The main purpose of the reference speed (thermal) is to provide a quick assessment of the speed capabilities of a bearing based on standardized reference values for the heat flow density as established in ISO 15312:2003 (diagram 1).
This ISO standard provides reference operating conditions and standard values for the heat flow under those reference conditions. The standard reference heat flow densities shown in diagram 1 are within the values found in bearing applications, shown as the shaded area.
To assess the actual temperature rise and heat flow in a particular application, a detailed analysis of the cooling conditions surrounding the bearing would be required. This is outside the scope of the present ISO speed rating standard. For detailed temperature rise calculations, contact the SKF application engineering service.
The values of the reference speeds are based on ISO 15312:2003. The ISO standard, established for oil lubrication, is also valid for grease lubrication. Outer ring rotation is not covered by the ISO standard. Therefore, it may be necessary to reduce the reference speed ratings in applications where the outer ring rotates. For additional information, contact the SKF application engineering service.
For bearings with contact seals, speed capability of the bearing is not determined by the frictional heat generated in the rolling element / raceway contacts. Therefore, these bearings are not included in the ISO reference speed standard and only the limiting speeds are listed in the product tables.
The ISO reference speed is based on open bearings under the following operating conditions:
- radial load P = 0,05 C0 for radial bearings
- axial load P = 0,02 C0 for thrust bearings
- nominal temperature increase of 50 °C (90 °F) above an ambient reference temperature of 20 °C (70 °F)
- good lubrication and clean conditions
- Normal internal clearance (→ Bearing internal clearance)
SKF values for the reference speed comply with these conditions, except for the clearance condition. The SKF values are valid for sufficient operating clearance (→ Bearing internal clearance), which in some cases might require bearings with an initial clearance greater than Normal.
Grease lubricated bearings may undergo a temperature peak during initial start-up, requiring a running-in period before they reach normal operating temperature.