Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/572212
14 September - October 2015 | www.machinerylubrication.com COVER STORY A short dwell or recovery time of less than a minute was allowed between each test within the sequence. Use of a multiple test sequence enabled each grease sample to be tested in a similar manner and with a similar level of pre-conditioning. The test that produced consistent pre-conditioning was a frequency sweep test, which was set to run for approximately seven minutes. The entire sample testing took only a few minutes longer. The yield stress rheometer test required less than a minute of the overall test time to complete. Test Data A population of 54 new or used field samples were tested using both the one-quarter-scale cone penetration method and the yield stress rheometer test. This population consisted of six different types of new greases including a polyurea, lithium complex, calcium complex, and clay and calcium sulfonate. In-service used grease samples included clay- and calcium-complex-thickened greases. Two of the samples tested were provided by NLGI to help determine cone penetration values from a group of collaborators. The mate- rial type of these samples was unknown. All grease samples were treated as unworked, with the only pre-conditioning being the process of obtaining the test sample from the bulk grease and placing it into the cup. No efforts were made to determine the level of working that the used samples received while in the field. The cone was dropped into each sample three times, and the average measured test value was reported. The tests were conducted at a laboratory ambient temperature, which was typically at 22-24 degrees C. When compared to rheometer-generated data, the non-linear aspect of the NLGI scale is clearly seen in Figure 3. Figure 4 shows samples measured by both methods and how the points correlated. The rheometer measurements are compared to the NLGI scale in Table 1. Note that the gaps between the defined consistency ranges become progressively larger with the stiffer greases. Repeatability To determine instrument and test sequence repeatability, three grease types were used in the test sequence, with multiple runs performed on each grease. The source of the grease was a single container for each type. The grease types tested included lithium 12 hydroxystearate, calcium sulfonate and clay-thickened greases. The test results are provided in Table 2. The clay- and lithium-thickened greases had a deviation of approximately 15 percent from the mean, while the third grease had a deviation of about 5 percent. The repeatability of the instru- ment was not stated. The differences found in repeatability are expected to be larger from in-service grease than from grease orig- inating from a single batch of fresh grease. The non-linear aspect of the NLGI scale can be seen in the overlays shown in Figure 4. Grease with consistencies above grade 4 as measured by NLGI were not found in machines. The accuracy of the equation for data beyond NLGI 4 is unknown. Other Applications Not only can a rheometer be used to replace an arguably outdated industry test and with less sample and mess than cone penetration, but it can also be utilized with a variety of test methods to learn more about a grease. Figure 3. Comparison of NLGI and rheometer scales Table 1. Comparison of rheometer-generated consistency to the NLGI scale Figure 4. Correlation of raw test data from cone penetration and a rheometer Table 2. Repeatability of data from the yield stress test NLGI GRADE RHEOMETER LOW END (PA) RHEO- METER HIGH END (PA) RHEOMETER GAP TO NEXT GRADE (PA) % DELTA FROM PREVIOUS GRADE HIGH TO NEXT LOW 00 56 98 32 32.6% 0 130 227 73 32.1% 1 300 527 160 32.2% 2 697 1,221 396 36.9% 3 1,617 2,833 917 32.4% 4 3,750 NUMBER TESTED AVERAGE (PA) 1 SIGMA (PA) Lithium 12 Hydroxystearate 41 797 105 Calcium Sulfonate 20 316 16 Clay 20 1,247 187