Spiral / Power Spring design? 1

[AXNRXN]

Howdy,

I'm looking for any equations to help ballpark a spiral spring design. I'm working on a retractable lanyard for keys / tools, etc. I'd like to find some equations that will estimate pull force for a given mat'l thickness, O.D., I.D. etc. This is the same type of spring you would find in a tape measure, starter recoil,...

I've found computer programs for purchase that will do this, but I'm just looking for the equations.

Thanks for any info,
--> Karl

 

[desertfox]

Hi AXNRXN

Try this site:-

http://www.roymech.co.uk/Useful_Tables/Springs/Springs_Spiral.html

desertfox

 

[AXNRXN]

YES!!!!

I think this is what I'm looking for. Thanks a bunch!

--> Karl

 

[desertfox]

Your Welcome

desertfox

 

[israelkk]

The spring suggested by desertfox is a spiral spring. This is not the spring used in a tape measure. The spring in a tape measure (the tape itself) is a "constant force" spring or "motor spring" that could make up to 40 turns. The spring suggested by desertfox can not make more than 3 turns without sideways bucking.

 

[desertfox]

Hi AXNRXN

Here is a mch better link on spring design, also it gives design on constant force springs on page 159.

http://www.scribd.com/doc/29044778/Spring-Design-Handbook

desertfox

 

[AXNRXN]

Many thanks again, Desertfox. That is a great resource.

From what I've seen, even an everyday tape measure doesn't rely on just the tape to pull it back. Mine has an additional flat spring contained in an inner casing. I'm not sure if its a normal constant-force spring or a power spring, since I never pulled it out of the casing.

The lanyard I'm working with uses a pre-stressed power spring. It looked like a spiral spring until it flew out of the casing and nearly decapitated me. The general equations for this type of spring are covered in the spring handbook.

Thanks again!