During the course of a project, Seabees often need to hoist or move heavy objects. Wire rope is used for heavy-duty work. The characteristics, construction, and usage of many types of wire rope are discussed in the following paragraphs. We will also discuss the safe working load, use of attachments and fittings, and procedures for the care and handling of wire rope.
Wire rope consists of three parts: wires, strands, and core (figure 4-5). In the manufacture of rope, a number of wires are laid together to form the strand. Then a number of strands are laid together around a core to form the rope.
Figure 4-5.Parts of wire rope.
The basic unit of wire rope construction is the individual wire, which may be made of steel, iron, or other metal in various sizes. The number of wires to a strand varies, depending on the purpose for which the rope is intended. Wire rope is designated by the number of strands per rope and the number of wires per strand. Thus, a 1/2-inch 6-by- 19 rope will have 6 strands with 19 wires per strand; but it will have the same outside diameter as a 1/2-inch 6-by-37 wire rope, which will have 6 strands with 37 wires of much smaller size per strand. Wire rope made up of a large number of small wires is flexible, but the small wires are easily broken, so the wire rope does not resist external abrasion. Wire rope made up of a smaller number of larger wires is more resistant to external abrasion but is less flexible.
The core is the element around which the strands are laid to form the rope. It may be a hard fiber (such as manila, hemp, plastic, paper, asbestos, or sisal), a wire strand, or an independent wire rope. Each type of core serves the same basic purpose-to support the strands laid around it.
A fiber core offers the advantage of increased flexibility. Also, it serves as a cushion to reduce the effects of sudden strain and acts as a reservoir for the oil to lubricate the wires and strands to reduce friction between them. Wire rope with a fiber core is used in places where flexibility of the rope is important.
A wire strand core not only resists heat more than a fiber core, but also adds about 15 percent to the strength of the rope. On the other hand, the wire strand makes the rope less flexible than a fiber core.
An independent wire rope core is a separate wire rope over which the main strands of the row are laid. It usually consists of six, seven-wire strands laid around either a fiber core or a wire strand core. This core strengthens the rope more, provides support against crushing, and supplies maximum resistance to heat.
Wire rope maybe made by either of two methods. If the strands or wires are shaped to conform to the curvature of the finished rope before laying up, the rope is termed "preformed." If they are not shaped before fabrication, the rope is termed "nonpreformed." When cut, preformed wire rope tends not to unlay, and it is more flexible than nonpreformed wire rope. Wire nonpreformed wire rope, twisting produces a stress in the wires; and, when it is cut or broken, the stress causes the strands to unlay. In nonpreformed wire, unlaying is rapid and almost instantaneous, which could cause serious injury to someone not familiar with it.
The main types of wire rope used by the Navy consist of 6, 7, 12, 19, 24, or 37 wires in each strand. Usually, the rope has six strands laid around a fiber or steel center. Two common types of wire rope, 6-by-19 and 6-by-37 rope, are illustrated in views A and B of figure 4-6, respectively. The 6-by-19 type of rope, having 6 strands with 19 wires in each strand, is commonly used for rough hoisting and skidding work where abrasion is likely to occur. The 6-by-37 wire rope, having 6 strands with 37 wires in each strand, is the most flexible of the standard 6-strand ropes. For that reason, it is particularly suitable when small sheaves and drums are to be used, such as on cranes and similar machinery.
Figure 4-6.Two common types of wire rope.
GRADES OF WIRE ROPE
Wire rope is made in a number of different grades. Three of the most common are mild plow steel, plow steel, and improved plow steel.
Mild plow steel rope is tough and pliable. It can stand up under repeated strain and stress, and it has a tensile strength of from 200,000 to 220,000 pounds per square inch (psi). Plow steel wire rope is unusually tough and strong. It has a tensile strength (resistance to lengthwise stress) of 220,000 to 240,000 psi. This rope is suitable for hauling, hoisting, and logging. Improved plow steel rope is one of the best grades of rope available, and most, if not all, of the wire rope in your work will probably be made of this material. It is stronger, tougher, and more resistant to wear than either plow steel or mild plow steel. Each square inch of improved plow steel can withstand a strain of 240,000 to 260,000 psi.
