A piston pump includes a first piston pump including a first piston and a first pump chamber, a second piston pump including a second piston and a second pump chamber, the second piston operating to follow the first piston, a pressure release passage formed in communication with the second pump cham
A piston pump includes a first piston pump including a first piston and a first pump chamber, a second piston pump including a second piston and a second pump chamber, the second piston operating to follow the first piston, a pressure release passage formed in communication with the second pump chamber, and a relief valve provided at the pressure release passage and defining an upper limit discharge pressure of the second piston pump. When a discharge pressure of the second piston pump is equal to or smaller than the upper limit, a discharge fluid of the second piston pump is added to that of the first piston pump so as to flow into an outlet port. When the discharge pressure exceeds the upper limit, the discharge fluid is released to the pressure release passage and the discharge fluid of the first piston pump only flows to the outlet port.
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The invention claimed is: 1. A piston pump comprising: a first piston pump including a first piston and a first pump chamber; a drive apparatus for driving the first piston pump; a second piston pump including a second piston and a second pump chamber having an annular shape to surround the first p
The invention claimed is: 1. A piston pump comprising: a first piston pump including a first piston and a first pump chamber; a drive apparatus for driving the first piston pump; a second piston pump including a second piston and a second pump chamber having an annular shape to surround the first piston, the second piston operating to follow an operation of the first piston; a pressure release passage formed in communication with the second pump chamber; a relief valve provided at the pressure release passage and defining an upper limit discharge pressure of the second piston pump; wherein when a discharge pressure of the second piston pump is equal to or smaller than the upper limit discharge pressure, a discharge fluid of the second piston pump is added to a discharge fluid of the first piston pump so as to flow into an outlet port, and when the discharge pressure of the second piston pump exceeds the upper limit discharge pressure, the discharge fluid of the second piston pump is released to the pressure release passage through the relief valve and the discharge fluid of the first piston pump only flows to the outlet port; and when a volume of the first pump chamber decreases during movement of the first piston, a volume of the second pump chamber decreases in a state where an entrance of the second pump chamber is closed. 2. A piston pump comprising: a first piston pump including a first pump chamber defined by a first cylinder and a first piston of which an end portion faces the first pump chamber; a drive apparatus for driving the first piston pump; a second piston pump including a second pump chamber defined by a second cylinder and a second piston of which an end portion faces the second pump chamber, the second pump chamber having an annular shape and formed at an outer peripheral side of the first piston, the second piston having an annular shape and arranged at the outer peripheral side of the first piston, the second piston performing a reciprocating movement by following an operation of the first piston; a pressure release passage formed in connection with the second pump chamber; a relief valve provided at the pressure release passage and defining an upper limit discharge pressure of the second piston pump; the first piston pump and the second piston pump including inlet valves respectively, an exit of the second pump chamber being connected to the first pump chamber via the inlet valve of the first piston pump, the pressure release passage being connected to the exit of the second pump chamber; and when a volume of the first pump chamber decreases during movement of the first piston, a volume of the second pump chamber decreases in a state where an entrance of the second pump chamber is closed. 3. A piston pump according to claim 2, wherein the fluid from the second piston pump is sent to the first pump chamber via the inlet valve of the first piston pump and flows into an outlet port via an outlet valve of the first piston pump. 4. A piston pump according to claim 2, wherein the second piston is arranged at the outer peripheral side of the first piston so as to be axially movable relative thereto, the first piston including a stopper at an outer periphery for restricting an axial relative movement between the second piston and the first piston to be equal to or smaller than a stroke of the first piston, and a sliding resistance between the second piston and the second cylinder is larger than a sliding resistance between the second piston and the first piston. 5. A piston pump according to claim 4, wherein the first piston includes a flange at the outer periphery for pressing the second piston in a compression process. 6. A piston pump according to claim 5, wherein the inlet valve of the second piston pump is opened and closed by the flange to make contact with and separate from the second piston. 7. A piston pump according to claim 4, wherein an inner peripheral surface of the second piston and an outer peripheral surface of the first piston form a gap therebetween. 