Hydropneumatic Suspension Systems

Hydropneumatic Suspension Systems

Wolfgang Bauer

Hydropneumatic Suspension

Systems

123

Dr. Wolfgang Bauer

Peter-Nickel-Str. 6

69469 Weinheim

Germany

[email protected]

ISBN 978-3-642-15146-0 e-ISBN 978-3-642-15147-7

DOI 10.1007/978-3-642-15147-7

Springer Heidelberg Dordrecht London New York

Library of Congress Control Number: 2010935667

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To Jingbo and Linda

for their support and patience

Preface

Many people probably use daily life commodities with gas springs without even

knowing or thinking about it. Like many other things in our life they’re simply

there. Moreover there are quite a lot of things that are intrinsically tied to gas as an

elastic medium. Maybe just in this moment you are actually sitting on a gas spring:

your office chair, especially if it’s a swivel chair, is most likely equipped with such

a system. In contrast to simple gas springs, like for example those used for the trunk

lid of your car, the gas spring in your swivel chair is a rather sophisticated suspension

system. Via a button or a lever you have the possibility to allow the transfer of gas

between separate internal chambers. This feature provides the adjustment function

for the seat level and you can easily adapt it to your body height – much easier than

with older mechanical spindle systems like those from, for example, piano stools.

If you use gas as an elastic, suspending medium, basically you always take advan￾tage of the equation of state for the ideal gas. However, since usually the suspension

motions are quick and allow little heat exchange, it is not possible to calculate with

an isothermal change of state but rather with the polytropic approach. It is among

others this special behavior of the gas which makes the respective spring characteris￾tic disproportionately higher. Another advantage of gas springs is the just described

possibility of easy adjustment of the suspension level. This is especially favorable

in applications with different spring loads.

Due to their undoubted positive characteristics, gas springs are used in many

applications. However, when looking at the small hysteresis of the gas forces while

cycling the spring between compression and rebound, it becomes directly obvious

that a simple gas spring always needs the assistance of an additional damping ele￾ment – usually a hydraulic damper. Like their mechanical counterparts (for example

helical springs or torsion bars) the gas spring can dissipate only a little amount of

energy during the suspension motion (except for the special so called air damping

systems). The gas spring of the above mentioned swivel chair is rather special since

it is only dampened by an (intentionally) high solid body friction of the setup. This

is fully sufficient since this arrangement is mostly used as a shock absorber (when

sitting down) and is not exposed to frequent excitation – well, except for the rather

unpleasant case of an earthquake.

Now is the time to take the step towards hydropneumatic suspensions. Here too,

a gas volume acts as the elastic medium, so basically the same laws apply as for

vii

viii Preface

the pure gas spring. The only difference here is that the gas pressure is not directly

in contact with the active surface of the spring element but is transferred by an

additional component – the hydraulic fluid. It can be called a coupling medium

since it acts just like a mechanical coupling rod.

The fluid connection offers numerous advantages: on one hand fluids can be

sealed better than gas which basically increases the possible working pressures

and therefore reduces the space requirements for the suspension element. On the

other hand the fluid offers the possibility to dissipate some of the motion energy

into heat, just like in a regular hydraulic damper. This viscous friction inside the

hydraulic fluid is more favorable for the damping of oscillations than for example

the above mentioned solid body friction and it can quite easily be adapted to certain

applications or even be made adjustable. So the bottom line is: a hydropneumatic

suspension provides spring and damping function always in direct concurrence.

Speaking for myself, I came in contact with hydropneumatic suspensions rather

late, after graduation, through my employment at the John Deere Mannheim facili￾ties (formerly Lanz tractor factory). My work on the wide field of hydraulics and, in

particular, hydropneumatic suspension systems made me aware of the advantages of

this technology. One important field for hydropneumatic suspensions is agricultural

tractors. This is underlined by the fact that today almost every suspended tractor

front axle is suspended with hydropneumatics. The reasons for this and much more

is explained in the following chapters. This book is based on experience in design

and testing which I gathered in the past decade. It is a translation of my initial

German edition [BAU08] with some updates and additions. The intention of this

book is to create a basic understanding of what is possible with a hydropneumatic

suspension system and which particular advantages and peculiarities this system

includes. In doing so, it is hoped that this technology will benefit many different

applications in the future.

I would like to express my gratitude to my parents and to all friends who

encouraged me to write this book. Furthermore I am indebted to my professional

colleagues, who supported me on my way from the raw version to the printable

version and who created a fertile ground for new ideas in many inspiring discus￾sions. Last but not least I thank Dr. Alastair McDonald who polished the linguistic

roughness out of my English translation.

