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 a 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 that causes unnecessary wear. A wire rope too large for a sheave tends to be pinched that damages the sheave. Also, the wire will be damaged because of too short a radius of bend. A wire rope too small for a sheave lacks the necessary bearing surface,

Figure 13-45.-Heavy-duty blocks.

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 maybe 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 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.

Block Safety

Safety items when using block and tackle are as follows: l Always stress safety when hoisting and moving heavy objects around personnel with block and tackle.

. 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 which have become worn, chipped, or corrugated must not be used, because they will injure 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.

. You must NOT use wire rope in sheaves and blocks designed for fiber line. They are not strong enough for that type of service, and the wire rope will not properly fit the sheaves grooves. Likewise, sheaves and blocks built for wire rope should NEVER be used for fiber line.

CHAIN HOISTS

Chain hoists provide a convenient and efficient method for hoisting by hand under particular circumstances. The chief advantages of chain hoists are that the load can remain stationary without requiring attention and that the hoist can be operated by one man to raise loads weighing several tons. The slow lifting travel of a chain hoist permits small movements, accurate adjustment of height, and gentle handling of loads. A ratchet handle pull hoist is used for short, horizontal pulls on heavy objects. Chain hoists differ widely in their mechanical advantage, depending upon their rated capacity.

Three general types of chain hoists for vertical operation are the spur gear hoist, the differential chain hoist, and the screw gear hoist.

The spur gear hoist (fig. 13-46, view A) is the most satisfactory for ordinary operations. This type of hoist is about 85 percent efficient. The differential chain hoist (fig. 13-46, view B) is only about 35 percent efficient and is satisfactory for occasional use and light loads. The screw gear hoist is about 50 percent efficient and is satisfactory where less frequent use of the hoist is required.

Chain hoists arc usually stamped with their load capacities on the shell of the upper block. Chain hoists

Figure 13-46.-A. Spur gear chain hoist; B. Differential chain hoist.

are constructed with their lower hook as the weakest part of the assembly. This is done as a precaution, so the lower hook will be overloaded before the chain hoist is overloaded. The lower hook will start to spread under load, indicating the approaching overload limit. Under ordinary circumstances the pull, exerted on a chain hoist by one or two people, will not overload the hoist.

Chain hoists should be inspected before each use. Any evidence of spreading of the hook or excessive wear is sufficient cause to require replacement of the hook. If the links of the chain are distorted, it indicates that the chain hoist has been heavily overloaded and probably unsafe for further use. Under such circumstances the chain hoist should be condemned. Before using any permanently mounted chain hoists, you should ensure that the annual certification is current.