Variable displacement piston pump

A variable displacement piston pump of a type which varies an output flow by controlling a hydraulic pressure to displace a variable element against a spring force, said pump comprising:

• pressure detecting means for generating an electric signal output corresponding to a pump output pressure;
• a proportional electro-hydraulic control valve means for communicating a pressure chamber of the variable element with a tank or a pump outlet port with an opening proportional to an input current;
• control amplifier means for receiving an externally applied pressure setting signal and the electric output signal from said pressure detecting means to control the input signal to said proportional electro-hydraulic control valve means in such a manner that said pump output pressure is introduced into the pressure chamber of said variable element to cut off said pump when a pressure detected by said pressure detecting means attains a setting pressure value;
• a safety valve for supplying said output pressure to said variable element to cut of said pump when said pump output pressure reaches an upper limit value predetermined by a pressure regulating spring;
• and
• said safety valve including pressure adjusting means responsive to said pump output pressure regulating spring at a value higher than said output pressure by a predetermined pressure value.

A variable displacement piston pump of a type which varies an output flow by controlling a hydraulic pressure to displace a variable element against a spring force, said pump comprising:

• pressure detecting means for generating an electric signal output corresponding to a pump output pressure;
• a proportional electro-hydraulic control valve means for communicating a pressure chamber of the variable element with a tank or a pump outlet port with an opening proportional to an input current;
• and
• control amplifier means for receiving an externally applied pressure setting signal and the electric output signal from said pressure detecting means to control the input signal to said proportional electro-hydraulic control valve means in such a manner that said pump output pressure is introduced into the pressure chamber of said variable element to cut off said pump when a pressure detected by said pressure detecting means attains a setting pressure value, said control valve means including supply means for supplying the output pressure to the variable element to cut off said pump in the absence of said input current.