Hydraulic control system for load supporting hydraulic motors

Claim 1

A load supporting fluid system comprising: load supporting means including a fluid motor for raising and supporting a load, said motor having a load supporting end including a fluid chamber; a source of pressurized fluid for operating said motor; means including a pilot operated directional control valve for selectively communicating pressurized fluid between said source and motor for raising, lowering and positioning said load supporting means; said directional control valve comprising a housing, a valve spool mounted in a bore in said housing for communicating with inlet and outlet ports for defining raise, lower, and hold positions; load check valve means disposed between said control valve and said motor for permitting free flow of fluid from said control valve to said motor and for normally blocking flow of fluid from said motor to said control valve; said check valve means having a control chamber including restriction means communicating with said load supporting chamber, and said check valve means being responsive to fluid pressure in said load supporting chamber to bias said valve means to said fluid blocking position; and vent valve means including a valve spool mounted in a bore coaxially with said directional control valve spool at one end thereof and engageable by abutment thereby upon shifting of said directional control valve spool to said lower position to vent said control chamber to effect opening of said check valve to permit flow of fluid from said load supporting chamber to effect lowering of said load supporting means, said vent valve means comprises a separate housing, means detachably securing said vent valve housing to said control valve housing, and said vent valve spool comprises a spring biased closed cylindrical spool having an annular groove for communicating between a first port in communication with said control chamber and a second port in communication with a sump, wherein said directional control spool is pilot operated by means of pressurized pilot fluid selectively directed to either end thereof, said vent valve means being exposed to said pilot fluid and said vent valve spool includes pressure balancing means for preventing actuation thereof by said pilot fluid.