Low Frequency Vibration

[Stlebo]

This machine has 3 rotating assembles that run at 13.6 Hz, 17 Hz and 19.7 Hz. I am seeing a very strong 3.4 Hz signal which translates into a very large displacement. I can't figure out the source. It only exists when the machine is running other nearby operations don't contribute. The closest thing I found is a bearing FTF at 5.5 Hz.

I'm at a loss, any ideas?

 

[Compositepro]

17 - 13.6 = 3.4 beat frequency.

 

[BrianE22]

If my addition is correct you may be getting a beat frequency at 17-13.6 = 3.4 hz.

 

[Stlebo]

Thanks. I thought that too and forgot to mention it. However, my understanding is that beat frequencies won't be present in structural measurements. The data is from accels. There actually has to be a forcing to get a response.

We also have bellow actuators in the system. they have a natural freq near 3Hz. Still, a forcing frequency near that freq would be necessary to excite.

Another clue; we tested this same exact machine on the same test stand in May. responses are consistent with the exception of this 3.4Hz.

Any other thoughts?

 

[Compositepro]

"my understanding is that beat frequencies won't be present in structural measurements".

Why would you think that?