Timewaveform 101

[flacrkr]

Being admittedly poor in timewaveform analysis skills, I have a question concerning flattened/truncated waveforms.
If waveform is "clipped" through full cycle (+ to -) is this rotor rub, data taken on overhung belt driven fan, inboard bearing vertical.If motor bearing is bad could this transfer to fan shaft in the form of rotor rub.What is full cycle flattening telling me?

 

[steve383]

A clipped waveform indicates a rub. Do you have half orders in the spectrum? Can you use your software to see the HZ from one clip to the next? This would tell you if it is turning speed. Truncation of the waveform (higher + then -)indicates more movement(towards the acellerometer) in one direction then the other. Possible soft foot\loose bearing. The bad motor bearing would not transfer to the fan in the form of a rub. An out of round fan sheave will give you a high fan turning speed 1x's in the spectrum and truncated waveform inline with the belts on the inboard fan bearing as the belts tighten and loosen. Using the courser from peak to peak in the waveform and noting the HZ is very informative. Be aware that the HZ in the waveform from peak to peak may be off a few HZ from the spectrum peak due to sampling. A high gearmesh at 1251.6 HZ may show up in the waveform peak to peak at 1254. Is the cipping only on the fan inboard? I would think the fan rub would be more dominant on the outboard closest to the fan.

 

[electricpete]

Since this is belt driven machine I assume rolling bearings and I assume these are housing acceleration measruements.

I see the clipped pattern on the Diagnostic chart indicating rub as mentioned above. However, imho by physical reasoning one would expect this pattern for sleeve bearing machine monitored by prox probe, but not for rolling element machine monitored by housing accelerometer (rub on rolling element machine monitored by housing accelerometer would look like impacts, would it not?).

I will be glad to understand if someone thinks I am mistaken on the above opinion.

Sorry I don't have any constructive suggestions. I have not seen that pattern.

 

[vanstoja]

Berry's vibration diagnostic chart [Sound and Vibration, Sept.1990, pg.17] says that type B mechanical looseness causes a truncation of the time waveform. The illustrated frequency-amplitude spectrum for type B mechanical looseness shows 1X-11X peaks plus 0.5X ,1.5X and 2.5X. He says "Type B is normally generated by improper fit between component parts which will cause many harmonics due to nonlinear responses of loose parts to dynamic forces from the rotor...Type B is often caused by a bearing liner loose in the cap, excessive clearance in either a sleeve or rolling element bearings, or a loose impeller on the shaft. Phase is often unstable and may vary widely from one measurement to the next, particularly if rotor shifts position from one startup to next...looseness will often
cause subharmonic multiples of 1/2 or 1/3 times RPM...[and]will often allow excessive vibration for relatively small unbalance or misalignment."
Continuous or intermittent rubbing between bearing, shaft, impeller components would seem to be one possible manifestation of mechanical looseness as suggested by electricpete. Constraint of any periodic motion whether rotational or linear seems to be the key to time-amplitude waveform truncation which may or may not entail physical contact or rubbing. For instance, in fluid film bearings, truncation of transient rotor displacement by an impenetrable bearing fluid squeeze-film might truncate the 1X waveform without allowing any physical contact between the shaft journal and the bearing wear surface.
A paper by F.F.Ehrich (not presently onhand) discusses "beat-frequency truncation" of vibration waveforms and mentions some interactions to produce other spectral peaks that are not represented in Berry's Type B mechanical looseness spectrum.

looseness spectrum

 

[flacrkr]

Sorry for the "truncated" info, bad pun I do have fan speed harm's out to 17x run speed and 1/2x subs to 13.5x
The components that have been ck'd are - brg' loose on shaft,sheave loose/concentric, and motor supports with all checking out o.k. The phase is unsteady. My data is limited to two permanently mtd. transducers, vertical on pillowblock roller bearings.

 

[steve383]

If the phase is unsteady you have a resonance, rub or looseness. The multiple harmonics and half orders are an indicator of looseness. The bearing to shaft could be loose. I had loose set screws once that would allow the inner race to spin on the shaft. I have seen looseness from the outer race to the pillow block. I have also seen where the key way was worn allowing the impeller to move back and forth on the shaft. Also check for cracked welds on the frame work and be sure it's frame to floor bolts are tight. Loosen one at a time and see if you can slide shims in. Tighten down and go to the next.