Condition monitoring: modern answers - age-old methods, Part 2

July 2006 Maintenance, Test & Measurement, Calibration

In the June 2006 issue of SAI&C, we looked at a bit of the history of SPM Instrument, specifically in South Africa. We did not discuss why it was so popular in its heyday or even what the method does. We trusted that the bit of nostalgia was enough to trigger your memory.

Now we will get down and dirty (or wet). SPM orginated from the shipping industry. A Danish shipowner was getting very upset about untimely bearing failures on his ships. He then commissioned a certain Eivind Sohoel to develop a method of predicting these failures. But how to do it? This was the sixties remember - no Internet or computer to help search for methods. The screwdriver was the answer! When people listen to a bearing they can get a fairly good idea of how well the bearing is doing. However, everybody hears and interprets the sound differently.

First take out the ear by replacing it with a microphone. But we still have a problem! Environment noise can distort the noise picture. Is it not too late when a bearing is making an audible noise? Should one not try and pick it up sooner?

Using a microphone that measures in the ultrasonic range resolves this issue. During the research Sohoel found that sharp repetitive shocks (ultrasonic sound waves) are generated by bearings.

It was also established that the shocks are generated by impact. But what influences the amplitude of these shocks?

Tests also showed that the impact amplitude is a function of the impact speed as well as the shape of the impacting edges. If one takes the shape and depth as an indication of damage (one certainly does not want craters in the rolling element or bearing races) then the only variable left is the speed. If one knows the rotational speed of the bearing and its diameter one can calculate the circumference speed. By then testing a lot of 'good' bearings one can empirically determine the relationship between speed, the bearing diameter and the impact shock amplitude.

Thus having done this one is able to use the absolute shock level measured on the bearing and by subtracting the speed and diameter influence one has a residual value which can be linked to bearing damage. By then testing various bearings in different damaged states one is able to empirically determine the shock levels that spells disaster. It is that simple! Next issue we will look at what else the shock pulse method can reveal.

For more information contact Vegter Condition Monitoring, 011 849 6876, [email protected]

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