Page 130 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

The fixture is now attached to a rigid pipe or shaft, and the
bar sag can be measured. Bar sag is very important in any preci-
sion alignment technique. Not compensating for bar sag can create
errors sufficient to void the best alignment process. It should be
noted that the error created by not properly compensating for bar
sag is multiplied over the length of the machine. Thus an error of
2.5 mils over a 12" distance becomes an error of 11.7 mils on a
machine foot 56" away.
To measure the bar sag, the pipe with the fixture attached is
rotated so that the rim indicator is in the bottom or 6 o’clock
position. The indicator is then set to indicate zero.
The entire fixture and pipe are now rotated to the top or 12
o’clock position and the indicator read. It will indicate a plus (+)
reading which is the bar sag. The bar sag must be recorded on the
alignment form for future reference.
The fixture should now be re-attached to the machine in the
12 o’clock position. By leaving the rim indicator set to the (+) bar
sag reading, no further compensation will be required for bar sag.
Always assure the rim indicator is set to the (+) bar sag when it
is in the 12 o’clock position.

BASIC MACHINE MEASUREMENTS

The next step is to measure the machine distances. As with all
measurements, care should be exercised to measure the distances
as accurately as possible. The most critical measurement is the
diameter traced by the face indicator as it is rotated around the
machine. This distance should be measured to within ±1/8 of an
inch.
Normally, it is easier to rotate the fixture so that the face
indicator is in the 3 o’clock or 6 o’clock position, then measure
from the center of the shaft to the center of the indicator stem, and
multiply the measured distance by two. This distance becomes the
A distance on the alignment work sheets.
Next the distances to the hold-down bolts are measured.