IDB-ORG-024
Seals · elastomers · gland design · AS568 / ISO 3601
O-ring selection guide
Choosing the elastomer, hardness, size and groove for a reliable O-ring seal — with the squeeze, gland-fill, extrusion and failure-mode rules that decide whether it lasts.
Abstract
An O-ring seals by being squeezed inside a groove (the gland) so it presses against both sealing faces; system pressure then energises it harder against the downstream wall. Get four things right and it lasts: the elastomer (chemical and temperature compatibility), the hardness (durometer), the size (AS568 inch or ISO 3601 metric), and the gland (squeeze, fill, stretch, finish, clearance).
Section 1 explains how the seal works. Section 2 is material and hardness selection. Section 3 is sizing and standards. Section 4 is gland design with the squeeze / fill / finish tables. Section 5 covers pressure, extrusion and back-up rings. Section 6 is failure modes and fixes. Section 7 is a step-by-step selection checklist.
1.How an O-ring seals
An O-ring is a circular elastomer torus installed in a groove and compressed between two surfaces. That initial compression — the squeeze — creates the seal at zero pressure. When system pressure is applied, it pushes the ring across the groove against the low-pressure wall and energises the contact: the higher the pressure, the harder it seals. This is why a correctly sized O-ring seals across a huge pressure range from one part.
1.1Key terms
1.2Static vs dynamic
- Staticno relative motion across the seal (face/flange seals, caps, fixed fittings). Higher squeeze is fine; the main risks are compression set and extrusion.
- Dynamica surface slides or rotates against the ring (pistons, rods, rotary). Squeeze is kept lower to limit friction, wear and heat, and surface finish and lubrication become critical.
2.Material selection
Pick the compound that survives every fluid it touches (including cleaning agents) across the full temperature range. This is the decision that most often determines seal life.
| Elastomer | Temp range (°C) | Good with | Avoid |
|---|---|---|---|
| NBR (nitrile / Buna-N) | −35 to +120 | Petroleum oils, fuels, hydraulic fluid, water, air; low cost — the default | Ozone & weather, ketones, esters, strong acids, brake fluid |
| HNBR (hydrogenated nitrile) | −35 to +150 | NBR media plus heat, abrasion, refrigerants; automotive, A/C | Aromatics, strong acids, ozone (better than NBR) |
| FKM (fluoroelastomer / Viton) | −20 to +205 | Fuels, oils, many solvents and chemicals, high temp; broad resistance | Low temp, ketones, esters, amines, hot water / steam, brake fluid |
| EPDM | −50 to +150 | Water, steam, brake fluid (glycol), weather, ozone, polar fluids, outdoors | Petroleum oils, fuels, greases (swells badly) |
| VMQ (silicone) | −60 to +200 | Extreme hot/cold, dry air, food & medical, inert; static only | Dynamic use (poor wear), fuels, oils, steam |
| FVMQ (fluorosilicone) | −55 to +175 | Silicone temperature range plus fuel / oil resistance; aerospace | Steam, high-pressure dynamic service |
| CR (chloroprene / Neoprene) | −40 to +110 | Refrigerants (ammonia), weather, ozone, mild chemicals | Strong acids, aromatics, oxidisers |
| FFKM (perfluoro / Kalrez) | −15 to +260 | Near-universal chemical + high-temp resistance; semiconductor, aggressive media | Cost (very high); confirm low-temp grade |
| AU / EU (polyurethane) | −30 to +90 | Outstanding abrasion & extrusion resistance, high-pressure hydraulics | Hot water / steam (hydrolysis), high temp, ketones |
2.1Hardness (durometer, Shore A)
- **70 Athe default.** Best balance of sealing, friction and assembly. Use unless you have a reason not to.
- **90 Ahigh pressure / anti-extrusion / hard, precise mating faces.** Higher friction and assembly force; resists being forced into the clearance gap.
- **50 Alow closure force, rough or compliant surfaces, vacuum.** Seals easily but takes more compression set and extrudes sooner.
Always confirm the specific compound against the fluid supplier's chemical-compatibility chart — "FKM" covers families with different low-temperature and chemical grades.
3.Size and standards
Specify a standard size wherever possible — non-standard tooling is slow and expensive.
- AS568 (SAE, inch-based) is the de-facto global standard. Sizes are given by a dash number whose series sets the cross-section: −0xx ≈ CS 1.02 mm, −1xx ≈ 1.78 mm, −2xx ≈ 2.62 mm, −3xx ≈ 3.53 mm, −4xx ≈ 5.33 mm.
- ISO 3601-1 (metric) specifies by ID × CS, e.g.
20.0 × 2.5. Common cross-sections: 1.0, 1.5, 1.8, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0, 6.0, 7.0 mm. - Tolerance & quality: ISO 3601-1 gives dimensional class A/B; ISO 3601-3 gives surface-quality grades (flash, defects) N (standard) or S (critical). State both for sealing-critical parts.
