August 4, 2015
Shimming when there are more than four feet on the moveable element.
By Tom Shelton
When more than four feet are on the movable machine, it is necessary to shim under the additional feet. How do we determine what amount of shim is required to properly support these feet? This is debatable. Some would say that it is necessary to calculate the needed shims. Others might say “Why bother? Just fill it.”
During a recent training class, the crew chose a piece of equipment that consisted of a pump coupled to a gearbox that had a c-face motor direct bolted to it. The “combined” motor and gearbox was “the moveable element” for this precision shaft alignment with essentially 8 feet. The question of how to properly shim this moveable machine was debated.
The class determined that the feet on the gearbox closest to the coupling and the motor feet on the non-drive end would be the correct feet to use to correct any misalignment. This was definitely the right decision.
This left the 4 “inboard” feet of the element unsupported. Due to base conditions, foot thickness variation, and other considerations, the crew determined that calculating the needed shims was a practice in math and not practical for this application. So what did they do?
The class took the first set of alignment readings, to determine the alignment condition, which was out of tolerance. They then corrected the vertical misalignment by shimming at the “4 corners of the movable machine (gearbox inboard and motor outboard feet) per the shimming screen. They then completed the horizontal adjustment as normal.
Before tightening the motor and gearbox hold-down bolts, the gaps under the unsupported feet were filled with shims. No science here, just good hand feel. The shims under all 8 feet had similar feel of pressure on them.
Typically, when performing the Verti-Zontal Precision Alignment Process the sensors are left in the horizontal plane during tightening of the hold-down bolts to detect any movement. Due to the multiple feet, the crew decided to rotate the sensors to the vertical plane to monitor movement as they tightened the hold down bolts. This insured that the shim adjustments of the “4 inboard feet” did not result in any frame distortion (soft foot) or cause undue stress on the two machines of the moveable element as they tightened the bolts.
No vertical movement was detected so the sensors were rotated to the horizontal plane to verify if any horizontal movement had occurred. It hadn’t so all was good.
A second set of alignment reading were taken to verify, the correction process they chose was the correct. It was! One Verti-Zontal Compound Move, multiple feet, no problem, done!