Mahindra 3535, Service Manual

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H-8 35 Series 4WD, Model - 3535, 4035, 4535 and 5035 SM June’08
4. DESCRIPTION
The power steering is of the hydrostatic type,
linked by fluid, there being no mechanical
connection between the steering wheel and
front axle. The system consist of an engine
driven pump, control valve, a hydraulic
cylinder and suction line filter. Manual pump
and control valve are integrated to form a
compact steering unit. Figure show a
schematic drawing of the system. This is open
centre, non load reaction type system.
Turning the steering wheel delivers fluid to
move the spool valve against its centering
spring. This then directs the delivery of fluid
to the inlet side of the manual pump, passing
through the pump and the control valve to
the steering cylinder. This in turn supplies the
required steering effort according to the rate
of steering wheel turn. The only manual effort
required is that necessary to over come the
control valve centering spring. The stiffness
of the spring has been selected to give the
desired feel and self-straightening effect.
Manual steering is accomplished in the same
manner when the engine is running, except
that there is no power assistance. By rotating
the steering wheel, the manual pump delivers
fluid through the control valve to the steering
cylinder. A check valve in the control valve
provides for the return of fluid from the
cylinder to the manual pump instead of to
the reservoir, to complete the circuit.
a. Manual Steering Effort
This circuit consists of the manual pump,
control valve and steering cylinder. Hydraulic
fluid circulates in the closed circuit from
manual pump, through the control valve to
the steering cylinder, and returns from the
steering cylinder through the control valve
back to the manual pump.
Initial movement of the steering wheel in
either direction develops pressure (15 to 20
lb/in
2 ) which actuates the control valve spool.
The movement of the control valve spool
opens the ports from the manual pump to
the steering cylinder and return ports from
the steering cylinder to the manual pump.
The resistance of the wheels determines the
steering effort required by the operator to
develop the necessary hydraulic pressure at
the steering cylinder piston to turn the
wheels. The manual pump attached to and
driven by the steering wheel develops the
necessary pressure.
The initial effort to turn the steering wheel
in either direction instantly increases hydraulic
pressure in the manual pressure line, while
the pressure in the suction line remains static
or in lowered. Since the manual pump and
the control valve are part of the closed
hydraulic circuit, this same difference in
pressure exists between the ends of the
control valve spool. Were it not for the
centering spring, the spool would instantly
move toward the lower pressure end. Very
little force is required at the steering wheel
rim to create a pressure difference of 15 to
20 lb/in
2 , the pressure necessary to over come
spring resistance. The spool then moves,
diverting fluid to the steering cylinder.
Continued turning of the steering wheel
moves the fluid to and through the control
valve to the steering cylinder and against the
cylinder piston.
Therefore, as pressure against the cylinder
causes it to move. Turning the wheels an
equal amount of displaced fluid moves, under
pressure from steering cylinder to the control
valve, it unseats a re-circulating check ball
connecting the engine driven pump pressure
inlet chamber and the cylinder return
chamber. This allows the fluid to move
through the passage in control valve spool
and back to the manual pump to complete
the cycle.
The displaced fluid does not return to the
reservoir even though the reservoir return line
is open because of the suction created in the
manual pump.
The engine driven pump being stopped, the
fluid in the power circuit and the reservoir
is under no pressure and is therefore static.
The re-circulating check ball is seated under
pressure when engine driven pump is
operating.
b. Power Steering
This circuit consists of reservoir, engine driven
pump, control valve, manual pump and
steering cylinder. The initial turn of the
steering wheel develops the 15 to 20 lb/in
2
pressure differential in the manual pump lines
at the control valve spool to direct the
Steering