[tvb12]

Where can you find supplies for these kinds of projects? Each 6ft. section of rack would cost $81 plus shipping from McMaster-Carr.Never mind the other components, that would add up quickly. I was trying to find some spur gears for a project, but wow those are expensive. I reworked it to use pulleys, but those weren't as cheap as I'd hoped, either.

[_fs]

I don't believe these kind of projects are designed to be economical :). He is building this to drive traffic to his youtube channel, which in turns, pays for the next outlandish project.

[nine_k]

They can be made economical if you have a lot of salvaged parts for cheap / free.

(As a boy, I could not have built much of the stuff I've built from 74xx ICs if I had to buy all of them new; instead, I had a bunch of PCBs from a decommissioned computer.)

[aliqot]

The motor for the little trolley in the video is a salvaged wiper motor.

[iancmceachern]

Use mcmaster to figure out what you need, and then many of these drive component type things like rack, gearboxes, bearings, etc. are made to standards. Once you know what you need, say a 50:1 nema 23 gearbox just search that on eBay or ali express. Even if there isn't a listing for exactly what you need, just message a factory/supplier thatbhas something similar posted and they can likely get it to you.

[Prcmaker]

I've built scale track out of 25x6mm flat bar steel, plenty strong enough for people to ride on. I wouldn't call the time spent 'fun', but even after buying equipment you'd break even over the mcmaster track very quickly.

[HeyLaughingBoy]

With mechanical stuff, if you're not being paid for it, the best approach is to design around cheap, easy to find parts.

You need cheap rack? Find a way to secure bicycle chain to a flat surface and use a sprocket for the drive.

[adrianmonk]

Can't answer the supply question, but it seems to me his problem could have been solved by drilling a line of holes into the sheet metal rail. Then have a sprocket stick into those holes.

It works for film in movie projectors (https://en.wikipedia.org/wiki/Film_perforations), so why not for a trash train?

It uses fewer parts, at least. But maybe cutting a bunch of holes in already-installed rail is more work than welding rack onto it.

[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.

---

[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.)

[xg15]

I think he made his design such that he could reuse tools/skills he already had. From what he tells in the video, he seems quite experienced with welding, whereas he doesn't seem to have a good way to ensure precise dimensions or distances: There are lots of thoughts in the video how to ensure that things still work if they are wobbly.

So maybe he favored a design that let him weld components together vs a design that would require high precision when drilling the holes and making the sprocket.

Of course, when designing a commercial, mass-produced version, you'd approach this differently.

[TylerE]

Movie film doesn’t have to go around corners. Trains have a lot of side to side slop. That’s why cog railways use rack and pinion drive, not sprockets.

Also, track has a not-trivial amount of thermal expansion and needs to be replaceable in-situ.

[itronitron]

If you're near a metal fabrication shop (stairs, metal fences, signs, etc.) then they should be able to source it for you if they don't have it in stock and also easily bend it for you. Total labor hours charged for this would likely be under four hours.

[dotancohen]

McMaster-Carr is for "I need a very specific thing right now at any cost", not for projects like this. Use it as a guide to discover what is available, but only order from there if every day your project is held up is costing you real money.

[imtringued]

I don't know what racks he uses but one meter of rack doesn't cost very much on Aliexpress.

[nine_k]

Hos is the shipping?

[notwokeno]

You could probably use aluminum stock from eg Lowe's as rails. I built a bunch of solar panel brackets from that last year and remember being surprised by how cheap it was.

McMaster-carr is great but it's almost always crazy overpriced.