Dynamic vs Static Seal
Also known as: static seal · dynamic seal · reciprocating seal · rotary seal
A static seal prevents leakage between non-moving surfaces (flange faces, port plugs); a dynamic seal operates with relative motion between mating surfaces — requiring harder compounds and tighter dimensional tolerances.
Technical Detail
Static seals (O-rings in flange grooves, face seals, end-cap seals) experience no relative motion between mating surfaces. Standard 70 Shore A compound with 15–25% squeeze works well; compression set ≤35% acceptable; surface finish Ra 1.6–3.2 µm on groove. Dynamic seals experience reciprocating motion (hydraulic cylinder rod/piston), oscillating motion (quarter-turn valves), or continuous rotation (pump shaft seal). Requirements are more demanding: squeeze 10–20% to minimise friction and heat; compression set ≤20%; surface finish Ra 0.2–0.4 µm on shaft (dynamic contact surface); harder compound 70–80 Shore A to resist extrusion; lubrication typically required. Rotary shaft seals above 1 m/s surface speed require a dedicated lip seal design (TC/TB oil seal) rather than a standard O-ring. For pressures above 7 MPa in reciprocating duty, add PTFE back-up rings in the groove to prevent extrusion.
Reference Standards
- ISO 3601-2
- ASTM D2000
- SAE AS568B
Related Terms
- O-Ring Groove / Gland Design — O-ring groove (gland) design specifies groove depth, width, and surface finish to achieve 15–25% dia…
- Compression Set — Compression set (CS%) measures how much of an O-ring's original thickness is permanently lost after …
- Oil Seal TC / TB (Lip Seal) — TC and TB oil seals are double-lip and single-lip rotary shaft seals with a spring-loaded PTFE or ru…