Fill quantity, cleaning and run-in for grease lubricated bearings
Too much grease runs hot and too little wears. A few fixed rules give the right fill, and a controlled run-in finds the steady temperature.
Cleaning
Bearings usually come with a preservative film that mixes with mineral oil greases and does not need removal. It must be removed before silicone or perfluorinated lubricants, and for high speed or critical bearings. Use a clean non-residual solvent. As cleaned parts dry they rust quickly, so lubricate at once or coat them with a dispersion of the intended lubricant.
How much grease
The aim is a steady lubricating film thick enough to prevent metal contact. Over-lubrication raises internal friction and temperature; under-lubrication causes boundary contact and starvation. The fill is stated as a percentage of the free space inside the bearing, and falls as the speed factor rises. The speed factor is the correction factor for the bearing type multiplied by n times Dm (rpm times mean diameter in mm).
| Bearing type | nDm correction |
|---|---|
| Deep groove ball, cylindrical roller | 1.00 |
| Angular contact ball, double row ball | 0.90 |
| Needle roller with cage | 0.70 |
| Spherical double row, taper roller | 0.60 |
| Full complement needle roller | 0.40 |
- Relubrication quantities are typically one third to one half of the original fill.
- If the housing has no way for excess grease to leave, take extra care with the quantity.
- A grease cavity larger than the bearing free space gives room for excess and a reservoir that feeds the contact.
- Get free space from the bearing maker where possible; use general charts only as a guide.
Run-in for high speed or critical bearings
The first running distributes the grease, expels the excess, forms a grease collar and brings the bearing to a low steady temperature.
- Start at about 20 percent of maximum speed and watch temperature.
- Raise the speed in steps once the temperature is steady.
- If the temperature rises quickly, stop; it indicates preload from thermal expansion. Keep the maximum below 70 degC.
- Let it cool, then restart from the last good speed.
- Continue until a steady temperature is reached at full speed; 35 to 40 degC is ideal.
- Where speed cannot be varied, run for about 30 seconds at full speed, then stop for at least five times as long, and repeat until the temperature levels off.
Notes
Run the bearing in the direction it will turn in service and, if possible, on site. Some thickener types orient on the raceway and need run-in; polyurea thickeners usually need less.