Repeated 6316 Bearing Failures on an Elevator Motor
A 75 kW, 4-pole TEFC motor (1475 rpm) drives a bucket elevator through a V-belt. Its drive-end bearing is a single-row deep-groove ball bearing 6316 (80 × 170 × 39 mm), grease-lubricated in a cast-iron housing with a relief plug. The same position has failed three times in 14 months. Every failed bearing shows a dull grey-to-blue outer-race track, spalling in the loaded zone at the bottom (6 o'clock) and reddish-brown staining on the balls; the grease found in the housing is black and hard. The PM schedule says 'grease monthly - 2 strokes with the hand gun', but the fitter greases until clean grease escapes the relief, roughly 350–400 g each time. The store issues a lithium 12-hydroxystearate EP2 and a polyurea EP2 interchangeably, whichever drum is open. After each belt change the pulleys are aligned with a straight-edge; a check now shows about 0.15 mm offset, and the belt is tensioned by feel. The bearing runs at 82 °C after three hours at full load in a 38 °C ambient.
1. Interpret the failure evidence and rank the contributing causes.
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Blue-grey track and black hard grease. Discolouration above roughly 200 °C means the bearing ran hot, and black, hardened grease is oxidised grease that has lost its oil. Both point to churning from over-greasing: a housing packed solid has nowhere for the grease to escape, the balls plough through it, power loss and temperature rise, the soap oxidises, and the bearing ends up starved on hard soap. 350–400 g per month is far more than the bearing can use.
Mixed greases. Lithium and polyurea thickeners are incompatible. Mixing them commonly softens the grease so it leaks away, or hardens it so it channels and starves the raceway - either outcome matches the findings and the practice of using whichever drum is open is indefensible.
6 o'clock spalling. The load zone is at the bottom, consistent with the belt pull plus rotor weight. Excessive belt tension set by feel increases radial load and shortens life; the 0.15 mm pulley offset adds a cyclic axial component and edge loading.
Reddish-brown staining. Moisture ingress or rust from a failed seal or washdown, or dye bleed from an incompatible grease; in either case the seal and the housing breather need checking.
Ranking: over-greasing and grease mixing first, belt tension and alignment second, moisture third.
2. Calculate a sensible grease charge for this bearing and rewrite the lubrication instruction.
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Charge. A common shop approximation for the initial fill of a ball bearing is G ≈ 0.005 × D × B, with D and B in millimetres: G = 0.005 × 170 × 39 ≈ 33 g. The housing should not be filled solid; with a relief plug the free space stays partly empty so that grease can escape the rolling path. The manufacturer's manual governs where available, but the order of magnitude is 30–35 g, one-tenth of what the fitter is putting in.
Rewritten instruction:
- Standardise on one grease - the lithium 12-hydroxystearate EP2 or the polyurea EP2, chosen with the bearing supplier's advice - and remove the other drum from the store for this area. Never mix.
- Clean the grease nipple before coupling the gun.
- Re-lubricate with the machine running if it is safe and permitted, with the relief plug removed, adding a small measured quantity (typically 2–3 gun strokes, about 5–10 g) until fresh grease just appears, then stop and refit the plug.
- Frequency: at n·dm = 1475 × 125 ≈ 184 000, this bearing does not need monthly greasing; a 3–6 month interval with the correct small quantity is more realistic. Confirm against the motor maker's table.
- Record date, grease type and quantity; a 400 g entry should be a red flag at the next audit.
3. The bearing runs at 82 °C in a 38 °C ambient. What does that tell you, and what condition-monitoring and design changes would you propose?
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Temperature. The rise is 44 K, above the roughly 40 K rule of thumb for grease-lubricated machines, and much of it is self-inflicted by churning. Grease life approximately halves for every 15 K above about 70 °C, so at 82 °C the grease is degrading several times faster than at 70 °C - which is exactly how a bearing fails three times in 14 months. Bring the running temperature down and the failure rate follows.
Monitoring. Add the drive-end bearing to a routine vibration route, measuring velocity (mm/s rms) and, where possible, envelope acceleration; a rising trend in envelope level catches spalling months before noise. Add a contact or infrared temperature reading to the monthly round, with an alarm at, say, 75 °C and a trip-level investigation at 85 °C. Record belt tension and pulley alignment at every belt change.
Design and practice changes.
- Replace the straight-edge alignment with a laser or dial-indicator check, and specify a target offset and angularity with a tolerance rather than 'check alignment'.
- Set belt tension with a tension gauge and the drive manufacturer's deflection force figure, not by feel.
- Verify the seal and breather path, and consider a sealed-for-life or a relubricable bearing with the correct housing fill, whichever the supplier recommends for this duty.
- If failures continue after these corrections, examine the bearing fit and housing bore, since a spinning outer ring or a tight fit also produces heat and bottom-zone spalling.