A Theoretical Model For The Gear Rattle Analisys In Automotive Manual Transmissions Accounting For The Gears Lubrication

Due to the irregularity of the I.C. engine torque, the impacts between the tooth of the unloaded gear pairs of a gear box produce vibrations originating an acoustic phenomenon, known as “gear rattle noise”. The paper presents a theoretical study on the dynamic behaviour of automotive transmissions focusing on the presence of oil between the impacting tooth of the meshing gears. This rattle phenomenon is particularly audible at low regimes and regards all the gear pairs different from the gear inserted to transmit the power. Another operating condition that manifests the gear rattle phenomenon is typically the “idle condition”, when all the gear pairs are meshing but without loading. The presence of oil between the impacting tooth of the gear pairs can play a positive role to decrease the vibrations and than the rattle noise level. In the paper some driveline complete models comprising many components as flywheel, clutch, primary and secondary shafts, are described accounting for the presence of oil lubricant between the teeth of the unloaded gear pairs. The described models consider an oil film lubrication between the tooth of the gear pairs, starting from the Reynolds equation and adopting some hypotheses regarding in particular the oil squeeze effect and also hypotheses about the teeth shape. Moreover some dissipative terms as friction and hysteresis in the clutch springs together with the oil drag torque in the gear box elements have been considered and modelled in the article.