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January 16, 2013

In Shaft Alignment, Low Can Sometimes Mean High!

By Stan Riddle

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A customer in the northeast US was concerned that his laser wasn’t working properly.  He called into our office stating “the motor shaft is a ¼ inch low at the coupling, but the laser shows I need to remove a lot of shims from the motor feet – both front and rear”.  His thought was something must be wrong with his laser.

I asked him to rough in the machines with a straightedge until it was pretty close then re-measure with his Fixturlaser Shaft Alignment System.  He did, and called back to tell me the offset at the  coupling was fairly close (slightly low), but the vertical foot values showed the motor front feet were +191 mils (too high) while the rear feet were +346 mils (too high)!

I asked if he had a smart phone (which he did), and if he would take a photo of the screen, and send it to me.  Sorry for the glare, but hopefully you can make out the position of the front and rear motor feet outlined in yellow.

I did some good old fashioned plotting on graph paper, and a little math, and came up with a good answer –His Fixturlaser Shaft System was working fine, and here’s why.

  • The motor was too high, but much higher at the rear feet then the front feet.
  • The slope, or difference between the front and rear feet, was 155 mils over 16.50″, or 9.4mils/inch – exactly what the angular coupling value shows (outlined in red in the first photo).
  • The length of the motor shaft from the front feet to the coupling center was 22 inches.  So as the slope of the motor shaft keeps moving towards the coupling center, it’s getting closer and closer to the stationary machine’s reference rotational centerline (zero), until it finally goes to the negative side. In this case 16 mils low at the coupling center.

Think of it as a see-saw.  When one end is higher, the other end is lower.In our case, the fulcrum would be the front motor feet.

 

Happy customer – good learning experience.

7 Comments

  1. Mike Keohane on January 17, 2013 at 9:24 am

    I think this confuses a lot of people and this information with screen shots and graphing is fantastic. Thanks for the post.



  2. Hamidreza on January 18, 2013 at 4:54 am

    As Stan stated clearly,we are measuring our data at coupling.The OEM usually advise the maximum misalignment tolerances.Considering this amount is 0.05mm at coupling center,it will be much higher at motor rear foot.Although the 0,05mm at coupling center is acceptable,this would not be acceptable for machine bearings specially at rear feet.I am confused why machine makers advise the maximum misalignment tolerances shall be 0.05mm at coupling center NOT at machine front and rear feet.



  3. Bill Snyder on January 23, 2013 at 10:14 am

    Please add my email to your Blog list.



  4. Stan Riddle on January 24, 2013 at 6:37 am

    Done!



  5. precision angle plates on February 20, 2013 at 4:02 am

    Yes, i agree that in shaft alignment low can sometimes mean high and this fact is so nicely explained here with the example of a customer facing such problem! Great job done with the blog!



  6. Hamid on December 19, 2013 at 12:51 pm

    Hi,
    I would like you share your information about my bellow sentences.
    As Stan stated clearly,we are measuring our data at coupling.The OEM usually advise the maximum misalignment tolerances.Considering this amount is 0.05mm at coupling center,it will be much higher at motor rear foot.Although the 0,05mm at coupling center is acceptable,this would not be acceptable for machine bearings specially at rear feet.I am confused why machine makers advise the maximum misalignment tolerances shall be 0.05mm at coupling center NOT at machine front and rear feet.



  7. Brad Case on January 9, 2014 at 6:43 pm

    Hamid – check out this previous blog “Foot Tolerances vs. Coupling Center Tolerances” from May 4, 2012 on this subject.

    http://thealignmentblog.com/blog/2012/05/04/foot-tolerances-vs-coupling-center-tolerances-for-shaft-alignment/

    Click on the link in the blog to an article that appeared in Maintenance Technology Magazine title “Don’t Look at your feet”.