13 replies to this topic

[kevins]

Hi, this is not an automotive application, just a project I'm working on (and as is obvious I'm no mechanical engineer).

 

I have a small rod (5mm dia) that is being pushed over a distance of 11mm with a decreasing force and want to resist that force on the other side with a spring.

 

I figure I need a compression spring with a falling rate. Here is a rough force curve.

 

 

 

 

 

I found this solution, but not sure if it will work?

 

https://www.mwspring...ng-spring-rate/

 

I don't need to match my curve exactly so 2 or 3 individual rates would do.

 

Thanks

[Greg Locock]

It will work. Softening springs are hard to do without a mechanism, but if you can do it with two links and a rotational spring at the hinge between them you'll get the characteristic you've drawn.

[manolis]

Hello Kevins.

 

Here is a solution of your problem:

 

 

 

 

 

 

And its application on a V-belt CVT:

 

 

For more: https://www.pattakon...ttakonPatEf.htm

 

Thanks

Manolis Pattakos

 

[Greg Locock]

That'll give a rising rate.

           o

          / \

         /   \

----->/     \<------

 

Is my suggestion where the o is a pivot and a torsional spring

[mariner]

Ironically I think you are trying to do what generations of single seater designers in particular have tried to avoid !

 

Most single seaters have to have inward sloping coil spring damper unts. They weaken the effective wheel rate as the suspension compresses.

 

Manolis's top animation is basically looking at single seater head on. 

[manolis]

That'll give a rising rate.

           o

          / \

         /   \

----->/     \<------

 

Is my suggestion where the o is a pivot and a torsional spring

 

Hello Greg.

 

Thanks

Manolis Pattakos

[kevins]

Greg, Manolis and Mariner, many thanks for your replies.

 

Unfortunately, I don't really understand them! Nor do I think I could fabricate them as I just have access to very basic tools such as a hacksaw and drill.

 

Looking at the solution I linked to, after giving it more thought, I don't think it will work for me. When the first (higher rate) spring is fully compressed, this means my rod has moved out say half way. But, by that point, the the force is can apply will also have decreased, so the first spring will start to open again? I'm sure I did not explain that well.

[manolis]

Hello Kevins.

 

These drawings may help:

 

 

As the angle of the srings increases, and despite the fact that the springs are further compressed, the total "spring force" reduces.

 

 

The above from another view point:

 

 

Think the case (not shown) wherein the springs get completely vertical to the long axis of the two rods. The srpings are "completely" compressed, however the "spring force" between the rods becomes zero.

 

Thanks

Manolis Pattakos

 

 

[gruntguru]

You need a bellville washer (disc spring/ conical spring). Clutch diaphragms have used them for years.

 

The force and rate you require is very low - you might be able to find an off-the-shelf clutch diaphragm (eg from a motorcycle) where you can use the force at the tips of the "fingers". (The clutch assembly uses the much higher force at the base of the disc for clamping)

 

 

Or you can use off-the-shelf bellville washers and stack them in series or parallel to adjust the rate down or up.

Edited by gruntguru, 11 February 2023 - 09:01.

[manolis]

Hello Gruntguru.

 

You write: "You need a bellville washer (disc spring/  conical spring)."

 

 

If you mean that "conventional springs cannot do the job", take a look at the following drawing:

 

 

In the mechanism shown in my previous post, at the first part of the compression (area inside the blue rectangle, springs at small angles relative to the “axial motion”) the mechanism behaves as a conventional spring wherein the more you compress it, the larger the reaction force.

 

But after a point (area inside the red rectangle, springs at large angles from the “axial motion”) the more you compress the mechanism, the less the reaction force.

 

At the last part of its compression (springs almost vertical to the “axial motion”), the mechanism behaves as a spring that gets softer and softer as it is compressed.

 

Compare the part of the curve A inside the red rectangle with what Kevins is looking for (first post in this thread).

 

The reason for using a pair of coil springs is to cancel out bending loads on the rods and reduce friction.

 

Thanks

Manolis Pattakos

 

[gruntguru]

Hi Manolis and welcome back - its great to have your expertise posting here.

 

I did not mean that "conventional springs cannot do the job"

I meant mean that a Belleville washer is a better solution than conventional springs plus a mechanism.

[manolis]

Hello Gruntguru and thanks.

 

At the end, with the leaning coil springs what is actually made is a long stroke “belleville” spring / washer.

 

If it is not easy for Kevins to find the required set of belleville springs/ washers, explain to him (in better English than mine) how to use conventional coil springs to achieve what he wants.

 

Thanks

Manolis Pattakos

[kevins]

Hi all and sorry for my delay in getting back.

 

I *think* I now understand the solution proposed by Manolis.

 

In the resting position, the springs are nearly aligned with the force pushing against them and thus "push back" with all their force.

 

But, as they get compressed they become more aligned with the top of the "T" and a larger portion of the spring force is actually pushing against the middle of the "T", in effect against each other? Is this it?

 

I'll look into bellville washers too, thanks.

[manolis]

 

In the resting position, the springs are nearly aligned with the force pushing against them and thus "push back" with all their force.

 

But, as they get compressed they become more aligned with the top of the "T" and a larger portion of the spring force is actually pushing against the middle of the "T", in effect against each other? Is this it?

 

 

Hello Kevins.

 

Yes.

 

Thanks

Manolis Pattakos