PULLEY. The pulley is one of the simple machines or mechanical powers employed in the construction of machinery and in the transmission and modification of force. The kinds of pulley in use are very numerous; but they all consist of combinations of a grooved wheel, moveable on an axis, and a rope lying in the groove; and the manner in which this rope passes over and under a system of these wheels, so as to connect the force with the resistance, or the power with the weight, de termines the species or kind of pulley. The following are the combinations by which the principle of the pulley is made available:— The single fixed pulley possesses no me• chanical advantage, the power and the weight being equal to each other. The single move able pulley consists of one wheel fixed and the other moveable ; the power acts at one end of the rope, and the other end is fixed to an immoveable obstacle ; the weight or resistance is attached to the sheave, or block, of the moveable wheeL In this the power is one-half the resistance when there is equilibrium. The first system of pulleys consists of an upper and lower set of wheels, called the upper and lower block ; the upper being fixed, and the lower, to which the resistance or weight is attached, being moveable. The power is to the
weight or resistance as unity to twice the num ber of wheels in the lower block. A system consisting of one fixed and two moveable pullies is called a Spanish Barton. Smeaton's Tad; so called after the celebrated engineer of that name, contains two tiers of wheels, one above the other, in each block. This arrange ment is ingenious, but it is attended with great friction and inequality of wear. TTrliile's Pulley consists of a single wheel in each of the two blocks. The when has a conical form, with grooves on its convex surface; and the diameters of the grooves are in arithmetical progression, by which they revolve with velo cities equal to that of the passing over them. The advantage is that the friction is reduced to that of the pivot and the two lateral faces.
There are two systems in which each block contains a single pulley, and has a separate string.. In one system the string is made fast at one end to the block next above it, and at the other to an immoveable object. In the other system each string is attached to the weight. These are superior to the other systems, with respect to the ratio of the re sistance to the moving power, but they are of no practical use.