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Thrust bearings are a specialized type of bearing designed to handle axial stresses while allowing rotary motion between two different moving parts. Thrust, in this case, actually refers to the axial loads under which such bearings are designed to operate. Some of the different types of thrust bearing include ball, roller, fluid and magnetic. In automotive applications, ball and roller thrust bearings are typically used in wheel hubs, while manual transmissions can utilize what is commonly known as a throw-out bearing as part of the clutch mechanism.
The two varieties of thrust bearing most commonly used in automotive applications are the ball and roller types. Ball bearings consist of a number of balls held within two circular rails and can handle a comparatively lower amount of thrust. Tapered roller bearings are similar in appearance, but the cylindrical roller bearings provide more surface area and can handle significantly higher levels of thrust. Both of these types of thrust bearing are commonly used within wheel hubs. Bearings are required for the wheels to rotate while supporting the weight of the vehicle, and the presence of a thrust bearing is necessary because of the inherent axial stresses.
Another use of thrust bearings in automotive applications is in manual transmissions. In these cases, the clutch release bearing, or throw out bearing, is actually a specialized type of thrust bearing. It sits on a spindle within the transmission and is activated by a fork when the driver pushes down on the clutch pedal. The fork presses it into the pressure plate, freeing the clutch disc and allowing the driver to shift or brake. The pressure plate is bolted to the flywheel and spins when the clutch release bearing contacts it, so the bearing may have to deal with high levels of thrust without breaking.
Thrust bearings are also found in non-automotive applications anywhere that a bearing might undergo high levels of axial stress. In airplanes and helicopters, thrust bearings will be used in landing gears and rotors to deal with the thrust involved in these applications. Where roller and ball bearings are most commonly used in automotive and aerospace applications, fluid thrust bearings are often used in the thrust block of ships. These bearings can be much smaller than ball or roller bearings and provide for lower levels of friction. These factors have allowed more powerful engines to be created, and fluid thrust bearings see wide use in both marine and land-based power plants.