Weinheim, Germany Wolfgang Bauer

April 2010

Contents

1 Suspension Systems Basics ....................... 1

1.1 Requirements for Suspension Systems . . ............. 1

1.1.1 Minimize Accelerations on the Isolated Side . . . .... 2

1.1.2 Equalize Variations of Vertical Wheel Forces . . . .... 4

1.2 General Setup of a Suspension System . . ............. 5

1.3 Hydropneumatic Suspensions Compared to Other

Suspension Methods . ....................... 7

1.3.1 Comparison of Spring Characteristics . . . . . . . . . . . 7

1.3.2 Comparison of Damping Characteristics . . . . . . . . . 11

1.3.3 Level Control . . . . . . . . . . . . . . . . . . . . . . . 12

1.3.4 Non-functional Requirements . . . . . . . . . . . . . . . 13

1.4 Applications for Hydropneumatic Suspensions . . . . . . . . . . . 15

2 Spring and Damping Characteristics of Hydropneumatic

Suspension Systems ........................... 19

2.1 General Setup and Working Principle . . . . . . . . . . . . . . . . 19

2.2 Spring Characteristics . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.1 Thermodynamic Background . . . . . . . . . . . . . . . 21

2.2.2 Calculation Predeterminations . . . . . . . . . . . . . . . 25

2.2.3 Non-preloaded Hydropneumatic Suspensions . . . . . . . 25

2.2.4 Systems with Mechanical Preload . . . . . . . . . . . . . 35

2.2.5 Systems with Constant Hydraulic Preload . . . . . . . . . 41

2.2.6 Systems with Variable Hydraulic Preload . . . . . . . . . 48

2.3 Damping Characteristics . . . . . . . . . . . . . . . . . . . . . . 50

2.3.1 Boundary Friction Damping . . . . . . . . . . . . . . . . 51

2.3.2 Fluid Friction Damping . . . . . . . . . . . . . . . . . . 55

2.3.3 End-of-Stroke Damping . . . . . . . . . . . . . . . . . . 62

2.4 Combined Operation of Spring and Damper . . . . . . . . . . . . 64

3 Dimensioning of the Hydropneumatic Suspension Hardware .... 67

3.1 Dimensioning of the Hydraulic Spring Components . . . . . . . . 67

3.1.1 Cylinder . . . . . . . . . . . . . . . . . . . . . . . . . . 69

3.1.2 Accumulator Gas Precharge . . . . . . . . . . . . . . . . 71

3.1.3 Detailed Calculation of p0 and V0 . . . . . . . . . . . . . 73

ix

x Contents

3.2 Dimensioning of the Hydraulic Damping Elements . . . . . . . . 85

3.2.1 Single-Acting Cylinder in a System Without

Hydraulic Preload . . . . . . . . . . . . . . . . . . . . . 85

3.2.2 Double-Acting Cylinder in a System Without

Hydraulic Preload . . . . . . . . . . . . . . . . . . . . . 88

3.2.3 Double-Acting Cylinder in a System with

Hydraulic Preload . . . . . . . . . . . . . . . . . . . . . 91

3.2.4 End-of-Stroke Damping . . . . . . . . . . . . . . . . . . 91

4 Hydraulic Components Design ..................... 95

4.1 Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

4.1.1 Function and Requirements . . . . . . . . . . . . . . . . 95

4.1.2 Types of Cylinders . . . . . . . . . . . . . . . . . . . . . 96

4.1.3 Sealing Elements . . . . . . . . . . . . . . . . . . . . . . 101

4.1.4 End-of-Stroke Damping . . . . . . . . . . . . . . . . . . 106

4.1.5 Types of Support Elements . . . . . . . . . . . . . . . . 109

4.2 Accumulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

4.2.1 Function and Requirements . . . . . . . . . . . . . . . . 111

4.2.2 Types of Accumulators . . . . . . . . . . . . . . . . . . 113

4.2.3 Methods to Reduce Diffusion Pressure Loss . . . . . . . 116

4.2.4 Integration into Available Design Space . . . . . . . . . . 118

4.3 Flow Resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

4.3.1 Non adjustable Orifices and Throttles . . . . . . . . . . . 120

4.3.2 Flow Direction Depending Resistors . . . . . . . . . . . 122

4.3.3 Adjustable Flow Resistors . . . . . . . . . . . . . . . . . 126

4.4 Hydraulic Lines and Fittings . . . . . . . . . . . . . . . . . . . . 130

4.4.1 Function and Requirements . . . . . . . . . . . . . . . . 130

4.4.2 Required Flow Cross Section . . . . . . . . . . . . . . . 132

4.4.3 Tubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

4.4.4 Hoses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

4.4.5 Fittings . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

