How Does a Thrust Bearing Work?

by Pauline Gill

Thrust Bearings

Thrust bearings absorb axial loads from rotating shafts into the stationary housings or mounts in which they are turning. Axial loads are those transmitted linearly along the shaft. Good examples of axial loads are the forward thrust on boats or prop-driven airplanes as a result of their propeller's rapid rotation. Thrust bearings are also used in power drills, where the user puts their weight into a rotating bit to drill through tough materials. Merry-go-rounds have massive thrust bearings to support all the rotating weight.

Pure Thrust Bearings

Pure thrust bearings are so termed because they only resolve axial forces from the rotating component into their mounting and not radial forces. As in other types of bearings, there are two major groupings of these bearings, sliding bearings and rolling bearings. An example of a sliding thrust bearing is a thrust washer which is a low-friction material between the shaft and the bearing journal along the rotating component. Types of rolling thrust bearings are ball thrust bearings, and specialized tapered roller bearings.

Combination Bearings

Many motors and machines use combination bearings which resolve both axial and radial forces with one bearing. These can be tapered roller bearings such as those used on automotive wheels, cup type ball bearings working in unison such as those on small ball bearing wheels on wagons and carts. They can also be deep groove caged ball bearings. Combination bearings control the rotational motion around the shaft, and carry the weight of the vehicle. They also limit side to side movement along the shaft, such as when cornering hard in a sports car. In this capacity, they function as thrust bearings. Combination bearings are used in applications where the thrust loading might be coincidental or relatively small compared to the radial loading. Pure thrust bearings are used in applications where thrust loads are the predominant forces transmitted by the rotating components into their stationary containment.

Thrust Bearing Operation

Sliding thrust bearings such as Oilite washers are used in some electric devices. The washers slide against flat shaft faces to keep the shaft from moving back and forth sideways. In automotive main bearings, they are implemented as a two sided shell between crankshaft throws as the middle bearing journal. Most aircraft engines, either piston or jet and propeller driven water craft, including everything from outboard boats to ships at sea, have separate pure thrust bearings as well as combination or radial bearings to support the spinning shafts, gears, and propellers. As their engines turn the propellers, the propellers provide a linear thrust or push that moves the vehicle forward. This thrust tries to push the driveshaft or propeller shaft through the end of the engine. Bearing balls rotate between two grooved washers in this case which act as axial races to contain the balls. These bearings have the appearance of a turntable since they are flat. The thrust is resolved from the spinning propeller shaft into the respective mountings and the ship is thrust forward.

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