Falling rate spring design 2

[mountainous]

I have seen compression coil springs that are rising rate, where the number of active coils decreases as the spring is compressed.

Has anybody ever seen a compression spring with a falling rate? Thinking about this for a couple minutes, I'm not sure how/if it can be done. I know it can be accomplished by using linkages that produce a falling rate from a constant rate spring, etc. But, I would like to do this via spring only. Any ideas?

 

[ivymike]

use a rising-rate spring loaded in the opposite direction?

 

[israelkk]

ivymike

I assume mountainous wants a falling rate with a rising force not with a falling force.

 

[israelkk]

I can be done with a belleville spring with an appropriate h/t ratio. You cab design the end rate to be zero or even negative.s Such belleville spring can not be found in the shelf. They had to be custom designed. The short come is the small deflection for such springs compared to the high loads.

 

[GregLocock]

You can also do it with a leaf spring, but that is a mechanism.

We often do it with jounce bumpers made of high density foam, through much the same effect as the Bellevilles, such that a short section buckles and then operates in bending.



Cheers

Greg Locock

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

 

[MikeHalloran]

The domed rubber springs in computer keyboards have a falling rate.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

Some air springs have a falling rate characteristic.

http://www.firestoneindustrial.com/pdfs/industrial/Airstroke_Airmount/EMDG.pdf

Some of the isolator products even have a negative rate.

 

[zekeman]

Picture a semicircular leaf spring with the edges on roller bearings. If you push it at the 90 deg position, the distance between supports increases and thus the stiffness diminishes-- just like a belleville washer works as isaelk stated.

 

[GDavet]

This can be done with a Belleville spring or a dome disc (similar to the bottom of an old-style oil can.)

If one of these designs is used, you will be limited on the load and deflection by your space constraints and material properties. In this case, what is your load desired (negative) rate? also, what are your space constraints?

 

[ornerynorsk]

I've seen a conical spring in some arrangement like this before. As the spring is compressed, the angular moment of each coil acting against the next one gives you a mechanical advantage. You'd have to play around with the geometry of it to optimize it, though.

 

[mountainous]

Thanks for the replies! I'm still in the concept phase of my design, but I have plenty of ideas now on how to make it work.

 

[drawoh]

mountainous,

I have designed a falling rate spring. It was a rotating shutter. I used a catalogue extension spring. As the shutter rotated open, the spring tension transitioned from tangential to the circle, to normal to the circle.

While the spring force increased as the shutter opened, the spring torque, which is what mattered, dropped.

This is mechanism design. Sketch stuff out. Do calculations. Try thinking backwards from your requirement.

JHG

 

[israelkk]

mountainous:

It would be helpful to know the approximate force, deflection, space and size that you are after.