How to choose a material for hydraulic seals
A step-by-step decision framework for choosing an O-ring material for hydraulic applications — from working fluid to dynamics.
- Published:
- Author:
- Ing. Filip Meheš
- Category:
- Applications
When choosing a material for a hydraulic seal, the working fluid is the most important input. Temperature is second. Pressure and dynamics come third. Work the problem in that order — the right material against the wrong fluid will fail faster than any other engineering compromise.
Step 1: identify the fluid
Mineral hydraulic oil (HL, HLP, HV) → NBR is the default. Water or water-glycol (HFC) → EPDM. Phosphate esters (HFD-R, e.g. Skydrol) → EPDM (peroxide-cured), optionally FFKM or PTFE; never NBR or FKM. Biodiesel (B100/FAME) → HNBR; synthetic esters (HEES) → HNBR or FKM. NBR is only marginal in biodiesel — at higher FAME ratios choose HNBR. When in doubt, consult the compound manufacturer's chemical compatibility table.
Step 2: temperature and pressure
NBR handles -30 to +100 °C; above +100 °C consider HNBR (up to +150 °C) or FKM (up to +200 °C). At high pressure (over 200 bar) dynamic seals typically use NBR 90 Shore A or PTFE back-up rings to prevent extrusion. Static seals can run softer 70 Shore A even at higher pressures.
For dynamic seals (sealing moving parts like pistons and rods) friction coefficient and wear matter — NBR or polyurethane usually outperform FKM here. Static seals (no relative motion) tolerate a wider material range and are mostly driven by chemical and thermal compatibility alone. That's why NBR 70 Shore A is the right first call for standard mineral-oil hydraulics at temperatures up to 100 °C — proven, cheap and immediately available.