MEASURING WIRE ROPE
The size of wire rope is designated by its diameter. The true diameter of a wire rope is the diameter of a circle that will just enclose all of its strands. Correct and incorrect methods of measuring wire rope are illustrated in figure 4-7. In particular, note that the correct way is to measure from the top of one strand to the top of the strand directly opposite it. The wrong way is to measure across two strands side by side. Use calipers to take the measurement. If calipers are not available, an adjustable wrench will do.
Figure 4-7.Correct and incorrect methods of measuring wire rope.
To ensure an accurate measurement of the diameter of a wire rope, always measure the rope at three places, at least 5 feet apart. Use the average of the three measurements as the diameter of the rope.
SAFE WORKING LOAD
The term "safe working load" (swl), as used in reference to wire rope, means the load that can be applied and still obtain the most efficient service and also prolong the life of the rope. Most manufacturers provide tables that show the safe working load for their rope under various conditions. In the absence of these tables, you must apply a thumb rule formula to obtain the swl. There are rules of thumb that may be used to compute the strength of wire rope:
This particular formula provides an ample safety margin to account for such variables as the number, size, and location of sheaves and drums on which the rope runs. Also included are dynamic stresses, such as the speed of operation and the acceleration and deceleration of the load. All can affect the endurance and breaking strength of the rope.
Lets work an example. In the above formula, D represents the diameter of the rope in inches. Suppose you want to find the swl of a 2-inch rope. Using the formula above, your figures would be: swl = 22 x 8, or 4 x 8 = 32. The answer is 32, meaning that the rope has a swl of 32 tons.
It is very important to remember that any formula for determining swl is only a rule of thumb. In computing the swl of old rope, worn rope, or rope that is otherwise in poor condition, you should reduce the swl as much as 50 percent, depending on the condition of the rope. The manufacturers data concerning the breaking strength (BS) of wire rope should be used if available. But if you do not have that information, one rule of thumb recommended is BS = C2 x 8,000 pounds.
As you recall, wire rope is measured by the diameter (D). To obtain the circumference (C) required in the formula, multiply D by pi (usually shown by the Greek letter p which is approximately 3.1416. Thus, the formula to find the circumference is C = pD
WIRE ROPE FAILURE
Wire can fail due to any number of causes. Here is a list of some of the common causes of wire rope failure.
HANDLING AND CARE OF WIRE ROPE
To render safe, dependable service over a maximum period of time, wire rope must have the care and upkeep necessary to keep it in good condition. In this section, well discuss various ways of caring for and handling wire rope. Not only should you study these procedures carefully, you should also practice them on your job to help you do a better job now. In the long run, the life of the wire rope will be longer and more useful.
Coiling and Uncoiling
Once a new reel has been opened, it may be either coiled or faked down like line. The proper direction
of coiling is counterclockwise for left-laid wire rope and clockwise for right-laid rope. Because of the general toughness and resilience of wire, however, it occasionally tends to resist being coiled down. When this occurs, it is useless to fight the wire by forcing down a stubborn turn; it will only spring up again. But if it is thrown in a back turn, as shown in figure 4-8, it will lie down properly. A wire rope, when faked down, will run right off like line; but when wound in a coil, it must always be unwound.
Figure 4-8.Throwing a back turn to make wire lie down.
Wire rope tends to kink during uncoiling or unreeling, especially if it has been in service for a long time. A kink can cause a weak spot in the rope, which will wear out quicker than the rest of the rope. A good method for unreeling wire rope is to run a pipe or rod through the center and mount the reel on drum jacks or other supports so the reel is off the ground or deck (figure 4-9.) In this way, the reel will turn as the rope is unwound, and the rotation of the reel will help keep the rope straight. During unreeling, pull the rope straight forward, as shown in figure 4-9, and try to avoid hurrying the operation. As a safeguard against kinking, never unreel wire rope from a stationary reel.
Figure 4-9.Unreeling wire rope (left) and uncoiling wire rope (right).
To uncoil a small coil of wire rope, simply stand the coil on edge and roll it along the ground or deck like a wheel or hoop, as illustrated in figure 4-9. Never lay the coil flat on the deck or ground and uncoil it by pulling on the end because such practice can kink or twist the rope.
To rewind wire rope back onto a reel or a drum, you may have difficulty unless you remember that it tends to roll in the direction opposite the lay. For example, a right-laid wire rope tends to roll to the left.
Carefully study figure 4-10, which shows drum-winding diagrams selecting the proper lay of rope. When putting wire rope onto a drum, you should have no trouble if you know the methods of overwinding and underwinding shown in the illustration. When wire rope is run off one reel onto another, or onto a winch or drum, it should be run from top to top or from bottom to bottom, as shown in figure 4-11.
