These dynamic seals are typically single-acting, pressure acting on one side only, and double-acting, pressure acting on both sides, seals. Also known as piston rings, they ensure fluid or other media does not by-pass the piston as the system pressure pushes the piston down the cylinder during a pressure cycle. The choice of piston seal is determined by the way the cylinder operates.

Common applications include agricultural and industrial machinery, automotive applications and off-road equipment, and hydraulic press. Different designs to suit varying operating conditions, with brass filled PTFE outer and Nitrile O-Ring Inner a common setup.