high-speed sliding contact. Lubrication is particularly difficult as the rings have an oscillating motion rather than continuous rotation (such as in a bearing journal). At the limits of piston movement, the ring stops and reverses direction. This disrupts the normal oil wedge effect of a hydrodynamic bearing, reducing the effectiveness of the lubrication.

Rings are also sprung to increase the contact force and to maintain a close seal. The spring force is provided by either the stiffness