What is a Static O-Ring Seal?
A static o-ring seal is one where the o-ring doesn’t move during operation — it just sits in a groove and gets compressed between two mating parts. Think flanges, covers, manifolds, and housings. If you’re not familiar with standard o-ring sizing, start with AS568 O-Ring Sizes first.
These guidelines cover low-pressure static applications (0–200 psig). They’ll work for higher pressures too, but above ~1,500 psi you’ll need to think about backup rings and extrusion gaps — that’s a topic for another day.
Two Seal Types: Face vs. Radial
There are two ways to orient a static o-ring seal:
- Face seal — the o-ring is sandwiched between two flat surfaces (like a flange gasket)
- Radial seal — the o-ring is sandwiched between two concentric cylinders (like a piston in a bore)
The Three Design Factors
Every static o-ring groove design comes down to three things: stretch, compression, and fill.
1. O-Ring Stretch (Radial Seals Only)
For radial seals, the o-ring should be stretched 2–5% when installed on the shaft or in the bore. This keeps it seated in the groove and prevents it from rolling or walking.
Example: For a groove with a 10.000″ OD, pick an o-ring with an ID between:
- 9.524″ (10.000″ ÷ 1.05 = 5% stretch)
- 9.804″ (10.000″ ÷ 1.02 = 2% stretch)
For face seals, stretch doesn’t apply — just match the o-ring ID to the groove OD.
2. O-Ring Compression
Compression is how much you’re squeezing the o-ring’s cross-section. For static seals, aim for 25–35% compression. I typically design to ~30%.
Since the cross-section diameter is determined by the first digit of the AS568 dash number, here’s a quick reference:
| Series | Cross-Section (CS) | Groove Depth @ 30% | Groove Depth @ 25% | Groove Depth @ 35% |
|---|---|---|---|---|
| -0XX | 0.070″ | 0.049″ | 0.053″ | 0.046″ |
| -1XX | 0.103″ | 0.072″ | 0.077″ | 0.067″ |
| -2XX | 0.139″ | 0.097″ | 0.104″ | 0.090″ |
| -3XX | 0.210″ | 0.147″ | 0.158″ | 0.137″ |
| -4XX | 0.275″ | 0.193″ | 0.206″ | 0.179″ |
3. Percent Fill
When you compress an o-ring, the rubber has to go somewhere. The groove needs to be wide enough to give the compressed o-ring room to spread — but not so wide that it has no support.
The rule of thumb: the compressed o-ring should fill 60–85% of the groove volume. Below 60% and the o-ring can move around too much. Above 85% and you risk hydraulic lock (the o-ring has nowhere to go and the parts won’t seat properly).
Calculating fill:
- O-ring cross-section area = π × (CS/2)²
- Groove cross-section area = groove depth × groove width
- Percent fill = o-ring area ÷ groove area × 100
For a -2XX o-ring (CS = 0.139″) with a 0.097″ groove depth:
- O-ring area = π × 0.0695² = 0.01518 in²
- At 75% fill: groove width = 0.01518 / (0.097 × 0.75) = 0.209″
Quick Design Checklist
- Pick your o-ring size from the AS568 tables
- Set groove depth for 25–35% compression (see table above)
- Set groove width for 60–85% fill
- Verify stretch is 2–5% for radial seals
- Add surface finish — groove surfaces should be 32 µin Ra or better (see surface finish guide)
- Specify tolerances — groove depth ±0.002″, width ±0.005″ for most applications
Material Selection
For most industrial applications, Buna-N (Nitrile, NBR) in 70 durometer is the default. It’s cheap, widely available, and handles oils, fuels, and water up to ~250°F. For higher temps, look at Viton (FKM). For food/medical, look at silicone or EPDM.
Need help with a specific application? Get in touch — I’m happy to help.