Patent ID: 8196401

Claim:
A hydraulic system for controlling a belt-driven conical-pulley transmission, said hydraulic system comprising: a pump that draws a working medium from a working medium tank and through a cooler return valve from a cooler return line, wherein the cooler return valve is a minimum pressure valve that closes a connection through the cooler return valve between the cooler return line and a pump inlet line while a pressure in the cooler return line remains below a predetermined minimum pressure, and that opens the connection through the cooler return valve between the cooler return line and the pump inlet line through the cooler return valve when the predetermined minimum pressure in the cooler return line is exceeded, wherein the cooler return valve includes a valve spool movably received within a housing and having an effective end area that is biased against a sealing edge within the housing when the valve spool is in a closed position against the sealing edge, wherein the valve spool includes a return pressure effective area that is pressurized with a pressure from the cooler return line against a biasing force acting on the valve spool, wherein the return pressure effective area includes a first incremental area that is in fluid communication with the cooler return line.