A piston ring is a ring with a split joint which fits into a groove on the outer surface of a piston in a combustion engine or pneumatic compressor or hydraulic cylinder/ tube, etc.
The main functions of piston rings are:
The ring axial depth must be sufficient and right fit inside the piston groove to provide a good seal against the liner. Loose fit inside the piston groove will result in hammering and low life.
The ring must be free in its groove and the correct clearance is required. The butt clearance must be sufficient to allow for thermal expansion. Insufficient clearance will cause jamming and seizure while excessive clearance will lead to excessive blow-past and leakage of pressure.
The ring must float freely in the groove within the piston so that it can stay in contact with the cylinder. The ring radial depth must be sufficient to allow adequate support for the ring in the piston groove. Radial clearance must be sufficient to allow a gas cushion to build up.
The rings are subject to wear so they are made of wear-resistant materials such as cast iron alloy, ductile iron or steel.
The ring outer periphery is constantly rubbing against the liner so this outer periphery is usually made slightly harder by hard chrome plating or case hardening.
Piston rings are classified according to the following criteria: