## PSN-L Email List Message

Subject: RE: Zero-length spring
From: "Jack Ivey" ivey@..........
Date: Fri, 1 Dec 2006 11:44:08 -0500

```I think he was talking about using a clock spring in an
odd manner.  If you let the spring relax completely, it becomes an
open spiral in a plane.  If you take the outside end of the=20
spring as one end and the inside end as the other, then
pull _axially_ (perpendicular to the plane of the coil,
you have a weak spring that starts at zero force at=20
zero axial displacement, and goes up linearly in both
the "negative" and "positive" direction.  It's not constant
force.

Also, I'd just like to clarify that for a standard coil
spring, having the coils contact when there is no external
force applied (like a screen door spring) is a necessary
but not sufficient condition to have a zero-length spring.
Most such springs are not tightly wound enough to be=20
zero-length, (i.e. they have a positive length at=20
the extrapolated zero force).

Jack

-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@...............
On Behalf Of Charles R. Patton
Sent: Friday, December 01, 2006 11:27 AM
To: psn-l@..............
Subject: Re: Zero-length spring

I beg to differ.  A standard open coil spring (where there is space=20
between the coil turns) has a finite lenght with zero tension.  The=20
tension increases with  stretch, but that tenssion starts wth that=20
offset length.  On the other hand a spring such as some types of screen=20
door closer springs which are wound such that that even from the start=20
position there is  tension.  If a plot of the tension vs length is done,

it is possible for the tension to go through zero at an imaginary zero=20
length, hence the term "zero length spring.."=20
On the other hand, a clock spring such as you describe may be the=20
reverse -- a constant force spring -- wherein at any length, it is=20
exhibiting constant force.  The only versions of these I'm aware of is a

type of roll up spring where the spring is slightly crowned to give it a

constant straightening force.  I believe is is also called a "negator"=20
spring.  But I also believe that watch springs are in general just stand

wind-up helical coils which really follow a small positive spring curve,

i.e., increasing force for increasing wind-up and since it starts out=20
too big and open, it fails the definition of a zero-lenght spring.
Regards,
Charles Patton

ChrisAtUpw@....... wrote:
> In a message dated 2006/11/30, tchannel@.............. writes:
>
>> I think its k that I am unclear of.=20
>
>
> Hi Ted,
>
>        'k' is the force constant of the spring in say, pounds per inch

> of stretch. This is constant until you get to very large extensions=20
> which change the spiral.
>
>        An example of a zero length spring is an ordinary clock spring=20
> when you pull it along it's shaft axis.
>
>        Regards,
>
>        Chris Chapman

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