Reciprocating (rod and piston)
- Squeeze: 10 to 20 percent (lower than static) to limit friction.
- Stretch: below 3 percent for piston seals, avoid stretching in rod grooves.
- Hardness: 70 to 90 Shore A. Harder rings resist extrusion and spiral failure but run higher friction.
- Finish: moving surface about Ra 0.2 to 0.4 micrometres, with a rounded profile (not a mirror). Too rough abrades. Too smooth cannot hold a lubricating film.
- Hardware: hard chrome, nitrided steel or ceramic-coated surface is typical for long life.
- Lubrication: the fluid or a compatible grease must wet the surface.
- Lead-in chamfers: 15 to 20 degrees, long enough to guide the ring without cutting.
Spiral failure
A long-stroke seal with uneven friction can twist and shear in a spiral pattern. Causes: wrong groove proportions (groove too wide), rough surface, side loading, low squeeze, lubrication loss. Use a quad ring, lip seal or PTFE-based seal when the geometry risks twisting.
Rotary seals
Ordinary O-rings are unsuitable for continuous rotation at speed because friction heat shrinks the stretched rubber (the Gow-Joule effect). Where used, keep the ring slightly compressed, not stretched, at low surface speed. For real rotation use lip seals, PTFE rotary seals or mechanical seals.
Oscillating
Small angle motion is a middle case: rings can be used, but keep low squeeze, good lubrication and low friction compounds.
The Gow-Joule effect
A stretched rubber band shortens when heated. Under frictional heating a stretched O-ring on a rotating shaft contracts and grips harder, causing more heat. This is a positive feedback and a common failure on rotary shafts.
Friction estimates
Break-out friction exceeds running friction, particularly after standing under pressure. Friction depends on squeeze, hardness, fluid pressure and surface. Use supplier data, or measure on a test rig, when torque or breakout force is critical.
Compound choice
For dynamic duty choose compounds with low friction and good wear: filled NBR or HNBR, or PTFE-filled elastomers for lower friction, and avoid heavily plasticised stocks that shed plasticiser. Specify dynamic properties and run a lab simulation before volume buying.
