pressure between a control pump and a pair of fluid chambers defined in the servopiston a control piston connected through a control lever pivotably supported by a pivot pin to the spool and the servopiston at least two helical compression springs for biasing the control piston and controlling torque and flow so as to approximate same to a constant torque curve a floating spring seat interposed between the helical compression springs and a guide rod for guiding the floating spring seat and accommodated in a bore formed in the housing at a base portion thereof to define a pressure chamber pressurized by a biasing spring the pressure chamber being communicated through a conduit to a discharge passage of the fixed displacement hydraulic pump application_number US 69847085 A description referring to fig1 reference numerals 1 2 3 and 4 designate first and second variable displacement hydraulic pumps a fixed displacement hydraulic pump and a control pump respectively which are driven by a common prime mover e discharge conduits 1a 2a and 3a of the first and second variable hydraulic pumps 1 and 2 and the fixed hydraulic pump 3 are connected through operating valves 5 6 and 7 to hydraulic equipments 8 9 and 10 a discharge conduit 4a of the control pump 4 is connected to a displacement control system a the displacement control system a comprises a servopiston 30 accommodated in a housing 31 and connected through a rod or actuator arm 32 to swash plates 1b and.