[JudgeWapner]

I read and re-read this for a solid 5 minutes and I have no idea how it produces the result. I get that gear D spins orbitally around the left-right axis instead of rotating clockwise, but I can't seem to grok how that would produce 2x rotations.

[Animats]

A differential is an adder.[1] This is a differential hooked up as a doubler.

Mechanical analog computers, especially gun predictors, had lots of setups like that.

[1] https://www.youtube.com/watch?v=s1i-dnAH9Y4&t=757

[TeMPOraL]

Thanks for that video. Very informative... like many of the videos from that age. There's something about training videos and physics videos from the 50s / 60s that makes them much better than what people produce today; it's hard to say exactly what it is, but they do feel cruft-free.

[Animats]

The editing, pacing, and style were better.

Educational videos today tend to go to one of two extremes. Either they're a talking head with PowerPoint, like most of the "massively online" courses. Or they have way too many jump cuts, like theatrical movies which want your attention, not your understanding.

There's also an annoying tendency to have distracting music and irrelevant graphics during narration.

Here's an old Jam Handy film, "Spinning Levers", on how a transmission works.[1] There's a whole series of these Chevrolet films on the Internet Archive, covering major vehicle systems. Things to note:

- There's a narrator and a demonstrator. We never see the narrator, and the demonstrator never talks. So the viewer can focus.

- The demo models of parts are really good. They start with a simple version and add features until a full transmission has been built up.

- There's some simple animation. Animation is used to point out how power flows through the gears. This is much clearer than someone using a pointer.

- The editing and narration are very well synchronized.

- There's an entertaining part at the beginning and end, so you don't feel like they're beating you over the head with the boring stuff.

[1] https://www.youtube.com/watch?v=JOLtS4VUcvQ

[dTal]

Not just the 50s/60s - this video explaining differentials is from 1937 and is a marvel of pedagogy:

https://www.youtube.com/watch?v=yYAw79386WI

[TeMPOraL]

Oh yes, that's legendary! I forgot it was pre-war! Speaking of war, I also recall pretty well-made video explanations for bomber pilots, about how to fly the planes to avoid anti-aircraft fire.

So I guess I should extend my original statement to the 1930s-1970s range.

[Grustaf]

They are not trying too hard to make it fun and entertaining. But it ends up much more entertaining than modern chewing gum tutorial videos.

[Anarch157a]

I think you're gonna like this one [1]. The B-25 Mitchell instructional that was linked here some few weeks ago is also amazing [2].

[1]: https://www.youtube.com/watch?v=Ac7G7xOG2Ag

[2]: https://www.youtube.com/watch?v=-YQmkjpP6q8

[TeMPOraL]

[1] reminds me of this: https://www.youtube.com/watch?v=bZe5J8SVCYQ :).

[jcims]

I wish they would make some for convolutional neural networks.

[spandexcajun]

3blue1brown videos are probably the best equivalent. Maybe a little basic? But really, all his math videos are just ridiculously good quality.

https://www.youtube.com/watch?v=aircAruvnKk

[rolandog]

Thanks for the link. Those mechanical computers are really amazing; such an elegant solution.

[neilk]

Not a mechanical engineer but I think this is what is happening.

Note that Shaft F and shaft D are made to rotate in opposite directions.

Then another gear is made to pit those motions against each other, by means of that wacky rotating frame A.

To see why let’s unwrap it from a polar coordinate to something more linear.

Imagine a wheel with gears resting on a rail with teeth. The wheel has an axle going through it. If you drag the wheel’s axle forward horizontally that’s like what Frame A does to impart 1x rotation. But then the rail is also moving backwards, giving you 2x rotation.

I think!

[Kliment]

You're entirely correct - it's a modified differential drive.

The two shafts are coaxial to each other and drive their corresponding gears in opposite directions. The frame is fixed relative to one shaft but mobile relative to the other, which is where the extra 1/2 of relative motion comes from.

The traditional way to do this was with a crossed belt. This is the higher-friction lower-slippage version of that.

[abdullahkhalids]

I think what happens is that E is moving the whole gear D move around. I think you realize this. In other words the motion of E does not change the teeth of E and the teeth of D which are in contact with each other.

But then the gear LeftC is making D rotate around the circumference of E i.e. changing the teeth of E and teeth of D which are in contact. Since this is in the same direction as above, you get 2x speed.

Essentially imaging you sitting on the edge of a plate. E is making the whole plate spin. And leftC is making you run around the circumference of the plate in the same direction as the direction of rotation. So your angular speed becomes twice as much as the speed of the plate.

[kd5bjo]

E is the output gear, and B is the input. The arrangement on the right is to make the frame A and gear C turn in opposite directions, and D adds the two rotations together again to turn E twice for each revolution of B.

[ElCapitanMarkla]

This is blowing my mind at the mo, I need to see it working...

[JudgeWapner]

I think I got it. The simplest way I can describe it is the gear on the far right is turning the whole square frame on the left because the little shaft turns inside the big shaft. The 2nd gear from the right (that is connected to the big shaft) is turning the "thing-thats-already-turning", so you get 2X rpm.