Could any of you give a practical description of the the change in the nature of the motion of a system that has a gradual increase in the ration of 1x to 2x?

I have a system with practically non-existent 1x, dominated by 2x.

I would expect (and it is in this example) that the 2x element (in WF) is roughly equal as it cycles through time, thereby "covering up" the 1x.

Agree/disagree?

Martin,

I disagree.
If 2xSS is dominant in spectrum, then it should be dominant in waveform. Measured vibration is the result of dynamic force and structural response. If vibration increases over time, then it can be due to an increase in dynamic force (say unbalance or misalignment) or a change in structural response (change in natural frequency or damping). Forget the coverup or conspiracy theory and just deal with the basic physics and dynamics.

Walt

Ok, maybe I created a misunderstanding.

The 2x is dominant in the waveform and the spectrum. And I expect that. The question is this:
If you still have the mass of the system moving at 1x, but the 1x is nonexisent in the spectrum and wf, then isn't what used to be a adominant 1x peak, spread out in the wf (and spectra) into 2 2x waves? Essentially the same energy, expressed in a different way.

And furthermore, if you have a perfect 2x waveform, would not you expect a 1x spectral peak to appear if the perfect 2x waveform began to have every other wave of different amplitudes?

No conspiracy theory!

The fact that spectrum is dominated by 2x is indicative rather of unisotrpoic properties of the shaft (due to the keyway, crack, etc.)then shaft misalignemnt. This vibration is caused by a force acting at 2x. Presence of 1x could be viewed (interpreted) either as imbalance or modulation of 2x force by the imbalance force. The TWF is going to show in this case a higher excursion every second cycle.

In cases of shaft misalignment 2x is caused due to non-linearity in vibration response vs. force. Sometimes 2x won't show up even if physical misalignment is present depending on system response and coupling type. On another hand, phase across the coupling will definitevely indicate misalignment if present.

There is 180 dg Phase shift across the coupling. I was more intererted in the 1x to 2x ratio and its implications.

1X WF is the carrier of the 2X, isnt it?. In fact residual unbalance is always present. When you see a clear 2X WF could be due to high spot of both occur at the same time. What would happen if 1X phase is untuned?, maybe you would see 1X WF like carrier of 2X.