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I have a conceptual design that I've been evaluating which can be summarized as a simple pendulum extended to 3D (i.e. swing not limited to x-z plane). If I want to actually build the conceptual design, is it possible to have a pivot point that does not permit rotation about the pendulum leg axis, but does allow for it to swing "freely" in 3D space?

An easier way to probably ask is, if the pendulum is hanging statically, can I constraint the motion such there is no rotation about the leg-axis (statically coherent with z-axis) and later, dynamically, allow for it to swing "freely" in 3D space?.

I should emphasis, I'm looking for a "product" idea that permits such motion. I thought a universal joint may work, but I think that in order to swing "freely" the joint must be able to rotate about the z-axis.

In the end, the more and more I think about it, I'm wondering if it's theoretically possible? Does this give rise the the "elastic pendulum" idea, where a spring must be involved?

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About the simplest approach is to use two single-axis joints (hinges) in line, 90 degrees to each other. That way one provides X-axis swinging, the other Y-axis, and nothing allows rotation.

If you want to maintain tip orientation ("parallel to the floor"), you'll need frame constructions, a single parallelogram shape using four hinges in the corners maintains the bottom's orientation relative to the floor. A rectangular prism based on two walls of such parallellograms, swinging on four to eight (4 is geometrically sufficient but 8 provide better rigidity) hinges perpendicular to the original 8 (2x4), will provide two-axis swinging retaining orientation of the bottom part relative to the floor.

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Universal and very low friction 360degree joints can be made by linking two spherical magnets.

While the contact force is limited by the size of the magnet this type of connection satisfies the requirement for free low friction 3D motion. The contact force is approximately 5Kg for a pair of 2cm rare earth magnetic spheres.

These joints are sometimes used on hex manipulators such as articulating 3D printer heads used in delta style printers. Additional mechanical parts are obviously required. enter image description here

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  • $\begingroup$ Yes, this was one of the ideas I was also considering, but you can rotate the spherical magnet pivot joint about the leg-axis. So it is not a solution to my question. I'm sure with multiple pivot configurations you can constrain the problem, but my question references a pendulum having a single pivot joint. $\endgroup$ – ThatsRightJack Nov 9 '17 at 22:20
  • $\begingroup$ Out of curiosity, where do you buy the spherical magnet pivot joints? They seem a bit difficult to track down on the internet. I might be able to use one if it has a strong attractive force (can hold 30+ lbs). $\endgroup$ – ThatsRightJack Nov 9 '17 at 23:09
  • $\begingroup$ Magnetic spheres are available in many sizes from McMC for example. I am not aware of manufactured "joints". My assemblies use a 3D printed cup to attach the magnets to structure. $\endgroup$ – Donald Gibson Nov 10 '17 at 0:32
  • $\begingroup$ What is the name they go by on McMC? I couldn't find them. $\endgroup$ – ThatsRightJack Nov 10 '17 at 1:21
  • $\begingroup$ Product is 3945K4 $\endgroup$ – Donald Gibson Nov 10 '17 at 16:48

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