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To select an appropriate penetration time, one must examine the penetration as a function of time. Figure 4 examines the gap height over time as a constant normal force is imposed on grease samples. This figure considers a range of forces between 2 N and 8 N and uses greases ranging from grade 00 to grade 2. For some grease types, a low normal force will cause the plate to barely move or, in some cases, not move at all. In cases where the top plate moves, the initial velocity (slope of this plot) is approximately the same but begins to level off as the plate nears a steady state. Because of this, it takes longer for the top plate to reach a steady state when the plate must travel a longer distance. Nevertheless, for all cases, it appears that 20 s is sufficient, confirming the results of a previous study [6]. Therefore, 20 s was used for all subsequent measurements.

 **Figure 4.** Rheometer gap height as a function of time for various greases and forces.

The next thing to look at is how penetration is affected by changing the normal force. For this examination and all future rheometer penetration test results, data will be presented as net penetration to provide a similar meaning as cone penetration. This will be defined as the final gap height subtracted from the initial gap height. In this way, the higher the penetration, the less firm a given grease is. In addition, the units of mm are converted to mm/100, which also corresponds to the percentage of the initial gap.

Figure 5 shows that there is a clearly nonlinear trend relating net penetration and normal force for a given grease sample. It is possible to separate the trend into three regions: a region where the force is too low, a region where the force is too high, and a region in the middle where the force is appropriate for measuring consistency. For greases below grade 1, the first region is not visible on the plot because imposing a force of less than 1 N using the rheometer often led to no movement. A force of 1.2 N would work well to determine the consistency of a grade 00 or grade 0 grease. However, it would not be able to determine any significant difference between grade 1 and grade 2 grease. Similarly, a force of 4 N would work well for a grade 1 or grade 2 grease but would not be useful for any grease below grade 1. Generally, the goal of testing greases with the rheometer penetration method is to keep results within the middle region. This means that the rheometer penetration test needs to vary the force according to the consistency of the grease chosen in order to keep results within the region of approximately between 15 mm/100 and 80 mm/100. Unfortunately, this means that if the rheometer penetration results are to be correlated with other tests, each normal force must be considered individually.

**Figure 5.** Rheometer net penetration as a function of force for various grease grades.

The final procedure developed for assessing the consistency of a grease using the rheometer penetration test involves first applying the sample to the base plate and/or upper plate of the rheometer. The top plate is then lowered to 2 mm where the sample is trimmed, and then lowered to the measurement gap of 1 mm. The desired normal force is imposed for 20 s, and the difference between the initial and final value is reported as the net penetration value. If the net penetration is not between 15 mm/100 and 80 mm/100, then the force should be changed.

## **4. Comparison of Tests**

The procedure developed for testing grease consistency using a rheometer penetration test will now be compared with the cone penetration test, yield stress measurements, and crossover stress measurements to observe correlations among these tests at room temperature.

#### *4.1. Materials and Procedures*

In order to test as wide a range of consistencies as possible and to obtain various intermediate consistencies, greases (given in Table 2) were sheared for various intervals in a standard grease worker specified by ASTM D217. For many of the greases, such as PU2 and AlC2.1, this led to considerable changes in consistency. However, some of the other grease types, such as all the lithium complex greases tested, showed a minimal change in consistency from the mechanical shear of the grease worker. In order to obtain an even wider array of consistencies, the lithium complex greases were mixed together in various ratios and the calcium sulfonate grease was contaminated with small amounts of water (under 10 percent by weight). The procedure for mixing involves placing the desired ratio of materials together in the grease worker and working for a minimum of 500 strokes. Overall, the exact proportions of greases, the exact concentration of water, and the exact number of strokes are not relevant to this investigation. This study is exclusively focused on examining the similarities and differences among the various consistency tests using the same sample in each.

Unless indicated otherwise, all results of oscillatory tests (calculation of yield stress and crossover stress) were conducted using a 25 mm flat plate with a 1 mm gap at 1 Hz with no relaxation or pre-shear. These tests, as well as cone penetration measurements and rheometer penetration measurements, were done three times, and the average value is presented.
