What contributes to thicker grease films ?

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Grease

A rolling bearing preferably runs under EHL conditions. The film thickness between the rolling elements and the rings is determined by the lubricant properties, operating conditions and bearing design. During the initial phase of operation, macroscopic flow (churning) of the grease takes place. It is usually during this churning phase that fully flooded conditions prevail where the load is carried completely by the lubricant. After this, fully flooded conditions can only be maintained if the speeds are not too high. Earlier it was thought that the grease film would consist of a part formed by elastohydrodynamic action and a part formed by residual layers due to thickener deposition on the running track. However, during the last decade it was shown that this model is not universally applicable.

In our paper we contributed to the understanding of grease lubrication by studying the effect of rheology, thickener particle geometry and thickener concentration on grease EHL films under fully flooded conditions by using commonly available rolling bearing greases varying the thickener – oil systems. As the complexity of the grease and the internal bearing geometry makes it difficult to study the mechanisms of grease lubrication in full rolling bearings, the research was done using the ball on flat disc configurations with optical interferometry.

The grease film thickness using the single point contact configuration was found to be larger than corresponding bled oil suggesting the presence of thickener in the contact. The film thickness of the greases were found to be independent of rheological properties characterized by steady and dynamic shear. AFM measurements of the thickener micro-structure, from which the dimensional properties of the thickener particles (fibers/platelets/spheres) were estimated showed that the relative increase in the film thickness and viscosity due to entrainment of the thickener was proportional to the ratio of thickener volume fraction and the size of the fibers/platelets/spheres (Figure 1). Therefore, the contribution of the thickener to the film thickness is largest for a high concentration of small particles. Having the effective viscosity of the grease in the contact, Saito’s equation for the viscosity versus particle dimensions and concentration can be used to calculate the actual concentration of thickener particles inside the contact. With the limited number of greases that we studied this varied between 1% and 70%. Therefore, it is now possible to predict the fully flooded film thickness based on the microstructure of the grease.

Further details can be found in the original article: “Effect of Thickener Particle Geometry and Concentration on the Grease EHL Film Thickness at Medium Speeds”.

Febin Cyriac
About Febin Cyriac 2 Articles
PhD (tribology), grease rheology and lubrication expert.

2 Comments

  1. From the Paper: “The film thickness can change
    rapidly due to changes in rheological flow properties
    caused by, for example, shear degradation of the thickener or due to side flow induced by pressure and centrifugal force. In the absence of a suitable mechanism for track replenishment, the film thickness decreases with time below that estimated by classical EHL theory, which can be ascribed to starvation.”

    Considering the above, one can see the advantages to ISN technology being incorporated into greases. Permanent polishing/smoothing of the contact surfaces reduces friction long after the grease has gone from the bearing.

    Single-row deep groove axle bearings tested with Phantaslube® incorporated into the grease continued to operate normally after the grease-only test bearings seized due to complete grease thermal breakdown.

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