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## Inventor General

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Posts: 11
Registered: ‎11-10-2012
Message 1 of 7 (318 Views)

# Gears and Relative Motion

318 Views, 6 Replies
11-12-2012 01:09 AM

Okay, so I have a problem in Inventor 2012. I'm trying to design a gear system wherein a small gear rotating on an axle interacts with a large gear, held on an arm. The arm rotates about the same axis as the small gear, in the opposite direction. I applied the relevant motion constraint between the two gears. When I rotate the arm, the motion of the large gear rotates the small gear. However, if I just rotate the small gear, it freewheels, and the large gear doesn't move at all. This is a problem, because the machine depends on the independent rotation of both gear and arm-- the small gear has an input applied to it, but if I try to rotate it it has no effect on the large gear. I assume this is to do with the fact that the large gear's axis of rotation moves with the rotation of the arm.

I can't just throw it all into dynamic simulation, because I don't know yet what kind of torques will be rotating the arm and the small gear-- I know how they rotate relative to each other in terms of degrees, but that's all.

Essentially what I need to do is make it so that rotating the small gear turns the big gear, even when the arm is free to move (when it's grounded, the gears turn normally). I thought about using a contact set, but the gears (generated by Inventor) actually slightly intersect at the teeth, so that doesn't work.

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Message 2 of 7 (302 Views)

# Re: Gears and Relative Motion

11-12-2012 04:36 AM in reply to: wth_187

Do not use Contact Sets.

Attach your assembly here - it should work in either direction with the one motion constraint, you have something missing.

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Message 3 of 7 (263 Views)

# Re: Gears and Relative Motion

11-12-2012 08:58 PM in reply to: wth_187

Sure. Here's a simplified version of my assembly-- for all intents and purposes the same, with the same issue. As you can see, if one rotates the large gear, the arm turns and the gears behave normally. But if you rotate the small gear (on the same axis of rotation as the arm), the large gear doesn't turn at all (neither does the arm, obviously). Also, when you rotate the arm, the gears don't interact properly, either.

I assume it's somehow related to the fact that the arm and the small gear are rotating about the same axis.

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Posts: 11
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Message 4 of 7 (219 Views)

# Re: Gears and Relative Motion

11-14-2012 06:41 PM in reply to: wth_187

Hey, hate to double post, but was wondering if anyone's had any experience with this kind of stuff before? Really bashing my head against the wall here.

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Posts: 23,645
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Message 5 of 7 (217 Views)

# Re: Gears and Relative Motion

11-14-2012 06:44 PM in reply to: wth_187

wth_187 wrote:

Hey, hate to double post

I would call this a bump - not a double post.
Bumping topics back to the top is perfectly acceptable and I recommend it.
I have been too busy to come to reliable solution (other than using Dynamic Simulation if you have Inventor Pro).

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Message 6 of 7 (212 Views)

# Re: Gears and Relative Motion

11-14-2012 08:10 PM in reply to: JDMather

Okay then, thanks.

I'm afraid I can't run a Dynamic Simulation-- unless there's some way to apply torques in degrees per second rather than newton metres, because I have relative rotations but no weights or torques yet. Thus it seems like constraints are the ideal way to go-- as we can define them as functions involving the other angles in the system-- and yet, critically, that one constraint is unsuitable... Interesting.

Cheers, let me know if you find the time to have a crack at it.

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Message 7 of 7 (182 Views)

# Re: Gears and Relative Motion

11-15-2012 10:24 PM in reply to: wth_187

ALSO. Equally important to my project. When specifying equations to define motion (say, you want one wheel to rotate at a rate proportional to another wheel, defined by an equation), is it possible to use regions?

For example, say I have to wheels, A and B, with rotational angles of ThetaA and ThetaB respectively. I might want wheel A to rotate 2*BTheta degrees for rotational angles of B between 0 and 180 degrees (so ThetaA = 2*ThetaB), but rotate only at ThetaA = ThetaB for angles between 180 and 360 degrees.

Once again, that's a simpler case than the one I'm facing, but the concept's the same. Anyone help?

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