MARKETING MATTERS PdM TIPS CAST TIP
A Bearing Deserves to Live a Long Life A
ALAN F. GAMBSKY, ARROWCAST INC. (SHAWANO, WISCONSIN)
s we age, most of us begin to realize our abilities to do some activities begin to change.
T ings that used to show our strength and endurance suddenly reveal our weakness or require longer periods of rest. Our bodies begin to show the physical wear and tear of a lifetime. T is is because we have a control cen- ter telling us what hurts, and how to avoid more hurting. We fi gure out how to do things without risking more hurt or collateral damage. Machines are not so equipped.
T ey still have a birthday, an an- ticipated life cycle, and either a slow, controlled death or a catastrophic, violent collapse. Our job in main- tenance is to assist in that birthing with proper preparation and correct procedures in the right environment using the best tools applied by a trained technician. T roughout the life of that asset, we must operate it within designed limits and protect it from accidents or illness while mak- ing periodic adjustments to compen- sate for any changes that occur to the machine, application, or environment. Figure 1 shows the ultrasound
time wave from a bearing that has been in service for nearly eight years. When you look closely, you can see a few spikes or impacts that exceed the height of the average ultrasonic peak. Microscopic defects are exposed as scratches and dents in the components of the bearing and reveal their exis- tence every time the rolling elements collide and react with them. Of course, the machine doesn’t know it’s getting older and has no way of correcting for this opera- tional restriction. It will just consume more energy to do the job it was designed to do. It might require lubrication more often now that seals aren’t quite as functional as they used to be. Or, if this had been a new bearing, it wouldn’t know the reason it’s show- ing this early distress level
is because it wasn’t installed prop- erly or it was contaminated. At this point in the bearing’s
life, a signifi cant amount of ser- vice is still possible if proper re- medial action is taken. In the case in Figure 1, the proper process was followed. Corrective measures included realigning the bearing on the shaft and tightening the holding screws. No interruption to the production schedule was required, and the maintenance was completed in a very short time. No collateral damage occurred to the shaft or the remainder of the system. Fast forward a couple years, and the
Fig. 3. The peaks and valleys relate to the frequency of a failure in this FFT chart.
Fast Fourier Transform (FFT). An FFT chart can be displayed in
bearing is talking again (Fig. 2). Notice the peaks are more pro- nounced and the average wave shape is bigger. Adding more lubricant at this point doesn’t have the desired eff ect of lowering the average height of the wave form anymore. Even though while applying the lube we see a tem- porary reduction in noise level, it al- ways comes back to the same “shape.” Most often, as we’re adding lubricant, the noise may increase as well as the temperature. Routine oscillations (because this bearing was installed on an eccentric shaft) suddenly get peaks or even peaks between peaks. We can analyze these frequency peaks using what is called a “frequency analysis method” that will take the sound wave and reveal the vast num- ber of noise frequencies that are most dominant. Mathematicians call this a
many ways. Figure 3 is one of them. T e vertical lines that seem to be high- lighting the peaks and valleys on the chart relate to the frequency of a cer- tain type of failure that the bearing is beginning to experience. At this stage of the bearing’s failure, it would take a very experienced technician to know a failure was starting without the chart. T ere is no audible noise or heat being generated this early. Will the bearing ever get better? No. Stage 1 failure describes when
the bearing simply needed lubrica- tion. Stage 2, as shown by the FFT, is irreversible, and diligent maintenance intervention will only mildly delay the progression to failure. It’s time to be careful with how you run this machine; you may want to pick up the pace of monitoring and planning for the eventual replacement. Stage 3 is signaled by audible noise and noticeable heat. You will easily feel the vibration, and if you have a vibra- tion monitor, you’ll be getting warning or alarm levels. All-out failure might still be way off , but your time is running short. When the bearing
Figs. 1-2. (Top) Shown is the ultrasound wave from a bearing in service for eight years. (Bottom) This chart is the same bearing a few years later.
reaches Stage 4, the useful life of the bear- ing is over. Those bear- ing component fre- quencies may no longer even be distinguished among all of the noise and vibration being generated. It’s already at a point where col- lateral damage to other machine parts may be taking place.
March 2017 MODERN CASTING | 73
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