Subject: Re: New Seismo
****Dave, Just to clarify...........on my Lehman's, I use exactly that Chri=
s=20
ecommended in his drawing. And that works for approx. 2 lbs mass. This=20
onsist of 4 very strong magnets, again as Chris noted, but I sometime sub.=
=20
he magnets, going thicker, up to 1/4". The plate I use is Alum about=20
..5" wide 2" long and 3/16" thick. Thin Alum just doesn't have the body.
Hi Ted,
Are you SURE that you are using ~pure Aluminium and NOT a higher streng=
th=20
Aluminium Alloy ? These can have a markedly higher resistivity. I have used=
=20
1/16" sheet Aluminium quite OK for a 4 lb mass. But I switched to copper an=
d=20
even 1/32" sheet gave OK results. Copper has a resistivity of 1.54 as oppos=
ed=20
to Aluminium of 2.42 and it is a lot lower than some of the Aluminium alloy=
s,=20
which may exceptionally be up to 8.2. Values from 3 to over 5 are common.
.
I don't like Aluminium much for a very good reason: it is diamagnetic.
When the damping blade on my Lehman drifted to one side, as it is wont to d=
o
and the edge of the blade got near the edge of the damping magnets, it then=
=20
pushed the arm right out to the side stop ! I was using about 1 mm clearanc=
e=20
between the blade and the magnets. The effect is not so marked for wider=20
gaps. Remember that the longer the period that you use, the less the dampin=
g=20
force required.
.
****On my experiments with my 15# mass, I used Chris's arrangement, but 4=
=20
even stronger magnets, and even a bigger hunk of Alum.=20
have used copper and Alum. I use Alum, because it is easier for me to=20
et. =20
=20
There is another cheap and easy way to ~double the damping. Just add=20
another pair of magnets to give a NSN triplet opposing a SNS triplet !!=20
But you do need to use zinc plated mild steel bolts linking the backing=20
plates, to provide the magnetic return path for one pair of magnets. Add=20
yet another pair of magnets to ~triple the damping. You can also use=20
longer magnets. The 1/4" thick mild steel backing plates tested OK for=20
1"x1/2"x1/4" NdFeB magnets, but any greater magnet thickness or width is=20
likely to require a thicker backing plate. Check for field penetration=20
using attraction to another mild steel plate. =20
Single and opposing singles of circular magnets are not much use for=20
damping, since the induced currents have a much longer path through the=20
damping plate. Quads of circular magnets can give quite significant=20
damping, but the correct position of the damping plate is much more=20
difficult to adjust than for rectangular magnets.
.
Regards,=20
.
Chris Chapman
=
Subject: Re: New Seismo
****Dave, Just to clarify...........on my Lehman's, I use exactly =
that Chris=20
recommended in his drawing. And that works for approx. 2 lbs mass. This=20
consist of 4 very strong magnets, again as Chris noted, but I sometime sub.=
=20
the magnets, going thicker, up to 1/4". The plate I use is Alum about=20
1.5" wide 2" long and 3/16" thick. Thin Alum just doesn't have the body.
Hi Ted,<=
/FONT>
Are y=
ou SURE that you are using ~pure Aluminium and NOT a higher strength
Aluminium Alloy ? These can have a markedly higher resisti=
vity. I have used
1/16" sheet Aluminium quite OK for=
a 4 lb mass. But I switched to copper and
even 1/32=
" sheet gave OK results. Copper has a resistivity of 1.54 as opposed
to Aluminium of 2.42 and it is a lot lower than some of th=
e Aluminium alloys,
which may exceptionally be up to 8.2. Values from 3 to over 5 a=
re common.
.
I=
don't like Aluminium much for a very good reason: it is diamagnetic.
When the dampi=
ng blade on my Lehman drifted to one side, as it is wont to do<=
/PRE>and the edge of the b=
lade got near the edge of the damping magnets, it then
pushed the arm right out to =
the side stop ! I was using about 1 mm clearance
between the blade and the magnets. The effect i=
s not so marked for wider
<=
TT>gaps. Remember that the longer the period that you use, t=
he less the damping
force required..
****On my experiments with my 15=
# mass, I used Chris's arrangement, but 4
even stronger magnets, and even a bigger =
hunk of Alum.=20
I have used copper and Alum. I use Alum, because it is easier for me to=20
get.
<=
/FONT>
There =
is another cheap and easy way to ~double the damping. Just add =
another pair of magnets to give a NSN triplet opposing a SNS tri=
plet !!
=
But you do need to use zinc plated mild steel bolts linking the backing
plates, to =
provide the magnetic return path for one pai=
r of magnets. Add
yet another pair of magnets to ~tr=
iple the damping. You can also use
longer <=
/FONT>magnets. The 1/4" thick mild steel backing pl=
ates tested OK for
1"x1/2"x1/4" NdFeB magnets, but any greater magnet thickness or =
width is
likely to require a thicker backing plate. =
Check for field penetration
using attraction to another mild steel plate. <=
/TT>
Single and o=
pposing singles of circular magnets are not much use for
=
damping, since the =
induced currents have a much longer path through th=
e
dampin=
g plate. Quads of circular magnets can give quite significant
damping, but the correct position of the damping plate is much more
difficult to adjust than for rectangular magnets. <=
/PRE>.
Regards, .
Chris Chapman<=
/PRE>