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-   -   magnet damped unipivot (http://www.diyaudio.com/forums/analogue-source/218474-magnet-damped-unipivot.html)

dtut 24th August 2012 09:57 PM

magnet damped unipivot
 
4 Attachment(s)
I stumbled across a picture of a Pickering/Stanton Unipoise arm, which I think is very attractive, and that got me thinking about how to build one and unipivots in general. It seemed to me the most obvious disadvantage is rotational instability along the longitudinal axis and I'm not convinced outrigger weights really help. So why not a magnet placed above the pivot with an attractor above that for stability? Well, it would probably lift the arm off the pivot if the pivot was a typical point and cup. OK, how about a ball and socket, instead? When I went to my local small parts source, one of the guys there said "would a Heim joint work?" Heim joints are ball joints usually found on the ends of various kinds of control rods. The one I got is well made and relatively cheap.

The arm in the photos is the result. I've been listening to it for the last couple of night and it works pretty well. The magnetic damping is effective so a normal soft lift at the finger lift doesn't produce wiggle, VTF is easy and accurate, and it tracks well.

It obviously owes more than a bit to Frank Schroeder and quite a lot to Nanook.

Joachim Gerhard 24th August 2012 10:09 PM

Pretty cool and very simple ! I like it.

Joachim Gerhard 24th August 2012 10:13 PM

This joint was even designed by the Germans, unfortunately for WW2. Interesting story nevertheless.
Rod end bearing - Wikipedia, the free encyclopedia

sreten 24th August 2012 10:50 PM

Hi,

Is the friction of a heim joint really low enough to work well ?

(Is a heim joint a unipivot, no its not the same).

Interesting arrangement though I'd say you've got it the
wrong way up, magnetic stabilisation by attraction under
the arm has to work better than above in my book.

If you think about it, I'm sure you will agree.

rgds, sreten.

awkwardbydesign 24th August 2012 11:07 PM

Well. I've used rose joints on the bike, gear lever connector and steering damper, but would never have thought of this. So, is the magnet the only thing stopping the arm from flopping sideways? Scary! And I also wouldn't have thought it was low friction.

dtut 25th August 2012 04:47 AM

2 Attachment(s)
These photos are of the first effort. Aside from appearance, the major difference is that the wand goes through the pivot rather than above it because the wand fits the Heim joint ID. The wand in the second version is mounted above the Heim joint because the joint is smaller and I couldn't figure out another way to do it. In that respect, the first version is inherently more stabile the the second. There'll be a third version that incorporates elements of both previous versions including mounting the wand at the bearing height.

Joachim - Thanks. Simplicity is what I wanted. And thanks for the history link. I didn't recognize the name, but I did recognize the part as soon as I saw it.

Awkwardbydesign - Yes, the magnet is the sole source of anti-rotational stability, but it does work as a vertical damper to some extent, too. The gap between the magnet and the attracting cap screw has to be pretty carefully gauged so the anti-rotation is effective, the vertical isn't over-damped, and the upward pull doesn't increase friction in the bearing. When the adjustments are correct, the bearing turns very freely and the arm's motion is excellent both vertically and horizontally.

sreten - I had my doubts about this project from the beginning, but decided it was worth the try. I'm glad I went ahead with it because the arm does work well. It started as a unipivot exercise and that name just stuck. Formally, I'd concede it isn't a unipivot so I'd be glad to call it a unibearing. As I said to ABD, the Heim bearing is very low friction if it is not subjected to problematic exterior forces, which is the reason the magnet is overhead, rather than under hung. Properly adjusted, the overhead magnet counters gravity instead of adding to it as it would from underneath.

Thanks to all of you for your interest and comments.

awkwardbydesign 25th August 2012 08:27 AM

I love the high tech materials. :D Tempted to try it myself; wonder if my Decca would like it? :eek:

Mark Kelly 25th August 2012 10:24 AM

The functional problem with this design is that the distance between the pivot and the magnet system which provides stability around the long axis also provides the same amount of stability around the axis orthogonal to this in the horizontal plane.

The net result is that you have compromised the vertical response of the arm - in order for it to tilt in response to, say, a warp it must fight the magnet system.

Frank's system works quite differently and does not suffer this problem.

dtut 25th August 2012 07:02 PM

3 Attachment(s)
Mark,

Thanks for for response. At your prodding, I came up with a way to measure the warp-caused VTF change. The first photo is the equivalent of one record, the second of two, and the third of three. A three-record warp is a fairly significant bump. I understand that this is a static test and may or may not be relevant to the dynamic conditions of a moving record - especially vertical acceleration - but I do think it is indicative. In the first run through, the measurements were equal, but naturally as soon as I got the camera out, that 1.8 showed up.

When the magnet gap is adjusted to the point that VTF change is minimal and the rotational stability stays good, for instance no wiggle due to finger lift, the arm is good to go. This could be set up for simultaneous measurement. After I made the adjustments, I could tell the forces on the bearing had been reduced and the arm was moving more freely.

I think the crucial factor in the different amplitudes of the forces acting rotationally vs vertically is the different distances involved. The rotational coupling is quite close - about 25mm - and the vertical forces are applied at a distance approximately ten times farther. My math ability doesn't come anywhere near yours, so I may be way off track here, formally, but I believe the basic explanation is correct.

My reference to the Schroeder arm was mainly because of the admittedly tenuous magnet connection and because that arm was the one that got me started building. IIRC, the Schroeder arm is vertically damped and that is considered a feature, not a glitch.

berlinta 25th August 2012 09:39 PM

Hi Doug,
Helm joints are not a very good choice for a tonearm bearing. Friction could be held at an acceptable(but not super low) level, but the need for lubrification and the open/large surface of such bearings attracts dust and in this application a frequent cleaning would be neccessary( read: it's a pita).

A magnetically stabilized unipivot can be built without the addition of an outrigger(requiring its own bearing) la Graham Phantom. Just allow a magnet exactly underneath the unipivot to act against a ferromagnetic circle segment with the unipivot forming the center of said circle (segment). The orientation of the circle segment (ideally) ought to be at the same angle as the headshell offset. Damping is more pronounced for vertical movement, but exists for horizontal movement too. Built it 25 years ago, worked like a charm, but I prefer some damping in the horizontal plane and little to none in the vertical plane instead.
No need for a "low slung" counterweight, so(if the mass distribution is otherwise "neutral") no changes in VTF when changing VTA...

Cheers,

Frank


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