Algodoo

This is a small, ready-to-use gearbox based on keys.isDown. It uses A and Z to shift up and down.

Uses collision layer A, though you can change it if you change the reverse gear script (on collideSet, you can use {_c == 0 ? 8 : 0} for collision layer D for example), and then change the gears' collision layers to the same collision layer.

The gears have constant mass. They will always weigh 20 kg unless you change the scripts of the 3 over at density (the mass is the divided number, such as {20 / (math.pi * radius ^ 2)}, so you simly change the 20 for whatever mass in kg you want)

The gears are attached via "spring axles" to the box, so that they withstand virtually infinite angular velocity. Also don't fixate anything to the gears that change radius because fixjoints slowly drift from their initial position when you change the object's size (in this case radius).

To use just delete the blue fixjoint that's on the demo engine and copy the gearbox (you can copy the red stretched axles and connecting circles if you want, they are not essential anyways). It's phunlet safe since it doesn't use any scene.my.variable