A THREEFOLD PURCHASE consists of two triple blocks and has a mechanical advantage of 6 (fig. 6-13).

A COMPOUND TACKLE is a rigging system using more than one line with two or more blocks. Compound systems are made up of two or more simple systems. The fall line from one simple system is secured to the hook on the traveling block of another simple system, which may have one or more blocks.

To determine the mechanical advantage of a compound tackle system, you must determine the mechanical advantage of each simple system in the compound system. Next, multiply the individual advantages to get the overall mechanical advantage. As an example, two inverted luff tackles, each has a mechanical advantage of 4. Therefore, the mechanical advantage of this particular compound system is 4 x 4 = 16.

Figure 6-13. - A threefold purchase.

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.