Seal materials
A kit is the right size or it is not, and that part is arithmetic. The compound is the other half, and getting it wrong costs you the same strip-down a second time. Six materials cover nearly everything that runs on a jobsite.
If you know the fluid but not the material, look the fluid up instead — nearly a thousand of them are rated against fourteen compounds.
Nitrile
Also sold as Buna-N, NBR
- Temperature
- -40°F to +240°F
- Used for
- The default. O-rings, back-up rings, wipers, most kit seals.
Handles
- Petroleum hydraulic oil, the fluid in most machines
- Diesel and petrol
- Water and water-glycol fire-resistant fluid
- Holds its shape under compression, so it seals a static joint well
Fails in
- Phosphate ester fire-resistant fluid — Skydrol will destroy it
- Ozone and sunlight, so it wants covered storage
- Sustained heat above about 240°F, where it hardens and cracks
Cheapest of the common compounds and right far more often than not. If nobody has told you the machine runs something unusual, it is nitrile.
Polyurethane
Also sold as AU, EU
- Temperature
- -65°F to +200°F, briefly to +300°F
- Used for
- Rod and piston seals, wipers, back-up rings. Anything that moves.
Handles
- Abrasion and extrusion resistance far beyond nitrile
- High tensile strength, so it survives a high-pressure rod
- Cut resistance, which matters on a scored rod
- Oil swell, ozone and oxidation
Fails in
- Hot water and steam — it hydrolyses and goes soft
- Phosphate ester fluids
- Sustained heat above about 200°F
The reason a urethane rod seal outlasts a nitrile one on a working machine. Most loaded U-cups are 95 durometer urethane.
Fluorocarbon
Also sold as Viton, FKM
- Temperature
- -20°F to +400°F
- Used for
- Hot oil, aggressive chemicals, biodiesel.
Handles
- High temperature, well past where nitrile gives up
- Chemical resistance across most of the chart
- Alcohol and aromatic fuels
- Ultraviolet light and ozone
Fails in
- Low temperature — it stiffens and stops sealing in the cold
- Aircraft hydraulic fluids
- Hot water and steam
Several times the price of nitrile. Worth it when the oil runs hot; a waste when it does not.
PTFE
Also sold as Teflon, usually glass- or bronze-filled
- Temperature
- to +500°F, limited by the energiser
- Used for
- Piston seal caps and rings, back-up rings, wear bands.
Handles
- Chemically inert against nearly everything
- Lowest running friction of any seal material
- Very high and very low temperature
- Non-stick, so it does not grab on a slow stroke
Fails in
- No memory of its own — it needs an O-ring or rubber energiser behind it
- Cold flow under sustained load
Rarely used alone. The rating on a PTFE piston seal is usually set by the nitrile energiser behind it, not the PTFE.
Ethylene propylene
Also sold as EPDM
- Temperature
- -20°F to +300°F
- Used for
- Phosphate ester systems, brake systems, steam.
Handles
- Phosphate ester fluids such as Skydrol — the one thing nitrile cannot do
- Steam, hot water, acetone
- Dilute acids and bases
- Brake fluid, in the right compound
Fails in
- Petroleum oil of any kind — it swells badly
The exception that catches people out. EPDM in mineral oil fails as fast as nitrile in Skydrol, and the two mistakes look identical from the outside.
Silicone
Also sold as VMQ
- Temperature
- -75°F to +450°F
- Used for
- Static seals over a wide temperature range.
Handles
- The widest working temperature range of any of these
- Dry heat, ultraviolet light and ozone
Fails in
- Anything dynamic — poor abrasion resistance and high friction
- Most petroleum fluids at the top of its range
Not a cylinder seal. It appears in gaskets and static joints where the temperature swing is the problem.
Durometer
Durometer is hardness on the Shore A scale, and the number is printed on most of what we sell: 70 and 90 for nitrile O-rings, 95 for urethane U-cups. Lower is softer. A softer seal conforms to a worn surface and seals at low pressure; a harder one resists being pushed into the extrusion gap at high pressure. Substituting one for the other changes how the kit behaves even when every dimension matches.
Next: what the terms mean.