Toothed Lock Washer

A toothed lock washer, also known as a serrated washer or star washer, has serrations that extend radially inward and/or outward to bite into the bearing surface. This type of washer is especially effective as a lock washer when used with a soft substrate, such as aluminum or plastic, but can still resist rotation more than a plain washer on hard surfaces, as the tension between washer and the surface is applied over a much smaller area (the teeth).

There are four types: internal, external, combination, and countersunk. The internal style has the serrations along the inner edge of the washer, which makes them more aesthetically pleasing. The external style has the serrations around the outer edge, which provides better holding power, because of the greater circumference. The combination style has serrations about both edges, for maximum holding power. The countersunk style is designed to be used with flat-head screws.