Keywords: cushion spring, dry clutch, sensitivity analysis.

The recent automated manual transmissions (AMT) systems are generally constituted by a dry or wet clutch assembly and a multi-speed gearbox, both equipped with electro-mechanical or electro-hydraulic actuators, which are driven by the transmission control unit (TCU). The car manufacturers are developing these transmissions because they could offer many advantages with respect to other automatic transmissions in terms of driving performance together with reductions of fuel consumption and pollutant emissions.

An appropriate clutch engagement control strategy is fundamental to obtain all the AMT advantages listed above. Therefore, the knowledge of the clutch engagement law is necessary to realize a good model of the nonlinear torque transmission vs. throwout bearing position characteristic to be implemented into the TCU. In [1], the influence of the cushion spring load-deflection characteristic on the torque transmissibility by the clutch is shown. A common shape of the cushion spring is a wavy thin steel disk, placed between the clutch facings. The nonlinearity of the torque transmission characteristic is mainly due to the nonlinearity characteristic of the cushion spring [1]. Therefore, in this paper a sensitivity analysis on the main factors about the shape of the cushion spring has been proposed to analyse how they influence the load-deflection curve and accordingly the torque transmissibility characteristic.

The analysis has been realized after a reverse engineering phase and after the validation of the finite element model with experimental tests. Four geometrical parameters, of the CAD model of the cushion spring, has been analyzed to obtain their influence on the load-deflection curve.

All curves have shown a non-linear trend. In fact in all curves there is a first part with a slight reaction rise. This part can be modified by changing the mix of parameter values, but it cannot be eliminated for the given cushion spring here described. Nevertheless, this behaviour with low slope of the first compression path is also required to dampen out some driveline oscillations, known as judder phenomenon.

Since the cushion spring accounts for the main contribution to the shape of the clutch transmissibility characteristic, i.e., the torque vs. throwout bearing position curve, it could be useful to develop the main shape of the cushion spring to be customized in order to match a polynomial shape for the load-deflection characteristics as from the TCU control designers' demands.

1

F. Vasca, L. Iannelli, A. Senatore, M. Taglialatela Scafati, "Modeling Torque Transmissibility for Automotive Dry Clutch Engagement", American Control Conference 2008, 306-311, 2008. doi:10.1109/ACC.2008.4586508

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