June 26, 2014
How To Align a “Moving” Motor.
By Stan Riddle
After 34 years, aligning a few thousand machines, teaching millwright training for a technical college, and being an instructor for VibrAlign, I sometimes think that no alignment challenge can beat me! Then something comes along to really put me in my place!
I was put in such a place while training at a large manufacturer in South Carolina. This company has dozens of small utility pumps. I will not name the pump manufacturer, and no photos will be used, but I will explain the set-up. This machine was used for the “hands-on” portion of the training, since the company had been experiencing excessive vibration from the unit.
The pump assembly skid consists of two rails, which bolt to the floor. This skid should normally be grouted, but this one was not. Underneath the motor is a plate steel riser, approximately 1/8″ thick, and about 4 inches tall. On top of this riser sat a 40 HP, rolled steel plate motor. Rolled plate motors are not uncommon, but this was the largest horsepower motor I had seen using this type of motor construction.
The normal pre-alignments steps were completed, with a special emphasis on minimizing soft foot, since motors of this type do not have the typical four cast iron feet. The motor housing is broken in a sheet metal brake to provide two long “feet”-one on each side of the motor.
The first alignment measurements were completed, and the motor was shimmed and moved to the values calculated using the Verti-Zontal method. The 2nd set of result measurements were close, but not on target. While performing live moves with the laser alignment tool, the mechanic leaned on the motor. We noticed movement at the coupling of over 10 mils (0.010″)! So we stopped what we were doing, and checked every hold down bolt on the motor, riser, and base. Everything was tight. So, we checked again, and we still got over 10 mils of movement.
While we did not conduct any detailed study, due to time constraints, we could only assume the movement was due to insufficient stiffness of the frame, riser, and motor. A recommendation was made to this facility to grout the base, and add some horizontal stiffening to the frame and riser.
So, how do you align a moving motor? You do not. You make modifications to the structure to make it rigid enough to be aligned. If you do not, you are asking for problems.