Piston seals or piston rings are used in pneumatic cylinders for fluid sealing and are designed to ensure that pressurized fluid does not leak across the cylinder head as the system pressure pushes the piston down the cylinder bore.
The choice of piston seal is decided by the way in which the cylinder operates. For a cylinder which is exclusively single-acting, capable of sealing dynamic pressure from one side (unidirectional), it is always best to choose the type of seal designed to provide optimum sealing qualities for single-acting functions.
The best sealing capacity of a double-acting cylinder, capable of sealing dynamic pressure from both sides (bidirectional), is achieved by choosing a double-acting seal.
A piston design where two single-acting seals on the piston for a double-acting cylinder are used can easily give rise to a breakdown. The reason is that a very high pressure can be trapped between the seals.
Piston seals, both single and double-acting, can be designed for and used with integrated or separate back-up rings and guide rings.