Variable displacement vane pump

1. A variable displacement vane pump comprising:

a pump body that includes a front body having a receiving space defined therein and a rear body attached to the front body to close the receiving space;

a drive shaft that passes through the pump body and rotatably supported in the same;

a rotor that is mounted on an outer cylindrical surface of the drive shaft and received in the receiving space;

a plurality of vanes that are retractably and projectably received in a plurality of slots formed in the rotor in a manner to extend radially outward;

a cam ring that is swingably arranged about the rotor to form a plurality of pump chambers, each pump chamber defined by adjacent two vanes, the rotor, and a part of the cam ring;

a pressure plate that is arranged in a manner to be put between inside surfaces of the rotor and cam ring and a bottom surface of the receiving space and biased toward and pressed against the inside surface of the rotor to slidably contact with the rotor by a pump discharge pressure led from the bottom surface of the receiving space;

first and second fluid pressure chambers that are formed around the cam ring to control an eccentric amount of the cam ring;

a pressure control means that controls the pressure in the first or second fluid pressure chamber; and

an arrangement that includes:

an intake port that is provided by one of an inside surface of the rear body and an inside surface of the pressure plate facing the rotor and opened to a range where each pump chamber increases a volume;

a discharge port that is provided by the inside surface of the pressure plate and opened to a range where each pump chamber decreases the volume;

an axially extending through opening that is formed in the pressure plate for receiving the drive shaft;

a back pressure groove that is formed on the inside surface of the pressure plate at an area that slidably contacts the rotor to feed a pressurized fluid to bottom portions of the slots;

a seal surface that is formed between the back pressure groove and the axially extending through opening and slidably contacts with the inside surface of the rotor, and

a circumferentially extending lubricating groove that is formed on the seal surface,

wherein a radial width of the lubricating groove is set to a range from 10% to 25% of a radial width of the seal surface, and a distance from a center of the radial width of the lubricating groove to an inside cylindrical surface of the axially extending through opening is set to a range from 24% to 70% of the radial width of the seal surface.

2. A variable displacement vane pump as claimed in claim 1 , wherein a depth of the lubricating groove is set to 25% or more of the radial width of the lubricating groove.

3. A variable displacement vane pump as claimed in claim 1 , wherein the radial width of the lubricating groove is set to a range from 15% to 20% of the radial width of the seal surface.

4. A variable displacement vane pump as claimed in claim 3 , wherein the distance from the center of the radial width of the lubricating groove to the inside cylindrical surface of the axially extending through opening is set to a range from 30% to 45% of the radial width of the seal surface.

5. A variable displacement vane pump as claimed in claim 1 , wherein the distance from the center of the radial width of the lubricating groove to the inside cylindrical surface of the axially extending through opening is set to a range from 30% to 45% of the radial width of the seal surface.