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.

 

[israelkk]

This is not a common spring for spring makers. It is basically a solid one piece disk made of thin strip with reliefs that creates the leaves. The leaves are all connected in the center of the disk and the outer of the disk. This is commonly called flexure (disc) spring. They are mainly used to support free vibrating pistons in such applications as in Stirling mini cryocoolers. The disks function as flexure bearings to support and guide the piston on both sides to make it friction-free and at the same time they are used as loading/damping springs. They are usually manufactured by chemically etching. Such spring are mainly designed using FEA.

See fig. 5 in the following document

http://www.waset.org/journals/waset/v56/v56-50.pdf

or

http://mydocs.epri.com/docs/public/000000000001007317.pdf

fig. A-16 Stirling Technology Company – Flexural Bearings on page A-54 (page 128 of the document)

 

[desertfox]

Hi israelkk

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

 

[ChristianThomas]

Hi all,

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

 

[ChristianThomas]

I see on Jacob's stirling engine that they use this design in order to *avoid* radial movement. I, of course, want it. I can see how it serves their purpose with the wide leaves. It seems it may have to get more complicated.

C

 

[desertfox]

Okay Christian

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

 

[ChristianThomas]

Yes, FEA is probably the only way. Another concern is linearity, meaning that to some extent it is like a Belville spring and, if one isn't careful - or lucky - one will start to tug along the length of the material. Ie run out of things to flex.