Normally, it is faster and more accurate to perform shaft alignment while the machines are coupled together.  This offers several benefits such as:

  •  Keeping the relative angular positions of the shafts the same allows the technician to align the true shaft rotational centerlines.
  • Coupling Gap is less likely to change while adjusting the moveable machine, since the insert normally keeps axial spacing intact.
  • If the machines are coupled, time is not wasted separating the coupling, removing the insert, re-packing grease, etc.

However, there are occasions when the coupling might need to be separated as part of the alignment process.


Here are three examples:

  • When the shafts are so misaligned as to cause shaft deflection, or bending, when the coupling is connected.  This is normally detected when the coupling does not easily assemble.  The easiest way to minimize this is by roughing in the alignment with a straight edge before mounting the laser alignment tool.
  • When the flexible coupling has sufficient stiffness to deflect, or bend the shafts during the process of rotation.  Some flexible couplings are less flexible than others as part of their design, so a stiff coupling is not necessarily a bad coupling.  Some examples of stiff couplings are disc-pack and tire couplings.
  • Long, narrow shafts which extend several inches from the inboard bearing can deflect even when the coupling is not abnormally stiff.

If your laser alignment tool has built-in inclinometers, in the sensors, an uncoupled shaft alignment should not be difficult.  Use the inclinometer values to set both sensors to the first measurement position, and collect your initial measurement.  Rotate one sensor to the second position then rotate the other sensor to the second position, matching the inclinometer values for each and measure.  Repeat this process for the third measurement.  Make sure to check your inclinometers before each measurement.

A simple method of detecting whether or not the coupling should be removed for the alignment is simply by monitoring the coupling values. After performing the pre-alignment steps, measure the misalignment then shim/move to correct the misalignment as indicated.  Re-measure.  If the values are not very close to being in tolerance, and if you have carefully taken your measurements, suspect that the coupling stiffness is influencing the shaft’s rotational centerlines. Re-measure for misalignment, this time with the shafts uncoupled, and see if the measurement changes.

Once the machines have been aligned, re-couple the shafts, and re-measure your alignment.  Your values should not change more than 1-2 mils.

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  1. Hamidreza on December 12, 2012 at 11:19 am

    Dear Stan,
    In a recent case,we aligned our machine while we take out GEAR coupling spacer.I mean uncoupled condition. Having performed alignment we installed gear coupling spacer. However,we found our alignment is off by 0.20mm.As you can see at uncoupled condition the alignment was okay but when the machines are coupled together the alignment is very bad.

  2. Stan Riddle on December 12, 2012 at 11:37 am

    Hamidreza, if the shafts are truly aligned while uncoupled, then they are truly aligned while coupled. The only way the shafts could become misaligned is if the gear coupling is either machined incorrectly, or if the shafts are somehow deflected from the process of bolting the coupling together.

    I would recommend remeasuring both coupled and uncoupled alignment again to confirm. If you are not getting repeatable readings both coupled and uncoupled, something is changing.

  3. Hamidreza on January 5, 2013 at 1:24 pm

    My good friend,I did in reverse.That is,I alined while coupled.Then while uncoupled,I measured alignment.However,I found big differences.

  4. John Walden on January 9, 2013 at 6:52 am

    This is an interesting conundrum we find with very stiff coupling, and gear type meet this criteria. In fact, when aligning these coupling types with the coupling made up, it can be advisable to re-energize the machine, and then do a final measurement. Yes, I know – more time, more effort. While Stan’s reference to bending is very relevant, these couplings also create considerable stress within the bearing constraints (when run misaligned). A machine run in poor alignment opens up the tolerances over time, making the alignment even more challenging. Many machines SHOULD be aligned un-coupled!

  5. precision surface equipment on January 10, 2013 at 10:16 am

    I have read the benefits and examples given above in the post and all are worth to read and even I agree to the benefits also..Having performed alignment we installed gear coupling spacer.

  6. Themistoklis on April 12, 2013 at 6:11 pm

    There is a possibility to have a coupling face which is not straight. Is the coupling assembly new? If not, was it hit or heated(in spots) while it was pulled out?

  7. Silvio Balieiro on August 6, 2015 at 8:38 pm

    Excelent article

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