Thoughts on this unipivot bearing design? - diyAudio
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Old 4th January 2011, 11:12 PM   #1
martyh is offline martyh  United States
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Default Thoughts on this unipivot bearing design?

Howdy, I’m wondering if anyone can offer comments on this design for a unipivot bearing with eddy current damping. The idea is to use the ring of magnets to damp resonances in the arm through eddy currents induced in the copper plate. I’m thinking I would need to experiment a bit with shaping the fields by introducing an air gap around the magnets. If I understand the physics correctly, forces that tend to rotate the arm in a horizontal plane would more strongly resisted than those in the vertical plane and all of the damping would be proportional to velocity of the arm.
Thanks in advance,
Marty

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Old 4th January 2011, 11:43 PM   #2
kevinkr is offline kevinkr  United States
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Interesting idea, no idea how well it is likely to work. Damping is often needed most in the vertical plane - in my limited experience. (warps) Of the two uni-pivots I've owned the better one was fluid damped and almost entirely in the vertical plane.
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Old 4th January 2011, 11:56 PM   #3
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Like Kevin said, I think you have the damping in the wrong plane. I also think that the design will not provide very effective damping. The usual arrangement is with magnetic pole pieces connected by a magnetic circuit on either side of a thinner conductive plate. The idea is to maximize the flux through the conductive plate. With the magnets open-circuited, you won't get much flux and therefore won't get much damping.
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Old 5th January 2011, 12:17 AM   #4
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Ditto on the wrong plane, and ditto on the damping.

I think the problem with magnetic isolation is when there is opposing magnets, there really is not much damping at all ... once you get the right magnetic opposition to compensate for the mass of the arm, it's almost a direct connection.

There are some resonances that will be damped out, but are they the right ones? I think at some frequencies it's almost like there is a direct connection, not dampening, and I also think it's at the cartridge resonance frequency where this is most likely (broadly speaking ... at the low end, anyway).

I believe that is why other unipivot designs used the paddle-in-silicon fluid damping; because it was needed at those frequencies (and these arms did use a direct connection, although it was minimal and transmitted little).

I don't mean to discourage you ... it's an interesting design concept and my suspicions are just that; feel free to prove me wrong. Even better, feel free to confirm it and then find a novel solution.
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Old 5th January 2011, 12:46 AM   #5
martyh is offline martyh  United States
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Default Thanks for the quick replies…

I guess I was confused on which plane was more important to damp. My thought was that if the arm was too heavily damped in the vertical plane it would tend to want to “dig in” at a warp. As I have it drawn there would still be some damping in the vertical plane as the magnets would rotate about the bearing point in the vertical plane so there would be movement in the horizontal plane. I think it is possible to flip the direction of maximum damping to the vertical plane by adding a matching pole in the mounting plate (I.E. north faces north with the conductor plate in the center) One problem I see with that is that it might tend to float the bearing off of the ball and increase chatter.

As an aside… if a stylus is tracking a warp at what point would it reach maximum velocity?
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Old 5th January 2011, 01:25 AM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by martyh View Post
I guess I was confused on which plane was more important to damp. My thought was that if the arm was too heavily damped in the vertical plane it would tend to want to “dig in” at a warp. As I have it drawn there would still be some damping in the vertical plane as the magnets would rotate about the bearing point in the vertical plane so there would be movement in the horizontal plane. I think it is possible to flip the direction of maximum damping to the vertical plane by adding a matching pole in the mounting plate (I.E. north faces north with the conductor plate in the center) One problem I see with that is that it might tend to float the bearing off of the ball and increase chatter.

As an aside… if a stylus is tracking a warp at what point would it reach maximum velocity?
Damping in the vertical plane helps damp out oscillations in the arm/cartridge/suspension excited by the very low frequency energy typical of a warp.. Note that the damping also needs to be easily adjustable as the amount required may vary depending on cartridge compliance and mass, headshell mass, vertical bearing friction, and position of the counter weight relative to the fulcrum.

Based on admittedly limited experience it seems to me that the damping should be symmetrical regardless of the direction the arm is moving in the vertical plane. (Most are)

Warps vary as to amplitude and duration, and I am not sure they can be counted on to be symmetrical either. The old NARTB standard for warps was something like 1mm pk-pk relative to flat, (recollection may be faulty, but not much more than this IIRC) but modern records can be much worse IMLE, and you should expect to successfully track something like 3X this as a minimum. (I have three albums out of hundreds that could destroy my cartridge if given a chance due to excessive warp or eccentricity as they all excite the arm/cartridge resonant frequency which is definitely a bit too low, but generally not problematic.)

Maximum velocity would be reached at the point of maximum slope, and that somewhere between the top and bottom of the warp entering and exciting that warp - obviously the minima is at the crest of the warp when talking about vertical plane. (And this is where arm inertia takes over and can create some interesting mischief) I suspect there must have been a fair amount of discussion of this issue in old AES/JAES papers during the first vinyl era..
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Old 5th January 2011, 02:19 AM   #7
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I'm not convinced that the damping requirements are the same in the horizontal plane as they are in the vertical plane. Unwanted excitation in the horizontal plane will be dominated by eccentricity error, and will there fore occur at approximately 0.5 Hz, which should cause no difficulties for an arm/cartridge with a resonant frequency of 8-10 Hz. In the vertical plane the main source of excitations will be record warps and other surface defects, which will be all over the place. That is where I think you'll see the most potential benefit from damping.
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