A variable-displacement vane-pump has a rotor which is rotatably supported by a pump housing in coaxial relationship therewith, and a cam ring interposed between the rotor and the pump housing in eccentric relationship with the pump housing. The rotor has radially outwardly slidable vanes on its outer peripheral surface in contact with an inner surface of the cam ring. Two side plates are disposed on both sides of the vanes, thereby defining pump chambers between every adjacent vanes. An arc-shaped intake port and an arc-shaped exhaust port are formed in one of two side plates, and respectively open into the pump chambers along one side face of the cam ring. In the other side face of the cam ring, a groove is formed except for the region corresponding to the exhaust port, and a partially cut off annular friction ring is disposed within the groove while urged by a seal ring into close contact with the other side plate. A pressure compensating recess having a configuration which conforms to that of the exhaust port and communicating with the pump chambers is formed in the other side face of the cam ring except for the region wherein the groove for the friction ring is formed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a variable displacement vane pump and, 
more particularly, to a structure of a pump which effectively prevents 
stick slip of a cam ring during its swinging movement for varying the pump 
displacement. 
2. Description of the Prior art 
Vane-pumps are known as small-sized and light-weighted pumps having a high 
efficiency, and have been used in various fields. 
One example of variable-displacement vane-pumps out of such vane-pumps will 
be explained with reference to FIG. 3. 
FIG. 3 is a cross sectional view of a pump portion of the 
variable-displacement vane pump. A drive shaft 8 is rotatably supported by 
a pump housing 7, and extends into a circular inner cavity of the pump 
housing 7 in coaxial alignment with the center axis of the inner cavity. A 
columnar rotor 1 is securely fixed to and rotates integrally with the 
drive shaft 8 in the direction shown by the arrow in FIG. 3. An annular 
cam ring 3 is interposed between the rotor 1 and the pump housing 7. The 
uppermost portion of the cam ring 3 contacts the inner surface of the pump 
housing 7 through a pivot member 32 while the lowermost portion of the cam 
ring 3 also contacts the inner wall of the pump housing 7 through a 
sealing member 33. Thus, the cam ring 3 swings on the pivot member 32. 
The cam ring 3 is provided with a spring seat 34 at its lower portion. A 
coil spring 35 is disposed between the spring seat 34 and the bottom 
surface of a concave formed in the pump housing 7. The cam ring 3 is urged 
by a spring force of the coil spring 35 into its maximum eccentric 
position. 
A plurality of vanes 2 are provided in the rotor 1 at regular intervals in 
a circumferential direction thereof. These vanes 2 are radially slidable 
inward and outward in contact with the inner surface of the cam ring 3. 
Upon receiving the pump discharge pressure, each vane 2 outwardly slides 
toward the inner surface of the cam ring 3 until a top end thereof 
contacts the inner surface of the cam ring 3, thereby defining closed pump 
chambers P together with a pair of side plates 4A, 4B, each closely facing 
each of both side faces of each vane 2. With the rotation of the rotor 1, 
each pump chamber P rotates while changing its volume. 
An arc-shaped line port 41 is formed in the side plate 4B in facing 
relationship with the pump chamber P of which the volume gradually 
increases while an arc-shaped exhaust port 42 is formed in the side plate 
4B in facing relationship with another pump chamber P of which the volume 
gradually decreases. Thus, working fluid is sucked from the intake port 41 
and pressurized fluid is discharged from the exhaust port 42. 
The pressurized fluid is led into a space 5a defined by a half portion of 
the outer periphery of the cam ring 3 ranging from the pivot member 32 to 
the sealing member 33, and the inner surface of the pump housing 7 by way 
of a regulator 92 (FIG. 5) while a space 5b defined by the remaining half 
portion of the outer periphery of the cam ring 3 and the inner surface of 
the pump housing 7 is communicated with a reservoir tank 94. 
With the increase in the flow rate of the discharge fluid from the pump, 
and accordingly, with the increase in the discharge pressure, the cam ring 
3 starts to swing leftward in FIG. 3 on the pivot member 32 against the 
spring force of the coil spring 35, and the center of the cam ring 3 
approaches the rotational center of the rotor 1. 
As the eccentricity of the cam ring 3 decreases, the volume change of the 
pump chambers P decreases with the result that the discharge rate 
decreases. 
