Page 17 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

Here, the phase angle is expressed in degrees, where there
are 360 degrees in one cycle. Although both weights are vibrating
in this example, it is possible to express a phase angle of a single
vibrating weight with respect to a fixed object. Using the upper
limit of motion as a reference point, the phase angle can be ex-
pressed in degrees from a fixed reference point. In Figure 1-4,
weight A has a phase angle of zero degrees and weight B has a
phase angle of 180 degrees.
It should also be noted that both vibrating weights have the
same frequency, and thus will remain 180 degrees out of phase as
long as they both are in motion. As will be seen later, this is not
always the case. When the two vibrating weights have different
frequencies, they will come in and out of phase with one another.
This will result in what is known as a BEAT frequency. Generally,
beat frequencies are minimal unless the two sources are within 20
degrees of each other. The beat frequency is the frequency of the
two vibrating sources coming into and out of phase with one
another.
In Figure 1-5, the two weights are vibrating 90 degrees out of
phase with one another. In Figure 1-5, the two weights have the
same frequency and will remain 90 degrees out of phase as long
as they continue to vibrate. In Figure 1-6, the same two weights
are shown vibrating with different frequencies. Note the resulting
vibration when the two amplitudes are added to one another. Also

90°
A

A B

B
1 Cycle
(360°)

Figure 1-5. Phase Relationship 90° Out of Phase