The Geneva drive
Turning steadily into stopping and starting, a quarter at a time.
A pin on a steadily turning crank drops into a slot in a wheel, moves it round by a quarter, and comes out again. Between one slot and the next, a curved shoulder holds the wheel absolutely still. Continuous in, indexed out, and no electronics anywhere.
This is what advanced the film in a cinema projector twenty four times a second for a century. It is still what indexes a rotary table on a machine tool.
| Quantity | How it is worked out |
|---|---|
| Step | 360° ÷ the number of slots |
| Centres | a = c ÷ √2 for a four slot wheel |
| Slot depth | (c − a) − the pin's radius |
The centres are the whole trick: get them wrong and the pin either misses the slot or jams in it. The app's scene is built to the numbers above.
Open the geneva scene in the app to watch this one run.