PSN-L Email List Message

Subject: Re: Lehman
From: "David Saum" DSaum@............
Date: Wed, 13 Jun 2001 19:20:30 -0400

> From:    "Bryan&Regina Goss" 
> Date:    Tue, 12 Jun 2001 13:28:56 -0500
> I have read that a ball bering was used in place of a knife edge

I read this also, so when I built my Lehman I used a ball bearing
but I had trouble getting a smooth metal surface that it could
rest against without making a dent.  So I used a vertical piece of
glass about the size of a microscope slide.  It works very well as
a low friction pivot.  It would seem that the bearing would gradually
work its way off the glass since there is nothing to center it, but
this has not happened in months of operation.  I suppose you
could superglue a big washer to the glass to form a keeper for
the bearing if you expect major motions near your unit.

For my next Lehman I plan to build a shorter boom (now ~30") so the
entire system can sit under an inexpensive 24" glass fish tank.
I am tired of looking at the ugly cardboard box I use as an air motion
shield now. Your plexiglass enclosure sure looks great, but expensive.

I would also like to get rid of my oil damping because it is going to spill
and cause a big mess one day.  Right now I am working on a way of
using a rare earth magnet assembly from a disk drive to do inexpensive
and compact magnetic damping.  These magnet assemblies are only a
couple of  $$.   Maybe if the magnets are used in the paired assembly
they do not have to be separated as far from the pickup coil as
a horseshoe magnet does with it is used for magnetic damping.

The Inexpensive Seismometer Project


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Larry Cochrane <cochrane@..............>