PSN-L Email List Message
Subject: Re: Best Lehman pivot
From: "Robert O. Green" rog@..........
Date: Thu, 09 Apr 2009 20:23:48 -0700
Thank you kindly for your advice. Do you have pictures of your set up
pivot and top suspension?
> 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?
> 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.
> 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.
> 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.
> Chris Chapman
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