Drive transmissions

In a highway vehicle as defined herein comprising ground wheels, a prime mover, and a drive train connecting the prime mover to the ground wheels, a gearbox forming part of said drive train and comprising:

• an input shaft;
• an output shaft;
• an input shaft gear on the input shaft;
• an output shaft gear on the output shaft;
• at least two layshafts having corresponding constant mesh layshaft gears;
• clutch means for each of said layshafts for selective transmission of drive through the layshafts;
• and
• a gearbox housing;
• said layshaft gears comprising layshaft change speed input gears and layshaft output gears with one layshaft input gear and one layshaft output gear for each layshaft;
• the layshaft input gear of each of said layshafts meshing directly with said input shaft gear and the layshaft output gear of each of said layshafts meshing directly with said output shaft gear;
• one layshaft and its corresponding clutch means and layshaft gears being provided for each layshaft-transmitted transmission ratio of the gearbox, and said clutch means being located within said housing at one end thereof and at the ends of the layshafts, the clutch means each comprising an assembly of interleaved clutch plates whereby automatic or semi-automatic power shifting between the ratios provided by said gearbox can be provided;
• liquid coolant supply means to supply liquid coolant to said clutch means and coolant control means being provided to control the supply of liquid coolant to said clutch means whereby in use the flow of coolant to at least one of said clutch means may be greater than the flow to another thereof according to the usage of the clutch means, said one of said clutch means being associated with a layshaft providing a lower transmission ratio for lower output shaft speed than said other of said clutch means;
• and
• clutch control means operable to cause at least one of said clutch means to be partially engaged in addition to the clutch means engaged for driving purposes at a given time, to provide a vehicle retarder.