Piston Seals or Piston Rings are used in hydraulic cylinders for fluid sealing. They are internal to the cylinder head and seal against the cylinder bore, preventing fluid from flowing across the cylinder head. This allows pressure build up on one side of the piston, making the cylinder extend or retract.

A piston seal works by containing pressure on one side of a piston without leakage. This allows maximum mechanical effort to be applied to moving the piston along the bore of a cylinder. A piston seal can be driven either hydraulically or pneumatically, with an appropriate piston seal design for each system and application.

Piston seals intended to prevent leakage past the piston and therefore maximise the efficiency of a system. Pressure can either be applied in one direction only (single acting) or from either direction (double acting) to drive the ram to which the piston is fixed both in and out.