Experimental stability analysis of tractor drivelines affected by cold judder

Clutch judder is a friction-generated self-excited vibration affecting automotive drivelines. Stick-slip, negative gradient of the friction coefficient and geometric disturbances are the only causes currently identified in the scientific literature. Nevertheless, they are not sufficient to explain the vibration occurring in drivelines (typically of tractors) with dither (oscillating component) added to clutch hydraulic actuation signals. In this study, experimental data acquired on a specifically designed test bench are processed to show that this particular kind of judder is an instability due to parametrical excitation exerted by the actuation pressure signal. A minimal model with four degrees of freedom is proposed, able to fit the experimental data, to explain the onset of the parametrically excited vibration, and to identify which parameters have a major role in controlling this phenomenon. Stability charts obtained from the proposed model also represent a useful tool to prevent instability already in the design phase.