We just learned about the Cam.
Another part of moving things like machines is a Valve.
This is something that opens and closes to let air or water in or out.
We use valves every day, like when we turn on the water to wash hands.
Turning the water on opens the valve, and you can usually open it a little or a lot.
Then turning the water off closes the valve.
(from: wikipedia - valve)
Kid Facts - Blast from the past: Great Art of Artillery, the First Part
