two metal framed, heavy-duty blocks. Block A is designed for manila line, and block B is for wire rope.
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.
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.
To reeve blocks in simple tackle, you must first 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 bends pointing toward each other.
To start reeving, lead the standing part of the falls through one sheave of the block that has the greatest number of sheaves. Begin at the block fitted with the becket. Next 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 having the fewest number of sheaves, using a becket hitch for temporary securing or an eye splice for permanent securing.
When blocks have two or more sheaves, the standing part of the fall should be led through the sheave closest to the center of the block. This places the strain on the center of the block and prevents the block from toppling and the lines from being chafed and cut through by rubbing against the edges of the block.
Falls are normally reeved through 8-inch or 10-inch wood or metal blocks, in such away as to have the lower block at right angles to the upper. Two 3-sheave blocks are the traditional arrangement, and the method of reeving is shown in figure 6-6. The hauling part has to go through the middle sheave of the upper block or the block will tilt to the side and the falls will jam under load.Continue Reading