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Shaft Orbit Simulator

On machines with fluid-film bearings, two orthogonal proximity probes measure how the shaft moves inside its bearing. Combined, the X and Y signals form the orbit, which helps tell apart unbalance, misalignment, preload and oil whirl.

1X
2X
1X precession —
1X ellipse major axis (pk-pk)
— µm
1X ellipse minor axis (pk-pk)
— µm
Ellipticity (minor/major)
—
Largest peak-to-peak motion
— µm
Rotation frequency
—

Orbit and X/Y time signals (dot: once-per-rev reference)

These tools are intended for preliminary assessment and training. Decisions on critical equipment should be based on on-site measurement and expert evaluation.

How to use

  1. Enter the speed and direction of rotation.

  2. Enter the 1X amplitude and phase of the X and Y probes (pk-pk, µm); equal amplitudes 90° apart give a circular orbit.

  3. Add a 2X component to explore the banana and figure-eight orbits seen with misalignment and preload.

Theory & formulas

Each probe gives x(t) = Axcos(ωt + φx), y(t) = Aycos(ωt + φy); the orbit is the X–Y plot of the two.

The 1X ellipse is the sum of a forward and a reverse circular component: rf = |X + jY|/2, rb = |X − jY|/2. Major axis rf+rb, minor axis |rf−rb|.

If rb > rf the precession is reverse, a warning sign of rub or heavy preload.

A very flat orbit points to directional stiffness differences; inner loops from 2X suggest misalignment or preload.

Frequently asked questions

What does the dot on the orbit show?

It marks the moment of the once-per-revolution (keyphasor) pulse. Its position gives the 1X phase; several dots on the orbit point to subsynchronous vibration.

On which machines is orbit analysis used?

On turbines, compressors, large pumps and generators with fluid-film bearings and two orthogonal proximity probes. On rolling-element bearing machines, casing vibration is measured instead.