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Re: Block Foundation - Vibration Modes

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On Nov 21, 2006, at 9:55 AM, Padmanabhan Rajendran wrote:

Intuitively, it seems to me that only tthree modes of vibration will be important: The vertical mode, the rotational mode about the X-axis and its coupling with translation in Y-direction. Am I unconservative in ignoring the vibration modes in other 3 planes (rotation about Y-axis with coupled translation in X-axis and the torsional mode about Z-axis)?
If you're looking at the motor as a rigid body supported elastically it isn't that hard to find all six modes. It's hard to hipshoot non-trivial modes, and even a trivial mode may produce some resopnse if the excitation is just right. I'd sure look at rocking about the Y axis because the mass moment of inertia will be relatively high.

You can find a complete solution to this problem in Harris and Crede's Shock and Vibration Handbook. The arithmetic is tiresome, but not difficult. It's easy to code up with simple FEA software as a lumped mass, the hardest part is finding mass moments of inertia and spring rates.

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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