A pulley is a modification of a first or second class lever. A single pulley does not produce any mechanical advantage, just a change of direction in force, Figure 4.13.

If the power loss to friction is not ignored, then a single pulley has a mechanical advantage of less than one. When pulleys are combined in pairs, a mechanical advantage is produced. This use of pulleys is commonly called a block and tackle, Figure 4.14.

FIGURE 4.14. Block and tackle.

FIGURE 4.14. Block and tackle.


The amount of mechanical advantage produced by a block and tackle is proportional to the number of ropes that support the weight. The block and tackle in Figure 4.14 has four ropes, but only three, RB, RC, and RD, support the weight. In this arrangement the amount of weight that can be lifted is three times the amount of force being applied. Expressed mathematically:

where W = Amount of weight to be lifted; F = Amount of force applied to the block and tackle; Rn = Number of ropes.

Problem: How much pull (pounds of force) would it take on the block and tackle rope in Figure 4.14 to lift a 545-lb ball?

Solution: From Figure 4.14, the number of ropes supporting the load is three. The pull can be found by rearranging the pulley equation to solve for F:

Rn 3

With a three rope, block and tackle, 182 lb of force will lift a 545-lb load, but the rope where the force is applied will move a three times the distance the load moves.

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