all the drivetrain parts are reinstalled, lower the vehicle and reconnect any disconnected hardware under the hood, including the battery negative cable. Install the boot to the floor inside the vehicle and screw the gear shift knob back if removed.
After everything has been reconnected, operate the clutch pedal a few times to see that the clutch engages and disengages properly. Also, move the gearshift lever to check operation of the shift linkage.
Carefully road test the vehicle, making sure the transmission and clutch operate properly. If any problems are detected, correct them before releasing the vehicle. Make any other adjustments as needed and recheck the oil level.
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The transfer case is the mechanical unit that splits power between the front and rear wheels (axles) (Figure 19). It is attached to the rear of the transmission or transaxle and receives power from the transmission or transaxle output shaft. Power leaves the transfer case through two output shafts. In addition to various drive gears, transfer cases may contain drive chains, a differential assembly, or a viscous coupling. The term all-wheel drive (AWD) in heavy-duty trucks usually refers to a chassis with a front drive axle in addition to rear tandem drive axles. Figure 20 shows the location of the transfer case on a typical truck chassis.
Figure 19 - Three-shaft design transfer case.
Figure 20 - All-wheel drive vehicles
Transfer cases can transfer drive torque directly using a 1:1 gear ratio, or can be used to provide low gear reduction ratios of 2:1 additional to those in the transmission. The drop box design of a transfer case housing permits its front driveshaft output to clear the underside of the main transmission
Most transfer cases are available with power takeoff (PTO) capability and front axle declutch. The front axle declutch is used to option-drive to the front axle when negotiating steep grades or slippery or rough terrain. Both the PTO and front axle drive declutch are driver engaged by dedicated shift levers.
Although it is relatively small, the transfer case can develop problems that affect the entire drive train. Accurately determining the source of the problem is the first step in repairing a transfer case. Figure 21 illustrates some of the components that can develop problems in a typical transfer case.
Figure 21 - Exploded view of a gear-type transfer case showing components that can develop problems.
Lubricant leakage in a transfer case is a loss of lubricating fluid from the housing, gaskets, or seals. Leaks may be noticed as oil drips on the pavement.
Always begin troubleshooting an oil leak by making sure the leak is from the transfer case. Engine oil, transmission fluid, or power steering fluid can be mistaken for transfer case lubricant. As a vehicle is driven, oil leaking from the front components is sometimes blown onto the rear components. This makes it difficult to determine the origin of the leak.
One way to verify a transfer case leak is to check the oil level in the case. In most transfer cases, the oil level is correct if the oil just wets the bottom of the fill plug threads (Figure 22). If the oil level is normal, the leak is probably from some other part of the vehicle. If the oil level is low, there is generally a leak somewhere in the transfer case. If the oil level is too high, overfilling may be the cause of the leak. Overfilling may cause oil to be forced out of a seal or out through the case vents.
Figure 22 - Checking oil level.
When looking for transfer case leak, remember that oil will drip downward and is often blown to the rear of the vehicle. Look for the leaks from loose or broken bearing retainers or from retainer gaskets. Check the seals in the area where the drive shaft yokes enter the transfer case. Leaks may also originate from loose case housing bolts and defective housing gaskets. Check for leaks from the drain and fill plugs, and from bolts at the bottom of the transfer case. Some bolts must be installed with a thread sealant to prevent leaks.
When a transfer case fails to engage properly, no power will be transmitted to one or both sets of wheels. However, a transfer case rarely fails to drive at least one set of wheels. If the vehicle will not move in any transfer case mode, the problem is usually caused by a defective transmission or by a binding axle. If the transfer case fails in only one gear, the problem may be caused by defective internal parts or linkage.
Transfer case failure causing a no-drive condition in all gears is usually caused by stripped gears or splines, a broken chain, a broken shaft, or a viscous coupling that has lost its fluid. Generally, a transfer case must be removed from the vehicle to repair these defects.
Abnormal noises in the transfer case include whines and rumbles that occur during operation. Other abnormal noises include grinding, knocking, popping, or snapping sounds. Since the transfer case is mounted directly behind the transmission, it is easy to mistake transmission or drive noise for transfer case noise. Always make sure the noise is actually coming from the transfer case and not from some other part of the vehicle.
Transfer case noises usually vary from driving conditions. If the transfer case is the part- time type, road test the vehicle to determine the mode in which the problem occurs. If the transfer case is the full-time type, the vehicle should be placed on a lift to listen to for abnormal noises, such as grinding, knocking, or excessive whining. Remember that normal drive train noises will seem louder when you are under the vehicle.
Typical causes of whining or rumbling noises in the transfer case include a low lubricant level, worn or damaged input gear, worn or damaged driven gear, worn bearings, worn planetary gears (where used), and damaged input or output shaft bearings. If the speedometer gears are installed on the transfer case, check them for damage. A worn drive chain can cause popping or snapping noises as the vehicle is accelerated.
Sometimes, the transfer case will be noisy because the front and rear axle ratios do not match due to modifications. This can cause friction in the transfer case planetary gears or coupling. To repair damaged or worn transfer case parts, the unit must be removed and overhauled.
When the transfer case jumps out of gear, the shift mechanism will move into Neutral at undesirable times. The first step in troubleshooting this type of problem is to determine the gear in which the problem occurs. If the problem occurs in only one gear, the teeth on that particular gear may be chipped or worn. Further, a blocking ring of a sliding clutch of the affected gear may be worn.
In some cases, it may become difficult to change transfer case gears. Hard shifting may be caused by a shift linkage that is bent or worn. In some cases, the linkage simply needs lubrication. Figure 23 shows shift linkage adjustment. If the linkage is okay, the problem may be caused by bent or damaged shift forks inside the transfer case.
Figure 23 - Transfer case shift linkage adjustment.
Transfer case shudder is a jerking motion that typically occurs during acceleration. Possible shudder causes are low fluid level, loose transfer case fasteners, worn gears or bearings, or defective internal clutches. A shudder may also occur at low speeds on vehicles with a defective viscous coupling or clutch pack.
When diagnosing this problem, make sure the shudder is not caused by the engine, clutch, transmission, limited-slip differential, or universal joints. Check the fluid level, and make sure all fasteners are tight. Pay particular attention to the fasteners holding the transfer case to the rear of the transmission. If fluid level is at the normal mark, and the fasteners are tight, it will probably be necessary to remove and disassemble the transfer case to locate the problem. If the vehicle has a viscous coupling, check for silicone in the transfer case lubricant. Because silicone does not mix with oil, it will appear as globules in the lubricant. This will indicate the silicone has leaked from the viscous coupling and the coupling requires attention. If the viscous coupling has failed, check the different front and rear axle ratios, which may have caused it to fail. Refer to Figure 24 for quick reference to troubleshooting.
Figure 24 - Transfer case quick reference troubleshooting guide.
Many of the problems encountered in all-wheel drive systems result from improper transfer case operation. Worn or damaged transfer cases must often be removed from the vehicle for service.
Transfer case removal is similar to manual transmission removal. Nevertheless, recommended removal procedures vary among manufacturers, and for that reason only the basics of transfer case removal will be discussed. Refer to the manufacturer's service manual for your particular transfer case for detailed procedures for overhaul, such as details regarding whether or not the transmission and transfer case should be removed as a unit. The general procedure for removing a transfer case is as follows:
If the transfer case is not properly supported, it will drop when the bolts holding it to the transmission are removed, causing injury or damage.
After removing the transfer case, drain any remaining oil. Clean the outside of the housing and mount the unit securely on a clean workbench. Make sure the transfer case will not fall off the bench during the disassembly procedure.
If not removed previously, remove the drive shaft yokes. Most drive shaft yokes are secured to the output shafts with large nuts. The nuts can be removed with the proper wrench. The yoke should be held securely with a special tool or a large pipe wrench as the nut is loosened. Mark the front and back yokes to ease reassembly.
Remove the speedometer gear and housing from the transfer case. Also, remove electrical switches and other external transfer case parts as necessary.
Figure 25 - Transfer case with a two-piece or split-type housing.
If the transfer case housing is the common two-piece type as the one shown in Figure 25, remove the bolts holding the front and rear halves together. Notice the identification tag on the transfer case. The same as previously discussed with the transmission, this should be used to order parts and to determine which service specifications you should use. The halves can then be split apart. If the gasket sticks, pry the halves apart with pry bars (Figure 26).
Figure 26 - Prying apart a transfer case housing.
