Service Manuals, User Guides, Schematic Diagrams or docs for : . Car Manuals Subaru Impreza 2001-2002.2004-2007 Approved Subaru Impreza 2004 WRX STi 08. MECHANISM AND FUNCTION 46. POWER ASSISTED SYSTEM (POWER STEERING) 02. Power Steering System
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W1860BE.book Page 5 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
2. Power Steering System
A: HYDRAULIC SYSTEM
The fluid pump is directly driven by the engine through a belt.
The fluid flow is maintained almost constant regardless of change in the engine speed by the
function of the flow control valve. The flow-regulated fluid is delivered to the control valve through
hose A.
When the steering wheel is turned, the rotary control valve connected to the pinion shaft opens
the hydraulic circuit corresponding to the direction in which the steering wheel is turned. The fluid
then flows into chamber A or B via pipe A or B.
The fluid pressure in chamber A or B acts on the rack piston in the same direction as that in which
the rack shaft is moved by rotation of the steering wheel. This helps reduce the effort required of the
driver to operate the steering wheel.
Movement of the rack piston causes the fluid in the other chamber to return to the reservoir tank
via pipe A or B, control valve, and hose B.
As the steering shaft is connected to the pinion shaft mechanically via the rotary control valve,
the steering system can operate as a manual system even if the hydraulic system becomes inop-
erative.
To control the maximum fluid pressure, a relief valve is built into the fluid pump to prevent build-
up of an excessive fluid pressure.
PS-5
W1860BE.book Page 6 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
(1) (2)
(3)
(4)
(5) (6)
(8)
(7)
(9)
(12)
(13) (10)
(11)
(14)
(16)
(17)
(15)
(18)
(19)
PS-00191
(1) Power cylinder (11) Steering wheel
(2) Rack piston (12) Tank
(3) Rack shaft (13) Vane pump
(4) Pinion shaft (14) Relief valve
(5) Chamber A (15) Hose A
(6) Chamber B (16) Hose B
(7) Pipe A (17) Pump control valve
(8) Pipe B (18) Fluid pump
(9) Control valve (19) Engine
(10) Steering shaft
PS-6
W1860BE.book Page 7 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
B: GEARBOX ASSEMBLY
1. POWER CYLINDER
The gearbox integrates the control valve and power cylinder into a single unit. The rack shaft serves
as a power cylinder piston. The rotary control valve is located around the pinion shaft.
The rotary control valve and power cylinder are connected to each other by two pipes through which
hydraulic fluid flows.
(4)
(1) (2) (3)
(5)
(6)
PS-00192
(1) Piston (4) Power cylinder
(2) Rack shaft (5) Pinion shaft
(3) Cylinder (6) Control valve
2. ROTARY CONTROL VALVE
The rotary control valve consists of a rotor (which rotates together with the steering shaft), a pinion
(which is connected to the rotor and torsion bar), and a sleeve (which rotates together with the pin-
ion). The rotor and sleeve have grooves C and D, respectively, which form fluid passages V1 through
V 4.
The pinion is in mesh with the rotor with adequate clearance, which enable the rack to be moved
manually by rotating the steering shaft (fail-safe feature).
PS-7
W1860BE.book Page 8 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
(B)
(A)
(2)
(3)
(4)
(1)
(5)
(A) (B)
(17)
(8)
(7) (10)
(11) (11) (18)
(16)
(6) (9)
(10) (10)
(12)
(13) (15)
(14) PS-00011
(1) Torsion bar (12) Torsion bar
(2) Sleeve (13) Rotor
(3) Rotor (14) Sleeve
(4) Pinion (15) Fluid return line (to reservoir tank)
(5) Pinion-to-rotor engagement (fail-safe feature) (16) Pinion
(6) Fluid passage V1 (17) Torsion bar
(7) Fluid passage V2 (18) Rotor
(8) Fluid passage V3
(9) Fluid passage V4 (A) Cross-sectional view A (fluid passage switching cir-
cuit)
(10) Groove C (B) Cross-sectional view B (pinion-to-rotor engagement)
(11) Groove D
PS-8
W1860BE.book Page 9 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
Principle of operation
When the torsion bar is twisted by a rotational force applied to the steering wheel, the relative posi-
tion between the rotor and sleeve changes. This changes the cross-sectional area of fluid passages
V1, V2, V3 and V4. The fluid passages are thus switched and the fluid pressure is controlled in ac-
cordance with the operation of the steering wheel.
