Machinery Lubrication

Machinery Lubrication Nov Dec 2013

Machinery Lubrication magazine published by Noria Corporation

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Page 30 of 81

GREASES BY WES CASH AND RICK JAMES, NORIA CORPORATION Grind for In-Service Gage Grease Analysis Using a O Oil analysis is commonly used across industry not only to analyze oil health but also machine health. Recently, advances have been made in grease analysis. With the increasing demand for machines to operate fault-free, it has become even more critical to understand what is occurring inside of them. Although technologies such as vibration, thermography and motor-current analysis can offer early signs of machine failure, analyzing in-service oil and grease can help refine your detection ability. By periodically sampling the oil in a machine, you can obtain information about its health. The same is true for grease-lubricated components. With 90 percent of bearing applications being grease lubricated, it makes sense to apply the same methodology for analyzing the lubricant in these cases. test other properties of the grease and its constituents. Some of these tests can be expensive and may take several days before results are received. This has created a need to analyze in-service grease in the field and obtain instant feedback on condition, contamination and wear debris content. A study published by SKF indicates that roughly 70 percent of bearing failures are due to contamination. Another study by NSF found that contamination caused nearly 50 percent of bearings to fail. By applying the Pareto principle (the 80/20 rule), you can see that addressing contamination in bearings should prevent some if not most failures. Accessories such as shields and bearing isolators can be added to make bearings last longer, but you must be able to analyze the amount of contaminants in the lubricant to know how clean or dirty they are. Field tests for oil analysis are readily available for almost any property you want to test, including acid number, viscosity, water content, etc. While there are some field tests for greases, they are primarily limited to rough estimations of consistency and oil content. Whether in the field or lab, few practical methods of analyzing particle concentration exist for in-service grease. In fact, there are only three methods available: microscopic analysis (FTM 3005.4), scratched acrylic plates (ASTM D1404) and fineness-of-grind gages (Hegman gages). This article will discuss the fineness-of-grind gage as a practical tool for both lab and field applications. Gage Design Freshly purged grease should be collected and analyzed for wear debris and contaminants. The value of grease sampling has been recognized and is now even an ASTM standard (D7718-11). This standard describes a method for taking a representative grease sample from an in-service bearing. While this is the first step in grease sampling, the second phase involves conducting tests on the sample. Several laboratories can perform grease analysis and check for contamination, changes in viscosity or consistency, as well as 28 November - December 2013 | The gage's design features a large block of steel (typically stainless or chromium plated) and tapered grooves or "raceways" machined into the surface. One groove may range from a depth of 0 to 250 microns, with a second groove from 0 to 50 microns. A wiper blade is fabricated from the same material as the block. It draws the sample across the block's surface for analysis. The wiper blade and the surface of the block are milled as smooth as possible to allow for zero clearance between them during the testing process. After many uses, the wiper may lose its profile and may need to be retooled.

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