PSN-L Email List Message

Subject: RE: How Pivotal is the pivotal relationship?
From: "Paul Cianciolo" Paulc@........
Date: Fri, 15 Jun 2007 13:22:13 -0400

Hi Mark,

Thank you for that information.
So one should strive for a level pendulum, and a near vertical as feasible
support points?


-----Original Message-----
From: psn-l-request@..............
[mailto:psn-l-request@................. Behalf Of Mark Robinson
Sent: Friday, June 15, 2007 12:57 PM
To: psn-l@..............
Subject: Re: How Pivotal is the pivotal relationship?

Paul Cianciolo wrote:
> Hello Folks,
> Now that I have my instrument somewhat operational, there are another
> few "improvement"
> Here is the deal.
> When constructing my first experimental Lehman units I used a piece of
> steel channel iron mounted in a vice at the vertical member.
> The idea being it would for an experimental platform for different
> pendulum lengths, weights, suspension wires, bearing surfaces, material
>  etc. etc.
> The first bearing surface was the head of  # 5 bolt polished, and
> mounted into a holed drilled into the vertical member. The arm had a
> similar bolt mounted in the end of an aluminum tube. This bolt had a
> hole in the center of the head and a hard stainless ball glued into it.
> The suspension was a .020 piano wire and the mass was a 5 Lbs roll of
> copper wire slide over the end of the aluminum pipe.
> The top of the suspension wire was simply drooped over the top of the
> vertical member and  "C" clamped to it in order to made the pendulum as
> close as possible to horizontal.
> By moving the pivots laterally I could see the effect, and cause the
> mass to stop where ever I wanted within reason.
> OK  comes the problem.  After putting guitar tensioner on the top of the
> vertical member, so I could adjust the pendulum level, I could no longer
> cause the mass to stop anywhere.
> It seemed that no combination of level the pendulum, changing the mass
> position moving the bottom pivot would allow the pendulum to stop.
> After a couple hours of messing around, I reverted back to the original
> setup and Bingo... perfect balance.
> So... blah. blah, blah,
> Finally I realized that the adjustment apparatus had moved the top wire
> suspension point back slightly so that it was a bit behind the vertical
> member.
> No longer directly vertical of the bottom pivot.
> I find that I need to have the top pivot just slightly forward of the
> bottom pivot to get any pendulum I build to balance.
> Is there some rule of thumb...  It seems the closer to 90 degrees the
> angle between pendulum and vertical becomes the easier the pendulum will
> shift off balance.
> Currently my bottom pivot is crossed hard cylinders, and I want to do
> the top this way, but I would like to know more about the "Pivotal
> Relationship" before I construct the bearing.
> Phew!!!.. I hope this makes sense to someone.
> Thank you
> PauLC

Hi Paul,

You discoveries are spot on.

As the line through the pivots approaches vertical the period gets longer
the system becomes less stable. The first is a Good Thing, the second isn't.

Most people use three fine pitched bolts through the base to make this
adjustment. One at the centre of one end and the others either side at the


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