Axles are classified as either live or dead. The live axle is used to transmit power. The dead axle serves only as a support for part of the vehicle while providing a mounting for the wheel assembly. Many commercial trucks and truck-tractors have dead axles on the front, whereas practically all passenger vehicles use independent front-wheel suspensions and have no front axles.
The shaft in a live axle assembly may or may not actually support part of the weight of a vehicle, but it does drive the wheels connected to it. A live axle is involved with steering when it is a front drive axle. Some live rear axles are also designed to steer. The rear axle of conventional passenger vehicles is a live axle, while in a four-wheel drive vehicle both front and rear axles are live. In some six-wheel vehicles, all three axles are live axles.
A front drive axle is very similar to a rear drive axle (Figure 10-17); however, provisions must be made for steering the front wheels. Power is transmitted from the transfer case to the front axle by a drive shaft. The differential housing may be set off center in the axle housing to permit the drive shaft to pass beside the engine oil pan and maintain sufficient road clearance without excessive height at the front end of the vehicle.
Figure 10-17 — Front drive axle.
Since the front wheels must turn on the spindle arm pivots, they must be driven by the axle shaft through universal joints, which are located on the outer ends of the axles. The universal joints allow the front wheels and hubs to swivel while still transferring driving power to the hubs and wheels.
The cross and roller joint shown in Figure 10-17 is similar to conventional U-joints used on the rear drive shaft, and in some cases they are interchangeable.
This type of U-joint is limited to use in light-duty vehicles. Other types of universal joints are used in the axles of heavy-duty vehicles. The types you will encounter in military designed vehicles are the Rzeppa and Bendix-Weiss constant velocity joints (Figure 10-18).
Figure 10-18 — Constant velocity universal joints.
The front drive axle of a four-wheel drive axle requires locking hubs. Locking hubs transfer power from the driving axles to the driving wheels on a four-wheel drive vehicle. There are three basic types of locking hubs:
Manual locking hub—requires the operator to turn a latch on the hub to lock the hub for four-wheel drive action.
Automatic locking hub—hub locks the front wheels to the axles when the operator shifts into four-wheel drive.
Full time hub—front hubs are always locked and drive the front wheels.
Manual and automatic locking hubs are the most common. Used with part-time, four- wheel drive, they enable the drive line to be in two-wheel drive for use on dry pavement. The front wheels can turn without turning the front axles. This allows for increased fuel economy and reduces drive line wear.
The rear drive axle connects the differential side gears to the drive wheels. The axle may or may not support the weight of the vehicle. Rear axles are normally induction hardened for increased strength. There are several types of rear axle designs: semi- floating, three-quarter floating, and full-floating. However, the semi- and full-floating types are the most common. Most automobiles use the semi-floating type, whereas four-wheel drive vehicles and trucks use full-floating axles.
The axle housing may be of the one-piece or split (banjo) type construction. The former, known as the banjo type because of its appearance, is far more common (Figure 10-19). Notice that openings, both front and rear, are provided in the center housing. The front opening is closed by the differential carrier, while the rear is closed by a spherical cover plate.
Figure 10-19 — Axle housing.
Since the assembly must carry the weight of the vehicle, the axle housing in heavy trucks and tractors is a heavy cast unit. In light-duty trucks it may be a combination of cast and steel tube; in general, the center or differential and final drive case is a cast and machined unit, whereas the axle housings themselves may be welded or extruded steel tubing.
Items such as brake backing plates, mounting flanges, spring mounting plates, and accessory units may be riveted, welded, or cast into the axle housing. Inspection covers are often provided, through which the internal parts can be inspected, removed, and installed. Lubricant filler plugs are usually incorporated in the housing inspection cover.
To prevent pressure buildup when the axle becomes warm, a breather vent or valve is provided atop the housing. Without this valve, the resulting pressure could force the axle lubricant past the rear wheel oil seals and damage the brake linings. The valve is constructed so air may pass in or out of the axle housing; however, dirt and moisture are kept out.
A solid axle is a hardened-steel shaft that has splines on the inner end and an axle flange on the outer end that makes a mount for the brake and wheel assembly. Solid drive axles can either be semi-floating or full-floating.
The semi-floating axle is used in passenger vehicles and light trucks. In vehicles equipped with this type of axle, the shaft as well as the housing supports the weight of the vehicle. The inner end of the axle is carried by the side gears in the differential housing. This relieves the axle shafts of the weight of the differential and the stresses caused by its operation that are taken by the axle housing. The inner ends of the axle transmit only turning effort, or torque, and are not acted upon by any other force.
The outer end is carried by a bearing located between the shaft and the housing. A tapered roller of ball-type bearing transfers the load from the shaft to the housing.
