This one's my favorite (although I haven't read the whole thing):
"This movement is designed to double the speed by gears of equal diameters and numbers of teeth—a result once generally supposed to be impossible. Six bevel-gears are employed..."
"This movement is designed to double the speed by gears of equal diameters and numbers of teeth—a result once generally supposed to be impossible. Six bevel-gears are employed..."
Thanks for sharing!
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!
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.
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.
Mechanical analog computers, especially gun predictors, had lots of setups like that.
So I guess I should extend my original statement to the 1930s-1970s range.
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.