FIG. 5 shows a diagram showing the flow route of working fluid, wherein 91 
designates a vane pump, and 93 designates a load. A part of pressurized 
fluid is led to the space 5a of the vane pump 91 through the regulator 92, 
which controls the pressure of the fluid led to the space 5a in response 
to a control signal(not shown). As a result, the pressure in the space 5a 
of the vane pump 91 changes in proportion to the control signal so that 
the displacement of the vane pump 91 is controlled in accordance with the 
control signal. 
In the conventional vane pump having the above described construction, when 
a preceding vane 2 of each pump chamber P reaches the intake port 41 with 
the rotation of the rotor 1, and when the preceding vane 2 reaches the 
exhaust port 42 with the rotation of the rotor 1, the inner pressure of 
each pump chamber P suddenly changes. This results in eccentric loads 
periodically acting upon the cam ring 3 in its swinging directions, 
generating undesirable hunting of the cam ring 3. This hunting of the cam 
ring 3 causes the unstable control of the discharge rate of the pump, and 
causes the generation of noise. 
Accordingly, conventionally, as shown in FIG. 3, the vibrations of the cam 
ring 3 have been prevented by providing a friction ring 6 having a 
rectangular cross section along one side face of the cam ring 3 over the 
entire length thereof. More specifically, a circular groove 36 is formed 
in the entire side face 3a of the cam ring 3, as shown in FIG. 4. A 
circular seal ring 61 is disposed within the groove 36. The friction ring 
6 is brought into close contact with the side plate 4A by an elastic force 
of the seal ring 61. thereby generating a friction force between the 
friction ring 6 and the side plate 4A, and preventing the vibrations of 
the cam ring 3. 
One example of the friction ring employed in the variable-displacement pump 
is disclosed in Japanese unexamined Utility Model publication No. Sho 
59-160875. 
However in the above-described conventional variable-displacement 
vane-pump, the discharge pressure from the exhaust port 42 acts upon the 
other side face 3b of the cam ring 3, which faces tthe exhaust port 42. 
This discharge pressure causes an excessively large pushing force to act 
upon the friction ring 6, thereby excessively increasing the friction 
force between the friction ring 6 and the cam ring 3, and accordingly, 
causing the generation of undesirable stick slip of the cam ring 3 during 
its swinging movement. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a variable-displacement 
vane-pump which effectively prevents vibrations of a cam ring without 
generating stick slip during the swinging movement of the cam ring. 
The variable-displacement vane pump in accordance with the present 
invention has a pump housing, a drive shaft rotatably supported by the 
pump housing in coaxial relationship therewith, a rotor supported by the 
drive shaft to be rotated integrally with the drive shaft within tee pump 
housing. The rotor is provided with a plurality of vanes at regular 
intervals in the circumferential direction on the outer peripheral surface 
thereof. These vanes are radially slidable outward and inward with the 
rotation of the rotor. A cam ring is interposed between the rotor and the 
pump housing in eccentric relationship with the pump housing, thereby 
defining two spaces between respective half portions of an outer periphery 
of the cam ring and an inner surface of the pump housing. One of the two 
spaces between the cam ring and the pump housing is communicated with a 
pressure source. Two side plates are respectively provided on both sides 
of the vanes. A top end of each vane contacts the inner surface of the cam 
ring, thereby defining pump chambers between the rotor and the cam ring. 
An intake port for successively sucking working fluid into the pump 
chambers, and an exhaust port for successively discharging pressurized 
working fluid from the pump chambers, are provided in one side plate, and 
the inner end of the exhaust port faces one side face of the cam ring. A 
friction ring is provided between the cam ring and one of the side plates. 
The friction ring is urged by an elastic member into contact with the 
facing side plate. A pressure compensating recess is formed at the other 
side face of the cam ring, which is opposite to the one side face upon 
which the exhaust port opens. The pressure compensating recess has a 
configuration which conforms to that of the exhaust port, and is 
communicated with the pump chambers. 
In operation, working fluid is successively sucked from the intake port 
into pump chambers, and the sucked fluid is successively pressurized and 
discharged from the pump chambers into the exhaust port. The cam ring is 
swung from its eccentric position toward its coaxial position with respect 
to the pump housing in accordance with the pressure of the pressure 
source, and accordingly adjusting the discharge rate. 
