The Use of a Stress Rheometer in Lieu of Cone Penetration

The Cone penetration test is used to provide a measurement of grease stiffness and is considered the most important of laboratory performed grease tests. The design of this test was intended to support new grease manufacture and sales. This test is not well suited for used grease as it requires a volume of grease which in most cases exceeds the volume of sample obtained from an in service field machine. The reporting method for cone penetration is very crude. The results are misleading in that the NLGI numbering system appears to be linear (0, 1, 2, 3 etc.) but the grease consistency changes are not linear. For example, one would reasonably think that NLGI 4 grease would be twice as stiff as an NLGI 2 and 4 times as stiff as an NLGI 1. Rheometer data is reported in Pa and these units avoid the awkwardness of the NLGI scale. When comparing this data to cone penetration, the non linear nature of the NLGI scale becomes apparent. A Good correlation was obtained between the test methods and it is concluded that use of a rheometer in lieu of cone penetration is both desirable and acceptable when monitoring the consistency of new grease or in service greases.