Page 207 - Machinery Component Maintenance

Machinery Alignment 189

Zero rim indicator on top and add or “lay on” face indicator, noting
rim indicator deflection that occurs - (Step 2). Double this __
(Step 3).
Add it to original total single indicator rim sag (Step 1). - 4).
(Step
This figure, preceded by a plus sign, will be the sag correction for the
rim indicator readings taken at bottom.
With field measurement setup as shown, zero all indicators, and de-
flect the indicator end of the upper bracket upward an amount equal to
the total rim sag (Step 4). Note the face sag effect by reading the face
indicator. This amount, with opposite sign, is the face sag correction to
apply to the readings taken at the lower position - (Step 5).
Now deflect the upper bracket back down from its “total rim sag” de-
flection an amount equal to Step 3.
The amount of sag remaining on the face indicator, preceded by the
same sign, is the sag correction for the single face indicator being read at
the top position - (Step 6).
All of the foregoing refers of course to bracket sag. In long machines,
we will also have shaft sag. This is mentioned only in passing, since
there is no need to do anything about it at this time. Our “point-by-point”
alignment will automatically take care of shaft sag. For initial leveling of
large turbogenerators, etc., especially if using precision optical equip-
ment, shaft sag must be considered. Manufacturers of such machines
know this, and provide their erectors with suitable data for sag compen-
sation. Further discussion of shaft sag is beyond the scope of this text.

Leveling Curved Surfaces
It is common practice to set up the “rim” dial indicators so their con-
tact tips rest directly on the surface of coupling rims or shafts. If gross
misalignment is not present, and if coupling and/or shaft diameters are
large, which is usually the case, accuracy will often be adequate. If,
however, major misalignment exists, and/or the rim or shaft diameters
are small, a significant error is likely to be present. It occurs due to the
measurement surface curvature, as illustrated in Figures 5-13 and 5-14.
This error can usually be recognized by repeated failure of top-plus-
bottom (T + B) readings to equal side-plus-side (S + S) readings within
one or two thousandths of an inch, and by calculated corrections resulting
in an improvement which undershoots or overshoots and requires re-
peated corrections to achieve desired tolerance. A way to minimize this
error is to use jigs, posts, and accessories which “square the circle.”
Here we attach flat surfaces or posts to the curved surfaces, and level