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National Fluid Power Centre Steering Systems

Presented by John R Savage Director NFPC

Steering Systems

The purpose of these notes is to introduce the subject of HYDROSTATIC STEERING

These notes are not intended to give a total explanation but merely provide an overview of a very

complex subject

To study this subject in greater detail you are recommended to visit the Sauer Danfoss website on www.sauer-danfoss.com and the Eaton Hydraulic website on www.hydraulics-eaton.com We wish to thank our Advisory and Strategic Planning Group, representing: Bosch Rexroth, Eaton Corporation, Denison Hydraulics, Hydac Technology, BFPA, Moog Controls, Linde Hydraulics, Rotary Power and Witham Oils, for their kind support in giving their permission for us to use some of their illustrations in the formulation of notes presented by the National Fluid Power Centre Acknowledgement Contents Page • Introduction 3

• General Machine Layout 4

• Hydrostatic Steering and Associated Parts 5

• Basic Steering Operation 8

• Why Use This Type of Steering System? 11

• Types of Systems Available 11

• Introduction to Load Sensing 12

• Priority Flow Control Valves 15

• Priority Valve Operating Principles 16

• Full Load Sensing System 21

• Dynamic Steering System 22

• Dual Displacement System 25

• Load Reaction System 27

National Fluid Power Centre Steering Systems

If we look at a typical modern wheeled excavator we see that the hydraulic system is

sub-divided into a number of individual circuits: (1) the closed hydrostatic circuit for providing machine travel (in some cases machine movement is accomplished by an open circuit transmission

(2) The auxiliary circuit ( usually supplied by an axial piston swash plate pump)

incorporating load sensing and used in conjunction with a multi-function direction control valve block for operating the various services such as boom lift and lower, arm and bucket operations and machine swing This pump may be driven independently by the engine or as a “piggy-backed” unit driven from a through shaft on the main hydrostaticpump

(3) The steering and braking circuit It is also normal practice to incorporate an

additional fixed displacement pump ( usual gear type) to provide a supply to the steering system In some cases this pump also provides the supply of oil to the service brake system

Introduction

Steering Systems

Closed Hydrostatic System (Machine Travel Circuit)

Parking Park Brake

Gearbox

Axle

Service BrakePriority Valve

Main Hydrostatic Pump

Auxiliary Pump

Displacement Control

Steering / Brakes Pump

SteeringControlUnit

Service ValvesProportionalBang - Bang

Pilot Pressure Circuit

Motors Cylinders

Load Holding and Motion Control Valves

Pressure Relief and

Unloading Valves

Direction Flow Pressure

Steering Cylinders

Service Brake Control

Auxiliary Services

National Fluid Power Centre Steering Systems

Lets now look at the individual parts needed to create the steering function

First of all we need a steering wheel and steering column (although some

machines use a joystick arrangement) and instead of a mechanical connection to

a steering rack, in a hydrostatic steering system the steering column is connected

to the steering control unit (SCU)

The service ports of the SCU are then piped to the steering cylinders and these cylinders provide the mechanical

connection to the steering linkage on the axle

When the steering wheel is turned the SCU provides a metered volume of pressurised oil to the steering cylinders

Let us now focus on the major component parts of a HYDROSTATIC STEERING SYSTEM.The system is fully fluid linked, this means that there is no mechanical connection between thesteering control unit, the steering pump and the steering cylinders

A suitable axle arrangement

incorporating one or two hydraulic

cylinders provides the steering action

Some machines have both front and rear wheel

steering with the steering actions working

together, opposed or individually depending on

the machine requirements and terrain

Hydrostatic Steering and Associated Parts

SCU

Steering Systems

Splined Coupling to

fit the STEERING COLUMN

Steering Cylinder Ports

Pressure / Tank Ports

Cardan Shaft

Neutral Position Springs

Rotary Servo Valve(spool and sleeve)

Rotary Metering Unit

(gerotor set often termed

orbit set )

STEERING

CONTROL

UNIT

Internally, the SCU comprises a manually operated rotary servo valve and manually operated

rotary metering unit both housed in a common body The initial deflection of the steering wheel

causes the rotary valve to open up a supply of pressurised oil to the rotary metering unit (also

connected to the steering wheel) Rotation of the steering wheel rotates the gerotor set via the

cardan shaft connection and thus allows a precise volume of oil ( based upon its displacement)

to be metered to the steering cylinders Oil will be directed to the left or right cylinder ports

depending on which direction the steering wheel is turned.