Figure 4-10.Drum windings diagram for selecting the proper lay of rope.
Figure 4-11.Transferring wire from reel to drum.
If a wire rope should forma loop, never try to pull it out by putting strain on either part. As soon as a loop is noticed, uncross the ends by pushing them apart. (See steps 1 and 2 in figure 4-12.) This reverses the process that started the loop. Now, turn
the bent portion over and place it on your knee or some firm object and push downward until the loop straightens out somewhat. (See step 3 in figure 4-12.) Then, lay the bent portion on a flat surface and pound it smooth with a wooden mallet. (See step 4 in figure 4-12.)
Figure 4-12.The correct way to takeout a loop in wire rope.
If a heavy strain has been put on a wire rope with a kink in it, the rope can no longer be trusted. Replace the wire rope altogether.
Used wire rope should be cleaned at frequent intervals to remove any accumulation of dirt, grit, rust, or other foreign matter. The frequency of cleaning depends on how much the rope is used. However, rope should always be well cleaned before lubrication. The rope can be cleaned by wire brushes, compressed air, or steam. Do not use oxygen in place of compressed air; it becomes very dangerous when it comes in contact with grease or oil. The purpose is to remove all old lubricant and foreign matter from the valleys between the strands and from the spaces between the outer wires. This gives newly applied lubricant ready entrance into the rope. Wire brushing affords a good opportunity to find any broken wires that may otherwise go unnoticed.
Wire rope is initially lubricated by the manufacturer, but this initial lubrication isnt permanent and periodic reapplications have to be made by the user. Each time a wire rope bends and straightens, the wires in the strands and the strands in the rope slide upon each other. To prevent the rope wearing out by this sliding action, a film of lubricant is needed between the surfaces in contact. The lubricant also helps prevent corrosion of the wires and deterioration of fiber centers. A rusty wire rope is a liability! With wire rope, the same as with any machine or piece of equipment, proper lubrication is essential to smooth, efficient performance.
The lubricant should be a good grade of lubricating oil, free from acids and corrosive substances. It must also be of a consistency that will penetrate to the center of the core, yet heavy enough to remain as a coating on the outer surfaces of the strands. Two good lubricants for this purpose are raw linseed oil and a medium graphite grease. Raw linseed oil dries and is not greasy to handle. Graphite grease is highly resistant to saltwater corrosion. Of course, other commercial lubricants may be obtained and used. One of the best is a semiplastic compound that is thinned by heating before being applied. It penetrates while hot, then cools to a plastic filler, preventing the entrance of water.
One method of applying the lubricant is by using a brush. In doing so, remember to apply the coating of fresh lubricant evenly and to work it in well. Another method involves passing the wire rope through a trough or box containing hot lubricant (figure 4-13). In this method, the heated lubricant is placed in the trough, and the rope passed over a sheave, through the lubricant, and under a second sheave. Hot oils or greases have very good penetrating qualities. Upon cooling, they have high adhesive and film strength around each wire.
Figure 4-13.Trough method of lubrication.
As a safety precaution, always wipe off any excess when lubricating wire rope. This is especially important where heavy equipment is involved. Too much lubricant can get on brakes or clutches, causing them to fail. While in use, the motion of machinery can throw excess oil onto crane cabs and catwalks, making them unsafe to work on.
Wire rope should not be stored in places where acid is or has been kept. The slightest trace of acid coming in contact with wire rope damages it at that particular spot. Many times, wire rope that has failed has been found to be acid damaged. The importance of keeping acid or acid fumes away from wire rope must be stressed to all hands.
It is especially important that wire rope be cleaned and lubricated properly before it is placed in storage. Fortunately, corrosion of wire rope during storage can be virtually eliminated if the lubricant film is applied properly beforehand and if adequate protection is provided from the weather. Bear in mind that rust, corrosion of wires, and deterioration of the fiber core greatly reduce the strength of wire rope. It is not possible to state exactly the loss of strength that results from these effects. It is certainly great enough to require close observance of those precautions prescribed for protection against such effects.
Wire rope should be inspected at regular intervals, the same as fiber line. In determining the frequency of inspection, you need to carefully consider the amount of use of the rope and conditions under which it is used.
