Motor bearing overheating after overgreasing and grease mixing
A 15 kW, 4-pole totally enclosed fan-cooled motor drives a centrifugal pump through a belt. Its drive-end bearing is a 6310 single-row deep-groove ball bearing, grease-lubricated and fitted with a C3 clearance. After a routine regrease, the bearing temperature rises from 62 °C to 94 °C within 40 minutes and the vibration reading at the drive-end bearing goes from 1.8 to 4.6 mm/s RMS, with no dominant 1× or 2× peak but a broad high-frequency floor. The maintenance technician used a lithium-complex EP grease, NLGI 2, and filled the bearing cavity completely because “more grease means better protection”. Records show the previous grease was a polyurea NLGI 2, and the motor had run 6,000 h since the last clean-out. The bearing housing drain plug is painted over and the motor is in a food plant where washdown occurs daily.
1. Diagnose the most likely cause of the temperature and vibration change after regreasing. Explain why bearing wear alone is not the best explanation.
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The event is time-linked to the regrease, so the grease is the first suspect. Filling the cavity completely causes churning: the rolling elements must push a solid mass of grease, which generates heat, raises temperature and can break down the grease. The technician also mixed a lithium-complex grease with the existing polyurea grease. These thickeners are not generally compatible; mixing can soften or harden the grease, cause bleeding or form lumps, all of which disturb the lubricant film.
Bearing wear is less likely because the change was sudden, not gradual. A worn bearing usually develops discrete defect frequencies such as 1×, 2×, BPFO or BPFI. The broad high-frequency floor is more consistent with lubricant film disturbance and churning. The painted-over drain plug and daily washdown are additional risks, but the immediate cause is overgreasing and probable grease incompatibility.
2. Set out the immediate corrective actions and the correct regreasing procedure for this bearing.
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Stop the motor, isolate and lock off. Remove the drain plug, clean away the excess grease and inspect the discharged grease for discolouration, hard lumps or contamination. Do not add more grease. Run the motor unloaded with the drain open for 20–30 minutes and monitor temperature and vibration; some excess grease will purge. If the temperature does not fall towards normal, dismantle, clean the bearing and housing and repack with the correct grease.
For routine regreasing, clean the nipple, use the specified polyurea NLGI 2 grease, and add slowly while the motor runs. Fill only about 30–40% of the free space, not the whole cavity. Stop when fresh grease appears at the drain, then refit the drain plug. Never mix greases without a compatibility check; if changing type, clean out the old grease first. Record the grease type, quantity, date and running temperature.
3. Recommend follow-up condition monitoring and design changes to prevent recurrence.
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Trend vibration and bearing temperature weekly for at least a month, and compare against the 1.8 mm/s baseline. Check the grease condition and relubrication interval against the 6,000 h history and the motor speed. Inspect the seals and breather, because daily washdown can drive water into the housing; use a food-grade grease and a protected breather or drain arrangement if washdown is unavoidable.
Make the drain plug accessible and clearly identified, and remove paint from it. Train staff that more grease is not better and that grease types must not be mixed. Check belt tension and alignment because overhung belt load affects bearing life. If the bearing has been overheated, inspect it for discolouration and clearance; replace it if the C3 clearance has been lost or the grease has carbonised.