Flat Spiral spring design
Flat Spiral spring design
(OP)
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.





RE: Flat Spiral spring design
RE: Flat Spiral spring design
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).
RE: Flat Spiral spring design
RE: Flat Spiral spring design
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.
RE: Flat Spiral spring design
RE: Flat Spiral spring design
RE: Flat Spiral spring design
Best
Christian Thomas
RE: Flat Spiral spring design
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?
RE: Flat Spiral spring design
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
RE: Flat Spiral spring design
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
RE: Flat Spiral spring design
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.
RE: Flat Spiral spring design
Can you make a pdf of your uploaded file its a dxf at present which makes it difficult to open.
desertfox
RE: Flat Spiral spring design
I can't do that at home for some reason, but I can do it when I get into work tomorrow.
RE: Flat Spiral spring design
Here's a pdf of the dxf I uploaded yesterday.
RE: Flat Spiral spring design
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
RE: Flat Spiral spring design
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
RE: Flat Spiral spring design
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
RE: Flat Spiral spring design
RE: Flat Spiral spring design
RE: Flat Spiral spring design
See fig. 5 in the following document http://www.waset.org/journals/waset/v56/v56-50.pdf
or
http://
RE: Flat Spiral spring design
Good document, I understand the spring in the document you reference however I cannot see how the OP's sketch fits in with it.
The OP sketch appears to have overlapping leaves and not just a single piece of material chemically etched out or have I misunderstood.
regards
desertfox
RE: Flat Spiral spring design
Israel has it right, and the leaves don't overlap. The idea is to create a cheap (ie. with water jet cutting or laser cut) and compact spring which is internally well damped. A spring in steel with a nominal resonance with the supported mass of about 10Hz would have self resonances in the 120Hz region. Here the hope is that if one can push that self resonance up high enough (and nominally it should double being 1/4 of the mass) then the damping of the matrix should keep it controlled.
The spring in the strimmer handle could be equally practical - and a lot easier to draw.
That could also address the torsion question. Although I don't foresee any direct torsional loads - this is part of an isolation mechanism - it is not beyond the realms of possibility that some sort of couple could arise. My shape of cuts wouldn't probably be very good at this without some deformation.
It would also seem to point to a greater thickness and more cuts.
Christian
RE: Flat Spiral spring design
C
RE: Flat Spiral spring design
I misread your sketch, but the spring israelkk posted acts as a spring in the radial plane, your talking about it acting as a spring at 90 degrees to its 1.6mm thickness unless I have misunderstood.
I doubt you will get 3 to 5mm deflection of your spring by cutting leaves in a flat plate as you yourself have said a solid disc as only about one tenth this deflection.
I think you need to get it FEA programme on this to get any meaningful answer as indicated by others.
desertfox
RE: Flat Spiral spring design