Hi Chris,  I know how the roller on roller works, but I can't find a =
picture close up of one.   Do you know of one?   I was trying to explain =
it to someone, and picture would help.

Thanks, Ted
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Thursday, April 09, 2009 9:43 AM
  Subject: Re: Best Lehman pivot


  In a message dated 09/04/2009, rog@.......... writes:
    Is a scalpel blade against a steel ball bearing the current best =
consensus for a Lehman pivot?
    rob

  Hi Rob,

      Crossed cylinders can be a bit better. The surfaces need to highly =
polished. The SEP uses 1/8" OD polished Tungsten Carbide needle rollers, =
but these may be rather expensive / difficult to obtain depending on =
where you live. See =
http://www.bgs.ac.uk/education/school_seismology/seismometer.html You =
mount both the vertical rollers on the frame.=20

      However, a polished SS plane rolling on a SS ball bearing works =
perfectly well and it may cost only 50 c! See http://www.smallparts.com/

      I use a 1/2" SS bearing and a large scalpel blade 'trimmed' oval =
with a carbide disk and glued to the arm with two part acrylic adhesive. =
You can use bearings down to 1/4" OK, but mount them on the vertical =
frame, NOT on the arm! My top suspension is an 8 thou music wire. I can =
set the period to 60 seconds OK and it works for some days, but it has a =
significant tilt drift. My 'normal' set period is 20 seconds. I measured =
+/-2mm drift over a fortnight with this, in a max drift range of =
+/-10mm.

      The two 'suspensions' which are very likey to give trouble are the =
knife edge on a plane and a point in a cup. The materials in contact are =
loaded close to or over the maximum allowed stress. I advise against =
trying to use either of these types.=20

      It may be beneficial to attach the top support ~3/4 along the arm =
as shown above. The balance between the mass and the weight of the arm =
itself can be set to minimise the vertical load on the bottom bearing. =
This improves the stability.

      Regards,








Hi Chris,  I know how the roller on roller works, but I can't =
find a=20
picture close up of one.   Do you know of one?   I =
was=20
trying to explain it to someone, and picture would help.
 
Thanks, Ted

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Thursday, April 09, 2009 =
9:43=20
  AM
  
Subject: Re: Best Lehman =
pivot
  


  

  
In a message dated 09/04/2009, rog@.......... writes:
  Is a=20
    scalpel blade against a steel ball bearing the current best =
consensus for a=20
    Lehman pivot?
rob

  

  
Hi Rob,
  
 
  
    Crossed cylinders can be a =
bit better.=20
  The surfaces need to highly polished. The SEP uses 1/8" OD polished =
Tungsten=20
  Carbide needle rollers, but these may be rather expensive / difficult =
to=20
  obtain depending on where you live. See http://www.bgs.ac.uk/education/school_seismology/seismometer.html&n=
bsp;You=20
  mount both the vertical rollers on the frame. 
  
 
  
    However, a polished SS plane rolling on a =
SS ball=20
  bearing works perfectly well and it may cost only 50 c! See http://www.smallparts.com/
  
 
  
    I use a 1/2" SS bearing and a large =
scalpel blade=20
  'trimmed' oval with a carbide disk and glued to the arm with two =
part=20
  acrylic adhesive. You can use bearings down to 1/4" OK, but mount them =
on the=20
  vertical frame, NOT on the arm! My top suspension is an 8 thou music =
wire. I=20
  can set the period to 60 seconds OK and it works for some days, but it =
has=20
  a significant tilt drift. My 'normal' set period is 20 =
seconds. I=20
  measured +/-2mm drift over a fortnight with this, in a max drift =
range of=20
  +/-10mm.
  
 
  
    The two 'suspensions' which are=20
  very likey to give trouble are the knife edge on a plane and a =
point in a=20
  cup. The materials in contact are loaded close to or over the maximum =
allowed=20
  stress. I advise against trying to use either of these types. 
  
 
  
    It may be beneficial to attach the top =
support=20
  ~3/4 along the arm as shown above. The balance between the mass and =
the weight=20
  of the arm itself can be set to minimise the vertical load on the =
bottom=20
  bearing. This improves the stability.
  
 
  
    Regards,
  
 
  
    Chris=20
Chapman