If the housing is a one-piece type, the front and rear retainers that hold the input shaft bearings should be removed to gain access to the internal parts (Figure 27). As you see in View A, most bearing retainers can be unbolted from the case and then lightly pried up, always being careful not to damage any machined parts. View B shows the basic procedure for removing the bearing retainer and shaft on the one-piece case. Always use a soft-face hammer to avoid damaging the shaft or gears.
Figure 27 - Disassembly of a one-piece transfer case.
After accessing the internal parts, remove any snap rings that hold the transfer case gears to the shaft or secure the shafts to the housing. Then remove the internal parts as necessary. All shafts, sliding gears, and planetary gears, if used, should be removed at this time. Refer to the service manual for detailed disassembly procedures for your particular transfer case.
Figure 28 shows a mechanic removing a mainshaft, complete with gears, shift fork, and rail. Removal procedures for other parts are similar. Some transfer cases do not have an input shaft. Instead, the transmission output shaft is splined to the inside of the transfer case input gear. Remove the differential unit, viscous coupling, or clutch drum as applicable.
Figure 28 - Removal of mainshaft, complete with gears, shift fork, and rail.
If the transfer case is equipped with a drive chain, lift the front output shaft, sprocket, and chain out of the case. In many transfer cases, the chain and sprockets are removed as an assembly. In some designs, however, it is necessary to slide the chain off the mainshaft drive sprocket during the removal process. After removing the chain and sprockets, carefully remove any thrust washers that were located under the sprocket.
While disassembling the transfer case, note the relationship of all parts so that they can be reinstalled properly. If necessary, mark the parts with a punch or a scribe to ensure proper assembly.
Before inspecting the transfer case parts, scrape all gasket material from the transfer case housing, being careful not to damage the surfaces. Check the bottom of the housing for needle bearings or other small parts. Clean the inside of the housing and all internal parts. Make sure all sludge and metal particles are removed.
After cleaning, thoroughly inspect the transfer case housing, bearing retainers, and extension housings for cracks or other damage. Additionally, check all bushings and seals for wear.
All internal transfer case parts should be inspected for wear and damage. Examine all shaft bearings, needle beatings, shift forks, sliding clutch sleeves, and washers. Replace worn components as needed.
Figure 29 illustrated the checks that should be made to a transfer case planetary gear assembly. Examine the gears for worn, cracked, or chipped teeth. Damaged gears should be replaced. Check for wear between gear bushings and the shafts. Look for wear or damage to the splines on the shafts and to the planetary housings. Splines can strip off so cleanly that splined surfaces appear to be machined smooth. This makes it very important to compare these parts to specifications.
Figure 29 - Check planetary gear assembly.
If used, check the transfer case blocking rings of the sliding clutch assembly for wear on the outer teeth. Also, inspect the areas where the inner cone ridges contact the gear cone. Replace blocking rings showing signs of wear or damage.
It may be necessary to disassemble the mainshaft to inspect the shaft-mounted components. In the service manual there may a sectional view similar to the one shown in Figure 30 which is helpful in guiding you through the proper steps in disassembling and reassembling the mainshaft without damage or incorrect assembly.
Figure 30 - Assembly details of a transfer case mainshaft.
Check the sliding clutch sleeves where the shift forks ride. If the shift forks are equipped with separate pads at the riding surfaces as illustrated in Figure 31, the pad should be replaced whenever the transfer case is disassembled. Replace the entire shift fork if it is bent or worn. Seals used in areas where the shift forks or levers pass through the case should be replaced.
Figure 31 - Checking shift fork for wear.
If the transfer case uses a differential assembly or a viscous coupling, it should be checked for wear and damage. Always follow the inspection procedures outlined in the factory service manual.
If the transfer case has a clutch pack, carefully check the condition of the clutch plates. Replace any clutch plates that are worn or burned. Clutch pack clearance is vital to proper operation and must be carefully checked. Adjust clearance by adding or subtracting shims or replacing the existing shims with shims of the proper thickness.
The transfer case should be reassembled in the reverse order of disassembly. Before beginning, replace worn bushings and seals (Figure 32).
Figure 32 - Replacing worn bushings and seals.
Begin reinstalling the parts in the transfer case. The service manual will provide an exploded view such as the one shown in Figure 33 and, along with sequential numbering, is very helpful in determining where parts are placed. Take your time and do it right the first time, especially installing the internal parts. If not, you will just have to disassemble and reassemble correctly. Reassembly procedures may vary with the transfer cases you will overhaul. The following procedures will serve as a general guide to reassembly.
Figure 33 - Exploded view of a typical one-piece transfer case.
Assemble the mainshaft gears and hubs onto the mainshaft. Use new parts as needed.
Install all internal parts in the transfer case housing. When referring to Figure 34, notice that the shift forks are usually installed at the same time as the gears they operate. Make sure the shafts are properly seated in the bottom of the housing to avoid binding.
Figure 34 - Reassembly of internal components.
Install the drive chain, making sure the sprockets and shafts are properly aligned with the chain as Figure 35 illustrates. After installing the chain, make sure all thrust washers are installed as shown in Figure 36. Next, install the snap rings that hold the chain and shafts in place.
Figure 35 - Reinstalling drive chain and sprockets.
Figure 36 - Installing thrust washers.
On two-piece housings, place a new gasket, an appropriate sealer, or both on the mating surface of the front housing. Make one last check of all internal parts and install the rear housing on the front housing (Figure 37). Install and tighten the housing bolts. Then, make sure both shafts turn. If either shaft is binding, the transfer case must be disassembled to determine the cause.
Figure 37 - Recheck all components before installing rear housing.
On one-piece housings, install the bearing retainers using new gaskets. After installing the retainers, make sure the shafts turn. Do not install the transfer case if the shafts are binding; correct the problem first. Adjust the bearing preload according to the manufacturer's service manual.
Manufacturers provide specifications for shaft endplay. If specifications are provided, the endplay should be measured. If endplay is incorrect, the rear half of the housing or the bearing retainer must be removed, and thicker or thinner thrust washers must be installed.
If hypoid drive and driven gears are used in the transfer case, they may require adjustment. The adjustment procedure is similar to that used for a ring or pinion (Figure 38). Be sure to refer to the service manual for detailed procedures.
Figure 38 - Checking the adjustments of the hypoid gear on the transfer.
Finally, install the external housing components, such as the speedometer drive gear, front and rear drive shaft yokes, electrical components, and drain and fill plugs.
The transfer case is installed in the reverse order of removal. Because the transfer case is heavy and unbalanced, exercise caution when lifting it or working around it.
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A variety of accessories on heavy-duty tucks and construction equipment require an auxiliary drive. Auxiliary drive can be sourced directly from the engine or by means of the transmission or transfer case. Auxiliary drive systems are power takeoffs or PTOs. When a PTO is coupled to the transmission or transfer case, it is an assembly such as the two shown in Figure 39. Vehicles and equipment that have a PTO are wreckers, fuel tank trucks, dump trucks, cement transit mixers, fire trucks, water well drilling rigs, auger trucks, aerial bucket trucks, lube trucks, and cranes.
Figure 39 - Power takeoffs are used to operate auxiliary equipment.
The PTO is simply a means of using the chassis engine to power accessories, eliminating the need for an additional auxiliary engine. There are six basic types of PTOs classified by their installation or drive source:
The objective of a PTO is to provide driving torque to auxiliary equipment such as pumps, compressors, and winches. The driven equipment can be mounted either directly to the PTO or indirectly using a small drive shaft. The PTO input gear is placed in constant mesh with a gear in the truck transmission. The PTO may couple into a transmission by a means of a dedicated PTO gear on the transmission countershaft. Any rotation of the PTO countershaft gear drives the PTO input gear. The on/off capability required by a PTO is provided within the PTO assembly.
Gears in most PTO units can either be spur or helical. Establishing the correct mesh between the PTO drive gear and its partner in the transmission is critical: too little or too much backlash can produce problems. Backlash is defined as the space between meshing surfaces of the gears in gearbox devices. Space is needed for expansion caused by heat and viscosity changes in lubricants. Refer to the PTO service manual for the correct backlash adjustment procedure which is to be performed on every PTO installation. Use of a dial indicator is recommended. The recommended backlash between the transmission and PTO is from .006 to .012 inches.
Too many gaskets will create too much backlash and cause the PTO to rattle when running at no load. To correct - remove one or more gaskets and recheck backlash. Too few gaskets will cause PTO to whine and may cause difficult shifting of the PTO and transmission. To correct - add one or more gaskets and recheck backlash. PTOs will not always make noises when improperly spaced. Correct backlash must also be established when gear adapters are used. Transmissions using automatic transmission fluid may have higher noise levels caused by the thinner consistency of the lubricant and the large PTO drive gear in the transmission.
Gear ratio is also critical in PTO operation. Gear ratio must be set to the torque capacity and operating speed required of the driven equipment.
As previously mentioned, some PTOs function when the vehicle is running and they can operate at a fixed ratio of engine speed, while others vary depending on which gear the transmission is in. Other PTOs are designed to work only when the transmission has been shifted to the neutral position. All transmission-mounted PTOs are clutch- dependent in that they transmit torque only when the vehicle clutch is engaged. Flywheel and forward crank-shaft-driven PTOs operate independently of vehicle clutch engagement, so they are used in applications where continuous auxiliary power is required as previously mentioned; a cement transit mixer is an example.
Several types of shift mechanisms are used to connect the power takeoff with operator controls. Cable, lever, electric-over air, and electric are options used on trucks. The newest version is electronic-over electric PTOs.
Most PTOs feature simple designs and rugged construction. Care should be taken to properly mount a PTO. Setting backlash to specification is critical; this operation is performed by setting the gasket thickness at the PTO mounting flange. Contaminated transmission oil can damage a PTO, so regular maintenance is important.
The power takeoff, being an integral part of the transmission, should be serviced at the same intervals as the transmission. Transmission fluid changes should follow the interval recommended by the vehicle manufacturer for severe service. Transmission oil level is important. Checking for PTO leaks and checking the transmission oil level should be done on a regular basis.
The PTO is also part of a system. The PTO system may include the activation control parts, a driveshaft, or hydraulic pump. This PTO requires periodic checks and service. Typically the interval for maintenance checks of the PTO system depends on the application of the system. Every time the chassis is lubricated or a mechanic is under the vehicle, the PTO system should be checked and serviced. For severe duty PTO system applications, it is recommended that the system be checked for service every 100 hours of use. Service should include checking and lubricating direct mount pump shaft connections. PTO gears can be checked for wear by removing the inspection or shifter cover. If pitting, galling, cracking, or deformation of the gears has occurred, then the PTO needs to be rebuilt or replaced.
After installing a new PTO or overhauling, recheck the PTO within the first week of use. Check for leaks and loose mounting hardware such as studs, cap screws, and nuts. Recheck the cable or lever connections for proper adjustment and tighten any loose connections. At regular maintenance intervals, check adjustments, lubricate moving parts, and tighten and repair the connections, mounting hardware, and cable or lever linkages.
Pumps that are mounted directly to the PTO output require the application of an antiseize or a high temperature, high pressure grease. The purpose of this grease is to help make the PTO easier to service and to reduce the effects of fretting corrosion on the mating PTO and pump shafts. PTO applications under severe duty cycles and/or high torque requirements may require servicing this shaft connection, periodically re- greasing the vibrations inherent in these vehicles. Fretting corrosion cannot be stopped by applying grease; the grease is only a deterrent.
If the system utilizes a driveline between the PTO and another device and if you have noise in your system that was not there before, the angularity or phasing of your driveline may be the cause. Check driveline angularity and reduce total angularity per the recommendation in Figure 40, and be sure the PTO shaft is parallel within 1.5° to the pump shaft (or driven unit). Drivelines must be in phase, that is, the yoke ears on the PTO and pump shafts must be in alignment; Figure 41 provides a quick reference for troubleshooting a power takeoff.
Figure 40 - PTO driveline angularity.
Figure 41 - Power takeoff quick reference diagnosis.
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Propeller shafts or driveshafts have a simple function: to transmit drive torque from one driveline component to another. This should be accomplished in a smooth, vibration- free manner. In a heavy-duty truck, that means transmitting engine torque from the output shaft of the transmission to a rear axle or to an auxiliary transmission. Driveshafts also are used to connect forward and rear axles Figure 42 shows the driveshaft arrangement used in a tandem drive tractor equipped with an auxiliary transmission.
Figure 42 - Driveshafts transmit torque between driveline components.
In most cases, a driveshaft is required to transfer torque at an angle to the centerlines of the driveline components it connects to. Because the rear drive axle is part of the suspension and not connected to the rigid frame rails of the vehicle, the driveshaft must be capable of consistently changing angles as the rear suspension reacts to road or terrain profile and load effect. In addition to being able to sustain constantly changing angles, a driveshaft must be able to change in length when transmitting torque. When the rear axle reacts to terrain or road surface changes, torque reactions, and braking forces, it pivots both forward or backward, requiring a corresponding change in the length of the driveshaft.
A driveshaft assembly is made up of the following:
The propeller shafts have a tubular construction designed to sustain high torque loads and be light in weight. Figure 43 shows the components of a typical heavy-duty truck driveshaft.
Figure 43 - Exploded view of a heavy-duty truck driveshaft.
Two types of driveline configurations are used to transmit drive torque to the drive wheels: the parallel-joint driveline and the non-parallel or broken-back driveline. In the parallel-joint type, all the companion flanges or yokes in the driveline are on a parallel plane to each other with working angles of the joints of a given shaft being equal and opposite (Figure 44). For instance, if the transmission output shaft centerline angles down 5 degrees from a true horizontal plane, the centerline at the front of the auxiliary main shaft or rear axle pinion shaft must angle 5 degrees up.
Figure 44 - Parallel joint driveline arrangement.
With the no-parallel or broken-back installation, the working angles of the U-joints of the driveshaft are equal, but the companion flanges and/or yokes are not parallel (Figure 45). For example, the transmission yoke is angled 3 degrees down from horizontal plane, while the rear axle pinion flange is angled up at 12 degrees. Providing that the U- joint angles of this propeller shaft remain equal, the shaft will run vibration free.
Figure 45 - "Broken back"- type driveshaft, angles A and B are equal.
Proper U-joint working angles are required for vibration-free and long-lasting driveline operation. Most drivelines are angled on a vertical plane as shown in Figure 46, View A, but on some trucks, the drivelines are also horizontally offset (angled) as shown in Figure 46, View B. When a driveshaft is angled on both the vertical and horizontal planes, a compound angle exists.
Figure 46 - One-plane-angle and two-plane angle driveshafts.
All U-joints have a maximum working angle at which they can smoothly transmit torque. This working angle depends in part on the U-joint size and design. Exceeding the maximum recommended working angles of U-joints can destroy a U-joint rapidly and also damage interconnected driveline components.
High working angles combined with high rpms tend to result in a reduced U-joint life. Unequal U-joint working angles can cause vibrations and contribute to U-joint transmission, and differential problems. Ideally, the operating angles on each end of a driveshaft should be equal or within 1 degree of each other and have a 3-degree-maximum operating angle. Driveshaft rpm is the primary factor in determining the maximum allowable operating angles in a give application.
Figure 47 correlates expected driveshaft rpms with maximum working angles. You can see that the faster a driveshaft turns, the lower the allowable working angle drops. Angles are calculated at the U- joints as the driveshaft angles downward from the transmission to connect to the input yoke on the rear axle differential carrier yoke. For example, a 2,100 rpm engine running through a fast overdrive transmission and a fairly slow axle ratio might turn a driveshaft faster than 3,000 rpm. This would limit the maximum permissible angle to 5 degrees, measured as the truck sits on level pavement.
Figure 47 - Maximum U-joint angles.
With equal working angles, the rear U-joint will slow down by the same amount that the forward joint speeds up during a rotation, resulting in U-joint cancellation. The driving and driven shafts will turn at constant and identical speeds. If the working angles of two opposed U-joints vary more than 1 degree, the driveshaft will not rotate smoothly because it accelerates and decelerates during a cycle. The result is vibration and ultimate U-joint failure.
U-joint working angles become greater when the vehicle suspension flexes over uneven road surfaces. If this occurs at slow speeds, it should not be a major problem unless driveline angles are high to begin with. Driveline angles tend to present fewer problems these days than when tractors with a short wheelbase were mandated to keep vehicle overall length within legal limitations. Relaxation of those legal restrictions has led to the use of a longer wheelbase that allows driveshafts to be longer and eliminates aggressive U-joint working angles.
The longer the driveshaft, the greater the weight and therefore the greater the radial forces, especially as driveshaft rpm increases. At high speeds, balance becomes more critical. This is why manufacturers limit tube length. For example, at 3,000 rpm the length of any single driveshaft section, measured between the centerline of the U-joints at either end, should not usually exceed 70 inches.
Heavy-duty U-joints have a unique characteristic. Because they are always operating at an angle, they do not transmit constant torque or turn at a uniform speed during their 360-degree rotation (Figure 48).
Figure 48 - A driveshaft will increase and decrease speed once each revolution.
With the drive yoke turning at a constant rpm, a driveshaft increases and decreases speed once each revolution. To counterbalance this fluctuation above and below mean (average) driveshaft speed, the U-joints must be positioned in-phase with each other as demonstrated in Figure 49. An out-of-phase condition produces an effect similar to what occurs when two children are turning a rope, and the rope is snapped on one side; by contrast, if the rope is snapped at both ends at the same time, the resulting waves through the rope cancel each other, and neither child feels the reaction of the wave the snap causes.
Figure 49 - Two universal joints in-phase will cancel out the speed fluctuations in the driveshaft.
When in phase, the slip yoke lugs (ears) and welded yoke at the opposite end should be perfectly in line as illustrated in Figure 50. If a driveshaft is assembled one spline out, the driveshaft is out of phase and the result can be a significant vibration. There should be opposing alignment arrows stamped on the slip yoke and on the tube shaft so you can assemble a driveshaft in phase. If a driveshaft is not marked with timing arrows, do it yourself. Make a light score mark using a steel scriber.
Figure 50 - Driveshaft in phase.
Driveshafts should be routinely inspected and lubricated. Driveline vibrations, U-joint failures, and hanger bearing problems are caused by such things as loose end yokes, excessive radial play (side-to-side), slip spline play, bent driveshaft tubing, and missing lube plugs in slip joint assemblies.
You can limit driveshaft performance problems by performing some simple inspection procedures each time the driveshaft is lubed.
One of the most common causes of U-joint and slip joint problems is the lack of proper lubrication. If U-joints are properly lubricated at recommended service intervals, they will meet or exceed their projected operating life span. Regular lubing ensures that the bearings have adequate grease and, additionally, that the trunnion races are flushed, which removes contaminants from the critical surface contact areas.
Clean grease fittings thoroughly to remove accumulated grease and abrasives before use. Any contaminants and abrasives can be forced through the grease fitting into the bearing during lubing.
A hole in each trunnion leads to the center of the cross (Figure 51). A grease fitting often connects to the center of the cross. Grease injected through the fitting flows through the holes in the trunnions to each needle bearing. Some universal joints are sealed at the factory and cannot be greased unless they are disassembled.
Figure 51 - Internal passageways in this cross-and-roller universal joint act as a grease reservoir.
Heavy duty driveshafts typically use lithium soap-based extreme pressure (EP) grease, National Lubricating Grease Institute classification grades 1 and 2 specifications. Grades 3 and 4 are not recommended; because of their greater thickness they are less effective in the cold. Most lubed-for-life bearings use synthetic greases. When replacing a U-joint, note that the grease in the U-joint is only there to protect it during shipping and storage. Lubing the U-joint after installation is a service requirement and should never be put off. Lubrication schedules can vary greatly depending on operating conditions. Off-road equipment generally requires that the severe duty cycle recommendations on lube charts be followed. Off-road classifications are generally considered to apply to equipment that operates on unpaved roadways for 10% or more of the time.
If grease is seen to exit only from three of the four trunnion seals, the bearing is not properly lubed and will almost certainly fail. You must take appropriate corrective action when U-joints fail to take grease. If a driveshaft separates because of a U-joint failure, it can take out air tanks and fuel tanks, and can generally cause damage that greatly exceeds the cost of replacing a single U-joint.
Half-round end yoke self-locking retaining bolts should not be reused more than five times. If in doubt as to how many times the bearing bolts have been removed, replace with new bolts.
The chassis lubricant used for U-joints is satisfactory for use on slip splines. An EP grease meeting NLGI grade 1 or 2 specifications is required. Slip spines should be lubed on the same service schedule as U-joint intervals.
In cold temperatures, you should drive the vehicle immediately after lubricating the driveshafts. This activates the slip spline assembly and removes excessive lubricant. Excess lubricant in slip splines can freeze in cold weather to a wax consistency and force the breather plug out. This would expose the slip joint to contaminants and eventually result in wear and seizure.
Hanger bearings are usually lubed for life by the manufacturer and are not serviceable. However, when replacing a support bearing assembly, fill the entire cavity around the bearing with chassis grease to shield the bearing from water, salt, and other contaminants. You should put enough grease to fill the cavity to the edge of the slinger around the bearing.
When replacing a hanger bearing, make sure you look for and do not lose track of the shim pack that is usually located between the bearing mount and cross member. The shims set the driveshaft angles, and omitting them will result in a driveshaft vibration.
Replacing U-Joints is a routine shop task and usually requires no special tools. However, an arbor press or U-joint puller can make the procedure easier and reduce the risk of damaging the yoke. In most truck applications, you do not have to raise the vehicle to perform a U-joint replacement. You should begin by removing the grease fittings because these are easily sheared during the removal process.
When removing a driveshaft with half-round or flange type yokes, support the weight of the driveshaft with a sling before separating the U-joints.
Before removing a driveshaft, mark the slip yoke assembly and tube shaft with a paint stick to ensure the correct phasing alignment on reassembly. If the shaft assembly is to be cleaned before reassembly, use a steel scribe to indent alignment marks (Figure 52).
Figure 52 - Scribe mark the driveshaft, rear yoke, and universal joints before disassembly.
The key here is to use the least amount of force possible.
Although U-joints are relatively inexpensive and commonly replaced, the driveshaft, yokes, and slip spline assembly are expensive and designed to last the life of the vehicle.
The removal procedure is as follows:
Figure 53 - Universal joint puller.
The main problem when replacing U-joints is the potential to damage the driveshaft and slip yoke assembly. It is important to exert only enough force to separate the U-joint. Remember that using heat to free a seized bearing cap almost always destroys the U- joint, so this is not acceptable when you are removing the driveshaft for reasons other than replacing the U-joint. Many shops use a 50-lb. slide hammer to separate U-joints. This can result in damaging the driveshaft tube when the U-joint cups seize in the yoke bores.
Snapring U-joint. If the U-joints are secured in the yoke by snaprings, remove the snaprings and raise the driveshaft using either the jack or puller methods described earlier to separate the cups.
Half-round yoke assemblies. If the driveshaft has half-round end yokes, remove the strap retaining bolts or the U-bolts. Then collapse the driveshaft by moving it inboard to separate the bearing cups from the yoke. An advantage of this design is the ease it lends to removing driveshafts.
Flange type yokes. If the driveshaft has flange type yokes, loosen and remove the fasteners securing the flange yoke to the transmission or drive axle carrier flange. Hold the shaft firmly when tapping the flanges free. When separated, compress the driveshaft by forcing it inboard and lower the assembly to the floor.
Never use a sledge hammer directly on a yoke to separate a U-joint. The result will almost certainly be a damaged driveshaft.
Do not distort the driveshaft tube by applying excessive clamping force. Using an appropriate puller such as the one shown in Figure 54 is the best way to remove plate-type bearing caps. If a puller is not available or if the U-joint is not equipped with bearing plates, you can use an arbor press or hammer and soft round drift to remove the bearings.
Figure 54 - Plate-type bearings can be removed with a puller.
Another way to remove U-joints is to support the cross on vise jaws, and then tap the yoke to drive the bearing cup forward. This is the least preferred method because of its potential to damage either the yoke or the slip spline assembly. Use a minimum amount of aggression. When the bearing cup can be pulled out by hand, reverse the yoke and U-joint and repeat the procedure to remove the opposite bearing cap.
Inspect the U-joints and bearing cups for signs of wear and damage.
After removing the U-joint cross and bearing cups, inspect the yoke bores for damage or burns. Some bore irregularities can be removed with a rat tail or half-round file, followed by finishing with emery cloth.
Check the yoke bores for wear, using a go-no-go wear gauge. Use an alignment bar (a bar with approximately the same diameter as the yoke bore) to inspect for misalignment of the yoke lugs. Slide the bar through both yoke bores simultaneously. If the alignment bar will not pass through both yoke bores simultaneously, the yoke has been distorted either by disassembly malpractice or excessive torque and should be replaced. Next, clean and inspect the mating yoke with an alignment bar gauge. Do not risk reusing a defective yoke.
Use the following procedure to assemble a driveshaft installing new U-joints.
If the bearing cap binds in the yoke bore, gently tap with a ball peen hammer in the center of the bearing cap. Do not tap the outer edges of the bearing cap because this could damage either the bearing cup or the yoke.
You now have the U-joint at each end of the driveshaft assembly. Make a final inspection of the driveshaft, checking the following:
Once in use, bearing caps and their trunnions should remain matched. Also, never take assembly short cuts by installing only the new bearing caps on a used trunnion, as this will usually result in a rapid failure. Regard a U-joint cross, its four bearing assemblies, and mounting hardware as a unit, and replace as such.
It makes sense to remove the grease fittings when installing a driveshaft; if they are knocked against the yoke, they intend to shear. You can easily reinstall them after the driveshaft has been installed.
For driveshafts using half-round end yokes, first support the driveshaft more or less in position using slings. Then install the bearing cups onto the cross trunnions. Install the bearing cups into the end yoke shoulders and place the retaining straps over the bearing assemblies. Thread the self-locking capscrews into the threaded holes and torque the bolts to specification. Lubricate the cross and bearing assemblies.
Using slings to support the driveshaft, align the (permanent end) flange pilots of the driveshaft, flange yoke, and drive axle companion flange with each other. Align the bolt holes and then install bolts, lockwashers, and nuts to temporarily secure the driveshaft to the axle. Compress the slip joint assembly to position the opposite end of the driveshaft to the transmission companion flange. Align bolt holes and install bolts, lockwashers, and nuts. Torque the fasteners to OEM specifications.
Driveline vibrations are the source of many trouble cards, and some of them can be very difficult to isolate. Vibration is not as apparent on the off-road equipment as it is on the over-the-road vehicles. Operators are not likely to complain about vibration on off-road equipment. When investigating the source of a driveline vibration, you always should be aware that the cause can originate in an area other than in the driveline. It is probably not good practice to rely solely on a driver's report of a driveline vibration, so make it part of your normal routine to road test the vehicle.
The first challenge is for you to determine the source of the complaint. The cause of a vibration can be in the steering or suspension systems, in the engine or transmission systems, in the wheels or tires. Road test the vehicle loaded and unloaded, if possible, while recording engine rpm and road speed. Note any irregularities and at what engine or road speeds they occur. If the problem is noticeable only when pulling a trailer, try coupling to a different trailer to see if the problem persists.
An unbalanced driveline causes transverse vibrations or bending movements in a driveshaft. This type of vibration is directly related to driveshaft rpm and usually is most noticeable at a specific driveshaft speed. The cause is imbalance in the driveshaft and, without using specialized equipment, it can be difficult to pinpoint.
Attempting to balance a driveshaft without spinning it up is so much of a hit-and-miss process that it is unrealistic to attempt it. We will outline here a simple method of dynamically balancing a driveshaft assembly using a balance sensor and strobe light.
You need a balance sensor and strobe light, preferably from a kit designed to dynamically balance driveshafts. You will need to raise the truck off the ground and place it on stands ensuring the following:
Fit the balance sensor to the chassis to be tested. The test procedure is similar to that used when balancing tire and wheel assemblies. Using the strobe light and white machinist's crayon, mark the driveshaft and spin it up. If it is out of balance on this initial test, begin by removing any counterweights tacked onto the driveshaft with a cold chisel. Remove any weld by lightly grinding with an angle grinder.
Run the vehicle at those equivalent road speeds noted on the road test when the vibration was most noticeable. Use hose clamps to clamp counter weights (steel washers or blockweights) into position with the clamp worm screw located over the weight washer (Figurer 8-55). When the imbalance has been neutralized, tack weld the weight or weights into position. The weight of the weld tacks should be approximately equivalent to that of the hose clamp worm, so make a practice of always locating the worm over the balance weight.
Figure 55 - Balancing a propeller shaft.
Use small tack welds to attach weights. Larger welds can create another imbalance or, worse, distort driveshaft tubing.
To determine whether vibrations are caused by improper driveline angles, run through the following routine:
Figure 56 - Digital inclinometer.
Figure 57 - Checking the driveshaft angle.
Figure 58 - Method for calculating the driveshaft operating angles between the transmission and axle differential carriers.
The recommended method for correcting severe U-joint operating angles depends on the type of vehicle suspension and driveshaft design. On vehicles with a leaf spring suspension, thin wedges called axle shims can be installed under the leaf springs on the axle saddle to tilt the axle and thereby adjust U-joint operating angles. Wedges are available in a range of sizes to alter pinion angles.
On some vehicles with tandem drive axles, shimming of longitudinal torque rods can be used to adjust the drive carrier operating angle. Longitudinal torque rod shims fractionally rotate the drive axle pinion that alters the U-joint operating angle. A longer or shorter torque rod might be available from the manufacturer if shimming is not practical. Some torque rods are adjustable.
As a rule, the addition or removal of a ¼-inch shim from the rear torque arm will alter the axle angle approximately ¾ degree. A ¾ degree change in the pinion angle will typically change a U-joint operating angle by about ¼ degree.
Factors that can cause the U-joint operating angle to change are the following:
Any runout in a driveshaft can result in driveline vibrations. Driveshaft runout can be checked by using a dial indicator. Figure 59 shows the locations used to measure driveshaft runout and the tolerance limits for total indicated runout (TIR). Note that these are fine measurements, so before measuring, clean the surfaces from where you are going to take the dial indicator readings.
Figure 59 - Checking driveshaft runout.
Not only must the driveshaft be straight but the yokes attached to the transmission, auxiliary transmission, and drive axles must also be true and straight and in alignment with the shafts to which they are attached.
Mount a dial indicator to the transmission or axle housing as shown in Figure 60 and Figure 61. If the yoke is the half round type, place the tip of the indicator measuring plunger onto the machined surface of the yoke shoulder. If the yoke is the full round type, pop a bearing cup out just enough to insert the indicator plunger onto the cup as shown in Figure 61. Take the measurement, and then rotate the yoke 180 degrees and measure again. If runout exceeds 0.005 inch, the yoke should be replaced. Note that the reason for a yoke damaged in this manner is usually mechanic abuse caused by aggressive separation of a Y-joint.
Figure 60 - When checking half-round end yoke for runout, place the indicator plunger on smooth surface of shoulder.
Figure 61 - When checking a full-round end yoke for runout, be sure to leave enough surface for the indicator plunger.
The machined shoulder of a half-round yoke or yoke bores in a full-round yoke should be exactly 90 degrees to the centerline of the shaft to which the yoke is attached. If the shaft is horizontal (0 degrees), the yoke should be vertical (90 degrees); if the shaft angles away from a true horizontal plane, the yoke should be at an angle equal to 90 degrees minus the inclination of the shaft.
Before checking vertical alignment, make sure the vehicle is on a level surface. The engine and transmission mounts should be secure. Also check the yoke for looseness and tighten to specification if required.
When measuring the alignment of a yoke attached to the main transmission, first disconnect the driveshaft from the end yoke. Next, put the transmission in neutral and rotate the transmission output shaft until the yoke lugs are positioned vertically one above the other. Then, if the yoke is a half-round type, place a protractor with a magnetic base in a vertical position across the mechanical surfaces of the yoke (Figure 62). The protractor dial should read 90 degrees minus the inclination of the engine/transmission plane.
Figure 62 - Checking vertical alignment of a half-round yoke.
Figure 63 - Checking vertical alignment of a full-round end yoke.
If the yoke is the full-round type, place the protractor base on the outside surface of the machined shoulder as shown in Figure 63. The protractor should read the angle of the engine/transmission inclination. If the output yoke is more than ½ degree out of vertical alignment, it is either loose or it is distorted and should be replaced.
When checking the input yoke attached to the pinion shaft of a drive axle, jack up one wheel and rotate the wheel until the lugs of the yoke are aligned vertically. Then measure the vertical alignment of the yoke, using the foregoing procedure just described for checking the transmission output yoke. Refer to Figure 64, which outlines a driveshaft troubleshooting guide.
Figure 64 - Driveshaft Troubleshooting Guide
In some applications, an auxiliary power unit, such as a pump, can be directly mounted to the power takeoff (PTO) assembly (Figure 65, View A). However, it is more common to locate PTO driven units remotely and drive them using a driveshaft (Figure 65, View B).
Figure 65 - Power takeoff mounted without and with driveshaft.
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When symptoms point to differential troubles, remove the differential carrier or rear inspection cover. Inspect the ring gear, pinion drive gear, bearings, and spider gears.
A differential ID number is provided to show the exact type of differential for ordering parts and looking up specifications. The number may be on a tag under one of the carrier or inspection cover fasteners. It may also be stamped on the axle housing or carrier. Use the ID number to find the axle type, axle ratio, make of unit, and other information.
To remove a differential carrier from the banjo housing, use the following procedure:
Figure 66 - Loosen the tapered dowels with a brass drift and large hammer.
Figure 67 - Support the differential carrier with a hydraulic jack.
Most of the weight of a differential carrier assembly is on the inside of its mounting flange. Ensure that the assembly is properly fastened to the jacking device and that you do not damage the flange faces or gearing.
Differential Carrier Disassembly
Before disassembling the carrier, visually inspect the hypoid gearset for damage. If no damage is apparent, you should be able to reuse the gearset. Measure the backlash of the gearset and record the dimension. On reassembly, the crown and pinion backlash should be adjusted to the same dimension. The best overhaul results are obtained when used gearing is adjusted to run in established wear patterns. Omit this procedure if the gearset is to be replaced. To remove the differential and ring gear from the carrier, use the following procedures:
Figure 68 - Thrust screw, jam nut, and thrust block.
Figure 69 - Mark the carrier components for reassembly.
Figure 70 - Removing the cotter key and lock plate.
Figure 71 - Removal of the bearing cap and adjusting ring.
Differential and Crown Gear Assembly
To disassemble the differential and crown/ring gear assembly, you should use the following procedure:
Figure 72 - Disassemble the differential and crown gear.
- Carefully center punch each rivet head in the center on the crown gear side of the assembly.
- Drill each rivet head on the ring gear side of the assembly to a depth equal to the thickness of one rivet head. Use a drill bit that is 1/32-inch smaller than the body diameter of the rivets (Figure 73).
- Drive the rivets through the holes in the ring gear and flange case half. Press from the drilled rivet head.
Figure 73 - Removing rivets on a ring gear.
Do not remove the rivet heads or rivets with a chisel and hammer because this can damage the flange case half or enlarge the rivet holes, resulting in loose rivets.
Figure 74 - Pressing the flange case half out of the crown gear.
Removing Drive Pinion and Bearing Cage
Figure 75 - Removing the A input flange or B yoke.
Figure 76 - Bearing cap removal.
Figure 77 - Removing the drive pinion and bearing cage.
Drive Pinion and Bearing Cage Disassembly
Figure 78 - Pressing the drive pinion from the bearing cage.
Figure 79 - A snap ring secures the spigot bearing to the pinion shaft.
Differential Carrier Reassembly
The reassembly procedure is essentially a reversal of the disassembly procedure. The procedure outlined is general. Always refer to the service manual for the proper and detailed procedures for the unit being overhauled.
Drive Pinion and Bearing Cage Reassembly
To reassemble the drive pinion and bearing cage, use the following procedure:
Figure 80 - Drive pinion assembly.
Install the Drive Pinion and Bearing Cage
When the drive pinion and bearing cage are assembled and adjusted, install them into the carrier as follows:
Figure 81 - Shim pack installation.
Figure 82 - Cover and seal assembly, gasket and bearing cage.
Crown Gear and Differential Assembly
During assembly, do not attempt to press a cold crown gear into the flange case half. A cold crown gear will damage the case half because of the interference fit.
The scraping that results produces metal particles that lodge between the components, resulting in gear runout that exceeds specifications.
Figure 83 - Crown ring gear installation to flange case half.
Figure 84 - Installing rivets to crown gear.
Figure 85 - Bearing cone installation in case half.
Figure 86 - Side gear installation.
Figure 87 - Spider, differential pinion, and side gear installation.
Installing the Crown Gear Assembly into the Carrier
To install the assembled crown gear and differential assembly into the carrier, use the following procedure:
Figure 88 - Installation of adjusting rings.
Figure 89 - Installation of bearing caps using match marks.
Continue the overall procedure by performing the following checks or adjustment:
Installing the Differential Carrier into the Banjo Housing
Now that the differential carrier has been assembled, you can install it into the axle banjo housing using the following procedure:
Figure 90 - Application of silicone gasket material to the mounting surface of
Figure 91 - Installing the gaskets and axle shafts into the axle housing and carrier.
End Play Adjustment
Input shaft end play requirements will vary with operating conditions, mileage, and rebuild procedures. To measure and adjust end play, do the following:
Crown/Ring Gear Runout Check
To check the runout of the crown/ring gear, do the following:
Figure 92 - Checking crown gear runout.
Check/Adjust Crown Gear Backlash
If the used crown and pinion gearset is installed, adjust the backlash to the setting that was measured before the carrier was disassembled. If a new gearset is to be installed, adjust backlash to the correct specification for the new gearset.
To check and adjust ring gear backlash, do the following:
Figure 93 - Check crown gear backlash.
Figure 94 - Adjustments to increase backlash.
Figure 95 - Adjustments to decrease backlash.
Pinion and Crown Tooth Contact Adjustment
Correct tooth contact between the pinion and crown gear cannot be overemphasized because improper tooth contact results in noisy operation and premature failure. The tooth contact pattern consists of the lengthwise bearing (along the tooth of the ring gear) and the profile bearing (up and down the tooth). Figure 8-96 shows crown gear tooth nomenclature.
Figure 96 - Crown gear tooth nomenclature.
Checking tooth contact pattern on a new gearset. Paint or coat 12 crown gear teeth with a marking compound such as white grease and roll the gear to obtain a tooth contact pattern (Figure 97). Machine blue, also called Prussian blue, can be used. Machine blue is a deep blue dye mixed in a grease-like substance. Use a small stiff bristle brush to apply a light even coat. A correct pattern should be well centered on the crown gear teeth with lengthwise contact clear of the toe (Figure 98). The length of the pattern in an unloaded condition will be approximately one-third to two- thirds of the crown gear tooth in most models and ratios.
Figure 97 - Application of a compound to check tooth contact.
Figure 98 - Correct tooth contact pattern for new gearing.
Checking tooth contact pattern on a used gearset. Used gearing will not usually display the square, even contact pattern found in new gearsets. The gear will normally have a pocket at the toe-end of the gear tooth that tails into a contact line along the root of the tooth (Figure 99).
Figure 99 - Correct tooth contact pattern for used gearing.
The more use a gear has had, the more the line becomes the dominant characteristic of the pattern.
Adjust tooth contact pattern.
When disassembling, make a drawing of the gear tooth contact pattern so that when reassembling it is possible to replicate approximately the same pattern. A correct pattern should be clear of the toe and should center evenly along the face width between the top land and the root. Otherwise, the length and shape of the pattern can be highly variable and are usually considered acceptable tooth contact pattern for new gearing-providing the pattern does not run off the tooth at any time. If necessary, adjust the contact pattern by moving the crown gear and drive pinion. Crown gear position controls the backlash setting. The adjustment also moves the contact pattern along the face width of the gear tooth (Figure 100). Pinion position is determined by the size of the pinion bearing cage shim pack. It controls contact on the tooth depth of the gear tooth (Figure 101).
Figure 100 - Two incorrect patterns during adjust pinion position.
Figure 101 - Two incorrect patterns when adjusting backlash.
These adjustments are interrelated. As a result, they must be considered together even though the pattern is altered by two distinct operations. When making adjustments, first adjust the pinion and then the backlash. Continue this sequence until the pattern is satisfactory.
In the semi-floating axle shaft, drive torque from the differential carrier is delivered by each axle half-shaft directly to the drive wheels. A single bearing assembly, located at the outer end of the axle, is used to support the axle half-shaft. The part of the axle extending beyond the bearing assembly is either splined or tapered to a wheel hub and brake drum assembly. The main
disadvantage of this type of axle is that the outer end of each axle shaft is required to support the weight of the vehicle, and the weight is placed on the axle over the wheels and tires. These loads will shift as the axle rotates, placing flexing stresses on the shaft. If an axle half-shaft were to break, the wheel would fall off.
Figure 102 shows a semi- floating axle using a ball bearing. This is a pre-greased bearing.
Figure 102 - Ball bearing semi-floating axle.
There is an axle seal behind the bearing. The axle collar is pressed onto the axle shaft. The bearing and axle are held in the housing by an axle retainer plate mounted on the outer end of the rear axle housing. The retainer plate and bearing control endplay during turns.
Figure 103 shows a roller bearing version of the semi-floating axle. This bearing is lubricated by rear end lubricant. The axle seal is installed in front of the bearing. When this kind of bearing is used, the axle is held in the housing by a clip on the inboard end of the shaft at the differential assembly. This kind of axle is sometimes called a C-lock axle because of the shape of the locking clip. Endplay on turns is controlled by the fit of the axle shaft between the C-lock and the other parts of the differential assembly.
Figure 103 - Roller bearing semi-floating axle.
Figure 104 shows a semi- floating axle using a tapered roller bearing. This type of axle is usually found on older vehicles. When this type of bearing is used, there is usually some provision for adjusting the bearing preload to control endplay. This is generally done by using axle shims or by turning an adjustment nut. Tapered roller bearings may be packed with grease or lubricated from the rear axle housing, depending on the particular manufacturer's design.
Figure 104 - Tapered roller bearing semi- floating axle.
In Figure 104, notice the use of the tapered axle. This is one of two methods used to secure a wheel hub to its axle. The tapered end wedges into a tapered hole in the wheel hub, and the key keeps the axle from rotating in the hub. The other method has the wheel hub (axle and flange, in this case) solidly mounted to the axle.
The axle shafts in a three-quarter floating axle may be removed with the wheels that are keyed to the tapered outer ends of the shaft (Figure 105 ). The inner ends of the shafts are carried as in a semi-floating axle. The axle housing, instead of the shafts, carries the weight of the vehicle because the wheels are supported by bearings on the outer ends of the housing.
Figure 105 - Three-quarter floating axle.
However, axle shafts must take the stresses caused by the turning or skidding of the wheels. Three-quarter floating axles are used in some trucks but in very few passenger cars.
A full-floating axle is used on large, slow moving off-road equipment with a drive wheel on each end of the axle housing. Figure 106 shows two large tapered roller bearings that are mounted to the wheel hub and carry the full weight of the equipment and load. On larger equipment, the bearings are mounted on spindle bolts to the axle housing. The axle itself only transmits torque of the engine through it and does not carry any weight; hence the term "floating" is used to describe its function.
Figure 106 - Typical heavy-duty full- floating axle.
On smaller equipment, the axle is connected to the drive wheels through a bolted flange that can be removed to gain access to the axle without removing the wheels.
On larger equipment with outboard planetary drives, the axle is floating and held in place with a thrust plate that can be removed to gain access to the planetary drive gears for servicing. The axles are easily removed when the equipment needs to be towed in a non-run situation. Caution should be used when removing the axles to allow the wheels to free- wheel. The equipment should be secured either with wheel chocks or a tow hook to another piece of towing equipment to prevent accidental movement.
Wheels attached to live axles are the driving wheels. The number of driving wheels is sometimes used to identify equipment. You, as a mechanic, may identify a truck by the gasoline or diesel engine that provides the power. Then again, you may refer to it as a bogie drive.
Wheels attached to the outside of the driving wheels make up dual wheels. Dual wheels give additional traction to the driving wheels and distribute the weight of the vehicle over a greater area of road surface. They are considered as single wheels in describing vehicles; for example, a 4 x 2 could be a passenger car or a truck having four wheels with two of them driving. A 4 x 4 indicates a vehicle having four wheels with all four driving. In some cases, these vehicles will have dual wheels in the rear. You would describe such a vehicle as a 4 x 4 with dual wheels.
A 6 x 4 truck, although having dual wheels in the rear, is identified by six wheels, four of them driving. Actually, the truck has ten wheels, but the four wheels attached to the driving wheels could be removed without changing the identity of the truck. If the front wheels of this truck were driven by a live axle, it would be called a 6 x 6.
The tracks on track-laying vehicles are driven in much the same manner as wheels on wheeled vehicles. Sprockets instead of wheels are driven by live axles to move the tracks on the rollers. These vehicles are identified as full-track, half-track or vehicles that can be converted.
The efficiency and life of mechanical equipment are as dependent on proper lubrication as they are on sound engineering design.
Proper lubrication depends on using the right type of lubricant at the proper intervals and maintaining the specified capacities. The recommended lubrication practices and specifications are general in nature and typical of manufacturer's procedures. It is always advisable to refer to the service manual for detailed procedures.
Mechanics who perform service on vehicles and off-road equipment must understand the importance of following the manufacturer's maintenance procedures. Many of the axle-related failures are caused by using a lubricant that is not meant for the application. This will result in a shorter component life than could be otherwise achieved. Many failures could be prevented by following sound maintenance practices concerning lubrication selection and use. Axle failures can and have produced many undesirable consequences. When lubrication levels are not properly maintained in axles, the life of the bearings and gears will be adversely affected, shortening their life or often leading to catastrophic failures. Regardless of how well the equipment is designed and operated if it is not properly maintained, premature axle component wear will occur and most definitely lead to early failure. Although some failures are caused by improper installation of components, mechanics need to make themselves aware of correct methods and ensure that the proper replacement parts and tools are used to assemble the components.
Lubrication-related damage may take place even if the correct lubricant is used. Often the intervals between servicing are inadequate for the application, resulting in component damage. Lubricants that are used in axle assemblies have three general functions:
If and when lubricating damage occurs to axle components, there are generally three basic problem areas that are responsible:
Lubricant practices that we will discuss are general examples, and you should always refer to the OEM manufacturer's service manuals for detailed procedures. Many manufactures allow the use of synthetic lubricants in axles; this extends service intervals significantly. The initial cost of synthetic lubricants is much higher upfront, but in the long run can be significantly less costly since drain intervals can often be increased by two or three times when compared to conventional crude-based lubricants. Synthetic lubricants have many qualities that conventional oils do not have, such as a much higher boiling point when compared to conventional oil, 600°F versus 350°F. Another important factor to consider when deciding on the use of synthetic oil is the amount of cold weather operation to which your equipment is exposed. Synthetic oil has much better fluidity than conventional oil in extremely cold weather; this could mean the difference between having lubrication instantly or operating for a few minutes before the oil can flow properly.
Many differentials have been ruined by a phenomenon called "channeling," which occurs in cold weather; the thickened oil is parted by the rotating ring gear and is too thick to flow back. This results in inadequate lubrication until the oil heats up. Often, by then the gears and bearing have sustained damage from the lack of lubrication. A good solution to this type of problem is to use a high quality synthetic lube-one that meets API-GL-5 specifications. All lubricants used in axle assemblies must meet the American Petroleum Institute (API) and Society of Automotive Engineers (SAE) Gear Lubrication (GL) standards. Today, the use of GL-1, GL-2, GL-3, GL-4, and GL-6 is no longer approved for newer axle assemblies by many manufacturers and should be discontinued. The recommended lubricant to use in the drive axles now is API-GL-5, which is rated as an extreme pressure lubricant and is required for use in axle assemblies that use hypoid gears. Using the correct viscosity will be operating- temperature dependent, and should be adhered to. Figure 107 shows the proper grade of lubricant for a given operating temperature.
|Ambient Temperature Range||Correct Grade|
|-40°F to -15°F||75W|
|-15°F to 100°F||80W|
|-90 -15°F and above||80W|
|-140 10°F and above||85W-140|
Figure 107 - Proper grade of lubricant.
Most OEMs approve the use of synthetic lubricants as long as they meet API-GL-5 classification standards. Due to the wide variety of operating extremes that vehicles and off-road equipment are subjected to, it is difficult to determine ideal drain intervals for all vehicles and equipment. Vehicles invariably use mileage, whereas off-road equipment service intervals are usually measure in hours of operation.
It is not recommended to mix different brands of lubricants. Not all lubricants are compatible with each other. Every effort should be made to prevent the mixing of different lubricants in axle assemblies. Some additives used by certain manufactures are not compatible with other manufacturer's additives. Never mix different brands of gear lube. Some synthetic oils, when mixed with petroleum-based oil, will thicken. This can lead to foaming, which can result in early component failure. Water, dirt, and wear particles can cause extensive damage in a short period of time. Water can enter the axle assembly through a faulty shaft seal or through the carrier to a banjo housing joint. Wear particles are generally a result of normal wear and can be minimized by the use of magnetic drain plugs. The magnets collect metal particles that settle to the bottom of the axle housing during shutdown periods. Some manufacturers use an oil pump in the drive axle to distribute oil and a filter in the circuit to filter out dirt particles.
Whenever a lubricant does not perform up to the vehicle or equipment application standards, the life of the internal components will be shortened. Following is a list of causes and effects that may occur as a result of lubrication failures:
When filling the axle housing and planetary wheel ends, allow enough time for the oil to flow through the components and find its natural level. When filling is completed, allow a few minutes and then recheck the level again. The lubricant should be warm when drained; this ensures that contaminants are suspended in the oil and get flushed out.
Lubrication levels for vehicles and off-road equipment should be checked according to the OEM service manual. Oil levels in drive axles must be checked properly; the oil level must be even with the bottom of the oil filler plug hole. Oil replacement can be adjusted according to your particular operations.
Remove the plug in the axle banjo housing; the oil should be even with the bottom of the oil level plug hole (Figure 108). Note that if the pinion angle is more than 7 degrees, a different oil level is used. To properly check the oil levels in a differential drive axle and the planetary wheel ends, the axle first should be run and then allowed to stand for approximately 5 minutes on level ground before checking the oil level. This allows time for the oil drain back down to the sump. Some axles have seals in the axles separating the oil in the wheel ends from the planetary drive. In this case, the oil level is checked independently in each wheel end and the differential banjo housing. When checking the oil level in a top mount or inverted pinion drive axle, a slightly different procedure must be followed. The oil hole on these units is located in the axle banjo housing. Note that some axles have another smaller hole located just below the oil level hole; this hole is for a temperature sensor and should not be used to fill or check the oil level.
Figure 108 - Checking the oil level in an axle housing.
Lubricants used in axle assemblies are necessary to provide lubrication to the internal components and, equally important, they distribute additives throughout the axle and wash away small wear particles that would otherwise cause accelerated wear to the gears and bearings. The oil also carries away excess heat from high friction areas. All these conditions, along with constant exposure to heat, lower the performance properties of the oil. Oil analysis should be used to determine optimum oil change intervals. Guidelines that are given in the manufacturer's service manual are based on average operating conditions. The average temperature of lubricants in axles is between 160°F and 220°F in summer months. Operating equipment with temperatures above 200°F causes the oil to oxidize at a higher rate, depleting the additives that give the oil its increased load-carrying capacity. This reason alone makes more frequent oil change intervals desirable.
Refer to Figure 109 for drive axle quick reference for troubleshooting the drive axle.
Figure 109 Drive axle quick reference troubleshooting guide.
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This manual presented extensive information on troubleshooting the drivechain assemblies of the transmission, transfer case, power takeoff, propeller shafts, and differentials.
Of the vast number of vehicles and construction equipment within the Seabee table of allowance, it is safe to say that almost every piece of CESE includes one of these assemblies. These assemblies give the vehicle or equipment its particular characteristic.
Even in the electronic age, working on these particular assemblies still requires the mechanic to possess the skills of disassembly and reassembly of components, inspecting components for wear and damage, and general hands-on mechanical know- how.
Lastly, and most importantly, you still will always be required to refer to the service manual for each vehicle or piece of equipment you are working on for the detailed specifications that will enable you to do the job correctly in order to turn out a finished product that operators can operate safely.
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1. When bearings wear and retainers start to break up in the standard transmission, the noise this creates is generally referred to as what?
2. Possible causes of noise while transmission is in gear include noisy speedometer gears.
3. Of the many places a transmission can leak oil, what is the one location that can cause a complete failure of the transmission should it leak?
4. A damaged O-ring in the range air shift lever will bleed air into the transmission, pressurizing it and resulting in oil leakage in what component?
5. Among the few causes, what is the most likely cause for the standard transmission to slip out of gear?
6. Even though you have the repair manual for your vehicle, before ordering any parts for your transmission, what should you check to be sure it is the correct transmission?
7. What safety feature does a transmission jack have to prevent the transmission from falling?
8. When removing the transmission, what should you do before removing any cross members?
9. What tool might you use to drive out the counter shaft when disassembling the standard transmission?
10. What parts are generally recommended to be replaced during reassembly of the standard transmission even though they may show no indication of any problem before disassembly?
11. One of the most tedious procedures is replacing needle bearings in countershaft gears. What can you do to make this procedure easier?
12. When installing the standard transmission, one helpful action is to use the transmission bolts to draw the transmission into the clutch housing.
13. What special tool or rod can you use to adjust the linkage on many types of standard transmissions ?
14. What mechanical unit splits power between the front and rear axles?
15. What should be the next step if you detect an oil leak around the transfer case, and upon checking you find the oil level is satisfactory?
16. As a mechanic you may encounter abnormal noises coming from the transfer case which unfortunately could be caused by one or more of its various components. What is the solution to this problem?
17. What component has failed if, when checking the transfer case for shudder, you discover silicone in the oil?
18. What is the first step when removing the transfer case for repairs?
19. Which component(s) will you probably have to remove as an assembly if your transfer case has a drive chain?
20. What component(s) can strip so cleanly as to appear machined smooth when you are inspecting the planetary gear assembly of the transfer case?
21. What parts are usually installed at the same time as the gears they operate when you are assembling the transfer case?
22. What should you install after removing the rear half of the housing or the bearing retainer after discovering the endplay of the transfer case to be incorrect?
23. You will find PTOs only on dump trucks.
24. Of the six basic types of power takeoffs referred to in the manual, which is the most common type found on trucks?
25. What term refers to the space between meshing surfaces of the gears in the gearbox devices?
26. What type of grease should be applied when pumps are mounted directly to the PTO output?
27. The basic driveshaft assembly is made up of U-joints, slip splines, propeller shafts, and .
28. In what type of driveline configuration are the working angles of the U-joint's driveshaft equal, but the companion flanges and/or yokes are not parallel?
29. The faster a driveshaft turns, the higher the allowable working angle rises.
30. What two components of the driveshaft must be perfectly in line with each other to be in phase?
31. What is the typical minimum OEM specification, in inches, when you are inspecting the looseness across the U-joint bearing caps and trunnions?
32. Heavy-duty drive shafts typically use what type of grease?
33. What service schedule could you use for servicing the slip splines?
34. When replacing the hanger bearings, make sure you look for and do not lose track of the .
35. When removing the U-joint cups, ensure the bearing cups stay matched with the they came from.
36. Although acceptable, what is the least preferred method to remove a U-joint?
37. Regard a U-joint cross, its four bearing assemblies, and mounting hardware as a unit, and replace as such.
38. With the full round end yoke, what is the next step after both bearing cups have been seated in the yoke, the lock plate tab is in place, and the bolts are in the yoke threads?
39. An unbalanced driveline causes transverse and bending in the driveshaft.
40. What tool is used to measure operating angle on the driveline?
41. What can be installed to correct the U-joint operating angle on a vehicle with leaf spring suspension?
42. Driveshaft runout requires the taking of fine measurements, and generally the tool used is a/an .
43. How many degrees should the machined shoulder of a half-round yoke or yoke bores in a full-round yoke be to the centerline of the shaft the yoke is attached to?
44. The PTO will always be driven by a driveshaft.
45. As you go through the steps to remove the differential, as a safety precaution, you should not remove the axle retaining nuts until you have loosened which components?
46. When removing the rivets on the crown gear, use a drill bit that is inch smaller than the body diameter of the rivets.
47. When removing the drive pinion and bearing cage, what tool(s) might you use if the yoke or flange is tight on the pinion?
48. When removing the pinion oil seal during disassembly of the drive pinion and bearing cage, what is the procedure if it is a one-piece design?
49. Aligning the oil slots in the shims with the oil slots in the bearing cage and carrier is part of the installation of which component(s)?
50. What can you do to the ring gear for easy installation?
51. What is the proper procedure for the fasteners after installing the hardware into the case halves of the differential?
52. To install the bearing caps over the bearings and adjusting rings, tap each cap into position with which tool?
53. What should the end play measurement typically be, in inches, when you are checking a rebuilt power divider with reused components?
54. The runout of the crown gear must not exceed inches.
55. If the used crown and pinion gearset is installed, you can use the setting that was measured before disassembly.
56. How should the correct pattern appear when you are checking the tooth contact pattern on a new gearset?
57. There are a couple of variations of the semi-floating axle. Which type is sometimes referred to as the C-lock axle because of the shape of the locking clip?
58. The full-floating axle will typically be found on what type of vehicle/equipment?
59. What will cool components that are subject to friction?
60. How many degrees is typically the difference in boiling points of synthetic oil, which is higher, and conventional oil, which is lower?
61. What is the recommended lubricant to use in drive axles?
62. For approximately how many minutes should you wait before checking the oil level in the differential drive axle after operation?
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