Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/572212
Table 3. Comparison of "same" greases Figure 5. Frequency sweep of similar greases with different manufacturing processes CONE PENE- TRATION (MM) YIELD STRESS (PA) FREQUENCY SWEEP 0.1 R/S (PA) FRE- QUENCY SWEEP 1 R/S FRE- QUENCY SWEEP 10 R/S FREQUENCY SWEEP 100 R/S Kettle 277 1,136 13,290 22,219 28,596 51,933 Contactor 292 871 7,669 10,304 13,974 19,737 As mentioned previously, variations within a grease manufacturer's product label can and do occur. These are captured as part of a manu- facturing tolerance, which includes instrument accuracy, and in the case of cone penetration, a range defined by the NLGI scale. However, the cone penetration test is limited and cannot be expected to determine if a product manufac- tured by a different process and under a single manufacturer's label is truly the same or if significant fundamental differences may exist. A recent study compared simple lithium grease manufactured from the same starting materials but with different manufacturing processes. The grease samples came from batches made with either a Stratco contactor or a kettle. These greases were manufactured as equivalents and compared well through several tests. Both were given an NLGI 2 classification. Samples from these greases were then measured using rheometer techniques and a common rheometer test method defined as a frequency sweep. This method applies a constant stress between a fixed lower plate and an oscillating upper plate, beginning at 0.1 radians per second and ramping up to 100 radians per second at a constant temperature of 30 degrees C. The grease in this test is eval- uated in a dynamic environment, and its response measured. The samples from the kettle and contactor were tested for yield stress, cone penetration and frequency sweep. The data obtained is shown in Table 3. It should be noted that these samples are the same according to the NLGI scale but vary by nearly 25 percent based on the yield stress rheometer test. Although described as similar, the greases tested in a very different manner. Under the defined test conditions, one grease appeared to have almost twice the elastic strength of the other grease. The magnitude of the G stresses measured for each grease was so different that an argument could be made that the greases were not the same and might be best suited for different applications. Without a discrimi- nating test, the potential exists that these greases could be packaged under the same label. Indeed, this is likely to occur. In conclusion, a rheometer has proven to be capable of providing data similar to cone pene- tration with better accuracy and test results in accepted scientific units. This avoids the use of a seemingly arbitrary scale that could easily cause users to make incorrect assumptions about a grease. In addition, rheometer testing can be performed with a very small sample, which is an important consideration when condition monitoring based on used grease samples is the objective.