ALLOWANCE FOR FRICTION

Because of friction, some of the force applied to tackle is lost. Friction develops in tackle by the lines rubbing against each other or the shell of the block. It is also caused by the line passing over the sheaves or by the rubbing of the pin against the sheaves. Each sheave in the tackle system is expected to create a resistance equal to 10 percent of the weight of the load. Because of fiction, a sufficient allowance for loss must be added to the weight being moved in determining the power required to move the load.

As an example, you have to lift a 1,000-pound load with a twofold purchase. To determine the total force needed to lift the load, you take 10 percent of 1,000 pounds, which is 100 pounds. This figure is multiplied by 4 (the number of sheaves), which gives you 400 pounds. This value is added to the load; therefore, the total load is 1,400 pounds. This figure is divided by 4, the mechanical advantage of a twofold purchase, which results in 350 pounds being the force required to move the load.

BLOCK SAFETY

Safety rules you should follow when using blocks and tackle are as follows:

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.

SLINGS

Slings are widely used for hoisting and moving heavy loads. Some types of slings come already made. Slings may be made of wire rope, fiber line, or chain.

SLINGS AND RIGGING GEAR KITS

The NCF has slings and rigging gear in the battalion Table of Allowance to support the rigging operations and the lifting of CESE. The kits 80104, 84003, and 84004 must remain in the custody of the supply officer in the central toolroom (CTR). The designated embarkation staff' and the crane test director monitor the condition of the rigging gear. The rigging kits must be stored undercover.

WIRE ROPE SLINGS

Wire rope slings offer advantages of both strength and flexibility. These qualities make wire rope adequate to meet the requirements of most crane hoisting jobs; therefore, you will use wire rope slings more frequently than fiber line or chain slings. FIBER LINE SLINGS

Fiber line slings are flexible and protect the finished material more than wire rope slings; however, fiber line slings are not as strong as wire rope or chain slings. Also, fiber line is more likely to be damaged by sharp edges on the material being hoisted than wire rope or chain slings.

CHAIN SLINGS

Chain slings are frequently used for hoisting heavy steel items, such as rails, pipes, beams, and angles. They are also handy for slinging hot loads and handling loads with sharp edges that might cut the wire rope.

Chain sizes, inspection, safe working load, and handling and care will be discussed after wire rope and fiber line, as their characteristics have been discussed in previous chapters.

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