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Has anyone modeled one-way limits in mechanism?

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Has anyone modeled one-way limits in mechanism?

I'm trying to model the motion of a one-way ratcheting joint in mechanism and I don't see any way to do it. If anyone has done this before, please chime in. Your help is invaluable. I'd basically like a body to rotate freely in one direction, but not at all in the reverse direction.

I'm using Elements/Pro 5.0 but let me know if you've made it work in any release.

I actually submitted an idea for this already because I was convinced that the functionality didn't exist. But given the creative minds here, I wonder if somebody has found a way.

Please vote up the idea if you think it's valid: http://communities.ptc.com/ideas/1394

Thanks!

Steve

5 REPLIES 5

Re: Has anyone modeled one-way limits in mechanism?

I'm not too experienced with mechanism, but could you do it with a stepper motor? Or a cam and follower type linkage? I've had a lot of problems with Mechanism lately and am pretty frustrated with it and stopped using it.

Re: Has anyone modeled one-way limits in mechanism?

Thanks Frank. I'm looking for a kinematic expression that does not depend on time, so I think that rules out a servo motor. I thought about using a cam, but not really sure how it would be set up for this use.

A corollary to this problem is that I can't seem to get rotational joint limits to extend beyond 360 degrees, which makes the traditional joint limit essentially useless for my purposes.

Re: Has anyone modeled one-way limits in mechanism?

Steve,

You should be able to do what you want using multiple cam definitions with cam lift off.
As long as the motion is in one plane it should work. Pro/e does not allow 3D cam motion (only 2D).
I put in a request a long time ago to add 3D cam motion.

You will need a snap shot to get it back to the starting position.

Would need to see more to help further.

Hope this helps,
Don

Re: Has anyone modeled one-way limits in mechanism?

Thanks Don. I tried setting up a cam, but since I'm working with a cylindrical rotating body there isn't really a surface or curve to use for definition.

Here's an illustration of what I'd like to do: I have a planetary gear set. I'd like the outer ring (gray) to rotate clockwise, but not counter-clockwise with respect to ground. It doesn't necessarily need to ratchet a fixed increment, it could slip infinitely clockwise but be fixed infinitely counter-clockwise.

2012-06-05_Planetary_1.PNG

Re: Has anyone modeled one-way limits in mechanism?

Steve,

Two ideas come to mind here. First, try a rachet mechanism to keep it from rotating CCW. You can do this by creating a spring loaded rachet arm and a pattern of teeth around the OD of the ring gear. Create a cam-pair between the end of the ratchet arm and the continuous surface of the teeth. Backlash would depend upon on the size and quantity of teeth you create. The other way would be to simulate a clutch/sprag bearing. You may only need to create one roller, depending on torque capacity, its spring and ramp. Cams with friction would be required here, but it should work and have very little backlash.