Whist pondering the design of this arm :
http://www.aiko.com/roscoe/airbearingarm.html
and dice45's airpumpless arm :
http://home.t-online.de/home/bernhard.kistner/LT-2_index.htm
It struck me that the airflow of the Ladegaard design
could be replaced by a magnetic repulsion bearing.
My estimates indicate that as long as modern magnetic
materials can support say > 500% of their own weight
this should be entirely feasible, at 500% the lateral
mass of the arm would only be increased by 20%.
A small price to pay for no airpump.
Or have I missed something ?
Anyone know what the actual percentage is for modern
magnets ? I would of thought its a lot more than 500%.
sreten.
http://www.aiko.com/roscoe/airbearingarm.html
and dice45's airpumpless arm :
http://home.t-online.de/home/bernhard.kistner/LT-2_index.htm
It struck me that the airflow of the Ladegaard design
could be replaced by a magnetic repulsion bearing.
My estimates indicate that as long as modern magnetic
materials can support say > 500% of their own weight
this should be entirely feasible, at 500% the lateral
mass of the arm would only be increased by 20%.
A small price to pay for no airpump.
Or have I missed something ?
Anyone know what the actual percentage is for modern
magnets ? I would of thought its a lot more than 500%.
sreten.Why not indeed. Only contra-indications I "feel" are that the magnetic circuit of a magentic Ladegaard version would be very "open". So it would generate a large stray field. And while a lot of people atround here dismiss magnetic bearings for platter because of possible cogging, I'm afraid that this is a far larger problem with an arm that is not actively driven. I mean a platter has a large mass and even larger inertia and is driven, so any cogging will be not much of a problem. But the Ladegaard is "driven" by the groove only.
I really need to put mine together. All parts are already finished (more or less) but I never get around finishing the job. Only problem I have is that making the holes 0.3mm drilled through the AL profile, it generates a lot of resistance, so I need a bloody large compressor instead of an aquarium pump....
I really need to put mine together. All parts are already finished (more or less) but I never get around finishing the job. Only problem I have is that making the holes 0.3mm drilled through the AL profile, it generates a lot of resistance, so I need a bloody large compressor instead of an aquarium pump....
analog_sa said:
you make something very similar to the Ladegaaard arm except
the guide and slide are magnetic material and add magnets.
The slide could be split into two small slides spaced apart,
for stabililty and minimum added mass.
The guide uses bog standard ceramic magnets.
The slide(s) use small high power modern magnets, these
need to be just powerful enough for levitation in the guide.
Should work ?
sreten.
Pjotr said:Hi Sreten,
It’s true that magnetic bearings can carry a lot of weight. But they also act as a (non-linear) spring and as such are, uhm well, very springy. Wonder how you gonna deal with this. This is also the case with air-bearings but that “bearing” is very thin and much stiffer.
Cheers
Yeah, I was wondering if it would simply refuse to stay in the guide.
But no fear, I have a solution for that - you add another
repulsion system above the arm to keep it in the guide.
This would be a very simple overhead flat linear system.
But I'm beginnning to see the problems :
If you make a stiff triangular repulsion bearing any misalignment
of the surfaces will impart a side force. And any uneveness of
the magnetic field would do the same, this is cogging ?
And there's the vertical resonant frequency to consider
. sreten.
Hello all,
not wanting to be a naysayer, but Nd-magnetic stray field? in the presence of 4 air-spaced unshielded phono leads with a 0dB-level of 200-600 µV? ... and cogging too has been mentioned ... that alone is reason enough not to do it.
While you are musing about Ladegaard-improvements, why not try to feed the air to the slider instead to the rail? Why not figur out a way to lead/guide the (ultra-thin & flexible) air hose in a way that the air hose pull helps the slider to move continuously centerwards?
One hint: with the air fed to the slider, you need only 4 nozzles (with pockets) at the ends of the slider's bearing surfaces and you have 100% nozzle coverage. Hence you can carry much more weight with the same pump as the air pressure does not collapse.
And another hint .... keep the nozzle pockets as small as even possible as they store pressurized air ... => could cause oscillations at higher frequencies.
not wanting to be a naysayer, but Nd-magnetic stray field? in the presence of 4 air-spaced unshielded phono leads with a 0dB-level of 200-600 µV?
... and cogging too has been mentioned ... that alone is reason enough not to do it.While you are musing about Ladegaard-improvements, why not try to feed the air to the slider instead to the rail? Why not figur out a way to lead/guide the (ultra-thin & flexible) air hose in a way that the air hose pull helps the slider to move continuously centerwards?
One hint: with the air fed to the slider, you need only 4 nozzles (with pockets) at the ends of the slider's bearing surfaces and you have 100% nozzle coverage. Hence you can carry much more weight with the same pump as the air pressure does not collapse.
And another hint .... keep the nozzle pockets as small as even possible as they store pressurized air ... => could cause oscillations at higher frequencies.
Was just a thought, wondering if magnetic is possible. I don't
pretend to know much about air bearing arms, other than what
appears to be common sense, and I'm sure their are people
who've given air bearing arms a great deal of thought.
DC magnetic fields are not a problem. Don't think cogging is either.
Just point out that the arm generally travels much further back
and forth at the eccentric frequency than it does at the very
slow average speed towards the centre. IMO helping the slow
average speed towards the centre is not a technical issue.
regarding the basic idea :
With the comments so far I can see as usual the problem is far
more complicated than it first appears. And modification of
design using a different system is far from a "good" solution.
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.
pretend to know much about air bearing arms, other than what
appears to be common sense, and I'm sure their are people
who've given air bearing arms a great deal of thought.
DC magnetic fields are not a problem. Don't think cogging is either.
Just point out that the arm generally travels much further back
and forth at the eccentric frequency than it does at the very
slow average speed towards the centre. IMO helping the slow
average speed towards the centre is not a technical issue.
regarding the basic idea :
With the comments so far I can see as usual the problem is far
more complicated than it first appears. And modification of
design using a different system is far from a "good" solution.
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.
sreten said:...........................
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.
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 µ_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
Pjotr said:
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 µ_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.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.
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.Attachments
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.
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.
Havoc said:
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.Attachments
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.
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.
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.
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