The National Lubricating Grease Institute (NLGI) scale is used to express the hardness of grease. It is important to understand the proper type of grease needed for a bearing—thermal variations within a bearing application will affect the performance of a grease. For example, elevated temperatures can soften some greases and lower temperatures can stiffen grease until it restricts rotation or limits oil bleeding.
Lubricating grease is made of mineral or synthetic oil combined with a thickener. Thickeners for grease range from metallic soaps (fatty acid + metal) to dyes, amides or ureas. The type of thickener and the percentage of it, determine the consistency. The consistency grades are as follows:
LGI Consistency Number Worked Penetration Range, 25°C (77°F)