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Mysteriously increased natural frequency in structure

Mysteriously increased natural frequency in structure

Mysteriously increased natural frequency in structure

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:
  • the original working frequency (set by a VFD) did not change at all since we originally shipped the unit
  • the customer removed some accessories from the second DOF (sieve shaker) amounting to 4% of the original mass of said second DOF
  • 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).
  • no physical damage was found anywhere

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?


RE: Mysteriously increased natural frequency in structure

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.


RE: Mysteriously increased natural frequency in structure

  • The inspection revealed no changes to the machine, other the removal of that 4% worth of mass
  • 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

RE: Mysteriously increased natural frequency in structure

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)' ?

RE: Mysteriously increased natural frequency in structure

On my web site (http://rmniall.com) you will find a spreadsheet that covers electricpete's 2DOF model.

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