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

What is the Difference between Unbalance and Runout?

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By on September 5, 2018

Unbalance and runout are two terms that we hear frequently in the predictive maintenance community. Both unbalance and runout can be serious issues and need to be addressed to prevent inefficient functioning and damage to machines.

Unbalance occurs due to corrosion, erosion of the fan wheel, or if a correction weight is removed. Runout occurs due to eccentricity (machining error) or bent shaft. Both cause an uneven distribution of mass around an axis of rotation. Both also cause destructive forces, contribute to misalignment, and result in decreased machinery life.

Both problems can be serious. Runout, however, can be viewed as more of a root problem that needs to be corrected by replacing parts, while unbalance can often be field remedied by adding to or taking weight from an accessible fan wheel.

In order to correct unbalance, you must also know two things:

  1. The force—how much vibration is being caused by rotational unbalance?
  2. Direction—where is the heavy (or light) spot

You must also have an accurate measure of runout before you can begin to correct this issue. An accurate way to measure runout on a shaft is to use the Fixturlaser runout probe. You can view this tool here.

To read more about unbalance and runout, watch the helpful videos below:

You can also check out other blog posts from VibrAlign. Here is one on runout and here is one on unbalance.


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2 responses to “What is the Difference between Unbalance and Runout?”

  1. Sam Jones says:

    I have found over the years the coupling hub or rim needs to be cleaned and free of damage such as hammer marks. This can throw off your readings and give you inaccurate findings. The bearings being worn out can come into play with run out also and can be checked when disconnected when you do an end play check. Let us not forget the all to often worn out insert to the coupling. Overall this is good information that during alignment checks should be considered.

  2. Brad Case says:

    Sam thanks for your comments. Very good items to remember. It’s always a good idea to check for worn bearings when aligning equipment that has been in service for a given period of time. This can be accomplished with a lift check to check bearing clearances. Check out this video;