Hydraulic control system for a V-belt transmission

Claim 1

An automatic transmission for a vehicle having an internal combustion engine with a throttle opening, comprising: a continusouly-variable speed transmision mechanism including an input shaft, an output shaft, an input variable-pitch pulley having a fixed pulley flange fixed concentrically to said input shaft and a movable pulley flange mounted concentrically with and axially slidably on said input shaft, an output variable-pitch pulley having a fixed pulley flange fixed concentrically to said output shaft and a movable pulley flange mounted concentrically with and axially slidably on said output shaft, and endless belt interconnecting said input and said output variable-pitch pulleys and hydraulic servomotors provided for said input and said output variable-pitch pulleys respectively for pressing the corresponding movable pulley flanges toward the corresponding fixed pulley flanges; and a hydraulic control circuit including a pressure generating source, regulator valve means to control the fluid pressure of said pressure generating source and adapted to generate a line pressure corresponding to pressure signals fed thereto, throttle pressure valve means to control said line pressure and adapted to generate a pressure corresponding to the throttle opening of the internal-combustion engine which has a predetermined value, when the throttle opening is zero, and to feed the pressure to said regulator valve means as a throttle pressure signal, ratio detecting valve means formed in said output shaft to control said line pressure and adapted to generate a pressure corresponding to an axial displacement of said movable pulley flange of said output variable-pitch pulley mounted on one of said shafts and to feed the pressure to said regulator valve means as a reduction ratio pressure signal, means to supply pressurized fluid at the line pressure to the hydraulic servomotor provided for said output variable-pitch pulley, reduction ratio control valve means provided with an axially slidable spool and two pressure chambers formed therein at both axial ends of the spool and adapted to selectively control the supply of said line pressure to the hydraulic servomotor of said input variable-pitch pulley through the movement of said spool and having fluid passages connecting said two pressure chambers of said reduction ratio control valve means, respectively, to the output side of said throttle pressure valve means and two solenoid valves for individually regulating the respective pressures in said pressure chambers.