System Description
Steering Pump (cont'dl
The flow control valve and sub-valve in the pump per-
forms the following steps @ through @ to control the
flow of fluid, that is to increase the discharge volume
when engine speed is low, and to decrease it when the
engine speed increases. The assistance thrust of the
steering gearbox changes in compliance with the
change in the discharge volume.
When the engine starts, fluid discharged from the
discharge port starts to flow through oil passage A,
the fixed orifice and the variable orifice to the steer-
ing gearbox. When the engine speed is extremely
low, the return port is closed by the flow control
Fluid pressure discharged from the discharge port
is applied to the top of the sub-valve, and the fluid
pressure that passed through oil passage A is applied
to the bottom of the sub-valve. When this happens,
the pressure difference between the ends of oil pas-
sage A, which is caused by the resistance oil passage
A when the fluid flows through the passage. is
applied to the sub-valve, However, the pressure dif-
ference applied to the sub-valve. that is the force
that pushes the sub-valve down, is too small to
overcome the spring force, and the variable orifice is
fully open when the engine speed is extremely low.
Because the fluid volume flowing through the fixed
orifice and variable orifice increases, a pressure dif-
terence is created between the ends of these ori-
fices, and it increases in proportion to the engine
speed. As the fluid pressure that passed the fixed
orifice and variable orifice is directed to the bottom
of the flow control valve. a pressure difference is
created between the top and bottom of the valve,
which pushes down the flow control valve and
opens the return port. As a result, pan of the fluid
discharged from the discharge port feturns to the
pump suction port, keeping the discharge volume
constant.
In this condition, the sub-valve does not move. and
the variable orifice stays fully open.
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PUMP R.P.M.
To STEERING GEARBOXSUB.VALVE
To STEEnING GEARBOX
FLOW CURVE
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FLOW CONTNOL VALVE
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ORIFICE
ToP
17-18
FLOW CONTROL VALVE
