Most seal failures are not sealing failures — they are material failures. The groove was right, the squeeze was right, but the elastomer met a fluid or a temperature it could not survive, and it swelled, hardened, or dissolved. Picking the right compound is the highest-leverage decision in any sealed joint, and it usually comes down to two questions: what fluid, and how hot.
The six elastomers you will actually use
There are dozens of seal compounds, but a handful cover the overwhelming majority of real designs:
| Compound | Loves | Hates | Temp range |
|---|---|---|---|
| NBR (Buna-N) | Petroleum oils, fuels, water, air | Ozone, brake fluid, ketones, weather | −40 to 120 °C |
| FKM (Viton) | Hot oils, fuels, aggressive chemicals | Ketones, esters, brake fluid, low temp | −20 to 205 °C |
| EPDM | Glycol brake fluid, hot water, steam, ozone | Petroleum oils & fuels (swells badly) | −50 to 150 °C |
| HNBR | Refrigerants, hot oil, better ozone/wear than NBR | Strong acids, ketones | −40 to 150 °C |
| Silicone (VMQ) | Wide static temp, dry heat, food/medical | Abrasion, dynamic use, fuels | −60 to 200 °C |
| PTFE | Almost everything chemically; very low friction | Elastic memory (needs an energizer) | −200 to 260 °C |
The classic mistakes
1. NBR in brake fluid
Glycol-based DOT 3/4/5.1 brake fluid destroys nitrile. This is the single most common seal error, and the fix is always EPDM — which, confusingly, is the one compound you must keep away from petroleum oil. Get the fluid family wrong in either direction and the seal swells shut or shrinks open within days.
2. FKM in the cold
FKM is the go-to for heat and chemicals, but standard grades stiffen badly below about −20 °C and lose their seal on a cold start. If you need chemical resistance and low temperature, reach for a low-temperature FKM grade or a different chemistry — do not assume “Viton handles everything.”
3. Silicone in a dynamic seal
Silicone tolerates a huge temperature range, which tempts people to use it everywhere. But it tears and abrades easily, so it belongs in static seals — face gaskets, static O-rings — not on a reciprocating rod. Put it on a moving surface and it shreds.
Compatibility is necessary, not sufficient
Choosing a chemically compatible compound gets you a seal that survives the fluid. It still has to be dimensioned correctly. Two failure modes hide in the geometry:
- Too little squeeze and it leaks; too much and it takes a compression set and never recovers.
- Wrong gland volume — the groove must give the swollen, thermally expanded seal somewhere to go, typically a 60–85% fill.
Screen the compound against your fluid with the Seal Compatibility checker, then size the hardware: the O-Ring Groove calculator for static and simple dynamic O-rings, or the Seal Groove calculator for profile seals. If you are working an actual O-ring you can measure but not identify, the O-Ring Identifier reverse-looks-up the size.
Dynamic and hydraulic seals
Reciprocating rod and piston seals live a harder life than static O-rings — extrusion, wear, and pressure spikes all come into play, and the seal profile matters as much as the compound. For a hydraulic cylinder, let the Hydraulic Seal Selector match the profile (rod, piston, wiper, buffer) to the pressure and speed, and use the Lip Seal / Rotary Shaft Seal lookup for rotating shafts. A quality kit from Parker, Trelleborg, or SKF is worth the premium — the seal is a few dollars; the rebuild to replace it is not.
A three-step selection method
- Fluid first — eliminate every incompatible compound with the compatibility checker. This usually leaves two or three candidates.
- Temperature second — cut anything that cannot cover your worst-case high and low.
- Duty third — static vs. dynamic, pressure, and cost decide among the survivors.
Follow that order and you rarely get it wrong. Skip step one and you get a seal that looked perfect on paper and failed in a week.
Reading a compatibility rating
Chemical-resistance charts usually score a compound/fluid pair on a 1–4 scale (or A–D): 1 means negligible effect, 4 means not recommended. What matters is what “effect” means physically — it is almost always volume swell and hardness change. A little swell (under ~10%) can actually help a static seal; a lot (over ~25%) turns the seal to mush and lets it extrude. Shrinkage is worse than swell, because a shrinking seal simply loses contact and leaks. When a chart gives you a “2/fair,” it is telling you to test at your real temperature before committing.
Temperature cuts both ways
Every compound has a glass-transition temperature below which it stops behaving like rubber and starts behaving like plastic — it stiffens and loses its seal. That is the real reason standard FKM struggles in the cold. But heat is the more common killer: for every 10 °C above its rating, an elastomer’s useful life roughly halves, and it takes a permanent compression set that never recovers on cool-down. This is why the selection method insists on the worst-case high and low, not a comfortable midpoint.
Back-up rings and extrusion
At high pressure, even a compatible, correctly sized O-ring can extrude into the clearance gap and nibble away until it fails. The fix is a back-up ring — a stiffer anti-extrusion ring on the low-pressure side of the gland. Rule of thumb: consider back-ups above roughly 70 bar (1,000 psi) for dynamic seals, sooner if the clearance gap is generous. The O-Ring Groove calculator accounts for the extra axial space a back-up ring needs.
Check your fluid before you order
Run your fluid and temperature through the Seal Compatibility checker, then dimension the gland with the Seal Groove calculator — compound and geometry in two steps, no chemical-resistance chart hunting.
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