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# The SKF model for calculating the frictional moment

A sketch of the frictional moment in a typical bearing, as a function of rotational speed or viscosity, is presented in diagram 1. During the start-up period (zone 1), as speed or viscosity increases, the frictional moment decreases as a hydrodynamic (lubricant) film is being formed. As speeds or viscosity continue to increase and the bearing enters into the full elasto-hydrodynamic lubrication (EHL) zone, the thickness of the hydrodynamic film increases (increasing κ value, refer to Viscosity ratio κ), which also increases friction (zone 2). Eventually, speed or viscosity increase to the point where kinematic starvation and inlet shear cause friction to reach a plateau or even decrease (zone 3).
To accurately calculate the total frictional moment in a rolling bearing, the following sources and their tribological effects must be taken into account:
• the rolling frictional moment and eventual effects of high-speed starvation and inlet shear heating
• the sliding frictional moment and its effect on the quality of the lubrication
• the frictional moment from seal(s)
• the frictional moment from drag losses, churning, splashing etc.
The SKF model for calculating the frictional moment closely follows the real behaviour of the bearing as it considers all contact areas, design changes and improvements made to SKF bearings as well as internal and external influences.
The SKF model for calculating the frictional moment uses

M = Mrr + Msl + Mseal + Mdrag

where
 M = total frictional moment Mrr = rolling frictional moment Msl = sliding frictional moment Mseal = frictional moment of seals Mdrag = frictional moment of drag losses, churning, splashing etc. (→ Drag losses)
The SKF model is derived from more advanced computational models developed by SKF. It is designed to provide approximate reference values under the following application conditions:
• grease lubrication:
• only steady state conditions (after several hours of operation)
• lithium soap grease with mineral oil
• bearing free volume filled approximately 30%
• ambient temperature 20 °C (70 °F) or higher
• oil lubrication:
• oil bath, oil-air or oil jet
• viscosity range from 2 to 500 mm2/s
• 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 loads in magnitude and direction
• normal operating clearance
• constant speed but not higher than the permissible speed
For paired bearings, the frictional moment can be calculated separately for each bearing and added together. The radial load is divided equally over the two bearings; the axial load is shared according to the bearing arrangement.