Categories: Shaft Alignment

Coupling or Shaft Runout

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By on October 3, 2013

On rotating machinery, runout is defined as the degree to which a shaft or coupling deviates from true circular rotation.  Every shaft or coupling has a center or rotation, or centerline.  Any stray from concentricity is considered runout.  If runout is severe, it can cause many problems with equipment, such as:

  • Excess vibration
  • Seal wear
  • Bearing damage

Runout can be easily overlooked during the alignment process, regardless of the tool being used to measure for misalignment. But it is easy to check for runout before the alignment process begins.

Use a dial indicator and magnetic base.  Zero the indicator on the outside diameter of the coupling hub.  Slowly rotate the shaft to look for runout, and measure the amount.  Then move the indicator to the shaft adjoining the coupling hub.  Measure the same way.  If the hub and shaft travel by approximately the same amount, the runout is probably due to a bent shaft.  To confirm, measure the shaft in different places.  The bend normally decreases the closer the indicator is moved toward the bearing.  In this case, if the runout is excessive, the shaft should be replaced.

If runout is measured on the coupling hub, but not the shaft, the runout is probably due to either a casting or machining error (such as the hub being bored out of center).  In this case, the coupling hub should be replaced, but the shaft should be OK. 

In addition, axial runout of coupling hubs should be checked as well.  This can often indicate either a damaged hub, or one that was not bored concentrically, but at an angle.

Most engineering manuals recommend no more than 2-3 mils of runout.  On machines rotating at 3600 rpm or faster, this number should be cut in half (1 to 1 1/2 mils).

A quick check for runout can identify machinery problems, increase reliability, and extend component life.

 

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.

2 responses to “Coupling or Shaft Runout”

  1. Mike Keohane says:

    Often overlooked. There is a great video on the VibrAlign youtube channel that shows exactly how to check for runout.

  2. Ramesh Kumar.R says:

    Rotating equipment alignment procedures for both radial & axial fans of 600 MW Thermal Power Stations.