# simple harmonic mechanical motion

Discussion in 'Physics' started by strantor, Aug 15, 2011.

1. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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I have been contemplating a new design for a levelwind. A levelwind is a device most commonly recognized in a fishing reel. it travels side to side, evenly distributing the line on the reel (takeup) as it spools on (figure A). We have many machines in my plant (wire & cable making plant) which do exactly the same thing but on a larger scale. This is more of a thought exercise than a real plan to reinvent the wheel; I do not actually plan to modify anything, just looking for ideas. So what I was thinking is that instead of all the mechanical wizardry to make the levelwind travel back and forth, stopping and reversing direction, to just replace it with a rotating disk (figure B). the disk would have a hole in it and the wire (or fishing line) would pass through the hole. as the disk travels around, the line would be brough outward to each side of the reel and back again. the problem with this is that the position of the hole on the rotating disk, if plotted out, would be a sine wave. As in a sine wave, its average position over time would mostly be to the outsides of the reel, resulting in a concave wire load on the reel (figure C). My idea is that if the speed of the rotating disk could be controlled by seperate equal sine wave 90degrees out of phase of the rotating hole, the wire load would be even. the rotating disk would slow as it passed the center of the reel, then speed up as it went to the side, then slow back down as it went past center again, and so on. I can't think of a (simple) mechanical way to rotate the disk at sinousidal variable speed though. The best I can come up with is an equally sized drive gear whose center shaft is off-center, but then the shaft of the drive gear would have to be on a movable arm and would just be wacky. any ideas?

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2. ### shortbus AAC Fanatic!

Sep 30, 2009
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I retired from Packard Electric division of GM and the made wire too. If I remember right they used 'linear cams' for their reel-up operations. Didn't work in that area so didn't pay that much attention to it. Some thing like this is how it worked - http://www.notesandsketches.co.uk/Mechanisms_Demo_Animation.html click on the cams to see it.

3. ### MrChips Moderator

Oct 2, 2009
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As shortbus suggests, you need something like a "linear cam". In electronics this would be the equivalent of generating a triangular wave instead of a sine wave.

The only thing I can think of is a screw drive driven by a motor that reverses direction at the end of the travel.

Last edited: Aug 15, 2011
4. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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yes, the screw drive reversing direction is currently how it's done. I'm not looking for any-old alternative to the way it's done. I am more interested in learning how to generate this opposing mechanical sine wave than find an efficient alternative to the screw drive system. It's just a thought exercise.

5. ### BillO Well-Known Member

Nov 24, 2008
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How about a chain drive using a circular drive sprocket and an elliptical driven sprocket?

6. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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That's a good idea; That's similar to my off-center drive gear idea. It would probably work if the drive sprocket were on a movable arm to accommodate the shortening and lengthening chain. the drawback is that all the sprockets and pivots would probably make it as complicated as the reversing screw.

7. ### Alchymist New Member

Apr 16, 2011
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Use the rotating disk with the hole in it- drive the disk with a stepper motor, program the stepper for desired rotation.

Sep 30, 2009
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Oct 3, 2010
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10. ### shortbus AAC Fanatic!

Sep 30, 2009
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Another way it could be done, using you rotating disk, is rotate a 'crank' from the coil shaft( by bevel gears) Then from the crank a connecting rod would move a pivoted arm back and forth guiding the wire onto the coil/reel.

I'll make a sketch if that helps. The main thing with any solution is for there not to be too much dwell at the ends of the movement. Too much dwell will cause the wire to stack at the rims of the reel.

11. ### THE_RB AAC Fanatic!

Feb 11, 2008
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A DC motor operating a leadscrew. DC motor is the speed and a limit switch at each end would reverse direction.

One big advantage is that it requires no mechanical attachment to the coil. Disadvantage is if you need to match it to a varying coil speed.

For a purely mechanical system, you could use the chain and 2 small pulleys and at 1 point in the chain you fix the eyelet on a swivel. That will move forward and backlinearly with a small pause at each extremity because of the pulley size (hence the need for small pulleys).

12. ### atferrari AAC Fanatic!

Jan 6, 2004
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The mechanical "tables" (cannot recall the actual name) used long time ago to solve the (elevation / azimuth) problem of gunnery in battleships had lot of cams.

Try to get the description of one of them. That could help.

The disk driven right-left-right... following a sine law looks interesting. A pendulum comes to mind, obviously.

Anything else, involving a threaded screw will suffer backlash which, I learnt it, is an ample field with lot done and different degrees of success (with dearly costing solutions).

Myself, in the quest of my own coil winding machine I would be very pleased to put my hands in a not-expensive voice coil motor. It seems impossible.

My project was put in abeyance because of that.

13. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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Yes DC motors are a coomon find in these systems. The fancier "all the bells & whistles" type has a DC drive for the screw and an AC drive for the "coil" (usually referred to as a winder or takeup). The AC drive outputs a 0-10V signal proportional to it's 0-60Hz motor output; this signal goes through a proportioning potentiometer (so set the spacing between the wire wraps) and into the 0-10 reference of the DC drive. This system works flawlessly. Like I said; I don't want to change the way it's done; only interested in satisfying my own curiosity about generating the mechanical sine wave to oppose the sine wave of the rotating disk.

in the purely mechanical systems (still employing a screw) the screw shaft at one end has a gear and a sprocket, each with a small electric clutch attached. There is a second shaft which spins in only 1 direction and has a complimentary gear and sprocket. this second shaft is driven off of the winder drive shaft. in order to reverse direction, either the gear clutch or the sprocket clutch will be energized.

14. ### Wendy Moderator

Mar 24, 2008
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I like the lead screw idea. Simple contact switches feeding a flip flop, and a cutoff. Simple.

15. ### BillO Well-Known Member

Nov 24, 2008
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This is the design, one way or another, used in most good spinning reels that I have. Why re-invent the barrel cam? Good find!