Over-Greased Fan Bearing Running at 195 °F
A forced-draft fan runs at 1,800 rpm on a 22220 spherical roller bearing (100 mm bore, 180 mm OD, 46 mm width) in a pillow block, grease lubricated. The bearing has run at about 195 °F (90 °C) for a month. A new automatic lubricator was set to deliver 1 g per day, and the previous greaser also pumped a manual shot every Friday. Vibration shows a raised high-frequency noise floor with a small BPFO peak. The grease at the relief port is dark and stiff, and grease is being pushed past the seal. The technician wants to increase the grease rate to "flush out the contamination," and the operator wants to run until the next shutdown in six weeks.
1. Diagnose what is happening. Would more grease help or hurt? Estimate a reasonable initial grease quantity for this bearing and explain how you would set the relubrication rate.
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Diagnosis: over-greasing, not under-lubrication. The clues line up:
- Grease being pushed out of the seal and stiff, dark grease at the relief port.
- Two lube sources at once (automatic 1 g/day plus a weekly manual shot) - the bearing is being fed far more than it can use.
- A raised high-frequency floor is what churned, overheated grease looks like in vibration data; the small BPFO peak says damage has started but is not yet dominant.
More grease makes it worse. Excess grease in the housing is churned by the rollers, which raises drag, raises temperature, and cooks the grease - and cooked grease stiffens, which raises temperature further. Adding grease to "flush" a bearing that has no room for it just accelerates the failure and can push the seal out.
Quantity: a common rule of thumb for initial fill is G ≈ 0.005 × D × B in grams with D and B in mm: 0.005 × 180 × 46 ≈ 41 g. After that, relubrication is a small top-up, not a refill, and the housing should not be packed solid - free space is filled only partially.
Rate: use the bearing manufacturer's relubrication chart (based on speed, bore size, and temperature) rather than a guess. As a sanity check on the effect of temperature, grease life roughly halves for every 15 °C above about 70 °C; at 90 °C you are about 20 °C over, which is roughly a 2.5× reduction in grease life. That justifies shortening the interval - it does not justify increasing the daily quantity.
Immediate action: shut off the automatic lubricator temporarily, remove the manual greasing from the route, clean out the relief port, and let the bearing run. If temperature drops noticeably within a shift or two, over-greasing is confirmed.
2. The bearing is scheduled for replacement. List the installation checks that determine whether the new bearing lasts, and explain what a BPFO peak tells you about the cause.
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BPFO meaning: a ball-pass-frequency-outer-race peak indicates a defect on the outer raceway. Outer race defects are often installation- or environment-related - a distorted housing, a cocked bearing, a tight or loose fit, or contaminated grease - not necessarily a load problem. That is why you check fits and alignment rather than just pressing in a new bearing.
Installation checks:
- Shaft fit: the inner ring should be a light interference fit on a rotating shaft - on the order of 0.001–0.002 in tight for this size. A shaft worn below nominal will spin the inner ring and fail again.
- Housing fit: the outer ring should not be pinched; check for out-of-round and fretting dust in the housing.
- Heating: if heating the bearing to mount it, stay within the bearing maker's limit (commonly around 250 °F/120 °C) and use an induction heater or oil bath - never an open flame, and never heat beyond what preserves the pre-packed grease.
- Seals and housing cleanliness: new seals, clean housing, correct grease type. Mixing incompatible thickeners (for example lithium complex and polyurea) can soften the grease and cause leakage and heat.
- Alignment and belt/fan balance: re-check shaft alignment and fan balance before start-up; a BPFO recurring in months usually means the root cause was never corrected.
- Correct grease quantity at installation, and a documented relubrication interval with one owner - not two people greasing the same bearing.
Records: log the shaft and housing measurements, the grease type and quantity, the installation temperature, and the start-up vibration and temperature readings, so the next failure can be compared.
3. The operator wants to run six more weeks to the shutdown. How would you decide? What information would you gather, and what conditions would force an earlier replacement?
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Frame it as a risk decision, not a preference.
Gather:
- Temperature trend, not a single reading. A slow rise with a stable load is manageable; a climbing trend over days is not.
- Vibration trend at the same load and speed. Watch for the BPFO growing, and especially for 1X running speed or harmonics appearing - that suggests the race is breaking up and the shaft is beginning to move. Rising overall levels plus a widening high-frequency floor is a bad combination.
- Grease condition - sample it. Iron content and color tell you how far the damage has progressed.
- Consequence of failure: is the fan on a critical process, is there a spare, is there a redundant unit, what does an unplanned outage cost, and is the fan in a position where a seized bearing could damage the shaft, housing, or ductwork?
- Lead time on the replacement bearing and the crew available for the job.
Conditions that force earlier replacement:
- Bearing temperature rising without a load change, or approaching the grease/oil limit.
- A bearing turning on the shaft (fretting, audible change, sudden vibration shift) - this escalates from a bearing replacement to a shaft replacement.
- Vibration increase that puts the fan near a shutdown limit, or any sign of seal failure letting contaminants in.
Decision rule: if the trend is flat, the load is stable, and a failure would not be a safety or process-critical event, monitored run-to-shutdown with an increased surveillance frequency (daily temperature and vibration, weekly grease sample) is defensible in writing. If the trend is rising or the consequence is severe, replace now. Either way the decision should be documented, with the monitoring plan and the trigger points named.