Abstract
Because of the influential role of consistency in selecting a grease for a given application, accurate and meaningful methodologies for its measurements are vitally important. A new method, recently introduced, uses a rheometer to compress a grease sample to evaluate a relative consistency between a fresh and degraded grease; however, the results of this approach compared to a standard penetrometer and other methods of assessing consistency have not been studied. This paper takes a closer look at the relevant parameters involved in the rheometer penetration test and establishes a recommended procedure for its use. The consistency of various greases is then tested using this method and compared to results obtained from yield stress, crossover stress, and cone penetration tests. The results indicate that rheometer penetration may be used to assess the change in consistency for a given grease but should not be used to compare different greases. For this purpose, the crossover stress method is recommended, which is shown to correlate very well with cone penetration while using a simple procedure and allowing the use of a substantially smaller sample. A strong power law correlation between crossover stress and cone penetration was found for all greases tested and is presented in Figure 12.
Highlights
The consistency of a lubricating grease is often considered to be its most important rheological property
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, only recommended to quickly and monitor the change in consistency to a particular grease
Summary
The consistency of a lubricating grease is often considered to be its most important rheological property. The penetration depth obtained from any of these tests is usually used to assign a grade to a grease for succinctly characterizing its nature to a broad market of consumers Using specialized equipment such as a rheometer is becoming increasingly popular for assessing the rheological properties of grease. The liquid-like behavior is characterized by the loss modulus, which describes the ability of a material to lose energy through viscous dissipation and is out of phase with the input These two parameters both change as the state of stress within material changes, so imposing an increasing stress or strain amplitude (often called an amplitude sweep) in an oscillatory test is often an ideal way of examining viscoelastic behavior. The flow point will be considered the point where the storage modulus decreases enough to cross over the loss modulus, temporarily signifying a material with properties more closely resembling a liquid than a solid.
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