## PSN-L Email List Message

Subject: Re: Zero-length spring
From: "Charles R. Patton" charles.r.patton@........
Date: Fri, 01 Dec 2006 08:55:17 -0800

```Apologies.  I think I want to eat a few words!  Since it is possible for
a helical spring to be relaxed and exerting zero force at some arbitrary
position, then increasing rotation is from zero and hence meets a
zero-force spring definition.  Although to be truthful, at the moment, I
don't know how to usefully apply that to a seismometer. in particular a
LaCoste.
Regards,
Charles Patton

I beg to differ.  A standard open coil spring (where there is space
between the coil turns) has a finite lenght with zero tension.  The
tension increases with  stretch, but that tenssion starts wth that
offset length.  On the other hand a spring such as some types of screen
door closer springs which are wound such that that even from the start
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
length, hence the term "zero length spring.." On the other hand, a clock
spring such as you describe may be the reverse -- a constant force
spring -- wherein at any length, it is 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" 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 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.
>
>
> 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
> which change the spiral.
>
>        An example of a zero length spring is an ordinary clock spring
> when you pull it along it's shaft axis.
>
>        Regards,
>
>        Chris Chapman
ChrisAtUpw@....... wrote:
> In a message dated 2006/11/30, tchannel@.............. writes:
>
>> I think its k that I am unclear of.
>
>
> 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
> which change the spiral.
>
>        An example of a zero length spring is an ordinary clock spring
> when you pull it along it's shaft axis.
>
>        Regards,
>
>        Chris Chapman

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