Shimming when there are more than four feet on the moveable element.


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!

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  1. […] of shim is required to properly support these feet? This is debatable. Some would say […] Read More Source: The Alignment […]

  2. Andrew Martin on August 6, 2015 at 9:42 am

    Another great procedural tip that will definitely be useful for others with similar applications.

  3. Silvio Balieiro on August 6, 2015 at 7:40 pm

    We have here in the company 02 electric motors with 08 feet and generally distribute the shims in 08 feet. Very good information. Thanks.

  4. N Puri on April 24, 2019 at 4:16 am

    refer to API 686 Para The use of shims under special-purpose machinery gearboxes to correct for soft-foot or gear tooth contact is NOT permitted, what does it mean??
    is it mean that the gear box will not be shimmed, only the movable machinery (driver/ motor) will be shimmed for the alignment of equipment train? some one to clarify!!

  5. Tom Shelton on April 25, 2019 at 12:14 pm

    In the case presented in the original blog, the motor and gearbox had to be adjusted (shimmed etc.) as one unit, to correct the misalignment, due to the C face motor mounted to the gearbox.

    The statement you are quoting in API 686 Para “The use of shims under special-purpose machinery gearboxes to correct for soft-foot or gear tooth contact is NOT permitted”; appears to be addressing “shimming” for correcting soft foot or gear tooth contact not for correcting misalignment. Soft foot is a base or machine foot flatness issue that needs to be corrected before a precision shaft alignment is performed.

    API has flatness specifications for installed machine bases as well as machinery feet that manufactures must meet so there is no soft foot present. If you need to shim a gearbox to correct the alignment it would be best to check with your engineering department or the gearbox manufacturer to see if it is permitted.

    Thanks for your comment.

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