Machinery's Handbook, 31st Edition
2722
SHAFT ALIGNMENT
Fig. 3. Indicators Set Up for a Runout Check In field applications, mount the face dial as near as possible to the rim indicator. The face indicator is drawn at the 6 o’clock position in both views for clarity.
Concerning Item 3) in the previous list, it is possible (however unlikely) for a coupling that was bored off center and would have shown up as coupling runout with a rim indicator test except that a slight bend in the shaft has canceled out what would normally have been evident due to the off-center bored coupling. This odd situation can cause vibration even when a standard test proves there is no misalignment. Coupling runout can also be caused by too large a coupling bore, with the slack being taken up by set screws. Interpreting the Indicators: The actual amount of runout registered by the rim indicator is half of the difference between the plus and minus reading extremes. Example: A rim indicator is mounted as shown in Fig. 3 and set on zero. Then, several full turns of the shaft reveals a maximum minus of − 0.006 inch and a maximum plus of +0.012 inch on the indicator. The total variation is 0.018 inch and half of 0.018 is 0.009. Thus, the result of Items 1), 2), or 3) is 0.009 inch of eccentricity, or 0.018 inch TIR (total indicator reading) of coupling runout. Note: Much less coupling runout than is described above is usually expected. If the face indicator shows zero at all points around a coupling while the rim indicator moves between +0.XXX inch and − 0.XXX inch, it is probable that the coupling was bored off-center. If the rim indicator shows practically zero movement while the face indicator moves between − 0.XXX inch and +0.XXX inch, the cause could be Item 2) or 3) above, or the coupling was bored essentially centered, but at an angle other than perpendicular to the face of the coupling, as is shown in Fig. 4.
Fig. 4. Coupling Bore Centered but Not Perpendicular to Coupling Face Face Indicator Shown, Rim Indicator Not Shown
In Fig. 4, both the indicators shown will alternate from plus to minus and back again as the coupling being checked is rotated. Even if the shaft centerline goes through the center of the coupling, it will do so at an angle other than perpendicular to the face of the cou- pling. The result is serious coupling runout on the face indicator and less serious runout on the rim indicator. Face runout can easily show up when there is none. If a machine has sleeve bearings and if the shaft is not axially restrained by a thrust bearing during the runout test, severe face runout will likely be indicated when little, if any, exists. Note: Unacceptable runout of either or both couplings should be of little concern in the alignment process. The extended centerlines of the two shafts can be aligned even when
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