## PSN-L Email List Message

From: "Jerry Payton" gpayton880@.......
Date: Wed, 1 Aug 2007 16:35:15 -0500

```Brett,

Thank you for an EXCELLENT reply and explanation.  You study to the
Servalite #59 springs was very helpful too.  That is exactly what I hope to
use in my present project.  Ted has been kind enough to obtain it for me,
since it apparently is difficult to find here in my area.

I really appreciate your patience and explaining it in such detail.  I can
actually understand and do not feel like a knuckle-dragging caveman.

Best regards,
Jerry

----- Original Message -----
From: Brett Nordgren
To: psn-l@..............
Sent: Wednesday, August 01, 2007 3:56 PM

Jerry,

Your question comes up fairly regularly on the list and sometimes generates
some confusing information.  I'll try to go at it from a 'hardware store'
approach.

Most close-wound extension springs have some amount of pre-tension.  That
is, you have to pull on them with significant force before they begin to
stretch.  A so-called zero-length spring has its pre-tension force carefully
controlled to be an exact value relative to its length and spring constant
(its force increase/length increase).  To be precise, the pre-tension force
'F' must be designed to equal the spring's unstretched length 'L' x its
spring constant 'k'.  Most springs from the hardware stores have
pre-tensioning that is substantially less than what is needed for them to be
"zero-length".

Actually, however, there is only one situation in which such a spring is of
much interest, and that is in the particular geometry discovered by Lucien
LaCoste when he was studying designs for vertical seismometers.  Some form
of that geometry, using a zero-length spring, had been employed for many
years in LaCoste & Romberg Gravity Meters.

** When used with any other geometry, there is nothing all that magical
about the zero-length spring characteristic. **

The particular LaCoste geometry, combined with a zero-length spring, has the
property that regardless of how you position the seismic mass, up or down,
it always remains exactly balanced, that is, the spring-mass has an infinite
period of oscillation.  However, having such a setup may not be all that
good for home seismometers.  In general when folks start trying designs
which have very long natural periods, they tend to have lots of problems
with position stability, of the sort that Ted mentions in his 28 July PSN-L
posting "Testing the Folded Pendulum".  When working with a very long
natural-period design, I would expect that you would need to use some sort
of electronic feedback in order to have a chance of its working reliably.

To see what a real spring looks like, I measured a Servalite #59 from the
hardware store, a spring that some others have used for their verticals.
The results, data and a graph, are at
http://bnordgren.org/seismo/Servalite59.pdf  Its unstretched length was
about 13.8 cm, and its pre-tension was near 205g, which meant that you might
be able to call it a "6.4cm-length" spring.  In order for it to be
"zero-length" its pre-tension would need to have been about 382g, slightly
more than 86% greater than what it was.

Hope all this doesn't just add to the confusion.

Brett

At 06:36 PM 7/31/2007 -0500, you wrote:

Not being a physics student and rather dense (yes, I admit it), will
someone please explain what a "zero length spring" is?  I have searched on
the net and found this law and that law, which means nothing to me.
Finally, I found a webpage that also called it a constant pressure spring
and displayed a large heavy duty spring that wound around itself, much like
an alarm clock spring.

It a zero length spring simply one that returns to its original shape
after the force is released?

Thank you,
Gerald Payton

My e-mail address above should be working, but if not
you can always use my mail form at: http://bnordgren.org/contactB.html

Brett,

Thank you for an EXCELLENT reply and explanation.  You study =
to the=20
Servalite #59 springs was very helpful too.  That is exactly what I =
hope to=20
use in my present project.  Ted has been kind enough to obtain it =
for me,=20
since it apparently is difficult to find here in my area.

I really appreciate your patience and explaining it in such =
detail.  I=20
can actually understand and do not feel like a knuckle-dragging =
caveman.

Best regards,
Jerry

----- Original Message -----=20
From: Brett=20
Nordgren
To: psn-l@..............
Sent: Wednesday, August 01, 2007 3:56 PM
Jerry,Your question comes up fairly regularly on =
the list=20
and sometimes generates some confusing information.  I'll try to go =
at it=20
from a 'hardware store' approach.Most close-wound extension =
springs have=20
some amount of pre-tension.  That is, you have to pull on them with =

significant force before they begin to stretch.  A so-called =
zero-length=20
spring has its pre-tension force carefully controlled to be an exact =
value=20
relative to its length and spring constant (its force increase/length=20
increase).  To be precise, the pre-tension force 'F' must be =
designed to=20
equal the spring's unstretched length 'L' x its spring constant =
'k'.  Most=20
springs from the hardware stores have pre-tensioning that is =
substantially less=20
than what is needed for them to be "zero-length".Actually, =
however,=20
there is only one situation in which such a spring is of much interest, =
and that=20
is in the particular geometry discovered by Lucien LaCoste when he was =
studying=20
designs for vertical seismometers.  Some form of that geometry, =
using a=20
zero-length spring, had been employed for many years in LaCoste & =
Romberg=20
Gravity Meters.  ** When used with any other geometry, =
there is=20
nothing all that magical about the zero-length spring characteristic.=20
**The particular LaCoste geometry, combined with a zero-length =
spring,=20
has the property that regardless of how you position the seismic mass, =
up or=20
down, it always remains exactly balanced, that is, the spring-mass has =
an=20
infinite period of oscillation.  However, having such a setup may =
not be=20
all that good for home seismometers.  In general when folks start =
trying=20
designs which have very long natural periods, they tend to have lots of =
problems=20
with position stability, of the sort that Ted mentions in his 28 July =
PSN-L=20
posting "Testing the Folded Pendulum".  When working with a very =
long=20
natural-period design, I would expect that you would need to use some =
sort of=20
electronic feedback in order to have a chance of its working =
reliably.To=20
see what a real spring looks like, I measured a Servalite #59 from the =
hardware=20
store, a spring that some others have used for their verticals.  =
The=20
results, data and a graph, are at http://bnordgren.org/seismo/Servalite59.pdf  =
Its=20
unstretched length was about 13.8 cm, and its pre-tension was near 205g, =
which=20
meant that you might be able to call it a "6.4cm-length" spring.  =
In order=20
for it to be "zero-length" its pre-tension would need to have been about =
382g,=20
slightly more than 86% greater than what it was.Hope all this =
doesn't=20
just add to the confusion.BrettAt 06:36 PM 7/31/2007 =
-0500,=20
you wrote:
Not being a physics student and rather =
dense=20
(yes, I admit it), will someone please explain what a "zero length =
spring"=20
is?  I have searched on the net and found this law and that law, =
which=20
means nothing to me.  Finally, I found a webpage that also called =
it a=20
constant pressure spring and displayed a large heavy duty spring that =
wound=20
around itself, much like an alarm clock =
spring. It a zero length spring simply one =
that returns=20
to its original shape after the force is =
released? Thank you,Gerald=20
Payton      =
;       =20
My e-mail address above should be working, but if notyou can always =
use my=20
mail form at: http://bnordgren.org/contactB.html=20
&nb=
sp;           &nbs=
p;  =20