Standards Development
This document specifies a method for evaluating the change of consistency, as measured by cone penetration, of lubricating grease when worked in the roll stability test apparatus. The method only applies to greases with cone penetration in the range 175 1/10 mm to 385 1/10 mm as measured using ISO 2137.
The present method, instead of extruding the grease through holes or centrifuging it from a non-sealed tapered roller bearing, laminates the grease between a roller weight and a hollow cylinder. This is closer to the stresses that a grease encounters in a bearing.
Mechanical stability of a grease depends on the type of thickener, the manufacturing procedure, and of its consistency grade. But any grease in a bearing suffers rolling, with the effect of destroying the thickener and its structure. After a while, only a very tiny quantity of grease remains on the races of the external and internal ring, in the cage pockets, leading to starved elasto-hydrodynamic lubrication regime. The lubricant is fed to the contact by capillary flow from the surrounding remaining grease. As long as the capillary flow of lubricant remains, the bearing is lubricating. Once this capillary flow ceases, the risks of seizure are important.
Checking of the mechanical stability of a grease by rolling at laboratory scale is therefore important.
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