August 13, 2012
Shaft Alignment of Small Machines
By Brad Case
The VibrAlign Training Faculty sees a lot different machines during our training classes across the USA. The one statement that we hear repeatedly from mechanics and millwrights is, “I would rather align a 500 HP motor & pump set than a 10 HP one”. We agree!
While small machines are easier to “physically” adjust during a shaft alignment, their small size presents another set of alignment issues. The electric motors are typically lightweight, the mounting feet are a thin stamping, the bases are rather flimsy when compared to larger and heavier machines, and there are no jacking bolts to control the horizontal adjustment.
What does this mean for the aligner? Well, his/her alignment technique will need to be excellent and a “light” touch is also needed while making the live horizontal adjustment. After roughing in, soft foot absolutely needs to be corrected and a cross-pattern bolt tightening sequence with three passes needs to be followed to minimize movement of the motor. This, however, may not be enough.
During an alignment of an 10 HP motor/pump set during a training class the vertical alignment corrected easily, however the horizontal alignment was extremely “sensitive” and would not stay in tolerance when the hold down bolts where tightened. The bolts where checked for cupped washers which where okay.
The problem? – the distance between the motor feet was only 5-1/2”. The slightest movement at the feet caused the horizontal alignment to move out of tolerance at the coupling.
The solution? – move the sub-base! This application has a motor sub-base bolted to the pump base with a bolt spacing of approximately 10-1/2”.
So for this motor alignment we adjusted the Verti-Zontal compound move slightly.
After getting results, the vertical alignment was corrected by shimming the motor feet and re-tighten those hold down bolts. The sub-base bolts were then loosened and the horizontal alignment adjusted and the bolts re-tightened. It took two modified Verti-Zontal adjustments to get the alignment in tolerance. By the way, when making the horizontal adjustment you don’t need to change the feet dimensions, just move the motor the direction indicated until the coupling values are in tolerance!
Pretty cool, huh?