Seal materials and fluids: choosing what the seal is made of
Rubber, reinforced rubber, polyurethane and PTFE each do something different. Fire-resistant hydraulic fluids need extra care. Use the table as a first check, then confirm with the seal maker.
Why material matters
The same seal profile can last months or years depending on its material. Heat, abrasion, chemicals and the fluid being sealed decide which one to use. A wrong choice can fail in days, even when the design is right.
Material groups at a glance
| Group | Strengths | Watch out for |
|---|---|---|
| Elastomers (rubbers) | Flexible lips, wide choice of grades, can be reinforced with fabric or metal | Each grade has its own temperature and fluid limits |
| Reinforced rubber | A strong body with a soft flexible lip | Check that the reinforcement suits the fluid |
| Polyurethane | High strength, very good abrasion and wear resistance, low compression set | Limits in hot water or some fluids |
| PTFE compounds and engineering plastics | Resist chemicals, high temperature and wear; low friction | Harder materials need good shaft finish and alignment |
Lower friction at the lip
Some lip seals have a low-friction band bonded to the sealing lip. It reduces friction and heat and can raise seal life. It is a design feature to ask about for high-speed or dry-running duty.
Fire-resistant hydraulic fluids: first-check guide
| Fluid category | Service temperature | Materials usually considered |
|---|---|---|
| HFA-E and HFA-S (water-based) | +5 to 60 °C | Specially formulated polyurethane, NBR, HNBR, specially formulated or properly formulated FPM; EPDM for some. Individual tests needed for HFA-S |
| HFB (water-in-oil) | +5 to 60 °C | Specially formulated polyurethane, NBR, HNBR, specially formulated FPM |
| HFC (water-glycol) | -20 to 60 °C | NBR, HNBR, EPDM, MVQ |
| HFD-R, HFD-S, HFD-T (synthetic) | Above 100 °C | FPM, EPDM or MVQ depending on the fluid |
| HFD-U (synthetic esters) | Above 80 °C, up to 100 °C | Specially formulated polyurethane or FPM, with FPM plasticiser-free |
| Silicone oils | Above 100 °C | Specially formulated polyurethane up to 100 °C, FPM (plasticiser-free) |
How to use the table
It is a first screen, not a release. Fluid brands and additives change compatibility, so confirm with the seal supplier and the fluid supplier before fitting, and test a sample when the fluid is new.
Quick material checklist
- Name the fluid, its temperature range and any water content
- List abrasives, chemicals and wash-down
- Check the shaft or rod surface and speed
- Ask whether a split or weldable design is needed
- Record the material on the maintenance record so the next change matches