What is Spike Energy and why is it useful for Bearing Health Monitoring?
Introduction to Spike Energy
IRD Mechanalysis have developed “
Spike Energy” to detect rolling element bearing condition in its very early stage. “Spike Energy” detects low amplitude transient impacts generated within the ultrasonic frequency range by microscopic surface flaws in rolling element bearings and gears. As the vibrations are at very high frequencies, circuit utilises an acceleration signal and a high passfilter to filter out low frequencies. A true Peak-to-Peak value detecting circuit is incorporated as these signals have low RMS values.
The acceleration signal processed via a high pass filter and a peak detection circuit produces a numerical value which is the product of the number and amplitude (intensity) of the impacts in a unit of time. This is Spike Energy.
There are four failure stages in a bearing:
Stage 1: Microscopic Flaw, only seen in Spike Energy
Spike Energy gives earliest indications in the ultrasonic range. It may first appear at about 0.25 gSE or lower in the first stage which will gradually increase.
Stage 2: Size of Flaw increases, natural frequency of bearing appears in Zone C
Slight defects and ensuing impacts begin to ring bearing component natural frequencies. These frequencies occur in the range of 30k-120k CPM
Stage 3: Bearing Defect Frequencies appear in Zone B
At the end of stage 2, sideband frequencies appear above and below natural frequency. Spike Energy grows from 0.25 to 0.50 gSE. Wear is now visible and may extend around the periphery of the bearing
Stage 4: Size of the flaw keeps increasing, Spike Energy may actually decline
Spike Energy increases to between 0.5 -1.0. Discreet bearing defect frequencies disappear and are replaced by random broad band vibration in the form of a noise floor. Towards the end, even the amplitude at 1 X RPM is affected. High frequency noise floor amplitudes and Spike Energy may in fact decrease at this stage. Just prior to failure gSE may rise to high levels.
Stages of Bearing Failure:
Noise level normal
Measurable increase in ultrasonic sound, acoustic emission, spike energy
Overall vibration low;
No discrete spikes at bearing frequencies
Remaining life more than 10% to 20% of L10 life of bearing.
- Slight increase in noise level
- Temperature normal
- Large increase in ultrasonic sound, acoustic emission, spike energy.
- Slight increase in overall vibration acceleration and velocity
- Bearing frequencies barely visible on vibration spectrum;
Remaining life less than 5% to 10% of L10 life of bearing.
- Noise level quite audible
- Slight increase in temperature
- Very high ultrasonic sound, acoustic emission, spike energy.
- Large increase in overall vibration acceleration and velocity
- Bearing frequencies with harmonics and sidebands clearly visible on vibration spectrum;
Remaining life less than 1% to 5% of L10 life of bearing.
- Change in pitch of noise level
- Significant temperature increase
- Gradual decline followed by rapid increase in ultrasonic sound, acoustic emission, spike energy
- Significant increase in overall vibration displacement and velocity; decrease in acceleration.
SPIKE ENERGY IS A TRENDING TOOL, HOWEVER THE FOLLOWING CHART CAN BE USED AS A GUIDE LINE
Spike Energy Chart
Readings need to be directly taken on bearing, minimise interfaces. For consistency between measurements always use the same accelerometer. Transducer mounting makes a difference and hence follow mounting suggested in IRD manual. Spike energy or shock pulse propagate through structure at the speed of sound in steel (16000 ft/sec). Energy is lost at interfaces of two material (60 to 80%). PLEASE KEEP ULTRASONIC TRANSDUCER AS CLOSE TO THE BEARING AS POSSIBLE. As ultrasonic energy is localised it is easier to isolate the bearing with problem.
meters have Spike Energy measurement facility.
Note : Some companies use high pass filter and check vibration acceleration and call it vibration spikes or high frequency defects. This signal is just a filtered acceleration and does not indicate the true bearing condition. This is due to the fact that bearings in their nascent stage of deterioration emit very low signals in the ultrasonic range which is lost in the filtered signature carpet.
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