An O-ring that extruded and leaked at 100 bar. The O-ring was correct for the groove size (3.55 mm cross-section, Ø50 bore). The pressure was 100 bar. The customer thought the O-ring material was wrong. It was NBR for hydraulic oil — correct. The issue: the groove clearance gap was too large for the pressure. At 100 bar, the O-ring material extruded through the gap. This is about O-ring groove design.
The compression ratio
The O-ring must be compressed in the groove to create a seal. The compression ratio is:
CR = (d – h) / d × 100%
Where d is the O-ring cross-section (mm) and h is the groove depth (mm). For a 3.55 mm O-ring with a 3.0 mm groove depth: CR = (3.55 – 3.0) / 3.55 = 15.5%. That’s correct for static seals. Too little compression (under 10%): the O-ring doesn’t seal. Too much (over 25%): the O-ring squashes out and the seal fails on pressure reversal.
| Application | Recommended compression |
|---|---|
| Static piston seal (hydraulic) | 10-15% |
| Static face seal | 15-25% |
| Dynamic (reciprocating) | 5-10% |
The extrusion gap
The O-ring sits in a groove with a clearance gap between the groove wall and the mating surface. Under pressure, the O-ring is pushed toward the low-pressure side. If the gap is too large, the O-ring material extrudes through the gap. At 100 bar, the maximum recommended gap for NBR is 0.1 mm. For FKM (viton), it’s 0.15 mm.
The failed seal had a gap of 0.3 mm. At 100 bar, the NBR O-ring extruded through the 0.3 mm gap. The material wore. After 50 cycles, a leak path formed.
What I changed
1. Reduced the clearance gap. I changed the piston clearance from 0.3 mm to 0.1 mm. The tolerance on the piston OD and bore ID was tightened. The O-ring no longer extrudes. The seal holds at 100 bar.
2. Added a back-up ring. For pressures over 70 bar, I add a PTFE back-up ring on the low-pressure side of the O-ring. The back-up ring blocks the extrusion gap. The O-ring can’t push through. For 100 bar and above, back-up rings are mandatory — even with a tight gap.
3. Checked the groove width. The groove must be wide enough to allow the O-ring to compress without binding. For a 3.55 mm O-ring, the groove width is 4.8 mm. If the groove is too narrow, the O-ring is trapped and extrudes sideways. If too wide, the O-ring rolls (dynamic seal). I always check the groove dimensions against the manufacturer’s datasheet.
The O-ring material selection
| Material | Temp range | Fluid resistance | Max pressure |
|---|---|---|---|
| NBR (nitrile) | -30 to 100°C | Mineral oil, water | 100 bar (with back-up) |
| FKM (viton) | -20 to 200°C | Oil, chemicals | 200 bar |
| EPDM | -40 to 150°C | Water, steam, glycol | 100 bar |
| Silicone | -60 to 200°C | Air, low pressure | 10 bar |
The gap I check: at 100 bar, clearance under 0.1 mm plus a PTFE back-up ring. The leaking O-ring wasn’t the wrong material — it was a 0.3 mm extrusion gap at 100 bar. Tighten the clearance or add a back-up ring. For dynamic seals, use 5-10% compression (not 15%).
