The basics of lubricating grease and in-use testing
Written by Amy Rishell and John Sander
In-Service Condition Monitoring
Analysis of in-service grease can provide useful information about the condition of the lubricant and the equipment in which it is installed. While condition monitoring of used fluid samples has been performed for many years and is mainstream, testing of semisolid grease samples is just now becoming more common, due to the development of new sampling and testing procedures within the industry.
Sampling
Grease has been found to provide more of a challenge as compared to contemporary lubricating fluids when collected for condition monitoring. This is mainly because it is not a liquid, so it does not flow as much in the application. This means that a sample collected from the improper location within a bearing may not truly represent the condition of the bearing or grease in the contact zone. In addition, many grease-containing bearings only contain a small amount of grease. Finally, due to spatial limitations, it is often difficult to collect a sample of grease from the equipment, especially when it is running (which is the recommended time to collect in-service lubricant samples).
Collecting enough sample of the used grease out of an application for testing can be challenging. The size and shape of the part, the re-lubing interval, and the quantity of grease used in the application can influence the amount that can be collected for testing. Each application can have more than one of these difficulties to overcome during sample collection.
The size of the recovered sample comes into play when the part that uses the grease is very small and the total quantity of in-service grease is less than an ounce. In this case, the part typically has to fail or be changed out in order to get a sample. Even then the sample size is normally very small, which limits the range of testing that can be performed.
The ASTM method for obtaining in-service samples of grease is ASTM D7718 Standard Practice for Obtaining In-Service Samples of Lubricating Grease. To collect grease samples from an application, available techniques include the use of a scraper, straw, spoon or any other utensil or tool that might aid in the removal of the sample from the application. On rare occasions, a sample can be collected when the grease is purged. Testing kits can be purchased that include a sampling device, such as the Grease Thief sampling tool made by MRG Laboratories.
Another challenge is purging a grease application when trying to collect an in-service or used sample. When adding grease to an application, in theory the used grease will be pushed out and can be collected. In the real world, sometimes there is no purged grease when grease is added to an application. There also is the chance that the amount put in, isn’t the same amount that is purged. This has to do with how grease is used to lubricate and the design of the application or part itself.
The decision whether the sample should be tested in the field or at a laboratory is based primarily upon the need for immediate results, but secondarily upon what information is needed. It may seem like common sense, but a more thorough analysis can be performed in a laboratory than can be done in the field. Yet, if expediency is required due to equipment production requirements, a field screening test may be sufficient.
Shipping & Documentation
If sending the sample to a laboratory, it is important to select a proper container so that the lubricant does not leak out of the container. All sorts of containers have been used to ship grease samples, including plastic bags, plastic bottles, rags, cans and disposable plastic containers. It is better to use a new container, not one that has been cleaned. Reused containers – even when they appear clean – create the possibility of contamination.
Anytime a sample is collected, in order to aid in the most thorough analysis, there are various background requirements. As much background as possible should be collected from the sample and either recorded on the sample container or sent on a data sheet accompanying the sample. If a sample of the new grease is available, it is preferable to include a new sample of grease from the same batch number, as test data compared to the used sample can be very beneficial. The side-by-side comparison highlights the changes that occurred in the used sample. This type of sample is often referred to as a baseline sample.
