Hi Charles and other PSN members,
I think too much is being made of the "unstable" characteristics of the
Lehman. Both of mine (T Max and Old Lehman) are set up in my garage with a
sloping concrete floor. They are in the same garage with a washing machine
and drier with only about six or eight feet separating them from my ham
shack in a small garage room.  They are next to the front garage door and
only about 20 feet from a busy street. A car and a van park only about six
feet from them.  Foot traffic on the porch is several feet away.  Here on
the Gulf Coast of Texas (near Houston) our houses are constructed with
concrete slabs resting on clay soil. All the houses in my subdivision have
constant problems with concrete slabs that shift with each rain or dry
period.  Our sheet rock walls frequently crack and the wall corners shift
and open for this reason.  My modified Lehman has been in constant operation
resting on my garage floor since January, 1997.  After a heavy rain I have
learned to check the pendulum to see if it is centered. If it is not, it
only take me a few seconds to center it.  I usually check it every night
before going to bed to see if it needs centering.

I have worked on my Old Lehman only once and that was to change the
pendulum.  It was producing a period of only 12 seconds or so.  It had a
pendulum with its end ground to a point that rested in a dimple of a steel
bolt head.  12 seconds seemed to be the limit.   Then along came John Cole
with his ball bearing method.  John lives only a few miles from me and he
presented me with a polished bolt and a pendulum with a ball bearing
inserted in one end.  We changed the pendulum out to where the ball bearing
rested on the polished bolt.  I was able to immediately increase the period
to about 18 seconds and after an adjustment or two the period has remained
over 20 seconds.  John recently presented me with his T Max detector which
is a modified Lehman with a nickel chrome ball bearing resting upon a
slightly convex nickel chrome polished steel surface at the lower pivot
point. The upper suspension point has a nickel chrome ball bearing resting
on a slightly convex polished nickel chrome surface. The nickel chrome steel
convex surface is impervious to dimpling or scratching by the ball.  The
period of this instrument remains at 27 to 30 seconds.  It is difficult to
time the period exactly because the pendulum moves so slowly.

I checked the pendulum tonight and it is still centered from 24 hours ago.
If polishing a nickel chrome surface is too much trouble I suggest using the
ball bearing with a polished bolt at the lower pivot point and fine piano
wire at the upper suspension point.  The Lehman is a simple instrument that
produces great results (especially with a ball bearing pivot point) but the
owner must devote a couple of minutes a day to keep the pendulum centered.
I do not try to upload all the quakes I detect to PSN but there are enough
posted for anyone to check how my simple sensors resting on a concrete
garage floor with much nearby traffic produces results.

Pictures and descriptions of my Lehman detectors are included in my web page
at http://pages.prodigy.net/fxc/

May you keep your pendulum centered,
Frank Cooper, Friendswood, Texas, USA



----- Original Message -----
From: Charles R. Patton 
To: 
Sent: Tuesday, November 19, 2002 11:00 AM
Subject: stability of a Lehman


> There have been numerous discussions about lengthening the period of a
> Lehman beyond the approximately 10 sec period and it becomes
> "unstable."
>
> The question is - unstable in what way?  The period changes, the
> centering fails, sensitivity goes haywire?  In particular, if the answer
> is, "The centering fails," does the center just move around, or does the
> beam "flop" to one side or the other?
>
> I'm doing some thought experiments during my long commute, and I'm
> thinking about the sources of error.  In particular it's been mentioned
> before that the center of rotation changes on the flexible hinge
> designs.  That's intuitive, but how much and in what direction - toward
> stability or instability? Has anybody figured out the actual path of the
> center of rotation for a typical Lehman?  Crossed-X flex hinges were
> mentioned, but in this application with side force, I don't think they
> would be any less susceptible to the same de-centering.
>
> Another question is the upright's rigidity.  Several pounds of weight at
> the end of a boom is a fair torque moment which is resisted by the
> spring constant of the 'pipe' (in the pipe constructed versions)
> vertical.  How much does this constant vary with temperature in standard
> steel?
>
> Anyway, if there is an answer I should have looked up in the past posts,
> just flame me, but please include an approximate date and subject header
> I can go looking for!
>
> Thanks,
> Charles R. Patton
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