When performing a precision laser shaft alignment, it is best to add or remove shims exactly as the display unit indicates. There are circumstances when this may not be possible due to your assortment of shims, or when you want to “fudge” your shims slightly to keep the total number of shims low. (Clink for link to shim count blog.)
It is quite possible to do this if needed and still bring the alignment into tolerance. When fudging the shims, always fudge as little as possible, maybe just .001”. If you fudge by .002” or more, you may not be able to achieve your shaft alignment tolerances. For example, if the alignment tolerance is set to 3600 rpm, then .002” would set you at the limit of your tolerance.
If you fudge the front feet (drive end) in one direction and the rear feet (non-drive end) in the opposite direction, your coupling can bind and the alignment will be unacceptable, this typically will cause excessive angularity at the coupling. By fudging both the front and the rear feet in the same direction, you may avoid this bound coupling situation. You could possibly end up with slight offset misalignment, which is better than a coupling having angular binding in most situations.
That being said, if you do decide to fudge the shims slightly at both the front and the rear of the machine, it is best to fudge both ends in the same direction. Let me explain. As a good alignment practice, there are 2 guidelines that should be followed. 1) Keep both the front and rear feet the same sign (+ or -). 2) The front feet should always be a lower number than the rear feet. By following these guidelines your machine will stay in the “Zone of Acceptance” and the machine shafts will be pointing toward the coupling.
If the feet values are different signs or the front feet are a larger value than the rear, then this will cause the coupling to be in a bind. As a rule of thumb, the front feet will affect the offset more and the rear feet will affect the angularity more.
In photo 1, you can see that the alignment is not perfect, however the front and rear feet are the same sign (+) and the rear feet are a larger amount than the front. The machine is still well within tolerances.
Photo 2, shows the alignment results after we were instructed to add .036” shims to correct the vertical misalignment. To minimize the shim count .035” was added to both the front and rear feet. That left the moveable machine 1.9 mil (.0019”) low at the coupling but still in the good alignment zone and there were fewer shims under the feet.
Photo 3, shows the effect of the front feet being a higher value than the rear feet. The front feet are -48 mils (-.048”) with a vertical offset at the coupling of -61 mils (-.061”). As the rear feet are a smaller value (-34 mils) than the front feet the direction of the angular misalignment causes the movable machine to be outside the “Zone of Good Alignment” by 61 mils (.061”) at the center of the coupling.
When following the 2 guidelines mentioned above, you will always have a better chance for your precision shaft alignment to be in tolerance and your coupling to be relaxed and unbound.