Transmission for a walk-behind lawn mower

A transmission device between a primary motor shaft and a drive shaft for the wheels of self-propelled lawn mowers having a driving member connectable to a motor shaft, an output shaft, and a helically threaded driven gear rotatably mounted and moveable axially on the output shaft, the driven gear including an exterior driving surface and an interior surface, the interior surface including a plurality of teeth fixed in rotation with the gear, the teeth having a sloped leading edge and trailing edges substantially on a plane extending radially outward from the center of the driven gear, the outer engaging surface in constant engagement with the driving member so that the driven gear rotates whenever the driving member rotates. A control member configured to move the driven gear along the axial axis of the output shaft from a first position to a second position, the control member including a fork with two branches, each branch of the fork being provided with a radial internal tooth so as to ensure contact with the driven gear. A projection secured in rotation with the output shaft and extending from the output shaft such that opposite ends of the projection project from the output shaft to engage the interior teeth of the driven gear, projection being positioned on the output shaft such that when the driven gear is in the first position, the projection is disengaged from the interior surface of the driven gear such that the driven gear is uncoupled from the output shaft, and when the driven gear is moved to the second position, the projection engages the interior surface of the driven gear in a locking relationship to produce a driving rotation of the output shaft.