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)