Vibration Isolation

[scotland6]

Hi,

Hopefully someone can give me some assistance. I have a mass that I wish to isolate with 4-off AV mounts. The mass (40kg) is subject to a 1/2 sine input of 30g 18ms. The isolators I have in mind have a 'Q' of 5. How can I calculate the velocity due to the shock, the natural frequency of the supported mass and the displacement due to the shock? Can I also work out the mount dynamic stiffness? How does this relate to static stiffness?

Any pointers appreciated.

Regards

Scotland6

 

[GregLocock]

It's a single degree of freedom system, set up the ODE and solve it.

"Can I also work out the mount dynamic stiffness? How does this relate to static stiffness?"

You can't in the real world. If this is a textbook question then it probably tells you how.

For instance there are spring materials where the dynamic stiffness equals the static stiffness, and there are others that have dynamic stiffness two times the static stiffness. The Q may be a clue, I'm not really sure.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[scotland6]

Thanks Greg for this. Can you expand?

I have the following variables to set up the ODEs for a single-degree-of-freedom system :-

m is mass (known)
c is damping coefficient (unknown)
k is spring (mount) stiffness (unknown)
y1 is displacement of the mass (unknown)
y2 is input displacement (known)

Is this too many unknowns?

Regards

Scotland6

 

[GregLocock]

Ah, ok you have base excitation.

That looks about right.

There is probably some fancy pants equation that links Q to c via wnat (rad/s), a Q of 5 implies little damping, so you 'll be pretty close if ou assume wnat=sqrt(k/m).







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[scotland6]

Hi Greg,

But I don't know k or wn I only know m.

I have worked out the velocity change due to the 1/2 sine input. Can I use this to establish wn?

Sorry for labouring this.

Regards

Scotland6

 

[IRstuff]

You're trying to solve for too many unknowns. As with ALL vibration and shock isolation problems, you need to decide the allowable residual vibration or shock; it cannot be zero. That means that the displacement of the mass is known. Whatever you choose determines the k and wnat.

TTFN

FAQ731-376