Drag losses for oil bath lubrication

The SKF model for calculating the drag losses in oil bath lubrication considers resistance of the rolling elements when moving through the oil and includes the effects of the viscosity of the oil. It provides results with sufficient accuracy under the following conditions:
  • The oil reservoir is large. Effects from reservoir size and geometry or external oil agitation are negligible.
  • The shaft is horizontal.
  • The inner ring rotates at a constant speed. The speed is not higher than the permissible speed.
  • The oil viscosity is within the limits:
    •  500 mm2/s when the bearing is submerged up to half or less (oil level H  D/2)
    •  250 mm2/s when the bearing is submerged more than half (oil level H > D/2)
The oil level H is measured from the lowest contact point between the outer ring raceway and the rolling element (fig. 1). The postion of the lowest contact point can be estimated with sufficient accuracy using:
  • for tapered roller bearings: outside diameter D [mm]
  • for all other radial rolling bearings: outer ring mean diameter [mm] = 0,5 (D + D1)
The frictional moment of drag losses for ball bearings can be estimated using

Mdrag Frictional moment of drag losses for ball bearings

The frictional moment of drag losses for roller bearings can be estimated using

Mdrag Frictional moment of drag losses for roller bearings

The rolling element related constants are:

Kball Rolling element constant

Kroll Rolling element constant

The variables and functions used in the equations for the frictional moment of drag losses are:

Cw Variable for frictional moment of drag loss

lD Variable for frictional moment of drag loss

ft = sin(0,5 t), when 0  t  π

ft = 1, when π < t < 2 π

Rs Variable for frictional moment of drag loss

t Variable for frictional moment of drag loss 

When H > 1,2 dm, use H = 1,2 dm

fA Variable for frictional moment of drag loss

where 
Mdrag =frictional moment of drag losses [Nmm]
VM =drag loss factor (diagram 1)
B=bearing width [mm]:
  • for tapered roller bearing → width T
  • for thrust bearings → height H
dm =

bearing mean diameter [mm] 

= 0,5 (d + D) 

d=bearing bore diameter [mm]
D=bearing outside diameter [mm]
H=oil level (fig. 1) [mm]
irw=number of ball rows
KZ=bearing type related geometric constant (table 1)
KL=rolling bearing type related geometric constant (table 1)
n=rotational speed [r/min]
ν
=kinematic viscosity at operating temperature [mm2/s]

Drag losses for vertical shafts

To calculate drag losses for vertical shafts, the model for fully submerged bearings can be used to get an approximate value. The obtained value for Mdrag should be multiplied by a factor equal to the width (height) that is submerged relative to the total bearing width (height).
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