Soft foot; what are you look at correcting?

Motor anchor bolt holes & feet area.

the dwg didn't post - see attachment for complete report. Sorry.
1 4



Jack



3 2

When checking for soft foot we normally loosen one foot’s anchor bolt at a time. A four-footed motor works like a chair that will wobble whereas a three-legged stool has no rock or soft foot. Hence, some machine are three-footed.

On our model; let’s loosen anchor bolt #1 – is there deflection? No! Move to foot #3. Deflection? Yes! None at foot #2; OK. We have a relative simple correction problem. Or is it?

Our 3-4 (blue line) diagonal is the soft diagonal. The 1-2 (green line) is the hard diagonal. Since #3 was soft, will #4 be equally as soft? Maybe if textbook perfect. Our standard tolerance is 2 mils (0.002”). We like zero will settle for 2 mils.

Let’s say we had 8 mils soft foot at #3. Since bolts 1 & 2 were tight the unit won’t respond in a linear fashion but some fraction thereof. With bolts 1 & 2 remaining tight loosen #4 – did the dial indicator show deflection and did #3 decrease in the amount of soft foot? You may want to shim under both feet. Was #4 under 2 mils? If #4 was under 2 mils you can opt to shim #3 only. If so, you can tighten #’s 2 & 4 and loosen #1. This gives the greatest amount of machine flexibility at foot #3 and keeps the machine basically at its present location (your particular machine may not allow this practice and it may not be practical). Providing you have machine dimensions that give flexibility you can jack at or near the foot to make the adjustment. As you’ve already seen, just because you had 8 mils deflection doesn’t mean that an 8 mil shim will make the final correction.

We can check soft foot as long as mass doesn’t overcome stiffness. If mass overcomes stiffness – all four feet down on their respective pads and adding ~20 mils shimming to one foot doesn’t create a soft foot on an adjacent foot.

Soft_foot.doc (22 Kb, 37 downloads) Soft foot