topic Re: Motion links/joints able to do multiple motions? in Fusion Design, Validate & Document Forum

Motion links/joints able to do multiple motions?

sebastiask4NFSD — Mon, 15 Dec 2025 14:19:08 GMT

Hi!

I am creating a design with three bevel gears, two to the sides (A and B), and one on top (C), see the picture below. All the gears are connected to an axle in the middle, and gear C is free to rotate around this axis due to a ball bearing (not visible) in the centre of bevel gear C.

When bevel gear A and B spin in opposite directions, I want C to rotate around its own axis. This I have accomplished by using rotating joints and motion links. Good!

However, I also want C to spin around the axle in the middle of the picture if both A and B spin in the same direction. To put it differently: I want A and B to spin in the same direction if i move C around the axle, and A and B to spin in opposite directions if I spin bevel gear C around its own axis.

Are motion links able to do double duty like this in Fusion?

Re: Motion links/joints able to do multiple motions?

jhackney1972 — Mon, 15 Dec 2025 15:08:17 GMT

A Motion Link has a "Reverse" option if this helps you accomplish what you desire. According to your description, the gears will not be functioning properly if both outside gears rotate in the same direction.

Re: Motion links/joints able to do multiple motions?

sebastiask4NFSD — Mon, 15 Dec 2025 15:16:20 GMT

The stalled "not fully functioning" gears, are a feature, not a bug to this system. When both bevel gears move in the same direction, they arrest the turning motion of bevel gear C and instead force it to turn around the axle. See for instance this youtube short: https://www.youtube.com/shorts/tJ4rwWNeLlE

In short I want the joints to be able to either spin both A and B gears in the same direction (so that the C gear tilts), or spin A and B in opposite directions (so that the C gear rotates around its own axis).

Re: Motion links/joints able to do multiple motions?

jeff_strater — Mon, 15 Dec 2025 16:54:58 GMT

there is no "conditional evaluation" of Motion Links in Fusion. So, there is no way to explicitly say "when both side gears are turning in the same direction, the top gear should not spin". However, in this mechanism in real life, (I believe), there is also not explicit locking of that gear - it is just physics. Do you have a model with these gears already modeled? I'd like to try to see what happens if you just put the right set of Revolute Joints on this mechanism.

Re: Motion links/joints able to do multiple motions?

sebastiask4NFSD — Mon, 15 Dec 2025 17:04:12 GMT

Sure! Have a look and download from the link below.
https://myafk104.autodesk360.com/g/shares/SH28cd1QT2badd0ea72b6d9342e81f9ebc10

Re: Motion links/joints able to do multiple motions?

jeff_strater — Wed, 17 Dec 2025 00:31:17 GMT

Thanks for the model. After a day of thinking about it, I am sad to say that I've failed. I just don't think that you can come up with one set of motion links that will show both behaviors. The best I could do was to set a user parameter for the two different "modes" of rotation - one where both side gears rotate in the same direction, and one where they rotate in opposite directions. Doesn't really answer your question, though. Also, I had to do some hacks to get it to work at all, because there is no way today to conditionally suppress or unsuppress a motion link. So, I had to set really large ratios for the case where the top gear does not move.

video:

(view in My Videos)

Re: Motion links/joints able to do multiple motions?

sebastiask4NFSD — Wed, 17 Dec 2025 08:37:05 GMT

Thank you for your time and solution. I think your work around is good enough, at least for the moment!