February 12, 2013
Soft Foot Caused by a C-face Motor
By Chris Troutt
Our service group received a call from a water treatment plant in the western US for assistance to align a gearbox with C-face motor to a rotary lobe pump. After numerous failed attempts and multiple hours of frustration they had determined the gearbox to be Bolt Bound.
We were asked to bring turned down bolts and perform the final alignment. We packed up our Fixturlaser GO Pro and undercut bolts then headed out for some in the trenches training.
The Initial GO Pro readings found the gearbox to within tolerance in the vertical plane, but out of tolerance in the horizontal plane with an angular misalignment of 2.0 mil/1” and offset misalignment of -52 mils at the coupling center.
We loosened the foot bolts and our tech immediately found soft foot at the feet on the low-speed side of the gearbox. We decided that the soft foot was caused by the gearbox rocking back and forth due to the overhung load of the C-faced motor. Knowing soft foot to be the undoing of many good alignments, we devised a way to keep it to 2.0 mil or less. We loosened the gearbox feet bolts just enough to allow for side to side movement of the gearbox which kept the soft foot under 2.0 mils.
We utilized the live reading for our horizontal correction and left it “live” while we tightened the feet down in a cross torque sequence looking for any gross changes indicating the return of soft foot. It took us only two turns of the shaft to accomplish the alignment, but still accomplished it to an 1800 RPM tolerance in under 30 minutes!
We were also asked to align another rotary lobe pump and accomplished the shaft alignment in short order as well, since we had deduced a way to handle the soft foot. Both alignments took our techs less than an hour and a half, total time.
So in the end the gearbox was not bolt bound, but had soft-foot induced by the overhung load of the C-face motor causing reading inaccuracies. This inaccurate data cost our customer many man hours and 4 machined bolts, which of course weren’t needed.
Being vigilant for soft-foot needs to carry through the entire alignment, not just during the “pre-alignment steps”. If soft foot is not controlled, it will hamper even the most seasoned alignment technician.