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Elastic Center

Elastic Center

I working on a design for an electronics box-type assembly that weighs about 50 lbs.  We have vibration isolators mounted at each of the 4 corners.  The vibration isolators work well in the vertical direction, but they appear to stiff in the two horizontal directions.  

I was told by the vibration-isolator manufacturer that the lack of isolation in the two horizontal directions is due to the fact that the center of gravity of the box does not coincide with the system's elastic center.  The system is comprised of the very-rigifd 50 lb box (10" X 12" X 15) and the 4 isolators.

I've modeled the vibration isolator in Cosmos and have found that the spring rates in all three directions are about the same.

Any help if pointing me in the right direction to understand how the elastic center comes into play would be greatly appreciated.


Dave Maxham

RE: Elastic Center

I don't know what your vibration isolators look like but there mounting might make a differents.


"In this house, we obey the laws of thermodynamics." Homer Simpson

RE: Elastic Center

Thanks MotorVib,  I think that's what the manufacturer is getting at - the way the isolators are mounted.  I suspect that the the mounting affects the ability to get the elastic center and the center of gravity to coincide.

Obviously, I'm still grasping at straws.  I'll be hitting the books this weekend.  I have Steinberg's vibration book for electronics - I hope something's in there about elastic centers.


RE: Elastic Center

What the manufacturer said seems to make sense.

The isolators generally are flexible in the vertical direction.  This can translate to horizontal motion in the form of rocking.  

How stiff is the horizontal motion compared to the original vertical spring stiffness is a matter of geometry.

If I mount a very tall box and push near the top of it, I don't have to push hard at all to distort the springs because I have a large moment arm due to the height of the box.  Therefore, the effective horizontal stiffness at that height is very low.   

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RE: Elastic Center

I should point out the horizontal spacing of the springs will be important also, but for simplicity just change one variable (height) and you should be able to imagine that higher height means easier to push horizontally (less horizontal stiffness) for the same springs.

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RE: Elastic Center

Thanks electripete,

Our arrangement is such that the isolators cannot rock because each one of the isolators' posts is mounted up against the flat surface of the unit.  


RE: Elastic Center

Try raising the mounts so they line up with the cg of the box, and see how your isolation changes. Frankly I'm a bit surprised by your experience and the manufacturer's comments - for example many engines are mounted via mounts that are low compared with the cg.



Greg Locock

SIG:Please see FAQ731-376: Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Elastic Center

I think the old Lord catalog suggested the benefit of mounts at the same height as the CG is to keep the translational modes dominant, which are easier to calculate from mount stiffness.

My Volvo 240 has very squishy mounts close to the engine and rocks like a mother at idle, but keeps the whopping secondary vibration from the 2.3 liter 4 fairly well isolated.
Our Volvo 740 using ~the same red block 4 cylinder had similar looking mounts on outriggers attached to the block, but was a much more refined car.

RE: Elastic Center

Thanks GregLocock and Tmoose,

Look's like everything is pointing to having the mounts higher up to be in line with the center of gravity.  I've also gone up the learning curve in regard to rocking modes, etc.


RE: Elastic Center

Thanks for all your help everyone - it's greatly appreciated. I'd like to ask a follow up question:

We have an in-house vibration table that has a vertical motion only.  Whenever we have to do lateral tests, we go to an outside testing facility that has a horizontal-motion table.

I have this idea (good or bad) that we could use our vertical-motion table for at least some of our horizontal tests.  This would be done by first mounting a large enough right angle structure (like a big L) onto the vertical-motion table.  Then we would mount the unit under test to the vertical section of the big L.  Then when the table went up and down, the unit would be subjected to a vibration in one of it's employed, horizontal axes.  Then the other horizontal axis could be tested by rotating the unit 90 degrees.

I don't know if my idea makes any sense.  I worked at a place a number of years ago where we did such a thing, but the units or objects under test were much smaller. In our case, the unit under test weighs about 50 lbs.  I understand that the isolators described above will be initially compressed in their employed, radial direction, but to me that seems the same thing as when the unit is mounted directly on the vertical-motion table without any angle structure, just in a different axis.

Any help and pointing in the right direction is greatly appreciated.


RE: Elastic Center

It is certainly possible - do you know the effect of static loading on the rates of your isolators though? That could be critical.



Greg Locock

SIG:Please see FAQ731-376: Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Elastic Center

No, I don't know the effect of static loading on the rates of the isolators, but I see your point - thanks.


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