In a message dated 13/06/2007, davenn@............... writes:

Hi Jon

going by your pic ... It seems  you have no mass (weight) at the end of the 
arm
and that is why the wire  line is slack  also the terminating of that line at 
the top
end is  critical to overcome free swinging problems   follow Geffory's ideas  
using piano wire and turnbuckles and a good pivot point etc,  as shown in 
the original Lehman design.

a  good mass would be ~ 5 - 10 kg of brass or lead
My commercial horizontal  seismo had 10kg brass mass
Hi Jon,
 
    You choose your mass to go with the arm. I use  either 15 mm SS water 
pipe for preference, or 3/4" to 1" OD Al tube. The SS  pipe weighs ~213 gm per 
metre, it is quite rigid and works very well for arms of  <40 to ~100 cm. You 
can buy inexpensive brass compression fittings for water  tube. SS is more rigid 
than Al. Use tube, not solid rod, to get longer  periods. Another possibility 
is thin wall brass 'telescope' tube. You  used to be able to buy brass 
curtain rod tube? Do not make the arm or the mass  from magnetic material - they 
tend to pick up magnetic field fluctuations from  the Earth and from the power 
wiring. 
 
    You don't say what sort of top and bottom bearings  you are going to use? 
SS Ball on a SS plane or crossed cylinders are fine. See  
_http://pages.prodigy.net/fxc/_ (http://pages.prodigy.net/fxc/)  and  
_http://pages.prodigy.net/fxc/JC.html_ (http://pages.prodigy.net/fxc/JC.html) 
 
    You put the balls or the vertical rollers on the  upright support column. 
This defines the axis of rotation of the arm, so  that once you have set the 
system up for cross balance, you only need to trim  this slightly 
subsequently. Even if the flats / horizontal rollers move slightly  during operation of 
the seismometer, they have only a small effect on the  balance or the period. To 
get a long period, the '~vertical rotation axis' has  to be set at 1/3 degree 
or less to the true vertical.
 
    If you put the ball on the arm and the flat on the  vertical, every time 
that you have to reassemble / adjust the seismometer,  you have to completely 
reset the cross balance and then the period. If the  bearing moves slightly 
during normal operation, the arm will probably get  unbalanced / drift.
 
    DON'T use a POINT PIVOT or a KNIFE EDGE. I don't  know who started using 
these, but they are UNSATISFACTORY and WILL  seriously limit the period that 
you can achieve.   
 
    Assuming that your 60 cm arm is light compared to  the end mass, you may 
need ~1 kg mass, not over 2.5 kg ---> most of the  mass needs to be at the end 
of the arm. If you try to use a very heavy mass, you  could get into problems 
with bearing loading and with suspension wire stretch /  breakage. The 
commercial units like Sprengnether used two single short piano  wires in tension for 
the suspension. They use a heavy twin wire V suspension to  prevent the arm 
from rotating / oscillating about it's long axis. 
 
    See the Sprengnether photos at 
_http://www.geocities.com/meredithlamb/page023.html_ (http://www.geocities.com/meredithlamb/page023.html)  These  heavy 
units were probably designed before the MTQ noise limit had been  worked out. 
 
    Regards,
 
    Chris Chapman 
    







   





In a message dated 13/06/2007, davenn@............... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>   Hi Jon

    going by your pic ... It=
 seems=20
  you have no mass (weight) at the end of the arm
and that is why the wir=
e=20
  line is slack  also the terminating of that line at the top
end is=
=20
  critical to overcome free swinging problems   follow Geffory's i=
deas=20
  
using piano wire and turnbuckles and a good pivot point etc,  as shown in 
the original Lehman design.

&nb=
sp; a=20
  good mass would be ~ 5 - 10 kg of brass or lead
My commercial horizonta=
l=20
  seismo had 10kg brass mass
Hi Jon,
 
    You choose your mass to go with the arm. I=20=
use=20
either 15 mm SS water pipe for preference, or 3/4" to 1" OD Al tube. Th=
e SS=20
pipe weighs ~213 gm per metre, it is quite rigid and works very well for arm=
s of=20
<40 to ~100 cm. You can buy inexpensive brass compression fittings for wa=
ter=20
tube. SS is more rigid than Al. Use tube, not solid rod, to get longer=20
periods. Another possibility is thin wall brass 'telescope' tube.=20=
You=20
used to be able to buy brass curtain rod tube? Do not make the arm or the ma=
ss=20
from magnetic material - they tend to pick up magnetic field fluctuations fr=
om=20
the Earth and from the power wiring. 
 
    You don't say what sort of top and bottom beari=
ngs=20
you are going to use? SS Ball on a SS plane or crossed cylinders are fine. S=
ee=20
http://pages.prodigy.net/fxc/&nbs=
p;and=20
http://pages.prodigy.net/fxc/J=
C.html
 
    You put the balls or the vertical rollers on=
 the=20
upright support column. This defines the axis of rotation of the arm, so=
=20
that once you have set the system up for cross balance, you only need to tri=
m=20
this slightly subsequently. Even if the flats / horizontal rollers move slig=
htly=20
during operation of the seismometer, they have only a small effect on the=20
balance or the period. To get a long period, the '~vertical rotation axis' h=
as=20
to be set at 1/3 degree or less to the true vertical.
 
    If you put the ball on the arm and the flat on=20=
the=20
vertical, every time that you have to reassemble / adjust the seismomet=
er,=20
you have to completely reset the cross balance and then the period. If the=20
bearing moves slightly during normal operation, the arm will probably get=20
unbalanced / drift.
 
    DON'T use a POINT PIVOT or a KNIFE EDGE. I don'=
t=20
know who started using these, but they are UNSATISFACTORY and WILL=20
seriously limit the period that you can achieve.   
 
    Assuming that your 60 cm arm is light compared=20=
to=20
the end mass, you may need ~1 kg mass, not over 2.5 kg ---> most of=20=
the=20
mass needs to be at the end of the arm. If you try to use a very heavy mass,=
 you=20
could get into problems with bearing loading and with suspension wire stretc=
h /=20
breakage. The commercial units like Sprengnether used two single short piano=
=20
wires in tension for the suspension. They use a heavy twin wire V suspension=
 to=20
prevent the arm from rotating / oscillating about it's long axis. 
 
    See the Sprengnether photos at http://www.geoci=
ties.com/meredithlamb/page023.html These=20
heavy units were probably designed before the MTQ noise limit had been=20
worked out. 
 
    Regards,
 
    Chris Chapman