Variable displacement pump

1. A variable displacement pump, comprising:

a housing;

a cam ring arranged in the housing, the cam ring being swingable in the housing;

a seal member provided in a chamber formed between the housing and the cam ring, the seal member dividing the chamber into two portions that defines first and second fluid-pressure chambers;

a rotor rotating within the cam ring and formed with slots, the rotor having an axis offset with respect to an axis of the cam ring;

a plurality of vanes inserted retractably into the slots;

a biasing device arranged in the second fluid-pressure chamber, the biasing device biasing the cam ring to the first fluid-pressure chamber in the direction that increases volumes of pump chambers defined between the cam ring, the rotor, and the vanes;

an orifice provided to a discharge passage, the discharge passage supplying to a hydraulic device a fluid discharged from a discharge port;

a control valve operated by a pressure difference between upstream and downstream sides of the orifice, the control valve comprising a spool slidably arranged in a valve hole, the control valve controlling, in accordance with a slide position of the spool, a pressure of the first fluid-pressure chamber to oscillate the cam ring for variable control of a flow rate of the fluid, the control valve being formed with a high-pressure chamber for introducing therein a high-pressure fluid on the upstream side of the orifice, a pressure chamber for introducing therein a pressure fluid on the downstream side of the orifice, a low-pressure chamber arranged between the high-pressure and pressure chambers for introducing therein a low-pressure fluid, and a communication passage for providing fluid communication between one of the high-pressure and low-pressure chambers and the first fluid-pressure chamber; and

a communication device which provides fluid communication between the low-pressure and high-pressure chambers through the communication passage when the spool carries out selective switching between the high-pressure and low-pressure chambers to supply the fluid to the communication passage,

wherein the second fluid-pressure chamber introduces therein a low-pressure fluid at least under a condition that the spool of the control valve is maximally displaced so as to provide fluid communication between the low-pressure chamber and the first fluid-pressure chamber,

wherein the communication device comprises a first recessed groove formed in one end of a valve element of the spool and a second recessed groove formed in the other end of the valve element,

wherein a length of the first recessed groove in an axial direction of the valve element is longer than a length of the second recessed groove in the axial direction of the valve element, the first recessed groove being on a side of the low-pressure chamber of the control valve, and the second recessed groove being on a side of the high-pressure chamber of the control valve,

wherein the first recessed groove is located to partly face an open end of the communication passage when the second recessed groove begins to open.

2. The variable displacement pump as claimed in claim 1 , wherein the first and second recessed grooves are formed in respective outer-peripheral surfaces of the valve element of the spool.

3. The variable displacement pump as claimed in claim 2 , wherein the spool comprises a land between the first and second recessed grooves.

4. The variable displacement pump as claimed in claim 2 , wherein at least one of the first and second recessed grooves is shaped in a taper way to be larger to the side of one of the high-pressure and low-pressure chambers of the control valve.

5. The variable displacement pump as claimed in claim 2 , wherein at least one of the first and second recessed grooves is shaped stepwise to be larger to the side of one of the high-pressure and low-pressure chambers of the control valve.

6. The variable displacement pump as claimed in claim 1 , wherein the communication device has a transitional state in which the first and second recessed grooves communicate with the communication passage at a time while the spool carries out the selective switching.

7. The variable displacement pump as claimed in claim 6 , wherein the low-pressure fluid introduced in the second fluid-pressure chamber is supplied from a fluid tank without pressurizing.

8. A variable displacement pump, comprising:

a housing;

a cam ring arranged in the housing, the cam ring being swingable in the housing;

a seal member provided in a chamber formed between the housing and the cam ring, the seal member dividing the chamber into two portions that defines first and second fluid-pressure chambers;

a rotor rotating within the cam ring and formed with slots, the rotor having an axis offset with respect to an axis of the cam ring;

a plurality of vanes inserted retractably into the slots;

first means, arranged in the second fluid-pressure chamber, for biasing the cam ring to the first fluid-pressure chamber in the direction that increases volumes of pump chambers defined between the cam ring, the rotor, and the vanes;

an orifice provided to a discharge passage, the discharge passage supplying to a hydraulic device a fluid discharged from a discharge port;

