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September 29, 2011

Why Should I Align A Flexible Coupling?

By Stan Riddle

In a recent training class, a mechanic asked, “Why should I worry about aligning a flexible coupling? I thought the reason we used flexible couplings is because they didn’t have to be aligned.” That’s a great question, and one that deserves an honest answer.

Let’s pick a commonly used coupling, and look at its alignment specifications: T.B. Woods Sureflex Coupling (with an 8JE insert).

  • Cost – about $40.00 for the sleeve, or insert.
  • Maximum RPM – 4500
  • Parallel misalignment – 0.020 in.
  • Angular misalignment – 0.094in. (about 0.012”/inch)

 

Based on the manufacturer’s recommendations, this coupling can tolerate a fairly substantial amount of misalignment. And, since the insert is relatively inexpensive, why go to all the trouble of precision alignment?

Because we don’t align couplings – we align shafts! The coupling sleeve, or insert, can tolerate that amount of misalignment; but, the bearings, seals, shafts, gears, and so on, can not!

The coupling insert will bend or flex, but it also resists bending. It would prefer not to be flexed at all. And when it is flexed, it requires a good amount of energy to counteract that force. That energy manifests itself in:

  • increased heat due to friction
  • increased radial and axial loading on the bearings, seals, gears, and shafts.
  • Premature wear, and eventual failure, of these components, including the insert.

Think of it this way. Take a wide rubber band, and place it across the ends of your index fingers. Stretch it until it just begins to straighten out. You could hold it there for a long time. Now, stretch the rubber band until just before it breaks. It won’t take long until your knuckles start hurting. It’s still the same rubber band, but it requires much more effort to keep it stretched.

So to answer the question, “Why should I worry about aligning a flexible coupling?” – you shouldn’t worry about it at all. But aligning the shafts to a precision alignment value is extremely important. And it you align the shafts, the coupling will “go along for the ride”.

You’ll increase the life of your machines, and your couplings!

18 Comments

  1. hamidreza on January 4, 2012 at 12:37 pm

    Hi,
    Although the coupling can withstand the forces brought by misalignment,those forces are still transmitted to the equipment resulting in severely shortened life span,catastrophic failure.



  2. Stan Riddle on January 4, 2012 at 4:12 pm

    Hamidreza, you are correct! The radial forces can be tolerated by many couplings, but not by the bearings and seals! Thanks for your comment!



  3. hamidreza on January 24, 2012 at 4:57 am

    Hi,
    In my last comment I was talking about misalignment and force. In this comment I am talking about misalignment and vibration.
    As far as I am concerned flexible coupling absorb some misalignment without creating vibration. I am of the opinion that this coupling can absorb misalignment WITHOUT CREATING HIGHER VIBRATION up to parallel 0.02in and angular 0.094in.Once the misalignment increase more than these values(0.02,0.094in)we should face with increasing vibration. Please confirm if I am correct.
    If you are with me ,I should say that taking vibration data while running is not going to detect misalignment.
    Assuming we have misalignment by 0.015 and 0.08in,in this condition as these amounts are less than coupling makers maximum allowable misalignment ,we don’t see higher vibration but other parts like bearings and seals will fail as are under increased force brought by misalignment.
    Br
    Hamidreza



  4. David Zdrojewski on January 24, 2012 at 7:13 am

    I really appreciate your participation….I hope more people will find our little blog and join in!



  5. hamidreza on January 25, 2012 at 1:23 am

    Thank you for your kind words.
    Honestly I was novice in alignment and I have learned many valuable information from your friends. My friend Patrick have given me many helps.I know him for long time and I already have very many communication with him via email.
    I am really interested in your excellent blog. I am of the opinion that this blog is very good for everyone working in industry and hope all people will be familiar with this great blog.
    Sorry if my English is poor.
    Br
    Hamidreza



  6. What's Eating Your Couplings? on January 30, 2012 at 5:33 am

    […] remember an entry in which Stan Riddle discussed why we should care about precision alignment of flexible couplings. Today, I would like to offer up some evidence that many of you may well relate to. We were called […]



  7. Patrick Lawrence on January 30, 2012 at 4:19 pm

    Back to your comment on 1/24, I think we’re getting into some theoretical areas now. All these systems are very complex. To try to pin down a system and say vibration will only occur after so much misalignment, and have it do so repeatedly, would be difficult.

    If that is the case, I would love to see some manufacturers present some data.



  8. hamidreza on March 28, 2012 at 2:16 am

    Concerning detection misalignment ,If you misalign a driver system the vibration may go up,go down or stay the same but in the majority of cases the vibration levels go down.This is because misalignment is considered as static force and these forces can actually reduce vibration levels by diminishing the capacity for the rotors to move as freely if the static forces were not present.
    Br
    Hamidreza



  9. Stan Riddle on March 29, 2012 at 8:37 am

    Hamid, I would say you are correct. Vibration, in this case a mechanical looseness, would probably go down, since the coupling clearances were reduced. However, in the long term, forces acting upon the mechanical system would go up, eventually causing bearing problems. Even though a flexible coupling is flexible, it does resist movement, and eventually these forces must be transferred to the bearings, and other machine components.



  10. Hamidreza on April 10, 2012 at 7:47 am

    Hi,
    My understanding is vibration forces will first cause bearing damages .Afterwards, we see the machine vibrating. That is, the bearing has already gone when we find high vibration.
    This means that we must not only rely on vibration to detect the misalignment. I strongly believe the ONLY way to detect misalignment at early stage is physically disassembling the coupling and check the alignment by hand. With this we can realize misalignment very soon and before bearing failures.
    Br
    Hamidreza



  11. Hamid on June 30, 2012 at 3:08 pm

    If anyone else has any comments.I have very much hope that other people join in and share their comments.
    Br
    Hamid



  12. ravindra alpe on December 13, 2012 at 12:54 am

    sir,
    I wanted some basic data in simple language about couplings,its types,allignment,trouble shooting chart,maintenance of couplings etc. for giving presentation on it



  13. Stan Riddle on December 13, 2012 at 7:13 pm

    Please see our web site (www.vibradev.wpengine.com). Under the Blog tab, you will find much of the information you seek.



  14. Saira on July 3, 2014 at 2:24 am

    HI Respected seniors,

    i just want to ask whether vibration at IX is due to coupling misalignment, coupling we use is flexible shaft coupling,
    this coupling used between gas turbine and Dynamo-meter.
    my driver’s rpm is 15500.



  15. Stan Riddle on July 3, 2014 at 7:37 am

    1X vibration can be caused by many things, but misalignment is a common reason for 1x vibration on coupled machines. So is imbalance.



  16. Ugur on December 4, 2014 at 2:23 am

    Hi,
    One of the couplings we use are star couplings and we use laser alignment sysytem. As you know when we try to align that shafts (which are connected with star coupling) we can’t take precision result because of the flexibility of the coupling. It causes ; laser get away from the axis.What kind of solution do you suggest?



  17. Stan Riddle on December 4, 2014 at 9:07 am

    If there is play or looseness due to the coupling insert, there are methods to control the backlash, and improve accuracy and repeatability. You can use a toothpick, or small wooden shims, to take up the looseness. I have used duct tape as well. Simply remove the wood or tape once the alignment is complete.

    Depending on the type of laser tool you are using, you can even remove the insert, align the two shafts, and install the insert once the alignment is completed.



  18. micheal diri on February 12, 2018 at 7:40 am

    please am working on misalignment of flexible shaft coupling as my dissertation in uni please can any one help me on how i should go about it thank you