Thanks Chris and All,   =
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.

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

Ted
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Friday, April 23, 2010 4:52 PM
  Subject: Re: Lehman set up


  In a message dated 23/04/2010 19:11:49 GMT Daylight Time, =
tchannel@............ writes:
    Many of you have more experience in setting up a Lehman. I have a =
basic question. Let's assume the frame and the horz arm are parallel. =
The vertical column has two pivots points, for the arm, top and bottom. =
The bottom pivot is a roller on roller and is not adjustable. The top =
pivot is a wire, which carries the weight of the arm. This wire has two =
adjustments. One is a slip nut, which allow the suspension wire to be =
shorten or lengthen, and keeps the arm parallel to the base. (The next =
adjustment on this top pivot wire is an adjustment screw which moves =
this point forward or behind that of the bottom pivot)

    This tilt off vertical is also controlled using the front adj. leg.
  Hi Ted,=20

      I suspect that you may be making life hard for yourself. You need =
one vertical top clamp for the wire. Then tilt the base to set the =
period. If the roller mounted on the frame is vertical, the arm swings =
about it's axis. The top wire should rotate about the edge of the clamp. =
The period is only stable when the vertical from the top suspension =
intersects the arm between the centre of rotation of the lower bearing =
and the mass. =20
      See =
http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node15.ht=
ml Fig 8a.=20
      The period of the pendulum T =3D 2xPixSqrt( L / gxsin(A)), where A =
is the angle between the vertical and the suspension axis. Using a 56cm =
pendulum, this is about 1/3 degree to get a 20 second period. g =3D 9.81 =
m/s/s and L is the length in metres from the suspension to the centre of =
gravity of the arm. If the vertical separation is 40cm, the offset is =
~2.3mm.
    My question is "as a starting point, should the top pivot point be =
perfectly vertical to the bottom pivot point"
      No, see above. That would be neutral stability.
    I have set up several Lehman's, and really not paid much attention =
to this. I think I have always started with them vertical and used the =
front leg to move the top point behind the bottom pivots.
      The actual angle that the pendulum makes with the vertical has no =
effect for small angles. The period depends on the angle that the swing =
axis makes with the vertical.

      Regards,








Thanks Chris and All,   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 =
the pivots?)  My=20
Lehman and others have the ability to move this top pivot behind or in =
front of=20
this line, before anything else is done.   I am not saying I =
should,=20
but I can make such an adjustment.
 
In setting up the frame I can align these two pivots as shown in =
this=20
illustration, in line......   The most obvious answer is to =
line both=20
pivots up on the same vertical line, as =
illustrated.    =20
BEFORE attempting to set the period, using the front=20
leg.   
 
Ted

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Friday, April 23, 2010 =
4:52=20
PM
  
Subject: Re: Lehman set =
up
  


  
In a message dated 23/04/2010 19:11:49 GMT Daylight Time, tchannel@............ =
writes:
  
    
Many of you have more experience in =
setting up=20
    a Lehman. I have a basic question. Let's assume the frame and the =
horz arm=20
    are parallel. The vertical column has two pivots points, for=20
    the arm, top and bottom. The bottom pivot is a roller on =
roller=20
    and is not adjustable. The top pivot is a wire, which carries the =
weight of=20
    the arm. This wire has two adjustments. One is a slip nut, which =
allow the=20
    suspension wire to be shorten or lengthen, and keeps the arm =
parallel to the=20
    base. (The next adjustment on this top pivot wire is an adjustment =
screw=20
    which moves this point forward or behind that of the bottom=20
    pivot)
    
 
    
This tilt off vertical is also =
controlled using=20
    the front adj. leg.
  
Hi Ted, 
  
 
  
    I suspect that you may be making life =
hard for=20
  yourself. You need one vertical top clamp for the wire. Then tilt the =
base to=20
  set the period. If the roller mounted on the frame is vertical, the =
arm swings=20
  about it's axis. The top wire should rotate about the edge of the =
clamp. The=20
  period is only stable when the vertical from the top suspension =
intersects the=20
  arm between the centre of rotation of the lower bearing and the=20
  mass.  
  
    See http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_ht=
ml/node15.html Fig 8a. 
  
    The period of the pendulum T =3D =
2xPixSqrt( L /=20
  gxsin(A)), where A is the angle between the vertical and the =
suspension=20
  axis. Using a 56cm pendulum, this is about 1/3 degree to get a 20 =
second=20
  period. g =3D 9.81 m/s/s and L is the length in metres from the =
suspension to=20
  the centre of gravity of the arm. If the vertical separation is =
40cm, the=20
  offset is ~2.3mm.
  
    
My question is "as a starting =
point, should the=20
    top pivot point be perfectly vertical to the bottom pivot=20
    point"
  
    No, see above. That would be neutral=20
  stability.
  
    
I have set up several Lehman's, and =
really not=20
    paid much attention to this. I think I have always started with them =

    vertical and used the front leg to move the top point behind the =
bottom=20
    pivots.
  

  
    The actual angle that the pendulum makes =
with the=20
  vertical has no effect for small angles. The period depends on the =
angle that=20
  the swing axis makes with the vertical.
  
 
  
    Regards,
  
 
  
    Chris=20
Chapman