[TheForge] Treadle Hammer Springs

[Bruce Freeman]

The garage door springs I've tested had essentially
linear spring constants, BUT were wound with an
initial tension.  To test a spring, I simply hung it
from a beam, measured its length, then sequentially
added known weight and remeasured the length.

A plot of the length looked something like this
(weight is plotted horizontally, lenght vertically):

          /
         /
        /
       /
      /
     /
____/

The initial horizontal line represents the weight that
had to be added before the spring started stretching
at all.  If a treadle hammer is made such that the
springs are not subjected to at least a little
stretch, then the users leg has to overcome this extra
tension before the hammer even begins to move.

If a spring doesn't have an initial tension, this is
not an issue.  That is my guess as to why some people
prefer small springs to garage-door springs.

Bruce
NJ


--- xlch58 at swbell.net wrote:

> Steve Smith wrote:
> >
> >
> > Bruce Freeman wrote:
> >
> >> Rumor has it that multiple small springs are
> better
> >> than one large spring, and you see some treadle
> >> hammers so constructed.  I haven't given this
> enough
> >> though to explain why this should be true.  It
> MIGHT
> >> be true because garage-door springs take a load
> to
> >> even BEGIN to extend, whereas most small springs
> >> don't.
> >
> > This never made sense to me. Spring constant is
> spring constant, no 
> > matter whether it comes from a bunch of small
> springs or one large one.
> >
> > Changing the spring constant, however, is another
> issue.
> >
> > Steve
> Some springs are progressively wound, so depending
> on extension ( or 
> compression) the spring constant changes.  Good
> reasons for doing this 
> are to allow easy installation on a strong spring. 
> Other springs are 
> wound with an initial tension.    Finally, common
> extension springs do 
> not have a constant rate, their behavior is most
> linear in the middle of 
> their range.   There are in fact a special class of
> extension spring 
> call a constant rate spring.  So depending on the
> design, a bunch of 
> small springs could be put together in such a way to
> give a more 
> constant rate, or  to provide a progressive rate.
> 
> Charles
> 
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