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Friction

The friction in a spherical plain bearing or rod end depends primarily on the sliding contact surface combination, the load and the sliding velocity. Because there are so many influencing factors that are not mutually independent, it is not possible to quote exact values for the coefficient of friction. Under laboratory conditions, however, it is possible to record the coefficient of friction for different sliding contact surface combinations. The friction during the running-in phase is higher than the value recorded during the subsequent test period. Guideline values for the coefficient of friction μ will be found in table 1. They have been determined in laboratory trials.
The coefficient of friction for maintenance-free steel/PTFE fabric and steel/PTFE sintered bronze sliding contact surface combinations decrease with increasing specific load. At a constant specific load, friction is reduced to the given minimum value as soon as the transfer of PTFE from the sliding layer to the opposing steel surface is complete. The frictional moment for a spherical plain bearing or rod end can be calculated using

M = 0,5 μ P dm

where
M=frictional moment [Nm]
μ=coefficient of friction (table 1)
P=equivalent dynamic bearing load [kN]
dm=mean bearing diameter [mm]
  • for radial spherical plain bearings and rod ends d= dk
  • for angular contact spherical plain bearings, dm = 0,9 dk
  • for thrust spherical plain bearings, dm = 0,7 dk
dk=inner ring sphere diameter [mm]

After the bearing has been in operation for an extended period of time, negative influences (contamination, inadequate lubrication) may cause the bearing to approach or exceed the maximum values for the coefficient of friction listed in the table. Bearings are susceptible to this phenomenon even under light loads and especially under harsh operating conditions. In applications where friction is particularly important, SKF recommends determining the power ratings by using the maximum values for the coefficient of friction that are listed in table 1. For bearings operating under conditions of mixed or dry friction, there may be slight differences between adhesive and sliding friction. Experience has shown that it is not possible to eliminate stick-slip entirely and that it most frequently occurs when support elements lack adequate stiffness. In most applications, however, the effects are negligible.
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