[joatmon-snoo]

What you're describing is still a rack and pinion system, but likely to wear much more aggressively. I would be happy to be proven wrong, but my gut instinct is that such a system would wear out very quickly, because due to the involved forces, the sprocket teeth would wear out the holes really quickly.

Film, I would assert, works because the teeth just need to align the film; force transfer just uses the filmstrip itself.

Gear teeth are a reasonably complex design - settled, for the most part, I think, but still non-trivial - and do their job very well. It's a lot of "meshing together heterogeneous surfaces to maximize force transfer" problem solving.

[adrianmonk]

> Film, I would assert, works because the teeth just need to align the film; force transfer just uses the filmstrip itself.

I'm not a projectionist or a mechanical engineer, but I reasoned the opposite way:

* Film sprockets have a difficult job because the film has to be stopped and started 24 times a second[1]. It needs to be held still while light is shining through it, and then (to reduce flicker) it needs to get moved to next frame quickly.

* Film seems soft and easy to damage, but somehow all that seems to work anyway.

* So it's probably OK if you have a stronger material like steel and all you need to do is move at a constant speed.

> Gear teeth are a reasonably complex design

That's a really good point. Here's a page that covers film sprockets: https://www.sprocketschool.org/wiki/Sprockets

And sure enough, it covers the evolution of sprocket design, saying a manufacturer "created the VKF ('very kind to film') sprocket in order to improve presentation and reduce film wear".

So, although you could probably steal ideas from film, there is indeed some engineering to do.

Also, the trash train designer said he wanted smooth, quiet operation. He 3D-printed the pinion gear out of plastic, which achieves that. If he'd done a sprocket and hole thing, my guess is the sprocket teeth need to be strong, so it probably has to be made out of steel. So it'd be harder to make and less quiet.

I guess the main reason film does it this way is because it has to. It's flat.

I still think the film-style approach is probably feasible, but now I'm convinced it has some significant drawbacks.

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[1] Apparently the sprockets that do this are called intermittent sprockets. Wikipedia has a good explanation and illustration here: https://en.wikipedia.org/wiki/Movie_projector#Film_gate_and_...

Note the blue arrows showing slack between the intermittent sprockets and the other sprockets that move continuously.

[joatmon-snoo]

Ah, so both of us were in the dark about the others' expertise - I hadn't really thought about the fact that FPS is basically a start-stop frequency in analog film.

> Film seems soft and easy to damage, but somehow all that seems to work anyway.

I still think it likely that force transfer relies more on the filmstrip, but I'm also assuming that the filmstrip stays under tension (the shock absorber in the Wiki link appears to be that slash a belt tensioner for the filmstrip).

(Also, I wasn't aware that Geneva drives had actual real-world use cases. Thanks for the Wiki link!)

> He 3D-printed the pinion gear out of plastic, which achieves that.

Yeah- this part actually surprised me a bit. Plastic gears are great for low-torque applications, but my understanding is that with high-torque/high-force applications, you start running into shear stress material limitations. This is one of those "just try it out and see how it holds up" questions.

I'll also say that when I read your suggestion, I was really confused because the first sprocket that came to mind was a bike sprocket... :)

(FWIW, I'm not a MechE, just a software person who spent enough of their high school years in a shop.)