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Author Topic: Wire Size for Dirt Cheap Torque Meter  (Read 580 times)
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frash
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« on: January 15, 2018, 03:24:34 PM »

Laser-Cut Planes supplies a "Dirt Cheap Torque Meter" suitable for Science Olympiad Wright Stuff planes. Our SO team acquired one, broke the torsion wire, and has two replacements on order. If someone knows the wire diameter, I probably can buy a guitar string, and repair it to get us through this week's practice flying. I'm guessing it to be slightly larger than 0.014-in since I tried to measure an old Jim Jones A-size meter. The wire length is almost the same and the distance between laser-cut plywood front and back is about the same distance as between the Jim Jones front and back metal plates. Laser-Cut torque meter has 12 subdivisions to the circular meter dial, Jim Jones has eight.

Thanks to all.

Fred Rash
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strat-o
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« Reply #1 on: January 15, 2018, 03:40:19 PM »

K & S sells or sold 0.015" wire typically in 36" lengths.  You might find some at a LHS.  Might be available at Hobby Lobby or similar.  Look for the red tubes.  Not sure if this is what you need for replacement but I bet it would get you close, especially if you can calibrate somehow.
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frash
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« Reply #2 on: January 15, 2018, 03:45:11 PM »

I think that you are right. I am headed for an (inside) walk with a mechanical engineer friend. He will help also, I think. Thanks.

Fred Rash
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lincoln
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« Reply #3 on: January 15, 2018, 04:21:42 PM »

If you hold the proposed replacement and the original wire with your fingertips, I'll bet you can tell, to a reasonable degree of accuracy, which one is larger. The last time I needed small music wire, I went to a local music store and it turned out they had a drawer full of broken guitar strings. I imagine many guitarists have old strings too. You might consider a slightly larger size, since the previous wire broke. If the previous meter was calibrated, you can calibrate this one, almost as if you were trying to teach those kids about science and engineering. ;-)

BTW, I'm sure you or a student could make one of these from pieces of a wood yardstick, a photocopy of a clock face, a Peck nose button*, and your favorite wire. It would only take a few minutes, I think. I've made homemade torque meters before, but those are the "outdoor" style, which double as extensions for using a blast tube. I already have the Jim Jones ones.


*A hole drilled in suitable plastic would probably work as well.
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Hepcat
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« Reply #4 on: January 15, 2018, 07:37:33 PM »

Fred,
It seems strange replying to you when I have always found that you Chemical Engineers are far better at maths than we engineering draughtsmen!  I hope the three charts below might be of some use. One gives wires about the sizes used on Indoor meters and one for wires for outdoor meters.  The third one is realy just of interest, emphasizing that a wire needs to be 323 times as long as its diameter to have a safe stress at one full turn twist.  I can send the equation for stress in twisted wires if you don't have it to hand.
John

SORRY, i GAVE 2 COPIES OF ONE GRAPH AND MISSED THE GRAPH FOR SMALL WIRES.  i HAVE ATTACHED IT IN A LATER POST i HOPE.
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« Last Edit: January 15, 2018, 09:06:22 PM by Hepcat » Logged

John Barker UK - Will be missed by all that knew him.
Hepcat
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« Reply #5 on: January 15, 2018, 08:38:33 PM »

Fred as you have brought up the subject of Dirt Cheap I thought you might be amused to see something that is.  Below is my last torque meter. I am not proud of it but it works. There is a balsa base on which sits my digital scale. In the foreground is a piece of wood with a bit of ply on either end. It used to help me make small springs. Now it just holds a bit of wire with a hook at one end for the motor being wound and the other end is bent to touch the of the scale pan at a length that gives a read out in in.oz.  It is out of date now because I realize that if just clip a small stick of balsa at the correct radius on the propeller leading edge then all I need to do is touch the stick on the scale pan.

John   
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