Download - Hydraulic Power Assist.ppt
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Hydraulic Power
Assist
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Definition
Hydraulic power assist means that a
hydraulic system is incorporated with
mechanical steering
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Full Time Part Time Power
Steering Part Time
The force of the center springs of the valvegives the driver the feel of the road at thesteering wheel.
Full Time
The valve is installed without centeringsprings. Any movement of the steeringwheel results in hydraulic boost beingapplied.
(Vickers, 1967)
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Hydro-mechanical Power
Steering (HMPS)Hydro-mechanical power steering was
the first type to be used on agriculturaltractors in the early 1950s.
Suitable for small to medium tractorswhere power steering can be an optionto manual steering.
Once the size of the tractor becomes toobig for manual steering, hydro-mechanical steering is usually not costeffective.
(Wittren, 1975)
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Hydro-mechanical Power
Steering There are many different types of hydro-
mechanical power steering, but they allcan be grouped into four basic groups.
Steering linkage mounted integral valve andactuator
Steering wheel mounted control valve,linkage mounted actuator
Separate control valve and actuator mountedin best position
Integral valve and actuator mounted atsteering wheel
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HMPS Type 1
Easiest to adaptto an existing
mechanical
steering layout.
Only two hoses
are needed.
The integral valve and actuator
coupled to steering linkage.
(Wittren, 1975)
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HMPS Type 2
Creates a highly
congested area with
all of the hoses.
Four Hoses are
needed: supply,
return and a pair to
the actuator.
Creates noise, heatand vibration
(Wittren, 1975)
Steering column mounted control
valve with separate, remote actuator
coupled to a linkage member.
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HMPS Type 3
Keeps the area around
the steering wheel from
becoming too
congested.
Four hoses are
required.
(Wittren, 1975)
Control Valve and actuator separately
mounted in the steering linkage.
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HMPS Type 4
Most sophisticated
design
Requires little or no
steering linkage
modification.
Requires larger
space envelope than
other types.
(Wittren, 1975)
The control valve and the actuator aremounted on the steering column, the
actuator drives the pitman arm by rack
and pinion or by crank arm means.
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Integral Linkage Power
System-hydraulic assist Pitman arm operates
steering gear throughdrag link (B)
Power cylinder thrust at
steering arm (C) Boosters actuate left
wheel steering arm, rightwheel steered by crosssteering arm (D)
Only lines to booster are
pressure and tank Frame absorbs shock
instead of steering gear,easy to service
(Vickers, 1967)
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Remote Linkage System
Steering valve is
remote mounted, not
with cylinder
This linkage system
allows for
mechanical steering
(Vickers, 1967)
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Combined Integral Remote
System Two cylinders
One cylinder has
integral linkage
Second cylinder is
operated by the
same valve
The steering valve
has an extra set ofports for the
connection
(Vickers, 1967)
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Remote Dual System
Two cylinders
operated by single
valve
Valve connects
pitman arm and left
cylinder
Common on rear
wheel steer
(Vickers, 1967)
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Power Steering Circuits
General Circuit
Integral Steering Unit Circuit
Remote Linkage System Circuit
(Vickers, 1967)
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General Circuit Components
Manual Steering Gear
Power Steering Pump
Power Cylinder
Valves Relief
Flow Control
Steering
Filters
Oil Air Breather
Oil Reservoir
Hydraulic Lines
(Vickers, 1967)
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General Circuit
Manual Steering Gear
Transmits motion of the steering wheel to
the turning of the wheel. Could be eliminated, but there are two
reasons for not doing so.
Hydraulic system failure.
The public is not ready for a 1 to 1 ratiosteering system.
(Vickers, 1967)
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General Circuit
Power Steering Pump
Usually a vane-type pump or similar.
Driven by the engine. Power Cylinder
Double-acting differential cylinder.
Steering response to left and right turns is
slightly different. Hardly noticeable.
(Vickers, 1967)
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General Circuit
Valves
Relief Valve
Required to protect the pump.
Flow Control Valve
Helps maintain a constant flow.
Variations in engine speed would affect pump flow
without flow control valve.
Steering ValveA four way valve that functions as a positioning
servo valve.
Most are open-center.
(Vickers, 1967)
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General Circuit
Filters
Oil Filter
Preferably installed in the
return line.
A 10-micron or smaller
filter is recommended.
Air Breather Filter
The breather or vent inthe reservoir.
A 3-micron filter is
recommended.
(Vickers, 1967)
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General Circuit
Oil Reservoir
Must be large enough to hold more than all
of the oil for the system. Should be capable of dissipating heat in
oil.
Hydraulic Lines
Flexible hoses due to the steeringcomponents movements.
(Vickers, 1967)
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Special Power Steering
Circuits Integral Steering Unit
Circuit
A simple circuit with the
valve and cylindermounted together.
Remote Linkage System
Circuit The valve and cylinder are
mounted separately.
(Vickers, 1967)
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Integral Steering Unit
S20 Steering Unit Consists of a power cylinder
connected to a steeringvalve.
Two external and internalports The inlet port is connected
to the pressure line.
The outlet port is the tankreturn.
The upper internal portconnects between thecoaxial tubes of thecylinder to the cylinder rod
end. The lower internal port
connects to the head endof the cylinder.
(Vickers, 1967)
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Integral Steering Unit
View A The valve is in neutral
position. The spool iscentered, and the oil from thepump is directed back to thetank
View B The valve is in retract
position. The spool ispushed to the left and the oilis directed to the rod end ofthe cylinder, thus moving thesteering unit to the left.
View C The valve is in the extend
position. The spool is movedto the right and the oil isdirected to the head end ofthe cylinder. The steeringunit is moved to the right.
(Vickers, 1967)
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Integral Steering Unit
Check Valve
Helps to avoid hydrostatic lockand allow for manual steering
Relief Valve
Optional.
Can be incorporated if the flowcontrol and relief valve is notused.
Ball Stud mounting
The control valve ball stud canbe mounted in any fourpositions relative to the portconnections.
(Vickers, 1967)
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Remote Linkage System
Auxiliary Side Ports
(Vickers, 1967)
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Remote Linkage System
Pitman Arm Stops
Helps to protect
against overheating.
Adjusted so that thePitman arm stops just
before the wheels
must stop.
Should be used with
any system that has a
separate steering
valve.
(Vickers, 1967)