Basics of Lubrication
Rolling bearings need to be adequately lubricated to operate reliably and to prevent direct metal-to-metal contact between the rolling elements, raceways, cages and other components. The lubricant also inhibits wear and protects the bearing surfaces against corrosion. The choice of a suitable lubricant and lubrication method is important, as is proper maintenance.
A wide assortment of greases, oils and alternative lubricants like graphite-based compounds are available to lubricate rolling bearings. Selecting a suitable lubricant and lubrication method depends primarily on the operating conditions such as the required speed or permissible operating temperature. However, other factors like vibration and loads can also influence the selection process.
Generally, the most favourable operating temperatures can be obtained when the minimum amount of lubricant needed to reliably lubricate a bearing is provided. However, when the lubricant has additional functions, such as sealing or removing heat, additional amounts of lubricant may be required.
The lubricant in a bearing arrangement gradually loses its lubricating properties as a result of mechanical working, ageing and the build-up of contaminants. It is therefore necessary for grease to be replenished or renewed and for oil to be filtered and changed at regular intervals.
The information and recommendations in this section relate to bearings without integral seals or shields. SKF bearings and bearing units with an integral seal and/or shield on both sides are factory-greased. Information about the standard greases used by SKF is provided in the relevant product chapters, together with a brief description of the performance data.
Under normal operating conditions, the service life of the grease in sealed bearings exceeds the service life of the bearing so that, with some exceptions, no provision is made for the relubrication of these bearings.
For purposes of this discussion, normal operating conditions can be defined as:
- constant loads in magnitude and direction
loads equal to or larger than the recommended minimum load and at least:
- 0,01 C for ball bearings
- 0,02 C for roller bearings
- constant speed, but not higher than the permissible speed
- appropriate operating clearance
for grease lubrication:
- only steady state conditions (after several hours of operation)
- lithium soap grease with mineral oil
- bearing free volume filled approximately 30%
- minimum ambient temperature 20 °C (70 °F)
for oil lubrication:
- oil bath, oil-air or oil jet
- viscosity range from 2 to 500 mm2/s
Ring centred cages
Bearings with ring centred cages are primarily designed for oil lubrication. Grease lubrication can be used for moderate speeds. Information about the cage designs and the limits are provided in the relevant product chapters, if applicable.
Differences in the lubricating properties of seemingly identical lubricants – particularly grease – produced at different locations, or even different production batches from the same location, can exist. Therefore, SKF cannot accept liability for any lubricant or its performance. The user is therefore advised to specify lubricant properties in detail to obtain the most suitable lubricant for the application.