When learning vibration analysis, we are taught that determining bearing defect frequencies is as simple as a mathematical formula. Simply input some numbers and presto! While this is the fundamental technique needed to determine these frequencies, there are some other important considerations that need to be made.
If you find that you have a frequency that you believe is a defective bearing but you just cannot get the frequencies to line up with your fault frequency overlays, here are some things you need to consider.
Check the true running speed of the machine. You may have the machine identified as an 1,800 rpm machine that is actually running at 1,780 rpm.
Ensure the bearing characteristics match those in your vibration database. Most vibration software comes with a database of bearings. You may find your bearing number in that database; however, the actual bearing may be slightly different. The bearing may have one more or one less ball than the selected bearing.
Determine if axial loading is occurring. High axial loading will change the roller path and thus effect the frequency calculations. As the axial load increases, the rollers have less distance to travel because they are being forced on the raceway shoulder.
Determine if wear is an issue. As a bearing wears over time, the distance the rollers have to travel will increase as a result. The rollers will actually decrease in diameter as wear occurs.
Determine if you have identified the right bearing. Occasionally when equipment is rebuilt, the original bearings are replaced with a similar bearing of a different brand. This can lead to differences in bearing defect frequencies.
Ensure you have enough resolution to separate defect frequencies from running speed harmonics.
If you follow these simple guidelines for determining the proper bearing defect frequencies, you will find that your fault overlay will match your suspected bearing defect frequencies a much higher percentage of the time.
About Ludeca Inc.
Ludeca is the exclusive distributor and factory-authorized service and training center for PRUFTECHNIK alignment systems and condition monitoring products in the United States, the Caribbean and Venezuela. It is also the manufacturer and distributor of DotLine Laser and SheaveMaster Pulley alignment tools. For more information, visit www.ludeca.com or call 305-591-8935.