CM

Condition Monitoring (by CRASys)

Driving your car to the shops, you notice that there is a bump, rattle or squeak that was not there yesterday, so you decide to take the car in for a service. What you have decided to do is ``preventative maintenance'' --- the car has not broken down, but you have decided that it may breakdown in the near future if you do not do something about it.

Hearing the unexpected squeak is a form of condition monitoring. You have subconsciously been monitoring the condition of your car by simply knowing what it sounds like. If that sound changes markedly, for whatever reason, it can be a cause of concern.

In the manufacturing industry, where the whole production line depends on all the machines in the line working correctly, condition monitoring is one tactic used to schedule preventative maintenance. In industry, a machine breaking down can mean hundreds of thousands of dollars in lost production while the machine is fixed.

The condition of any critical component can be monitored. People with heart conditions often have electro-cardiograms (ECG) to monitor a variety of heart abnormalities. The industrial critical component is more likely to be a bearing, a gearbox, a motor or a transformer than a heart, although the chief executives of the manufacturing company may think otherwise!

Rolling element bearings are those that you might find supporting the pedals on your bicycle or the wheels of your skates. A rough picture of one of these sorts of bearings is shown in the first Figure.

Failure of these bearings can mean several things, depending on the application. It may mean that they stop turning, do not turn smoothly or break completely. You might think that your skate bearings have failed only after one of the wheels stops turning; a steel-plate manufacturer might think their bearings have failed if the bearings no longer turn smoothly enough to produce a suitably flat sheet of steel.

As indicated in the Figure, failures in these bearings are usually caused by problems with one of the four parts which make up the bearing: the inner race, inside which the shaft sits; the outer race, around which the wheel is mounted; the rolling elements, which separate the inner and outer races; and the cage, which keeps the rolling elements separated from each other.

Suppose a rolling element of a particular bearing has a chip in it. Every time the chipped surface meets the inner race, a small impact will be generated; every time the chipped surface meets the outer race, an impact will also be generated. These small impacts will cause a small vibration of the bearing and the frequency with which these vibrations occur will change as the rotation speed of the shaft changes.

As the bearing condition deteriorates, the energy or force of the vibration increases. Condition monitoring engineers use this energy level to determine when a bearing is no longer serviceable.

If the shaft speed is constant, then the chip-induced vibration will show up regularly. In this case, standard techniques for determining the frequency and energy of the impacts may be used. However, these standard techniques often fail when the shaft speed changes --- even if this change is only small.

The difference between the constant shaft speed vibration and the case where the speed varies is illustrated below in the second Figure.

Many bearings do not rotate at constant speed. The bearings on the wheels of your bicycle do not, nor do those on your skates. In industry, many bearings also vary their speed.

CRASys researchers are investigating the case where, over a short time-frame, the speed of the shaft turning the bearing changes significantly. Being able to do this will give CRASys clients in the manufacturing industry a clear advantage in the scheduling of maintenance stops in production.

Next time you get into a car and you hear the familiar sounds of the starter motor as the ignition key is turned, the engine coming to life and the car being driven, remember that what you are doing is monitoring the condition of your car. That new squeak which just appeared may mean the difference between getting to where you are going and being stranded by the side of the road on the way there.

Below is an example situation where condition monitoring is being used by CRASys to determine the expected lifetime of bearings.