C H A P T E R 1
rotational center is moved. Machinery performance is adversely
affected by a soft foot’s effect on bearings. Precision alignment
is nearly impossible to perform unless soft foot is corrected.
Correcting gross soft foot can be accomplished by loosening
all the mounting bolts and finding any obviously loose shim
packs. Add shims as needed to make a snug fit.
TIGHTENING SEQUENCE
You will find that specifying, recording and maintaining a
tightening sequence will result in a better and quicker alignment.
The fact is that the machine will move (both vertically and
horizontally) as we tighten down the bolts. This is true even
after we correct soft foot. If you follow a known sequence you
will minimize and make this movement predictable during the
alignment process.
Often when making vertical corrections, people will only
loosen two bolts at a time to try to hold horizontal position.
This practice is acceptable, but some horizontal movement
will likely occur anyway. Therefore, you should re-loosen all
bolts and retighten in the recorded sequence after all vertical
adjustments are complete.
In practice, make at least three passes around the bolts: Snug
on the first pass, ~50% on the second pass and completely
tight the third pass. Follow the same sequence throughout
the alignment.
FINAL SOFT FOOT
Soft foot issues account for most alignment repeatability
issues. For that reason, we address it twice. Now that the
obvious soft foot issues are resolved and all feet are tight,
we’ll make one final check on soft foot. Loosen
one bolt at
a time
and check with a 2 mil (0.002") shim or feeler gage
at that foot. Correct any foot with 2 mils or more softness.
Re-tighten the bolt and proceed to the next foot.
If the foot has an angular relationship to the base, you can
correct this by cutting a partial shim to make up the angular
correction. When cutting partial shims from pre-cut stock,
always leave the shim “tab” intact. The tab can be used to
make each shim pile organized and the partial shim will return
to the original position. Never “feather” or step shims out
to fix an angled foot. It is unlikely the angular correction will
be duplicated when the shim piles are changed during the
alignment.
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