From: Dave Nelson dave.nelson@...............
Sent: Sun, 24 Jul 2011 23:16
Subject: Re: New Seismo


****  hmmmm  am a bit worried that there is something fundamentally=20
wrong with it. There was a M6.2 off Japan overnight only 8000km=20
away and nothing recorded.  I would have expected that even with=20
under dampening that that event would have shown up =20
you will see that there is a pretty much steady low freq signal=20
there, varying in intensity, in winquake it has a~ 3.5 sec period.=20
=20
****ok got around to taking a pic of the new Lehman
http://www.sydneystormcity.com/IMGP2023a.jpg

Hi Dave,=20
=20
    I can't see from the photo how you have made the bottom bearing.=20
I presume that you are using Stainless Steel Bearings? It looks like=20
you have the bearing on the end of the arm trying to roll up against=20
the Al column ? If so, this DOESN'T WORK. The Al is far too soft and=20
the bearing makes a dimple hollow - tiny but it effectively prevents=20
any low amplitude motion of the arm. A bearing rolling on another=20
spherical bearing is unlikely to be stable.
    I suggest that you keep the bearing on the column and cut off the=20
end of the arm flat. Then stick a hardened stainless steel flat onto=20
the end of the arm with two component Acrylic Glue - forget Epoxy, it=20
sets brittle and it doesn't stick Aluminium alloys at all well.
    I use a section cut from a Swann-Morton SS scalpel blade for the=20
hard flat counter-surface stuck to the end of the arm. You can also=20
use a section cut from a polished SS tea knife blade. Dremmel sell=20
thin carbide cutting disks and mandrels suitable for doing this.=20
=20
****The frame is made of 10mm thick, 50mm wide aluminium bar. The boom=20
is 16mm diam aluminium bar.  The 2 lead weights are 2 pound diving belt=20
weights, holes bored in them and then slid onto the end of the boom.


    You need to clamp the weights to the boom and they need to be=20
bolted together, if you use two. A single 2 lb mass is usually OK, when=20
using a light arm. I suggest that you drill a suitable tap hole through=20
the top of the weight and through the arm. Slide the weight off, tap a=20
thread in the arm and drill a clearance hole through the lead. Then use=20
a stainless steel bolt, spring washer and washer to clamp the lead to=20
the arm. Yacht / power boat / marine suppliers usually stock SS hardware.=
=20
=20
****The pickup coil sits between 2 rare earth disc magnets=20
~ 1.5 cm diameter.
The dampening at the moment is just a cylindrical rare earth=20
magnet below the lower plate of the plates that support the=20
pickup magnets.


    You need square or rectangular NdFeB magnets. See the PSN web site=20
for suppliers. I use two opposing magnet pairs of 1/8" thick magnets,=20
for both applications.
    Check http://www.jclahr.com/science/psn/chapman/lehman/index.html=20
for drawings of highly efficient adjustable dampers and also sensors.

****I am not totally happy with the lower pivot.  I was going to have=20
ball bearings butted against each other. But they wont sit together. =20
Even just the single ball bearing in the end of the boom won't sit=20
happily against the vertical bar.=20
=20
    Use a flat SS plate on the end of the arm rolling on a SS ball=20
mounted on the vertical support. This is better than putting the ball=20
on the end of the arm. Look for B3295 syrupy structural acrylic glue.=20
This is much easier to use than the thick paste glues.
=20
**** There is just so much upwards force at that end of the boom=20
caused by the downwards force of the weight at the other end.

    This is easy it solve. You just need to add weight to the ball end=20


    This is easy it solve. You just need to add weight to the ball end=20
of the arm ! Remove the arm and support it at the V trace attachment=20
points. Measure the weight required to balance the arm, either with a=20
set of digital scales or by placing weighing scale weights near the=20
end of the arm. Then either make a suitable brass ring weight to clamp=20
to the arm, or use a strip of lead to wrap around the arm. Slide the=20
weight along the arm until it balances and clamp it. With lead strip,=20
I first fit a stainless steel hose clamp over the middle, then I slide=20
the strip along the arm until it balances and tighten the clamp.
    =20
     I hope that this helps.


     Regards,

     Chris Chapman
























=
From: Dave Nelson dave.nelso=
n@...............

