hydraulic power assist
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Hydraulic Power Assist
Definition
Hydraulic power assist means that a hydraulic system is incorporated with mechanical steering
Full Time Part Time Power Steering Part Time
The force of the center springs of the valve gives the driver the “feel” of the road at the steering wheel.
Full Time The valve is installed without centering
springs. Any movement of the steering wheel results in hydraulic boost being applied.
(Vickers, 1967)
Hydro-mechanical Power Steering (HMPS) Hydro-mechanical power steering was
the first type to be used on agricultural tractors in the early 1950’s.
Suitable for small to medium tractors where power steering can be an option to manual steering.
Once the size of the tractor becomes too big for manual steering, hydro-mechanical steering is usually not cost effective.
(Wittren, 1975)
Hydro-mechanical Power Steering There are many different types of hydro-
mechanical power steering, but they all can be grouped into four basic groups.
Steering linkage mounted integral valve and actuator
Steering wheel mounted control valve, linkage mounted actuator
Separate control valve and actuator mounted in best position
Integral valve and actuator mounted at steering wheel
HMPS Type 1
Easiest to adapt to an existing mechanical steering layout.
Only two hoses are needed.
The integral valve and actuator coupled to steering linkage.
(Wittren, 1975)
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, heat and vibration
(Wittren, 1975)
Steering column mounted control valve with separate, remote actuator coupled to a linkage member.
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.
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 are mounted on the steering column, the actuator drives the pitman arm by rack and pinion or by crank arm means.
Integral Linkage Power System-hydraulic assist Pitman arm operates
steering gear through drag link (B)
Power cylinder thrust at steering arm (C)
Boosters actuate left wheel steering arm, right wheel steered by cross steering arm (D)
Only lines to booster are pressure and tank
Frame absorbs shock instead of steering gear, easy to service
(Vickers, 1967)
Remote Linkage System
Steering valve is remote mounted, not with cylinder
This linkage system allows for mechanical steering
(Vickers, 1967)
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 of ports for the connection
(Vickers, 1967)
Remote Dual System
Two cylinders operated by single valve
Valve connects pitman arm and left cylinder
Common on rear wheel steer
(Vickers, 1967)
Power Steering Circuits
General Circuit Integral Steering Unit Circuit Remote Linkage System Circuit
(Vickers, 1967)
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)
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 ratio
steering system.
(Vickers, 1967)
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)
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 Valve
A four way valve that functions as a positioning servo valve.
Most are open-center.
(Vickers, 1967)
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 in
the reservoir. A 3-micron filter is
recommended.
(Vickers, 1967)
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 steering components movements.
(Vickers, 1967)
Special Power Steering Circuits Integral Steering Unit
Circuit A simple circuit with the
valve and cylinder mounted together.
Remote Linkage System Circuit The valve and cylinder are
mounted separately.
(Vickers, 1967)
Integral Steering Unit
S20 Steering Unit Consists of a power cylinder
connected to a steering valve.
Two external and internal ports
The inlet port is connected to the pressure line.
The outlet port is the tank return.
The upper internal port connects between the coaxial tubes of the cylinder to the cylinder rod end.
The lower internal port connects to the head end of the cylinder.
(Vickers, 1967)
Integral Steering Unit
View A The valve is in neutral
position. The spool is centered, and the oil from the pump is directed back to the tank
View B The valve is in retract
position. The spool is pushed to the left and the oil is directed to the rod end of the cylinder, thus moving the steering unit to the left.
View C The valve is in the extend
position. The spool is moved to the right and the oil is directed to the head end of the cylinder. The steering unit is moved to the right.
(Vickers, 1967)
Integral Steering Unit
Check Valve Helps to avoid hydrostatic lock
and allow for manual steering Relief Valve
Optional. Can be incorporated if the flow
control and relief valve is not used.
Ball Stud mounting The control valve ball stud can
be mounted in any four positions relative to the port connections.
(Vickers, 1967)
Remote Linkage System
Auxiliary Side Ports
(Vickers, 1967)
Remote Linkage System
Pitman Arm Stops Helps to protect
against overheating. Adjusted so that the
Pitman arm stops just before the wheels must stop.
Should be used with any system that has a separate steering valve.
(Vickers, 1967)
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