Apparatus for changing the traveling speed of a hydraulically actuated working machine, said apparatus comprising:
- a first hydraulic traveling motor,
- a first traveling motor capacity control cylinder connected to said first hydraulic traveling motor,
- a first traveling motor capacity control valve connected to control said first traveling motor capacity control cylinder,
- a second hydraulic traveling motor,
- a second traveling motor capacity control cylinder connected to said second hydraulic traveling motor,
- a second traveling motor capacity control valve connected to control said second traveling motor capacity control cylinder, each of said first and second traveling motor capacity control valves having a pilot port,
- a hydraulic pump for supplying hydraulic oil to said first hydraulic traveling motor and to said second hydraulic traveling motor to provide driving pressure in said first and second hydraulic traveling motors,
- a first operation valve connected between said hydraulic pump and said first hydraulic traveling motor for determining the rate and direction of rotation of said first hydraulic traveling motor,
- a second operation valve connected between said hydraulic pump and said second hydraulic traveling motor for determining the rate and direction of rotation of said second hydraulic traveling motor,
- a source of hydraulic control pressure,
- a first control pressure changeover valve for controlling the supply of hydraulic pressure from said source to the pilot port of said first and second traveling motor capacity control valves for changing the capacity of said first and second hydraulic traveling motors over two stages in accordance with the operation of the first control pressure changeover valve,
- a capacity control element for controlling the capacity of said hydraulic pump,
- a load sensing valve having first and second pilot chambers and a spring for providing a differential pressure setting force, said load sensing valve being connected to said capacity control element for controlling the capacity of said hydraulic pump,
- a first conduit for applying to said first pilot chamber the pressure of the hydraulic oil discharged by said hydraulic pump,
- a second circuit for applying to said second pilot chamber a pressure representative of the driving pressure in said first and second hydraulic traveling motors,
- an adjusting device capable of changing the differential pressure setting force of said spring, whereby said hydraulic pump is controlled in accordance with the balance of the thrust forces produced in said first and second pilot chambers and the differential pressure setting force of said spring,
- a second control pressure changeover valve for supplying hydraulic control pressure from said source to said adjusting device to vary the differential pressure setting force of said spring,
- a speed changeover switch having at least a first stage speed position, a second stage speed position, and a third stage speed position, and
- a controller responsive to the position of said speed changeover switch for providing a first changeover command to said first control pressure changeover valve to vary the capacity of said first and second hydraulic traveling motors in accordance with said first changeover command, and for providing a second changeover valve to vary the differential pressure setting force of said spring in accordance with said second changeover command, whereby the traveling speed of said hydraulically actuated working machine can be changed over at least three stages corresponding to the first, second and third stage speed positions of said speed changeover switch while for each of said first, second and third stage speed positions of said speed changeover switch said first traveling motor is operated at a rate corresponding to the amount of operation of said first operation valve and said second traveling motor is operated at a rate corresponding to the amount of operation of said second operation valve, to thereby provide an optimum traveling speed according to the condition of the work while improving simultaneous operation characteristic during fine control of the working machine.