The gear – lever mechanism is based on the slot - crank motion (like Geneva wheels) configuration. When oscillated motion is applied at the input gear (by an oscillated lever about fixed axis A -not shown), which meshes with the second gear, it forces it to oscillate in opposite direction. On the second gear is mounted rigidly a crank whose pin slides along slot of output lever which turns freely about axis A. Output lever transmits reciprocating motion to an intermediate link (not shown) which is connected to the driven part of the mechanism. Thus oscillation of input gear (and lever) is converted into different oscillation of output lever.
The advantages of the gear - lever mechanism against the cam actuated is the lower cost of gear trains, and the adaptability to different positions of the input lever (remember that a lever oscillates the input gear) or the input shaft. It can be used reversely (but not in many positions) in order to produce a larger oscillation angle. At the configuration shown at the figures there is a formula for calculation of the angular displacement of the output lever b corresponding to crank angle a, which is equal (if the gear train ratio is one) to input gear angle.