Calculating Pedestal Stiffness and Shaft Critical

[sloth4z]

In thread384-156111 some one said that a bump test could be used to calculate pedestal stiffness. Can some briefly explain to me how to do this? I know how to perform a bump test to get the resonant frequencies, but what do you do with the results to get a value for stiffness? If I have FEA at my disposal should I use frequency analysis or some other analysis (if a stiffness value is the goal)?

Does anyone have any books the I can reference on this topic (shaft critical, pedestal stiffness or rotor dynamics in general)?

 

[GregLocock]

If you have FEA just do a static run with a force of 1N applied in the direction and at the point of interest.

Estimating the static stiffness in a hammer test is easy - integrate your acceleration/force transfer function twice in the frequency domain, so give you compliance. Then invert it, and you have stiffness.

If you are reasonably lucky there will be a near horizontal line down near 0 Hz, that is a good estimate of the static stiffness. If you have rigid body modes that'll mess things up, you may be able to convince yourself that a flat line just above them is usable.



Cheers

Greg Locock

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