Block Construction

The terms used to describe the parts of a tackle (fig. 6-3) and various assemblies of tackle are as follows: • The block(s) in a tackle assembly change(s) the direction of pull, provides mechanical advantage, or both. • The fall is either a wire rope or fiber line reeved through a pair of blocks to form a tackle. • The hauling part of the fall leads from the block upon which the power is exerted. • The fixed (or standing) block 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. • The frame (or shell), made of wood or metal, houses the sheaves. • The sheave is a round, grooved wheel over which the line runs. Usually the blocks have one, two, three, or four sheaves. Some blocks 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. • The straps inner and outer) 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. • To overhaul means 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). • The term two blocked means that both blocks of a tackle are as close together as they can go. You may also hear this term called block and block. BLOCK CONSTRUCTION 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. Since fiber line is more flexible and pliable, it does not require a sheave as large as the same size that wire rope requires, According to the number of sheaves, blocks are called SINGLE, DOUBLE, OR TRIPLE blocks. Blocks are fitted with a number of attachments, such as hooks, shackles, eyes, and rings. Figure 6-4 shows Figure 6-3.—Parts of a tackle. Figure 6-4.—Heavy-duty blocks. 6-2