CAE Based Benchmarking of Shaft Deflection for Transmission Gear Rattle Noise

Vehicle transmission gear rattle is one of the most critical NVH irritants for refined vehicles. It is perceived more dominantly in lower gears of vehicle running. It depends on various design parameters like engine input torque amplitude & fluctuations, driveline torsional vibrations, gear micro & macro geometry, shaft flexibility, etc. Establishing exact contribution of each of these parameters to transmission rattle, thru experimental or simulation technique, is very challenging. Current paper explains the NVH CAE benchmark approach deployed to understand difference in rattle behavior of two transmission designs. Paper focuses on simulation of gear impact power and its sensitivity to transmission shaft deflections. Impact power is one of the indicators of transmission rattle noise and transmission shaft deflection is one of the contributors for gear impact power. 3D MBD simulations are carried out to calculate loose gear impact power by applying angular acceleration input to transmission input shaft. Gear impact power results corroborate the perceived rattle behavior of two designs of transmission. For shaft flexibility, FEA based nonlinear static simulations are carried out to calculate shaft deflections by applying torque induced forces at each of the gear contacts for a given gear engaged condition. Shaft deflection results of two transmission designs, under identical loading and boundary conditions, indicate the contribution of shaft flexibility towards difference in gear impact power. Transmission design feature causing this difference in shaft deflection is identified and its sensitivity towards rattle is established.