The rotary servo valve comprises a spool and sleeve incorporating a complex series of

interconnecting galleries linked to the main ports The rotary spool and sleeve are held in a

neutral position by a group of leaf springs When the steering wheel is turned the viscous drag

and gerotor forces within the housing creates angular deflection between the spool and sleeve.

This relative movement represents the opening of the valve to allow oil to flow to the gerotor

set below.

A maximum deflection of up to 15 degrees relative movement between the spool and sleeve

can occur The degree of opening is based upon the rate of steering which in turn sets the

demand for pressurised oil to meet the steering operation If power fail should occur or there is

insufficient flow of oil from the steering pump then manual operation can be achieved but with a greater

National Fluid Power Centre Steering Systems

T P

Exploded view of the SCU

Here we see the rotary valve and gerotor set in more detail The amount of oil displaced by the gerotor set per revolution depends upon its displacement.Typical sizes range from 32 to

1000 cm3 per revolution

Rotary Servo Valve in Detail Rotary Servo Valve SimplifiedHydrostatic Steering and Associated Parts

4

Steering Systems

HYDROSTATIC STEERING

STEERING UNIT open centre non reaction

fixed displacement pump

main relief valve

shock valves and anti - cavitation check valves

back pressure check valve

to prevent cavitation in cylinders

(OSPC ON)

The diagram above shows the typical arrangement of an open centre non-reactive system.The flow from the fixed displacement pump returns to the reservoir when the steering wheel is in the neutral position Rotating the steering wheel opens a rotary servo spool valve inside the steering unit, thereby connecting the relevant service ports as shown below

HYDROSTATIC STEERING

fixed displacement pump RUNNING

steering to left Basic Steering Operation

NEUTRAL POSTION

National Fluid Power Centre Steering Systems

When the steering wheel of a vehicle is turned the unit provides a metered flow of oil to the steering cylinders based upon the degree of opening of the rotary servo valve and the

displacement of the rotary metering unit

At this stage it may be better to think of this as an adjustable throttle valve (rotary in design) that will open between zero flow and maximum flow based upon a deflection of

up to15 degrees between the inner and outer spool and sleeve

The degree of deflection being a function of the related to the physical steering rate

The diagram shows a simple open center-non reaction steering unit

With this type of system the flow rate though the unit is LOAD DEPENDENT

External force acting on wheels

F

closed ports

Overload / Shock Protection If the wheels are subjected to an external force with

the steering valve in the neutral position the shock valves will react as cross line relief valves.Where differential area steering cylinders are employed the re-distribution of fluid is

achieved via the inline check valves Any shortfall in fluid necessary to prevent cavitation isobtained from the return line

Basic Steering Operation

Steering Systems

So far we have introduced the basic open centre steering system, but there are alternative systems

- Tractors

- Loaders

- Combine Harvesters

- Forklift Tucks

Typical Applications for the Open Centre System

Steering Control Unit Priority valve

Steering cylinders

Shock valve block

Many machines employ load sensing hydraulic systems and include the operation of the steering system within this arrangement Priority flow is given to the steering system based upon its importance and

at this stage a valve termed a PRIORITY VALVE

is incorporated with the design of the circuitry When the steering system demand for fluid by the steering system is zero the priority valve directs the pump flow along its excess flow line This oil may be used to charge the accumulators for the service brake system or be used to supply a auxiliary valve bank

National Fluid Power Centre Steering Systems

WHY USE THIS TYPE OF STEERING SYSTEM ?

Hydrostatic Steering Control Units offer the following facilities:

TYPES OF SYSTEMS AVAILABLE

Open Centre

Open Centre – Power Beyond

Steering Systems

INTRODUCTION TO LOAD SENSING SYSTEMS

National Fluid Power Centre Steering Systems

in the CF at the point at which the steering wheel is turned This can lead to a time

delay and poor response causing the steering to feel heavier than normal

RSU

National Fluid Power Centre Steering Systems

To improve the overall performance of the steering system, load sensing is incorporated and the steering control unit is used in conjunction with a priority flow control valve

