hydraulic power assist.ppt

7/22/2019 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.


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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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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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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Auxiliary Side Ports

(Vickers, 1967)


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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)

hydraulic power assist.ppt

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