Categories: Shaft Alignment,Machinery Diagnostics,Condition Monitoring,Machinery Maintenance,Other Topics

Shaft Distortion and Electric Motors

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By on July 15, 2016

Although shaft distortion due to misalignment affects more than just the electric motor, here are a few things that are a telltale signs you may have an alignment issue.

Rotational Centerline

In a perfectly straight shaft, the centers of each shaft cross-section from end-to-end of the shaft lie on a straight line. A shaft is bent if that is not the case.

shftbndgwmisalign

In a bent shaft, the axis of the shaft is different than its axis of rotation. The range of the orbit or gyration caused by the shaft bend can increase with increased degree of misalignment.

Detection Using Vibration Analysis.

Symptoms: Twice line frequency (100 or 120 Hz) radial

Stator problems will generate high vibration at twice the line frequency (100 or 120 Hz). Stator eccentricity produces an uneven stationary air gap between the rotor and stator that produces a very directional source of vibration.

A stator may become eccentric due to soft foot.

Spectrum: The peak at 2xLF will be high.  For a two-pole motor this peak will be close to 2xTS running speed – you will need sufficient resolution to separate them.

A live spectrum may reveal beating – the 2xLF and 2X peak may appear to rise and fall if you do not have sufficient resolution to separate the peaks.

The vibration will be strongest at the point(s) where the stator is closest to the rotor.  Move the accelerometer around the motor housing to see if the peak is highest in one or two locations.

Waveform: The waveform will be a combination of 1xTS, possibly 2xTS and 2xLF and may therefore include a “wobble” or take on the “M” or “W” shape.  Beating between 2xLF and 2xTS (2-pole motor) may be observed if the time waveform covers more than a few seconds.

TS = Turning Speed

LF = Line Frequency (50 or 60

Eccentric rotor

Damage to the bearings, rotor and stator will also be evident due to the distortion of shafts due to misalignment.

shaft distortion electric motor

 

 

 

About the Author

Mac was a Journeyman Millwright at a Kimberly-Clark paper mill in Washington State before moving into a Maintenance Team Leader position. Later, he moved to Kentucky where he went to work for Wausau Paper as Maintenance Team Leader on a new mill start-up. This is where Mac was first introduced to VibrAlign. “I was so impressed with the people, passion, and products of VibrAlign. I had always hoped to one day give back some of the knowledge I have been fortunate enough to gain from others in my field. VibrAlign has given me that opportunity.”

2 responses to “Shaft Distortion and Electric Motors”

  1. Taylor Bishop says:

    Thanks for going over some signs of an alignment issue in electric motors. You mentioned that the range of the orbit from the bend can increase the more it’s misaligned. I’m interested to learn more about how you can fix this and what to do if the misalignment is small or if it is large. http://www.vibrationsystems.com.au/nea-neg-ned-neh-nes

  2. Mac MacCormack says:

    Taylor,
    Thank you for your comment. The fix is to perform a precision shaft alignment. The common alignment tolerances are based on rotational speed, for instance in the case of an 1800rpm motor the maximum misalignment is .004″ offset and .0007/1″ of coupling diameter angular. Of course, higher speeds require tighter tolerances.
    Hope this helps,
    Mac