PSN-L Email List Message
Subject: Re: optimal position of magnetic damper & 2 X 1 inch NdFeB magnets
From: "Robert O. Green" rog@..........
Date: Sun, 31 May 2009 18:51:41 -0700
Thank you as always the advice and pictures are wonderfully helpful.
I am experimenting with a pair of 2 X 1 X 0.25 inch NdFeB magnets with
spacers at each end and a 1.5 inch Al damping plate.
> In a message dated 31/05/2009, rog@.......... writes:
> What is the thinking on the optimal position of the magnetic
> damper (MD)
> along the lower boom?
> It seem by shifting the MD position towards or away from the lower
> one can tune the damping strength.
> Hi Rob,
> You need to have as low a vertical force on the bottom suspension
> as practicable. I suggest that you put the mass on the end of the arm
> and then slide the arm over a round horizontal rod until the weight of
> the mass on one side balances the weight of the arm on the other. Mark
> this point to attach the top suspension. Check the photos at
> You need the damping fairly close to the mass, but this is not
> critical. But the damping plate DOES need to be lie on the OFFSET line
> joining the centre of mass to the bottom suspension, hence the offset
> mounting hole in the mass. This prevents a quake motion from trying to
> rotate the mass as well as pushing it to one side. Please note that
> the next modification was to fit the sensor block with a V wire top
> suspension. See
> http://jclahr.com/science/psn/chapman/2008%20lehman/index.html This
> 30 lb fishing trace with looped ends worked very well indeed,
> preventing any rotation of the mass around the long axis of the arm.
> Have a look at the damper drawings at
> http://jclahr.com/science/psn/chapman/lehman/index.html These use a
> quad NdFeB magnet block. You only have to slide the block about 1/2"
> to 3/4" over the 1" magnets to get the correct damping. You adjust the
> separation of the top and bottom mild steel backing plates to set the
> damping in this range. But you also need to be able to remove the
> damping easily when setting the period and to set the damping
> afterwards to 0.7 critical, hence this layout where the damping block
> simply slides along the baseplate.
> This is a good, compact and easily adjusted design. I can set a
> period of 30 seconds, no problem. The trimming of the height of the
> arm, setting up the period and then setting the damping are all
> sequential and non interacting adjustments.
> Chris Chapman
> Internal Virus Database is out of date.
> Checked by AVG - http://www.avg.com
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