Piston Seals or Piston Rings are used in hydraulic cylinders for fluid sealing and are designed to ensure that pressurised 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.