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  • Advice for semi-independent pendulum axis

    Discussion in 'The main mechanical design forum' started by MaxHeinritz, Dec 14, 2016.

    1. MaxHeinritz

      MaxHeinritz New Member

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      Hello, I am looking to create a simple device with two semi-independent pendulums. I created a Google Doc with a rough overview (forgive the janky drawings):

      https://docs.google.com/document/d/1j44wmI84Odi-nf3Oc4ymygjO1y7X1vXWiUhHgPOdFlk/edit#

      I'm wondering how to design and build the top axis to support the semi-independent motion of the pendulums. Is there an existing mechanical engineering concept that supports this kind of thing? Where should I go to learn how to design and construct a prototype? What materials would you recommend?

      Sorry, total noob. I took a SolidWorks class in high school many years ago, but haven't thought about mechanical design for a while. Thanks!
       
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    3. Dana

      Dana Well-Known Member

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      Assuming it's only pendulum 2 that will be hitting an obstacle: Picture for sake of discussion that pendulum 1 is a cute girl on a swing (it does look like a swing set, doesn't it?) and pendulum 2 is a boy on his own swing, who has his arm behind her <choose your anatomy part> but not holding her. When he hits the obstacle, he stops but she keeps on swinging forward. He continues to hold his arm rigidly out and when she swings back, she hits his arm and pushes him back up again.

      It doesn't have to be an arm, it could be done with a couple of round sleeves serving as the bearings on the crossbar and offset cuts allowing them not to pass in the one direction. Or it could be an arm, mounted somewhere on pendulum 2 crossing behind pendulum 1 (or a rigid arm mounted on pendulum 1 crossing in front of pendulum 2). Whatever you do will have to be strong enough to withstand the impact.

      Remember, though, that if you stop pendulum 2, its energy is absorbed, so when pendulum 1 re-contacts it on the back swing part of its energy will be transferred to 2 so they both won't go as high.
       
    4. MaxHeinritz

      MaxHeinritz New Member

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      Thanks! The arm metaphor makes total sense. Good point about energy being absorbed. That's acceptable for me (at least, I anticipate it will be).

      I drew a sketch with what I believe to be "round sleeves" and "offset cuts", though I'm not sure. Something like this?

      https://goo.gl/photos/HntHufcEpgTbTHcs6

      Any thoughts on how to prevent the boy from touching the obstacle after hitting it? I'm considering a roller ratchet, but I'm not sure how to engage it only after the obstacle's been hit.

      Another question I have is how to physically construct the crossbar to minimize friction. One idea was to weld the pendulums to the outside portion of a bearing; then secure the inside of the bearing to the crossbear. With large sleeves as in the sketch, I'm not sure (a) whether there'd be too much friction and (b) where to get sleeves and the crossbar of the proper diameter (would I have to cast them myself?).

      Thanks again!
       
    5. Dana

      Dana Well-Known Member

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      Yes, that's more or less what I meant. But be aware that there will be a lot of load on the contact surfaces (and on the pivot end of the pendulum) if it has any significant mass.

      Not sure what you mean. If he doesn't touch the obstacle how does it stop him?

      The appropriate bearing will depend on how much weight you're suspending, how fast it's moving, how much friction is acceptable, how long it has to last, etc.
       

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