Sent: Sun, 24 Jul 2011 23:16

Subject: Re: New Seismo





****  hmmmm  am a bit worried that there is something fundam=
entally 


wrong with it. There was a M6.2 off Japan overnight only 8000km <=
/div>


away and nothing recorded.  I would have expected that even with =



under dampening that that event would have shown up  

you will see that there is a pretty much steady low freq signal 


there, varying in intensity, in winquake it has a~ 3.5 sec period.&nbs=
p;


 


****ok got around to taking a pic of the new Lehman

ht=
tp://www.sydneystormcity.com/IMGP2023a.jpg



Hi Dave, 


 


    I can't see from the photo how you have made the bo=
ttom bearing. 


I presume that you are using Stainless Steel Bearings? It looks like <=
/div>


you have the bearing on the end of the arm trying to roll up against <=
/div>


the Al column ? If so, this DOESN'T WORK. The Al is far too soft and <=
/div>


the bearing makes a dimple hollow - tiny but it effectively prevents <=
/div>


any low amplitude motion of the arm. A bearing rolling on another 


spherical bearing is unlikely to be stable.


    I suggest that you keep the bearing on the column a=
nd cut off the 


end of the arm flat. Then stick a hardened stainless steel flat onto <=
/div>


the end of the arm with two component Acrylic Glue - forget Epoxy, it =



sets brittle and it doesn't stick Aluminium alloys at all well.


    I use a section cut from a Swann-Morton SS sca=
lpel blade for the 


hard flat counter-surface stuck to the end of the arm. You can al=
so 


use a section cut from a polished SS tea knife blade. Dremmel sell 


thin carbide cutting disks and mandrels suitable for doing this. =



 


****The frame is made of 10mm thick, 50mm wide aluminium bar. The boom=
 


is 16mm diam aluminium bar.  The 2 lead weights are 2 pound divin=
g belt 


weights, holes bored in them and then slid onto the end of the boom.



 


    You need to clamp the weights to the boom and they =
need to be 


bolted together, if you use two. A single 2 lb mass is usually OK, whe=
n 


using a light arm. I suggest that you drill a suitable tap hole throug=
h 


the top of the weight and through the arm. Slide the weight off, tap a=
 


thread in the arm and drill a clearance hole through the lead. Then us=
e 


a stainless steel bolt, spring washer and washer to clamp the lea=
d to 


the arm. Yacht / power boat / marine suppliers usually stock SS hardwa=
re. 


 


****The pickup coil sits between 2 rare earth disc magnets 

~ 1.5 cm diameter.

The dampening at the moment is just a cylindrical rare earth 

magnet below the lower plate of the plates that support the 

pickup magnets.




 


    You need square or rectangular NdFeB magnets. See t=
he PSN web site 


for suppliers. I use two opposing magnet pairs of 1/8" thick magnets, =



for both applications.


    Check http://www.jclahr.com/science/ps=
n/chapman/lehman/index.html 

for drawings of highly efficient adjustable dampers and also sensors=
..



****I am not totally happy with the lower pivot.  I was going to have =



ball bearings butted against each other. But they wont sit together.&n=
bsp; 


Even just the single ball bearing in the end of the boom won't sit 


happily against the vertical bar. 


 


    Use a flat SS plate on the end of the arm rolling o=
n a SS ball 


mounted on the vertical support. This is better than putting the ball =



on the end of the arm. Look for B3295 syrupy structural acrylic glue. =



This is much easier to use than the thick paste glues.


 

**** There is just so much upwards force at that end of the boom 


caused by the downwards force of the weight at the other end.



    This is easy it=
 solve. You just need to add weight to the ball end 


of the arm ! Remove the arm and support it at the V trace att=
achment 


points. Measure the weight re=
quired to balance the arm, either with a 


set of digital scales =
or by placing weighing scale weigh=
ts near the 


end of the arm. Then either make a suitable brass ring weight=
 to clamp 


to the arm, or use a strip of lead to wrap around the arm. Slide the 


weight along the arm until it balances and clamp it. With lead strip, 


I first fit a stainless&=
nbsp;steel hose clamp over the m=
iddle, then I slide 


the strip along the arm until it balances and tighten the cla=
mp.

     

     I hope that this helps.




 


     Regards,




     Chris Chapman