Fact #2: Comparing
the phase angles at 2 separate transducer locations (vertical and horizontal
in our example) allows you to make the rough judgement of the shape of
the orbit - one that is more circular or one that is flatter in shape.
Before explaining
this, you must understand what is meant by the 'shaft orbit'. Consider
the following:
-
A shaft is generating vibration at only
1x rpm.
-
The amplitude horizontally is 'X' ips
or mm/sec.
-
The amplitude vertically is exactly
the same - 'X' ips or mm/sec.
-
In fact, no matter what direction the
transducer is oriented (pointed), the amplitude at 1x rpm is 'X' ips or
mm/sec.
-
By plotting the recorded amplitude values
in the appropriate direction (Figure 1), you can plot the 'orbit', or shape,
of the shaft center point.
-
In this particular example, the orbit
is a circle. In the 'real' world, this would be virtually impossible.
|
Figure 1
|
-
Another shaft is generating vibration
at only 1x rpm.
-
The amplitude horizontally is 'X' ips
or mm/sec.
-
The amplitude vertically is 80% of the
horizontal amplitude - 0.8(X) ips or mm/sec.
-
By plotting the recorded amplitude values
in the appropriate direction (Figure 2), you can again plot the 'orbit',
or shape, of the shaft center point.
-
In this example, an elliptical orbit
is plotted. These relative amplitudes would not be considered unusual.
|
Figure 2
|
-
A third shaft is generating vibration
at only 1x rpm.
-
The amplitude horizontally is 'X' ips
or mm/sec.
-
The amplitude vertically is 50% of the
horizontal amplitude - 0.5(X) ips or mm/sec.
-
By plotting the recorded amplitude values
in the appropriate direction (Figure 3), you can again plot the 'orbit',
or shape, of the shaft center point.
-
In this example, an elliptical orbit
is plotted. These relative amplitudes would not be considered unusual.
|
Figure 3
|
When the amplitudes are relatively equal,
you can see how the 'orbit' approaches a circle. But let's look at the
other extreme. What if the horizontal amplitude were 'X' and the vertical
amplitude were very, very low - even 0.00 ips or mm/sec. That 'orbit' would
be a straight line - linear movement (vibration). |
