The Parts of a Pulley
The three parts of a pulley are the hub (or boss), the arms (or spokes) and the rim. Cheeks (side plates) prevent lateral motion of the hub and catch the tackle (rope, string or chain) if it disengages from the pulley rim. The hub may be mounted on a shaft using a key, splines or a set screw.
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Rim
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The rim is concave on the arms side. This keeps the belt in the middle of the rim. When the belt is flat, the rim is flat, with flanges on the side to hold the belt in place. The rim may be V-shaped to accommodate a V-shaped belt.
A rim can have teeth to catch the links of a chain, for example, as in the rear derailleur of a bicycle.
Arms
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If the pulley wheel has arms or spokes, they are often cast curved. If the pulley is cast in one piece with the spokes at right angles to the rim, stresses during cooling can cause cracking. Curved arms absorb the stresses.
Cast arms are typically elliptical in cross section, to reduce wind loss.
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Hub
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A pulley can be either fast or loose. This means the hub can be affixed to the shaft or allowed to move around it. A fast and a loose pulley on the same shaft can enable a belt to stop, even if the shaft has to keep rotating. By shifting a rotating belt from the fixed pulley over to the loose pulley, the shaft can continue rotating while the belt stops. In short, a fast and loose pulley system can therefore be used to interrupt the transmission of motion in machinery.
A loose hub can be made to turn more freely with bushings (cylinder or sleeve) or ball bearings inserted between hub and axle. Pulleys can be made interchangeable by swapping in bushings that match the radius of the shaft.
Block
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If the cheeks hold multiple pulleys, they form what is called a block. Multiple blocks can be used to gain mechanical advantage. If one of the blocks has to move along with a load, then for the distance it is hoisted, the rope (tackle) stretched between separate blocks must each shorten the same distance. If there are three stretches of rope between two blocks, the mechanical advantage would be 3:1, because the rope would need to be pulled three times what the load is lifted. It is analogous to using a lever to reduce the force required to lift a heavy load by increasing the distance over which the input force must be applied. This type of system is called block and tackle.
Differential Pulley
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A variation of block and tackle is a differential pulley. The pulleys in a block are of different radius. The rims have engaging sprockets to prevent slip. The chain makes a complete loop so that it is unending. The merit of the system is that it affords great mechanical advantage without requiring the tackle/chain to be tied down or in need of a brake to keep the load from dropping. Keeping the radii (R and r) close, friction is enough to keep the pulley from turning when under load.
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References
Comments
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