Orbit Plots
Orbit plots are plots that appear on an oscilloscope screen when two proximity probes are connected to the horizontal and vertical inputs, respectively. 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 oscilloscope dot vertically, and horizontal motion of the shaft will move the dot horizontally. Thus, when the shaft is turning, the dot will be stationary if the center of the shaft is stationary in the bearing. If the shaft is migrating around in the bearing, the dot will follow, and it will trace out the motion of the shaft center in real time. The instrumentation setup is shown schematically below:
Under normal operating conditions, the scope 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 at the left above 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 the bearing is worn in an oval pattern, with more vertical clearance than horizontal clearance.
An orbit pattern like the one shown above indicates shaft motion that moves in a figure 8 pattern. 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.
DLI Engineering provides products and services for Predictive Maintenance including vibration analysis instruments, monitoring and diagnostic software, and consulting for CBM programs.
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