Pilot actuated override mechanism for holding valve

A holding valve assembly for use in a hydraulic operating system, the assembly having a valve port adapted to receive fluid from and deliver fluid to a control valve, a cylinder port adapted to receive fluid from and deliver fluid to a fluid actuator, a pilot port adapted to receive pilot fluid from and deliver pilot fluid to the control valve, and a tank port adapted to deliver fluid to a reservoir, comprising:

• a primary check valve assembly for permitting fluid to flow in a first direction from the valve port to the cylinder port of the holding valve assembly;
• a poppet valve assembly for permitting fluid to flow in a second direction from the cylinder port to the valve port of the holding valve assembly, said poppet valve assembly having a closed position for preventing fluid from flowing therethrough, an open position for permitting fluid to flow therethrough, and intermediate positions for modulating the rate at which fluid flows therethrough, the selection of which being determined by pilot fluid pressure communicated through a passage in the holding valve assembly from the holding valve pilot port to said poppet valve assembly;
• a piston assembly which is moved by pilot fluid pressure, communicated through a passage connecting said piston assembly to the holding valve pilot port, to engage said poppet valve assembly and move said poppet valve assembly to an open position;
• a secondary check valve assembly which closes said passage connecting said piston assembly to the holding valve pilot port, so that said piston assembly retains said poppet valve assembly in the open position;
• a valve which closes and opens a fluid passage connecting said piston assembly to the holding valve tank port, so that pilot fluid pressure is selectively applied to said piston assembly;
• and,
• an orifice device which restricts the flow of pilot fluid through said passage in the holding valve assembly connecting said piston assembly to the holding valve pilot port.

A holding valve assembly adapted for use in a hydraulic operating system comprising:

• a body having a valve port adapted to communicate with a control valve, a cylinder port adapted to communicate with a fluid actuator, a pilot port adapted to communicate with the control valve, and a tank port adapted to communicate with a reservoir;
• means for permitting the one-way flow of fluid from said valve port to said cylinder port;
• means for selectively permitting the one-way flow of fluid from said cylinder port to said valve port, said means for selectively permitting including a poppet valve assembly disposed within said body and movable between a closed position, wherein the flow of fluid from said cylinder port to said valve port is prevented, and an opened position, wherein the flow of fluid from said cylinder port to said valve port is permitted, the operation of said poppet valve assembly being normally controlled in response to the pressure of the fluid at said pilot port;
• and
• means for selectively maintaining said poppet valve assembly in said opened position regardless of the pressure of the fluid at said pilot port.