Chapter 9 Steering Systems 187
Flow Control Valve
The flow control valve allows the pump to produce
maximum flow for assist at low speeds, while reducing
flow at high speeds to preserve road feel. Allowing the
pressurized fluid to enter the inlet also reduces pump wear
and fluid temperatures. The goal of the flow control valve
is to keep the flow constant as the pump speed and the
fluid demands of the system change.
The flow control valve consists of a tapered pin
inserted into a small orifice. A spring holds the valve in the
closed position, Figure 9-47. Note that in the closed
position, fluid can still flow to the power steering gear.
As the engine and pump speed increases, excess fluid
flow through the orifice causes a low-pressure condition
(called the venturi effect) on the spring side of the valve.
Lower pressure on the spring side causes the valve to
compress the spring and begin moving. Valve movement
uncovers a passage to the pump inlet, and fluid is returned
to the pump. With more fluid bypassing back to the pump,
less flows through the steering gear, reducing sensitivity.
The tapered pin helps to control the amount of venturi
action, allowing the valve to move more or less depending
on the amount of fluid flowing. This helps to avoid sudden
changes in flow.
Inlet port
Discharge
port
Start of fluid intake:
Fluid intake:
Fluid discharge:
Fluid movement:
The vanes are pushed onto
the inner circumference of
the cam ring.
The volume of the vane
chamber increases so
that fluid is sucked in.
The sucked-in fluid moves
toward the discharge port.
As the vanes return to their
original position on the inner
side, the volume of the vane
chamber decreases so the
fluid is discharged from the
discharge port.
Figure 9-43. Study the operation of this vane-type power steering pump. (Honda)
Cam ring
Drive shaft
Slipper
Housing/
reservoir
Figure 9-44. Exploded view of a slipper-type power steering
pump. (Moog)
Cam
Roller
Housing/reservoir
Roller carrier
Drive shaft
Figure 9-45. Exploded view of a roller-type power steering
pump. (Moog)
Port housing
Housing seal
Pump
housing
Drive
gear
Driven
gear
Dowel pins
Figure 9-46. One type of dual-gear power steering pump. They
operate much like an engine gear oil pump. (Honda)