Pulley bearings are located at 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, on the outside it usually looks like a metal ring or set of rings built into the center of the pulley wheels. Inside the bearing, inner and outer tracks, balls, lubricating oil, and other components spin as the pulley wheel turns. In the case of ball bearings, each ball within the pulley bearing rotates, acting like dozens of small fingers to help turn the pulley wheel, distribute load weight, and absorb heat.
Since a pulley wheel needs the ability to rotate a full 360 degrees, most wheels are mounted through the center. Center mounting gives a pulley wheel an overall shape similar to a doughnut, with the center hole mounted on an axle. 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 structure and wheel from heat, allow for increased speed, and ensure smooth operation. Certain types of pulley bearing designs also reduce wobbling and vibrating, forcing the pulley wheel to turn on a level, even axis.
Regardless of specific construction, materials, or type of bearing used, pulley bearings allow for the smooth, continuous rotation of pulley wheels, while simultaneously reducing resistance and heat from friction. Types of pulley bearings vary, depending on the type of pulley wheel used. Most often, manufacturers of pulley mechanisms use ball bearings, a series of small, heavily oiled metal balls set in a metal track. As the pulley wheel turns, the individual balls inside the bearing help evenly spread load weight, reduce friction, and provide a smooth, even rotation with limited vibration or loss of speed.
Pulleys made from plastic, nylon, and other synthetic polymers are most commonly incorporate pulley bearings. Since the wheel itself is made of plastic, heat from the friction caused by turning can easily warp, melt, or otherwise shorten the pulley's lifespan. 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 carrying the weight and heat of a particular load, the bearing absorbs most of the stress and heat, ensuring the wheel maintains its shape and performance.