Flat Spiral spring design 1

[ChristianThomas]

Hi, I want to make a flat circular spiral spring which will be laser cut from sheet stainless steel and I need the spring constant to be the same horizontally as vertically. It is supporting a mass of about 1.25kg for a deflection of around 3 to 5mm. I also need to know about linearity and whether this would behave like a Belville spring or be more linear than that. The spiral doesn't need to be like a clock spring and I envisage four or so links from the perimeter to the centre. Thanks in advance.

 

[btrueblood]

A sketch might help. When you say vertically, do you mean perpendicular to the plane of the sheet? Is horizontal the same as parallel to the plane of the sheet? 4 links from rim to center - each of these is a continuous spiral?

 

[ChristianThomas]

"do you mean perpendicular to the plane of the sheet?" Yes, thanks, that is what I should have written. And therefore the same for the horizontal, too.

I don't quite have a mental image of the plan of the spring as viewed from above. It will probably have four fixing holes on the rim, each with a spiral going to a single centre, load bearing, fixing hole. The question is probably whether these go round (concentrically) a half turn, one turn or two turns or maybe more. I just haven't seen any literature beyond looking at them as leaf springs that describes the horizontal motion. Part of me thinks that there may need to be three or so parallel springs on each arm in order to avoid twisting in the horizontal plane - particularly if there is a pre load deflection.

Incidentally, my starting point for thickness was likely to be about 1.6mm (or an old-fashioned 1/16th).

 

[btrueblood]

I guess I'd try using a FEA program. Standard coil spring design criteria could get you close, but only if +/- 25% for the horizontal load case "spring constant" is close enough. And the conversion from coiled wire springs to rectangular sections is non-trivial, thus my first recommendation.

 

[ChristianThomas]

"Conversion to rectangular section is non-trivial." Oh dear. I was going to go assume it was easier, though I did have some doubts about, say, an identical vertical and horizontal displacement adding up to root two, especially with it working across the hypotenuse of the section.

Is your instinct to use a proper spring? It won't look quite as pretty but if it is going to be difficult - and linearity is also an issue because the dynamic behaviour is paramount - then I am quite happy to accept that this a task that is beyond my expertise (or at least the expertise needed to do it neatly).Thanks for your help so far.

 

[btrueblood]

No, I think you can get there, but you'll need to do the number crunching, or get somebody with FEA capabilities to do it for you.

 

[megabill]

FEA is difficult with these types of springs. Not sure if it would help, but this is a link to a site that has power spring design software. www.spiral-spring.com

 

[ChristianThomas]

Thanks Megabill. That looks a really useful site but probably for another time as I'm trying to do a spring out of a sheet (probably now carbon rather than s/s). It is a spiral, though I'm still no further ahead on it.

Best

Christian Thomas

 

[israelkk]

ChristianThomas

What you are looking for is probably impossible with the four/three link arranged from the center disk to the outer disk. This kind of spring is usually for out of the plane movement. Those springs are linear as long as the deflection is small. Radial deflection is a problem because the opposite links are "fighting". One link is tensioned and the opposite link is compressed. More than that, the radial rate will be different for different orientation with respect to the links.

What are your space restrictions?

 

[ChristianThomas]

The discs are almost certainly going to be 1.6mm carbon plate with a diameter of 60mm and a static deflection of about 5mm for a force of 12.5N. It should be quite well damped so I wouldn't expect dynamic displacement of more than 3mm or so.

There are actually two of these springs with an annulus in between.

Thanks for your points. I hadn't actually noted that radially the springs would effectively be in parallel (or fighting). Would the radial rate vary with orientation or does the trigonometry take care of that?

Christian

 

[desertfox]

Hi ChristianThomas

I think you would be better if you could provide a sketch, you can see clearly what you want but its more difficult for us to imagine.
What I will say is you can't just pick a diameter and thickness of spring and then expect it to deflect 3mm under a 12.5N load without doing calculations, otherwise as israelkk says it won't work.

desertfox

 

[ChristianThomas]

Hi Erwin and others

Here is a very rough sketch of what it might look like. The spirals could go round further, or more than once, but this is the basic outline (and the easiest to draw).

As to just picking a thickness the structure would seem to be the dominant factor here. The deflection as a solid disc would be about a tenth of what I'm after and as a plain single spiral (Ie fairly normal spring) one could easily make it extend four or five times the 3mm. Where one may need to optimise the thickness, towards the end, is in lateral displacement.

 

[desertfox]

Hi ChristianThomas

Can you make a pdf of your uploaded file its a dxf at present which makes it difficult to open.

desertfox

 

[ChristianThomas]

Hi desertfox

I can't do that at home for some reason, but I can do it when I get into work tomorrow.

 

[ChristianThomas]

Hi

Here's a pdf of the dxf I uploaded yesterday.

 

[desertfox]

Hi ChristianThomas

Thanks for the pdf, I have never seen a spring like that before, what does it look like in the other plane?
I can't honestly see how your sketch would work like a spring.

desertfox

 

[ChristianThomas]

It's not fantastically clear and one has to imagine an annulus at the centre and at the rim onto which each of the curved ribs attaches. It was very quickly drawn and I couldn't easily get the compound curve that I ideally wanted.

By way of explanation in this instance it is effectively 8 leaves. Lateral compliance (not much in this drawing but one can extrapolate) comes from the thinner sections of the leaves.

The other plane is just a 1.6mm plate.

Christian

 

[desertfox]

So when its loaded it extends downward in the plane of the 1.6mm plate I assume, but how do the leaves move relative to each other? I assume one leaf overlaps the adjacent one.
Truthfully I have no idea how to analyise your sketch and I am not sure its a practical proposition, have you spoken to any springmakers because that I feel would be your best bet.

desertfox

 

[btrueblood]

I think megabill should rescind his statement about FEA not being applicable to YOUR spring form. I think it's your only way forward. You might be able to analyse it using Roark's formulas for curved beams, but by the time you get about halfway thru that tangled web, you will be looking hard for some FEA capability.

 

[btrueblood]

I am going to have to back you up a step - do you need this spring to also resist torsional moments (i.e. rocking of the part about an axis parallel to the plane of the sheet)?