Parametric excitation as a cause of clutch judder: theoretical study and experimental validation

Judder is a friction-induced torsional vibration generated during clutch engagement in automotive drivelines. To the best of the authors’ knowledge, three are the causes attributed by the scientific community to the onset of the clutch judder: stick-slip phenomena, negative gradient of the coefficient of friction and geometric disturbances. With the help of a methodological approach that integrates the analysis of a mathematical model with the experimental data obtained on a specially designed test bench, this paper shows that, in some cases, the clutch judder may also be due to the presence of parametric excitation. This new understanding overcomes the limits of the existing explanations because it does not preclude the occurrence of clutch judder at high slip speeds, with positive gradients of the friction coefficient and at excitation frequencies different from the eigenfrequencies of the system. The four degrees of freedom model developed for the study of the transmission provides maps that allow to identify the instability conditions of the system. The analysis described in this paper has been aimed at solving a judder problem in a transmission already in production, but the same approach can be used to avoid the conditions that cause the onset of clutch judder on other transmissions in the design phase.