Hydraulic Seal Material Compatibility: Match the Seal to the Fluid and Temperature
Technical review: reviewed against the cited manufacturer guidance and practical hydraulic-seal/CNC fabrication experience. How HydraResolve reviews technical content.
“What material is strongest?” is usually the wrong first question. A seal material has to survive the actual fluid, temperature, pressure, motion and hardware. A material that performs well in mineral hydraulic oil can behave very differently in a water-based, fire-resistant or unusual synthetic fluid.
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Compatibility is more than “oil resistant”
SKF’s hydraulic-seal guidance publishes compatibility tables because temperature and fluid chemistry can change swelling, hardness and service life. Even within a material family, compounds are formulated differently. “NBR,” “PU” or “FKM” by itself is not a complete specification.
Why PU, NBR, FKM and PTFE are not interchangeable
Polyurethane is widely used where wear and extrusion resistance are important. NBR is a flexible elastomer common in mineral-oil hydraulic applications. FKM is often considered when higher temperature or particular fluid resistance is needed. PTFE-based seal systems can offer low friction and broad chemical resistance but typically use different seal constructions and installation procedures.
Those are screening characteristics, not permission to substitute one material for another without checking the exact compound and profile.
A safer selection order
- Identify the exact fluid and temperature range.
- Check the seal manufacturer’s compatibility data for the specific compound.
- Confirm the profile can handle pressure, speed, extrusion gap and direction of pressure.
- Check whether the application requires low friction, abrasive-wear resistance or contamination tolerance.
- Verify installation requirements and mating-surface condition.
What chemical incompatibility can look like
Depending on the material and fluid, incompatibility can show up as swelling, shrinkage, softening, hardening or loss of mechanical strength. If a removed seal has changed dimensions or texture significantly, record the fluid, temperature and service time before choosing the replacement.
Use the HydraResolve Material Selector as a screening tool, then verify its recommendation against the exact compound data. For a direct PU/NBR comparison, see PU vs NBR Hydraulic Seals.
To connect this inspection with the next repair decision, see Skydrol & Phosphate-Ester Seal Compatibility: Guide and Checker.
A five-gate material screening workflow
| Gate | Question | Evidence to record |
|---|---|---|
| 1. Fluid | What exact fluid or base chemistry is in the system? | Product name, fluid family, additives or recent fluid change |
| 2. Temperature | What are normal and peak temperatures at the seal? | Measured/known operating range, hot-only or cold-start symptoms |
| 3. Motion | Is the seal static or dynamic, and how fast does it move? | Rod/piston location, stroke pattern, speed and lubrication condition |
| 4. Pressure/support | What pressure and extrusion support does the gland provide? | Working/spike pressure, clearance, backup rings and guide condition |
| 5. Environment | What else attacks the seal? | Water, contamination, dust, cleaning chemicals and surface condition |
Only after all five gates are known should PU, NBR, FKM, PTFE or another family be treated as a serious candidate. Those names describe material families, not one universal compound; supplier-specific formulation and application data still decide the final choice.
Technical references
These sources support the technical principles above. Always follow the service manual and seal data for the exact machine.
Turn the finding into a repair decision
Use the tools to organize the symptom, dimensions and operating conditions before ordering a seal or committing to a rebuild.
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