August 20, 2012
Cutting Shims to Correct Soft Foot During A Shaft Alignment
By Patrick Lawrence
Here’s a thought-provoking question for you my fellow shaft aligners: is cutting shims to correct soft foot overkill? That concern was addressed last week during a shaft alignment training class where we aligned a nice little water pump.
The motor had one of the stamped out bases that are always fun to work with. We followed our process and during the obvious soft foot step, two feet needed additional shims–the front left and the back right feet were loose making the motor rock back and forth diagonally. This is what most folks in the industry call ‘rocking soft foot.’ Shims were added accordingly to both feet to fill the gaps and they torqued the motor back down. The final soft foot check initially showed no additional corrections needed to be made.
Cutting to the chase, while correcting the horizontal misalignment live and torquing down the motor, there was a lot of unintended movement that had to be controlled. After remeasuring, the alignment was somewhat successful but not completely acceptable. These are unmistakable signs of soft foot.
Soft foot troubleshooting began again. This time the final soft foot check revealed some additional movement at the same two feet as before. How can that be? During the final soft foot check, just one foot is loosened at a time with the other feet tight. Finding an angled foot is a little easier. The right rear foot lifted a couple mils and was corrected with a three mil shim. The front left foot was more severe. The front left foot took a whole five mil shim (the entire shim slid under the foot) and another five mil shim had to be cut to make sure just the angled portion was filled.
This is when one of the guys commented that this might be overkill. Cutting shims just didn’t seem necessary.
Well, the motor was much better behaved the second go around and the alignment was successful. We like eliminating and/or managing unintended movement. It makes for much less frustration. My partner in alignment agreed.
So what do you folks do for angled soft foot? Do you mind cutting shims? If so, how many? Any boundaries you don’t cross? Let me know.