Yes  Chris I understand your comments,
Thanks, Ted


----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Saturday, April 24, 2010 5:14 PM
  Subject: Re: Lehman set up


  In a message dated 24/04/2010, tchannel@............ writes:
    Thanks  =
http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node15.ht=
ml  Chris this is a helpful site. My question is similar to the =
illustration Figure 7 (a)
    In this illustration the top pivot and bottom pivot are vertical on =
the same line. (is this how I should align the pivots?)  My Lehman and =
others have the ability to move this top pivot behind or in front of =
this line, before anything else is done.  I am not saying I should, but =
I can make such an adjustment.
  Hi Ted,
     =20
      I deliberately did not refer to Fig 7a. This shows NEUTRAL =
stability. I use something equivalent to Fig 8a, with Alpha set to only =
about 0.3 degree - the top flexure is displaced from the vertical =
towards the mass end by about 2.5 mm for a 40 cm vertical position =
difference between the suspension points.

      There is a definite advantage in making the boom lie parallel to =
the frame, with the damping and sensor systems all aligned parallel, but =
then tilting the whole assembly up at the mass end to actually set the =
period. The setup is then separate sequential operations which are not =
interdependent.

      I deliberately avoided using a nipple for the top suspension. They =
are difficult to make and they don't actually CLAMP the wire! There must =
be NO 'free play'. Also, I use finer wire, about 12 thou OD. I use two =
bronze disks, about 0.15" thick. They are dished on the inside faces by =
about 15 thou to leave a 1 mm wide rim on the periphery. The wire is =
clamped between these rims on opposite sides of the disk. The hole for =
the clamp bolt is offset from the disk centre by 1/8" to allow for =
adjustment. The disks can then be rotated as a pair to offset the clamp =
position exactly. The wire goes through a hole in the clamp bolt, which =
is aligned and glued to the upright column. See =
http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_prototype.jpg =
  =20

  >>    In setting up the frame I can align these two pivots as shown in =
this illustration, in line......  The most obvious answer is to line =
both pivots up on the same vertical line, as illustrated.  BEFORE =
attempting to set the period, using the front leg.  =20

      If you set up the system to give neutral stability with the frame =
horizontal, you may not be able to lower the mass end of the frame =
enough to give the period that you want, before it hits the floor! Note =
that the three  adjustment bolts use a wavy spring washer and a locknut =
to prevent any rotation after they have been adjusted. This seems to =
prevent any rocking in the threads.=20

      Crossed foil and Cardan suspensions do need to be accurately =
aligned vertically.=20

      Note also that I have used a V wire top suspension made from 30 lb =
SS fishing trace with crimped loop ends. This prevents the arm from =
rocking about it's longitudinal axis. It proved to be very difficult to =
balance the drag from the damper accurately enough using a single wire. =
If the arm can rotate about it's long axis, you get spurious signals =
generated, especially since this motion is not usually damped.

      I tested my system by setting it up to give a stable period of 60 =
seconds. It behaved perfectly. I then reduced the period to about 25 =
seconds for normal operation. You are limited in the maximum period that =
you can set by the extreme sensitivity to tiny tilt drifts as the ground =
heaves naturally with changes in air pressure, temperature and rainfall. =
The shorter arm Sprengnethers were usually run at 15 seconds period to =
limit their natural tilt drift, although they were specified to be used =
at periods up to 30 seconds.

      I hope that this all makes sense.=20

      Regards,








Yes  Chris I understand your comments,
Thanks, Ted
 
 
----- Original Message ----- 

  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Saturday, April 24, 2010 =
5:14=20
  PM
  
Subject: Re: Lehman set =
up
  


  
In a message dated 24/04/2010, tchannel@............ =
writes:
  
    
Thanks  http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_ht=
ml/node15.html =20
    Chris this is a helpful site. My question is similar to the=20
    illustration Figure 7 (a)
    
In this illustration the top pivot and bottom pivot are =
vertical on the=20
    same line. (is this how I should align=20
    the pivots?)  My Lehman and others have the ability to =
move this=20
    top pivot behind or in front of this line, before anything else is=20
    done.  I am not saying I should, but I can make such an=20
    adjustment.
  
Hi Ted,
  
    
  
    I deliberately did not refer to Fig 7a. =
This=20
  shows NEUTRAL stability. I use something equivalent to Fig 8a, with =
Alpha set=20
  to only about 0.3 degree - the top flexure is displaced from the =
vertical=20
  towards the mass end by about 2.5 mm for a 40 cm vertical position =
difference=20
  between the suspension points.
  
 
  
    There is a definite advantage in =
making the=20
  boom lie parallel to the frame, with the damping and sensor systems =
all=20
  aligned parallel, but then tilting the whole assembly up at the mass =
end to=20
  actually set the period. The setup is then separate sequential =
operations=20
  which are not interdependent.
  
 
  
    I deliberately avoided using a nipple for =
the top=20
  suspension. They are difficult to make and they don't actually CLAMP =
the wire!=20
  There must be NO 'free play'. Also, I use finer wire, about 12 =
thou OD. I=20
  use two bronze disks, about 0.15" thick. They are dished on the inside =
faces=20
  by about 15 thou to leave a 1 mm wide rim on the periphery. The wire =
is=20
  clamped between these rims on opposite sides of the disk. =
The hole for=20
  the clamp bolt is offset from the disk centre by 1/8" to allow for=20
  adjustment. The disks can then be rotated as a pair to offset the =
clamp=20
  position exactly. The wire goes through a hole in the clamp bolt, =
which is=20
  aligned and glued to the upright column. See http://jclahr.com/science/psn/chapman/2008%20lehman/lehman_proto=
type.jpg    
  
 
  
>>    In setting up the frame I =
can align=20
  these two pivots as shown in this illustration, in line......  =
The most=20
  obvious answer is to line both pivots up on the same vertical line, as =

  illustrated.  BEFORE attempting to set the period, using the =
front=20
  leg.   
  
 
  
    If you set up the system to give =
neutral=20
  stability with the frame horizontal, you may not be able to lower the =
mass end=20
  of the frame enough to give the period that you want, before it hits =
the=20
  floor! Note that the three  adjustment bolts use a wavy spring =
washer and=20
  a locknut to prevent any rotation after they have been adjusted. This =
seems to=20
  prevent any rocking in the threads. 
  
 
  
    Crossed foil and Cardan suspensions do =
need to be=20
  accurately aligned vertically. 
  
 
  
    Note also that I have used a V =
wire top=20
  suspension made from 30 lb SS fishing trace with crimped loop ends. =
This=20
  prevents the arm from rocking about it's longitudinal axis. It proved =
to be=20
  very difficult to balance the drag from the damper accurately =
enough=20
  using a single wire. If the arm can rotate about it's long axis, you =
get=20
  spurious signals generated, especially since this motion is not =
usually=20
  damped.
  
 
  
    I tested my system by setting it =
up to give=20
  a stable period of 60 seconds. It behaved perfectly. I then reduced =
the period=20
  to about 25 seconds for normal operation. You are limited in the =
maximum=20
  period that you can set by the extreme sensitivity to tiny tilt drifts =
as the=20
  ground heaves naturally with changes in air pressure, temperature and=20
  rainfall. The shorter arm Sprengnethers were usually run at 15 seconds =
period=20
  to limit their natural tilt drift, although they were specified to be =
used at=20
  periods up to 30 seconds.
  

  
 
  
    I hope that this all makes sense. 
  
 
  
    Regards,
  
 
  
    Chris=20
Chapman