Issue 30

G. Belingardi et alii, Frattura ed Integrità Strutturale, 30 (2014) 469-477; DOI: 10.3221/IGF-ESIS.30.57 470 So, the behaviour of new automotive transmissions may be simulated with specific multibody programs, being these systems suitable to be described as a collection of subsystems called bodies and the motion of them is kinematically constrained. Basic to any analysis of multibody mechanics is the understanding of the motion of subsystems (bodies or components). Referring to this environment, a gear is a complex body with a specific geometry, as an example the modulus, the number of teeth, the face width and the contact ratio. Once a gear is designed as a function of the power to be transmitted, it is necessary to analyse it from both kinematic and dynamic point of view [1]. Some papers have been published about the numerical analysis of the dynamic meshing gears. Most of them are based on lumped parameter models [2-5], where the teeth contact is represented by an equivalent mass, a stiffness and damping of the teeth. Another way to address the contact analysis is related to FEM simulations; this approach is helpful to determine the gear contact stiffness and the corresponding strain and stress distributions in the teeth. In particular the multibody kinematic approach uses a contact stiffness which depends on parameters that are not yet well understood [6]. In the present work the overload effects due to both speed and meshing in a gear couple of an electric vehicle transmission have been analyzed. The electric vehicle is designed for urban people mobility and presents all the requirements to be certified as M1 vehicle (a weight less than 600 kg and a maximum speed more than 90 Km/h). The first aim of this research is to evaluate how the dynamic overloads may influence the fatigue life of the above quoted gears in term of durability. To this goal, a specific multiplicative factor to the applied load, called Internal Dynamic Factor (K v ), involved in ISO 6336 Standards [7-10] has been taken into account; K v values have been calculated by means of the related analytical equations (ISO 6336 Methods B and C [7]).Then, the dynamic analysis of the above quoted two gears has been performed by means of the multibody software RecurDyn (Functionbay), in order to obtain the real response of the components. The model has been developed by tuning step by step the contact parameters of engaging pair of gears and by optimizing the integration parameters. Forces values obtained by the simulations have been utilised to determine the dynamic factor K v involved in ISO Standard 6336 [7] for the calculation of gears load capacity. Analytical results in terms of K v values have been compared with those coming from multibody simulations, involving full rigid and rigid-flexible models. G EAR MODEL bject of this paper consists of two specific gears of a differential architecture for an electric vehicle. Its design has been developed in order to keep as low as possible the transmission weight; in this work, a single speed stage gearbox has been chosen. Transmission is constituted by an ordinary gear system and an epicyclical one named differential (see Fig. 1). The ordinary gear system consists of four helical gears, the differential of four bevel gears (two pinions and two suns). From the dynamic point of view, only the ordinary gears system have to be taken into account; the transmitted torque passes directly to the wheel axes through the pinion and the vehicle has been analysed in a rectilinear motion condition. Fig. 1 shows the complete transmission model that has been set up for the simulation. Figure 1 : complete transmission model. O