The respective stiffness level (or softness) of a grease is measured by an instrument called a penetration meter. A steel cone is ‘dropped’ into the grease sample, and the penetrating depth of the cone is measured. A deep intrusion of the cone into the grease signifies a soft grease, while a shallow penetration indicates a harder grease.
To simplify grease selection, the National Lubricating Grease Institute (NLGI), classified stiffness according the cone depth penetration levels, as illustrated on the table below.
Stiffer greases, NLGI 3 or higher, are recommended for applications where adhesiveness is paramount, such as with bearings under very high vibrations and shock loads, or open bearings exposed to the environment.
Softer semi-flowing greases such as NLGI 000, and 00, are used for open gear systems wherein greases are sprayed, or used as a bath.
NLGI 0 and 1 greases are appropriate for automatic lubricating systems, while NLGI 2 is the standard consistency most consistently used.