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Old 18th March 2017, 04:06 PM   #1131
directdriver is offline directdriver  United States
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Quote:
Originally Posted by 2wice View Post
I've completed the math for the tricycle curves. There's are no tangency errors on them.The direction of the tricycle arms can be translated in any direction and can form any triangle and it will still work, but this looks nice. Stylus drag pulls the tricycle along nicely as it's all downhill (they share vectors)
Excellent work! The tricycle idea was suggested years ago in post #19 and #21. It was a crude concept back then and I hadn't refined the idea until recently. Your placement of the 3 points are better and simpler. I just love your graphic skills!

I abandoned the idea due to my stubbornness about not using linear motion bearings and (after all, this is a thread about pivoting genre) thought a parallel tracker would do the same. But you're right about the shared vectors that the stylus drag force pulling the tricycle is much smaller than a parallel tracker. I think this has a lot going for it and easier to use ergonomically as it works just like any pivot arm. The main armwand is similar length to most 9" pivot arms and, for DIYers, it can be salvaged from broken arms and be repurposed to attach it to the tricycle, as long as you take out the 23 offset angle at the bearing. And yes, you can even use magnet to lessen the mass.

The key is to create the 3 curved tracks precisely and make the tricycle moving smoothly with low friction. I'll leave that to the more creative and resourceful members here.


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Originally Posted by 2wice View Post
might be a bit of a problem with records playing from the inside out as the motive force would have to come from the angular contact.
People who spend time playing inside out records probably don't care about tangency.

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Originally Posted by Hiten View Post
Can this linear slides be used for linear tracking tonearm?
Deltron Precision Linear Slides
These are precision slides with very low coefficient of friction.
They look amazing and I think they definitely have potentials for making linear tonearms.
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Old 19th March 2017, 05:04 AM   #1132
Straight Tracker is offline Straight Tracker  United States
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Default Stylus drag

Quote:
Originally Posted by 2wice View Post
Stylus drag pulls the tricycle along nicely as it's all downhill
Hi 2wice,

There is no doubt that your geometry would result in perfect tracking. However, the force pulling the tonearm along, will have to be controlled by an opposing force. If that is not done, the tonearm will overshoot and the cantilever will be deflected. I have been trying to design a tonearm moved by record fiction for years (since 2010) and I know what you're up against. The drag of the LP acts on the tonearm through a mechanical advantage which increases as the tonearm approaches the label area. That in turn means that the above mentioned opposing force has to increase as the tonearm nears the label area. On top of that, it is not a linear function. It is my opinion, that this problem cannot be solved without some kind of servo, be it mechanical, electronic or magnetic.

Sincerely,

Ralf
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Old 19th March 2017, 07:02 AM   #1133
2wice is offline 2wice  South Africa
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Hi Ralf

The problem of overshoot did trouble me, and was the reason I abandoned a Unipivot on the same carriage.
The overshoot is constrained by no horizontal bearings coupled to the rigid triangle of the carriage. The geometry solves true for any point in the groove with 3 points on the guide curves. For there to be any overshoot, that triangle will need to deform. I have a preliminary mechanical model where 3 jewel wheels, mounted on that rigid triangle, trace those guide curves. The contact points of the jewel wheels form the rigid triangle.
In simulation I can already see this effect, if I apply a constant force in the -y direction, the wheels bind onto the guide curves. The carriage only starts moving if I change the force vector to the tonearm vector.
I haven't measured it yet but suspect that the bind force on the wheels increases with the deviation of the two vectors linearly.
I do not yet fully understand the effects of the bind force on the wheels, but will have a clear picture once simulations are completed.
It certainly won't be a easy to build, and simulation to successfully implemented not guaranteed.

Last edited by 2wice; 19th March 2017 at 07:25 AM.
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Old 19th March 2017, 09:03 AM   #1134
2wice is offline 2wice  South Africa
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While looking for other triangles that would still solve, I found that removing one of the curves, translating one curve to form a right angle triangle with the stylus as the third point works fine.
This would be easier to built as the sharp curve on the innermost curve was problematic.

On the other hand, overshoot looks like a bigger a problem now, but the arm length can be reduced by another 30 mm.
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File Type: png Tricycle Tonearm6.PNG (66.6 KB, 188 views)
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Old 19th March 2017, 11:53 AM   #1135
Hiten is offline Hiten  India
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directdriver, good to know those bearing can work.
How do linear tonearms cope with bias ? Any Anti skating mechanism worth knowing ?
Regards.
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Old 19th March 2017, 02:21 PM   #1136
directdriver is offline directdriver  United States
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Quote:
Originally Posted by 2wice View Post
In simulation I can already see this effect, if I apply a constant force in the -y direction, the wheels bind onto the guide curves.
What if the wheels are angled 45 toward the curves instead of vertical, provided the curves are round rods and the wheels have U-grooves?

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Originally Posted by 2wice View Post
I found that removing one of the curves, translating one curve to form a right angle triangle with the stylus as the third point works fine.
That does look simpler and easier to build but in your drawing all the tonearm mass is concentrated in the front point/wheel. Is there a graph that places the arm location at the center of the triangle so mass is equally distributed?

The 3 points 2 curve approach reminds me of some curved linear slides.
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Old 19th March 2017, 02:40 PM   #1137
2wice is offline 2wice  South Africa
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Originally Posted by directdriver View Post
What if the wheels are angled 45 toward the curves instead of vertical, provided the curves are round rods and the wheels have U-grooves?
I'm thinking 90 Deg wheel running a knife edge along the side of the curve, but your idea would be easier and cheaper. Bending a rod accurately will be a mission though.

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That does look simpler and easier to build but in your drawing all the tonearm mass is concentrated in the front point/wheel. Is there a graph that places the arm location at the center of the triangle so mass is equally distributed?
Yes, those curves can be placed anywhere, it will make the arms a little bit longer, closer to a 9". I'll cook something up.

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The 3 points 2 curve approach reminds me of some curved linear slides.
Yep, that's what I'd like to do but with 90 deg wheels and magnetic lift.
The tracks can then be lasercut to +-0.25 micron.
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Old 19th March 2017, 03:12 PM   #1138
2wice is offline 2wice  South Africa
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Something like this?
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File Type: png Tricycle Tonearm7.PNG (87.1 KB, 176 views)
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Old 19th March 2017, 03:42 PM   #1139
directdriver is offline directdriver  United States
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Originally Posted by 2wice View Post
Yes, those curves can be placed anywhere, it will make the arms a little bit longer, closer to a 9". I'll cook something up.
To be honest, as a DIYer I prefer the arm to be 9" so I can repurpose existing conventional 9" arms. If arm length is a concern, the curves assembly can be placed ABOVE the arm (yes, a hanging tonearm) and even cantilever slightly over into some record area and you can have an armwand no more than 5 or 6 inches.

Obviously if you want the vertical pivot point located at the record height outside of the (12" type) platter size, then the arm must be longer than 6 inches. With your graphic and geometric skill you can play around with all the permutations and find the optimum geometry.

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Something like this?
Unless I'm not seeing correctly, looks like you're missing the third point/wheel of the triangle...

Last edited by directdriver; 19th March 2017 at 04:09 PM.
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Old 19th March 2017, 06:27 PM   #1140
2wice is offline 2wice  South Africa
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Doesn't need the 3rd curve anymore, still works with only 2 curves with the stylus now providing the 3rd point.
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