SKF - Rolling Bearings

The SKF model for calculating the frictional moment
Sliding frictional moment
The sliding frictional moment can be calculated
using
M sl = G sl μ sl
where
M sl = sliding frictional moment [Nmm]
G sl = variable († table 2, page 104),
depending on:
• the bearing type
• the bearing mean diameter d m [mm]
= 0,5 (d + D)
• the radial load F r [N]
• the axial load F a [N]
μ sl = sliding friction coefficient
Effect of lubrication on sliding friction
The sliding friction coefficient for full-film and
mixed lubrication conditions can be estimated
using
μ sl = f bl μ bl + (1 – f bl ) μ EHL
where
μ sl = sliding friction coefficient
f bl = weighting factor for the sliding friction
coefficient
1
= JJJJJJJLL
e 2,6 ¥ 10–8 (n n) 1,4 dm
(† diagram 3)
e = base of natural logarithm ≈ 2,718
n = rotational speed [r/min]
n = actual operating viscosity of the oil or the
base oil of the grease [mm 2 /s]
d m = bearing mean diameter [mm]
= 0,5 (d + D)
μ bl = coefficient depending on the additive
package in the lubricant, generally ≈ 0,15
μ EHL = sliding friction coefficient in full-film
conditions
Values for μ EHL 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 3 shows the influence of lubrication
conditions on the weighting factor for the
sliding friction coefficient:
• For full-film lubrication (corresponding to
large values of k, page 241), the value of the
weighting factor for the sliding friction
coefficient f bl 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 f bl tends to 1, as
occasional metal-to-metal contact may
occur and friction increases.
Diagram 3
Weighting factor f bl for the sliding friction coefficient
1,0
0,9
0,8
0,7
0,6
0,5
C
f bl
(n n) 1,4 d m
103
0,4
0,3
0,2
0,1
0
10 5 10 6 10 7 10 8