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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

A GENERAL MACHINE LAYOUT OF SUCH A SYSTEM IS SHOWN
Introduction
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National Fluid Power Centre
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

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

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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National Fluid Power Centre
Steering Systems
HYDROSTATIC STEERING
STEERING 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

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