pulley(redirected from Second-class pulley)
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arrangement of moving and stationary mechanical parts used to perform some useful work or to provide transportation. From a historical perspective, many of the first machines were the result of human efforts to improve war-making capabilities; the term engineer
..... Click the link for more information. consisting of a wheel over which a rope, belt, chain, or cable runs.
A grooved pulley wheel like that used for ropes is called a sheave. A single sheave mounted in a block and fixed in place simply changes the direction of force exerted on the rope passing over it. If the end of the rope that ordinarily would attach to the load is passed around a second, unfixed pulley and back to the fixed pulley, a load attached to the free pulley can be raised with half the effort, or with a mechanical advantage of 2. Thus arranged, the device is called a block and tackle. The number of pulley wheels mounted in the fixed and free blocks can be increased indefinitely to get a higher and higher mechanical advantage, the mechanical advantage equaling the number of strands running to the free pulley. Therefore if the rope is run over the first fixed pulley wheel, around the free pulley, over a second pulley wheel in the fixed block, and back to the free block, the mechanical advantage is 3. A 300-lb load can be raised by a pull of 100 lb on the free end of the rope. To raise the load 10 ft, however, the free end of the rope must be pulled 30 ft.
Disregarding friction, work output will always equal work input. If the action is reversed by attaching the load to the free end of the rope and pulling on the free block, the mechanical advantage becomes a mechanical disadvantage, but a speed advantage. A rope block and tackle is usually for hand operation. To lift larger loads by hand, a chain is substituted for the rope and the pulleys have grooves for gripping the links. A differential pulley consists of two pulleys of different radii connected and rotating as one on a common axle. The pulleys have their circumferences grooved and spiked so that a chain will run in them without slipping. Over the pulleys an endless chain is run, forming two hanging loops. In one loop is placed a movable block, whose pulley is shaped to take the chain. The load is attached to the movable block and is raised by pulling on the other loop of the chain.
Power-operated machinery usually has cables, as in vertical-lift drawbridges, power shovels, and cranes. Before the extensive use of electric motors, steam engines or water turbines often supplied the power for factory machinery. One engine or turbine might run a whole factory through a complicated system of shafts, pulleys, and belts. Pulleys for flat belts are crowned to keep the belt centered. Raised flanges will serve the same purpose, but they wear the edges of the belt. Drive pulleys for conveyor belts often have a covering, called lagging, to provide better grip. Individual electric motors usually provide drive by means of V belts, the pulleys having raised flanges to form slots that match the trapezoidal cross sections of the belts. Cone pulleys consist of a number of pulleys of varying diameters massed in the shape of a cone. They are used with belt drives for machines (e.g., lathes) requiring a variety of speeds.
a rotating part of a belt drive or a cable transmission, made in the form of a wheel around which a flexible link passes. Drive pulleys and driven pulleys, which transmit torque, are fastened to shafts by means of, for example, key or spline joints. Pulleys that do not transmit torque, such as idler and tension pulleys, turn freely on shafts or axles. Pulleys with grooved surfaces are sometimes called sheaves.
Pulleys are available in a wide variety of designs. Pulleys of small diameter are manufactured in one solid piece (Figure l,a). Pulleys of medium or large-diameter (Figure l,b) have a hub (1) and a rim (2) connected by a web (3) or by arms. Large pulleys are sometimes produced in two halves, which are bolted together. Pulleys may be made of cast iron, steel (Figure l,c), a light alloy, plastic, or—in some cases—wood.
Flat-belt pulleys (Figure 1,a) have a cylindrical or crowned peripheral surface to prevent the belt from being thrown off; flanges (4) are sometimes used for the same purpose. V-belt pulleys for single-belt or multiple-belt drives have grooves with a trapezoidal cross section (Figure l,b). Round-belt pulleys have a groove with a rounded bottom. Timing-belt pulleys have flanges and axial grooves for the teeth of the timing belt. Step pulleys (Figure 1,d) are used in drives in which the transmission ratio is adjusted by shifting the belt from one step to another. Doublecone pulleys in continuously variable transmissions with a wide V belt are produced with one or two springloaded movable cones; the displacement of one or both cones is forced.
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(Russian, espander), a piece of athletic training equipment with springs or weights, used to create an additional load on specific muscle groups during various physical exercises, such as stretching and lifting weights. Both stationary and portable types are used.