**6. Conclusions**

After a procedure was established for conducting rheometer penetration tests, the method was used to assess the consistency of various greases and compared to other methods of assessing consistency. It was found that the rheometer penetration test does not correlate well with any of the other tests and is, therefore, only recommended to quickly and easily monitor the change in consistency to a particular grease. Though using this method is acceptable to monitor the change in consistency to a particular grease, it is not acceptable to compare the consistency of different greases to each other. There is clearly some parameter responsible for the lack of agreemen<sup>t</sup> between the rheometer penetration test and other tests and investigating this lack of agreemen<sup>t</sup> can be beneficial for understanding the tests themselves as well as the general nature of grease.

An interesting result from the experiments is that the crossover stress test appears to be an excellent substitute for the cone penetration test. This test is simple to perform, can use a very small sample, is not particularly sensitive to sample preparation, allows for temperature control, and correlates very well with cone penetration. The main disadvantages are that it requires an expensive instrument and relies on arbitrarily chosen

parameters such as plate diameter, gap height, and oscillatory frequency. Nevertheless, many researchers use the same variables and if a 25 mm flat plate is used with a 1 mm gap at 1 Hz, the equation given by Figure 11 would be a valid way of estimating cone penetration using the crossover stress. This relationship holds true for all grease types tested and is expected to hold for any other grease type.

Overall, these methods of assessing consistency can be used as a tool for monitoring the degradation of grease due to processes such as shear, oil bleed, contamination, and chemical reactions. However, it is important to keep in mind that the consistency of a grease can appear to change simply because a given sample has a different balance of thickener and oil compared to the average. Thus, appropriate sample selection is an important factor for in-service sampling and even sampling of an unused grease that has been in storage. Once an appropriate sample is taken, it appears that using oscillatory rheometry to calculate the crossover stress has a significant value for reliably assessing the consistency of a grease.

**Author Contributions:** A.G. is the main author of the paper and received significant guidance, insight, and technical assistance from Professor M.M.K. during writing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
