Since each type of mechanical defect has its ownunique vibration characteristic, any change in vibration is a signal the mechanical health of a particularmachine is changing. Keeping this in mind and applying a systematic method of monitoring vibrationon a piece of equipment at set intervals, any increase in vibration level recorded over time providesan early warning of impending trouble. Once this change in vibration level exceeds preset tolerancescorrective action should be initiated to prevent a costly breakdown failure.

Maintenance personnelcarrying out routine inspections of plant equipment for machine health should have a goodfundamental understanding of the three basic characteristics of vibration.

1. The amplitude ofvibration or displacement (mils p-p)
2. The frequency characteristic of vibration (CPM)
3. Therelationship of the above (2) characteristics of vibration measured in the form of velocity (in/sec peak)

Throughout years of experience the maintenanceindustry has established general vibration standards, which in simple terms identifies that if thevibration level on a machine is below a certain level, good maintenance free operational life can beexpected from it. If the overall vibration of the machine rises above this tolerance level the operatinglife before mechanical failure is inversely reduced. As a general rule machines operating between 800to 10,000 RPM should have an overall vibration level of 0.2 in/sec. or less as measured at eachbearing location if good operating life is to be achieved.

Now keeping the discussed facts in mind andwith a good quality vibration meter a plant maintenance department can now proceed to set up avibration measurement program, that will establish the present mechanical health of any piece ofequipment using the following guideline.

1. Sketch a simple layout diagram of the machine to betested complete with bearing locations to be tested, marking the bearing points 1,2,3 etc.
2. Write inmachine details of identification, shaft speeds, normal loading conditions.
3. Record the units ofmeasurement that readings will be recorded in.
4. Record readings at each bearing point in thehorizontal, vertical and axial directions.
5. Examine the data to see if the recorded vibration fallsbelow the accepted tolerance level, if it does, then the machine tested can be considered to be in goodmechanical condition. If a vibration point is above the established tolerance then furtherinvestigation as to the cause would be in order. This could be carried out by a physical examinationof the components involved or by a person with vibration diagnostic experience and a moresophisticated vibration analyzer.
6. Establish baseline vibration levels for machines operating withingood vibration tolerances, by choosing one specific test reading, either a horizontal or vertical valuefrom each bearing point tested, and one axial value from each machine tested.
7. Take readingsinline with the values chosen for baseline comparison over successive months and record the resultson a graph. The results will produce a mechanical deterioration curve which will accurately pinpointthe time frame in which the machine should be serviced.

The previously discussed procedure hasserved the maintenance industry very well in timely scheduling of overhaul work on machinery thatfailed due to low frequency vibration (vibration below 10,000 CPM) such as unbalance, mechanicallooseness due to wear, misalignment, dirt build-up, metal fatigue, foundation deterioration and plainbearing failures. But it has not provided reliable monitoring for anti-friction bearing wear and failures.

For this we must take a second reading at each bearing point to record the high frequency vibrationgiven off by defective bearings. To accomplish this, various instrument manufacturers have fitted theirvibration meters with a special frequency filter setting which cancels out the low frequency vibration,but amplifies the ultrasonic vibration given off by bearings as they deteriorate. Although allmanufacturers measure the same characteristics of the bearing vibration they often record them bytheir own patented units. For the purposes of this discussion we will use the IRD units of G/SE(Spike Energy).

Through a number of years of research we now have a set of vibration tolerances foranti-friction bearings which establish the level of wear that has occurred within a bearing. Using thistolerance as a guide line and taking successive readings at the same point on a machine bearinghousing over a period of time, the failure point of a bearing can be determined with fair accuracy.

When taking bearing condition readings with instruments that have this capability there are a numberof factors that must be taken into account which can produce misleading vibration data.

1. Readingsmust be taken on a solid casting perpendicular to the bearing.
2. Pumps that are cavitating willproduce spike-energy vibration.
3. Chain drives with dry chains or misaligned sprockets will generatefalse energy readings.
4. Worn gears in close proximity to the bearing also give off spike energyreadings.
5. Dry seals rubbing on a shaft produce high frequency noise which can produce misleadingdata.
6. Bearings loose in their respective bearing holder will create spike energy readings (althoughthis could be interpreted as a form of bearing failure).
7. Machines that operate near their criticalspeed or have resonant frequency structures produce false data.
8. Bearings with plastic cages willtend to produce low level spike energy readings even with advanced wear, but will produce unstableunbalance characteristic vibration.

When checking bearings on machines operating over 1000 RPMand using instruments calibrated in g/SE (spike energy) a quick check general rule for bearingcondition is as follows

1. Readings of 0.5 g/SE bearing is new or in good condition.
2. Readings of1.0 g/SE bearing is reading the advanced wear stage.
3. Readings of 2.0 g/SE bearing is starting tofail.

When checking bearings on shafts that rotate below 1000 RPM special considerations have tobe taken into account such as background noise before the condition of a bearing can be established.More sophisticated instrumentation with graphic readouts can better detect faulty bearings in thelower speed zone, but these instruments are too expensive for many maintenance departments andrequire special training of personnel to interpret the readings properly.
Larry Bowler is with BowtinIndustries Ltd, located in Saskatoon, Saskatchewan.