Orbit and shaft centerline analysis is very useful when dealing with rotor vibrations. This analysis is also called rotor-dynamics. The design of bearings in large rotating machinery or very high-speed shafts is so that a mechanical gap exists between the rotor and the bearing race. This gap is typically filled up with pressurized oil. The existence of this gap let the rotor vibrate by itself within the bearing independently of the rest of the machine. The rotor as so to speak, a space of freedom to move: this is called the clearance circle. The point of orbit analysis and shaft centerline analysis will be to display and analyze the displacement of the rotor center within the clearance circle. The combination of both analyses will in the Full Shaft Motion display as shown below.
The analysis can be achieved based on signals obtained from sensors called proximity probes that are typically equipping the bearings of rotating machinery for protection and condition monitoring purposes. Portable diagnostics instruments such as OROS systems are typically connecting to the buffered output BNC connectors. Provided signals can typically reach relatively high DC voltages. OROS instruments can acquire results up to +/- 40 Volts. The orbit will be representative of the vibration of the rotor and will show the displacement pattern of the rotor center during one or several rotations. The shaft centerline is representative of the position of the shaft center within the clearance circle;
The orbit and shaft centerlines displays are key tools for achieving proper diagnostics of turbomachinery trains composed of turbines, electric machines or even gear boxes. It is an efficient tool among others to carry an efficient diagnostic of typical rotating machine faults.
The shape of the orbit and its evolution as a function of RPM is representative of typical faults machines using fluid film bearings. In particular, sub-harmonic phenomena (below 1X), such as oil whirl and oil whip can be diagnosticated using these tools.