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Old 6th August 2013, 02:56 PM   #221
bgruhn is offline bgruhn  United States
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Quote:
Originally Posted by bear View Post
Ok, after looking at the last image posted, and thinking some more, I do have a question. You know that regarding the bearings and the vertical motion I have had some questions/reservations... so here is the big question: If you balance the arm for "zero" tracking force, so that the cartridge hangs above the platter, what happens if you very lightly blow on the cartridge side or the counter weight side?
I too have reservations regarding vertical motion with this design and why I want to build a test piece. I assume that you are blowing lightly from above the arm on either side of the "fulcrum". With Bo's 2 bearing design the arm will easily move away from the air "blast". You have the balls nicely centered in the ground races with a small amount of free motion (radial play) possible. The friction in this position is exceedingly small. Only when the amount of deflection up or down of the cartridge is greater than the amount which the radial play will permit does the bearing need to slide sideways on glass surface. With the 4 bearing on the outside of the glass tube the only motion allowed by the bearings radial play is constrained so that for any vertical displacement which can not be accommodated by the stylus deflection must cause the bearings to slide around the outside of the glass tube. This sliding will be controlled by the friction between the bearings and the glass. This friction by definition will be twice the value of the 2 bearing design and functioning full time, not just when the radial play limits in Bo's design are exceeded.

Theoretically this shoots down the 4 bearing design. In actual practice maybe it is of no consequence. Perhaps it could have unforeseen benefits. This I intend to find out. Thanks for raising the issue.

BillG
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Old 6th August 2013, 03:10 PM   #222
bgruhn is offline bgruhn  United States
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Originally Posted by bgruhn View Post
I too have reservations regarding vertical motion with this design and why I want to build a test piece. I assume that you are blowing lightly from above the arm on either side of the "fulcrum". With Bo's 2 bearing design the arm will easily move away from the air "blast". You have the balls nicely centered in the ground races with a small amount of free motion (radial play) possible. The friction in this position is exceedingly small. Only when the amount of deflection up or down of the cartridge is greater than the amount which the radial play will permit does the bearing need to slide sideways on glass surface. With the 4 bearing on the outside of the glass tube the only motion allowed by the bearings radial play is constrained so that for any vertical displacement which can not be accommodated by the stylus deflection must cause the bearings to slide around the outside of the glass tube. This sliding will be controlled by the friction between the bearings and the glass. This friction by definition will be twice the value of the 2 bearing design and functioning full time, not just when the radial play limits in Bo's design are exceeded.

Theoretically this shoots down the 4 bearing design. In actual practice maybe it is of no consequence. Perhaps it could have unforeseen benefits. This I intend to find out. Thanks for raising the issue.

BillG
A further thought reminds me of automotive shock absorbers (dampers on the other side of the pond). Before hydraulic dampers had been invented they used FRICTION dampers to control vertical motion. Perhaps the friction provided by the 4 bearing design will provide some needed damping action.

BillG
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Old 6th August 2013, 04:08 PM   #223
bgruhn is offline bgruhn  United States
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Quote:
Originally Posted by bgruhn View Post
I too have reservations regarding vertical motion with this design and why I want to build a test piece. I assume that you are blowing lightly from above the arm on either side of the "fulcrum". With Bo's 2 bearing design the arm will easily move away from the air "blast". You have the balls nicely centered in the ground races with a small amount of free motion (radial play) possible. The friction in this position is exceedingly small. Only when the amount of deflection up or down of the cartridge is greater than the amount which the radial play will permit does the bearing need to slide sideways on glass surface. With the 4 bearing on the outside of the glass tube the only motion allowed by the bearings radial play is constrained so that for any vertical displacement which can not be accommodated by the stylus deflection must cause the bearings to slide around the outside of the glass tube. This sliding will be controlled by the friction between the bearings and the glass. This friction by definition will be twice the value of the 2 bearing design and functioning full time, not just when the radial play limits in Bo's design are exceeded.

Theoretically this shoots down the 4 bearing design. In actual practice maybe it is of no consequence. Perhaps it could have unforeseen benefits. This I intend to find out. Thanks for raising the issue.

