The scotch yoke is a very simple mechanism for converting rotary motion into reciprocating (back and forth) motion using an absolute minimum of parts.

The dark blue pin, sits in the slot in the yoke. The pin is as close to the width of the slot as possible. As the wheel turns the yoke is forced back and forth with in its two bushes (dark orange). The speed of motion follows that of a sine wave with the speed reaching a maximum at the middle of the travel and momentarily coming to a complete stop at each end of the travel.

Scotch yokes have the advantage of being very simple and direct but the disadvantage of having a lot of parts rubbing together which can cause wear and friction.