Shaft Orbit Analysis

Think of the shaft of a running machine — a steam turbine, compressor, centrifugal pump, fan or motor — as a pen. The shaft center does not stay still but moves around the bearing center. Observing the shaft from horizontal and vertical directions at the same time and joining the positions of its center gives the shaft orbit, an electrocardiogram of the rotor.

The window shows the X and Y waveforms at the top, the orbit with the limit circles below, channel and criterion selection on the left, and the parameter table and zone result at the top.

Wherever a shaft runs in plain bearings with eddy-current probes, the shaft orbit applies.

Large low-frequency motions that accelerometers do not show well are the domain of eddy-current displacement probes.

Step 1: orbit from synchronized channels

Samples (Xi, Yi) of the X and Y channels from the same acquisition frame are joined in time order. Both channels must be sampled from the same clock; any phase offset distorts the orbit, which is why the orbit is always drawn from one frame. The DC component of each signal is removed to obtain the AC orbit, and the mean is kept for the mean shaft position (static offset).

Step 2: parameter extraction

Peak-to-peak values Xpp and Ypp are the differences of the time-domain extremes. The ellipse axes and main vibration direction come from a second-order statistical fit of the orbit points. Precession is determined from the accumulated sign of the cross product of successive shaft positions: motion in the direction of rotation is forward precession, otherwise reverse. Running frequency is estimated from the spectrum of the X channel AC component (Hanning window), taking the largest line between 1 and 400 Hz and converting it to r/min.

Step 3: harmonic components

The X and Y amplitudes at the estimated running frequency fr give the 1× component (the unbalance signature) and those at 2fr the 2× component (signature of misalignment and looseness). The 2×/1× ratio is a quick screening indicator. Non-power-of-two records are zero-padded and the frequency axis uses the true line spacing.

Step 4: zone evaluation

Spp = max(Xpp, Ypp) is compared with the three limits of the selected criterion:

The three limits are drawn as dashed circles (radius = limit / 2), so it is clear without reading values whether the orbit touches a boundary. The circles are a simplified visual aid; the formal evaluation is always the comparison of the Spp value with the limits.

Test-mode demonstration data, read in four steps.

1. Example display: Spp = 36.7 µm. The source gives inconsistent speed-tier and zone labels for its 90 / 170 / 280 µm limits, so the applicable tier and zone conclusion are withheld. The synthetic drawing illustrates geometry only; it does not authorize continued machine operation.
2. Shape and size: a stable ellipse without sharp corners or inner loops; peak-to-peak about 16 µm in X and 37 µm in Y — a slightly flattened ellipse, typical of motion dominated by one harmonic.
3. Frequency content: the running frequency is estimated at 50.00 Hz (3000 r/min); 1× is 30.0 µm and 2× 6.0 µm, a 2×/1× ratio of about 0.2. The energy is concentrated at running speed and the hint points to unbalance — the condition simulated by the demonstration.
4. Position and direction: the example has a steady mean offset and forward precession. Neither orbit direction nor a dominant 1× component proves a diagnosis or excludes rubbing or oil-film instability. Confirm probe polarity, speed, operating conditions, spectra and trends before a machine assessment.