pulley, poleyne, polive, polley, pullie, pully

[Fr. poulie a pulley. The form polive (in Chaucer: C. T., 10,498) is hard to explain, but poleyne ( Prompt. Parv.) is from Fr. poulain = "a foal, or colt, also the rope wherewith wine is let down into a cellar, a pulley-rope" ( Cotgrave ), from Low Lat. pull an us =a colt, from Lat. pullus the young of any animal (cogn. with Eng. foal ). For the transference of sense cf. horse a kind of frame; Fr. poutre a filly a beam; chèvre a goat a crane; Eng. crane in its double meaning; Gronos an a , a crane, a pulley, .] 1. Mech.: One of the six simple machines or mechanical powers. It consists of a small circular plate or wheel which can turn round an axis pa ing through the centers of its faces, and having its ends supported by a framework which is called the block. The circular plate has a groove cut in its edge to prevent string from slipping off when it is put round the pulley. With a single fixed pulley (that is one in which the block in which the pelley turns is fixed), there is neither gain nor lo of power; for, as the tension in every part of the cord is the same, if a weight be suspended at one extremity, an equal weight must be applied at the other to maintain equilibrium. Hence, the effect of a fixed pulley is simply to change the direction of a force. By means of movable pulleys one can gain mechanical advantage, greater or le , accord in to the number and mode of combination of the pulleys. This advantage may be computed by comparing the velocity of the weight raised with that of the moving power, according to the principle of virtual velocities. Thus: In a single movable pulley with the strings parallel when there is equilibrium the weight is twice the power. It may, therefore, be considered a lever of the second cla , in which the distance of the power from the fulcrum is double that of the weight from the fulcrum. In a system of pulleys in which each pulley hangs by a separate string and all the pulleys are parallel, when there is equilibrium the weight is equal to the power multiplied by 2n, where n is the number of pulleys. In a system of pulleys in which the same string pa es round all the pulleys, and the parts of it between the pulleys are parallel, when there is is equilibrium the weight is equal to the power multiplied by the number of strings at the lower block. In a system of pulleys in which each string is attached to the weight, and all the strings are parallel, when there is equilibrium the weight is equal to the power multiplied by 2 raised to a power whose exponent is one le than the number of pulleys. 2. Mach .: A wheel with a grooved, flat, or slightly convex rim, adapted to receive a cord or band, which runs over it. It transmits power or changes the direction of motion. (1) Cone pulley : [CONE-PULLEY.] (2) Conical pulley : [CONE-PULLEY, 2.] (3) Fast pulley : A pulley firmly attached to the shaft from which it receives or to which it communicates motion. (4) Loose pulley : A pulley running free on the shaft, to receive the belt and allow it still to traverse without being affected by, or affecting the motion of, the shafting. (5) Sliding pulley : A kind of coupling in which the band-pulley is slipped into or out of engagement with an arm freely attached to the shaft and rotating therewith. (6) Speed pulley: [CONE-PULLEY, 2.] pulley-block, subst. A shell with a sheave or sheaves. pulley-box, 8. Loom: A frame containing the pulleys for guiding the tail-cords in a draw-loom.