When no steering force is applied:
The rotor and sleeve are held at the neutral position. Fluid passages V1, V2, V3 and V4, which are
formed by grooves C and D are open equally. Under this condition, the fluid from the pump returns
to the reservoir tank so that neither fluid pressure builds up nor the rack piston moves in the power
cylinder.
(1) (2)
(4) (5)
(3) (6)
(7) (7)
(9) (8)
(8) (9)
(7) (7)
(9) (8)
PS-00012
(1) Chamber A (4) V2 (7) From fluid pump
(2) Chamber B (5) V3 (8) To A
(3) V1 (6) V4 (9) To B
PS-9
W1860BE.book Page 10 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
When steering force is applied:
When the steering wheel is turned to the right, for example, fluid passages V1 and V3 are opened
while fluid passages V2 and V4 are nearly closed.
At this point, the fluid pressure in chamber A of the power cylinder increases depending on the
degree of closure of fluid passages V2 and V4 so that the rack piston moves to the right. The fluid
in chamber B, on the other hand, is drained through fluid passage V3 into the reservoir tank.
(1) (2)
(4) (5)
(3) (6)
(7) (7)
(9) (8)
(8) (9)
(7) (7)
(9) (8)
PS-00013
(1) Chamber A (4) V2 (7) From fluid pump
(2) Chamber B (5) V3 (8) To A
(3) V1 (6) V4 (9) From B
Fail-safe function
If fluid pressure fails to build up due to, for example, a broken fluid pump drive belt, the steering
wheel rotating torque is transmitted from the valve rotor to the pinion through mechanical engage-
ment between them.
PS-10
W1860BE.book Page 11 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
C: FLUID PUMP AND RESERVOIR TANK
The fluid pump is a vane type driven by the engine via belt.
The reservoir tank is mounted on the vehicle body.
The fluid pump incorporates the flow control valve and relief valve, each performing the following
functions:
The flow control valve regulates the flow rate of discharged fluid to a constant level irrespective
of the engine speed.
The relief valve protects the system from an excessively high pressure which may occur, for ex-
ample, when the steering wheel is turned all the way.
(2)
(1)
PS-00014
(1) Fluid pump
(2) Reservoir tank
PS-11
W1860BE.book Page 12 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
(1)
(3)
(2)
(4)
(5)
PS-00015
(1) Reservoir tank (4) Fluid pump assembly
(2) Flow control and relief valve assembly (5) Steering gearbox
(3) Vane pump
PS-12
W1860BE.book Page 13 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
The fluid pump is belt-driven by the engine. The fluid flow is controlled according to the engine
speed so that an adequate steering resistance is given during high-speed operation.
The fluid pump is a variable capacity type vane pump whose delivery rate per rotation decreases
as the engine speed increases. The pump is integral with a pump control valve and relief valve.
The vane pump consists of a rotor, a cam ring, and eleven vanes.
When the rotor rotates, the vane in each slot of the rotor is radially moved out by centrifugal force
and pressed against the cam ring. The fluid from the suction port is confined in chambers formed
between two adjacent vanes and carried to the discharge port. Since the cam ring is movable in
relation to the rotor, the volume of each chamber is variable. This enables the delivery rate per ro-
tation of the pump to be changed.
(2) (1)
(3)
(5)
(4)
PS-00016
(1) Suction
(2) Vane
(3) Rotor
(4) Cam ring
(5) Discharge
PS-13
W1860BE.book Page 14 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
Flow control
The variable capacity pump changes its delivery rate per rotation by changing the degree of eccen-
tricity of the cam ring according to its rotating speed (engine speed).
