The (thermal) reference speed listed in the product tables represent a reference speed value that is to be used to determine the permissible operational speed of the bearing subjected to a certain load and running with a certain lubricant viscosity.
The values of the reference speed listed are in accordance with ISO 15312:2003 (where thrust ball bearings are excluded). This ISO standard has been established for oil lubrication, but is also valid for grease lubrication.
The reference speed for a given bearing represents the speed, under specified operating conditions, at which there is equilibrium between the heat that is generated by the bearing and the heat that is dissipated from the bearing to the shaft, housing and lubricant
The reference conditions according to ISO 15312:2003 for obtaining this heat balance are:
- a temperature increase of 50 °C above an ambient temperature of 20 °C, i.e. a bearing temperature of 70 °C, measured on the bearing stationary outer ring or housing washer
- for a radial bearing: a constant radial load, being 5% of the basic static load rating C0
- for a thrust bearing: a constant axial load, being 2% of the basic static load rating C0
- open bearings with Normal clearance
for oil lubricated bearings:
lubricant: mineral oil without EP additives having a kinematic viscosity at 70 °C of:
- ν = 12 mm2/s (ISO VG 32) for radial bearings
- ν = 24 mm2/s (ISO VG 68) for thrust roller bearings
- method of lubrication: oil bath with the oil reaching up to the middle of the rolling element in the lowest position
for grease lubricated bearings:
- lubricant: regular grease with a lithium thickener and a mineral base oil having a viscosity of 100 to 200 mm2/s at 40 °C (e.g. ISO VG 150)
- grease quantity: approximately 30% of the free space in the bearing.
A temperature peak may occur during the initial start-up of a grease-lubricated bearing. Therefore the bearing may have to be in operation for up to 10 to 20 hours before it reaches the normal operating temperature.
Under these specific conditions the reference speed for oil and grease lubrication will be equal. It may be necessary to reduce the ratings in applications where the outer ring rotates.
For certain bearings, where the speed limit is not determined by heat from the rolling element/raceway contacts, only limiting speeds are shown in the bearing tables. These include, for example, bearings with contact seals.