The overdrive is operated by an electric solenoid controlled by a switch, usually mounted on the steering column or fascia panel. Thus the gear train is locked and overrun or reverse torque is taken by the cone clutch, without which, the UDC would give a freewheeling condition. The cone clutch is held rearwards by spring pressure exerted through a thrust ring and ball race, loading the inner friction lining of the clutch in contact with the outside diameter of the annulus on the output shaft (the loading is further increased by the reverse thrust of the helical sunwheel). The forward drive is transmitted directly through the uni-directional clutch (UDC). The UDC enables the output member to rotate faster than the input member. Since the planet carrier is splined to the input shaft and driven by it, the planet wheels orbit round the stationary sunwheel and in so doing, rotate the annulus and output shaft at a speed greater than that of the carrier and input shaft. As the cone clutch is attached to the sunwheel, both come to rest and the sunwheel becomes the reaction member for the planetary train. HOW AN OVERDRIVE WORKSThe cone clutch is moved forward by hydraulic pressure in the operating cylinders, so that the outer friction lining of the clutch comes into contact with the stationary brake ring.
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