During an inspection, the rope should be examined carefully for fishhooks, kinks, and worn, corroded spots. Usually, breaks in individual wires are concentrated in those portions of the rope that consistently run over the sheaves or bend onto the drum. Abrasion or reverse and sharp bends cause individual wires to break and bend back. The breaks are known as fishhooks. When wires are only slightly worn, but have broken off squarely and stick out all over the rope, the condition is usually caused by overloading or rough handling. Even if the breaks are confined to only one or two strands, the strength of the rope may be seriously reduced. When 4 percent of the total number of wires in the rope are found to have breaks within the length of one lay of the rope, the wire rope is unsafe. Consider a rope unsafe when three broken wires are found in one strand of 6-by-7 rope, six broken wires in one strand of 6-by- 19 rope, or nine broken wires in one strand of 6-by-37 rope.
Overloading a rope also causes its diameter to be reduced. Failure to lubricate the rope is another cause of reduced diameter since the fiber core will dry out and eventually collapse or shrink. The surrounding strands are thus deprived of support, and the ropes strength and dependability are correspondingly reduced. Rope that has its diameter reduced to less than 75 percent of its original diameter should be removed from service.
A wire rope should also be removed from service when an inspection reveals widespread corrosion and pitting of the wires. Particular attention should be given to signs of corrosion and rust in the valleys or small spaces between the strands. Since such corrosion is usually the result of improper or infrequent lubrication, the internal wires of the rope are then subject to extreme friction and wear. This form of internal, and often invisible, destruction of the wire is one of the most frequent causes of unexpected and sudden failure of wire rope. The best safeguard, of course, is to keep the rope well lubricated and to handle and store it properly.
WIRE ROPE ATTACHMENTS
Many attachments can be fitted to the ends of wire rope so that the rope can be connected to other wire ropes, pad eyes, or equipment. The attachment used most often to attach dead ends of wire ropes to pad eyes or like fittings on earthmoving rigs is the wedge socket shown in figure 4-14. The socket is applied to the bitter end of the wire rope, as shown in the figure.
Figure 4-14.Parts of a wedge socket.
Remove the pin and knock out the wedge first. Then, pass the wire rope up through the socket and lead enough of it back through the socket to allow a minimum of 6 to 9 inches of the bitter end to extend below the socket. Next, replace the wedge, and haul on the bitter end of the wire rope until the bight closes around the wedge, as shown in figure 4-15. A strain on the standing part will tighten the wedge. You need at least 6 to 9 inches on the dead end (the end of the line that doesnt carry the load). Finally, place one wire rope clip on the dead end to keep it from accidentally slipping back through the wedge socket. The clip should be approximately 3 inches from the socket. Use one size smaller clip than normal so that the threads on the U-bolt are only long enough to clamp tightly on one strand of wire rope. The other alternative is to use the normal size clip and hop the dead end back as shown in figure 4-15. Never attach the clip to the live end of the wire rope.
Figure 4-15.Wedge socket attached properly.
The advantage of the wedge socket is that it is easy to remove; just take off the wire clip and drive out the wedge. The disadvantage of the wedge socket is that it reduces the strength of wire rope by about 30 percent. Of course, reduced strength means less safe working load.
To make an eye in the end of a wire rope, use new wire rope clips, like those shown in figure 4-16. The U-shaped part of the clip with the threaded ends is called the U-bolt; the other part is called the saddle. The saddle is stamped with the diameter of the wire rope that the clip will fit. Always place a clip with the U-bolt on the bitter end, not on the standing part of the wire rope. If clips are attached incorrectly, the standing part (live end) of the wire rope will be distorted or have mashed spots. An easy way to remember is never saddle a dead horse.
Figure 4-16.Wire rope clips.
You also need to determine the correct number of clips to use and the correct spacing. Here are two simple formulas.
Another type of wire rope clip is the twin-base clip (sometimes referred to as the "universal" or "two-clamp") shown in figure 4-17. Since both parts of this clip are shaped to fit the wire rope, correct installation is almost certain. This considerably reduces potential damage to the rope. The twin-base clip also allows for a clean 360° swing with the wrench when the nuts are being tightened. When an eye is made in a wire rope, a metal fitting (called a thimble) is usually placed in the eye, as shown in figure 4-16, to protect the eye against were. Clipped eyes with thimbles hold approximately 80 percent of the wire rope strength.
Figure 4-17.Twin-base wire clip.
After the eye made with clips has been strained, the nuts on the clips must be retightened. Occasional checks should be made for tightness or damage to the rope caused by the clips.
