I did vibration measurements on a 12 Cyl. V diesel generator set, 4500 KW with 900 rpm and found at the generator's NDE bearing vertical and horizontal vibration levels with app. 3.5 mm/s RMS at running speed as well as 8 mm/s at 67.5 Hz (in vertical direction only ), but in axial direction a very dominant frequency of 67.5 Hz of app 36 mm/s.
Vibration levels on generator's DE: vertical and horizontal app the same, axial by 10 mm/s, Diesel engine side: low vibration at app. 4 mm/s
Spectrum starts with 0.5x and harmonics

I said app. because I measured on 6 generator sets. All the same

Any suggestions ?

Regards

Sven

Sven,

Looks like a resonant structure in axial direction excited by 4.5x service speed. Does the D-G unit sit on isolation mounts (springs or pads) or a rigid foundation? I suggest:
1) Conduct impact test to identify/confirm natural frequency near 67.5 Hz and evaluate possible modifications to detune structure

2) Check torque on shaft coupling bolts to be sure no looseness is present

3) Check thrust bearing condition at engine or generator, depending on design

Walt
Need Assistance? w_f_strong at msn dot com

Walt, thank you, Yes the units are on a rigid foundation and the foundation is rubber mounted.
Last info I received that they had problems at the generator NDE + DE side in sealing defects, otherwise the bearings are good.

Thank you for the help, will try the impact test

Sven

hi sven,
i agree with walt. this might be a problem of resonance. also please check the alignment between engine and generator.