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, 
 
    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.  
    See 
_http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node15.html_ 
(http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node15.html)  Fig 8a. 
    The period of the pendulum T = 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 = 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,
 
    Chris Chapman





In a message dated 23/04/2010 19:11:49 GMT Daylight Time,=20
tchannel@............ writes:

  
Many of you have more experience in set=
ting up a=20
  Lehman. I have a basic question. Let's assume the frame and the horz arm=
 are=20
  parallel. The vertical column has two pivots points, for=20
  the arm, top and bottom. The bottom pivot is a roller on rolle=
r and=20
  is not adjustable. The top pivot is a wire, which carries the weight of=
 the=20
  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=
 which=20
  moves this point forward or behind that of the bottom pivot)
  
 
  
This tilt off vertical is also controll=
ed using=20
  the front adj. leg.
Hi Ted, 
 
    I suspect that you may be making life hard fo=
r=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 sw=
ings=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-stu=
ttgart.de/oldwww/seismometry/man_html/node15.html Fig 8a.&nb=
sp;
    The period of the pendulum T =3D 2xPixSqrt(=
 L /=20
gxsin(A)), where A is the angle between the vertical and the suspensi=
on=20
axis. Using a 56cm pendulum, this is about 1/3 degree to get a 20 second=
 period.=20
g =3D 9.81 m/s/s and L is the length in metres from the suspension to the=
 centre=20
of gravity of the arm. If the vertical separation is 40cm, the offset=
 is=20
~2.3mm.

  
My question is "as a starting point, sh=
ould 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 rea=
lly not=20
  paid much attention to this. I think I have always started with them ver=
tical=20
  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 Chapman