Yeah I'll give you that one. Dyneema looks like a better bet based on a quick google search. The other advantages of dental floss is it's cheep, hopefully you already have some and when you get your arm working (as I'm sure you will) you'll have a nice bright smile to celebrate with.
Niffy
When heated does it burn or melt? If it melts you may have a solution to the knot problem. Drill a small hole through a small grub screw, feed a fine strand of the dyneema through the hole, heat the end of the strand so it melts into a small ball then pull tight. The grub screw now gives you a tensioner. Alternatively do any adhesives adhere to it? Then you can tie a knot and secure it with glue and still use the grub screw tensioner. Dental floss does have the advantage here in that you can tie knots in it. I used a fine strand of it in the rig I built for measuring the friction of bearings for my arm and it worked superbly, flexible with very low stretch.
Niffy
Niffy
It melts (it is a longchained polypropylene) Kevlar instead burns and is quite rigid. Floss maybe is dacron at best, that is used for belays, but not for halyards or flying since it elongates, less than nylon, but elongates. Here i need a string that mantains his length in time: for tension is used a 1mm piano wire that can be considered constant. The way it's attached may be seen on the drawings: i think that will work, although dyneema is quite slippery (and super flexyble!)
Skate: maybe no one notices that i've used an adjustable headshell: on traditional tonearms i use this trick to correct small residual SF, adding or subtracting some offset. With the scope seems to work acceptably, so i've leaved this chance
here too
carlo
forgot adhesives nothing attaches to dyneema. kevlar instead loves epoxy ( i think is epoxy) so on boats old ropes were glued to glassfiber to reinforce the attchment of fittings (winches camcleats etc)
Skate: maybe no one notices that i've used an adjustable headshell: on traditional tonearms i use this trick to correct small residual SF, adding or subtracting some offset. With the scope seems to work acceptably, so i've leaved this chance
here too
carlo
forgot adhesives nothing attaches to dyneema. kevlar instead loves epoxy ( i think is epoxy) so on boats old ropes were glued to glassfiber to reinforce the attchment of fittings (winches camcleats etc)
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Is this an alternative to the Thales semi-circle? I notice all the arm lines converge into a single pivot point and yet the blue curved line is not Thales geometry. Fascinating!
Yes. It is an alternative to the Thales semi-circle. In Carlo’s design, he did it base on Thales if I understand it correctly. So, his tangential curve is different from my blue line. My math is not that good so I am not able to describe the blue line mathematically. If someone can do it mathematically, it will be really nice. The Beauty of this line is its tracking error is zero and skating force is zero, too.
Jim,
I think Ray is saying that we must account for the back side (counterweight side) of the pivot even though its front portion (cartridge side) is a straight line. If the opposite end exhibits side force then it would affect the other end. I guess it's the two sides of the same coin. I wonder if we change to a different guiding mechanism entirely will we have a skateless arm....
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All,
What if I dispense the strings and use the carriage itself to do the push & pull part of the elongation and add two horizontal pulleys riding on a curved track, will we have zero or less skating force? Or we can do the Schroeder way of a curved magnetic track?
I think Ray is saying that we must account for the back side (counterweight side) of the pivot even though its front portion (cartridge side) is a straight line. If the opposite end exhibits side force then it would affect the other end. I guess it's the two sides of the same coin. I wonder if we change to a different guiding mechanism entirely will we have a skateless arm....
======================================
All,
What if I dispense the strings and use the carriage itself to do the push & pull part of the elongation and add two horizontal pulleys riding on a curved track, will we have zero or less skating force? Or we can do the Schroeder way of a curved magnetic track?
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But I am not seeing a single pivot point though. In Jim's blue curve line, all the Arm lines converge into a single pivot point. Hmm....
I know the Schroeder LT (and Doug's build) uses similar geometry but the lines don't converge into a single point.
DD,
I understand what he was saying.
Let’s look the cartridge part first. If the cartridge is tangential to the groove. There is a force to drag the arm in same direction as the force. There is no force to pull the arm side ways. This force is the only force to move the arm.
Now let’s look the pivot part. Both green and red string should have same tension so the arm doesn’t skate. In reality, groove is a spiral. So, it will pull the cartridge towards center of record. The balance of red string and green string tension is broken. The red string should have slightly higher tension than green string’s. The arm moves towards to center. But such movements are not skating. Let’s put Carlo’s arm in my experiment. If the cartridge is in red or green areas, the arm will skate for sure. Yes. Red string exerts torque. But green string exerts opposite torque, too. Both forces should be same. If the design is perfect and arm is perfectly made, all the pulleys are frictionless or very little friction, you push the arm, the arm should be swing from left to right freely. The higher tension side will switch from green to red, red to green so on.