A block (figure 4-18) consists of one or more sheaves fitted in a wood or metal frame supported by a shackle inserted in the strap of the block. A tackle (figure 4-19) is an assembly of blocks and lines used to gain a mechanical advantage in lifting and pulling.
Figure 4-18.Nomenclature of a fiber line block.
Figure 4-19.Types of tackle: simple (view A) and compound (view B).
In a tackle assembly, the line is reeved over the sheave(s) of blocks. The two types of tackle systems are simple and compound. A simple tackle system is an assembly of blocks in which a single line is used (view A of figure 4-19). A compound tackle system is an assembly of blocks in which more than one line is used (view B of figure 4-19).
To help avoid confusion in working with tackle, you need a working knowledge of tackle vocabulary. Figure 4-20 will help you organize the various terms.
Figure 4-20.Parts of a tackle.
A fall is a line, either a fiber line or a wire rope, reeved through a pair of blocks to form a tackle. The hauling part is the part of the fall leading from one of the blocks upon which the power is exerted. The standing part is the end of the fall, which is attached to one of the beckets. The movable (or running) block of a tackle is the block attached to the object to be moved. The fixed (or standing) block is the block attached to a fixed objector support. When a tackle is being used, the movable block moves, and the fixed block remains stationary. The term "two-blocked" means that both blocks of a tackle are as close together as they will go. You may also hear this term called block-and-block. To overhaul is to lengthen a tackle by pulling the two blocks apart. To round in means to bring the blocks of a tackle toward each other, usually without a load on the tackle (opposite of overhaul).
Dont be surprised if your coworkers use a number of different terms for a tackle. For example, line-and-blocks, purchase, and block-and-falls are typical of other names frequently used for tackle.
The block (or blocks) in a tackle assembly changes (or change) the direction of pull or mechanical advantage, or both. The name and location of the key parts of a fiber line block are shown in figure 4-18.
The frame (or shell), made of wood or metal, houses the sheaves. The sheave is a round, grooved wheel over which the line runs. Ordinarily, blocks used in your work will have one, two, three, or four sheaves. Blocks come with more than this number of sheaves; some come with 11 sheaves. The cheeks are the solid sides of the frame, or shell. The pin is a metal axle that the sheave turns on. It runs from cheek to cheek through the middle of the sheave. The becket is a metal loop formed at one or both ends of a block; the standing part of the line is fastened to this part. The straps hold the block together and support the pin on which the sheaves rotate. The swallow is the opening in the block through which the line passes. The breech is the part of the block opposite the swallow.
CONSTRUCTION OF BLOCKS
Blocks are constructed for use with fiber line or wire row. Wire rope blocks are heavily constructed and have a large sheave with a deep groove. Fiber line blocks are generally not as heavily constructed as wire rope blocks and have smaller sheaves with shallower wide grooves. A large sheave is needed with wire rope to prevent sharp bending. Since fiber line is more flexible and pliable than wire rope, it does not require a sheave as large as the same size of wire rope.
Blocks fitted with one, two, three, or four sheaves are often referred to as single, double, triple, and quadruple blocks, respectively. Blocks are fitted with a number of attachments, the number depending upon their use. Some of the most commonly used fittings are hooks, shackles, eyes, and rings. Figure 4-21 shows two metal frame, heavy-duty blocks. Block A is designed for manila line, and block B is for wire rope.
Figure 4-21.Metal frame, heavy-duty blocks.
RATIO OF BLOCK SIZE TO LINE OR WIRE SIZE
The size of fiber line blocks is designated by the length in inches of the shell or cheek. The size of standard wire rope blocks is controlled by the diameter of the rope. With nonstandard and special-purpose wire rope blocks, the size is found by measuring the diameter of one of its sheaves in inches.
Use care in selecting the proper size line or wire for the block to be used. If a fiber line is reeved onto a tackle whose sheaves are below a certain minimum diameter, the line will be distorted and will soon wear badly. A wire rope too large for a sheave tends to be pinched and damages the sheave. The wire will also be damaged due to the too short a radius of the bend. A wire rope too small for a sheave lacks the necessary bearing surface, puts the strain on only a few strands, and shortens the life of the wire.
With fiber line, the length of the block used should be about three times the circumference of the line. However, an inch or so either way doesn't matter too much; for example, a 3-inch line may be reeved onto an 8-inch block with no ill effects. As a rule, you are more likely to know the block size than the sheave diameter. However, the sheave diameter should be about twice the size of the circumference of the line used.
