December 14, 2016
Back Off Jack Bolts for Improved Alignment Results
By Brad Case
Millwrights and mechanics know the benefits of using jack bolts to precisely control the horizontal position of the movable machine when performing a precision shaft alignment. However, we also see the tendency to tighten the jack bolts, on both sides of the movable machine, either before or after tightening the hold down bolts.
Tightening jack bolts after the hold down bolts have been tightened can lead to increased vibration levels as discussed in a previous blog, https://acoem.us/shaft-alignment/shaft-alignment-jacking-bolts-reduce-vibration/
Tightening the jack bolts before the hold down bolts, in an attempt to maintain the horizontal position of the movable machine, can result in an erroneous vertical alignment position when new measurements are taken.
I have witnessed this on numerous occasions, especially on smaller motors. The jack bolts dig into the motor feet holding up the motor a few or more thousandths, essentially preventing the hold down bolts from “pulling” the motor completely down. Typically, when the jack bolts were backed off, you could feel the motor “drop” thus allowing the hold down bolts to be fully tightened. After new alignment readings were taken the vertical alignment positions were essentially in tolerance.
Tightening the jack bolts on both sides of the motor feet can also cause “FALSE” horizontal alignment positions.
A recent alignment check of a 3600 RPM electric motor/air compressor set, with the jack bolts tight against all the motor feet, showed a horizontal offset of +3.1 mils which was slightly out of tolerance. After backing off the jack bolts and taking a new set of measurements, the horizontal offset changed to +5.5 mils which was almost 3 times out of tolerance.
When using jack bolts to correct horizontal misalignment, it’s best to back off the opposing jack bolts in the direction the machine needs to move. Once horizontal alignment is achieved, then loosen the jack bolts used to push the machine and THEN tighten the hold down bolts.
If the movable machine is moving out of tolerance as the hold down bolts are tightened (using 3 passes in a crossing pattern) don’t “force” the horizontal position with the jack bolts. Investigate the cause of the movement and correct. Typically, this movement can be traced back to the basics of soft foot issues, cupped washers, loose or broken bases, etc. You can find numerous blogs on these subjects elsewhere in the VibrAlign Blog.
Remember, jack bolts are for positioning a machine during an alignment, not for holding it in place.