Up Time Magazine - December 2008/January 2009 - (Page 53) Similar Beginnings Interesting enough, vibration analysis and ultrasound have similar beginnings. The accelerometers that vibration analysis uses rely on piezoelectric crystal to detect the changes in acceleration as a body oscillates or moves in a repetitive motion. The contact rod that airborne ultrasound uses in bearing inspection also has piezoelectric crystal that moves in response to the amount of ultrasonic signal generated from the friction energy as the bearing rotates in its housing. What differs is the specification of the crystal, the way the signal from the crystal is processed and the resulting information that comes out. followed, to ensure the validity of data. It is quite easy to get an erroneous reading. The inspector should always try to contact the spot closest to the bearing, because then it is likely that the reading is from the bearings and not from something else. What energy did he pick up? Friction? Impacting? Or Both? For example, I recently conducted both a vibration and ultrasonic test on the outboard bearing of a pump for a customer. The ultrasonic reading was higher than normal. Sound quality was rough, similar to the sound produced when you have sand between your wet hands and you rub your hands together in a rotational motion instead of a back and forth movement. What could I deduce from this information? Namely, that there was more ultrasonic energy being produced than before, and that the sound was rougher than it used to be. Could I say that there was an outboard bearing problem based on the higher reading and the sound quality that I heard through the headphones? No, at least not yet. Ultrasound will give an indication of the change in ultrasonic level and the quality of the sound heard (subjective information), but it cannot tell the specific cause of the change. Some highly experienced ultrasonic inspectors may be able to have a pretty good idea of the bearing condition based on the sound quality, but to transfer that experience to another ultrasonic inspector is difficult. This is when vibration tests come in. It is important to get more detailed information from another source in order to have a better picture as to the condition of this pump. And vibration analysis fits in perfectly in this type of situation. Vibration Analysis detected that there was serious impacting happening in the bearing housing. Visual inspection showed grease coming out from the bearing seals, a similar situation to that in Figure 4. All three inspection methods – ultrasound, visual, and vibration – gave me much more comprehensive information to make the proper recommendation. This is a simple example of how the different condition monitoring technologies can be used together to increase effectiveness. Figure 4 - Overgreased Bearing Sound Can Be Misleading We live in a real and complicated world, and sometimes the real world gives us combinations of events that throws us for a loop and changes our perception. One time I was in another customer’s facility doing routine machine inspections. The ultrasonic reading was higher than normal on one pump in particular – high enough that ultrasonic guidelines called for possible incipient bearing failure. Spectra from machine vibration did not indicate any problem. I began scratching my head in thought. Hmmm, why was there so much more ultrasonic signal being generated from this pump? I went to operations to ask a few questions. They told me that there was an increased flow going through the process. Of course, this meant that the machines were doing more work and, hence, a higher turbulence was being generated by the pumped liquid. The machines were designed to handle the increased workload. So the ultrasonic energy level changed, but not because of a physical defect on the bearing. I learned the lesson that going back to the basic understanding of where and how ultrasonic energy can be Figures 2 and 3 - It’s important to get the ultrasonic probe as close to the bearing as possible for accurate readings. Two (or more) is Better Than One For Ultrasound PdM, or the bearing inspection application of ultrasound, the inspector gets a reading from his ultrasonic detector that relates to the amount of ultrasonic energy generated from the bearing. Care must be taken, and proper procedure should be www.uptimemagazine.com 53 http://www.uptimemagazine.com
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