Fits, tapered-bore drive-up and seals: mounting causes of hot running
Wrong shaft or housing fit, a wrong drive-up on a tapered seat, and seal designs that rub or run dry.
Shaft and housing fits
The original fits can usually be found on the fan drawings, or measured on the bearing seats with proper tools. Compare them with the recommended fit for the bearing type, the load and the rotation of the ring. A too loose inner ring fit lets the ring turn on the shaft (creep, scoring on the bore). A too tight one removes internal clearance and the bearing runs hot. A too loose housing fit lets the outer ring turn in its seat and wear it.
Tapered bore: drive-up
For bearings on an adapter or withdrawal sleeve, or on a tapered shaft, the inner ring is pushed up the taper to take up the clearance. Too little drive-up leaves the ring loose on the sleeve so it rotates; too much preloads the bearing and heats it. Measure the drive-up either by the reduction in radial clearance with feeler gauges (the usual method for toroidal roller bearings) or, better, by a controlled axial drive-up with a hydraulic nut and gauge, which is the more accurate method.
Practice points
- Record the clearance before and after mounting and the final drive-up distance.
- Lock the nut with its washer tab only after the drive-up is complete.
- Check for signs of a ring turning on its seat when the bearing is removed: bright polished bore or scoring on the faces.
Seals that cause heat
Seals cause heat when the material, design or lubricant compatibility is wrong or when the counter-face is rough. A rough seal counter-face on the shaft is a good indication of a problem. A flattened lip or a gap in a contact seal, or an uneven gap in a non-contact one, points to misalignment or a worn shaft.
What to do
- Change the material or the type of seal (contact or non-contact) if the lip is cooking or hardening.
- Grease the seal lip when mounting, or lubricate it with oil in operation, to avoid running it dry.
- Where misalignment and thermal movement are too much for the seal, seek a different sealing or bearing arrangement rather than replacing the same seal again.