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


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By on March 24, 2016

Unbalance is another common machinery vibration problem.  Unbalance is the uneven distribution of mass around a rotating axis.

Balance Photo 1Any rotating machine can have an unbalance problem, not just fans.  It is important to note that once rotating machinery components are balanced, they should stay balanced until something changes.  These changes are normally:

  • Dirt, paint, or other foreign matter build-up.
  • Rust or broken blades, vanes, etc.
  • Missing fasteners in the rotating element.

In other words, something has to be either added, or taken away from, the rotating element for it to become unbalanced.


  • Cleaning- one of the most common causes of unbalance is material build-up around the rotating element. And cleaning the rotating element is one of the easiest ways to balance, or to confirm that balancing is needed.  Scraping, pressure washing, sandblasting-all are methods of removing unwanted material from the rotor.
  • Proper Machining and Assembly -Rotating elements should be manufactured so that mass is equally distributed. Mounting machine components eccentrically can cause an unbalance condition.
  • Balancing – When it is confirmed that the rotor is clean, and properly machined and mounted, the rotor can be balanced. The two most common methods for balancing are shop balancing as part of the manufacturing process (normally done on a dedicated balancing machine), or in situ balancing, or balancing the machine in the field.  While balancing need not be complicated, it can be dangerous.  Balancing training is recommended before attempting to balance a rotor.


  • Bent shaft
  • Improper assembly
  • Machining errors
  • Looseness
  • Improper or damaged machine bases or frames
  • Resonance
  • Over-or under loading rotors

If you are interested in browsing tools and equipment to help you diagnosis and correct unbalance, browse VibrAlign’s online store here.

About the Author

Stan Riddle joined VibrAlign in 2008. He has over 35 years experience in aligning industrial machinery. Stan received his AAS Degree in Machinist Technology from Surry Community College in Dobson, NC, and also holds a diploma in Industrial Systems Technology from Forsyth Technical Community College in Winston-Salem, NC, where he was also an instructor in the program.

Stan began his maintenance career working as a machinist and millwright for companies such as Weyerhaeuser, R.J. Reynolds, and Tyco Electronics. He also has over 25 years experience in Predictive Technologies, such as vibration analysis, thermography, oil analysis, and ultrasonic inspection. He is a certified Level III Vibration Analyst with the Vibration Institute, and is a Past Chairman and Board Member of the Piedmont Chapter.

Stan and his wife live in Yadkinville, NC.

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