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Coupling or Shaft Runout

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Coupling or Shaft Runout

Coupling or Shaft Runout

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:

  • Excessive 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.

For more information, visit VibrAlign.com.

 

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Comments

12 responses to “Coupling or Shaft Runout”

  1. adonis purisima says:

    how to measure the the coupling hub or shaft total runout base on the reading from 90º, 180º, 270º, 0º. Please give an example reading so i can understand. thank you

  2. Stan Riddle says:

    Please follow this link to an excellent training video on coupling runout.

    http://www.youtube.com/watch?v=JILP_4GJJH0

  3. Obaida Mohamed Gad says:

    What about performing alignment with permissible shaft runout, in other words, how to compensate this permissible amount of run out on indicator reading

  4. Stan Riddle says:

    Good question. Obviously, runout should be checked before coupling alignment. If it is excessive, you should make a determination if the shaft is bent, the hub is eccentric, or both.

    As to performing alignment on machines which have a small degree of runout, that depends on how the alignment is being measured. When using lasers or the reverse dial indicator method, it is preferable to rotate the shafts together. By measuring this way, the runout has no effect on the measurements, since the plungers of the indicators are always contacting the same area. In other words, the lasers or dial indicators do not measure the runout.

    If you are using the rim and face method, runout will be measured. One method I have used is to mark the high and low spots on the coupling hub. As an example, if I had a hub with 3 mils of runout, I would mark the high spot as +1.5, the low spot as -1.5, and the neutral spots as 0. You can make your calculations based on the numbers marked on the hubs, and adjust your numbers as needed.

    In many cases, the runout tolerance for the hub should be less than the alignment tolerance.

  5. Frimpong Tuffour says:

    What is the procedures for checking end float of a centrifugal pump using a dial indicator?

  6. Stan Riddle says:

    Good question. It depends on several factors, such as manufacturer, bearing type, seal type, and so on. I would recommend checking with the pump manufacturer. This information is often in the installation and maintenance procedures.

  7. jaimin says:

    Can we remove runout from the shaft which are more costly. ….?
    And during the measurements of runout ones I get reading at suppose 90 degree +0.02 and at 180 degree -0.02 can I say that ..that is ok….or it is runout…

  8. Stan Riddle says:

    Jaimin, I’m not sure what you mean by removing runout, other than by replacing the shaft. I have heard of people straightening shafts, but I personally have not had good experience with it. As to whether +.02 at 90 and -.02 at 180 is OK or not, I could not say. It could depend on the speed of the machine, as well as many other factors. I would recommend consulting with your engineering department.

  9. What should be acceptable shaft radial and axial play for Mechanical seal installation?

    • Stan Riddle says:

      Good question! It depends, based on the seal type and manufacturer. I would recommend checking with the seal manufacturer.

  10. alok kumar says:

    If the coupling have run out of 0.1 mm near the intersection point of shaft-coupling, what should be the run-out if shaft is 1 meter in length?

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