Rings of Gears and the Two Pound Coin
Rack and Pinion
The rack and pinion is used to convert between rotary and linear motion. The rack is the flat, toothed part, the pinion is the gear. Rack and pinion can convert from rotary to linear of from linear to rotary.
The bell crank is used to convert the direction of reciprocating movement. By varying the angle of the crank piece it can be used to change the angle of movement from 1 degree to 180 degrees. The bell crank was originally used in large house to operate the servant’s bell, hence the name.
“Jeeves, where’s my tea?!”
Download and make your own bell crank mechanism here.
There's an updated animation here: