Rolling bearing lubricants

Statistics make it very clear for the designer: 36% of premature damage or sudden bearing failures are caused by incorrect lubrication, especially in cases where:

Restoration of the bearing lubricating film is not possible, such as in engine compartment bearings, wheel hubs, washing machine tub support and personal computer fans
The area to be lubricated is difficult to access and the costs of any downtime would be incompatible with company productivity

Rolling bearings, by virtue of their versatility and centrality in the design of industrial machines, can involve predictive periodic maintenance or lubricant replacement.

In both cases, synthetic lubricants are preferable to petroleum-derived mineral lubricants since they guarantee a higher lubricant film resistance and longer maintenance cycles.

A careful analysis of the operating and boundary conditions of the rolling bearing is the starting point for selecting the most suitable synthetic lubricating grease, involving:

An assessment of the minimum and maximum operating temperature
Work loads and rhythms
Maximum rotation velocity
Presence of leaching contaminants
Operating frequency – continuous or intermittent
Possible requirements of quietness
Presence of protective shields
Geometry of rolling elements (rollers, spheres)
Type of contact (orthogonal or oblique)
Axial or radial load transfer
Time to restore the lubricating film

The design of a rolling bearing is the first requirement to be taken into account when choosing a lubricating oil or grease. In particular, the absence of protective shields – such as in the case of shaft bearings in axle reduction units – presupposes that the lubricant applied to the gear chamber also provides wear protection for the bearing itself.

Thus, rolling bearings have the dual need to be lubricated and protected. It follows that the choice of the right lubricant must balance two opposing risks:

An increase in viscous friction (with consequent rise in heat) to promote adhesion (or permanence) of the protective film
Or, on the contrary, the centrifugal effect due to insufficient adhesiveness

The use of a grease, as opposed to an oil, favours adhesiveness yet also raises the problem of increased friction during bearing rotation (especially in the transient start-up phase of rotation), which must therefore be regularised by adjusting the viscosity of the base oil.

The ideal level of dynamic viscosity in a bearing lubricant is calculated from the velocity factor – the numerical value obtained by multiplying the bearing's maximum rotational speed (rpm, as indicated by the bearing manufacturer) by its average diameter in dm. The higher the declared rotational velocity of the bearing, the lower the viscosity of the lubricant must be.

It goes without saying that standardised lubricants will never completely satisfy the velocity factor of a bearing, which can be achieved by designing and developing a customised solution.