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

Stages of Bearing Failure:

1st Stage:

1. Noise level normal
   
2. Temperature normal
   
3. Measurable increase in ultrasonic sound, acoustic emission, spike energy
   
4. Overall vibration low;
   
5. No discrete spikes at bearing frequencies
   

Remaining life more than 10% to 20% of L10 life of bearing.

2nd Stage:

1. Slight increase in noise level
   
2. Temperature normal
   
3. Large increase in ultrasonic sound, acoustic emission, spike energy.
   
4. Slight increase in overall vibration acceleration and velocity
   
5. Bearing frequencies barely visible on vibration spectrum;
   

Remaining life less than 5% to 10% of L10 life of bearing.

3rd Stage:

1. Noise level quite audible
   
2. Slight increase in temperature
   
3. Very high ultrasonic sound, acoustic emission, spike energy.
   
4. Large increase in overall vibration acceleration and velocity
   
5. Bearing frequencies with harmonics and sidebands clearly visible on vibration spectrum;
   

Remaining life less than 1% to 5% of L10 life of bearing.

4th Stage:

1. Change in pitch of noise level
   
2. Significant temperature increase
   
3. Gradual decline followed by rapid increase in ultrasonic sound, acoustic emission, spike energy
   
4. Significant increase in overall vibration displacement and velocity; decrease in acceleration.
   

Bearing fails.

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.

IRD Model 811 and 811D meters have Spike Energy measurement facility.

IRD Mechanalysis Training programme VT2 (Vibration Technology Level 2) discusses Spike Energy in greater details.

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.