Use the Right Thickness of Lubricant 

Selecting the right lubricant, the right amount of lubricant and the correct application of the lubricant are essential to the successful performance of any bearing. Why? Because bearing lubricants serve three purposes:

 

  • To reduce friction by separating mating surfaces.
  • To transfer heat (with oil lubrication).
  • To protect from corrosion and, with grease lubrication, dirt ingress.

 

The successes of these three factors depend heavily on the film thickness on the raceway (simulated according to elastohydrodynamic effects) and at the rib/roller end contact.

 

Elastohydrodynamic lubrication – What is it?

 

The formation of the lubricant film between the mating bearing surfaces is called the elastohydrodynamic (EHD) mechanism of lubrication. The two major considerations in EHD lubrication are the elastic deformation of the contacting bodies under load and the hydrodynamic effects forcing the lubricant to separate the contacting surfaces while the pressure of the load is deforming them.

 

Film thickness on the raceway

 

The lubricant film thickness between two contact surfaces can be related to bearing performance. The thickness of the generated film depends on operating conditions such as velocity, load, lubricant viscosity and the relationship of pressure to viscosity.

 

Analytical relationships have been developed for calculating minimum and average film thickness for successful bearing performance:

 

To determine minimum film thickness:
hmin = KD (µoV)0.7a0.54W-0.13R0.43

 

where :
hmin = minimum lubricant film thickness
KD = constant containing moduli of elasticity

 


µo = lubricant viscosity at atmospheric pressure
V = relative surface velocity
a = lubricant pressure viscosity coefficient
W = load per unit length
R = equivalent radius

 

To determine average film thickness:
h = 0.039 (µVa)0.728 (P/ l)-0.091 (S 1/ R)0.364

 

where :
h = lubricant film thickness (mm)
µ = viscosity of lubricant
V = surface velocity
a = lubricant pressure viscosity coefficient
P = load between inner race and rollers
l = effective length contact between rollers and inner race
S 1/ R = sum of inverses of contact radii

 

Film thickness at rib/roller end contact

 

The contact between the large end of the roller and the inner race rib is called elastohydrodynamic contact or a hydrodynamic contact (as elastic deformations are negligible). As the rib/roller loads are much lower than the roller/race loads, the film at the rib/roller end contact is usually twice as thick as on the roller/race contact.

 

However, scoring and welding may still occur in severe conditions, including high speeds, viscosity, load or inadequate lubrication. In these conditions, use a lubricant with EP (extreme pressure) additives to prevent bearing damage.