Piston seals maintain sealing contact between the piston and the cylinder bore. Differential pressures acting on the piston to extend or retract the piston rod can be in excess of 400 bar （5 800 psi）.
The pressure acting on the piston seal increases contact forces between the piston seal and cylinder surface.
Therefore, the surface properties of the sealing surfaces are critical to proper seal performance.
Piston seals are typically classified into single-acting （pressure acting on one side only） and double-acting （pressure acting on both sides） seals.