Seals, O-Rings and Gaskets
O-ring squeeze and groove fill, extrusion and back-up rings, rod and shaft seals, gaskets and flange bolting.
What a seal does
A seal keeps a fluid in, or a contaminant out, between two surfaces. Static seals (a cover, a flange, a plug) do not move; dynamic seals (a piston rod, a rotating shaft, a valve spindle) have to seal a moving surface, with friction and wear. A seal works because it is deformed against the surfaces with a contact stress higher than the fluid pressure; if the pressure exceeds the contact stress the fluid leaks. Most seal failures come from the wrong material, a poorly machined groove or a damaged surface, not from the seal itself.
The O-ring
An O-ring is a rubber torus in a rectangular groove (a gland). The ring is squeezed by a fixed fraction of its cross-section, which preloads the seal, and system pressure pushes it further against the sealing face (a self-energizing seal). Key numbers: the squeeze (compression), the gland fill (the ring volume as a percentage of the groove volume: must stay below about 85 percent because rubber is incompressible and expands with heat and swell) and the stretch of the inside diameter on installation (1 to 5 percent: more shortens the life).
| Seal | Squeeze (typical) | Gland fill |
|---|---|---|
| Static, face seal | 25 to 30 % | ≤ 85 % |
| Static, radial (piston/rod) | 15 to 25 % | ≤ 85 % |
| Dynamic reciprocating | 8 to 16 % | ≤ 85 % |
| Dynamic rotary | 5 to 10 % | lip seal preferred |
Worked example: the rod gland
Worked example: a 3.53 mm O-ring on the 45 mm piston rod
- Squeeze = (cross-section − groove depth) ÷ cross-section = (3.53 − 2.80) ÷ 3.53 = 20.7 percent (the target for a dynamic seal is 8 to 16 percent; this groove is more suited to a static seal: for dynamic use deepen the groove to about 3.1 mm).
- Gland fill = ring area ÷ groove area = 9.79 ÷ (2.80 × 4.80) = 73 percent: below 85 percent, so there is room for thermal expansion.
- Stretch: an O-ring of 44.17 mm inside diameter on a 45 mm rod stretches by 1.9 percent (negative means compressed: for a rod seal in a bore the ring is installed in the housing, stretched over the groove). The nominal cross-section shrinks by about 0.3 times the stretch.
- Friction of a rod seal: contact area × contact stress × friction coefficient. With a contact width of 2.5 mm, a contact stress of pressure + squeeze (16 + 2 = 18 MPa) and a lubricated coefficient of 0.10: Ff = 0.10 × π × 45 × 2.5 × 18 = 636 N, 0.8 percent of the press force, and heat of 93 W at the rod speed of 0.146 m/s (Cylinders and Actuators). A seal squeezed too hard wears fast and heats the oil; too little leaks.
- Surface finish: Ra 0.1 to 0.4 µm for the dynamic surface (smoother than that does not hold an oil film; rougher cuts the seal), 1.6 µm for the groove bottom, and a lead-in chamfer of 15 to 20° so that the ring is not nicked when it is pushed on.
Extrusion, back-up rings and pressure
Under pressure the rubber flows into the clearance gap between the moving parts (extrusion) and is nibbled away. The permitted gap depends on the pressure and the hardness (a 90 Shore A O-ring tolerates about 0.1 mm at 160 bar; a 70 Shore ring only about 0.05 mm): at the 160 bar of the press the radial gap must be under 0.10 mm with a 90 Shore ring, and a hard back-up ring (PTFE or nylon) on the low-pressure side is the cure above about 100 bar or with larger gaps. Pressure reversal needs back-up rings on both sides. Large clearance at heat (the bore grows more than the rod) also opens the gap: design for the temperature extremes (see Temperature, Coatings and Fit Problems).
Rod, piston and rotary shaft seals
| Seal | How it works | Use |
|---|---|---|
| U-cup / rod seal (polyurethane, PTFE) | a lip pressed against the rod by system pressure and a spring or O-ring energizer | hydraulic cylinders, high pressure and speed |
| Wiper (scraper) | wipes dirt and water off the rod on the way in | always with a rod seal |
| Piston seal and guide rings | a cap seal with wear rings (bearing strip) carries side load | double-acting cylinders |
| Radial lip seal (oil seal) | a spring-loaded rubber lip on a rotating shaft | gearboxes, pumps, motors to about 0.5 bar and 10 to 20 m/s |
| Mechanical (face) seal | two flat lapped faces pressed together by a spring | pumps and agitators: low leakage at high speed and pressure |
| V-ring, labyrinth, magnetic fluid | non-contact or low-friction exclusion | dust, splash, low speed to very high speed |
| Packing / gland | compressed braided rings round a spindle | valves and slow pumps; adjustable |
Gaskets and flange bolting
A gasket fills the irregularities of two flat faces so that the bolts, not the faces, carry the load. The bolts must supply a seating stress to bed the gasket in and enough residual load to hold it against the pressure force trying to separate the flange (the hydrostatic end force p π D² ÷ 4). The stress must stay between a minimum for sealing and a maximum above which the gasket crushes. Soft fiber and rubber gaskets seal at 5 to 20 MPa, spiral-wound at 70 to 200 MPa, and metal ring joints much higher.
