O-Ring Basics
Straight answers to the questions every seal conversation starts with: what an O-Ring is, how it seals, pressure and temperature limits, and why glands matter.
Straight answers to the questions every seal conversation starts with, from Darrell Hansen and the crew at The O-Ring Store, backed by our factory partners at Freudenberg-NOK Sealing Technologies and others.
New to specifying seals? Start with the homepage Find the right O-Ring three-step path (size, chemistry, physical properties), then come back here when you want the fundamentals.
What is an O-Ring?
A doughnut-shaped ring (a torus) with a round cross-section, usually molded from rubber. Special jobs also get PTFE and other plastics, but the rubber O-Ring is the most widely used seal in the world because it is simple, compact, and it works.
What is an O-Ring seal, exactly?
Two parts working together: the rubber ring and the gland, the groove machined into the harder part that contains and supports it. The ring alone is not the seal; the ring squeezed inside a properly sized gland is.
How does an O-Ring actually work?
Treat the rubber as an incompressible, very thick fluid with high surface tension. Squeezed in its groove, it flows into every microscopic scratch and closes the leak path. System pressure then pushes it tighter against the gland, so within its limits, more pressure seals better, not worse.
Why use an O-Ring instead of a flat gasket?
No critical bolt torque, because the joint closes metal to metal. It seals across wide swings of pressure, temperature, and tolerance, needs almost no room, installs by hand with no special tools, can often be reused, and costs less than any other seal this dependable.
How much pressure can an O-Ring handle?
Static seals hold to 5,000 psi and beyond with the right gland. The limit is extrusion: pressure forcing rubber into the clearance gap. A standard 70-durometer ring starts extruding around 1,500 psi with a typical gap; a harder compound, a tighter gap, or a back-up ring buys the rest.
Can O-Rings seal moving parts?
Yes. Reciprocating pistons and rods are everyday duty, and rotary shafts work at low surface speeds. Keep the surfaces polished, keep them lubricated, and never run a pressurized ring across a port edge; a trace of weep in dynamic service is normal and healthy.
What if pressure comes from both sides?
One O-Ring seals pressure from either direction, alternating included. For severe duty, arrange the design so each seal sees pressure one way. And know that seals in series do not share load: the first seal takes all of it, the second is a backup.
What temperatures can O-Rings take?
It is set by the compound. Everyday nitrile runs -40°F to +250°F, silicone -75°F to +400°F, and FFKM reaches +617°F. Cold stiffening is temporary and harmless once warmed; overheating hardens rubber permanently.
How do O-Rings fail?
Gradually, and that is a feature: leakage creeps up and warns you before letdown. The usual causes are extrusion at pressure, abrasion in dynamic service, and chemical attack from the wrong fluid, and each one leaves a signature you can read.
