Page 115 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Misalignment of Machine Shafts

able to transmit all the required power, there are other problems
as well. Belt tension directly affects the natural frequency of the
belts. Belts that are either too tight or too loose can go into reso-
nance. This in turn will cause a vibration which can be harmful to
the bearings and seals.

PHASE ANGLE RELATIONS

Although phase analysis is useful in determining whether a
bent shaft or misalignment is the source of the vibration, it is not
foolproof. The easiest solution is to check the alignment with dial
indicators. Precise methods are discussed in following chapters. It
is best to check the alignment with the equipment at operating
temperature to assure the problem is not due to thermal growth.
The first step in using phase angles as a diagnostic tool is to
locate a reference point on the shaft or coupling that is easily vis-
ible throughout the entire 360 degrees of rotation. Often, the key-
way in the shaft serves this purpose well.
A vibration analyzer with a strobe light attachment is re-
quired to take these readings. The direction of the vibration
pickup must not be changed from reading to reading. Changes in
the direction in which readings are taken will cause a phase shift.
The types of readings taken (displacement, velocity, accelera-
tion) cannot be changed when taking phase readings. To do so will
change the phase angle.
Since the objective is to determine a bent shaft from misalign-
ment, the strobe light should be set to trigger on the vibration, but
the filter should be used to assure the vibration source is a 1 ×
rpm. The filter should not be re-tuned during the other phase
readings.
For the initial test, four readings around the bearing should
be taken. Typically, the 12, 3, 6, and 9 o’clock positions should be
used. The end of the shaft can be viewed as a clock face so that the
phase mark can be referenced to the points of the clock. A reading
at 3:00 o’clock would be 90 degrees from the top; a reading of 7:00