VMI VIBER X5 MKIII, Instrument Manual, Page: 74 / 121

74
7.11. Orbit
The Orbit (XY Shape) Measurement will display a
diagram of X value versus Y value, using two
measurement channels.
From Main menu select Measure , Advanced and Orbit .
The measurement requires 2 transducers of same type
connected to the VIB 1 and VIB 2 inputs. The directions of
the two transducers must be perpendicular (90 degrees angle).
If the transducers used are displacement transducers, this diagram is the relative orbit of
the shaft.
The proximity probes are installed in a sleeve bearing, one oriented vertically and one
oriented horizontally. Under these conditions, vertical motion of the shaft center line will
move the value vertically, and horizontal motion of the shaft will move the value
horizontally. When the shaft is turning, the X and Y value will be stationary if the center of
the shaft is stationary in the bearing. If the shaft is migrating around in the bearing, the
value will follow, and it will trace out the motion of the shaft center on the diagram.
Under normal operating conditions, the instrument will show a circle, indicating the oil film
has equal stiffness and thickness in all directions, and there is some imbalance causing the
centerline to move in a circle.
The orbit plot at the left shows an ideal condition of the shaft in the
bearing, while the one on the right shows that the shaft is moving more in
the vertical direction than it is in the horizontal direction. This may mean that the bearing is
worn in an oval pattern, with more vertical clearance than horizontal clearance.
An orbit pattern that shows shaft motion in a figure 8 pattern, indicates that it is
vibrating twice as fast in one direction than in the other one. This can be caused
by excessive clearance in one direction, or a bearing worn into an oval shape. A
pattern like this is a danger sign, for the journal is likely to develop metal-to-metal
contact with the bearing, causing extensive damage to both.
7.11.1. Orbit Measurement Settings
In the Settings menu you can set the following parameters:

Number of periods to display (1, 2, 4, 8) related to
measured or estimated speed, function of reference
transducer usage.

Tacho (Enabled, Disabled) Reference transducer

Estimated speed . The estimated speed will be used to determine the acquisition
time when a reference transducer (Tacho) is not used, or as a default value for
speed.
The Settings menu has also shortcuts to the following related menus:
Figure 0-20: Orbit Selection
Figure 0-21: Orbit Settings