## PSN-L Email List Message

Subject: Re: optimal position of magnetic dampter
From: JAMES RIDOUT jamesridout@..............
Date: Sun, 31 May 2009 17:38:33 +0000 (GMT)

```I have the damping plate underneath of the boom on my Lehman seup. is this =
wrong ? As mentioned, quake movement would try to rotate the boom & weight.=
would i see any difference if i mounted the plate and magnet level with th=
e boom?

--- On Sun, 31/5/09, ChrisAtUpw@.......  wrote:

From: ChrisAtUpw@.......
Subject: Re: optimal position of magnetic dampter
To: psn-l@..............
Date: Sunday, 31 May, 2009, 5:45 PM

In a message dated 31/05/2009, rog@.......... writes:
What is the thinking on the optimal position of the magnetic damper (MD)=20
along the lower boom?
It seem by shifting the MD position towards or away from the lower pivot=20
one can tune the damping strength.

Hi Rob,
=A0
=A0=A0=A0=A0You need to have as low a vertical force on the bottom suspensi=
on as practicable. I suggest that you put the mass on the end of the arm an=
d then slide the arm over a round horizontal rod until the weight of the ma=
ss on one side balances the weight of the arm on the other. Mark this point=
to attach the top suspension. Check the photos at http://jclahr.com/scienc=
e/psn/chapman/school/MKII/index.html=A0
=A0=A0=A0=A0You need the damping fairly close to the mass, but this is not =
critical. But the damping plate=A0DOES need to be lie on the OFFSET line jo=
ining the centre of mass to the bottom suspension, hence the offset mountin=
g hole in the mass. This prevents=A0a quake motion from trying to rotate th=
e mass as well as pushing it to=A0one side. Please note that the next modif=
ication was to fit the sensor block with a V wire top suspension. See =A0ht=
tp://jclahr.com/science/psn/chapman/2008%20lehman/index.html=A0This 30 lb f=
ishing trace with looped ends worked very well indeed, preventing any rotat=
ion of the mass around the long axis of the arm.=20
=A0=A0=A0=A0Have a look at the damper drawings at =A0http://jclahr.com/scie=
nce/psn/chapman/lehman/index.html=A0These use a quad NdFeB magnet block. Yo=
u only have to slide the block about 1/2" to 3/4" over the 1" magnets to ge=
t 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 t=
o be able to remove the damping easily when setting the period and to set t=
he damping afterwards to 0.7 critical, hence this layout where the damping =
block simply slides along the baseplate.=20
=A0=A0=A0=A0This is a good, compact and easily adjusted design. I can set=
=A0a period of 30 seconds, no problem.=A0The trimming of the height of the =
arm, setting up the period and then setting the damping are all sequential =
and non interacting adjustments.
=A0
=A0=A0=A0=A0Regards,
=A0
I have the damping plate underneath of t=
he boom on my Lehman seup. is this wrong ? As mentioned, quake movement wou=
ld try to rotate the boom & weight. would i see any difference if i mou=
nted the plate and magnet level with the boom?
--- On Sun, 31/5/09, ChrisAtUpw@....... <ChrisAtUpw@a=
ol.com> wrote:
From: ChrisAtUpw@....... <ChrisAtUpw@.......&g=
t;Subject: Re: optimal position of magnetic dampterTo: psn-l@webtro=
nics.comDate: Sunday, 31 May, 2009, 5:45 PM

In a message dated 31/05/2009, rog@.......... writes:
What is the thinking on the optimal position of the ma=
gnetic damper (MD) along the lower boom?It seem by shifting the MD =
position towards or away from the lower pivot one can tune the damping =
strength.

Hi Rob,

You need to have as low a vertical force on th=
e 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 http://jclahr.com/science/psn/chapman/school/MKII/ind=
ex.html
You need the damping fairly close to the mass,=
but this is not critical. But the damping plate DOES need to be lie o=
n the OFFSET line joining the centre of mass to the bottom suspension, henc=
e the offset mounting hole in the mass. This prevents a quake motion f=
rom trying to rotate the mass as well as pushing it to one side. Pleas=
e 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 t=
race with looped ends worked very well indeed, preventing any rotation of t=
he mass around the long axis of the arm.
Have a look at the damper drawings at  http://jclahr.com/science/psn/chapman/lehman/index.ht=
ml 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. Y=
ou 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 alon=
g the baseplate.
This is a good, compact and easily adjusted de=
sign. 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 dampin=
g are all sequential and non interacting adjustments.

Regards,

Chris Chapman<=

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