a control valve operated by a pressure difference between upstream and downstream sides of the orifice, the control valve comprising a spool slidably arranged in a valve hole, the control valve controlling, in accordance with a slide position of the spool, a pressure of the first fluid-pressure chamber to oscillate the cam ring for variable control of a flow rate of the fluid, the control valve being formed with a high-pressure chamber for introducing therein a high-pressure fluid on the upstream side of the orifice, a pressure chamber for introducing therein a pressure fluid on the downstream side of the orifice, a low-pressure chamber arranged between the high-pressure and pressure chambers for introducing therein a low-pressure fluid, and a communication passage for providing fluid communication between one of the high-pressure and low-pressure chambers and the first fluid-pressure chamber; and

second means for providing fluid communication between the low-pressure and high-pressure chambers through the communication passage of the control valve when the spool carries out selective switching between the high-pressure and low-pressure chambers to supply the fluid to the communication passage,

wherein the second fluid-pressure chamber introduces therein a low-pressure fluid at least under a condition that the spool of the control valve is maximally displaced so as to provide fluid communication between the low-pressure chamber and the first fluid-pressure chamber,

wherein the second means for providing fluid communication comprises a first recessed groove formed in one end of a valve element of the spool and a second recessed groove formed in the other end of the valve element,

wherein a length of the first recessed groove in an axial direction of the valve element is longer than a length of the second recessed groove in the axial direction of the valve element, the first recessed groove being on a side of the low-pressure chamber of the control valve, and the second recessed groove being on a side of the high-pressure chamber of the control valve,

wherein the first recessed groove is located to partly face an open end of the communication passage when the second recessed groove begins to open.

9. The variable displacement pump as claimed in claim 8 , wherein the second means for providing fluid communication has a transitional state in which the first and second recessed grooves communicate with the communication passage at a time while the spool carries out the selective switching.

10. A variable displacement pump, comprising:

a housing;

a cam ring arranged in the housing, the cam ring being swingable in the housing;

a seal member provided in a chamber formed between the housing and the cam ring, the seal member dividing the chamber into two portions that defines first and second fluid-pressure chambers, the second fluid-pressure chamber constantly introducing therein a low-pressure fluid;

a rotor rotating within the cam ring and formed with slots, the rotor having an axis offset with respect to an axis of the cam ring;

a plurality of vanes inserted retractably into the slots;

a biasing device arranged in the second fluid-pressure chamber, the biasing device biasing the cam ring to the first fluid-pressure chamber in the direction that increases volumes of pump chambers defined between the cam ring, the rotor, and the vanes;

an orifice provided to a discharge passage, the discharge passage supplying to a hydraulic device a fluid discharged from a discharge port;

a control valve operated by a pressure difference between upstream and downstream sides of the orifice, the control valve comprising a spool slidably arranged in a valve hole, the control valve controlling, in accordance with a slide position of the spool, a pressure of the first fluid-pressure chamber to oscillate the cam ring for variable control of a flow rate of the fluid, the control valve being formed with a high-pressure chamber for introducing therein a high-pressure fluid on the upstream side of the orifice, a pressure chamber for introducing therein a pressure fluid on the downstream side of the orifice, a low-pressure chamber arranged between the high-pressure pressure and pressure chambers for introducing therein a low-pressure fluid, and a communication passage for providing fluid communication between one of the high-pressure and low-pressure chambers and the first fluid-pressure chamber; and a communication device which provides fluid communication between the low-pressure and high-pressure chambers through the communication passage when the spool carries out selective switching between the high-pressure and low-pressure chambers to supply the fluid to the communication passage,

wherein the communication device comprises a first recessed groove formed in one end of a valve element of the spool and a second recessed groove formed in the other end of the valve element,

wherein a length of the first recessed groove in an axial direction of the valve element is longer than a length of the second recessed groove in the axial direction of the valve element, the first recessed groove being on the side of a low-pressure chamber of the control valve, and the second recessed groove being on a side of the high-pressure chamber of the control valve,

wherein the first recessed groove is located to partly face an open end of the communication passage when the second recessed groove begins to open.

11. The variable displacement pump as claimed in claim 10 , wherein the communication device has a transitional state in which the first and second recessed grooves communicate with the communication passage at a time while the spool carries out the selective switching.