Worked example: a cover plate on the hydraulic tank
- A 200 mm cover, 0.6 MPa (6 bar) test pressure: end force = 0.6 × π ÷ 4 × 200² = 18.8 kN.
- Eight M10 class 8.8 bolts (stress area 58 mm², proof strength 580 MPa) preloaded to 70 percent of proof: 23.5 kN each, 188 kN in all, 10 times the end force: plenty of residual load.
- The gasket is 15 mm wide: contact area π × (200 + 15) × 15 = 10 132 mm², so the gasket stress is 188 384 ÷ 10 132 = 19 MPa. This is more than soft fiber gaskets can take (they crush above about 30 to 50 MPa): torque the bolts lower, widen the gasket, or use a stop (a metal face or a groove) that limits the compression. Tighten in a cross pattern in 3 steps, and re-torque after the first heat cycle.
- For flanges the standard (EN 1591, ASME VIII Appendix 2) calculation uses the gasket factors m and y, the bolt load for seating and operation, and flange rotation; do not rely on a rule of thumb for pressure vessels.
Materials
| Elastomer | Temperature range | Good with | Poor with |
|---|---|---|---|
| NBR (nitrile) | −30 to 100 °C | mineral oils, fuels, water | ozone, weathering, ketones, brake fluid |
| FKM (Viton) | −20 to 200 °C | hot oil, fuels, many chemicals | amines, steam, ketones; costly |
| EPDM | −50 to 150 °C | water, steam, brake fluid, alkalis | mineral oil (swells badly) |
| Silicone (VMQ) | −60 to 200 °C | wide temperature, food, medical | poor wear, tears easily; not for dynamic |
| PTFE (and filled) | −200 to 260 °C | almost all chemicals, low friction | creeps; needs an energizer or back-up ring |
| Polyurethane (AU/EU) | −35 to 90 °C | hydraulic rod and piston seals; wear | hot water, hydrolysis |
Choose the elastomer from the fluid first (compatibility tables), then the temperature, then pressure and wear. Hardness is usually 70 Shore A for general use and 90 for high pressure. The ring takes a compression set (permanent flattening) with time and heat, so a seal that is removed and reinstalled should be replaced. Safety: stored energy, hot fluid and hazardous liquids at leaks: depressurize before opening and wear eye protection.
Related
- Previous: Springs: Design, Stress and Fatigue. Next: Power Screws and Lead Screws.
- Fluids, Filtration and Reservoirs.
- Bolted and Welded Joints for bolt preload and torque.
- Machine Design: the whole guide.
FAQ
How much squeeze should an O-ring have?
About 15 to 25 percent for a static seal and 8 to 16 percent for a dynamic one. Too little leaks, too much wears and sets.
Why did my O-ring swell?
The fluid is incompatible with the elastomer (nitrile in brake fluid, EPDM in oil). Check a compatibility chart and choose another material.
Can I reuse an O-ring?
Usually not: it takes a compression set and may be nicked on removal. A new ring costs far less than a leak.
What does the number on an O-ring mean?
The AS568 dash size (e.g. 2-2xx) sets the inside diameter and cross-section; the metric series gives ID × cross-section in mm. Always specify the material and hardness too.
References
- Parker O-Ring Handbook (ORD 5700); Trelleborg Sealing Solutions, O-Ring Catalog; ISO 3601 (O-rings), AS568 and ISO 6149/6162.
- Flitney, R., Seals and Sealing Handbook, Elsevier; EN 1591-1 (flange connections: gasketed), ASME BPVC VIII Div. 1, Appendix 2.
- ISO 5597 and ISO 6195 (hydraulic cylinder seal housings and wipers), SKF and Freudenberg seal catalogs.
Last reviewed: October 2026. Squeeze, fill and friction values are typical; use the seal supplier’s design guide.