Transmission system in working vehicles

A speed change mechanism in a working vehicle which comprises two gears (40, 41) rotatably mounted on a transmission shaft (11) and two frictional clutches (45, 46) mounted on said transmission shaft for coupling said gears one at a time to said transmission shaft so as to perform a two-ratio speed change transmission, said speed change mechanism being characterized in:

• that one of said clutches (45) is formed into a fluid-actuated clutch which is actuated by an engagement between a first set of frictional elements (47, 48) under the action of fluid pressure applied to a first piston (49) and which is disactuated by the force of a return spring (50) acting upon said first piston, whereas the other of said clutches (46) is formed into a spring-actuated clutch which is actuated by an engagement of a second set of frictional elements (52, 53) under the action of a spring (56) and which is disactuated under the action of fluid pressure applied to a second piston (59), and
• that said transmission shaft (11) includes a single operating fluid passage (61) for supplying fluid to said first piston (49) and to said second piston (59), fluid pressure being applied to and drained from said first and second pistons respectively at a same time through said fluid passage so as to perform said two-ratio speed change transmission.