August 15, 2014
Don’t Overlook “Hidden Shims” Under the Feet!
By Brad Case
A recent Fixturlaser EVO class performed an alignment check on a 35HP, 1750 RPM electric motor and chill water pump set.
The initial set of alignment results indicated horrible angular misalignment (14 to 16 times tolerance) and offset misalignment (6 to 8 times tolerance) in the vertical and horizontal planes.
Before making corrections, pre-alignment checks were done which included consolidating the shim stacks under each foot, performing soft foot checks which revealed a 10 mil soft foot on the right front foot that was corrected.
While making corrections using the Verti-Zontal Compound MoveTM, they noticed some movement of the motor when tightening bolts. Too much movement, resulting in slightly inconsistent results.
All the mounting brackets, sensor clips, hold down bolts, etc. were tight so they took a closer look at the shim stacks and discovered a number of carbon steel “hidden shims” under 3 of the 4 motor feet.
So why weren’t they found during the pre-alignment checks? Several reasons.
First the shims are much smaller than the motor feet and the properly sized shims. Plus the shims had no safety tabs so they were hidden under the motor feet.
Second the pump skid OEM had used shims with holes instead of slots allowing the shims to spin around on the hold down bolts to where they were really tucked up under the backside of the feet which was very difficult to see.
Third, the rust had them stuck to the bottom of the feet.
(Shim shown on top of foot for clarity).
Once the old shims were replaced and shim stacks corrected the 10 mil soft foot previously found disappeared and the alignment was completed with another Verti-Zontal Compound MoveTM.
Other issues were also discovered during the alignment.
- The motor was slightly bolt bound which was corrected by using slightly smaller diameter bolts.
- The washers under the hold down bolts were cupped which pulled the rear feet approx. 40 mils to one side as the hold down bolts where tightened.
- Due to the large vertical and horizontal angular corrections the coupling axial spacing closed up, putting the two hubs in contact with each other causing a bind which prevented horizontal adjustment, and had to be reset.
With all these issues the alignment was still completed in a little over 1 hour and 45 minutes!
Lessons learned? Take nothing for granted! During pre-alignment or when shimming, double check under the feet for “hidden shims” or shims stuck to the underside of motor feet or the base by corrosion, rust, grease, or oil.
So when your laser tool (or dial indicators) shows inconsistent results after making (or during) corrections don’t keep chasing the alignment, stop to check out the big picture to find out why.