Abstract: It comprises a mechanical power input (1, 55) receiving the power preferably from an electric motor and a mechanical power output (3, 67), as well as transfer means (2, 2', 2", 2'", 4, 64) for transferring this power from the input to the output, and control means (2, 5, 6, 7, 59, 60, 61) for controlling the transfer means (2, 4, 64) to make sure that the power transferred at the output does not exceed a predetermined level. There are provided means to break down the power received at the input into a power applied at the output (3, 67) and excess power which is used to produce the cut-off of power supplied by the motor. This excess power is controlled by the pressure of element (6, 7, 61) whose tension is adjustable which actuates against a movable surface which defines a cam profile, thereby increasing the force or torque during the angular or linear motion of the cam (2, 2', 2", 2'"). The transfer mechanism includes a basic group with four different output alternatives (C1, C2, C3 and C4).