If the machine to be moved has 6, 8, or more anchor bolts, caution needs to be applied when deciding how to shim the middle machine feet.

A method still used by many is to calculate the shimming for the inboard and outboard feet of the MTBM and shim accordingly. Then, use a feeler gauge to determine the number of shims required to “fill in” under the middle feet. You tighten the inboard and outboard feet before making the shim correction to the middle feet. A common practice is to add a few mils of shim under the middle feet to compensate for any casing sag due to the weight of the machine. The problem with this method is deciding how many mils of shims to add when compensating for casing sag.

Some laser shaft alignment tools can calculate the middle feet shim requirement based on the distance from the front (or inboard) feet to the middle feet. This calculation is based on a simple rise over run and assumes that the base is completely flat. Some criticize this method of letting the laser alignment system calculate the middle feet corrections. The argument is that on a normal machine configuration, the four feet nearest the bearings establish the reference plane. But when additional feet are introduced, you should not assume they are in the same exact plane. This is true and shows up as a soft foot condition. Therefore, when a proper Rough soft foot and Final soft foot are performed, the calculated middle feet corrections by the laser alignment system are preferred over just guessing how much to shim the middle machine feet.

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, by Pedro Casanova CRL