Wire rope manufacturers issue tables that give the proper sheave diameters used with the various types and sizes of wire rope they manufacture. In the absence of these, a rough rule of thumb is that the sheave diameter should be about 20 times the diameter of the wire. Remember that with wire rope, it is diameter rather than circumference that is important. Also, remember that this rule refers to the diameter of the sheave rather than to the size of the block.
SNATCH BLOCKS AND FAIRLEADS
A snatch block (figure 4-22) is a single-sheave block made so that the shell opens on one side at the base of the hook to permit a rope or line to be slipped over a sheave without threading the end of it through the block. Snatch blocks ordinarily are used where it is necessary to change the direction of the pull on a line.
Figure 4-22.Top dead end snatch blocks.
Figure 4-23 shows a system of moving a heavy object horizontally away from the power source using snatch blocks. This is an ideal way to move objects in limited spaces. Note that the weight is pulled by a single luff tackle, which has a mechanical advantage of 3 (mechanical advantage is discussed below). Adding snatch blocks to a rigging changes the direction of pull, but the mechanical advantage is not affected. It is, therefore, wise to select the proper rigging system to be used based upon the weight of the object and the type and capacity of the power that is available.
Figure 4-23.Moving a heavy object horizontally along a floor with limited access using snatch blocks and fairleads.
The snatch block that is used as the last block in the direction of pull to the power source is called the leading block. This block can be placed in any convenient location provided it is within 20 drum widths of the power source. This is required because the fairlead angle, or fleet angle, cannot exceed 2° from the center line of the drum; therefore, the 20-drum width distance from the power source to the leading block will assure the fairlead angle. If the fairlead angle is not maintained, the line could jump the sheave of the leading block and cause the line on the reel to jump a riding turn.
The mechanical advantage of a tackle is the term applied to the relationship between the load being lifted and the power required to lift it. If the load and the power required to lift it are the same, the mechanical advantage is 1. However, if a load of 50 pounds requires only 10 pounds to lift it, then you have a mechanical advantage of 5 to 1, or 5 units of weight are lifted for each unit of power applied.
The easiest way to determine the mechanical advantage of a tackle is by counting the number of parts of the falls at the running block. If there are two parts, the mechanical advantage is two times the power applied (disregarding friction). A gun tackle, for instance, has a mechanical advantage of 2. Therefore, lifting a 200-pound load with a gun tackle requires 100 pounds of power, disregarding friction.
To determine the amount of power required to lift a given load by means of a tackle, determine the weight of the load to be lifted and divide that by the mechanical advantage. For example, if it is necessary to lift a 600-pound load by means of a single luff tackle, first determine the mechanical advantage gained by the tackle. By counting the parts of the falls at the movable block, you determine a mechanical advantage of 3. By dividing the weight to be lifted, 600 pounds, by the mechanical advantage in this tackle, 3, we find that 200 pounds of power is required to lift a weight of 600 pounds using a single luff tackle.
Remember though, a certain amount of the force applied to a tackle is lost through friction. Friction develops in a tackle by the lines rubbing against each other, or against the shell of a block. Therefore, an adequate allowance for the loss from friction must be added. Roughly, 10 percent of the load must be allowed for each sheave in the tackle.TYPES OF TACKLE
Tackles are designated in two ways: first, according to the number of sheaves in the blocks that are used to make the tackle, such as single whip or twofold purchase; and second, by the purpose for which the tackle is used, such as yard tackles or stay tackles. In this section, well discuss some of the different types of tackle in common use: namely, single whip, runner, gun tackle, single luff, twofold purchase, double luff, and threefold purchase. Before proceeding, we should point out that the purpose of the letters and arrows in figures 4-24 through 4-30 is to indicate the sequence and direction in which the standing part of the fall is led in reeving. You may want to refer to these illustrations when we discuss reeving of blocks in the next sections.
A single-whip tackle consists of one single-sheave block (tail block) fixed to a support with a rope passing over the sheave (figure 4-24.) It has a mechanical advantage of 1. If a 100-pound load is lifted, a pull of 100 pounds, plus an allowance for friction, is required.
A runner (figure 4-24) is a single-sheave movable block that is free to move along the line on which it is reeved. It has a mechanical advantage of 2.
