Hi,

Just got a call from my client. Here are the details.

The machine is turbine coupled to gearbox, 6 MW machine.

Its on MRT (Mechanical run test) on the test bench

Turbine speed: 7500 rpm
Gearbox output: 1500 rpm

no generator since its on the test bed

alignment is OK

Turbine casing and shaft vibs well within limits
Turbine front and rear max 25 microns

Gearbox (pinion) high speed side (HSS) DE and NDE 55 and 60 microns

Gearbox (bull gear) low seed side DE and NDE: data not taken and will be taken tomorrow

OK the complaint from the client was upto 7200 rpm the GB HSS DE and NDE measures around 25 microns. once the pinion speed crosses 7200 rpm the vib on GB HSS DE and NDE increases and reach 60 mic on shaft at 7500 rpm.

a predominant 3X freq component seen in both casing and shaft spectrums.

vib readings and spectrums awaited.

bump test/NFT not carried out, critical speeds of the GB not known. critical speed of the turbine 2700 rpm. client says alignment is OK and balancing as per norms.

any clues about this 3X freq component coming from the HSS DE and NDE side appearing after 7200 rpm.

will come back to you with more data.

Cheers,

Madhu

Madhu,
I had a case like this once. The bump tests proved nothing. The predominant 3x was present, no matter what load, and also happened at low speed and high speed motor rpm.
The answer?
The pinion gear had been hobbed in a three jaw chuck with too much tool pressure. The result? The shaft would deflect between the jaws and therefore there were three spots on the pinion that were 'high'.

What Ron describes could happen, but in my experience also yields a dominant 3X in the slow roll signal as well, not something that comes on with speed, and in your case relatively high speed. In addition in my experience this usually only happens on the end of the rotor that is chucked up (typically drive end); the end with a "dead" center won't have the issue (NDE). Also, the lathe chuck has to put pressure over the probe target areas. Any DECENT gear shop knows to watch out for all the above. Your slow roll should be 25% of running vibration limit or 0.25 mil, whichever is greater as recommended by API.

By the way 4-jaw chucks can also cause the issue with a 4X component.

What are your tooth combinations? Also coupling type? Not perhaps a polygon fit type of coupling; I have seen these when out of spec "open" due to bore growth at speed and cause a 3X since the bore shape is somewhat triangular.

John from PA

Madhu,

A shot at the Diagnostic Dart Board: Possible torsional vibration, since there is no generator (dynamic mass)connected and no load on the gear teeth. Looseness (chatter) could be in gear coupling (if used), quill shaft (if used) or in reduction gear.

Walt

Thanks,

OK i agree with how a 3 jaw chuck holding can create problems at site conditions.

but the 3X freq component is not to be seen until 7200 rpm.

its appearing only after 7200 rpm and will increase further when the train reaches 7500 rpm.

vibs are within 25 mics upto 7200 rpm and reach 60 mics at 7500 rpm on GB HSS DE and NDE.

Horizontal vibrations are high compared to vertical and axial

no 3X component found on turbine front and rear.

3X component seen in both casing and shaft vib data (GB HSS DE and NDE), what this indicates...???

should a physical check to be made on the pinion side. please advice

waiting for the bull gear HSS data.

Regards,

Madhu

Madhu,

Of course to suspect a resonance in the gearbox somewhere in the 7500 CPM region is reasonable. It could be a torsional also as has been mentioned above. The question is how to excite one, torsional in particular? I wish someone can give a suggestion. For the lateral resonance is better to do a run up instead of bump test considering oil film influence. The waterfall may possibly show resonance excited by the 4th harmonic at a lower speed.

Please keep us informed about your findings.

David