PSN-L Email List Message

Subject: Re: Lehman damping
From: Robert McClure bobmcclure90@.........
Date: Wed, 6 May 2009 16:20:24 -0400

Hi J=F3n

  I advised you to rotate the damping vane 90 degrees. I said nothing
about the magnet.

  It is difficult to make anything clear to you, especially without
pictures, but I will try.

  I assume that you are using a four-pole magnet assembly, having two
steel plates and a pair of side-by-side Neodymium block magnets of
opposite polarity on each plate. The steel plates are held apart with
bolts so that an air gap exists between the pair of magnets on the top
plate and the pair on the bottom plates. The plates are oriented in
the direction that gives strong attraction accross the gap. The
"cracks" between the pairs of block magnets should always be
perpendicular to the direction of motion of the damping vane.

  The damping vane should be made of thick copper sheet (or a stack of
thinner sheets) and rectangular in shape. Its width should be
appreciably greater than the width spanned by the magnet field
generated in the magnet assembly. The vane should be mounted below the
boom, with its long dimension parallel to the boom. The amount of
damping achieved by the penetration of the vane into the magnet gap is
then controlled by the longitudinal position of the magnet. Less
damping is achieved by moving the magnet so that it covers less of the
vane. Maximum damping occurs when the vane penetrates the entire gap.

  When you set up the vane to enter the magnet assembly from the side
rather than from an end, you do not have the ability to adjust damping
because the damping is then a nonlinear function of depth of
penetration into the magnet. Never use such a configuration.

  You could always elect to mount the damping vane perpendicular to
the boom, with the long dimension vertical. In this case, the magnet
and its "cracks" are vertical, and the magnet assembly is moved
up-and-down to adjust damping.

  The reason for making the vane wider than the field is this: pure
copper is slightly diamagnetic, and impure copper can even be
paramagnetic. Aluminum can be used, but it is paramagnetic. A narrow
vane is thus subject to magnetic forces from fringe fields inside the
magnet. Making the vane wider than the fringe fields keeps the edges
of the vane outside the fringe fields and minimizes such forces.


On 5/5/09, J=F3n Fr=EDmann  wrote:
> Hi all
> I was told to move the damping plate and the magnets 90=B0 from where the=
> currently are, so the movement would be in the same direction as the arm
> moves.
> Can anyone explain to me why that is better ?
> Regards.
> --
> J=F3n Fr=EDmann
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