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Effect of lubrication on sliding friction

The sliding friction coefficient for full-film and mixed lubrication conditions can be estimated using

µslφbl µbl + (1 – φbl) µEHL

where
µsl =sliding friction coefficient
φbl=weighting factor for the sliding friction coefficient equation

Φbl Weighting factor for the sliding friction coefficient

(diagram 1)
=

base of natural logarithm 

 2,718

=rotational speed [r/min]
ν =kinematic viscosity at operating temperature of the oil or the base oil viscosity of the grease [mm2/s]
dm =

bearing mean diameter [mm] 

= 0,5 (d + D)

µbl =coefficient depending on the additive package in the lubricant, generally  0,15
µEHL=sliding friction coefficient in full-film conditions
Values for µEHL are:
  • 0,02 for cylindrical roller bearings
  • 0,002 for tapered roller bearings
Other bearings
  • 0,05 for lubrication with mineral oils
  • 0,04 for lubrication with synthetic oils
  • 0,1 for lubrication with transmission fluids
Diagram 1 shows the influence of lubrication conditions on the weighting factor for the sliding friction coefficient:
  • For full-film lubrication (corresponding to large values of κ, refer to Viscosity ratio κ), the value of the weighting factor for the sliding friction coefficient φbl tends to zero.
  • For mixed lubrication, which can occur when lubricant viscosity or the bearing speed is low, the value of the weighting factor for the sliding friction coefficient φbl tends to 1, as occasional metal-to-metal contact may occur and friction increases.
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