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Thread: Sometimes it's the last simple thing

  1. #1
    Project Engineer
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    Oct 2013
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    92

    Sometimes it's the last simple thing

    Hi Folks, don't expect anyone to be up at this hour.

    Earlier I finished an assembly I needed to finish a small machine, This was a centering device on a processing machine. it's supposed to adjust automatically when the substrate enters the machine and pull it to the middle of the entryway. My first assembly which I could swear I've made before didn't work and I can't think of why it fails to do what I want.

    In the end I added a vertical element with a linear bearing and 2 links connected above the pivots to that to make the arms work evenly, That works fine but I originally wanted to have it simplified and not use that linear bearing in the middle.

    so now I'm back wondering why this won't work like the mock up in the attached picture. Thee green elements are bearings for the pivots. the arms are mostly vertical with round elements on the top and there is a link connecting the arms together. The attaching points are the same distance from the pivots. one above and one below. These only have to be opened about 40 and not quite touch in the middle when in the closed position. My thinking was that as long as those distances between the pivot and where the cross link attached were the same on both sides then the arms would travel out and back at the same rate, but in this case one arm travels faster and further than the other... so it can be correct only in the closed or only in the open but not both? I thought I gave up on it too quick.

    Hope this makes sense?
    Attached Images Attached Images

  2. #2
    Technical Fellow jboggs's Avatar
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    Mar 2011
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    Glenn, The geometry devils attack again! The bottom line with your setup is that you want an angular change in one arm to result in an equal but opposite angular change in the other arm. Right? The problem is that with a fixed distance between the two connection points, an angular change in the position of one point results in an unequal angular change in the position of the other point. This diagram might help.
    Drawing2.jpg

    You'll see that the distance between two points on the circumference of two equal circles varies as those points travel in opposite rotary directions.

    Can't say I have a specific suggestion for you right now, but I cam confident that the final solution will involve some method of anchoring the motion to a known centerline, such as a fixed pivot point or a linear slide.

  3. #3
    Project Engineer
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    Quote Originally Posted by jboggs View Post
    Can't say I have a specific suggestion for you right now, but I cam confident that the final solution will involve some method of anchoring the motion to a known centerline, such as a fixed pivot point or a linear slide.
    Thanks. Yes thats whats in the design now is basically a linear slide with a link from that to each arm on one side of the pivots.

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