The connecting rod-crank mechanism is an ancient mechanism that transforms a circular movement into a translational movement, or vice versa. The most common current example is found in the internal combustion engine of an automobile, in which the linear movement of the piston produced by the explosion of gasoline is transmitted to the connecting rod and is converted into circular movement in the crankshaft.

In schematic form, this mechanism is created with two bars joined by a revolute joint. The rotating end of the bar (the crank) is attached to a fixed point, the center of rotation, and the other end is attached to the connecting rod. The remaining end of the connecting rod is attached to a piston that moves in a straight line.