An engagement system for lawnmowers comprising, internally of a casing (1) supported by a frame of the machine, a rotating blade (40) on a shaft (4) of which a driven pulley (5) is keyed, a drive pulley (2) keyed onto a shaft (3) of a motor, a belt (6) transmitting drive between the drive pulley (2) and the driven pulley (5), and a chain stretcher maintained elastically against the belt (6) the system comprising a metal drum (9) which is coaxial of and solidly constrained to the shaft (4) of the blade (40), a brake shoe (91) positioned at an end of a lever (8) inserted on a pin (7) parallel to the shaft of the blade (40) and positioned in front of the drum (9), the lever (8) being positioned opposite the pin (7) supported by the frame bearing the stretcher (80), and elastic means (10) being supplied to keep the brake shoe (91) pressed against the drum (9) and, contextually, to keep the stretcher (80) away from the belt (6), and means (12) for distancing the brake shoe (91) from the drum (9) and for placing the stretcher (80) in contact with the belt (6).
