Steve’s Handy Guide to Orbits By Steven M. Schultheis, P.E. Houston, Texas U.S.A.

Basics The plots usually show 8 revolutions. From keyphasor dot to keyphasor dot is one revolution. A normal orbit should be slightly elliptical with one keyphasor dot, and low vibration level. Distorted, or twisted orbits are usually due either to rubs or to runout. Multiple keyphasor dots indicate a subsynchronous vibration. All the plots presented are unfiltered and uncompensated. I live in the U.S.A. so all units are mil pp. Multiply my units by 25.4 for microns ie 1 mil pp=25.4 micron

Instabilities The first one is a 0.37X frequency component due to an aero instability in a compressor. The second is a very low level instability due to oil whirl on a motor with plain journal bearings. The third is another aero instability in a compressor. The last one is actually due to surge, notice the way from revolution to revolution the amplitude changes drastically.

Aero Instability

Low level oil whirl

Aero Instability

Compressor Surge

Casing Vibration Orbit The next one is a casing vibration orbit from an ID Fan. OK, it is kinda fuzzy, but you can see that the motion is primarily vertical, and the amplitude is pretty high at 0.5 ips-pk. This is due to a vertical structural resonance.

Velocity orbit on ID fan bearing housing

Unbalance The first plot is high 1X due to unbalance on a motor. In this case the unbalance is due to a bow induced by a hot spot. The second plot is also due to motor unbalance. The first thought might be misalignment due to the highly elliptical orbit, but this is actually due the fact that the motor is much less stiff horizontally than vertically. Balancing would reduce this vibration, but the orbit would remain pretty elliptical.

High 1X on motor due to rotor bow

High 1X due to unbalance, and assymetric stiffness.

Rubs and Preloads The first is due to casing distortion in a compressor that causes so much preload on the shaft that a rub is probably occurring as well. The second is data from the same machine as the distortion load changes. The third is a rub in a compressor due a failed thrust bearing. The fourth is a rub in a compressor due to a diaphram breaking loose and contacting a wheel. The fifth is a pure 1/2X impeller eye seal rub in a compressor induced by unbalance.

Rubs continued The sixth shows high vibration going through the critical after a rub induced bow causes the turbine to trip off line. The seventh shows a rub induced bow on a turbine during startup due to improper warm up. Most likely rubbing on the high pressure packing.

Rub during thrust bearing failure.

Rub on 1st stage wheel when the diaphragm broke loose and moved into contact.

1/2X rub due to eye seal contact 1/2X rub due to eye seal contact. The squiggly trace is due to electrical noise. Notice two distinct sets of keyphasor dots indicating two vibration cycles per revolution, or 1/2X.

Orbit plots at the 1500 cpm critical speed of the turbine during trip due to rub induced bow. Vibration amplitudes are 9 mil pp on the outboard and 8 mil pp on the inboard. Inboard Outboard

Rub induced bow in a turbine

Runout Scratches, nicks, dings, magnetic spots, chrome, and other surface irregularities affect the proximity signal and cause noise errors. The first one is a pump where I installed temporary probes on an untreated surface. The orbit was not much use, but the bode plots, spectrums, an position plots helped diagnose the problem. The next two plots show very distinct scratches on the shaft surface.

High runout level on shaft makes unfiltered orbit meaningless