On the basis of DIN 732-1, calculation of the thermal reference speed n_B has been standardised in ISO 15 312. The calculation of reference speeds has been matched to this standard. As a result, the values are different from the previous catalogue data.

The symbols used in the formulae have been matched to the international standard.

The thermal reference speed n_B is used as an ancillary value when calculating the thermally safe operating speed n_per. This is the speed at which, under defined reference conditions, a bearing operating temperature of +70 °C is achieved.

The reference conditions are based on the usual operating conditions of the most significant bearing types and sizes.

They are defined as follows in ISO 15 312:

• mean ambient temperature ϑ_Ar = +20 °C
• mean bearing temperature at the outer ring ϑ_r = +70 °C
• load on radial bearings P_1r = 0,05 · C_0r
• load on axial bearings P_1a = 0,02 · C_0a
• the operating viscosities (axial bearings in accordance with DIN 732-1). These are selected for radial bearings such that similar reference speeds are achieved for both oil and grease lubrication
  • radial bearings: 12 mm²s^–1 (ISO VG 32)
  • axial spherical roller bearings: 24 mm²s^–1 (ISO VG 68)
  • axial cylindrical roller bearings and axial needle roller bearings: 48 mm²s^–1 (ISO VG 220)
• heat dissipation through the bearing seating surfaces, see formulae:

for radial bearings, bearing seat A_r ≦ 50 000 mm²:

for radial bearings, bearing seat A_r ＞ 50 000 mm²:

for axial bearings, bearing seat A_r ≦ 50 000 mm²:

for axial bearings, bearing seat A_r ＞ 50 000 mm²:

The limiting speed n_G is based on practical experience and takes account of additional criteria such as smooth running, sealing function and centrifugal forces.

The limiting speed must not be exceeded even under favourable operating and cooling conditions.

The thermally safe operating speed n_per is calculated according to DIN 732-2 (draft). The basis for the calculation is the heat balance in the bearing, the equilibrium between the frictional energy as a function of speed and the heat dissipation as a function of temperature. When conditions are in equilibrium, the bearing temperature is constant.

The permissible operating temperature determines the thermally safe operating speed n_per of the bearing. The preconditions for calculation are correct fitting, normal operating clearance and constant operating conditions.

The calculation method is not valid for:

• sealed bearings with contact seals, since the maximum speed is restricted by the permissible sliding speed at the seal lip
• yoke and stud type track rollers
• aligning needle roller bearings
• axial deep groove and axial angular contact ball bearings.

The limiting speed n_G must always be observed.

The thermally safe operating speed n_per is a product of the reference speed n_B and the speed ratio f_n:

The speed ratio is derived from Figure 1:

In the normal operating range covering 0,01 ＜ k_L ＜ 10 and 0,01 ＜ k_P ＜ 10, f_n can be calculated using an approximation formula:

Heat dissipation via the bearing seating surfaces _S, Figure 2:

Heat dissipation by the lubricant _L:

Total dissipated heat flow :

Lubricant film parameter k_L:

Load parameter k_P: