Is it Time to Retire the Grease Penetration Test?

摘要

It has been said that the most important property of a lubricant is the viscosity, the resistance to flow. This is a true statement for fluid lubricants but not most grease products that are semi-solid. Grease marketers often sell grease first by thickener type, and second by consistency number, often called the NLGI grade. Described in basic terms, grease consistency is a measure of the “hardness or softness” of a grease. The most common test for evaluating grease consistency is the penetration test. This test has stood for many years as the standard for evaluating grease consistency. In this test, a standardized cone is dropped onto a flat surface of grease packed in a standard cup, allowed to sink for five seconds, and then the depth the cone has sunk is measured. Perhaps it is the simplicity of the test that has helped it to maintain its position as one of the most important grease evaluation tests. Still, it may be argued that this simplicity has caused complacency within the industry about the true value of this test. Formulators and engineers may have noticed that while several grease products may all have the same NLGI grade per the penetration test, their physical properties can be noticeably different.