In industrial fans, bearings are the component most likely to end a fan's working life early, and most bearing failures trace back to lubrication: too little grease, too much, the wrong grease, or grease contaminated with dust and water. Good bearing maintenance is not complicated, but it needs a routine and a record, because the warning signs are trends rather than events.
Know which bearings you have
| Arrangement | Typical fans | Maintenance |
|---|---|---|
| Sealed-for-life ball bearings, often inside the motor | Small and medium direct-drive fans | No relubrication; replace the bearings or the motor at end of life |
| Regreasable ball bearings | Medium fans and motors with grease nipples | Periodic relubrication per the motor or fan documentation |
| Pillow-block or plummer-block bearings, often spherical roller bearings | Belt-driven and larger fans with a separate shaft | Periodic relubrication, housing seals, mounting checks |
Adding grease to a sealed bearing does nothing useful, and forcing it can damage the seals. Check the documentation before anyone picks up a grease gun.
How much grease, and how often
- Quantity. A widely used rule of thumb from bearing manufacturers for relubricating from the side of the bearing is grams of grease = 0.005 × outside diameter (mm) × width (mm). A bearing 120 mm across and 31 mm wide takes about 19 g. Use the fan or bearing manufacturer's figure where one is given.
- Interval. It depends on bearing size, speed, temperature and contamination. Manufacturers publish charts; the practical rule is that higher speed, higher temperature and dirtier air all shorten the interval, and a bearing running hot or in dust may need grease several times as often as the chart's clean-room figure.
- Method. Clean the nipple, add grease slowly with the fan running where safe to do so, and let old grease escape through the purge point if the housing has one.
Too much is as bad as too little
Over-greasing is one of the commonest causes of bearing overheating. A housing packed full churns the grease, the temperature climbs, the grease breaks down, and the bearing then fails from lack of lubrication. A bearing that runs hotter for a short time after greasing and then settles is normal; one that stays hot has probably been overfilled.
Do not mix greases blindly
Greases are made with different thickeners, and some are incompatible: mixed together, they can soften and run out of the bearing. Use the grease specified in the documentation, label the nipples, and if you must change grease type, purge the old grease thoroughly rather than topping up with the new one.
Watch the trends: temperature and vibration
- Temperature. Record the bearing housing temperature with a handheld thermometer at the same load on every visit. The absolute value depends on the fan and the ambient; what matters is change. As a rule of thumb, a steady rise of 10–15 °C above the established baseline without a change in load or ambient deserves investigation.
- Vibration. Measure on the bearing housings in the same positions every time. Rising vibration can mean bearing wear, imbalance from dirt on the impeller or looseness; how limits are set is covered in impeller balancing grades and vibration limits.
- Sound. A rumble, grinding or intermittent squeal from a bearing is a late warning, not an early one.
Belts, alignment and mounting
- Belt tension. Over-tightened belts load the bearings heavily and shorten their life dramatically; too slack and they slip and overheat. Tension to the documented value and recheck after the first hours of running. The trade-offs of belt drives are discussed in belt-driven versus direct-driven fans.
- Alignment. Misaligned pulleys or couplings add load and vibration.
- Mounting. Check locking collars or adapter sleeves, housing bolts and the rigidity of the base. A loose bearing housing looks like a bearing fault on a vibration meter.
Standby fans and long storage
A fan that stands still for months can be damaged without running. Vibration from nearby machinery makes the stationary rolling elements dent the raceways, a damage called false brinelling, and grease separates. Rotate standby fans by hand periodically, run them regularly where possible, and relubricate before returning a stored fan to service.
Signs a bearing is failing
| Symptom | Likely cause | Action |
|---|---|---|
| Temperature rising over weeks | Lubricant ageing, contamination, excess load | Check grease condition and belt tension; relubricate or replace |
| Hot immediately after greasing and not settling | Over-greasing | Purge excess grease |
| Grease leaking or discoloured | Seal failure, overheating, contamination | Inspect the seals and the bearing |
| Vibration rising steadily | Wear, imbalance, looseness | Analyse and plan replacement before failure |
| Grinding or rumbling | Advanced raceway damage | Replace at the next opportunity |
Replacing bearings properly
Most premature failures of new bearings come from installation. Heat bearings for mounting rather than hammering them, keep them clean, set the specified clearance on adapter sleeves, and replace seals at the same time. For belt-driven fans such as our AT series backward-curved centrifugal fans, we supply bearing and grease specifications with every order so the maintenance routine can be written on day one.