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: