Machinery Lubrication

Machinery Lubrication Sept Oct 2015

Machinery Lubrication magazine published by Noria Corporation

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Page 15 of 71

ML | September - October 2015 | 11 than is generally available, this test methodology is clearly a poor choice for an in-service or condition monitoring test. The cone penetration test can be performed in a worked or unworked condition. The intent is to test the grease samples with a similar level of pre-conditioning for a more consistent result. The common pre-conditioning of a worked cone penetration sample test is to be sheared for 60 strokes. However, this pre-conditioning practice is only intended for new greases. In-service grease receives no pre-conditioning prior to machine operation; the machine simply starts. The reporting method used for the cone penetration test is also very crude, and the results can be misleading. The NLGI numbering system appears to be linear (0, 1, 2, 3, etc.), but the grease consis- tency changes are not linear. For example, it would be reasonable to assume that an NLGI 4 grease would be twice as stiff as an NLGI 2 and four times as stiff as an NLGI 1. However, this is not the case. Figure 1 was developed by determining the surface area in contact with the grease for increasing depths of the cone. These depths were then compared and plotted. It is clear that the change in surface area in contact with the grease is non-linear. Use of the NLGI scale permits tremendous product variation, which becomes more pronounced with stiffer greases. This allows the producer a huge manufacturing margin in the product's rheo- · Series1 360 300 250 200 150 100 50 0 0 50 100 150 200 250 300 350 400 450 500 1/4 Scale Cone Area Measured Penetration Cone Area mm2 Figure 1. Cone area change with increasing depth ML

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