Simplified fluid circuit for shifting a transmission

Claim 1

A fluid circuit for shifting a multispeed transmission having a first and second group of fluid pressure actuated drive establishing devices, wherein at least one of said first group of devices is pressurized to select the drive ratio through said transmission while at least one of said second group of devices is pressurized to select the drive direction through said transmission, comprising: a source of pressurized fluid, a speed delector valve receiving pressurized fluid from said source through a first flow path and having means moveable to direct said fluid to any selected one of said first group of devices, a directional selector valve having an inlet for receiving pressurized fluid and having valve means moveable to direct fluid from said inlet to any selected one of said second group of devices and being moveable to a neutral setting, a pressure modulating valve having an inlet communicated with said first flow path, and having flow controlling means moveable to control the pressure of fluid in said first flow path and further having modulating means shiftable to exert a changing force on said flow controlling means for gradually raising said fluid pressure in said flow path, means forming a constricted fluid passage from said first flow path to said modulating means of said pressure modulating valve for gradually shifting said modulating means in response to rising fluid pressure in said first flow path, and a safety differential valve having an inlet communicated with said first flow path and having an outlet communicated with said inlet of said directional selector valve and having a valve member moveable to control fluid flow from said inlet to said outlet, said safety differential valve having resilient means for uring said valve member towards a first position at which flow from said inlet to said outlet is blocked and having means for causing fluid pressure from said inlet to urge said valve member away from said first position towards a fluid metering position at which fluid may pass from said inlet to said outlet and having means for causing fluid pressure from said outlet to supplement the force of said resilient means on said valve member when said valve member is at said metering position whereby the pressure at said outlet is caused to have a predetermined relationship to the pressure at said inlet, said safety differential valve further having means on said valve member thereof for relieving said fluid pressure at said outlet when said valve member shifts from said metering position towards said first position in response to a predetermined pressure drop at said inlet and having reset means on said valve member for relieving said fluid pressure on said modulating means when said valve member shifts in the opposite direction past said metering position in response to a subsequent pressure rise at said inlet following said relief of said pressure at said outlet.