Grease Quantity Formula
The standard formula for regreasing quantity for rolling element bearings:
G = grease quantity (grams)
D = bearing outer diameter (mm)
B = bearing width (mm)
This is the quantity to add during each regreasing event — not the total bearing fill. Example: a 6210 bearing (OD = 90 mm, width = 20 mm): G = 0.005 × 90 × 20 = 9 g.
For motor bearings specifically, the electric motor standard formula includes a factor for housing fill (typically 30–50% of housing space should be filled with grease — never 100%):
Regreasing Interval Calculation
The SKF/ISO regreasing interval formula for horizontal shaft bearings running at normal temperatures with normal contamination:
T = regreasing interval (hours)
n = shaft speed (RPM)
d = bearing bore diameter (mm)
K = correction factor (see table below)
| Condition | K factor |
|---|---|
| Normal load (C/P > 8), horizontal shaft, clean environment, <70°C bearing temp | 1.0 |
| High load (C/P 4–8) or vibration | 0.5 |
| Very high load (C/P < 4), shock loads | 0.25 |
| Vertical shaft (additional) | ×0.5 |
| Contaminated environment (additional) | ×0.5 |
| High temperature >70°C (additional) | ×0.5 per 15°C above 70°C |
Worked example: Fan motor bearing, bore d = 40 mm, speed n = 1,450 RPM, clean environment, normal load:
T = 1.0 × (14,000,000 / (1,450 × √40) − 4×40) = 1.0 × (14,000,000 / 9,170 − 160) = 1.0 × (1,527 − 160) = 1,367 hours ≈ every 57 days at continuous operation, or every 6 months if run 8 hours/day.
Regreasing Method
- Run the machine — grease must be added while the bearing is turning (typically at normal operating speed) so fresh grease can distribute through the bearing
- Clean the grease nipple (zerk fitting) before attaching the grease gun — contamination introduced during greasing is a leading cause of premature bearing failure
- Remove the relief plug or open the drain hole if fitted — this allows old grease to be displaced rather than forcing it through the seals
- Add grease slowly — 2–3 pumps, pause, check for temperature rise or unusual sound
- Continue until the correct quantity is reached or until temperature/sound indicates correct fill (see ultrasound method)
- Replace the drain plug after 15–20 minutes of operation (allows excess grease to purge)
Ultrasound-Assisted Regreasing
Ultrasound monitoring (40 kHz contact probe on the bearing housing) provides real-time feedback during greasing — the best method to prevent both over- and under-greasing:
- Baseline: Establish a dBµV reading at the bearing when the machine is running in good condition
- Trigger for regreasing: A rise of 8–10 dB from baseline indicates bearing lubrication has degraded — initiate regreasing
- During greasing: Add grease in small amounts while monitoring. The dBµV level should fall as fresh lubricant film is re-established
- Stop point: When the dBµV level stops falling and starts to rise — stop immediately. The rising level after the initial drop indicates you are over-greasing (excess grease is churning in the bearing and generating heat)
Common Greasing Mistakes
| Mistake | Consequence | Prevention |
|---|---|---|
| Greasing a stopped machine | Grease loads to one side; doesn't distribute evenly | Always grease while running (where safe to do so) |
| Mixing incompatible greases | Softening, oil separation, loss of structure — bearing flooding | Identify grease type in use; flush completely before changing type |
| Using a high-pressure gun without a relief | Seal damage, over-packing, grease ingress into motor windings | Remove relief plug or use low-pressure guns; follow quantity limits |
| Contaminated nipple or gun nozzle | Particle contamination shortens bearing life dramatically | Wipe nipple; use dedicated, covered grease guns per grease type |
| Ignoring the grease drain | Old grease not displaced; contaminated grease remains in bearing | Fit and use drain plugs on critical motors and fans |