Page 209 - Machinery Component Maintenance

Machiner! Alignment 191

them at top and bottom dead center. This corrects the error as shown in
Figure 5-14.
For this method to be fully effective, rotation should be performed at
accurate 90” quadrants, using inclinometer or bubble-vial device.
In most cases, however, this error is not enough to bother eliminat-
ing-it is easier just to make a few more corrective moves, reducing the
error each time.

Jig Posts

The preceding explanation showed a rudimentary auxiliary surface, or
“jig post,” used for “squaring the circle.” A more common reason for
using jig posts is to permit measurement without removing the spacer on
a concealed hub gear coupling. If jig posts are used, it is important that
they be used properly. In effect, we must ensure that the surfaces con-
tacted by the indicators meet these criteria:

As already shown, they must be leveled in coordination at top and
bottom dead centers, to avoid inclined plane error.
If any axial shaft movement can occur, as with sleeve bearings, the
surfaces should also be made parallel to their shafts. This can be
done by leveling axially at the top, rotating to the bottom, and re-
checking. If bubble is not still level, tilt the surface back toward level
for a half correction.
If face readings are to be taken on posts, the post face surfaces should
be machined perpendicular to their rim surfaces. In addition to this,
and to steps 1 and 2 just described, rotate shafts so posts are horizon-
tal. Using a level, adjust face surfaces so they are vertical. Rotate
180” and recheck with level. If not still vertical, tilt back toward ver-
tical to make a half correction on the bubble. This will accomplish
our desired objective of getting the face surface perpendicular to the
shaft in all measurement planes.

The foregoing assumes use of tri-axially adjustable jig posts. If such
posts are not available, it may be possible to get good results using accu-
rately machined nonadjustable posts. If readings and corrections do not
turn out as desired, however, it could pay to make the level checks as
described-they might pinpoint the problem and suggest a solution such
as using a nonpost measurement setup.