Many vibration programs fail because they become too complicated. Too much data can sometimes become more confusing than too little data. Many potential machinery problems can be eliminated with the analysis if one keeps in mind several simple concepts:
1. Vibration units such as acceleration are more sensitive to high frequencies than low frequencies.
2. Vibration units such as displacement are more sensitive to low frequencies than high frequencies.
3. Velocity units are evenly sensitive between about 60 CPM to 60, 000 CPM.
4. High-frequency vibration does not travel far and degrades rapidly through metal seams.
5. In general the closer your measurement is to the source of the vibration the higher the amplitude will be.
These differences can be used to zero in on machine faults.
Example: Take a generic 100 HP motor. If an outboard rolling element bearing begins to fail because of a lack of lubrication the first indicator is high-frequency ringing from the bearing. This is characterized by a large increase in acceleration amplitude and a small to no increase in velocity or displacement. Now you have identified that there is a high-frequency problem and not a low-frequency mechanical problem. You can eliminate low-frequency sources such as looseness, unbalance, or misalignment. What is the most likely source of high-frequency vibration on the back end of a motor? Probably a bearing or shaft or rotor rub. Now you can apply a simple test. Grease the bearing and see if the acceleration returns to normal. If it does, you have nailed the problem without knowing the bearing frequencies or even taking a spectrum. Come back the next day and see if the acceleration is back up. If it is, you either have a lubrication problem with contamination or a loss of grease, a damaged bearing, or both.
While not perfect, understanding the behavior of vibration units combined with a mechanical understanding of machinery can help you quickly identify machinery problems.
Maintenance Tips, Vibration Analysis by Ana Maria Delgado, CRL