The priority valve is essentially a type of three way flow control valve

“P” PORT

LS

PP

The pump supply is connected to the “P” port, the “CF” line is connected to the

steering control unit pressure port and the “EF” line to the secondary circuit The

secondary circuit may be an auxiliary valve block or service brake system

Pressure Port

Tank Port

Port L Port R

Steering Systems

Operating Principles

1 It forms a diverter valve for the pump supply when the demand in the “CF”

port is zero The rating of the spring used in the priority valve is normally low

4 bar, 7 bar or 10 bar In either case the pump supply is diverted to the

“EF” line by moving the spool against this low pressure

2 The spool of the priority valve forms the main stage of a two stage relief

valve The pilot stage being represented by the LS relief valve in the steering unit combined they form the main relief valve for the steering system

3 The spool in conjunction with its rated spring also forms a pressure compensator

in conjunction with the throttling section of the orbit steering valve

combined they form a pressure compensated flow control valve ensuring

that changes in steering load does not affect the steering rate

Now consider the operating principles of a LS steering system, carefully examine the orbit unit port connections and its interface with the priority valve

4

To Steering Cylinder

Steering Orbit Unit [flow metering device]

Flow adjusted by the rate of steering

CF

To Braking System

EF LS

LS P

T PP

Control Orifice

RV set at 150 bar

Note When the steering wheel is released the orbit unit is closed centre [ P is blocked] and the

LS connection is drained to tank With the pump running the priority valve is piloted down against the low

rated spring [4,7 or 10bar.] The oil is then directed in to the EF line towards the braking circuit.

National Fluid Power Centre Steering Systems

When the steering wheel is turned the demand for fluid at the “P” port of the orbit unit causes the CF signal level to fall The priority spool is now forced upwards by the spring creating a flow path into the CF port Turning the steering wheel also connects the “P” port of the orbit unit to the service “A” and the service port “B” to tank.

Simultaneously the service port pressure is connected to the priority valve LS connection on its spring side This action causes the priority spool to take up a modulation condition balanced by the pilot pressure “pp” from the CF line on one side and the service port pressure and spring on the opposite.

This results in a pressure drop across the metering valve section of the orbit unit equal to the priority spool

= P1 – P2 Therefore

At this stage the priority valve becomes a pressure compensator for the steering unit forming a

PRESSURE COMPENSATED FLOW CONTROL This ensures a constant rate of steering irrespective of changes in steering cylinder forces

Steering Orbit Unit [flow metering device]

Flow is adjusted by the rate of steering

T PP

Control Orifice

RV set at 150 bar

OPERATING PRINCIPLES Condition 2.

B A

Priority spool

takes up the

new position

Steering (Conditions 2)

Steering Systems

When the steering wheel is turned the demand for fluid at the “P” port of the orbit unit causes the CF signal level to fall The priority spool is now forced upwards by the spring creating a flow path into the CF port Turning the steering wheel also connects the “P” port of the orbit unit to the service “A” and the service port “B” to tank.

Simultaneously the service port pressure is connected to the priority valve LS connection on its spring side This action causes the priority spool to take up a modulation condition balanced by the pilot pressure “pp” from the CF line on one side and the service port pressure and spring on the opposite.

This results in a pressure drop across the metering valve section of the orbit unit equal to the priority spool

= P1 – P2 Therefore

At this stage the priority valve becomes a pressure compensator for the steering unit forming a

PRESSURE COMPENSATED FLOW CONTROL This ensures a constant rate of steering irrespective of changes in steering cylinder forces

Steering Orbit Unit [flow metering device]

Flow is adjusted by the rate of steering

T PP

Control Orifice

RV set at 150 bar

OPERATING PRINCIPLES Condition 2.

B A

Priority spool

takes up the

new position

Steering (Conditions 2)

National Fluid Power Centre Steering Systems

Supply from Pump P

To Steering Cylinder CF

To Braking System

EF

LS

LS P

T PP

ORBIT STEERING UNIT and Rotary Valve

Increasing the rate of steering, opens the rotary metering valve in the orbit unit to ensure that the incoming flow rate matches the displacement requirements of the orbit gear set The displacement angle of the rotary valve in the orbit unit is 0-15 o

Rotating the steering wheel immediately alters the port connections in the internal rotary valve assembly

T PP

ORBIT STEERING UNIT

P1 and P2 rise due to the increased loading in the service port, whilst maintaining the present position of the priority spool.

The demand for flow in the service port progressively reduces.

Steering (Conditions 3 and 4)

Steering Control Unit

T PP

ORBIT STEERING UNIT

Steering (Conditions 5 and 6)

T PP

ORBIT STEERING UNIT

P3

This action cause the priority spool to move downwards directing the flow in to the EF line

This represents the “2 stage principle”

and the LS relief valve in conjunction

Steering Control Unit Steering Control Unit