Large machine resonance

[testtech]

A demonstrated resonant condition exists in a gearbox. Vibration of 1"/sec is measured on the structure at 330 Hz. The gearbox is driven by a 1000 HP DC motor and runs a rolling mill. The gearbox is about 6' by 6' by 4'. It sits on a concrete foundation that reportedly extends 8 feet underground. An operation deflection shape animation indicates that the foundation is rocking back and forth at the resonant frequency. The resonant frequency is gearmesh frequency. Here is the question: is it reasonable to have a resonance on such as masssive structure at 330 Hz. or should I be seeking another source?

 

[GregLocock]

The frequency is not unreasonable, and the level of vibration is about right (5g), but the described mode shape seems wrong for such a frequency. I'd be expecting casing modes and shaft resonances at that frequency more than rigid body motion of the entire seismic block. You might want to post this in one of the vibration forums.



Cheers

Greg Locock

 

[testtech]

Greg

If the shaft or casing were resonant, would you expect all sections of the entire gearbox to be moving basically in phase?

 

[GregLocock]

No, almost definitely not. As I said, it is a puzzle. Thinking about it 5g is a fair bit at 330 Hz as well.


(1) how many locations were monitored?
(2) How sure are you that the signal is good?




Cheers

Greg Locock

 

[testtech]

Greg:

91 DOFS were monitored using a stationary and roving accelerometer. Data were taken twice on perhaps 1/3 of the points to verify repeatability. Data collection was consistent.

 

[GregLocock]

Can we follow this up on the thread in the mechanical vibrations forum?

Cheers

Greg Locock