Mysteriously increased natural frequency in structure

[ebarba]

Hello all,

a customer complained one of our vibrating-table machines was not working. These are reactive machines - i.e. two-degree-of-freedom systems - where the vibrating table is the second DOF and sits on springs (circular steel bars). The bars are attached to a structure (first DOF) where the vibrators are clamped. Finally, this structure then sits on a series of more flexible springs that serve as legs, resting on the floor.

The sieve shaker has a natural frequency very close to the operating frequency of the vibrators, so it vibrates while the structure and the vibrators are almost vibration free.

Upon inspection, we found that:
[ul]
[li]the original working frequency (set by a VFD) did not change at all since we originally shipped the unit[/li]
[li]the customer removed some accessories from the second DOF (sieve shaker) amounting to 4% of the original mass of said second DOF[/li]
[li]the operational frequency of the machine had INCREASED about 25% since we shipped the unit (i.e. we had to run the vibrators 25% faster in order for the machine to work correctly).[/li]
[li]no physical damage was found anywhere[/li]
[/ul]


We shipped back the machine with a new VFD configuration (i.e. higher operating frequency), but we haven't found any explanation for what happened. The decrease in mass cannot explain an increase of 25% in the operating frequency, at least not according to our calculations.

Any ideas?

Thanks!

 

[Strong]

I worked on a vibratory feeder (circular) with performance problem. I traced it to the vibration mounts at the floor. In you case perhaps there was a nonlinear stiffness in mounts as result of removing secondary feeder mass and other "non disclosed" changes.

Walt

 

[ebarba]

[ul]
[li]The inspection revealed no changes to the machine, other the removal of that 4% worth of mass[/li]
[li]The floor mounts are helical springs (steel) working on their lower range of load and they are not clamped in any way to the floor, they just rest on it[/li]
[/ul]

 

[electricpete]

So we have a 2DOF
Ground === k1 === m1 === k2 === m2
m2 decreased by 4% and a resonant frequency was found 25% above a previous resonant frequency.

You said you checked the calculation.
did you check two frequencies in the "before" scenario against two frequencies in the "after" scenario?
If you post the k1/k2/m2/m1before/m1after, I'd be glad to check.



=====================================
(2B)+(2B)' ?

 

[Denial]

On my web site (

http://rmniall.com)

you will find a spreadsheet that covers electricpete's 2DOF model.