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National Fluid Power Centre
Steering Systems
Carlton Road
Worksop
Notts S81 7HP
Tel (01909) 504709
Fax (01909) 484571
www.nfpc.co.uk
HYDROSTATIC
STEERING SYSTEMS
Presented by John R Savage Director NFPC
Issue 04/05
EDUCATION AND TRAINING BY PROFESSIONALS
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National Fluid Power Centre
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
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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 hydrostatic
pump.
(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.

A GENERAL MACHINE LAYOUT OF SUCH A SYSTEM IS SHOWN
Introduction
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Steering Systems
(General Machine Layout SIMPLIFIED)
Closed Hydrostatic System (Machine Travel Circuit)
Parking Park Brake
Gearbox
Axle
Service Brake
Priority Valve
Main Hydrostatic Pump
Auxiliary Pump
Displacement
Control
Steering / Brakes
Pump
Steering
Control
Unit
Service Valves
Proportional
Bang - Bang
Pilot Pressure Circuit
CF
EF
Motors Cylinders
Load Holding and Motion Control Valves
Pressure Relief and

Unloading Valves
Direction
Flow
Pressure
Steering Cylinders
Service
Brake
Control
Auxiliary Services
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National Fluid Power Centre
Steering Systems
Let us now bring all the parts together to form an open centre system
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 the
steering 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
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National Fluid Power Centre
Steering Systems
Hydrostatic Steering and Associated Parts
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
operator effort.
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Steering Systems
1. Check valve
2. Shock valve
3. Relief valve
4. Housing with
anti-cavitation
valves
5. Spool
6. Neutral position
springs

7. Sleeve
8. Cross pin
9. Cardan shaft
10. Distributor plate
11and 12. Gerotor Set
13. End cover
OVP
R
L
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 cm
3
per revolution.
Rotary Servo Valve in Detail
Rotary Servo Valve Simplified
Hydrostatic Steering and Associated Parts
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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
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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.
HYDROSTATIC STEERING
fixed
displacement
pump RUNNING
steering wheel in neutral
position and vehicle
moving
External force acting on wheels
F
closed
ports
shock valve
lifts as pressure rise
diverting fluid to
opposite side of
cylinders
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 is
obtained from the return line.
Basic Steering Operation
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National Fluid Power Centre

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
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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