A tolerance ring is a mechanical fastener that also acts as a flexible shim. The rings are formed from thin strips of corrugated metal. Waves in the metal are actually highly engineered springs. Tolerance rings are traditionally used to allow broad machining or casting tolerances, or to maintain a press-fit between dissimilar materials subjected to a wide range of temperatures and differing expansion rates.
In the Mercruiser sterndrive engine by Mercury Marine, Stillwater, Okla., waves on the tolerance ring act as radial springs to support a guide bearing in the gimbal housing. The bearing is a high-precision part that uses a combination spherical bushing/ball bearing with a line-to-line fit to accommodate misalignment with the drive-shaft. The tolerance ring, made by USA Tolerance Rings, West Trenton, N.J., snaps into a groove on the bearing carrier. The entire assembly then presses into the gimbal housing. The tolerance ring keeps the carrier from spinning and retains the bearing axially without risk of taking up internal clearance or crushing the outer carrier.
The tolerance ring also offers several advantages during service and repair to the sterndrive. The tolerance ring retains the bearing in the gimbal housing while the driveshaft can be slipped through the inner race and removed. If the bearing needs service or replacement, it can be more easily removed than if it were pressed into the housing with an interference fit.