Spun Motor Bearing - Reading the Journal
A 150 hp, 1,780 rpm motor on a forced-draft fan failed with a growling drive-end bearing. The 6316 C3 bearing came off with a blued inner ring and metal transfer, and the shaft shows a polished band. You mike the journal with a three-point micrometer at four positions 90 degrees apart in two planes: 3.1478, 3.1480, 3.1474, 3.1479 in (plane 1) and 3.1476, 3.1477, 3.1482, 3.1475 in (plane 2). The new bearing bore measures 3.1494 in. The OEM spec calls for 0.0002 to 0.0010 in interference, journal round within 0.0005 in, no taper over 0.0005 in, and a 63 microinch maximum finish. The housing bore is within spec. The motor was aligned 14 months ago and no alignment records exist since. A tube of Loctite 660 is in the shop.
1. From the micrometer data, assess roundness, taper and fit. Can the new bearing be installed as-is?
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Roundness: max 3.1482, min 3.1474 → out-of-round = 0.0008 in, against a 0.0005 in limit. Fails.
Taper: plane averages (3.1478 + 3.1480 + 3.1474 + 3.1479)/4 = 3.14778 and (3.1476 + 3.1477 + 3.1482 + 3.1475)/4 = 3.14775 - effectively no taper at this axial location, but taper must still be checked between the two ends of the journal, not just in one plane.
Fit: the closest approach is bore 3.1494 minus the largest shaft reading 3.1482 = 0.0012 in clearance, where 0.0002 to 0.0010 in interference is required. The journal is short of the minimum by 0.0014 in and short of nominal (80 mm = 3.1496 in) by 0.0014 to 0.0022 in.
Verdict: no. A new bearing pressed onto this journal has clearance instead of interference - it will spin, blue and fail again, probably faster than the first one. The out-of-round also means the inner ring would be distorted, changing internal clearance and loading.
2. What does the evidence say about why the bearing spun, and what else must be checked before the machine is reassembled?
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The blued ring plus a polished, undersized journal is a classic spun bearing: contact was lost, the ring slipped, friction heat blued it and metal transferred. Loss of interference is the mechanism, but it is usually a symptom, not a cause. Interrogate:
- Alignment - 14 months with no records on a 150 hp fan is the first place to look; check coupling alignment and for a soft foot before reassembly.
- Fit history - was the bearing pressed on cold with a hammer, or heated? Was it removed with a puller that gauled the seat?
- Fan condition - balance, blade deposits, foundation looseness; a fan that has been shaking puts the whole load through the bearing.
- Housing - measure the bore for roundness and fretting; an out-of-round housing pinches the outer ring and loads the inner ring.
- The other bearing and shaft straightness - check runout at the bearing seats and the coupling hub; heat that blue a ring often warps a shaft.
- Bearing selection - confirm the C3 clearance and the correct bearing number against the nameplate; the wrong clearance changes the operating preload.
Do not simply press on a new bearing and return to service.
3. Compare the repair options for the journal and recommend one, including whether the retaining compound is acceptable.
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Options:
- Replace the shaft - restores nominal fit, roundness and finish exactly; best for a 150 hp continuous-duty fan, but it is the longest lead time.
- Weld and grind, or metal-spray and grind - a qualified shop can restore the seat to 0.0002–0.0010 in interference, round within 0.0005 in and 63 microinch finish. Acceptable and commonly done; requires removing the rotor and stress control on the weld.
- Repair sleeve - adds a joint and reduces the section under the inner ring; workable on lightly loaded shafts, but a poor choice here where the whole point is a reliable interference fit.
- Retaining compound (Loctite 660) - designed exactly for worn bearing seats and can fill a gap up to about 0.020 in. It is a legitimate, OEM-recognized repair when the journal is round, clean, degreased and free of taper.
Recommendation: the journal currently fails the roundness limit (0.0008 in vs 0.0005 in), so a compound alone is not the answer - the seat must be brought round first. Plan for weld/grind or a new shaft as the permanent repair. If the fan must run now, polish and re-check roundness and finish, then use the compound as a documented temporary repair with a shortened vibration-monitoring interval, and re-check for a temperature rise at the DE housing at start-up.