From the Mailbag: Mounting to Coupling Hubs

Machine Train Alignment

QUESTION: When mounting the lasers, if it isn’t possible to mount to the shafts, can they be mounted to a flanged sleeve gear type coupling? (The outer “floating hub”, not the inner one pressed on the shaft.) I have seen this done and do not think it is accurate due to the radial clearance between the inner and outer hubs, but others disagree. Thanks!

ANSWER: You are correct. Laser shaft alignment tools work great when mounted to a solid surface, whether it is the shaft or the coupling hubs. But if you mount on the coupling insert, sleeve, or anything else that can flex during the shaft alignment process, it will give you inaccurate results. This is true for laser alignment systems or dial indicators. The reason is the insert, by definition, “floats” or moves during rotation.

If by “floating hub”, you mean something like a Falk Steelflex, with a grid-type connecting “spring”, the hub moves on O-rings, which are mounted to the couplings hubs. It is simply a “cover” to hold grease in and dirt out, and should not be used as a precision surface for shaft alignment either.

Great question!

You too can have your questions answered or disputes refereed–just go over to http://thealignmentblog.com/contact/ or http://www.vibralign.com/. We appreciate the opportunity.

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About the Author

Stan Riddle

Stan Riddle joined VibrAlign in 2008. He has 30 years experience in aligning industrial machinery. 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 where he enjoys wood carving, fishing, and anything else outdoors.


4 Comments
  1. Bryan

    Good day sir. I’m new at the laser alignment and I have a problem with it. I just have a couple of questions.
    What are common human errors in shaft alignment?
    What will happen if I was wrong with my 2nd distance? From M-sensor to center of coupling? Thanks.

    • Excellent questions! The most common errors in laser alignment are the same as the most common errors for dial indicator alignment. They are (1)-not minimizing the soft foot before you begin. (2)-not controlling looseness in the coupling during the measurement process. (3)-not tightening the motor feet down in a controlled, repeatable pattern. And there’s one more common error – not inputting your dimensions correctly. As to your second question, think of it like this. The second measurement tells the laser tool where the intersect points of the shaft would be if they touched. So, if your measurement is off slightly, say 1/2″ or so, then your values at the coupling will be off slightly as well. Normally, it becomes less of an issue as the shafts beome more closely aligned. If your final alignment target for angularity is .7 mils per inch, and your measurement to the center of the coupling is off by 1 inch, then your coupling values will be off by the same .7 mils, or 0.0007″.

  2. Stefannie Coleman

    I would like to see more blog posts written like this. ” From the Mailbag ” shows our customers that we take their insight and questions seriously. I think publishing a common question from the customer also helps out other people who have the same concerns. Good job guys, keep up the great work!

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