Loosen the Hold Down Bolts to Lower the Motor! ??

While assisting a recent Fixturlaser NXA Training Class with a precision shaft alignment we came across an unusual alignment challenge.  When tightening the hold down bolts, after making a Verti-Zontal Compound MoveTM, we were seeing insignificant amounts of horizontal movement at the motor feet but nothing unusual.

However, upon rechecking the alignment, we were out of tolerance vertically at the coupling and the foot values showed the motor to be sitting 80 mils high at the front feet (1.0 mil = .001”) and 111 mils high at the rear feet! This was huge.  So we took pause and began checking out what could be the issue.

Photo 1

The class verified the rough-in of the motor, re-checked obvious and final (residual) soft foot, and ensured a proper bolt tightening sequence was followed. We then we performed the shaft alignment.  Same results, the motor was sitting about 100 mils high at the feet!

We again re-checked the pre-alignment steps and performed a Verti-Zontal Compound MoveTM only this time before tightening the hold down bolts we rotated the sensors to 12:00 to monitor the vertical values (position) live. Sure enough, as we torqued the motor bolts down the motor was actually lifting up 100 mils!

The problem?

The hold down bolts were too long for the application, they were extending down through the baseplate and bottoming out on the grout with two full turns still left.  This was flexing the baseplate upward 100 mils resulting in the motor moving upwards and out of vertical tolerance.

The technician suggested stacking washers to compensate for the extra bolt length, but I advised against that and suggested they use the correct length bolts instead to eliminate the potential for future issues. After installing grade 8 bolts, 1/4″ shorter than the original ones, the problem disappeared.

After one vertical correction the vertical alignment was slightly out of the 3600 RPM tolerance by only 0.6 mils (0.0006”). A quick 2 mil shim change at the motor rear feet and re-check of the alignment and we were done!

Photo 2

Lessons learned from this shaft alignment?

  • If it moves the laser system (or dial indicators) will see it.
  • Don’t assume anything when investigating shaft alignment issues. The cause may be as simple as bolts being the wrong length.

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  1. Tom Shelton on August 23, 2014 at 12:47 pm

    Great catch!!! That’s a real booger that causes a lot of head scratching. I will most definitely remember this.

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