Why Mounting Fit Matters
Even the best bearing, perfectly lubricated and contamination-free, will fail prematurely if it's mounted incorrectly. The fit between the bearing's inner ring and the shaft, and between the outer ring and the housing, determines how load is transferred and how the bearing behaves under rotation.
The Basics: Interference vs Clearance
- Rotating inner ring with stationary load — the inner ring needs an interference fit on the shaft to prevent creep and fretting.
- Rotating outer ring with stationary inner — the outer ring needs the interference fit in the housing.
- Indeterminate load direction — both rings should have interference fits.
Standard Tolerance Recommendations
For most industrial applications with rotating inner ring and stationary load:
- Shaft tolerance: k5 (or m5 for heavier loads)
- Housing tolerance: H6 (or J6 for non-separable bearings needing axial float)
For high-precision or high-speed applications, tighter fits (js5, h5) may be needed.
Measuring Before Mounting
Before fitting, measure:
- **Shaft diameter** at the bearing seat — check for wear, taper and out-of-round
- **Housing bore diameter** — check for ovality and bell-mouthing
- **Shoulder face runout** — must be within 0.003mm for precision bearings
If the shaft is worn or fretted, don't just fit a new bearing — repair or replace the shaft first. A bearing on a worn shaft will fail again.
Mounting Methods
- Cold mounting — for small bearings up to ~100mm bore, use a bearing puller/press or the hot-oil method (heat to 80–100°C in clean oil).
- Induction heating — the fastest, cleanest method for medium and large bearings. Never heat above 120°C (loss of hardness).
- Never use a hammer — even with a drift, hammering causes brinelling and cage damage.
Common Fit Problems
- Too loose — the bearing creeps on the shaft, causing fretting, heat and eventual spin. Look for polished or fretted bore on removal.
- Too tight — excessive radial preload, heat generation and reduced internal clearance. The bearing may run hot and seize.
- Misaligned housing — uneven load distribution, rapid wear on one side of the raceway. Always check housing alignment with a dial indicator before fitting.
