Full definition
A dynamic seal mounted in the groove of a hydraulic or pneumatic cylinder piston, sealing the pressurized fluid on one side of the piston from the opposite side while the piston reciprocates inside the cylinder bore. The piston seal must: (1) seal against high differential pressure (70-350 bar for hydraulic, 6-10 bar for pneumatic), (2) minimize friction for smooth, controllable piston movement, (3) prevent bypass leakage across the piston, and (4) withstand millions of reciprocating cycles. Types: U-cup (single-acting — seals in one direction), double-acting (seals pressure from both sides — most common for hydraulic), crown seal (compact, combined double-acting), step seal (PTFE ring + O-ring energizer — highest performance), piston ring (split ring, like engine piston ring — for large bores), and wear ring/guide band (not a seal, but supports the piston to prevent metal-to-metal contact and side loading of seals). Materials: polyurethane PU (best abrasion/extrusion resistance), NBR (standard), FKM (high temperature), PTFE/bronze (lowest friction), and fabric-reinforced phenolic (wear rings). Per ISO 5597 (metric cylinder dimensions), ISO 6020/6022 (hydraulic cylinders). Brands: Parker, Trelleborg (Turcon, Zurcon), Hallite, SKF (Economos).