December 11, 2012
Aligning Uncoupled Machines
By Stan Riddle
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.