Oil Separation in Grease lubricated Rolling Bearings

Dipl.-Ing. Frank Reichmann

Summary

The functions of thickeners in lubricating greases may be subsumed as follows [2]:

1. Avoidance of material exchange

2. Fluid lubrication

3. Additivation

The first topic refers to the flow limit, that avoids leakage and the ingress of foreign from the environment into the bearing. However, due to the flow limit there will be no replenishment of the grease into the contact zone. Therefore, the fluid lubrication acc. to the second is only maintained by the separated oil. This oil does not have a flow limit and will replenish into the contact. Therefore, oil separation is considered to be most important for the lubrication of rolling bearings.

Oil separation may be described with the equations of Darcy, as well as Carman and Kozeny. Acc. to these equations the oil separation is driven by an external force. The higher this force, the stronger the oil separation.

Lugt suggests considering three phases of grease lubrication in rolling bearings:

1. Churning phase

2. Bleeding phase

3. Severe film breakdown

During the churning phase after starting the operation of the bearing, a rather fresh grease will be exposed to a certain force driving oil separation. After the churning phase is completed, the grease may already have been aged from shear stresses and, in some cases, from thermal stress. Furthermore, the driving force will be low.

However, to provide sufficient fluid lubrication over a long period of time, the oil separation characteristic of a grease should be in a way, not to separate too much oil under fresh condition when exposed to a high driving force. Otherwise, the grease will not have sufficient capacity to separate additional oil once it is aged and the driving force is low in the bleeding phase.

To assess this performance of the grease, tests may be conducted, where the oil separation of fresh grease under high driving forces is compared to the oil separation of aged greases under low driving forces.