Machinery Lubrication magazine published by Noria Corporation
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Not only does this field test provide a way to quantify particle concentration in grease, but it also offers a crude indication of particle size. When the test has been completed, inspect the channels for streaks left behind. Where the streaks begin relative to the depth of the channel will give you an estimation of the particle size. Hard particles will leave behind In a recent experistreaks in the grease and can be ment, two tests were given a rough size estimation based upon where they appear conducted: one with on the scale. new grease applied directly from a grease gun and another test with in-service grease taken from a wheel bearing. When the results were compared, a few differences were observed, including the amount of streaks found in the samples. The new grease spread evenly down both channels with little to no streaks, indicating no hard particles in the grease. On the other hand, the used grease sample showed obvious signs of hard contaminants. the grease in the bearing. However, dry, old grease that is simply scraped from the shaft/seal interface will have more ambient contamination (airborne particles), making it harder to obtain any current information about the state of the bearing. This type of grease is historical data from the bearing and doesn't represent what is happening currently. Cutting wear particles (top) are generally indicative of hard, solid contaminants. A spherical wear particle is typically seen from fatigue cracks in rolling-element bearings. This magnified sample particle appears to be a metal chunk possibly caused by overloading and poor lubrication. Be sure to conduct this test on new grease to find out what its "streak" profile looks like in the raceways. Keep in mind that some solid additives such as graphite, moly, etc., may appear in both new and used grease samples and should be accounted for when evaluating a sample's total contamination. As the demand increases for machines to operate without failure for longer and longer periods, the need for accurate information on the operating condition of these machine parts will continue to become more important. This simple test should not be used to replace laboratory testing but rather to supplement it and provide more immediate information. References Doubleday, D. & Barkman, A. (1950). Reading the Hegman Grind Gage. Paint, Oil and Chemical Review. This is an example of a hard particle found in a grease sample. Although knowing the size and concentration of particles suspended in grease is beneficial, this test can be taken a step further to analyze the particles and identify the contaminants. By looking at the particles under a microscope, you can begin to distinguish environmental contaminants from wear debris originating in the bearing/race/cage assembly. These findings can then be used to determine wear patterns and modes as well as give feedback on any contamination control devices being utilized. This test offers the most information on grease that has been directly purged from the bearing's core. If the grease can be sampled, either through a grease purge trap or a grease sampling device, it will provide direct information on the current state of Lafferty, G.J. & Gross, H.M. Application of the Hegman Gage to Medicinal Particle Fineness in Ointments. Journal of the American Pharmaceutical Association, Vol. XLIV, No. 4. About the Authors Wes Cash is a technical consultant with Noria Corporation. He is a mechanical engineer who holds a Machine Lubrication Technician (MLT) Level II certification and a Machine Lubricant Analyst (MLA) Level I certification through the International Council for Machinery Lubrication (ICML). Contact Wes at wcash@noria.com. Rick James is an industrial services technician with Noria Corporation. He holds a Machine Lubricant Analyst (MLA) Level I certification and a Machine Lubrication Technician (MLT) Level I certification through the International Council for Machinery Lubrication (ICML). Contact Rick at rjames@noria.com. www.machinerylubrication.com | November - December 2013 31