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Vibration Mounts

Vibration Mounts

(OP)
Hello Group,

I am working on selection of vibration isolators for one of our electrical cabinets which would be subjected to Sine sweep vibration test from 3 to 100 Hz (1 octave/ min for 2.5 hours) with peak acceleration of 0.7g(see attached pic for test plot).For preliminary analysis, I have selected 4 neoprene floor mounts (Grainger part# 4C988)and determined the natural frequency to be about 4.95 Hz. When I calculated the transmissibility (Q), I found that the max. Q of 50.4 is observed at resonant frequency 4.95 Hz (See attached excel sheet). Therefore, an input acceleration of 0.15 g is amplified to 7.5 g ( 50.4 X 0.15 g) at resonant frequency- kindly correct if my understanding is correct?

This brings to my next question, if Neoprene (damping ratio-0.05) is not sufficient, what other rubber materials I could use for my application. I had found that Butyl Rubber's damping ratio is about 0.2 which would be really good for my application but I am having hard time to find a vendor. If anyone can suggest vendors that would be great.

thanks
karthik

RE: Vibration Mounts

What happens if your mounts have a Fn of 2.0 Hz? 200 Hz?

Mike Halloran
Pembroke Pines, FL, USA

RE: Vibration Mounts

(OP)
Mike,

Thanks for the response. I understand that modifying the fn to fall out of test range will evade the resonance. However, to increase fn beyond 100 Hz I have to increase the stiffness of the mounts more than 2000 times (since mass is constant), which is impractical. To decrease it beyond 2 Hz, I can probably decrease the stiffness by decreasing the number of mounts which may not be ideal for shock absorption.

Please correct if my thinking is incorrect.

thanks
karthik

RE: Vibration Mounts

Fn > 200 Hz just means remove the mounts. Rigid mounting may be better than amplification, depending on what is being isolated.

To push the Fn down below the range of interest, the key is to increase the static deflection, which may be a limit for geometric reasons.

Mike Halloran
Pembroke Pines, FL, USA

RE: Vibration Mounts

Typically, for Navy ships, the equipment are mounted on wire-rope isolators. But, 0.7g in the realm where no isolation is doable, so long as everything in the cabinet is designed to handle that vibration.

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RE: Vibration Mounts

(OP)
Mike and IRstuff,

thank you for your input. I have a vendor who fabricates vibration mounts out of Butyl rubber that has higher damping ratio of 0.2 (http://www.avproductsinc.com) which is an option. Also, I will further analyze my cabinet and electronics to determine its strength against 0.7g acceleration input, and its natural frequency (-to see if I can modify the design to have its natural frequency fall beyond 100 Hz)

thanks for the support

-karthik

RE: Vibration Mounts

Great information I know this is most of the time the quick place to find it, I am designing a vibration isolation system for a metrology laboratory and I will contact the mentioned company TKS

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