Oversize bearing journal and housing bore on a gearbox input shaft
A 40 mm journal on the input shaft of a gearbox carries a single-row deep-groove ball bearing 6208. The drawing specifies the shaft fit as 40 k6 (+0.018/+0.002 mm) and the bearing bore is manufactured to 40 0/−0.012 mm. The gearbox has been stripped because of a rumble; the journal shows a polished band and slight fretting. The workshop micrometer, recently zeroed and checked against a 25 mm standard, gives three readings across the same cross-section: 40.024, 40.021 and 40.019 mm. A dial indicator shows 0.005 mm out-of-round. The replacement bearing is available, but the new housing bore measures 80.042 mm against a drawing requirement of 80 H7 (+0.030/0). The maintenance planner wants to polish the journal with emery cloth, fit the new bearing and return the unit to service the same shift.
1. Calculate the design interference range for the 40 mm bearing journal and decide whether the measured journal can be accepted. Show the effect of the ±0.002 mm measurement uncertainty.
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Shaft limits for 40 k6 are 40.002 mm to 40.018 mm. Bearing bore limits are 39.988 mm to 40.000 mm. Minimum interference = 40.002 − 40.000 = 0.002 mm. Maximum interference = 40.018 − 39.988 = 0.030 mm.
The readings are 40.019, 40.021 and 40.024 mm. All exceed the shaft upper limit of 40.018 mm. The smallest excess is 0.001 mm and the largest is 0.006 mm. With ±0.002 mm uncertainty, the 40.019 mm reading could possibly be acceptable, but 40.024 mm cannot be explained away, and the 0.005 mm out-of-round is an additional form error.
Decision: reject the journal as measured. Pressing an oversize journal into a bearing reduces internal clearance, adds hoop stress to the inner ring and can cause overheating or cracking. Emery-cloth polishing is uncontrolled: it removes material unevenly, may not correct ovality or taper, and can take the journal below the minimum limit. The journal should be cylindrical-ground to about 40.010–40.015 mm, then checked for taper, roundness and surface finish before fitting.
2. Assess the housing bore measurement of 80.042 mm against 80 H7 and explain the risk of fitting the new bearing into it.
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80 H7 has limits 80.000 mm to 80.030 mm. The measured 80.042 mm is 0.012 mm above the upper limit, so the housing is oversize and non-conforming.
The outer ring of a deep-groove ball bearing is normally stationary and should be supported around its circumference with a controlled locational fit. An oversize bore reduces that support, allows the outer ring to creep or spin, and produces fretting, wear debris, noise and heat. It also permits the bearing to sit off-centre, which misaligns the shaft and adds load to the bearing and seal. Retaining compound might fill a small gap, but it is not a proper repair for a gearbox input bearing carrying load. The correct action is to replace the housing, or bore and fit a sleeve and then machine the sleeve bore to 80 H7. The bore should also be checked for roundness and taper in at least two planes before reassembly.
3. Outline the rework and inspection sequence you would authorise before the gearbox is reassembled, including the bearing fit and housing.
Read the worked answer
- Clean and inspect the shaft; record all journal diameters, taper and roundness. 2. Grind the journal on a cylindrical grinder to 40.010–40.015 mm, maintaining the shoulder radius; do not hand-polish to size. 3. Check surface finish (Ra ≤ 0.8 µm) and carry out magnetic-particle inspection of the journal and shoulder for grinding cracks. 4. Repair the housing by replacing it or by boring and sleeving to 80 H7; verify roundness and taper. 5. Check the new bearing is correct and clean; heat it inductively or in oil to about 80–100 °C and fit it onto the shaft by supporting only the inner ring. Never hammer it. 6. Fit the shaft assembly into the housing, check axial float and seal alignment, and confirm the outer ring is not distorted. 7. Run the gearbox unloaded, then loaded, monitoring temperature and vibration. 8. Record the actual shaft and housing sizes, bearing number, fitting method and final readings.