A deep-groove ball bearing failed after 2000 hours on a fan application. The bearing was 6208, mounted with a pressed-in housing fit. The inner race rotated on the shaft, and the outer race spun in the housing. Both races showed fretting corrosion. The fan was rated for 40,000 hours. The issue was not the bearing quality — it was the fit specification. This article examines how housing and shaft fits determine which race rotates and how that choice affects fatigue life.
Rotating vs stationary rings
In a normal bearing installation, one ring rotates and the other is stationary. The rotating ring should have an interference fit so it cannot creep. The stationary ring can have a clearance or transition fit so it can float axially (to accommodate thermal expansion) and rotate slightly in its seat (to correct misalignment). If the wrong ring is free to rotate, the contact surface wears. The fretting powder seen in the failed bearing was the result of both rings creeping.
For a fan with a rotating shaft, the inner ring must be interference fit on the shaft. The outer ring can be transition fit in the housing. In the failed installation, the inner ring was a clearance fit (H7/h6). Under belt pull and vibration, the inner ring crept on the shaft. The outer ring was pressed too tightly (K7 in the housing), preventing axial float. The shaft grew 0.05 mm from heat and the bearing preloaded. Both races wore.
The fit table
| Condition | Inner ring (rotating) | Outer ring (stationary) |
|---|---|---|
| Light load, steady | k5 (slight interference) | H7 (clearance) |
| Normal load, vibration | m5 (interference) | H7 or J7 |
| Heavy shock load | n6 (heavy interference) | J7 |
| Precision, preloaded | j5 (transition) | J7 (light preload) |
The installation procedure
For a 6208 bearing on a 40 mm shaft, the shaft tolerance is m5 (40 +0.002/+0.013 mm). The bearing bore is 40 mm (0/-0.010 mm). The interference is 0.012 to 0.023 mm. The bearing must be pressed onto the shaft using a drift on the inner ring — never through the rolling elements. The housing bore is H7 (52 +0.030/0 mm). The bearing OD is 52 mm (0/-0.013 mm). The outer ring has 0.013 to 0.043 mm clearance. It slides in by hand. The shaft can grow axially without binding.
The thermal expansion check
A shaft running at 1500 RPM in a fan grows about 0.05 m per meter of length. If the outer ring is locked in a rigid housing, the axial growth pushes the rings together. The bearing preloads. Temperature climbs. The grease thins. Life drops. For long shafts, one bearing should be a floating design: the outer ring is free to slide in the housing. The other bearing locates the shaft axially. Two fixed bearings on a long shaft always overheat.
The fit rule: rotating ring gets interference, stationary ring gets clearance. The failed 6208 had a clearance inner ring that crept. Specify m5 on the shaft, H7 in the housing. For long shafts, make one bearing a floating locator. Bearing life is determined by the fit, not just the load rating.