Jim
I understand what he was saying.
Let’s look the cartridge part first. If the cartridge is tangential to the groove. There is a force to drag the arm in same direction as the force. There is no force to pull the arm side ways. This force is the only force to move the arm.
Now let’s look the pivot part. Both green and red string should have same tension so the arm doesn’t skate. In reality, groove is a spiral. So, it will pull the cartridge towards center of record. The balance of red string and green string tension is broken. The red string should have slightly higher tension than green string’s. The arm moves towards to center. But such movements are not skating. Let’s put Carlo’s arm in my experiment. If the cartridge is in red or green areas, the arm will skate for sure. Yes. Red string exerts torque. But green string exerts opposite torque, too. Both forces should be same. If the design is perfect and arm is perfectly made, all the pulleys are frictionless or very little friction, you push the arm, the arm should be swing from left to right freely. The higher tension side will switch from green to red, red to green so on.
Jim
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Are you saying in a static, non-moving, single pivot point, the only tangential tracking is the Thales geometry? If true, then Jim's graphic has some discrepancy, however tiny to the naked eye.
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fantastic jim i was sending this post with similar thoughts
I try to get into the discussion only with clarifications on my project, criticisms are really useful at this stage (well, always) - I just want to add some elements that I can not figure out.
The stylus drag pulls the arm in line with the pivot to the left. The arm is prevented from moving to the left by the red string. This puts the red string into tension. The level of tension in the string is equal to the stylus drag. The level of tension is equal at all points along its length. Where the string passes around the rear bearing it turns through roughly 90°. The rear of the arm will be pushed sideways with a force of approximately equal to the stylus drag. - Niffy
The reasoning seems to be right**, considering the string only from CP1 to point X. Instead the string continues till to CP2 (and if the bearings rotate, the tension is evenly distributed). So what exactly will happen?
I thought this gadget as a closed system, where action and feedback control the stylus's position and balance; when side force increases*, the stylus advances and rotates to the right, so reducing the side force & so on. Instead what do you think about?
Carlo
**right or reversed? at the beginning: tangency, stylus aligned with pivot, drag the same, thus no rotation to the left, no string tension... For me, another mistery. May be for making side force that the groove is a spiral, or just to load more music?
2wice - the curve described by the vert. blu pivot is a cicloid? (line+circle movement)
Dd - exploring Schuch geometry i met lots of curves, simply altering the distance of the flask. But Schuch has infinite pivots
I try to get into the discussion only with clarifications on my project, criticisms are really useful at this stage (well, always) - I just want to add some elements that I can not figure out.
The stylus drag pulls the arm in line with the pivot to the left. The arm is prevented from moving to the left by the red string. This puts the red string into tension. The level of tension in the string is equal to the stylus drag. The level of tension is equal at all points along its length. Where the string passes around the rear bearing it turns through roughly 90°. The rear of the arm will be pushed sideways with a force of approximately equal to the stylus drag. - Niffy
The reasoning seems to be right**, considering the string only from CP1 to point X. Instead the string continues till to CP2 (and if the bearings rotate, the tension is evenly distributed). So what exactly will happen?
I thought this gadget as a closed system, where action and feedback control the stylus's position and balance; when side force increases*, the stylus advances and rotates to the right, so reducing the side force & so on. Instead what do you think about?
Carlo
**right or reversed? at the beginning: tangency, stylus aligned with pivot, drag the same, thus no rotation to the left, no string tension... For me, another mistery. May be for making side force that the groove is a spiral, or just to load more music?
2wice - the curve described by the vert. blu pivot is a cicloid? (line+circle movement)
Dd - exploring Schuch geometry i met lots of curves, simply altering the distance of the flask. But Schuch has infinite pivots
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To do my best Mark Kelly imitation, I can't help it to say that an ARC is always part of a circle. The segment of an ellipse is just a, err, curve.
The red string and green strings are under equal tension when there is no stylus drag. As soon as the stylus hits the rotating record the stylus drag puts the red string under greater tension than the green, the difference being equal to the stylus drag. As the red is under greater tension the arm tries to rotate clockwise.
Niffy