BillG
Correction please, I stated tha the friction of the 4 bearing device will be twice the friction of the 2 bearing device. This is wrong. The 2 bearing device has both corners of the outer races in contact with glass for a total of 4 contact points. The 4 bearing device has only one corner in contact for each bearing for a total of 4 contact points. This friction component is the same for each case. All the other concerns remain the same.

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Old 6th August 2013, 04:32 PM   #224
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But the friction inside the four bearings will be twice as high, more or less.
And because they are mechanical, the friction in the bearings will be higher before the arm starts to move than when it moves.

I think.

edit:

I should probably add that I like the setup with four bearings better than the two bearings setup.
Makes more sense to me, and should be easier to work on.
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Last edited by slowmotion; 6th August 2013 at 04:56 PM.
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Old 6th August 2013, 06:33 PM   #225
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But the friction inside the four bearings will be twice as high, more or less.
And because they are mechanical, the friction in the bearings will be higher before the arm starts to move than when it moves.

I think.

edit:

I should probably add that I like the setup with four bearings better than the two bearings setup.
Makes more sense to me, and should be easier to work on.
seems to me that inside the tube gravity is your friend outside the tube all friendship is lost and gravity again just drags you down every chance it gets. as to the number of bearing I see no advantage to using more than two of this type.
Bo's Cantus could be built with four single ball in cup bearings (ballpoint pen nibs) far less slack far less mass all the stability more rigid. Less is more. Moray James.
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Old 6th August 2013, 06:41 PM   #226
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Just random thoughts... the 4 bearing would be killer in a servo controlled horizontal motion arm.

There was previously discussed the 2 bearing version as having had a rounded outer race and also a "flanged" (single knife edge) outer race. Apparently this was discarded as being not necessary. This is puzzling, except possibly a problem was created in terms of the assembly to rotate/pivot - the pivoting taking place where the point contacts were made on the glass tube?

Otherwise I would have expected the rounded or the knife edge outer races to be the best solution. Puzzling.
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Old 6th August 2013, 06:44 PM   #227
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Moray,

As the diameter of the surface (ballpoint pen or rolling bearing) gets smaller the smoothness of the glass surface becomes more important.
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Old 6th August 2013, 07:23 PM   #228
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Originally Posted by bear View Post
Moray,

As the diameter of the surface (ballpoint pen or rolling bearing) gets smaller the smoothness of the glass surface becomes more important.
Bo told me that he found that the rolling knife edge version as I like to call it still had energy loss as a result of the resonance of the rocking arm assembly so he went with the two bearing assembly (four points of contact) that you see today and said that this was a better arrangement. If you want to play very warped records the rolling knife edge would be the way to go but if you want to play good records and to optimize retrieval of data the current version is the first choice. Bo spent over 25 years working on his design I think that he got it right. Best regards Moray James.
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Old 6th August 2013, 07:59 PM   #229
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Perhaps so... but the question is what *is* this "resonance"?

If it is the self resonance of the assembly, like a tubular chime being struck (for example) that is a different matter than the LF resonance typical arm moving up and down... that can be damped by a variety of means I would expect.

In my view using the slop in a ball bearing to achieve limited vertical motion isn't really proper engineering.

I think what you are saying is that Bo wanted to have an effect similar to a record lathe, where the cutter head is fixed at a vertical height and the cutter moves up and down, but in this case for playback the arm more or less stays fixed and the stylus moves.

I'm guessing but if there is a problem in this regard when a short arm (like this one) tracks, then the issue lies in the mass of the assembly, the geometry, and maybe the lack of an "external" damping mechanism. Sounds like the arm "teeter tottered" (see-saw?) when something like a warp moved the arm?
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Old 6th August 2013, 08:20 PM   #230
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Lots of interesting comments .


Perhaps this may be the arm that should not work in paper but does in actuality, haha. Yes there is vertical friction, but as I e stated this is easily overcome while in motion and if set at zero you can move the assembly bu blowing on it but it is a damped motion. One must remember too that the other Achilles heel of a typical linear arm is its different vertical mass related to lateral mass. Though at the end if the day one must build it to see just how it can work, or we could argue for hours on the merits of applied physics without acheiving much .


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