A rotary shaft seal has two jobs: keep the lubricant in, and keep dirt, dust and moisture out. Most selection problems come from treating the seal as a part number to be matched rather than a component that has to suit the shaft, the housing and the operating conditions. This guide sets out the questions to work through.
Start with the application
Before looking at profiles or materials, collect the basic facts about the duty.
- Equipment and position: gearbox, pump, motor, axle, roller or other, and whether the shaft is horizontal or vertical.
- Dimensions: shaft diameter, housing bore diameter and the available seal width.
- Media: the oil, grease or other fluid being retained, including the grade if you know it.
- Temperature: the normal running temperature, any peaks, and the lowest start-up temperature.
- Pressure: whether the seal sees any pressure, vacuum or pressure pulses.
- Speed: shaft speed, and whether rotation is continuous, intermittent or reversing.
- Environment: dust, slurry, water spray, wash-down or outdoor exposure on the air side.
Measuring the three key dimensions
Rotary shaft seals are normally described by shaft diameter, housing bore diameter and width, in that order. Where possible, measure the shaft and the housing rather than the old seal. A used seal may have worn, swollen, shrunk or distorted during removal, so its dimensions can mislead. If the seal carries a size marking or a part number, note it exactly as it appears and use it as a cross-check.
Choose a suitable profile
The profile is the cross-sectional design of the seal. The main choices are:
- Single lip, or with a dust lip: an additional dust lip helps where the air side is dirty or wet.
- Rubber-covered or metal outside diameter: a rubber covering is more forgiving of the housing finish; a metal outside diameter gives a firm press fit in a well-finished bore.
- With or without a spring: a garter spring maintains lip load on the shaft for oil retention; springless designs are typically used for grease retention or as excluders.
- Special designs: pressure-capable lips, PTFE lips and other arrangements for duties beyond the scope of a standard seal.
The widely used TC profile is covered in our article on understanding TC oil seals. Profile designations vary between suppliers, so confirm any designation against a drawing or a sample.
Match the material to media and temperature
The lip material must be compatible with the fluid and stable at the temperature it actually sees at the sealing lip, which is usually higher than the bulk oil temperature because of friction. NBR is a widely used general-purpose choice for mineral oils and greases, and FKM generally offers better heat and chemical resistance. The actual limits depend on the compound, the seal design and the application, and should be confirmed against the supplier's data for the specific product. Our materials overview gives a starting point.
Check the shaft and housing
A correctly selected seal can still leak if the hardware is not suitable. Points to check:
- The shaft surface in the lip contact zone is smooth and free of scratches, rust and old wear grooves.
- The shaft has no machining lead (spiral marks) that could pump oil past the lip.
- The shaft is hard enough to resist grooving, especially in dirty conditions.
- Shaft runout and shaft-to-bore misalignment are within what the seal design can follow.
- The shaft end and the housing bore have lead-in chamfers, so the seal is not damaged during fitting.
The acceptable values depend on the seal type and the duty, so check them against the supplier's recommendations.
Consider speed, pressure and lubrication
What matters to the lip is surface speed, which depends on both the shaft diameter and the rotational speed. Higher surface speed means more frictional heat at the lip. Standard lip seals are intended for little or no pressure. Pressure presses the lip harder onto the shaft, which raises friction, heat and wear, so a pressurised duty calls for a design intended for it. The lip also needs lubrication: a seal that runs dry, even briefly at start-up, can be damaged quickly.
Share your application details
We supply a broad range of rotary seals and help customers match them to the duty. Send the shaft, housing and width dimensions together with the media, temperature, pressure and speed, and a drawing or sample if you have one, to our sales engineers through the enquiry form.