On turbo models, a constant flow as shown with the broken line in the drawing below is maintained.
(9)
C
(7) D
B
(1)
(8)
(3) (2)
A
(4) (5) (6)
PS-00437
(1) Fluid flow (6) High-speed
(2) Pump speed (7) Delivery rate per unit time
(3) Idling (8) Delivery rate per rotation of pump
(4) Low-speed (9) Turbo models
(5) Mid-speed
NOTE:
In the following description, pump speed ranges will be indicated using the speed points A through
D shown in the drawing above.
PS-14
W1860BE.book Page 15 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
Low-range-speed operation (A -- B range)
In this speed range, as well as in all the other speed ranges, two different pump discharge pressures
are always applied to the control valve; one is directly led from the discharge port to the left end of
the valve and the other is led through an orifice (variable orifice) to the right end of the valve. Since
the orifice has a pressure reducing effect, the latter pressure is lower than the former.
When the pump is operating at a low speed, its discharge pressure is also low, resulting in only small
difference between the two pressures. In this condition, the valve stays pushed leftward by the
spring, allowing the non-pressurized tank fluid to enter chamber A. To chamber B, on the other
hand, the orifice-reduced discharge pressure is applied, so the cam ring is pushed leftward by the
cam ring spring. This makes the eccentricity of the cam ring a maximum and, therefore, the delivery
rate per rotation of the pump become a maximum.
(2)
(3)
(4)
(1)
(5)
(7)
(6)
(8)
(9)
PS-00261
(1) Control valve (6) Variable orifice
(2) Reservoir tank fluid (7) Pressure chamber A
(3) Control valve spring (8) Pressure chamber B
(4) Gear box (9) Cam ring
(5) Cam ring spring
PS-15
W1860BE.book Page 16 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
Mid- to high-range-speed operation (B -- D range)
During mid-range speed operation, the pump increases its delivery rate. Since the pressure before
passing through the variable orifice increases, the control valve moves rightward, overcoming the
tension of the control valve spring. This movement of the control valve allows the pressure upstream
of the variable orifice to be directed to chamber A after being adjusted to a necessary pressure* by
the port opening area created by the control valve. On the other hand, chamber B receives the res-
ervoir pressure (suction pressure). This means that the pressure in chamber A is higher than that
in chamber B. As a result, the cam ring moves rightward against the tension of the cam spring. This
causes the delivery rate per rotation of the pump to be reduced, so that the flow rate of the fluid to
the steering gear box decreases accordingly.
The above control is performed when the pump is operating at a speed in the B - D range.
* The "necessary pressure" for chamber A is a pressure required to move the cam ring to the posi-
tion corresponding to each predetermined flow rate (pump delivery rate). The pressure is obtained
by changing the port opening area appropriately through displacement of the control valve. The dis-
placement of the control valve is determined by how much the pressure before the variable orifice
is different from that after the orifice.
(2)
(3)
(4)
(1)
(10)
(5)
(7)
(6)
(8)
(9)
PS-00262
(1) Control valve (6) Variable orifice
(2) Reservoir tank fluid (7) Pressure chamber A
(3) Control valve spring (8) Pressure chamber B
(4) Gear box (9) Cam ring
(5) Cam ring spring (10) Opening
PS-16
W1860BE.book Page 17 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
Maximum pressure control
When the overall delivery rate of the pump exceeds a preset value, the relief valve opens to allow
part of the discharged fluid to flow toward the suction side of the pump, thereby controlling the max-
imum pressure.
(1)
(2)
(3)
(4)
PS-00263
(1) Reservoir tank fluid
(2) Relief valve
(3) Gear box
(4) Relief spring
PS-17
W1860BE.book Page 18 Tuesday, January 28, 2003 11:01 PM
POWER STEERING SYSTEM
POWER ASSISTED SYSTEM (POWER STEERING)
MEMO
PS-18
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