Thrust washers aren't technically washers; they're bearings that happen to look like washers. A thrust washer is a flat bearing that sandwiches in between a rotating component and a stationary component and acts to keep the rotating component in place by giving it something to brush up against if it starts to move sideways.
Bearings come in two basic varieties, either flat or rollerized. Flat bearings, like thrust washers and automobile main bearings, work by spreading the load out over a large surface area. This reduces point contact force and, with it, friction. A rollerized bearing uses some sort of mechanical device, such as a ball or needle-like roller, between the flat bearing surfaces to further reduce friction.
Thrust washers differ from standard washers in that they're designed at the outset with lubricity in mind. Lubricity refers to how easily one substance slides against another. A pair of bricks will tend to have very poor lubricity because their rough surfaces grip each other. Metals, such as copper, have very high lubricity because they allow other materials to slide easily over them. Thrust washers are generally made of some sort of alloy that offers increased lubricity, which extends bearing life.
Roller bearings tend to outlast thrust washers partially because they offer a bit of space in which to introduce a liquid lubricant. However, purpose-built thrust washers often come from the factory impregnated with a dry lubricant, such as graphite, molybdenum disulfide of boron nitride. Manufacturers add the graphite into the metal during the ingot casting process; as the bearing wears, more graphite particles come to the surface and help to resist wear. Put two graphite-impregnated thrust washers together and you end up with a very slick surface that's highly resistant to wear.
One way to extend the life of a thrust washer assembly is to add one or more intermediate washers. Let's say that the force that a thrust washer must cope with is equal to 100 pounds. If you use a single thrust washer pressing on another thrust washer, each one receives 50 pounds of pressure. Add another washer in between and you have two more wear faces, so each now receives just 25 pounds of pressure. Add another one and you reduce the load to 16.6 pounds of pressure. This translates directly into an improvement in bearing life and possibly friction reduction, depending on the thrust washer type and design.
Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.