pulley

• WordNet 3.6
  • n pulley a simple machine consisting of a wheel with a groove in which a rope can run to change the direction or point of application of a force applied to the rope
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• • n Pulley (Mach) A wheel with a broad rim, or grooved rim, for transmitting power from, or imparting power to, the different parts of machinery, or for changing the direction of motion, by means of a belt, cord, rope, or chain.☞ The pulley, as one of the mechanical powers, consists, in its simplest form, of a grooved wheel, called a sheave, turning within a movable frame or block, by means of a cord or rope attached at one end to a fixed point. The force, acting on the free end of the rope, is thus doubled, but can move the load through only half the space traversed by itself. The rope may also pass over a sheave in another block that is fixed. The end of the rope may be fastened to the movable block, instead of a fixed point, with an additional gain of power, and using either one or two sheaves in the fixed block. Other sheaves may be added, and the power multiplied accordingly. Such an apparatus is called by workmen a block and tackle, or a fall and tackle. See Block. A single fixed pulley gives no increase of power, but serves simply for changing the direction of motion.
  • v. t Pulley To raise or lift by means of a pulley.
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• • n pulley Properly, a simple machine consisting of a wheel having a grooved rim for carrying a rope or other line, and turning in a frame, which, when movable, is termed a pulley-block.
  • n pulley A block containing several grooved wheels.
  • n pulley A tackle or apparatus consisting of one or more pulley-blocks with a rope or ropes reeved through them for use in hoisting. The pulley serves to balance a great force against a small one; its sole use is to produce equilibrium; it does not save work, unless indirectly in some unmechanical way. The pulley is a lever with equal arms; but when it turns, the attachments of the forces are moved. Fig. 1 shows a fixed pulley. The equal weights d and e are in equilibrium, because they hang from the equal arms of the lever ab, having its fulcrum at c. Fig. 2 illustrates the principle of the movable pulley. The equal-armed lever, with fulcrum at c, has on one arm the weight d and on the other the force of the stretched string be. If there is equilibrium, this force must be equal to the weight of d. Thus, the total downward pull on f, one arm of the equal-armed lever fg, with fulcrum at h, is twice the weight of d, which must, therefore, be the weight of i to keep it in balance. We may also use the axiom that when a cord is free to move along its length it must be under equal stress in all its parts. Consequently, when a movable block is supported by a number of parallel parts of the same cord, these must bear equal shares of the load. Thus, in fig. 3, the lower block with the weight b brings equal strains upon four stretches of the cord, one of which is balanced by adjective Consequently, the weight of b is four times that of adjective But the effects of friction and of the stiffness of the cord are of great importance in the calculation of the advantages of pulleys. There is a great mechanical advantage in having separate blocks for all the movable pulleys, as in figs. 4 and 5. Thus, in fig. 4, the weight a is balanced over the lowest pulley by the pull on b, and the sum of these forces drawing down the lowest pulley is balanced over the second pulley by the pull on c, which is therefore double the weight at b. Thus, by means of four pulleys a balances a + 2a + 4a + 8a = a (2—1), or fifteen times instead of (as by the arrangement of fig. 3) only four times its own weight. Another arrangement is shown in fig. 5. Here, by means of four pulleys, a balances eight times its own weight.
  • n pulley In anatomy: A trochlea, or trochlear surface of an articulation.
  • n pulley A ligamentous loop which confines or changes the direction of the tendon of a muscle passing through it: as, the digastric muscle of the chin and the superior oblique of the eye both pass through a pulley. See cuts under muscle and eye.
  • pulley To raise or hoist with a pulley.
  • n pulley A pulley which merely tightens the belt but takes off no power.
  • n pulley A pulley built up of segments or sectors.
  • n pulley A segmental pulley; a pulley made in two or more sectors for convenience in casting.
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• • n Pulley pōōl′i a wheel turning about an axis, and having a groove on its rim in which a cord runs, used for raising weights
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