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Excessive pressure build up in the lubrication pasages is prevented by the lube relief valve which operates at a predetermined pressure. Oil passes through the relief valve spill port and is conducted into the main drive shaft to lubricate the running gear. It then passes through to the relief valve and acts upon the solenoid valve which is closed. Oil is drawn from the sump through the suction filter by the reciprocating pump and delivered to the pressure filter. A uni-directional clutch (UDC) or freewheel connects the input and output shafts. Carried on an extension of the sunwheel, a double sided cone clutch engages with a stationary brake ring or with the outside surface of the annulus. The planet carrier is attached to the input shaft. The overdrive gears are epicyclic (planetary) and consist of a sunwheel, three planet gears and an internally toothed ring gear or annulus. An inhibitor switch is invariably fitted in the electrical circuit to prevent engagement of overdrive in reverse and some or all of the indirect gears. 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.