The axle shafts take the stresses caused by turning, skidding, or wobbling of the wheels. The axle shafts are flanged or tapered on the ends (Figure 11-20). When the tapered axle is used, the brake drum and hub are pressed onto the shafts, using keys to prevent the assemblies from turning on the shafts. In some cases, the outer ends of the shafts may have serrations or splines to correspond with those on the drum and hub assembly. Should the axle break with this type of axle assembly, the wheel can separate from the vehicle.
Figure 10-20 — Semi-floating axle installation.
The full-floating axle is used in many heavy- duty trucks (Figure 10-21). The drive wheel is carried on the outer end of the axle housing by a pair of tapered roller bearings. The bearings are located outside the axle housing. In this way, the axle housings take the full weight of the vehicle and absorb all stresses or end thrust caused by turning, skidding, and pulling. Only the axle shaft transmits torque from the differential.
Figure 10-21 — Full-floating axle.
The axle shaft is connected to the drive wheel through a bolted flange. This allows the axle shaft to be removed for servicing without removing the wheel.
Front-wheel drive axles, also called axle shafts or front drive shafts, transfer power from the transaxle differential to the hubs and wheel of a vehicle. Front wheel drive axles turn much slower than a drive shaft for a rear-wheel drive vehicle. They turn about one third slower. They are connected directly to the drive wheels and do NOT have to act through the reduction of the axle ring gear and pinion gears.
Front-wheel drive axles typically consist of the following:
Universal joints that connect the drive axle are called CV joints. The outer CV joint is a fixed (nonsliding) ball and cage or Rzeppa-type joint that transfers rotating power from the axle shaft to the hub assembly. The inner CV joint is called a plunging (sliding) ball and housing or tripod-type joint that acts like a slip joint in a drive shaft for a rear-wheel drive vehicle.
The plunging action of the inner CV joint allows for a change in distance between the transaxle and the wheel hub. As the front wheels move up and down over bumps in the road, the length of the drive axle (inner joint) must change.
Rear axle service is needed when an axle bearing is noisy, when an axle is broken, bent, or damaged, or when an axle seal is leaking. The rear axles must be removed to allow removal and repair of the differential assembly.
Worn or damaged bearings in the carrier or on the axles produce a constant whirring or humming sound. When bad, these bearings make about the same sound whether accelerating, decelerating, or coasting. When diagnosing and repairing bearing failures, do the following:
When an axle bearing is faulty, it must be removed from the axle or housing carefully and a new one installed. The type of axle configuration determines how the bearing is to be removed and replaced. Always refer to the manufacturer's service manual for instructions for the removal and installation of the bearing.
The procedures we will discuss are for a semi-floating axle with the bearing and collar pressed on. With the axle removed from the vehicle, proceed as follows:
Procedures for axle removal may be found in the service manual for the applicable vehicle.
Do NOT use a cutting torch to remove the collar and bearing. The heat will weaken and damage the axle.
Wear eye and face protection when grinding or chiseling the collar from the axle. Small metal particles may fly into your eyes causing eye damage.
NEVER press on the outer race; bearing damage or explosion will result.
Wear face and eye protection when pressing a bearing on or off the axle shaft. The tremendous pressure used can cause the bearing to shatter and fly into your face with deadly force.
Do NOT attempt to press the bearing and collar on at the same time. Bearing and collar damage can result.
Rear axle lubricant leaks can occur at numerous spots, such as at the pinion gear seal, carrier or inspection cover gaskets, and two axle seals. The leak will show up as a darkened, oily, dirty area below the pinion gear or carrier, or on the inside of the wheel and brake assembly.
Always make sure that a possible axle seal leak is not a brake fluid leak. Touch and smell the wet area to determine the type of leak.
Anytime the axle is removed for service, it is wise to install a new axle seal. This action ensures that the seal between the axle and axle seal is tight. The axle seal is normally force-fitted in the end of the axle housing.
To remove a housing-mounted seal, use a slide hammer puller equipped with a hooknose. Place the hook on the metal part of the seal. With an outward jerk on the puller slide, pop out the seal. If a slide hammer puller is not available, a large screwdriver will also work.
Be careful not to scratch the bearing bore in the axle housing.
Make sure that you have the correct new seal. Its outside and inside diameters must be the same as the old seal. A seal part number is stamped on the outside of the seal. This number and the seal manufacturer’s name will assist you when ordering a new seal.
Before installing the new seal, coat the outer diameter with a non-hardening sealer. Coat the inside of the seal with lubricant that is the same grade that is in the axle assembly. With the seal facing in the right direction (sealing lip toward the inside of the housing), drive the seal squarely into place using a seal-driving tool. Be careful not to bend the metal seal housing or a leak can result. Make sure the seal is fully seated.
3. What type of drive axle allows the axle shaft to be removed without removing the wheel?