Dual pilot counterbalance valve

Claim 1

A counterbalance valve for controlling the flow of high pressure fluid from a hydraulic motor which moves a load and is driven by a pump having an inlet and an outlet comprising a valve body, a fluid inlet in the body, a fluid outlet in the body, means for connecting the outlet of the motor to the fluid inlet whereby the motor supplies pressure fluid to the fluid inlet, a first fluid passage which connects the inlet and the outlet, a poppet bore, a seat, a poppet movable in the bore between a first setting in which the poppet engages the seat and blocks the first fluid passage and a second setting in which the poppet is spaced from the seat and cooperates with the seat to open the first fluid passage and form a control orifice through which pressure fluid flows from the inlet to the outlet, and spring means for biasing the poppet into the said first setting, characterized by a second fluid passage connected to the top of the poppet, a control port at the entrance to the second fluid passage, means for connecting the control port to pressure fluid in the fluid inlet, second means for connecting the control port to the fluid outlet, a pilot valve movable between a first position in which the control port is open to receive inlet pressure fluid, a second position in which the control port is blocked and a third position in which the control port is open to the fluid outlet, second spring means for biasing the pilot valve into the first position, a pilot port connected to the pilot valve, and means for connecting the pump outlet to the pilot port, wherein the pilot port receives pressure fluid from the pump to bias the pilot valve into the third position when the pump drives the motor and the counterbalance valve begins to operate to thereby open the control orifice to permit fluid to flow from the fluid inlet to the fluid outlet, and the pilot valve moves in response to changes in the pressure of the fluid in the pilot port alternatively between the first and third positions to modulate the opening of the control orifice and control the flow of pressure fluid from the hydraulic motor and the pilot valve remains in the second position and the control orifice maintains its opening when the pilot pressure is constant.