PULLEY, one of the mechanical powers (q.v.), consists of a wheel with a groove cut all round its circumference, and movable on an axis; the wheel, which is commonly called a sheave, is often placed inside a hollow oblong mass of wood called a block, and to the sides of this block the extremities of the sheave's axle are fixed for support; the cord which passes over the circumference of the sheave is called the tackle. Pulleys may be used either singly or in combination; in the former case, they are either fixed or movable. The fired pulley (fig. 1) gives no mechanical advantage; it merely changes the direction in which a force would naturally be applied to one more convenient. The single movable pulley, with parallel cords, gives a mechanical advantage = 2 (fig. 1), for a little consideration will show that as the weight, W, is supported by two strings, the strain on each string is and the strain on the one being supported by the hook A, the power, P, requires merely to support the strain on the other string, which passes round C. The fixed pulley, C, is only of service in ehang the naturally upward direction of the power into a downward one.
If strings in the single movable pulley arc not parallel, there is a diminution of mechanical advantage—i.e., P must be more than half of W to produce an exact counterpoise; if the angle made by the strings is 120°, P must be equal to W; and if the angle be greater than this, there is a mechanical disadvantage, or P must be greater than W. The fol lowing are examples of different combinations of pulleys, generally known as the first, second, and third systems of pulleys. In the first system, one end of each cord is fastened to a fixed support above; each cord descends, passes round a pulley (to the lowest of which the weight W is fastened), and is fastened to the block of the next pulley, with the exception of the last cord, which passes round a fixed pulley above, and is attached to tke counterpoise P. The tension of a string being the same in all its parts, the tension of every part of the string marked (1) in fig. 2 is that which is produced by the weight of P, consequently, as the last movable pulley is supported on both sides by a string having a tension P, the tension applied in its support is 2P. The tension of the string is therefore 2P. and the second movable pulley is supported by a force equal to 4P. It
may similarly be shown that the force applied by the strings marked (4) in support of the last pulley (which is attached to W) is 8P. Hence we see, that according to this arrangement, 1 lb. can support 4 lbs., if two movable pulleys are used; 8 lbs., if there are 3 movable pulleys; 16 lbs., if there are movable pulleys; and if there are 71 movable pulleys, 1 lb. can sup port lbs. It must be noticed, however, that in practice, the weight of the cords, and of the pulleys, and the friction of the cord on the pulleys, must be allowed for; and the fact that iu this sys tem all of these resist the action of the power P, and that to a large extent, has rendered it of little use in practice.—The second system is much inferior in producing a mechanical advantage, but it is found to be much more convenient in practice, and is modified according to the purpose for which it is to be used. In this system, one string passes round all the pulleys, and as the tension in every, part of it is that produced by the weight of P, the whole force applied to elevate the lower block with its attached weight, W, is the weight P multiplied by the number of strings attached to the lower block; the pulleys in the upper block being only of use in changing the direction of the pulling force. This system is the one in common use in architec ture, in dockyards, and on board ship, and various modifications of it—such as White's pulley, Smeaton's pulley, etc., have been introduced; but the simpler forms have been found to answer best.—The third system is merely the first system inverted, and it is a little more powerful, besides having the weight of the pulleys to support the power, instead of acting in opposition to it, as in the former ease. By this time, it will have been evident to the reader that the mechanical advantage is not produced by the pulleys, but by the strings, and that the pulleys arc merely useful in keeping the strings in a certain position, changing with as little friction as possible the direction of the pull, mid affording a convenient, means of attaching the weight. Theoretically, the larger the number of movable pulleys in one combination, the greater is the mechanical advantage afforded; but the enormous friction produced, and the want of perfect flexibility in the ropes, prevent any great increase ill the Dumber of pulleys.