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Friction between a bearing’s rolling elements and races produces ultrasonic sound which has an amplitude level directly proportional to the friction. This relationship is the fundamental principle for acoustic lubrication. As the lubricant film becomes less effective at separating the metallic surfaces, friction increases in concert with the measurable ultrasonic amplitude. Restoring the lubrication film will decrease the friction and, consequently, the amplitude.
Lubrication technicians use ultrasonic detectors equipped with special contact probes to evaluate qualitatively and quantitatively the state of lubrication in rolling element bearings.
Qualitative evaluation depends on the ears of the inspector to judge what is heard through the headphones of the ultrasound detector. Using his own criteria, experience and best judgment, the lube tech listens to the ultrasonic signal of the bearing and decides if it requires greasing and, if so, how much grease to add. While there is an art to this method, there may not be very much science. The reality is that all bearings produce some level of ultrasound and, in certain situations, it is hard to evaluate if the sound is normal or abnormal, simply based on what was heard. Furthermore, it is impossible to baseline, compare or consult because it is absolutely possible for two people to hear the same signal in completely different ways.
There are acoustic devices available on the market that are based solely on qualitative interpretation. Greasing decisions should not be based on hearing alone, but rather on a combination of qualitative and quantitative data.
Quantitative evaluation is the science that makes ultrasound effective for acoustic lubrication. By the very definition of the word, “quantitative” suggests some level of measurement, benchmarking and trending will help decide conclusively the lubrication needs of the bearing. Quantitative can refer to static measurements where digital decibel metering is used for trending. Or, it can go a step further and refer to Ultranalysis, the science of analysing dynamic ultrasonic signals in the time and spectrum domains.
Static dBµV measurements are the standard best practice for lube techs, and Dynamic Ultranalysis is the advanced best practice. Where static measurements are well established and understood, dynamic measurement methods are in their infancy and a lot of the rules are still being written. But that is a discussion for a future article. For now, let us focus on static digital dBµV testing (SDT). With SDT, methods lube techs can:
Define numerical values for amplitude (baseline) VS time
Write acoustic lubrication best practice procedures that anyone can follow
Eliminate the human factor due to differences in hearing perception and ability between two technicians
Create a source of historical information to optimize lubrication and identify problems
About the author:
Allan Rienstra is the president of SDT North America and the editor of The Ultrawave Technology Report, SDT’s e-newsletter. For more information, visit www.sdtnorthamerica.com or e-mail firstname.lastname@example.org.