Vibration Analysis

Finding Problems with Bearings and Rotating Equipment Using Vibration Analysis
Bearings with rolling elements generate several frequencies which can be calculated and detected if you know the physical dimensions of the bearings and the R.P.M at which they are running. These frequencies can be recorded with an accelerometer and a spectrum analyzer. Many companies sell equipment and software for trending of this data. When there is a difference from bearing fault frequencies, a person experienced in reading these printed charts and trends can predict when a bearing may fail.
Four Stages in Bearing Failure Are Detected with Vibration Analysis:
1. The first stage (normal operation) appears at ultrasonic frequencies from about 1,200K to 3,600K CPM (cycles per minute). At this point the frequencies are evaluated by Spike Energy and Shock pulse instruments which listen to these frequencies. Trending this information can tell a person if there is a change or not.
2. The second stage of bearing failure defects begin to ring bearing components natural frequencies, which are picked up with a spectrum analyzer in the middle of the spectrum, 3OK-12OK CPM.
3. In the third stage of failure, bearing defect frequencies and harmonics appear on the spectrum as bearing defect frequencies. At this time if you remove the bearing, you can see the defects in the rolling elements.
4. Stage four appears toward the end of bearing life. It shows up as random high frequency vibration spikes on the spectrum, all running together. With vibration analysis, many other problems with rotating equipment can be diagnosed without taking equipment out of service. This can save hours of downtime and thousands of dollars.
Bearing Vibration Data
Frequency – as related to vibration – is the number of times an impact occurs during a specific period. Frequency is measured in Hertz (cycles per second) and CPM (Cycles Per Minute).
1 Hz = 60 CPM
Predominant Frequencies generated by bearings are:
BPFO    Bearing Outer Race Frequency
BPFI     Bearing Inner Race Frequency
BSF      Ball Spin Frequency Rolling
FTF      Fundamental Train Frequency
These frequencies and multiples of these frequencies show up as spikes on a vibration analysis spectrum when bearings begin to fail.