MECHANICAL, ADVANTAGE

The push or pull a human can exert depends on the weight and strength of that individual. To move any load heavier than the amount you can physically move, a mechanical advantage must be used to multiply your power. The most commonly used mechanical devices are block and tackle, chain hoist, and winches.

BLOCK AND TACKLE

A block (fig. 13-42) consists of one or more sheaves fitted in a wood or metal frame supported by a shackle

Figure 13-42.-Parts of a fiber line block.

Figure 13-43.-A. Simple tackle; B. Compound tackle.

inserted in the strap of the block. A tackle (fig. 13-43) is an assembly of blocks and lines used to gain a mechanical advantage in lifting and pulling.

In a tackle assembly, the line is reeved over the sheaves 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 (fig. 13-43, view A). A compound tackle system is an assembly of blocks in which more than one line is used (fig. 13-43, view B).

Various terms used with a tackle, as shown in figure 13-44, are as follows: . The fall is either a wire rope or a fiber line reeved through a pair of blocks to form a tackle. l The hauling part of the fall leads from the block upon which the power is exerted. The standing part is the end which is attached to a becket.

. The movable (or running) block of a tackle 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. l "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" means to lengthen a tackle by pulling the two blocks apart.

Figure 13-44.-Parts of a tackle.

To "round in" means to bring the blocks of a tackle toward each other, usually without a load on the tackle (opposite of overhaul).

The block(s) in a tackle assembly change(s) the direction of pull, provide(s) mechanical advantage, or both. The name and location of the key parts of a fiber line block, as shown in figure 13-42, are as follows: 

l The frame (or shell), made of wood or metal, houses the sheaves.

. The sheave is around, grooved wheel over which the line runs. Usually the blocks will have one, two, three, or four sheaves. Some blocks will have up to eleven 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 the becket.

l The straps hold the block together and support the pin on which the sheaves rotate.

. The shallow is the opening in the block through which the line passes.

. The breech is the part of the block opposite the swallow.

Blocks are constructed for use with fiber line or wire rope. Wire rope blocks are heavily constructed and have large sheaves with deep grooves. Fiber line blocks are generally not as heavily constructed as wire rope blocks and have smaller sheaves with shallow, wide grooves. A large sheave is needed with wire rope to prevent sharp bending. Because fiber line is more flexible and pliable, it does not require sheaves as large as that required for wire rope of the same size.

Blocks, fitted with one, two, three, or four sheaves, are often referred to as single, double, triple, and quadruple blocks. Blocks are fitted with a number of attachments, such as hooks, shackles, eyes, and rings. Figure 13-45 shows two metal framed, heavy-duty blocks. Block A is designed for manila line, and block B is for wire rope.