**5. Discussion**

When considering tests of grease consistency, it is important to realize that consistency is an overall property of a given sample. Even within a sample, there are likely localized differences in consistency that can cause different samples of the same batch to show different results. This can be especially pronounced when a grease has been in storage and/or has experienced large temperature fluctuations. As is displayed in Figure 13, greases can even appear visually nonhomogeneous. The non-homogeneity of the grease by itself can cause a significant difference between an unworked sample and one that is worked for merely 60 strokes in a grease worker.

**Figure 13.** Smooth texture of worked grease in the cup below compared to the rough texture of unworked grease above it.

Though it is important to note that some of the apparent uncertainty of a grease's consistency is due to the grease sample itself, the method for testing consistency plays a dominant role.

## *5.1. Rheometer Penetration*

Overall, the rheometer penetration test shows different trends with different greases. It is possible that the results correlate with some property that was not investigated, but there is no clear correlation between it and any of the other tests considered. Perhaps finding the properties—such as tackiness, base oil viscosity, or other rheological properties— responsible for causing the poor correlation would help give a deeper understanding of the tests themselves as well as grease performance.

Nevertheless, an interesting metaphor between the rheometer penetration test and the cone penetration test can be drawn despite significantly different geometries. The minimum penetration value that corresponds to an NLGI grade (grade 6) is 85 dmm, while the maximum penetration value that corresponds to an NLGI grade (grade 000) is 475 dmm. This is despite the fact that the minimum possible value of penetration is 0 dmm and the maximum value (where the cone hits the bottom of the cup) is 635 dmm. If this is scaled to the rheometer test with a 100 mm/100 starting gap, this corresponds to a minimum penetration of 13.4 mm/100 and maximum penetration of 74.8 mm/100. Despite the completely different geometry, this appears to match somewhat closely with the desirable middle region for the rheometer penetration test. This is especially true for greases within the range of grade 0 to grade 2.

## *5.2. Cone Penetration*

Though the cone penetration remains one of the most common tests, many critics contend that the results of the test do not indicate useful information about a grease. For instance, some point out that it is likely more important to look at pumpability and other grease flow characteristics than cone penetration [26] when considering a grease for a given application. However, the test is such a fundamental tool in measuring a grease that it is unlikely to go away any time soon. In addition, it has an advantage over the other methods used herein in that it can test very firm greases. Greases corresponding to an NLGI grade of 4 or above are generally unsuitable for use in a rheometer, while the cone is designed such that it will give a meaningful penetration value.

A look at the interaction of grease, the cone, and the cup during the cone penetration test shows that there are different phases of the test. The first phase is where the cone penetrates initially and the portion of the cone with a very steep angle causes rapid penetration. This is useful for testing very firm greases where a normal wide cone design would show a negligible penetration.

The second phase involves shallow penetration of the "main" cone body, which is roughly independent of the diameter of the grease cup. According to ASTM D217, this takes place for penetration values below 265 dmm. In this phase, grease begins to be lifted out of the cup, but not enough to significantly impact results.

The third phase is a transition phase, where the grease is lifted out of the cup and begins to be squeezed between the cone and the cup rim. In this region, the geometry of the cup starts to become a significant factor in penetration measurement. This roughly corresponds to the region of penetration between 265 and 400 dmm.

Finally, the fourth phase of cone penetration is where the cup geometry plays a major role in penetration. A large amount of grease is squeezed between the cone and the lip of the cup, causing the cup geometry to play a major role in determining penetration. If a cup of grease with different dimensions were used, it is expected that the penetration values would be significantly different. As is mentioned in ASTM D217, in order to obtain consistent readings for grease within this region (above 400 dmm), it is imperative to center the cone exactly above the cup.

A final note on the cone penetration test is that there are scaled-down alternatives given by ASTM D1403 with correlations between these tests and the full-scale test. These linear correlations are clearly empirical, pointing out some difficulty in describing the behavior of grease during this test with an analytical method. In addition, these tests are further restricted by not allowing the testing of 000 and 00 greases.

## *5.3. Critical Stress*

The evaluation of critical stresses through oscillatory rheometry appears to be a useful tool in measuring grease properties. The yield stress method used appears to be more sensitive to variables such as pre-shear and overfilling compared to the crossover stress method. In addition, the yield stress method does not appear to correlate well with cone penetration. Results from this study as well as the paper defining the procedure [10] show that a grease with a higher cone penetration may or may not have a lower yield stress. This does not mean that this method has no value but does indicate that it is a poor choice for estimating cone penetration and NLGI grade.
