A new method for determining the mechanical stability of lubricating greases

Mechanical stability is of central importance when dealing with the long-te rm service-length of grease-lubricated roller bearings. Poor stability will lead to consistency degradation of the grease, because of mechanical force s between the rolling parts of the bearing. The result can be leakage of gr ease through seals, or at worst a total failure of the bearing. The present investigation was initiated because present-day methods for prediction of mechanical stability show weak correlation with real service-length. The ai m of the project was to develop a useful alternative. In order to fulfil th is, both field tests and laboratory tests were carried out. In the field te sts, nine different commercial greases were examined in the wheel bearings of five ore waggons, used for transporting ore by railroad from the Kiruna Mine in northern Sweden to Narvik in northern Norway for shipping to foreig n markets. The test ore waggons travelled a distance of about 300,000 km du ring a period of 3 years. Small samples of greases were taken, on eight dif ferent occasions, for consistency testing. After the end of the test period , the damage on the bearings was also studied. In the laboratory tests, new undestroyed greases of the same brand as in the field tests were examined using conventional methods, such as the V2F, the Roll Stability Test and th e Grease Worker. Comparisons between the field tests and these laboratory t ests indicate poor correlation. In addition to these conventional methods, the relevance of the shear strength of the greases to the prediction of the mechanical stability was also tested. The shear stress tau(L) depends on t he applied pressure p, thus tau(L)=tau(0)+gamma.p where tau(0) is the shear stress at atmospheric pressure, gamma is a property of the lubricant in th e same way as viscosity or density. It was found that gamma correlates well with the mechanical stability in service. Increased gamma values lead to a decrease in the mechanical stability. One reasonable explanation is that h igh gamma values correspond to high shear stresses in the grease, and thus severe conditions for the thickener. (C) 2000 Elsevier Science Ltd. All rig hts reserved.