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Old 17th April 2004, 07:43 AM   #11
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Location: Netherlands
Quote:
Originally posted by sreten

...........................

Pjotr :

You say magnetic bearing can carry a lot of weight - interesting.

If this is true then a very small system is required.

I need to give this some further thought.

P.S. all :

I'm just as interested in why it can't work as if it can.

sreten.
Hi Sreten,

The force a magnetic field applies onto two poles with an air gap in between can be expressed as:

F = B^2 x A / (2 x _0)

Where F in Newton, A in m2 and B in T (or Wb / m2). Taking into account a max saturation flux density of the necessary iron of 1.5 T, the max force you can get is then app. 100N/cm2. This is equivalent to a pneumatic force of 10 bar.

Cheers
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Old 17th April 2004, 10:56 AM   #12
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by ingvar ahlberg
Hi Sreten
What about trying Your idea but with fieldcoil instead, You would get an adjustable system...
If coil is in guide rod there would be several adjust/control options?
Hi IA,

Can't see anything wrong with the approach, especially for
an experimental set up, I'm now thinking along the line of
something that could use a coil or permanent magnets.

sreten.
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Old 17th April 2004, 11:03 AM   #13
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Pjotr


Hi Sreten,

The force a magnetic field applies onto two poles with an air gap in between can be expressed as:

F = B^2 x A / (2 x _0)

Where F in Newton, A in m2 and B in T (or Wb / m2). Taking into account a max saturation flux density of the necessary iron of 1.5 T, the max force you can get is then app. 100N/cm2. This is equivalent to a pneumatic force of 10 bar.

Cheers
Thanks Pjotr,

I can see this sort of force in attraction.
But I don't think we would be dealing with saturated poles.

One of the problems seems to be the "springyness" of the
repulsion, a geometry is needed that rapidly varies the air
gap as you approach the surface to "stiffen" the bearing.

I've got an idea that I'll draw up and post later.

sreten.
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Old 17th April 2004, 01:36 PM   #14
sreten is offline sreten  United Kingdom
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Hi,

I've pondering what could work well as magnetic bearing but a
given is that the "stiffness" of an air bearing is not possible.

Also thought it would be nice to include a magnetic vertical
bearing in the basic idea as well, though knife edges could
still be used if a v block is used top and bottom.

The basic principle is shown in the diagram, I'm not suggesting
this is the best way to do it, just trying to show the idea.

The slide consists of two hollow steel tubes energised
by two small high power magnets placed each end.

(For weight considerations these could be steel tube sections
at each end mounted on aluminium supporting hollow tubes)

The guide is fairly self evident and a DC field coil
could easily replace the ceramic magnet(s) shown.

Now as I see it the top is inherently unstable and the slide
would simply fly out sideways, to stabilise this the arm is
underhung from the slide, the length of the underhang being
enough to provide the classic slight increase of tracking of
force as the arm lifts from horizontal, for improved tracking.

Seems to me it would only work if the bottom slide piece sits
at equilibrium very close to the V-groove wall, I'm thinking
the \o/ interface would cause this to happen as long as the
pole piece field is not too strong.

Do you think the \o/ interface would be quite stiff ?

The upper |o interface I envisage having quite a gap.
(Some mileage in adding a concave section
to the pole piece , i.e. an "(o" interface ?)

Obviously the whole thing would need to be made overlength
so the ends do not affect field strength, theoretically
bucking magnets at each would help even the magnetic field.

Stiffness in all planes would be pretty good, my only concern
would be the vertical resonant frequency, not sure about this.

Don't bash me too hard if there is something fundamentally
wrong with this, I'd like to know what that problem is.

Just seems to me getting rid of the airpump is a great convenience,
at the cost I admit of rigidity, but I assume there would be be
some noise advantages over an air bearing arm - there must be ?

sreten.
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Old 17th April 2004, 01:57 PM   #15
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Don't think the stray DC fields are a problem on the leads. They might upset the cartridge, but that can be far-far away.

As for damping, making the carrier out of copper should give nice magnetic damping without adding much weight.

While supplying the air to the carrier in a conventional Ladegaard may have advantages, the stiffness of the feeding hose will be a problem. I was amazed when I found out how low friction this arm bearing was. Even my phono leads (two 0.1mm Cu twisted pairs) have influence.
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Old 17th April 2004, 02:06 PM   #16
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Havoc
As for damping, making the carrier out of copper should give nice magnetic damping without adding much weight.
Isn't using aluminium just as good ?

Though I'm having a hard time seeing the induced
current flow that would provide some damping.

I can see that an aluminium oval placed as shown would provide
lateral damping and could be incorporated into the construction.

Nice point though ! I wasn't considering it.

sreten.
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Old 17th April 2004, 02:42 PM   #17
sreten is offline sreten  United Kingdom
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Just realised the aluminium oval will not provide damping in
that position, the induced currents would cancel each other.

sreten.
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Old 17th April 2004, 07:16 PM   #18
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In order to provide damping, the fieldlines have to cross the material at right angles. A eddy current or foucault brake for a rotating axis works by putting a copper disk on the axis, and then have a C-type coil former with the disk running through the gap. Don't know if I'm clear, but the fieldlines would go through the thin disk section, the C-section going over the rim like a disc brake caliper.
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Old 17th April 2004, 07:25 PM   #19
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Havoc
Don't know if I'm clear, but the fieldlines would go through the thin disk section, the C-section going over the rim like a disc brake caliper.
Seems to me eddy current effects would be
minimal, but I stand to be corrected on this.

I was initially thinking of induced shorted turn
damping, but not possible as far as I can see.

sreten.
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Old 17th April 2004, 10:54 PM   #20
dice45 is offline dice45  Germany
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sreten,

i would not be happy concerning the hanging architecture, i prefer to have my vertical pivot axis as close to the records surface as possible; warps and pressing bubbles have the fewest effect on effective stylus speed then.
Nevertheless i think you should try out if your idea works Ready, steady, go! As you have drawn it i don't see any danger of cogging, just mighty stray fields ... If you think this doesn't affect sonics.

Maybe it's me only who is incompatible to the sonic effects caused by a small magnet or a piece of iron located in close proximity to a signal-conducting wire (or worse: around that wire, add to that list: faintest trace of magnetic attractability in any passive or active component except Xformers). If you have read my last post in the Nagaoka MP-50 thread, you may have wondered why i waste my energy in designing RCA connectors and such. That's why.

BTW, i also use a magnetic bearing inside my tonearm's lateral force compensation mechanism but the magnetic circuit is tinytiny and closed (not much stray field) and far apart from the phono leads.
__________________
Greets,
Bernhard
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