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I have an issue where I need to make a large wire through a bearing spin in 3" diameter circles, and have a weight hanging from the bottom of the wire. Essentially, I need a hole that moves in a trajectory such that the center of that hole makes a 3" circle path. To make the 3" diameter circles, I was just thinking of having a spinning disk driven by a motor and an arm, and have a circle offset 1.5" from the center. The problem with this is that the motor shaft would be secured in the center of the rotating disk, and there would not be enough room for the weight at the bottom of the wire to spin without interfering with this shaft. One of my ideas was to design some fixture similar to the one shown in this video: https://www.youtube.com/watch?v=VTkULChFXkk. I would have four motors driving holes in the corners of a plate driving the rotation, and put my bushing through the center. Another idea I had was to do something like the following image, where I have 3 or more servos on the edges of a center gear that has the 1.5" offset hole. Would this be secure enough? How would assembly work with this, as I am not too familiar with gears. Would I be able to put the three servos down and then put the center gear down in the middle? enter image description here

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  • $\begingroup$ The other post wasn't deleted? I am just trying to work out some other ideas for how to make this work. Unless you are referring to someone else posting something and deleting it. $\endgroup$ Jan 27 at 14:49
  • $\begingroup$ Sorry. I was rash there. Your two accounts weren't linked and the other question got buried super fast. $\endgroup$
    – DKNguyen
    Jan 27 at 14:50
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    $\begingroup$ the motion can be circular without passing the cable through a circular plate ... web search pantograph ... drive it at the end with a 6 inch wheel ... the cable is attached at middle $\endgroup$
    – jsotola
    Jan 28 at 2:58

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Can you even find a gear big enough with the appropriate flat center surface? Most gears have center holes in them after all unless you plan to EDM one yourself out of a plate.

With your new method you only need one driven gear if you wish. No need to drive all three gears, gear tooth strength permitting. If you choose to drive multiple planets with multiple motors, they will inherently synchronize and load share and not disturb the path (whereas the the corner discs on square plate method would need electronically synced motors because it had more degrees of freedom so the path could be disturbed. In other words the square plate can wobble.)

You will want your planet gears to have a flat above and below the toothed part with wider diameter for supporting the sun gear and to keep it captive and stop it from lifting away. Sort of like what you see in pinions for timing belts so the belt doesn't run off. Could also be done with discsc unattached to the planet gears but mounted on the same shaft above and below the planets with thrust bearings. You may need other stronger, independent bearing surfaces to support the weight at the edges if the motors or planet gear shafts aren't enough.

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  • $\begingroup$ Good question, I would need to look around to find a gear that could make this work, just wondering if it is possible. Also, I thought about only having one gear at first, the reason for additional gears was to keep the larger gear centered. Usually both gears will have shafts fixed to something, so they cant move away from each other, but if I had nothing attached to the center of the large gear, what stops it from just moving away as the servo starts spinning? $\endgroup$ Jan 27 at 14:56
  • $\begingroup$ You need three, but only one driven gear. The 3 planets keep it centered. See last paragraph edits. $\endgroup$
    – DKNguyen
    Jan 27 at 14:58
  • $\begingroup$ Thanks for all the help on this over the last few days! I am in a new engineering role and there aren't other design engineers to talk ideas through with. $\endgroup$ Jan 27 at 15:01

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