Hydraulic drive system and improved control valve assembly therefor

A control valve assembly for use in a hydraulic system adapted for use on a system having a drive-line operable to transmit driving torque to a drive axle, said hydraulic drive system including a pump-motor unit operable, in a pumping mode, to receive drive torque from said drive-line, and operable, in a motoring mode, to transmit drive torque to said drive-line;

• a high pressure accumulator in fluid communication with a first port of said pump-motor unit through said control valve assembly whereby, when said pump-motor unit is in said pumping mode, pressurized fluid is communicated from said first port to said high pressure accumulator, and when said pump-motor unit is in said motoring mode, pressurized fluid is communicated from said high pressure accumulator to said first port;
• said control valve assembly including a valve housing defining a poppet seat and a port in fluid communication with said accumulator, and being characterized by:
• (a) a main poppet member defining a control cavity, fluid pressure in said control cavity biasing said main poppet member toward said poppet seat;
• (b) substantially all flow between said high pressure accumulator and said first port of said pump-motor unit flowing past said main poppet member and said poppet seat in either said pumping mode or said motoring mode;
• (c) a pilot valve assembly having open and closed conditions and operable to control fluid communication from said control cavity to a source of low pressure, in response to an electrical input signal;
• (d) a step-orifice valve assembly operable to control fluid communication from said accumulator through said port, defined by said valve housing, to said control cavity, and defining a first, relatively larger flow orifice when fluid pressure in said control cavity is relatively high, and a second, relatively smaller flow orifice when fluid pressure in said control cavity is relatively low, to provide a relatively small flow to said pilot valve assembly.