8. A piston pump according to claim 2, wherein the second piston includes a slit at an inner peripheral side by of which an exit of the second pump chamber is in communication with an inlet passage of the first piston pump. 9. A piston pump according to claim 2, wherein the fluid from the second pump chamber that is released by passing through the pressure release passage flows back to an entrance side of the second pump chamber. 10. A piston pump according to claim 2, wherein the pressure release passage and the relief valve are arranged inside of the first piston. 11. A piston pump according to claim 2, wherein the second piston is formed by a cup seal made of resin or rubber. 12. A piston pump according to claim 1, wherein the relief valve includes a first valve portion that is opened when the discharge pressure of the second piston pump exceeds the upper limit discharge pressure and a second valve portion that is opened at a lower pressure than a pressure for opening the first valve portion, and a fluid passing through the first valve portion in the open state causes the second valve portion to open, the second valve portion being constituted to cause an opening degree of the first valve portion to increase. 13. A piston pump according to claim 12, wherein the relief valve includes a valve body having a first pressure receiving surface and a second pressure receiving surface having a larger area than an area of the first pressure receiving surface, a valve seat with which and from which the valve body makes contact and separates, and a spring for biasing the valve body in a closed direction, the first valve portion being constituted by the valve body and the valve seat, the second valve portion being constituted by the valve body and an inner peripheral surface of a bore into which the valve body is inserted, the first pressure receiving surface receiving the discharge pressure of the second piston pump in a case where the first and second valve portions are each closed, the second pressure receiving surface receiving a pressure of the fluid that passes through the first valve portion in a case where the first valve portion is opened, the valve body of which the first valve portion and the second valve portion are constituted being the same valve body. 14. A piston pump according to claim 1, wherein a first cylinder into which the first piston is inserted and a second cylinder into which the second piston is inserted are integrally formed at the cylinder member. 15. A piston pump according to claim 1, further comprising a feed pump including the second piston, a third pump chamber provided on an opposite side of the second pump chamber relative to the second piston, an inlet valve for opening and closing an entrance of the third pump chamber, an outlet valve for opening and closing an exit of the third pump chamber, wherein the feed pump pressurizes a fluid in the third pump chamber to be forcedly sent to the first pump chamber in a case where the first and second piston pumps are in a suction process in which volumes of the first pump chamber and the second pump chamber increase along with movements of the first piston and the second piston, respectively, the third pump chamber being defined by the second piston, and a volume of the third pump chamber decreasing when a volume of the second pump chamber increases along with a movement of the second piston. 16. A piston pump according to claim 15, wherein the relief valve causes the discharge fluid from the second pump chamber that is released by passing through the pressure release passage to flow into the third pump chamber. 17. A piston pump according to claim 15, wherein a second cylinder is constituted by a movable cylinder arranged at an outer peripheral side of the second piston so as to be axially movable and into which the second piston is inserted so as to be axially slidable, the second piston of which an axial one end faces the second pump chamber while the axial other end faces the third pump chamber, and the movable cylinder and a case for accommodating the first and second pistons constitute inlet valves arranged at an entrance of the second pump chamber and at an entrance of the third pump chamber respectively, the inlet valves being alternately opened and closed by means of an axial reciprocating movement of the movable cylinder, the first and second pistons constituting a valve portion arranged at an exit of the second pump chamber and an outlet valve arranged at an exit of the third pump chamber, the valve portion and the outlet valve being alternately opened and closed by means of an axial relative movement between the first and second pistons, the outlet valve being opened and the valve portion being closed so that the fluid within the third pump chamber is sent to the first pump chamber in a case where the first and second piston pumps are in the suction process. 18. A piston pump according to claim 17, wherein a fixed cylinder is provided at an outer peripheral side of the movable cylinder, the fixed cylinder including cutout portions by way of which the second and third pump chambers are in communication with the inlet port respectively, and a movement of the movable cylinder is guided by the fixed cylinder. 19. A piston pump according to claim 17, wherein a valve portion of each of the inlet valves provided at the entrance of each of the second pump chamber and the third pump chamber are opened and closed by means of the movable cylinder to make contact with and separate from a seal member provided at the case. 20. A piston pump according to claim 2, wherein the second piston pump supplies fluid, sent to the second pump chamber via the inlet valve of the second piston pump, to the first pump chamber via the inlet valve of the first piston pump.
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