Four-bar linkages are used to convert a uniform continuous rotation into a non-uniform rotation or oscillation. The apparatus has a disc with angular scale which is used to measure the exact input angle. The crank pin can be located at different radii on the disc. The four-joint link is a four-bar pivot gear. It converts rotary motion into oscillating motion. The unit can be used to investigate the crank-rocker mechanism, the double rocker mechanism and the double crank. The experimental unit comprises the drive disk with the crank and coupling and the output disk with the rocker. The elements are connected to each other via pivot joints. The positions of the pivot joints can be varied to adjust the crank radius, oscillation radius and coupling length. The drive disk can be driven by the crank. The input angle is adjusted using the drive disk, and an angle scale is integrated into the base plate. The output angle is read at the angle scale on the output disk. The elements are mounted on a base plate. Two handles make it easy to carry and stack the unit.