In a message dated 2008/07/04, gel@................. writes:

> I have been taking my time to get my first Lehman up and running. First=20
> just to learn how the thing works and then to optimize the operation. In t=
he=20
> last month we have had lots of discussion on various Tungsten rods and oth=
er=20
> materials for the 2 horizontal rods and ball bearing support system. I fir=
st=20
> tried various rod materials by trying to get as long resonant period, but=20=
came=20
> to the conclusion this was difficult to repeat.=20

Hi Gary,

       You set the longitudinal tilt to get the required period. I can get 3=
0=20
seconds repeatably and quite easily using an 8 thou wire top suspension and=20=
a=20
3/8" SS ball + a SS plane as the borrom.

       It is easier to use a simple vertical pensulum.

Meredith on his 6/25 message got me thinking to take a different approach. I=
=20
reset=20
> the Lehman for a short period (about 10 seconds) and then connected the=20
> sensor to the amplifier and watched the display as the amplitude decayed.=20=
Now we=20
> have something that can be easily measured and repeated. I standardized on=
=20
> measurements for 5 minutes or 300 seconds. The decay equation is Y=3D A e-=
 t/T=20
> where t is time and T is the time constant of the system. For materials I=20=
used=20
> hardened steel,=20

       No good. It corrodes quickly in use.

>> stainless steel,

       Unless you used EN57 or another HARDENED martensitic stainless, it=20
will be far too soft. The ordinary Austenitic Stainless 304, 316 etc is much=
 too=20
soft.

 tungsten carbide (as received), tungsten carbide (mirror polished)=20
> 1/8=E2=80=9D diameter rods. The ball bearing is =C2=BC=E2=80=9D diameter s=
ilicon carbide.=20
>=20
> Here is what I found for the time constants:
>=20
> Hardened steel: 154 seconds
>=20
> Stainless steel: 125 seconds
>=20
> Tungsten carbide ( as received): 155 seconds
>=20
> Tungsten carbide (mirror polish): 191 seconds

       These decay times are far too short to be correct. The decay also=20
depends on the rigidity of the support, any air resistance of the mass, the=20
roughness of the rods / balls, losses in the pendulum structure itself, the=20
dimensions of the rolling elements and on the swing angle. If you use an coi=
l + magnet=20
sensor, this usually damps the pendulum A LOT. Was there a resistor across=20
the input to the amplifier?

> The goal is to have a high time constant, indicating lower friction.
> Polishing: The tungsten carbide as received actually had a very good polis=
h=20
> when received, but not quite a mirror finish. I obtained 3 diamond polishi=
ng=20
> grit sizes, starting out with 35u, then 15u, and finally 3u. The 35 and 15=
u=20
> actually made the tungsten carbide rod rougher. I followed Chris Chapman=
=E2=80=99s=20
> method for polishing using a bent sheet of copper with the diamond paste.
> Conclusion: Polished tungsten carbide rods have the lowest friction.

>>   The decay equation is Y=3D A e- t/T=20

       Unfortunately this is a myth. A free swinging pendulum does not behav=
e=20
as a simple harmonic oscillator and decays with Coulomb type damping.

       See Randall Peters paper=20
http://arxiv.org/ftp/physics/papers/0306/0306081.pdf

       A SS ball bearing and a 1/4 section of polished skin graft scalpel=20
blade are the cheapest suspension <50c.

       Regards,

       Chris Chapman  =20
In a me=
ssage dated 2008/07/04, gel@................. writes:



I have been taking my time to g=
et my first Lehman up and running. First just to learn how the thing works a=
nd then to optimize the operation. In the last month we have had lots of dis=
cussion on various Tungsten rods and other materials for the 2 horizontal ro=
ds and ball bearing support system. I first tried various rod materials by t=
rying to get as long resonant period, but came to the conclusion this was di=
fficult to repeat. 




Hi Gary,



       You set the longitudinal tilt to get th=
e required period. I can get 30 seconds repeatably and quite easily using an=
 8 thou wire top suspension and a 3/8" SS ball + a SS plane as the borrom.


       It is easier to use a simple vertical p=
ensulum.



Meredith on his 6/25 message got me thinking to take a different approach. I=
 reset 

the Lehman for a short period=20=
(about 10 seconds) and then connected the sensor to the amplifier and watche=
d the display as the amplitude decayed. Now we have something that can be ea=
sily measured and repeated. I standardized on measurements for 5 minutes or=20=
300 seconds. The decay equation is Y=3D A e- t/T where t is time and T is th=
e time constant of the system. For materials I used hardened steel, <=
FONT  COLOR=3D"#000000" BACK=3D"#ffffff" style=3D"BACKGROUND-COLOR: #ffffff"=
 SIZE=3D2 PTSIZE=3D10 FAMILY=3D"SANSSERIF" FACE=3D"Arial" LANG=3D"0">



       No good. It corrodes quickly in use.


>> stainless steel,



       Unless you used EN57 or another HARDENE=
D martensitic stainless, it will be far too soft. The ordinary Austenitic St=
ainless 304, 316 etc is much too soft.



 tungsten carbide (as received), tungsten carbide (mirror polished) <=
FONT  COLOR=3D"#000000" BACK=3D"#ffffff" style=3D"BACKGROUND-COLOR: #ffffff"=
 SIZE=3D2 PTSIZE=3D10 FAMILY=3D"SANSSERIF" FACE=3D"Arial" LANG=3D"0">

1/8=E2=80=9D diameter rods. Th=
e ball bearing is =C2=BC=E2=80=9D diameter silicon carbide. 



Here is what I found for the time constants:



Hardened steel: 154 seconds



Stainless steel: 125 seconds



Tungsten carbide ( as received): 155 seconds



Tungsten carbide (mirror polish): 191 seconds




       These decay times are far too short to=20=
be correct. The decay also depends on the rigidity of the support, any air r=
esistance of the mass, the roughness of the rods / balls, losses in the pend=
ulum structure itself, the dimensions of the rolling elements and on the swi=
ng angle. If you use an coil + magnet sensor, this usually damps the pendulu=
m A LOT. Was there a resistor across the input to the amplifier?



The goal is to have a high time=
 constant, indicating lower friction.

Polishing: The tungsten carbide as received actually had a very good polish=20=
when received, but not quite a mirror finish. I obtained 3 diamond polishing=
 grit sizes, starting out with 35u, then 15u, and finally 3u. The 35 and 15u=
 actually made the tungsten carbide rod rougher. I followed Chris Chapman=
=E2=80=99s method for polishing using a bent sheet of copper with the diamon=
d paste.

Conclusion: Polished tungsten carbide rods have the lowest friction.



>>   The decay equation is Y=3D A e- t/T 



       Unfortunately this is a myth. A free sw=
inging pendulum does not behave as a simple harmonic oscillator and decays w=
ith Coulomb type damping.



       See Randall Peters paper http://arxiv.o=
rg/ftp/physics/papers/0306/0306081.pdf



       A SS ball bearing and a 1/4 section of=20=
polished skin graft scalpel blade are the cheapest suspension <50c.



       Regards,



       Chris Chapman