Designing a simple cantilever spring 2

[Ian63]

I am trying to design a simple cantilever spring that needs to bend through 90 degrees. Do I use simple cantilever beam theory to determine the stress?

I am going to use 1055-1065 spring steel approximately 0.5mm thick. The beam will be approx 50mm long and 4mm wide.


| force
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| \
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| force

Regards,


Ian.


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[GregLocock]

No, because simple beam theory assumes, well, several things.

But E/R=s/y=M/I still holds true

so all you need to do is determine the bending moment at each point along the beam, then you can work out R, and then you can work out how much it bends. The problem is that the BM will depend on the results from the rest of the beam. It is unlikley to have a nice analytical solution.

However, in theory that isn't a problem, since your boundary conditions are straightforward, and a numerical solution will be good enough.

Cheers

Greg Locock

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[israelkk]

Using a beam for 90 degrees deflection is impractical. If you need 90 degrees than you should use a torsion spring instead. Anyway, this is a large deflection case, therefore, the standatd formulations do not apply, see

http://users.cybercity.dk/~bcc25154/Webpage/largedeflection.htm

 

[israelkk]

See MECHANICAL SPRINGS, 1963 2nd Edition. McGraw-Hill Book Co. By A.M. Wahl. page 179, too.

 

[Ian63]

What I am designing is sort of like a automotive clutch pressure plate, where I want the fingers to bend through 90 degrees without yielding when a large shaft is inserted through it. The spring rate is not as critical as long as the fingers can spring back to their original position. Hence I thought if I use thin enough spring steel, I just might be able to do it.

I can get some pieces of light gauge spring steel and perform some bend tests to see if I am exceeding the yield strength, but am hoping to refine my design before hand (tapering the finger width).

Thanks Greg & israelkk for your posts. I will work through the large deflection attachment now and see if I can get my head around it. Of course the solution is linked to fly fishing!

Ian.


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[GregLocock]

Sounds entirely feasible. Quite a neat little problem. Only thing is, I think 'F' in your drawing will have a strong leftwards component (governed by the coefficient of friction).





Cheers

Greg Locock

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[GregLocock]

For E =210 000

I get the following results, using the approach I suggested. It agrees in the linear range with the usual equation to better than 0.1%.

It ignores axial and shear effects.





Cheers

Greg Locock

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[GregLocock]

mu was 0.3, I assumed that the contact surface was //to the y axis, units are consistent.



Cheers

Greg Locock

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[OHSpringGuy]

Also check out Mechanical Spring Design, by Carlson

 

[israelkk]

OHSpringGuy

To my best knowledge, Spring Designer Handbook by Carlson doesn't deal with the case of large deflection flat cantilever springs.

 

[OHSpringGuy]

Not in great detail you're right. Attached are the formulas and a great chart for calculating large deflections in cantilever springs

 

[GregLocock]

Incidentally the SAE handbook for leafspring design has many nomograms for this and similar problems.

Cheers

Greg Locock

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