In general, the ring is inserted in a groove incorporated in the piston and presses against the cylinder. It fulfils a sealing function between the pressurised and non-pressurised volumes and may also comprise a cylinder oil wiper. Sealing is provided with a combination of the following:
• The valve effect between the ring surface and the wall of the piston groove.
• The contact between the peripheral surface of the ring and the bore of the cylinder in which it is used.
In this way, the ring exercises a specific pressure on the bore. This pressure (P) varies according to the metal elasticity module (E), opening gap (m) and cube of the radial thickness. The tangential force (Ft) enables measurement of this pressure and must be chosen according to the specific ring application. The ring manufacturer defines the dimensions alm as well as the value Ft. (The standards DIN, BSI and ISO recommend certain dimensions depending on the applications.) and
|Specific pressure on the bore|
With a temperature increase of 100°C, grey cast iron expands 1.01 mm per developed metre.