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 6-4.-Heavy-duty blocks.

two metal framed, heavy-duty blocks. Block A is designed for manila line, and block B is for wire rope.

BLOCK TO LINE RATIO

The size of a fiber line block is designated by the length in inches of the shell or cheek. The size of standard wire rope block 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 becomes distorted which causes unnecessary wear. A wire rope too large for a sheave tends to be pinched which damages the sheave. Also, the wire will be damaged because the radius of bend is too short. 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 does not matter too much; for example, a 3-inch line may be reeved onto an 8-inch block with no ill effects. Normally, 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, with wire rope, it is the diameter, rather than circumference, and this rule refers to the diameter of the sheave, rather than to the size of the block, as with line.

TYPES OF BLOCKS

A STANDING BLOCK is a block that is connected to a fixed object.

A TRAVELING BLOCK is a block that is connected to the load that is being lifted. It also moves with the load as the load is moved.

A SNATCH BLOCK (fig. 6-5) is a single sheave block fabricated so the shell opens on one side at the base of the hook to allow a rope to slip over the sheave without threading the end through the block. Snatch

Figure 6-5.-Snatch blocks.

blocks are used when it is necessary to change the direction of pull on the line.

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