The crank slider mechanism is the basis of the models Flying Pig and Surfin' the Web. As the crank (yellow) turns the pushrod and slider is moved up and down, the end point tracing out a curved, but not necessarily circular motion.
By altering the various dimensions, different motions can obtained, the key distances are

• Diameter of crank
  
• Crank centre to slider fixed point (green)

The ratio of these distances changes the locus of the end of the pushrod.