Speeds above the reference speed
As mentioned in the previous example, it is possible to operate bearings at speeds above the reference speed provided the increase in operating temperature can be controlled and does not have a negative impact on the bearing or the application.
Prior to operating a bearing above its reference speed, make sure that all bearing components, including the cage(s) and seal(s), can accommodate the increased temperatures. Also, check that clearance or preload values and the lubricant can accommodate higher temperatures.
The operating temperature can be kept under control if friction within the bearing can be reduced or if heat can be removed from the bearing arrangement.
Friction can be reduced to some extent with an optimized lubrication system that applies accurately metered, small quantities of grease or oil to the bearing.
Heat can be removed from a bearing arrangement in a number of ways. Typical solutions to cool the oil in oil lubricated applications include fans, auxiliary coolers and circulating oil systems (→ Oil lubrication methods).
When applying any of these solutions to a non-locating bearing, it is important to check that reduced temperatures do not affect the bearing's ability to move axially. In some cases, it may also be necessary to improve other speed-limiting factors such as bearing running accuracy, the cage design and bearing materials. Any increase in bearing temperature lowers the viscosity and effectiveness of the lubricant, making it more difficult for the lubricant to maintain an effective hydrodynamic film.
In most cases, this further increases friction and frictional heat. When temperatures increase to the point that the inner ring becomes significantly hotter than the outer ring, the operating clearance in the bearing can be reduced to the point that the bearing seizes. Any increase in speed above the reference speed generally means that the temperature difference between the inner and outer rings is greater than normal. Therefore, a bearing with a larger internal clearance than initially selected might be required (→ Bearing internal clearance). It may also be necessary to look more closely at the temperature distribution in the bearing, as well as the temperature limits of the cage and lubricant as steady-state temperatures higher than 70 °C (160 °F) may reduce their service life (→ Polymer cages and Lubricating greases).
For specific assessments of applications operating above the reference speed (thermal), contact the SKF application engineering service. As a general rule, the limiting speed of the bearing must be observed even under the most favourable frictional and cooling conditions.