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Motion Amplification is a fantastic tool for visualizing many machinery problems, from structural issues to resonance problems. But what about a vibration problem at 100Hz which is electrically related. Can motion amplification help find the root cause?

One of our engineers recently picked up a Motor problem. It was showing high vibration levels of 10mm/sec RMS at a frequency of 100Hz. The motor runs close to 50Hz so he collected a high resolution spectrum to confirm the problem was electrical related and not 2xRPM. He confirmed it was electrical at 100Hz (2XLF) so decided to visualize with MA Camera. The MA video below clearly shows the motor dipping down to the right side. This has all the hallmarks of a machine soft foot problem and probably the root cause of the high 100Hz vibration due to the motor frame twisting. The motor is now planned in for the next shutdown to rectify this problem.

Soft foot is a common term used for machine frame distortion. The distortion is caused when one or more feet of a machine differ in height from the others. This in turn may be due to differences when the machine was manufactured, a squishy footage with oil film etc. between foot and base, a bent foot, or it may be induced by a pipe to which the machine is attached (e.g. a pipe on top of a pump), which prevents the machine from touching all its feet to its base.  – Wikipedia

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Bearing Defects

Identifying bearing defects using vibration analysis is not going to improve plant reliability. To do this we have to look at the root cause of the defect and take action to prevent the same problem re-occurring in the future. 

Here is a nice example from a large ID Fan, after a 3 month shut down of the plant. The fan was restarted, straight away an outer race defect was picked up on the Fan NDE bearing by an RMS engineer.

 

rms fan NDE bearing defect blog

rms fan NDE bearing defect data before shutdown blog
The damaged bearing was planned in for replacement. At RMS we are always trying to find the root cause so the bearing was cleaned and inspected. You can clearly see the bearing has suffered from corrosion whilst the fan was stationary. Spalling has started to occur in one area.

rms fan NDE bearing defect root cause spalling

In this case the root cause of the problem was that the fan was not rotated during its 3 month shutdown. This allowed corrosion to take place within the bearing leading to a defect upon start-up. Plans are now in place for future shutdowns to rotate the shafts to eliminate this type of defect.