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Two primary challenges exist when performing condition monitoring of in-service grease. These challenges include obtaining a representative sample of the bulk grease and in obtaining enough sample volume to be able to produce meaningful data. Grease testing is further complicated when mixtures of more than one grease is in place as grease mixtures can degrade the overall grease performance. This paper will discuss a technology that uses minimal grease to provide rheological information about the consistency of the grease and how this same technology may be used to gain insight on how a mixture may be expected to perform when placed in service. Grease is an important lubricant type largely because it remains in service in the proximity of what is being lubricated. When the grease structure no longer supports this function, the grease may become unable to properly lubricate simply because of geography. An important failure mode of grease is when the grease becomes overly stiff or conversely loses its structure or consistency. When this occurs, the grease is not positioned properly or may be incapable of providing or releasing oil. Cone penetration is the ASTM standard method for use in determining the consistency of grease. This test method may not be useful when applied to condition monitoring as large grease volumes are required for this test and this amount of grease is seldom available from a used or in-service grease application. A stress rheometer is a highly repeatable instrument that produces meaningful measurements that can be quickly obtained from an in-service grease sample. This instrument can provide additional insight beyond what a cone penetration test can accomplish into how grease is expected to perform. As only small volumes of grease are needed, the use of a stress rheometer is well suited for in-service condition monitoring as the actual in-service grease can be included in the mixture study.
grease, condition monitoring, rheometer
Palo Verde Nuclear Generating Station, Tonopah, AZ