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Pulley bearings are located in the center of the pulley wheels used in a given pulley mechanism. Typically, a pulley bearing is cast or forged from heat-resistant metal or metal alloys. Depending on the specific type of bearing, from the outside it usually looks like a metal ring or set of rings embedded in the center of the pulley wheels. Inside the bearing, the inner and outer rails, balls, lubricating oil and other components rotate as the pulley wheel turns. In the case of ball bearings, each ball inside the pulley bearing rotates, acting like dozens of tiny fingers to help turn the pulley wheel, distribute the weight of the load, and absorb heat.
Since a pulley wheel needs to be able to turn a full 360 degrees, most wheels are center mounted. The center mount gives the pulley an overall donut-like shape, with the center hole mounted on a shaft. Around the center mounting hole, a metal bearing is inserted, usually pressed into place when the pulley wheel is manufactured. The purpose of a pulley bearing is to protect the mounting frame and wheel from heat, allow for increased speed and ensure smooth operation. Certain types of pulley bearing designs also reduce oscillation and vibration by forcing the pulley wheel to rotate on a level, uniform axis.
Regardless of the specific construction, materials or type of bearing used, pulley bearings allow smooth, continuous rotation of the pulley wheels while reducing frictional resistance and heat. Pulley bearing types vary depending on the type of pulley wheel used. Most often, manufacturers of pulley mechanisms use ball bearings, a series of small, heavily lubricated metal balls placed in a metal track. As the pulley wheel rotates, the individual balls inside the bearing help to evenly distribute the weight of the load, reduce friction and provide smooth, even rotation with limited vibration or loss of speed.
Pulleys made from plastic, nylon and other synthetic polymers are most commonly incorporated into pulley bearings. Since the wheel itself is made of plastic, the frictional heat caused by turning can easily bend, melt or shorten the life of the pulley. By fitting the wheel with pulley bearings around the mounting hole, each wheel is able to handle a heavier load at higher speeds without causing damage to the plastic or other polymer. Instead of the plastic wheel bearing the weight and heat of a specific load, the bearing absorbs most of the stress and heat, ensuring the wheel maintains its shape and performance.