Page 200 - Machinery Component Maintenance

182 Machinery Component Maintenance and Repuir

9. For long spans, adjustable clamp-on jigs are available for reverse-
indicator application, without requiring coupling spacer removal.
Face-and-rim jigs for long spans, by contrast, are usually nonad-
justable custom brackets requiring spacer removal to permit face
mounting.
10. With the reverse-indicator setup, we mount only one indicator per
bracket, thus reducing sag as compared to face-and-rim, which
mounts two indicators per bracket. (Face-and-rim can do it with
one per bracket if we use two brackets, or if we remount indicators
and rotate a second time, but this is more trouble.)

There are some limitations of the reverse-indicator method. It should
not be used on close-coupled installations, unless jigs can be attached be-
hind the couplings to extend the span to 3 in. or more. Failure to observe
this limitation will usually result in calculated moves which overcorrect
for the misalignment.
Both coupled shafts must be rotatable, preferably by hand, and prefera-
bly while coupled together. If only one shaft can be rotated, or if neither
can be rotated, the reverse-indicator method cannot be used.
If the coupling diameter exceeds available axial measurement span,
geometric accuracy will be poorer with reverse-indicator than with face-
and-rim.
If required span exceeds jig span capability, either get a bigger jig or
change to a different measurement setup such as face-face-distance.
Cooling tower drives would be an example of this.

Face-and-Rim Method

This is the “traditional” setup which is probably the most popular, al-
though it is losing favor as more people learn about reverse-indicator.
Advantages of face-and-rim:
1. It can be used on large, heavy machines whose shafts cannot be
turned.
2. It has better geometric accuracy than reverse-indicator, for large di-
ameter couplings with short spans.
3. It is easier to apply on short-span and small machines than is re-
verse-indicator, and will often give better accuracy.

Limitations of face-and-rim:
1. If used on a machine in which one or both shafts cannot be turned,
some runout error may occur, due to shaft or coupling eccentricity.