Gentlemen,

I have the following:
Gas Turbine driven generator through a single helical gearbox. The GOB is on the exciter side. The inboard side is upported on load gearbox. So it is one bearing rotor.

I got the following orbit shape and shaft center line plots from Generator bearings.

I have high vibration on the inboard side in horizontal and vertical and very high axial on the exciter side.

I would like to have your feed back and share with others.

Thanks
SAM

This message has been edited. Last edited by: XYZ,

Tell us a bit more about this gearbox; manufacturer, model, etc.

Also, is this an arrangement like a GE load box that uses a quill shaft?

Can you post some time domain and spectrum plots in addition to the existing orbits?

John from PA

additonal data

Orbit.pdf (242 Kb, 77 downloads)

From the polar plot it appears as if the generator balance resonance or critical speed is around 2000 RPM.

Unfortunately, as far a the drawing of the gearbox it isn't too helpful. The Graffenstaden drive is often a double helical with a quill shaft arrangement. See if you can find a cross sectional drawing of the drive or let us know what type of machine you have (Frame 5 for example).

John form PA

From the spectrum you have two frequencies that pick up at, about 25 Hz and at 85 Hz, when you get to full speed. I am not sure what frequencies are in the gearbox. The 25 Hz could be a whirl condition. Normally I would expect it to lock into the 2030 critical speed. It looks like it is less than that.

Doesn't it look like GIB has a rub based on the TWF's "bounce-off" pattern. It is hard to make any conclusions in this regard from the orbit as it has so much noise. Is noise electrical?

It is hard to interpret GIB V and GIB H polar plots. It appears that phase lag at resonance ( around 2000 RPM) and that at 1x (3600 RPM) are the same. Maybe Bode plot will give a better representation.

Also, 25HZ=0.41x60HZ, looks like whirl.

I attached the bode plot.

This message has been edited. Last edited by: XYZ,

Bode.pdf (53 Kb, 31 downloads)

XYZ,

It appears to me that this is an unbalance issue possibly aggrevated by a rub causing shaft bow.

A big question mark also is source of 85 HZ.

Instability, signified by whirling at 25 HZ, is also a problem. From the orbit on IB bearing it is hard to see whirling precession. A filtered orbit may help here.

Shaft centerline position at IB bearing has to be evaluated. Possibly bearing became unloaded due to misalignment.

As far as bearing parameters go, one can see how much OB and IB bearings are different (from Bode plots).

Dave

The 85 Hz is the driver. Hard to see and I am too lazy to get out a ruler what frequency is the subsynchronous?

Mark,

Rulers can be safer to scratch with than letter openers. However, use with care.

Dave_man,

I completey agree with you regarding the unbalance aggrevated by rub. The kind of damage is clear indication of rub.

The problem is that for one bearing generator rotor, there is no balancing plane at the IB. The only balancing plane available is in the OB toward the exciter. This means I cant balance the generator rotor onsite using two plane balancing method.


The other point where I agree with you is the reason of 85 HZ. This frequency is reflected throughout the generator rotor on the GOB. If there is bow in the rotor, it should reflect 1X Generator (60Hz) not 1X turbine (85). I measured the axial in GIB it was too low. No side band in the GMF of the gearbox. So what is the reason to see this frequncy.

I attached the spectrum from casing vibration along with the layout.

Casing_Vibration.pdf (36 Kb, 21 downloads)

Are you discounting the turbine as the source for the 85 Hz? I think Mark is correct.

XYZ,

The GOB shaft centerline plot shows that shaft moved in the direction of rotation almost straight left about
260 mk = 0.26 mm = 10 mil

The GIB moved almost staight up about
90 mk = 0.090 mm = 4 mil

Having this in mind what is the radial clearance of the bearing?
Also, can you show 1x filtered orbits?

Of course, you have to find the source and elimininate 85 HZ. Instability is another problem.

David_G,

The radial clearance is as measured 300 micron

I attached the filtered Orbit.

Still cant understand the following: regarding 1X Turbine (85 HZ).

at Gearbox Input shaft BNG#3 and Gearbox Output shaft BNG#7 as per the layout: high directional vibration horizontally. However, it is on the same frequecny high in the axial direction on BNG#8.

If the turbine has problem, so why the vibraion at turbine bearing is normal.

I am suspecting the pinion gear is misaligned.

Regards

This message has been edited. Last edited by: XYZ,

Flitered_Orbit.pdf (90 Kb, 22 downloads)

Hi XYZ
Orbit No. 24, indicates impacts and rubbing.
seems input pinion gear getting impacts. The same thing will be indicating 1X in your FFT or Time.

Regards
Guru

Although 85 HZ appears to be the troublemaker affecting GIB, the GOB has also some troubling signs. Looking at CL of OB bearing position it could be concluded that the rotor is in contact with the left side of the sleeve having a 300 mk radial clearance (when constructing a vector from the bottom of sleeve: left 260 mk and up 60 mk ).