The classic crank, above, is from Five Hundred and Seven Mechanical Movements, originally published in 1868. 

Matt Keveney has put this classic book online, and animated many of the movements. It's like a periodic table for machines. 

A crank converts circular motion into reciprocating motion. It also works the other way around, but that doesn't interest me now. I want to use servos (circular) to get that connecting rod going -- to power automata. 

The animated crank online gets me started, but fortunately I can do better -- working with a few crank kits I found online, starting with an extremely simple one from PaperMech. 

The Hummingbird group, out of Pittsburgh, also has one. 

Rob Ives even has one in paper.

Ready to start cranking.