Variable displacement pump and power steering apparatus using the variable displacement pump

A variable displacement pump for supplying a working fluid to a steering apparatus for a vehicle, the variable displacement pump comprising:

• a pump housing including a pump element accommodating portion therein;
• a drive shaft rotatably supported by the pump housing;
• a cam ring disposed within the pump element accommodating portion to be eccentrically moveable with respect to an axis of the drive shaft;
• a pump element disposed on an inner circumferential side of the cam ring, the pump element being driven by the drive shaft to suck, pressurize and discharge the working fluid and vary a specific discharge amount of the working fluid which is a discharge flow rate per one rotation of the pump element in accordance with variation in an eccentric amount of the cam ring with respect to the axis of the drive shaft,
• a suction port that is disposed within the pump housing and opened to a suction area of the pump element;
• a discharge port that is disposed within the pump housing and opened to a discharge area of the pump element;
• and
• a solenoid that is drivingly controlled based on variation in an electric current amount to be supplied to the solenoid which is controlled in accordance with an operating condition of the vehicle, the solenoid being configured to control the eccentric amount of the cam ring,
• wherein the solenoid is configured such that a rate of change in the specific discharge amount relative to the electric current amount to be supplied to the solenoid under a condition that the specific discharge amount is smaller is reduced compared to a rate of change in the specific discharge amount relative to the electric current amount to be supplied to the solenoid under a condition that the specific discharge amount is larger,
• a variable orifice disposed in a discharge passage connected to the discharge port, the variable orifice being configured to variably control a flow passage sectional area of the discharge passage with an axial displacement of an armature of the solenoid, and
• a control valve disposed within the pump housing and configured to control a control pressure for controlling the eccentric amount of the cam ring based on of a pressure difference between an upstream side of the variable orifice and a downstream side of the variable orifice,
• wherein the variable orifice is configured such that variation in the flow passage sectional area with respect to the axial displacement of the armature of the solenoid has a nonlinear characteristic,
• wherein the variable orifice comprises a plurality of orifices disposed in the discharge passage, and a valve portion that varies an amount of overlap with the plurality of orifices in accordance with the axial displacement of the armature of the solenoid, and the plurality of orifices include at least two orifices which are different in diameter from each other,
• wherein the plurality of orifices are arranged along a direction of the axial displacement of the armature in order of sizes of diameters thereof, and the plurality of orifices are arranged such that as the valve portion is displaced relative to the plurality of orifices from a side of the orifice having a large diameter toward a side of the orifice having a small diameter, the flow passage sectional area of the variable orifice being decreased to thereby reduce the specific discharge amount.