Hi Friends

Vibration was measured on gas turbine frame 6 load gear bearings numbered as follow:
brg 3 : high speed shaft- turbine side
brg 4 : high speed shaft - gen. side
brg 5 : low speed shaft - turbine side
brg6 ; low speed shaft gen. side

the domenent vibtration on bearing 4 was found at the generator speed although this shaft is the high speed shaft (5000 rpm) (domenent from 3000 rpm shaft.
who can interpret is that normal or means a faulf in the generator or electrical fault - i know the vibration transmitted in the machine parts because of mech. impedance and more at low frequencies.
i attached the brg 4 spectra
best regrads

HIGH_SPEED_SHAFT.doc (62 Kb, 23 downloads) spectra

Could you brief up,like from when you started observing this trend?Is it a sudden change you have observed?I think the gears come with the quill shaft arrangement,Am i wright?In this case,Please check the alignment between Low speed gear and generator.

Hi

this is the first time i measure vibration in this gas turbine using the data collector/FFT analyser.
the load gear is a reduction gear consists of two helical gears, the high speed gear is 45 teeth and low speed shaft is 75 teeth.

thank you

What if any work has been recently done on this particular machine? Also, has this behavior been typical or is this something new?

It is typical for these gears to be double helical, and thus axial spacing of the pinion relative to the gear is critical. If the spline at the quill shaft locks up you can have a situation where the dominant frequency of one rotor appears on the other rotor. This component typically goes down in amplitude, or even disappears as load is increased since the centering forces increase as a function of load.

I'd also suggest you verify your tooth count, 45 x 75 would be highly unusual in a gear of this type. This would have 5 assembly phases so care must be used if the gearset is ever dissassembled to insure they go back together with the same phase.

John from PA

I agree with John,do try to increase the load,If the quill shaft is locked and causing vibration....this atempt may help.
Do you have any phase readings?

Hi

john the tooth counts are 44 for driver and 75 for driven.
The vibrations readings were taken at full load

thank you

HI
A spectrum with Fmax of 660 000 cpm in acc. and 1600lines resolution would help to see the gear mesh. A lock pinion on herringbone (similar to double helical) tends to produce strong n/3 gear mesh frequencies.
Best regard, Marcel