Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/572212
12 September - October 2015 | www.machinerylubrication.com COVER STORY logical properties within a given NLGI grade. The NLGI scale also provides gaps in the numbered measure- ments. These gaps are very large when considered in units of stress (pascals). For example, a grease defined as NLGI 2 can be manufac- tured at either end of the scale. The extremes, when compared in stress measurements, have differences in stiffness of 175 percent from the high to low end of the NLGI 2 grade. The range in pascals for the gap between the NLGI 3 and 4 greases is the same as the total range from the soft end of NLGI 00 through to the midpoint NLGI 2 (see Table 1). These gaps can be a bit of a minefield, and confusion is likely for those not familiar with the non-liner nature of the NLGI measuring system. Using a Rheometer A rheometer is an instrument capable of measuring the physical properties of a grease by varying or holding constant an applied load, a temperature and an oscillatory shear. It is successfully used in many industries with a broad variety of applications. A rheom- eter also requires minimal quantities of test material when compared to cone penetration, with typical grease test volumes of 0.4 to 0.25 ml. Use of a rheometer to test grease samples is very limited within the grease industry. While a rheometer measurement is likely to be unfamiliar to many, it can be reported in accepted scientific units (pascals) and in a manner that does not include gaps. Yield Stress Test The yield stress test is useful when comparing rheometer-de- rived data to cone penetration. This test produces a stress that ramps up from very low to high at an increasing rate. It plots a data point at even time increments. As the test runs, each successive data point plots in a near vertical pattern as long as the material is primarily elastic and in a more horizontal orientation when it becomes fluid and flows. The intercept/line fit from the flatter sloped portion of the plot to the Y axis is used to determine the yield stress. This measurement represents the stress at which the grease shifts from being a more elastic material to more of a fluid under the prescribed test conditions (Figure 2). Temperature can significantly influence the results of a yield stress test, which makes temperature control vital. This is accom- plished with a heat sink at the instrument. Some rheological tests are designed to take hours or days to complete. These lengthy tests can be expected to yield precise results but are not practical for most in-service grease samples within a production testing environment. The ideal production test would be one that provides a reasonably accurate test result and has a short test duration. The yield stress test results shown in Figure 2 were obtained by performing a series of tests to include a stress and frequency sweep within a common test sequence, with the yield stress test performed last. The tests chosen prior to the yield stress test were useful in determining the grease's characteristics. Some pre-conditioning of each grease sample resulted from the test sequence used. The initial sample was placed on a 25-millimeter flat plate and lowered to a typical 1-millimeter gap distance. The grease was then trimmed to ensure an even edge with no undercutting of the sample. Upon completion of each test within the sequence, the head was automatically lowered by software within the instrument to ensure contact between the fixture and no undercutting of the sample. The sample edge was not trimmed between tests within the sequence. The settings for gap reduction used to lower the head were the same for each sample. 0.0 30.0 Rate[s -1 ] 0.0 10.0 20.0 40.0 50.0 60.0 2500.0 2000.0 1500.0 1000.0 500.0 Figure 2. Example of a yield stress test