Figure 4-24.Single-whip and runner tackle.
A gun tackle is made up of two single-sheave blocks (figure 4-25). This tackle got its name in the old days because it was used to haul muzzle-loading guns back into the battery after the guns had been fired and reloaded. A gun tackle has a mechanical advantage of 2. To lift a 200-pound load with a gun tackle requires 100 pounds of power, disregarding friction.
By inverting any tackle, you always gain a mechanical advantage of 1 because the number of parts at the movable block is increased. By inverting a gun tackle, for example, you gain a mechanical advantage of 3 (figure 4-26). When a tackle is inverted, the direction of pull is difficult. This can easily be overcome by adding a snatch block, which changes the direction of the pull, but does not increase the mechanical advantage.
A single-luff tackle consists of a double and single block as indicated in figure 4-27, and the double-luff tackle has one triple and one double block, as shown in figure 4-28, The mechanical advantage of the single is 3, whereas the mechanical advantage of the double is 5.
A twofold purchase consists of two double blocks, as shown in figure 4-29, whereas a threefold purchase consists of two triple blocks, as shown in figure 4-30. The mechanical advantage of the twofold purchase is 4; the advantage of the threefold is 6.
In reeving a simple tackle, lay the blocks a few feet apart. The blocks should be placed down with the sheaves at right angles to each other and the becket ends pointing toward each other.
To begin reeving, lead the standing part of the falls through one sheave of the block that has the greatest number of sheaves. If both blocks have the same number of sheaves, begin at the block fitted with the becket. Then, pass the standing part around the sheaves from one block to the other, making sure no lines are crossed, until all sheaves have a line passing over them. Now, secure the standing part of the falls at the becket of the block containing the least number of sheaves, using a becket hitch for a temporary securing or an eye splice for a permanent securing.
With blocks of more than two sheaves, the standing part of the falls should be led through the sheave nearest the center of the block. This method places the strain on the center of the block and prevents the block from toppling and the lines from being cut by rubbing against the edges of the block.
Falls are generally reeved through 8- or 10-inch wood or metal blocks in such a reamer as to have the lower block at right angles to the upper block. Two, three-sheave blocks are the usual arrangement, and the method of reeving these is shown in figure 4-31. The hauling part must go through the middle sheave of the upper block, or the block will tilt to the side and the falls jam when a strain is taken.
Figure 4-31.Reeving a threefold purchase.
If a three- and two-sheave block rig is used, the method of reeving is about the same (figure 4-32), but, in this case, the becket for the dead end must be on the lower, rather than the upper, block.
Figure 4-32.Reeving a double-luff tackle.
Naturally, you must reeve the blocks before you splice in the becket thimble, or you will have to reeve the entire fall through from the opposite end.
SAFE WORKING LOAD OF A TACKLE
You know that the force applied at the hauling part of a tackle is multiplied as many times as there are parts of the fall on the movable block. Also, an allowance for friction must be made, which adds roughly 10 percent to the weight to be lifted for every sheave in the system. For example, if you are lifting a weight of 100 pounds with a tackle containing five sheaves, you must add 10 percent times 5, or 50 percent, of 100 pounds to the weight in your calculations. In other words, you determine that this tackle is going to lift 150 pounds instead of 100 pounds.
Disregarding friction, the safe working load of a tackle should be equal to the safe working load of the line or wire used, multiplied by the number of parts of the fall on the movable block. To make the necessary allowance for friction, you multiply this result by 10, and then divide what you get by 10 plus the number of sheaves in the system.
Suppose you have a threefold purchase, a mechanical advantage of 6, reeved with a line that has a safe working load of 2 tons. Disregarding friction, 6 times 2, or 12 tons, should be the safe working load of this setup. To make the necessary allowance for friction, however, you first multiply 12 by 10, which gives you 120. This you divide by 10 plus 6 (number of sheaves in a threefold purchase), or 16. The answer is 7 1/2 tons safe working load.
Lifting a Given Weight
To find the size of fiber line required to lift a given load, use this formula:
C in the formula is the circumference, in inches, of the line that is safe to use. The number 15 is the conversion factor. P is the weight of the given load expressed in tons. The radical sign, or symbol, over 15 x P indicates that you are to find the square root of that product.
