Ultrasonic lubrication monitoring is not practical for all situations. A bearing turning 25 rpm will not emit the same acoustic energy as one turning 1800 rpm. Quality of the sound is used instead of an increase in dB level. You also do not want to under grease a slow turning bearing that the seal has been blown on. For example, if a seal is blown in a conveyor head or tail bearing then enough lubrication should be added to flush the contaminates out of the bearing on a regular basis. This bearing should also be scheduled for replacement as plant conditions permits.
As the lubricant film reduces, the sound level will increase. A rise of about 8 dB over your established baseline accompanied by a uniform rushing sound will indicate lack of lubrication. When lubricating bearings slowly add one-third to one-half of the calculated volume. The technician should cease greasing if any of the following conditions are met: the dB level returns to baseline and then starts to rise or the pre-calculated amount has been met. Be careful because some lubricants will need time to run to uniformly cover the bearings surface. Lubricate it slowly - a little at a time. The dB levels at the start of lubrication and dB level after being lubricated, along with the amount of grease applied to achieve the desired dB should be recorded and kept for future surveys.
Over lubrication can cause more damage than if a bearing were not lubricated. When high-speed bearings are sufficiently lubed, extra-added grease can cause excessive viscous drag. This leads to a whole myriad of new problems. The elevated temperature can lead to the excessive bleeding of the oil from the thickener, making less available to be in place to lubricate properly.
The elevated temperature can also greatly increase the oxidation rate and reduce the life of the lubricant. Elevated base oil temperature may take the lubricant viscosity below the range it was originally designed for. This decrease in film thickness can result an increased wear and in some cases, component failure. If excess lubricant cannot be removed, than all one can do is wait for the temperature to recede and stabilize while the aforementioned wear may or may not be occurring. Obviously this would be a case to illustrate what can happen with a misguided application of a good thing.
Tip provided by By Melissa Smith, Regional Analyst-Cedar Rapids, IA client site and Mike McCarty, Oil Analyst-Eddyville, IA client site, Allied Reliability
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