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Re: Dyanmic Loading of vibrating Equipment on Dampers

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On Oct 5, 2004, at 1:51 PM, Faizal Rahiman wrote:

The natural frequency of the supporting steel structure is around 4 to 5 CPS
with Dead loads and operating loads using Rayleighs method.
First, operating loads will not affect the natural frequencies. Only mass, that is, dead load, will affect the frequencies. You might want to check this again.

Given that your pans are failing, it's likely that the pans are vibrating. The overall system frequency calculation won't predict the frequencies for the attachment. You didn't say where the pans are cracked, but that's an important piece of information. Also if you've weld repaired the pans, the likelihood is that the repair welds have reduced the fatigue life, unless the welding is done a lot more carefully than most repair welds. Therefore what's happened after the repairs may be an aggravated version of the original problem.

As a general rule, stiffing the structure is bad practice--you'll never get decent service without a decent tip ;->

Is there a possibillity of interference when there are two pans
with diffrent capacities and same RPM Close by.
Sounds like a guess. It's possible for the vibration of one pan to tickle a vibration mode in the other, but stiffening the pan is still the problem even if only one pan were actually vibrating.

I d appreciate if any one could tell me how to apply the
load of sprung masses to the mathematical model as a
Sine function taking in to account the viscous damping
of the rubber dampers supporting the equipment.
Do a harmonic vibration analysis with loads imposed at the mass CG. You need a pretty good finite element program to do a multi-degree of freedom problem like this with a practicable schedule and budget. You can probably apply the Holzer method to find natural frequencies and mode shapes and solve the problem manually, but it's an awful job.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)    | this distance" (last words of Gen.
...................................| John Sedgwick, Spotsylvania 1864)

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