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Old 4th January 2018, 07:27 AM   #1631
nocdplz is online now nocdplz
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thanks for the advices, Doug - always coming from an uncommon experience

Bearings - I'm using SKFs as usual, hoping they are worth the cost.
I know something about pre load: in my arms I have always used double bearings loaded by spring washers. Making the first gimbal 10 years ago I discovered that two bearings have less friction than one badly loaded

# 1605 - The B hypothesis was drawn to place the arm at a standard distance (225mm). The A convinces me a lot more (I think I have resumed your solution) because two well spaced bearings are stressed much better by this load, and maybe can be made very light.

Weight is definitely a problem - I'm thinking of fiberglass (pcb) for the "cranks" and thin aluminum tube instead of solid bar, but above all a spring instead of the counterweight.

Belt: the accuracy of ratio is crucial, and even some elasticity - troubles guaranteed. The Twice solution must be considered seriously: i've rotated two gears leaning on table and works surprisingly well

Frictions: I believe, however, that I will never reach the data reported by Niffy for linears (surprising even to be measured!). What I did in the second arm was much better than many commercial types lent from a friend who likes to build 3D printers, but still far from 0.9 mN on the stylus. That means just a small fraction able to push the cart.
I am comforted by the fact that your prototypes worked even with low VTF: and I believe that the guides, or the second crank were not friction less.

For now, I'm trying to understand something with faded memories of high school. Difficult that I could succed, because the system is really complex: two related rotational movements + one rolling (the pulley on the belt).
I decided to make a crude arm, reduced to the bone, instead of a mock-up. I will lose some more time but I will not risk surprises.

ciao - Carlo

Last edited by nocdplz; 4th January 2018 at 07:30 AM.
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Old 4th January 2018, 04:19 PM   #1632
alighiszem is offline alighiszem  Hungary
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I have the gut feeling that if we would apply a sideway force to the stylus (as would the groove in the vinyl), the tonearm would rotate clockwise relative to the crank and the crank would rotate counterclockwise.
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Old 4th January 2018, 06:09 PM   #1633
2wice is offline 2wice  South Africa
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Quote:
Originally Posted by Hiten View Post
How about if we discard the rubber belt and connect two pulley directly together by rubber sleeved pulleys ? (Like this example) Which method will have less friction. Two rubber pulleys directly connected OR connected by belt ?
regards.
That's how I would like to do it, but with a metal surface on the main pivot and traction on the moving pivot.
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Old 4th January 2018, 08:54 PM   #1634
2wice is offline 2wice  South Africa
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You can reduce the error at the outer groove to 0.55░, middle -0.55░ & inner groove to 0.55░ with 2 null points at 1/3 and 2/3 of the record by changing
the length.
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File Type: png Birch Gadget CAD5.PNG (65.5 KB, 147 views)
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Old 5th January 2018, 05:33 AM   #1635
Hiten is offline Hiten  India
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Quote:
Originally Posted by nocdplz View Post
quite the same imho Hiten, no relative movement. But without belt bigger pulleys, bigger mass. and we doný't need any reversed rotation. For me the belt must be glued in one point of the two pulleys, to avoid any slippage,
Hi
The mass can be reduce by hollow light weight stiff spoked pulley. Isn't it ?
Regards
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Old 5th January 2018, 05:35 AM   #1636
Hiten is offline Hiten  India
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Quote:
Originally Posted by 2wice View Post
That's how I would like to do it, but with a metal surface on the main pivot and traction on the moving pivot.
Hi,
By traction you mean main pulley surface of metal and moving pivot some other surface ?
Regards
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Old 5th January 2018, 05:56 AM   #1637
Hiten is offline Hiten  India
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Rolamite
thinking of somehow if we can incoporate rolamite mechanism. I pressume it is almost frictionless and no slippage of ribbon.
Regards.
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Old 5th January 2018, 06:30 AM   #1638
2wice is offline 2wice  South Africa
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Quote:
Originally Posted by Hiten View Post
Hi,
By traction you mean main pulley surface of metal and moving pivot some other surface ?
Regards
Yes
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Old 5th January 2018, 06:33 AM   #1639
nocdplz is online now nocdplz
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Hi 2wice (sorry, sometimes I've called you Twice). The more I think of your solution and the more I think is the right one, simpler and with better application of the forces (bigger lever arm),
To reduce hor. mass can be used only the necessary sector of the pulleys (especially for the mobile one) and for contact I thought like you, but the opposite, an O - ring on the fixed one. Something must be invented to keep them in touch with just the slightest possible pressure.
I tried to trace the curve with two gears and a pencil and I discovered that the curve described is a beautiful spiral. May you model one for the belt? I'm curious to see if it's the same

Alighiszem - hope not, even with a similar feeling. To do so it would mean that the resistance on the belt (which is fixed) is less than on HP bearing (wich is "frictionless"): seems improbable.
But what learned from Syrinx is that we can't waste even a bit of the minimal forces available. Even a beautiful geometry it's not a working arm. In this phase are needed calculations, graphs, not feelings

Hiten - Rolamite: no, please, I beg you... you're putting too much stuff in the soup
Rolamite is for linear displacement, and nobody uses since his brilliant invention
carlo

Last edited by nocdplz; 5th January 2018 at 06:42 AM.
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Old 5th January 2018, 06:39 AM   #1640
2wice is offline 2wice  South Africa
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Hi Hiten
I'd prefer no band at all, all that's needed is fixed main pivot pulley, CW moving crank carrying CCW moving pivot pulley.
Looks like stylus drag will assist with the CCW rotation.
For the loads involved slipping might not be an issue.
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