What the seal has to do
A bearing seal does two jobs that pull against each other. It must keep lubricant inside and keep water, dust and process media outside, which calls for firm contact with the inner ring. And it must do that without creating so much friction and heat that the bearing runs hot and the lip wears away. The elastomer, the lip geometry and the metal insert inside the seal are tuned together.
In a standard sealed deep groove ball bearing, the seal is a steel plate bonded to a rubber lip, snapped into a groove in the outer ring and touching or nearly touching the inner ring shoulder. The common suffixes are 2RS or RS for rubber seals and ZZ or Z for metal shields. The difference between those two is covered in 2RS vs ZZ bearings.
NBR: the everyday seal material
Nitrile butadiene rubber is the standard choice for bearing seals. It resists mineral oils and greases, water, and many common fluids, is easy to mould and bond to steel, and is economical. Widely published figures place its continuous working range at roughly minus 30 to plus 100 degrees C, with short peaks to around 120 degrees C. At the low end it stiffens; at the high end it hardens and loses elasticity.
NBR is the right material for most motor, fan, pump, appliance and general industrial bearings. It is weak against ozone, weathering, strong acids, ketones and some synthetic fluids. Outdoor equipment in strong sun for years may need a more resistant compound. Hardened rubber leaks, so the real service life of a seal is often set by temperature history, not by wear.
FKM: heat and chemical resistance
Fluoroelastomer, widely known as FKM or by trade names, handles much higher temperatures and many fluids that attack nitrile. Typical published continuous service is up to roughly 200 degrees C, with a low-temperature limit about minus 15 to minus 20 degrees C for standard grades. It resists hot oil, fuels, many acids and solvents, and ageing in air.
The trade-offs are cost and low-temperature flexibility. FKM is several times the price of NBR, and standard grades stiffen in cold. It also does not tolerate some media, such as hot water with steam under pressure or ketones, so the application medium must be checked, not assumed. Where you need both heat tolerance and a wide cold range, special compounds exist, and the plant should advise.
| Material | Typical continuous range | Strengths | Watch for |
|---|---|---|---|
| NBR (nitrile) | About -30 to +100 degrees C | Oil and grease resistance, low cost, easy to bond | Ozone, weathering, strong acids, high heat |
| HNBR | About -30 to +150 degrees C | Heat and abrasion resistance beyond NBR | Higher cost, some limited stock |
| FKM | About -15 to +200 degrees C | Hot oil, fuel, chemicals, ageing | Cost, cold stiffness, steam and ketones |
| Silicone | About -50 to +180 degrees C | Wide temperature range, food contact grades | Poor wear and oil resistance |
| PTFE | About -50 to +200 degrees C and higher | Chemical resistance, low friction | Stiff, needs careful lip design |
The ranges above are widely published reference values for the material families. Actual limits depend on the compound, the lubricant and the contact pressure, so confirm against the supplier datasheet.
Contact, low-friction and non-contact designs
- Contact seal. The lip rides on the inner ring. Best protection, highest friction, lowest limiting speed. This is the standard 2RS design.
- Low-friction seal. A lighter lip or lower contact pressure reduces torque and heat and raises the speed capability, at some cost in protection. Suffix names vary by plant, so confirm what the suffix means.
- Non-contact seal. A small gap leaves no wear and almost no added friction. It protects less against fine dust and water, but suits high speed and low torque. Metal shields are the classic non-contact type.
- Labyrinth and external seals. Multi-stage features in the housing add protection around the bearing, common in agricultural and conveyor housings.
The catalogue speed limits for sealed bearings are lower than for open versions of the same size, since the seal friction raises temperature. For high-speed applications, confirm the limiting speed for the sealed variant.
Choosing by application
- Estimate the operating temperature at the seal, which is usually higher than ambient because of bearing heat.
- List the fluids that can reach the seal: oils, coolants, cleaners, process chemicals, water.
- Define the contamination level, from clean indoor to mud, dust and washdown.
- Pick the speed and torque tolerance, which decide contact against low-friction or non-contact designs.
- Check the grease and seal together. Some greases swell or shrink certain elastomers.
For farm machinery, conveyors and other dusty sites, heavy-duty contact seals or double-lip designs on housed units are normal; for washdown, see 440C stainless bearings for the combined stainless and seal discussion. Housing-level seals and covers are described under bearing housing.
Why seals fail in service
Most seal failures trace back to a short list of causes. Heat is the first: rubber that runs above its rating hardens, cracks and stops sealing. Chemical attack is the second, when a cleaner, coolant or lubricant swells or degrades the compound. Misalignment and shaft runout wear the lip unevenly, and a dry start or contaminated lip wears it quickly. Finally, mounting damage is common: pressing on the seal side, or letting it catch a sharp shoulder, deforms the lip before the bearing runs a single hour.
Prevention is mostly specification and handling. Pick the compound for the real temperature and media, press the bearing by the ring that is being fitted and never through the seal, and keep sealed bearings clean and dry before use. A sealed bearing is not designed to be washed or soaked in solvent, since solvent can wash out the grease and swell the rubber. Where the seal shows early wear, ask the plant for the compound and the lip design, and compare against the operating conditions before changing suppliers.
Marking and ordering
Seal material is not shown in the standard designation, which only says the bearing is sealed. If NBR is the default, you do not need to name it, but if you require FKM or a particular colour, the order must say so. Colour is a loose clue only; NBR seals are often black, and FKM is often brown or green, but dyes vary, and a colour should never be treated as a material certificate.
State the suffix, material, temperature range and media in the quote request, and ask the plant to confirm compound and grease compatibility in writing. Leakage is easiest to resolve when seal material, grease and fit are all documented. Seal elastomers also age on the shelf, so check packaging and storage guidance for long-stock programs, and use the checks on the testing and inspection page for seal appearance and rotation torque.