Hydraulic Seal Materials Guide: Viton, Buna-N, EPDM and PTFE

A hydraulic seal fails for one of two reasons: it was the wrong material for the fluid, or the wrong material for the temperature. Everything else is a distant third. This guide covers the elastomers used in hydraulic pump and motor rebuilds, what each one survives, what destroys it, and how each manufacturer codes the material into its own part numbers.

If you only read one thing, read the next table.

Start here: match the seal to the fluid

Buna-N, also called NBR or Nitrile

Buna-N is the default. Unless a pump was ordered with a special seal option, the seals that came out of it are Buna-N. It handles petroleum oil, water glycol and water emulsions well, it stays flexible in the cold better than any other common hydraulic elastomer, and it is the cheapest of the four.

What kills it: heat above roughly 100 C, ozone and weathering on the exposed face of a shaft seal, and phosphate ester fluid, which it has essentially no resistance to at all.

You will see it written as Buna-N, NBR, or Nitrile. These are three names for the same rubber. We tag both Buna-N and Nitrile on our products so that a search for either one finds the part.

Viton, also called FKM or fluorocarbon

Viton is a DuPont trade name. FKM is the generic designation and fluorocarbon is the descriptive one. Again, three names for one family of material.

Viton is the high temperature and chemical resistance option. It is the correct seal for hot hydraulic oil, for industrial fire resistant phosphate ester, and for systems that see aggressive chemistry. It is also considerably more expensive, which is why it is almost never the factory default.

What people get wrong about Viton is that it is not a straight upgrade. Viton is worse than Buna-N in three common situations: cold starts, water glycol fluid, and hot water or steam. A pump running HFC water glycol wants Buna-N, and fitting Viton to it is a downgrade, not an improvement.

EPDM, also called EPR or ethylene propylene

EPDM is the phosphate ester material. On Eastman's own Skydrol compatibility chart, ethylene propylene is one of only two elastomer families rated Excellent. For a machine running Skydrol Type IV or Type V, EPDM is not a preference, it is the specification. Peroxide cured EPDM is the correct grade, because sulfur cured EPDM leaches plasticizer into the fluid.

EPDM is also the right material for water, steam and glycol ether brake fluid.

The practical problem with EPDM is availability. Most hydraulic pump manufacturers never cataloged an EPDM kit at all. Of the brands we handle, Denison and Oilgear are the two that published factory EPDM and EPR options. For everything else, an EPDM set has to be built to the dimensions on the pump's own bill of material, which we can do from the factory drawings.

PTFE, also called Teflon

PTFE is chemically close to inert. It tolerates essentially every hydraulic fluid and the widest temperature span of anything on this page. It is not an elastomer, though, so it does not behave like one. It has no rubber-like memory, which is why it turns up as shaft seal lips, back-up rings and energized seals rather than as plain O-rings.

Oilgear is the one brand in our catalog that published a Teflon shaft seal as a standard cataloged option, under the -3B suffix. It is a legitimate answer for phosphate ester as well as an extreme temperature answer.

Temperature limits, and the number that actually applies

Every seal material has a published range, and the installed limit in a real pump is always narrower. Both numbers are worth knowing, because the material range tells you what is possible and the pump maker's number tells you what is warranted.

Chemical compatibility at a glance

Ratings below follow the four step scale used on Eastman's Skydrol material compatibility chart: Excellent, Good, Poor and No. The Skydrol row is taken directly from that chart. The remaining rows follow standard elastomer practice for hydraulic service.

Two more materials appear on the Skydrol chart and are worth knowing. Perfluoroelastomer, sold as Kalrez and generically as FFKM, rates Excellent on phosphate ester and on almost everything else, at a price that only makes sense for critical seals. Polyurethane, common in cylinder seals, rates No on phosphate ester and should never be carried over into a Skydrol conversion.

How each brand codes the seal material

Every manufacturer buries the seal material somewhere different. Some put it in the pump model code, some only in the kit part number, and some never offered a choice at all. This is the short version. The full decoder for each brand, with the part number suffixes and links to the parts, is on the seal materials by brand page.

How to tell what is in your pump right now

In order of reliability:

1. Read the model code on the nameplate. On a Vickers, Denison or Rexroth pump the material is in the code, and it is the only method that is conclusive. Our decoders will translate it.
2. Read the old seal. Buna-N is black. Viton is very often brown or green, and it feels slightly heavier and more slippery than nitrile. EPDM is black and gives no visual clue at all, which is precisely why the nameplate matters.
3. Ask what fluid is in the tank. If the answer is a fire resistant fluid, the seals are not standard and the previous rebuild either got that right or is the reason you are reading this page.

Frequently asked questions

Is Viton better than Buna-N?

Not automatically. Viton is better at heat, at phosphate ester and at chemical attack. Buna-N is better in the cold, better on water glycol and water emulsion fluids, and better on hot water or steam. On ordinary petroleum oil at normal temperature both work, and Buna-N is the cheaper of the two. Choose by fluid and temperature, not by price.

What is the difference between Viton and FKM, or Buna-N and Nitrile?

There is no difference. Viton is a trade name and FKM is the generic designation for the same fluorocarbon family. Buna-N, NBR and Nitrile are three names for the same nitrile rubber, and EPDM and EPR both mean ethylene propylene. We tag both names on our listings so either term finds the part.

What seal material do I need for Skydrol?

EPDM, or PTFE. Buna-N is rated No and Viton is rated Poor on Eastman's own chart. The common error is to order the Viton kit because it is the only special option on the price list. First confirm which phosphate ester you have, because aviation Skydrol and industrial fire resistant fluid such as Fyrquel or Reolube are different answers. On industrial HFD-R, Viton is correct.

What material should I use with water glycol?

Buna-N. This surprises people who assume the exotic material is always safer. Viton performs poorly in water glycol and in hot water. Bosch Rexroth makes the same call on the A10VSO, where the NBR build is the one specified for HFA, HFB and HFC fluids.

Can I fit a Viton kit to a pump that had Buna-N seals?

On petroleum oil, yes, as long as you replace the entire set and the machine does not cold start below -25 C. The dimensions are the same, only the compound changes. Do not do it if the system runs water glycol, where Viton is the weaker material.

Do you supply EPDM seal kits?

Yes, though how depends on the brand. Denison and Oilgear published factory EPDM and EPR options, so those carry real part numbers we can quote. Most other manufacturers, Vickers included, never cataloged one. For those we build the set to the dimensions on the pump's own bill of material. Send us the full model code from the nameplate and tell us the fluid, and we will confirm which route applies.

How long do hydraulic seals last?

Correctly specified and installed, a shaft seal in clean oil at moderate temperature commonly runs for years. The failures we see are rarely wear. They are the wrong material for the fluid, a shaft worn at the seal track, contamination cutting the lip, or a blocked case drain raising pressure behind a seal that was never meant to hold it. If a seal failed early, find out which of those it was before fitting another.

Related guides and catalogs

• Seal materials by brand
• Viton and FKM seal parts
• Hydraulic seal kits
• Denison model code decoder
• Char-Lynn seal kit finder
• Vickers seal kits
• What a rebuild costs