Categories: Shaft Alignment

Why do Pre-Alignment checks when using a Laser System for Shaft Alignment?

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By on January 2, 2013

While working on a sludge pump, several issues arose that could have caused alignment headaches that would have lasted hours. There isn’t any medication that would have removed that pain. In this scenario and all alignment jobs, Pre-Alignment is a MUST!

I was asked to witness an alignment; a 400 HP motor driving a sludge pump with a new power end installed the day before. This should be easy! When we arrived the picture was clear, a very clean environment and base. Easy! Let’s attach the Fixturlaser GO Pro Sensors and get this done, 10 minutes, tops!

We mounted the GO Pro and started our pre-alignment checks. As it appeared the motor was rough aligned we proceeded to check obvious soft foot by loosening all four feet on the motor to see if the shims have any movement. The shims were indeed tight, but through further inspection, we could see there was an angular soft foot condition on all four motor feet. To our surprise, the amount was similar; about 35 mils at the back of each foot. What to do? Was this machine rough aligned when the power end was installed? Simply stated, no! This is easy to understand. Each power end is a “machined” component and all too often, people assume that machined components will be exactly the same as the machined component that it is replacing. How many of us have seen reused shims when we replace motors, pumps, fans, etc. Same company, same model, therefore the same alignment, right! Wrong!

Let’s take a step back remove the coupling and start over. After the coupling was disassembled, an elastomer wrap style, the obvious soft foot angle was cut almost in half at all four feet. Four angled feet on a good flat base is unusual unless the motor has been damaged so we looked at the motor and noticed that the steel plates under the front feet measures at 3/8” thick while the plates at the rear feet were 1-1/8” thick. Our game plan was to replace the existing plates with ¾” thick plates at all feet; measure using the GO Pro with the shafts uncoupled to rough it in.

After the ¾” plates were installed, all angles were gone. We measured and shimmed the front feet 128 mils and the back feet 143 mils, now we were roughed in. The coupling was re-assembled; measurements taken with the GO Pro and a slight Verti-Zontal Compound MoveTM made with a vertical adjustment of 10 mils to the front feet and pushed the motor horizontally into position.

Our final coupling values were Vertical Angularity 0.2mil/1” & Offset 0.2 mil; Horizontal Angularity 0.2mil/1” & Offset 0.4 mil, all completed in 41 minutes!

If the pre-alignment was not done, this alignment would have started off wrong by correcting an angle soft foot condition that did not exist, and we know the snowball would have started down the hill from there. Pre-Alignment Steps are the shaft alignment medication that reduces frustration and saves time in the long run!

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