Magnetic pull kevlar suspension tonearm

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Paul Barker
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#1 Magnetic pull kevlar suspension tonearm

Post by Paul Barker »

Well a very nice chap one Eggbough gave me an apple wand he had turned me on his laithe.

Layed it on my deck to let you see the scale of it. Looks rather large. No complaints about it handling a 103! or an SPU (which I forgot to mension I originally intended it for.

Image

Anyway in this day and age I am not a man with the ability to search the end;ess threads on building this type of arm. So I did a very low level search and found out what I already decided about the hwole concept first time those many years when I studied it with a fine tooth comb amd discussed by telephone with my greta ideas ouncing mate Darren.

In a nutshell Will's method will suffice. two outriggers fine kevlar twisted for antiscate. end of.
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#2 Re: Magnetic pull kevlar suspension tonearm

Post by Paul Barker »

I'll keep this alive. Its quite hard to pull together the smatterings of prior art on this technique as its had its hayday of diy experimenters amd gone quiet. loads of dead links about.

Will spoke of keflar thread which was 15lb. Most kevlar is way over that. But did will miprint that, did he mean 15kg? who knows!

So, the strand of kevlar is significant, too strong and you probably might aswell go unipivot, to weak and you trash a cartridge when it breaks. (unless you manage to design a want catcher thta stays out of way but averts disaster.

So anyway I have prdered the thpe of kevlar thread used by magicians. I could strand it for more strength if required. I'm going to have to do my own breaking strain tests on it. I suppose get variety of magnets and test kevlar to magnet before arriving at solution with sufficient margin of safety yet sufficient magnetism and sufficiently low friction of thread.

Anyway nothings arrived yet.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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Nick
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#3 Re: Magnetic pull kevlar suspension tonearm

Post by Nick »

Having spent hours trying to set up a commercial arm using a thread and magnet, I don;t understand how this sort of bearing can work in the face of warped records. The magnet will act to cause the tracking force to vary as the cartridge moves up and down.

Unless (as is likely) I have missed something.
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#4 Re: Magnetic pull kevlar suspension tonearm

Post by IslandPink »

On Schroeder's arm there's a hole drilled through the neodymium magnet and the thread it hangs from goes through & terminates at the bottom of this magnet, just above the lower magnet that is providing the force. Hence there's very little vertical offset between the effective pivot point and the magnetic gap. He talks at some length about this and how difficult it is to drill through a neodymium ( or samarium cobalt ? ) magnet , in the article that was published years ago about the Schroeder arms. May not be out there now to read.
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shane
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#5 Re: Magnetic pull kevlar suspension tonearm

Post by shane »

I got round that on mine by putting a short bar across th arm level with the bottom of the magnet to form an outrigger on each side. By putting an inverted y shape in the thread down to each end of the outrigger, the bottom of the thread was level with the gap between the magnets so there's was no force interfering with the vertical pivot. It also meant that the azimuth was fixed.
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#6 Re: Magnetic pull kevlar suspension tonearm

Post by Nick »

Both make sense. Wish clearaudio had thought of either.
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#7 Re: Magnetic pull kevlar suspension tonearm

Post by cressy »

As far as I can see, the magnets are not the bearing, its the thread that is the 'bearing'. All the magnets do is hold the thread taught in a static vertical position as the arm tracks laterally, so there is no mechanical contact point. it is the ability to twistleft or right of the thread that allows this movement. Its also why bias can be applied by twisting the string. Wills arm had a yoke made of kevlar thread which meant that the vertical movement was achieved by the cross pin turning in the loops at the ends of the yoke.

Edit: missed shanes post while I was writing this, the outrigger mentioned it what i mean by cross pin
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#8 Re: Magnetic pull kevlar suspension tonearm

Post by cressy »

Also, re the tracking weight changing as the magnet tilts in response to vertical movement, as it is so small maybe 6mm across, there isnt a great enough distance from the centre point to the edge of the magnet for it to have much effect on the tracking weight. It will have some effect of course, but the longer the arm, the less angle will be put on it for a given vertical movement, meaning less effect it will have
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#9 Re: Magnetic pull kevlar suspension tonearm

Post by IslandPink »

Been trying to find the article but it's not easy to find online at the moment. It was in HiFi+ magazine about 15 years ago, though - and I might still have that magazine, so will check & scan it if I find it. I think one important thing was that to get low bass and good timing you had to get the magnets pretty close together. He talks about the 'Y'-shaped thread or 'yoke' method in the article and some of the early arms were done this way, but I think he found the drilled magnet used on the 'Referenz' was the best way to go.
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#10 Re: Magnetic pull kevlar suspension tonearm

Post by cressy »

Thinking about the hole in the magnet, the pivot point will be at the top of the magnet where the thread comes out. This is where the thread will bend, allowing vertical movement.
I cant see how this makes a difference, other than lowering the pivot point, as ideally the point needs to be centred on the magnet gap. It will still be offset vertically by the thickness of the magnet
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#11 Re: Magnetic pull kevlar suspension tonearm

Post by Nick »

but the longer the arm, the less angle will be put on it for a given vertical movement, meaning less effect it will have
I would have to do the maths but would a longer arm means the force at the end would be greater because of the longer moment arm. It's possible one would cancel the other.
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#12 Re: Magnetic pull kevlar suspension tonearm

Post by IslandPink »

Response to Ant ( Nick jumped in there ) :
Not if the hole's a little big bigger than the thread. Depends very much on small details.
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#13 Re: Magnetic pull kevlar suspension tonearm

Post by cressy »

What I meant was that for the cart end to move e.g 1cm upwards, the longer the arm is, the shallower the upward angle will be.
The bearing will have to traverse move less for the same vertical movement
Eg
Image

If the hole in the magnet is cone shaped and allows for the angle of movement then fair enough, i dont know if it is. If its straight, the thread will bend and move the pivot point
Obviously 1cm is a large movement in respect to how much there would be in practice, but illustrates the point.
The further away the edge of the fixed magnet in the base that the movable magnet is, the less pull it will exert to try and bring itsself back into line with the fixed magnet? Unless the proximity of the 2 is so close that it makes no difference. But there again, the gap needs to take into account the angle that the moving magnet could be moved to as it rides the a warp. Too close and the back edge touches the fixed magnet and locks the arm
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#14 Re: Magnetic pull kevlar suspension tonearm

Post by cressy »

Mind you, if the moving magnet is spherical, the above isnt a problem
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#15 Re: Magnetic pull kevlar suspension tonearm

Post by Nick »

I can see that if the pivot point was in-line with the lower face of the magnet then you would not get a for trying to keep the arm level. But magnetic fields attract with the square of the distance, so as the two surfaces of the magnet angles the increase in force between the two faces getting closer would be more than the decrease in force from the two faces on the side getting further away. It mat be that having the pivot slightly higher than the lower face would allow the two effects to cancel a little.

And in regards to your diagram ant, instead of thinking of angles subtended, think in terms of bending moment at the pivot (torque) and how the longer arm multiplies that to create a larger force at the end of the longer arm. As I say, I am not sure which would be more, I would have to do the maths.
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