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Random Vibration and Isolators

cbra

New member
I am getting ready to perform a modal and a random vibration analysis on a chassis that has vibration mounts (Lord HT2 Series).



Here is my question... Do I model the isolators to get accurate modes from the modal analysis before performing the random analysis? Or perform a modal analysis on the chassis hard-mounted and input the transmissibility curve of the isolators into the damping section of the input form for the random analysis?



If modeling the isolators...How or what type of elements have you used to simulate the isolators (rubber mounts)?



I hope this is clear!



Thanks in advance for your help.



Brian
 

Acecarbone

New member
Brian,

I cannot answer in terms of mechanica because I am a new user. However, I have done a lot of random analysis using other codes. The key issue I would like to point out to you, is that typically, the displacements in a problem like this are too small for the damping effects of rubber isolators to take effect. As a result, the isolators will act linear elastic over the displacement range of the problem and exibit the damping properties typical of the rest of the structure. So the good news is that you can use any element you want, from a beam to solids as long as you use the modulus at low strain levels and do NOT add extra damping because you are using rubber. The bad news is that this modulus is higher than you think and is best determined by test. If your design permits, friction damping is the most effective way of reducing the random response of the payload on the isolators. This is never an easy problem so I hope this advice gets you thinking, and directs your analysis in the right direction
 

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