Drive Rattle Elastodynamic Response of Manual Automotive Transmissions 2011-01-1586
Modern automotive industry is driven by improved fuel efficiency, whilst simultaneously increasing output power and reducing size/weight of vehicle components. This trend has the drawback of inducing various Noise, Vibration and Harshness (NVH) concerns in the drivetrain, since fairly low energy excitation often suffices to excite natural modes of thin walled structures, such as the transmission bell housing. Transmission rattle is one of the many undesired NVH issues, originating from irregularities in engine torque output. The crankshaft speed fluctuations are transferred through the transmission input shaft. Transmission compactness also allows repetitive interaction of conjugate loose gear pairs. The engine fluctuations disturb the otherwise unintended, but orderly meshing of these loose gears. This often leads to radiation of a characteristic air-borne noise from the impact sites. Structure-borne vibrations also occur by travelling structural waves along the retaining shafts to their support bearings and onto the flexible transmission housing. The paper investigates drive/creep rattle, which is a distinct form of rattle at partial engine loading and low engine speeds. The gear teeth impacts induce vibrations, which couple with the elastodynamics of the transmission casing. A multi-body dynamics model developed in ADAMS comprises a full 7 speed manual transmission of a front wheel drive vehicle. The kinematics of the transmission's input shaft (obtained from an experimental rig for low speed, low gear driving conditions) are used as an input to the model. Routines describing the transfer functions in the supporting bearings establish the link between the shafts/gear pairs' assembly and the elastic response of the casing. The modal behavior of the latter is included in the model through use of Craig Bampton mode reduction method. The predictions obtained from the numerical results are validated against experimental data acquired from an experimental rig.