How to interpret Orbit and Timebaseplot Reference Point

[jenz]

Hi,

the orbit and timebase plot includes a "red point" which is called reference point. I don't realy understand that meaning.

Please, have a look at the picture "orbit_timbase_plot.PNG".

Many thanks for your help!

[jenz]

Next question I have is what does it mean, if the "red dot" changes his position while the speed of the shaft rises.

Please, have a look at the pictures:

(1) Input_Signal_Probes_200k_Drehzahl_2k.PNG ... it shows the input signals of the probes (x,y) and the shaft speed

(2) Orbit_1_rev_Timebase_8_rev.PNG ... it shows the timebase and orbit plot (the red dot is changing his position)

Thanks 🙂

[QiaoGuo]

As mentioned in the help of Sound and Vibration, "the most common use for the orbit, timebase, and shaft centerline plots is to monitor turbomachinery with fluid film bearings. Some turbomachinery mechanical faults have characteristic plot shapes."

For example, reference point in a 1X filtered orbit plot can indicate several phenomena,

1. The angle from reference point to the direction which has maximum length to the origin is the 1X phase.

2. Stable working condition has fixed shape of elliptical 1X orbit plot and fixed position of reference point. Position changes of reference point may indicate imbalance, rotor rigidity changes due to the temperature, etc.

You can compare the acquired plots with any known characteristics to detect faults and diagnose machine problems.

Hope my answer helps.

[preston_johnson]

The movement of the red dot, indicates a phase change of the X and/or Y vibration.  As speed changes of  a machine, the machine may pass through a resonance speed, in which there will be up to a 180 degree change in phase.  The orbit plot can become a straight line and then "flip over" in these cases. 

[jenz]

Thank you very much for your information, it helps to understand what's going on 🙂

But how can I extract the phase information of the red dot for offline analysis? In LabVIEW S&V you only have a reference value? How can I assign this values to the time signals (x and y)?

Regards Jens

[jenz]

This picture shows what I mean.

[QiaoGuo · Accepted solution]

SVX Orbit and Timebase Plot (Filtered) (N Ch - M Orders).vi accepts the magnitude input and phase input from OAT Order Magnitude and Phase (Even Angle Signal, N Channels).vi.

If you want to extract the phase information of the red dot, you can just get them from the 2D array of cluster "phases [phase lag]".

When the "bandwidth [order]" is set to 1 and "x-axis selection" is set to Time, the "phase" array in "phases [phase lag]" corresponds to the phase of each red dot, the "time/rev/rpm" array in "phases [phase lag]" corresponds to the reference value of orbit/timebase data. 

[CipherThis]

The red dot represents the keyphasor trigger time.  The "phase angle" of the waveform is measured from the "High Spot" or maximum amplitude of the waveform to the red trigger dot.  Time between trigger dots equals 1 revolution, or 360 degrees.   The phase relationship will change as speed changes due to mass, stiffness and damping of the rotor.  At critical speed, the phase angle will have shifted 90 degrees. At low speeds, the high-spot will be close to being in phase with mass eccentricity of the rotor.  As speed increases, the "phase lag" between the heavy spot (mass eccentricity) and the "high spot" or maximum amplitude will increase.  The delay is primarily due to damping and the relationship between shaft stiffness and the mass of the rotor.  Most books on vibration and rotor dynamics have this information in the first chapters.   A good example of this is if one suspends a tennis ball below a slinky spring from their hand.  At super-slow frequencies, the slinky doesn't stretch...it appears infinitely stiff and the ball moves the same as the hand...or "in phase".   Move the hand a little faster and suddenly the ball isn't moving in phase with the hand anymore.  If you had a good way to measure it, at resonance, the motion between the hand and the motion of the ball would have a 90 degree phase difference.  At frequencies above the natural frequency, the hand can actually be moving opposite the ball (a 180 degree phase shift between excitation and response).  At very high frequencies, the ball doesn't move at all.  That is because the mass of the ball controls vibration above resonance.  That is the region called "isolation" as the vibration is completely absorbed by the spring.  The same thing happens in rotors but it is a little more complicated.  Anyway, that's where the shifting red dot with speed changes comes from.  By the way, in this example, the red dot would measure the high spot of the hand motion and the waveform would measure the motion of the ball.  The phase relationship would be measured from the dot to the maximum amplitude spot of the waveform.

[jenz]

Thanks Cipher This, it's a very nice explanation.