Less "rubbery" hinges?
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Sorry if this newby question is covered, but I've been mining the forum pretty hard with no luck. I'm trying to model a linkage mechanism, but the looseness of the hinges make it fly around like crazy. Is there a way to make a hinge tighter, so it's not like its made of rubber bands that lose their axis at any shock?
PS: this page http://code.google.com/p/sketchyphysics/ should be put out of it's misery. It probably just confuses people into thinking SP has been left to rot. Read the comments on the tutorials page.
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The strength of a joint is related to the size of the bodies. You can increase the strength by increasing the size and mass of the connected bodies. There are various ways of doing that depending on the application. If you can post a model of what you are working on I can give specific advice.
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Here's the file, it's just a study for a wing opening mech for a costume. I discovered by accident the relationship of motor torque to volume of it's "armature".
I'm guessing I need to upgrade to V3 to change the mass of the long elements. I normally shy away from betas but SP seems to be following the Google model of never getting out of beta! (at least V2 didn't)
I've managed to stiffen it by making the "sticks" into boards. At least that allows me to use it to design the mech's geometry. But as a mechanical designer, I've never run across a hinge or bearing that was looser if it's structure was smaller. It seems an odd artifact, but then so does Sketchup's general sensitivity to scale in boolians and such, something I never encountered before in a CAD program.
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That doesnt look too bad. I dont think the problem is rubber hinges its more the amount of force the returning arm generates. I think if you built this in real life it might have the same problem.
The relationship between joint strength and size is something that is in the physics engine.
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So is that relationship a bug or a feature?
In real life, the long structural members would flex rather than the actual joint stretching, or the joint would fail catastrophically. It's almost ironic that the solution to strengthening the joints for my study, extruding all the components into boards, illustrates that the hinge joint doesn't recognize the overhung load, which would cause most real joints to fail!
Oh well, it's free, and I appreciate the efforts going on to make it better. I saw a comment about a possible future feature of setting density of objects, I'm sure that would be a real help. I could then make the long rod a carbon graphite tube, and all would be well.
BTW, the work on the wiki is tremendous, congratulations to all for making the door a little wider open. The last time I played around with SP was over a year ago, and the info was much thinner.
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I think it was a design decision. To favor numerical stability and speed over accuracy.
You are right, in the real world the arm would bend. But in this case the arm cannot bend so the joint has to or the forces head to infinity and then the simulation blows up.
But I took a look and it may be that the joints are not as stiff as they could be. I assumed the default stiffness was 1.0 and its really 0.9. The rational is here:
I'll see if I can expose it.
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Would love to see stronger joins in the next release.
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