Pulley

(pul'le.) [From Gr. polos, a pivot.] (Mech.) One of the six mechanical powers, consisting ofa small wheel with a grooved edge and turning on an axis. A great mechanical advantage is obtained by using a system in which the pulleys are separate and have separate strings, each string being attached by one extremity to the supporting beam, pa ing round one movable pulley, and having the other extremity fixed to the pulley immediately above it. The power is applied to the cord which pa es round the upper pulley. Another arrangement consists of separate pulleys suspended by separate strings, one extremity of each string being attached to the weight; but both this and the preceding combination are of little practical use. In the common arrangements all the movable pulleys are in one block. The most powerful combination is Smeaton's tackle, in which each block contains two rows of five wheels each, and one string pa es round all, commencing with the centre one of the lower block, and finishing with the middle wheel of the upper. [s. 741]