Evaluation of the effect of profile modifications in gears subjected to sudden torque inversion

The search for propulsion systems of vehicles with lower environmental impact is focusing on systems with exclusive or supplementary use of electric motors. In fully electric or hybrid vehicles, the electric power delivery is used not only to provide torque to the vehicle and the consequent movement of the vehicle, but also to transform the vehicle's kinetic energy back into electrical energy for storage in batteries when the vehicle is downhill or it must be slowed down. Under normal operating conditions, even in urban usage, it is common to have sudden inversions of the power flow. This means that in a really short time the motor switches from delivering torque to receiving it. Normally the electric motor is not directly connected on the wheels, but a geared transmission is present. The change of torque from positive (motor) to negative (generator) causes a sudden change of the working conditions. This induces vibrational and durability problems on the gears. In the present paper, an accurate model of mesh dynamics is proposed, capable of considering the inversion of the power flow. This model evaluates the forces exchanged between the teeth both with the variation of the torque and the rotation speed. Through the evaluation of the variation in gear stiffness it allows to evaluate the transmission response to different operating conditions. The possibility of establishing which side is in contact and which is the direction of the force exchanged between the mating teeth allows to evaluate the effect on the dynamic response of the system. In this way it is possible to decide which is the best shape of the teeth which minimises the dynamic response of the system in all operating conditions, including torque inversion.