By providing the pressure compensating recess, the discharge pressure of 
the exhaust port also acts upon the side surface of the cam ring, which is 
opposite to the side surface facing the exhaust port. This discharge 
pressure is substantially equal to that acting upon the side surface of 
the cam ring, which faces the exhaust port, because the pressure 
compensating recess has a configuration conforming to that of the exhaust 
port. This results in the cam ring being not excessively pressed in only 
one direction by the discharge pressure of the exhaust port, and the 
pushing force acting on the friction ring being reduced to a proper value. 
Therefore, stick slip is prevented from generating when the cam ring 
swings to its opposite side, and hunting of the cam ring can be restrained 
.

DETAILED DESCRIPTION OF THE EMBODIMENT 
Referring to FIG. 1, there is shown a pump portion of a first embodiment of 
a variable-displacement vane-pump in accordance with the present 
invention. 
A rotor 1 is fixed to a drive shaft 8 which is rotatably supported by a 
pump housing 7 and extends into its inner cavity in coaxial alignment with 
the center axis of the inner cavity. A cam ring 3 is interposed between 
the rotor 1 and the pump housing 7, and is pivotably supported at an 
uppermost inner surface of the pump housing by means of a pivot member 32. 
Vanes 2 are provided in the rotor 1 at regular intervals in the 
circumferential direction thereof, and are slidable radially outward and 
radially inward so that& a top end of each vane 2 is brought into contact 
with the inner surface of the cam ring 3. Two side plates 4A, 4B are 
respectively disposed on both side faces of the vanes 2, thereby defining 
pump chambers P between every two adjacent vanes 2. An arc-shaped intake 
port 41 and an arc-shaped exhaust port 42 are provided in one side plate 
4B, and a radially inner half portion of each of these ports 41, 42 opens 
into the pump chambers P. 
One side face of the cam ring 3 faces a radially outer half portion of the 
exhaust port 42. A partially cut off annular friction ring 6 is provided 
between the side face of the cam ring 3, which does not face the exhaust 
port 42, and the side plate 4A except for the region corresponding to the 
exhaust port 42. The friction ring 6 is urged by a partially cut off seal 
ring 61 toward the side plate 4A, as shown in FIG. 4. 
And a pressure compensating recess 31 having an acting surface 31a which is 
parallel to the inner surface of the side plate 4A, and equal to that of 
the radially outer half portion of the exhaust port 42 is formed in the 
side face of the cam ring 3, which does not face the exhaust port 42, 
except for the region wherein the friction ring 6 is provided, as shown in 
FIG. 1. 
Reference numeral 33 designates a sealing member for bringing the cam ring 
3 into sealing engagement with the inner surface of the pump housing 7. 
Reference numeral 34 designates a spring seat for a coil spring 35 which 
urges the cam ring 3 to be positioned at its eccentric position with 
respect to the pump housing 7. Reference numerals 5a, 5b respectively 
designate spaces defined by halves of the outer periphery of the cam ring 
3 and the inner surface of the pump housing 7. 
In operation, when the rotor 1 rotates, working fluid is sucked from the 
intake port 41 into each pump chamber P defined by adjacent vanes 2, and 
the pressurized fluid is discharged from the exhaust port 42. At this 
time, discharge pressure of the line port 42 acts upon both side faces of 
the cam ring 3. Namely, the discharge pressure acts upon one side face of 
the cam ring 3, which directly faces the exhaust port 42, and the 
discharge pressure also acts upon the other side face of the cam ring 3 
through the pressure compensating recess 31. This prevents the cam ring 3 
from being excessively pressed against the side plate 4A through the 
friction ring 6. 
Therefore, the friction ring 6 is pressed against the side plate 4A by a 
proper elastic force of the seal ring 61, so the cam ring 3 is restrained 
from hunting. Furthermore, when the cam ring swings with the change of the 
displacement of the pump, the generation of stick slip of the cam ring can 
be prevented because the friction ring 6 is provided over the entire side 
face of the cam ring 6 except for the region corresponding to the exhaust 
port 42. 
The present invention is not limited to the above-described embodiment. For 
example, the exhaust port is provided in only one of two side plates in 
the above-described embodiment. Instead, the exhaust port may be provided 
in both side plates. And these exhaust ports may have sizes different from 
each other. 
As described above, in accordance with the variable-displacement vane pump 
of the present invention, both the friction ring and the pressure 
compensating recess can be provided in the cam ring without increase in 
the outer diameter thereof. This construction restrains hunting of the cam 
ring, and realizes the smooth swinging movement& of the cam ring without 
stick slip thereof.