What Is a Thrust Washer?

by Pauline Gill

Thrust washers can be found in almost every appliance, machine, transportation conveyance, power tool, and recreational device that has moving parts, axles, bolts, pins, bearings, and rotating components. In their simplest form, thrust washers are long-wearing flat bearings in the shape of a washer that transmit and resolve axial forces in rotating mechanisms to keep components aligned along a shaft. Thrust washers are an economical alternative to rolling thrust bearings whenever forces velocities are moderate.

Beginnings

The need for thrust washers presented itself in antiquity almost as soon as the wheel was invented. These rugged washer shaped flat bearings are used to prevent wheels from moving sideways on axles whenever the bearing that handles the radial load such as a bushing or roller bearing has no specific provision for axial or thrust loads. Typical sideways or axial loads are encountered whenever turning a corner and the vehicle is thrust sideways towards the outside of the curve. Other applications from our ancestors would have been grain mills, water wheels, turntables, and rotary drills, wherever the primary movement may have had both radial and axial forces to contend with. Of course, propeller shafts on every propeller driven vessel from the largest to the tiniest must have thrust bearings to resolve the linear and axial propulsion forces of spinning propellers either forward or backward.

Construction

Thrust washers are made from many different materials, and are selected for the best combination of performance, maintenance and price. Most hardened steel shafting applications will use an Oilite type of washer which is a very porous bronze material that can contain more than 30% oil by volume. As such, Oilite is self-lubricating.

Variations

Sometimes the sleeve bearings around a shaft integrate their own thrust washer, and these are termed flanged sleeve bearings. One of these on each end of a motor shaft will resolve all of the radial and axial forces presented by an electric motor. Another variation on the standard thrust washer is to use several in series whenever the Vmax or maximum velocity in RPM exceeds the normal specification for a sliding (not rolling) bearing, such as a single thrust washer. When a highly viscous fluid such as silicone or heavy oil is applied to thrust washers, a damping function is added to the thrust washer's capabilities which absorbs rotational vibration and acts as a dynamic brake.

Common Applications

Virtually every appliance electric motor has at least one thrust washer in it to control axial displacement, also known as end play. The propeller shaft on most inboard and outboard boats uses at least one thrust washer to transmit the axial thrust of a spinning propeller into forward or backward motion. Turntables of all kinds use thrust washers both as weight bearings as well as erratic motion dampers.

Disadvantages

As all sliding rather than rolling bearings do, thrust washers do consume a portion of the rotational energy in the system as friction. So their efficiency is always less than a ball or roller bearing type thrust bearing system.

Summary

Thrust washers are the most economical and easy to apply of all the thrust bearing alternatives. As with all mechanical systems, they work best when kept clean and lubricated.

About the Author

Pauline Gill is a retired teacher with more than 25 years of experience teaching English to high school students. She holds a bachelor's degree in language arts and a Master of Education degree. Gill is also an award-winning fiction author.

Photo Credits

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