Brett, thank you so very much for that invaluable information...Beautifully=
 =0Aclear terms! Also I would like to recommend this incredible PDF to the =
group....=0A=0A=0A=0A=0A________________________________=0AFrom: Brett Nord=
gren =0ATo: psnlist@...................... Sun, May=
 1, 2011 12:52:02 PM=0ASubject: Re: Possible Spring To Use=0A=0AGeoff,=0A=
=0AAt 10:35 AM 5/1/2011, you wrote:=0A> Why must that be ?=0A> =0A> Noteboo=
k paper is only 0.003"=0A> Why must a spring be so thin ?=0A=0AWhen you ben=
d a leaf spring into the sort of curve that you want for typical =0Aseismo =
designs, if the spring is too thick it will be stressed beyond its yield =
=0Astrength and it will get bent into a permanent curve--not good.=0A=0AThe=
 relationship is: Radius =3D E x Thickness / (2 x Stress)=0Aor Thickness =
=3D 2 x Stress x Radius / E=0A=0AWhere E is the Elastic Modulus - Assume 32=
E6 psi for steel=0AYield Stress is well over 200,000 psi for good spring st=
eel, so a safe design =0Astress level might be 150,000 psi=0A=0AFor a minim=
um bend radius of 1 inch, which is about what we see in a 9" long =0Ainstru=
ment, you have: Thickness =3D 2 x 150,000 x 1 / 32E6 =3D 0.0094" maximum.=
=A0 =0AThicker than that and the stress will exceed 150,000 psi.=A0 Smaller=
 designs will =0Arequire thinner springs.=0A=0A> Also,=0A> =0A> This Zero L=
ength spring stuff ?=0A> =0A> Do I understand right when I think=0A> a zero=
 length spring does not change force as the spring length changes ?=0A=0ANo=
, it acts pretty much like any other coil spring, only that it is pre-stres=
sed =0Aso that the force needed to make it begin to stretch is exactly the =
right =0Avalue.=A0 Then, *if and only if*, that spring is used with the LaC=
oste pendulum =0Ageometry, you get an 'infinite' period.=0A=0ASimilar resul=
ts may be obtained with astatic leaf spring designs, however 'zero =0Alengt=
h' only applies to LaCoste coil springs.=0A=0AAlthough having an infinite p=
eriod pendulum sounds attractive, it has the great =0Aproblem that tiny tem=
perature changes will make the boom drift off to the stop.=A0 =0AYou will w=
ant to give the boom at least a small tendency to center itself, which =0Am=
eans that it will have a non-infinite period.=0A=0AFeedback designs pretty =
much eliminate that issue.=0A=0ARegards,=0ABrett =0A=0A____________________=
______________________________________=0A=0APublic Seismic Network Mailing =
List (PSNLIST)=0A=0ATo leave this list email PSNLIST-REQUEST@...............
 with the body of the =0Amessage (first line only): unsubscribe=0ASee http:=
Brett, thank you so very much for that invaluable information..=
..Beautifully clear terms! Also I would like to recommend this incredible PD=
F to the group....
=0A

=0A
=0A
=0AFrom: Brett Nord=
gren <brett3nt@.............>
To: psnlist@..............
Sent: Sun, May 1, 2011 12:52:02 PM
Subject: Re: Possible Spring To Use
Geoff,

At 10:35 AM 5/1/2011, you wrote:
> Why must that be ?<=
BR>> 
> Notebook paper is only 0.003"
> Why must a spring be=
 so thin ?

When you bend a leaf spring into the sort of curve that y=
ou want for typical seismo designs, if the spring is too thick it will be s=
tressed beyond its yield strength and it will get bent into a permanent cur=
ve--not good.

The relationship is: Radius =3D E x Thickness / (2 x S=
tress)
or Thickness =3D 2 x Stress x Radius / E

Where E is the El=
astic Modulus - Assume 32E6 psi for steel
Yield Stress is well over 200,=
000 psi for good
 spring steel, so a safe design stress level might be 150,000 psi

Fo=
r a minimum bend radius of 1 inch, which is about what we see in a 9" long =
instrument, you have: Thickness =3D 2 x 150,000 x 1 / 32E6 =3D 0.0094" maxi=
mum.  Thicker than that and the stress will exceed 150,000 psi.  =
Smaller designs will require thinner springs.

> Also,
> > This Zero Length spring stuff ?
> 
> Do I understand righ=
t when I think
> a zero length spring does not change force as the sp=
ring length changes ?

No, it acts pretty much like any other coil sp=
ring, only that it is pre-stressed so that the force needed to make it begi=
n to stretch is exactly the right value.  Then, *if and only if*, that=
 spring is used with the LaCoste pendulum geometry, you get an 'infinite' p=
eriod.

Similar results may be obtained with astatic leaf spring desi=
gns, however 'zero length' only applies to LaCoste coil
 springs.

Although having an infinite period pendulum sounds attract=
ive, it has the great problem that tiny temperature changes will make the b=
oom drift off to the stop.  You will want to give the boom at least a =
small tendency to center itself, which means that it will have a non-infini=
te period.

Feedback designs pretty much eliminate that issue.
Regards,
Brett 

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