A complete callout names: compound + hardness + size + tolerance class + surface grade — e.g. "FKM 75 Shore A, 20.0 × 2.5 ISO 3601-1 Class A, grade N."
4.Gland (groove) design
The groove turns the chosen ring into a working seal. Design it for the right squeeze and fill, and a clearance small enough to prevent extrusion.
| Parameter | Static radial | Static face (axial) | Dynamic reciprocating |
|---|---|---|---|
| Squeeze | 15–30% (≈0.25–0.50 mm) | 18–30% | 8–16% |
| Gland fill (volume) | 60–85% | 60–85% | 60–80% |
| Stretch at ID | 1–5% (≤5%) | n/a (axial) | ≤2% (never compress) |
| Sealing-face finish Ra | ≤0.8 µm | ≤0.8 µm | 0.10–0.40 µm |
| Groove side / bottom Ra | ≤1.6 µm | ≤1.6 µm | ≤0.8 µm |
| Groove width | 1.2–1.5 × CS | 1.2–1.5 × CS | ≈1.4 × CS |
4.1Rules that matter
- Never exceed ~90% gland fill at the hottest, most-swollen condition. The groove must have room for thermal expansion and fluid swell, or the ring over-stresses and extrudes. This is why the groove is wider than the ring.
- Face (axial) seals: place the groove so pressure pushes the ring toward the groove wall that backs it uptoward the OD for internal pressure, toward the ID for external pressure or vacuum.
- Dynamic: keep squeeze low (8–16%) to limit friction and heat, and the sliding surface very smooth (Ra ≤ 0.4 µm) with no axial scratches. A plunge-ground or honed counterface seals best.
- Stretch reduces effective CS (and therefore squeeze) and raises ID tensionkeep it under 5% and re-check squeeze after stretch.
5.Pressure, extrusion and back-up rings
Extrusion (the ring shaved or nibbled on its low-pressure edge) is the classic high-pressure failure. It gets worse with higher pressure, larger clearance gap, softer durometer and higher temperature — all of which let the ring creep into the gap.
Three levers, in order of preference: tighten the clearance gap, raise the hardness (90 A), then add a back-up ring (PTFE anti-extrusion ring) on the low-pressure side — both sides if pressure alternates.
| System pressure | 70 Shore A | 90 Shore A |
|---|---|---|
| ≤ 5 MPa (~700 psi) | diametral gap ≤ ~0.15 mm | gap ≤ ~0.25 mm |
| 5–10 MPa | gap ≤ ~0.08 mm, or add back-up | gap ≤ ~0.13 mm |
| > 10 MPa (~1500 psi) | back-up ring | back-up ring |
Values are indicative diametral clearances — always confirm against the seal supplier's pressure-vs-gap chart for the actual compound and temperature.
6.Failure modes and fixes
| Mode | Looks like | Cause | Fix |
|---|---|---|---|
| Extrusion / nibbling | shaved or chewed low-pressure edge | gap too large, pressure too high, ring too soft | back-up ring, 90 A compound, reduce clearance |
| Compression set | flattened, won't spring back | over-squeeze, heat, wrong compound, long static life | low-set compound (FKM/HNBR), reduce squeeze, lower temp |
| Spiral failure | clean ~45° cuts (dynamic) | ring slides and rolls in the groove | better finish & lubrication, correct squeeze, harder ring |
| Thermal / chemical attack | hardening, cracking, swelling or softening | over temperature or wrong media | reselect compound for media + temp (Section 2) |
| Explosive decompression | blisters / splits after rapid gas vent | absorbed gas expands faster than it escapes | ED-resistant compound, higher hardness, slow depressurisation |
| Abrasion (dynamic) | worn flat, scored surface | rough counterface, no lubrication, debris | finer finish (Ra ≤ 0.4 µm), lubrication, filtration, harder ring |
| Installation damage | nicks, cuts, twist marks | sharp edges, no lead-in, dry assembly | 15–20° lead-in chamfers, deburr, lubricate, protect threads/splines |
7.Selection checklist
- Medialist every fluid and gas the seal contacts, including cleaning and sterilisation agents.
- Temperaturerecord the minimum and maximum, including short transient spikes.
- Pressuresteady value, peak, and whether it cycles or alternates direction.
- Motionstatic, or dynamic (reciprocating / rotary)? This sets the squeeze band.
- Materialchoose the compound that survives media and temperature (Section 2), then verify on the supplier chart.
- Hardness70 A by default; 90 A for high pressure or extrusion risk; 50 A for low force or rough faces.
- Sizepick a standard AS568 dash or ISO 3601 ID × CS; favour common cross-sections for availability.
- Glandset squeeze, fill (≤ 90% hot), stretch, surface finish and clearance per Section 4.
- Extrusioncheck gap against pressure and hardness; add back-up rings if needed (Section 5).
- Assemblyprovide 15–20° lead-in chamfers, deburr all edges, and use a lubricant compatible with both the elastomer and the media.
- Specify fully on the drawingcompound, hardness, size, tolerance class, surface grade and any back-up ring.