Patent ID: 7146811

Claim:
A hydrostatic continuously variable transmission in which a hydraulic pump and a hydraulic motor are connected through a hydraulic closed circuit and the capacity of at least one of said hydraulic pump and hydraulic motor is varied to change the relative rotating speed of an input of said hydraulic pump and an output of said hydraulic motor, the transmission comprising: a valve spool movably provided in a spool hole which extends axially in a support shaft which rotatably holds said hydraulic pump and hydraulic motor; said support shaft including: a high pressure clutch oil path connected to a high pressure oil path of said closed circuit and a high pressure clutch port open to said spool hole; a low pressure clutch oil path connected to a low pressure oil path of said hydraulic closed circuit and having a low pressure clutch port open to said spool hole; and a communication groove between the outer periphery of said valve spool and the spool hole for enabling said valve spool to move between a clutch release position in which said high pressure clutch port and said low pressure clutch port are connected and a clutch engagement position in which an outer peripheral surface of said valve spool at least covers said high pressure clutch port; wherein said communication groove is connected to the outside of the spool hole through a main discharge oil path formed in said valve spool.