A seal of the correct size and profile will still fail early if the material does not suit the duty. Material selection is mostly a matter of asking the right questions about the application and then checking the answers against the supplier's data. This guide gives a qualitative overview; it is not a substitute for a compatibility check on the specific compound.
Start with four questions
What is the seal in contact with?
Identify the fluid as precisely as possible: mineral oil, synthetic lubricant, grease, fuel, water, steam, a process chemical or a mixture. Additives and cleaning agents count too. Consider what reaches the outside of the seal as well, such as wash-down fluids, ozone or sunlight.
What temperature does it see?
Note the continuous temperature, the peaks and how long they last, and the lowest start-up temperature. A dynamic seal usually runs hotter at the contact surface than the surrounding fluid, because of friction.
Is the seal static or dynamic?
A static seal mainly needs to keep its elasticity. A dynamic seal also needs wear resistance and acceptable friction, and it generates its own heat.
What pressure does it hold?
Higher pressure calls for a material that resists extrusion into the clearance gap, or for a design that supports the seal with back-up rings.
Common seal materials at a glance
| Material | General strengths | Points to check |
|---|---|---|
| NBR | Widely used general-purpose material for mineral oils and greases; economical and widely available | Heat, ozone and weathering; some fuels and chemicals |
| HNBR | Generally improves on NBR for heat, ozone and wear resistance | Cost; compatibility with the specific fluid |
| FKM | Generally better heat and chemical resistance than NBR | Low-temperature flexibility; certain chemicals; hot water and steam with some grades; cost |
| Silicone | Wide temperature range; stays flexible in the cold | Limited abrasion and tear resistance, so mostly used for static or lightly loaded duty |
| EPDM | Water-based media and outdoor weathering | Generally not suitable for mineral oils and greases |
| Polyurethane | Wear and extrusion resistance; widely used in hydraulic seals | Temperature; hot water and humid conditions with some grades |
| PTFE | Low friction and broad chemical resistance | Not elastic, so it needs an energizer or a special design; installation care |
All of these comparisons are general. Actual performance depends on the compound, the seal design and the application. There is more detail on our materials page.
The compound matters as much as the family
Names such as NBR and FKM describe families of materials, not single products. Within each family, suppliers offer compounds that differ in hardness, low-temperature behaviour, wear resistance and chemical resistance. Two seals described as the same material can therefore behave quite differently in service.
In practice this means a general compatibility chart is a starting point, the supplier's data for the specific compound is the reference, and any limit quoted for a family should be read as an indication rather than a rating. For critical or unusual duties, a trial in the actual fluid at the actual temperature is the most reliable check.
Common selection mistakes
- Choosing by temperature alone. A material that tolerates the heat may still be attacked by the fluid, and the reverse is equally true.
- Ignoring the cold end. A seal that copes when hot may be too stiff to seal at a cold start.
- Upgrading by default. A more expensive material is not automatically better. The choice between the two most common options is discussed in NBR vs FKM seals.
- Using a static material in a dynamic position. Silicone, for example, has a wide temperature range but limited abrasion resistance.
- Changing the fluid without checking the seals. A new lubricant or cleaning agent can affect seals that have run well for years.
- Blaming the material for a hardware problem. Shaft damage, misalignment and installation errors are not solved by a different compound.
Where heat is the main concern, see our guide to seal selection for high temperature applications.
Ask our sales engineers
We supply seals in a range of materials and help customers match the compound to the duty. Send the shaft, housing and width dimensions, the media, temperature, pressure and speed, and a drawing or sample if available, through the enquiry form. If a seal has already failed in service, tell us what it looked like when it came out; that is often the most useful clue.
