Transient Clunk Response of a Driveline System: Laboratory Experiment and Analytical Studies

A laboratory experiment is designed to examine the clunk phenomenon. A static torque is applied to a driveline system via the mass of an overhanging torsion bar and electromagnet. Then an applied load may be varied via attached mass and released to simulate the step down (tip-out) response of the system. Shaft torques and torsional and translational accelerations are recorded at pre-defined locations. The static torque closes up the driveline clearances in the pinion/ring (crown wheel) mesh. With release of the applied load the driveline undergoes transient vibration. Further, the ratio of preload to static load is adjusted to lead to either no-impact or impact events. Test A provides a ‘linear’ result where the contact stiffness does not pass into clearance. This test is used for confirming transient response and studying friction and damping. Test B is for mass release with sufficient applied torque to pass into clearance, allowing the study of the clunk. A set of non-linear differential equations describe the experiment and the applicable dry friction coefficients are experimentally found. Various test conditions (corresponding to no impacts, and single-sided or double-sided impacts) are successfully simulated. Numerical and experimental time histories compare well. INTRODUCTION Clunk is an impulsive response in the powertrain which is typically initiated by a sharp torque reversal such as from throttle (and engine torque) tip-in or tip-out [1]. The torsional vibration response at the lowest mode, or ‘driveline shuffle or surging’, causes the gears to impact after they pass through the clearance between their backlashes. The oscillation frequency is under 10 Hz and varies with transmission ratio [1-2]. Also, in relation to the size of the torque step there is a corresponding mean change in rigid body motion [2]. Some results (at the vehicle and drivetrain level) are available for clunk experiments [1-3]. Likewise simulations have been used to study clunk, e.g. [4] where the combined effect of transients in engine torque, braking and road load are considered. In this paper we report a test device reduced in complexity so as to isolate clunk from additional non-linear sources, however friction needed to be considered. Faced with results from any of these experiments, the nature of non-linear response may be difficult to fully understand. Correct diagnosis usually requires numerous tests. Benefits in terms of time and cost reduction could be realized by using analytical studies. We thus apply a non-linear mathematical simulation technique [2] to understand the physics related to the impact event. Any simulation model is limited by its simplifying assumptions so the bench experiment needs to be designed and conducted. In this paper, the steps involved in experiment, rig and model development and findings are discussed. The laboratory experiment helps to refine and then correlate the mathematical model with measured results. DEVELOPMENT OF AN EXPERIMENT FOR TRANSIENT The driveline set-up is shown in Figure 1 and includes driveshaft, rear axle and axle shafts. The axle flanges are rigidly attached to the test-bed. The front end of the driveshaft is connected to the torsion bar and supported by a bearing. The torque is measured via a Wheatstone bridge at each end of the driveshaft and at each axle. The length and section of the torsion bar were selected to give a driveline shuffle frequency of around 2-3 Hz. This is similar to 1 gear in a typical vehicle. The significant clearances in the set-up are in the pinion/ring gear mesh and in the differential gears. The torque is applied to the set-up via masses suspended on an electromagnet attached to the end of the torsion bar (Figure 2). The mass of the torsion bar and magnet provides a mean (static) load, and suspended masses provide an applied load, , giving total preload, s T1