PULLEY. A machine used in the arta for the purpose of trans mitting power, which consists of a case with one or more openings, in which a similar number of sheaves revolve upon an axle ; the case or block is surrounded in the direction of its length by a band ending in a hook able to be fixed to any point of support. The cord or chain working upon the sheaf, or sheaves, either increases the power exer cised, whilst it decreases the velocity of movement, according to the principles of mechanics ; or it simply changes the direction of the movement transmitted. Pulleysare either made of wood, or of metal : the former being almost exclusively used for ship-building. In the wooden pulleys the blocks are usually made of elm, the sheaves of lignum vitro, and the axles of either wood, or of iron ; in metal pulleys the blocks are made of iron, the sheaves of brass, the axles of iron or of steel, and the bushes of brass. The proportions of the various parts of a pulley are regulated by the width of the sheaf, and this module itself must be chosen so that the cord or chain shall ride easily on its gorge, without working against the sides; the proportion of the diameter of the axle to that of the pulley must be made as small as is consistent with the strength of the machine, and the sheaves must be so truly bored as to allow of their revolution without any friction against the sides of the blocks.
The term pulley is likewise applied to some of the narrow band wheels used for the purpose of driving machinery gearing, especially when they are of the description technically known under the name of fast and loose pulleys, or those in which one of the assemblage of wheels is made to revolve freely upon the shaft, without being able to communicate any motion to the latter. In these cases the velocity of the axle driven by the intervention of the pulley will be regulated by the proportions between the diarecters of the pulleys. [JLienrNL]