To square a number means to multiply that number by itself. Finding the square root of a number simply means finding the number that, multiplied by itself, gives the number whose square root you are seeking. Most pocket calculators today have the square root function. Now, lets determine what size fiber line you need to hoist a 5-ton load. First, circumference equals 15 times five, or C = 15 x 5, or 75. Next, the number that multiplied by itself comes nearest to 75 is 8.6, Therefore, a fiber line 8 1/2 inches in circumference will do the job.
The formula for finding the size of wire rope required to lift a given load is: C (in inches) = 2.5 x P (tons). You work this formula in the same manner explained above for fiber line. One point you should be careful not to overlook is that these formulas call for the circumference of the wire. You are used to talking about wire rope in terms of its diameter, so remember that circumference is about three times the diameter, roughly speaking. You can also determine circumference by the following formula, which is more accurate than the rule of thumb: circumference equals diameter times pi (p). In using this formula, remember that equals approximately 3.14.
Size of Line to Use in a Tackle
To find the size of line to use in a tackle for a given load, add one-tenth (10 percent for friction) of its value to the weight to be hoisted for every sheave in the system. Divide the result you get by the number of parts of the fall at the movable block, and use this result as P in the formula
For example, lets say you are trying to find the size of fiber line to reeve in a threefold block to lift 10 tons. There are six sheaves in a threefold block. Ten tons plus one-tenth for each of the six sheaves (a total of 6 tons) gives you a theoretical weight of 16 tons to be lifted. Divide 16 tons by 6 (number of parts on the movable block in a threefold block), and you get about 2 2/3. Using this as P in the formula you get
The square root of 40 is about 6.3, so it will take a line of about 6 1/2 inches in this purchase to hoist 10 tons safely. As you seldom find three-sheave blocks that will take a line as large as 6 1/2 inches, you will probably have to rig two threefold blocks with a continuous fall, as shown in figure 4-33. Each of these will have half of the load. To find the size of the line to use, calculate what size fiber line in a threefold block will lift 5 tons. It works out to about 4 1/2 inches.
Figure 4-33.Rigging two tackles with continuous fall.
TACKLE SAFETY PRECAUTIONS
In hoisting and moving heavy objects with blocks and tackle, stress safety for people and materials.
Always check the condition of blocks and sheaves before using them on a job to make sure they are in safe working order. See that the blocks are properly greased. Also, make sure that the line and sheave are the right size for the job.
Remember that sheaves or drums that have become worn, chipped, or corrugated must not be used because they will damage the line. Always find out whether you have enough mechanical advantage in the amount of blocks to make the load as easy to handle as possible.
Sheaves and blocks designed for use with fiber line must not be used for wire rope since they are not strong enough for that service, and the wire rope does not fit the sheave grooves. Also, sheaves and blocks built for wire rope should never be used for fiber line.
HOOKS AND SHACKLES
Hooks and shackles are handy for hauling or lifting loads without tying them directly to the object with a line or wire rope. They can be attached to wire rope, fiber line, or blocks. Shackles should be used for loads too heavy for hooks to handle.
Hooks should be inspected at the beginning of each workday and before lifting a full-rated load. Figure 4-34, view A, shows where to inspect a hook for wear and strain. Be especially careful during the inspection to look for cracks in the saddle section and at the neck of the hook.
When the load is too heavy for you to use a hook, use a shackle. Shackles, like hooks, should be inspected on a daily routine and before lifting heavy loads. Figure 4-34, view B, shows the area to look for wear.
You should never replace the shackle pin with a bolt. Never use a shackle with a bent pin, and never allow the shackle to be pulled at an angle; doing so will reduce its carrying capacity. Packing the pin with washers centralizes the shackle (figure 4-34, view B).
Figure 4-34.Hook and shackle inspection (views A and B) and packing a shackle with washers.
If you need a hook or shackle for a job, always get it from Alfa Company. This way, you will know that it has been load tested.
Mousing is a technique often used to close the open section of a hook to keep slings, straps, and so on, from slipping off the hook (figure 4-35). To some extent, it also helps prevent straightening of the hook. Hooks may be moused with rope yarn, seizing wire, or a shackle. When using rope yarn or wire, make 8 or 10 wraps around both sides of the hook. To finish off, make several turns with the yarn or wire around the sides of the mousing, and then tie the ends securely (figure 4-35).
Shackles are moused when there is danger of the shackle pin working loose and coming out because of vibration. To mouse a shackle, simply take several turns with seizing wire through the eye of the pin and around the bow of the shackle. Figure 4-35 shows what a properly moused shackle looks like.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015