5 Level Control ............................... 141

5.1 Self-Pumping Suspension Elements . . . . . . . . . . . . . . . . 141

5.2 Mechanical Level Control with External Hydraulic Power Supply . 144

5.3 Electronic Level Control with External Hydraulic Power Supply . 147

5.3.1 Function . . . . . . . . . . . . . . . . . . . . . . . . . . 147

5.3.2 Hydraulic Circuits . . . . . . . . . . . . . . . . . . . . . 148

5.3.3 Control Algorithms . . . . . . . . . . . . . . . . . . . . 150

6 Special Functions of Hydropneumatic Suspension Systems ..... 157

6.1 Suspension Lockout . . . . . . . . . . . . . . . . . . . . . . . . . 157

6.1.1 Lockout by Blocking the Hydraulic Circuit . . . . . . . . 158

6.1.2 Lockout at the Compression End Stop . . . . . . . . . . . 160

6.1.3 “Quasi-Lockout” Through High Spring Stiffness . . . . . 161

Contents xi

6.2 Adjustment of the Zero Position . . . . . . . . . . . . . . . . . . 162

6.3 Alteration of Roll and Pitch Behavior . . . . . . . . . . . . . . . 163

6.3.1 Coupling Cylinders on Corresponding Sides . . . . . . . 163

6.3.2 Decoupling Cylinders . . . . . . . . . . . . . . . . . . . 164

6.3.3 Coupling Double-Action Cylinders on Opposite Sides . . 166

6.4 Spring Rate Adjustment by Selective Connection

of Accumulators . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

7 Design Examples ............................. 173

7.1 Tractor Front Axle Suspension TLS by John Deere . . . . . . . . 173

7.2 Passenger Car Axle Suspension by Citroen . . . . . . . . . . . . . 180

7.2.1 Citroens First Hydropneumatic Suspension . . . . . . . . 181

7.2.2 Hydractiv Suspension . . . . . . . . . . . . . . . . . . . 183

7.2.3 Activa Suspension . . . . . . . . . . . . . . . . . . . . . 188

8 Important Patents ............................ 193

8.1 Improvement of Suspension Characteristics . . . . . . . . . . . . 193

8.1.1 DE1755095 . . . . . . . . . . . . . . . . . . . . . . . . 194

8.1.2 DE19719076 . . . . . . . . . . . . . . . . . . . . . . . . 195

8.1.3 DE10107631 . . . . . . . . . . . . . . . . . . . . . . . . 196

8.1.4 DE10337600 . . . . . . . . . . . . . . . . . . . . . . . . 196

8.1.5 DE4221126 . . . . . . . . . . . . . . . . . . . . . . . . 198

8.1.6 DE4234217 . . . . . . . . . . . . . . . . . . . . . . . . 198

8.1.7 DE4223783 . . . . . . . . . . . . . . . . . . . . . . . . 200

8.1.8 US6167701 . . . . . . . . . . . . . . . . . . . . . . . . 201

8.1.9 DE19949152 . . . . . . . . . . . . . . . . . . . . . . . . 201

8.1.10 US6398227 . . . . . . . . . . . . . . . . . . . . . . . . 203

8.1.11 DE102008012704 . . . . . . . . . . . . . . . . . . . . . 203

8.2 Roll Stabilization and Slope Compensation . . . . . . . . . . . . 205

8.2.1 GB890089 . . . . . . . . . . . . . . . . . . . . . . . . . 205

8.2.2 DE3427508 . . . . . . . . . . . . . . . . . . . . . . . . 206

8.2.3 DE10112082 . . . . . . . . . . . . . . . . . . . . . . . . 207

8.2.4 US4411447 . . . . . . . . . . . . . . . . . . . . . . . . 208

8.2.5 US6923453 . . . . . . . . . . . . . . . . . . . . . . . . 209

8.3 Suspension Lockout . . . . . . . . . . . . . . . . . . . . . . . . . 210

8.3.1 US3953040 . . . . . . . . . . . . . . . . . . . . . . . . 211

8.3.2 DE4308460 . . . . . . . . . . . . . . . . . . . . . . . . 211

8.3.3 DE4032893 . . . . . . . . . . . . . . . . . . . . . . . . 212

9 Looking into the Future ......................... 215

Index of Symbols and Abbreviations .................... 219

References .................................. 223

Index ..................................... 229

Chapter 1

Suspension Systems Basics

1.1 Requirements for Suspension Systems

As already mentioned in the preface, suspension systems have a broad range of

applications in our daily lives. Usually people do not even know that they exist, yet

they are doing a hard job in many cases. If they malfunction it is often the first time

that one starts thinking about them. For example, anybody who has ridden a bicycle

with too low tire pressure will probably remember how soft and wobbly the bike felt

on smooth roads and how badly he felt the bumps when there was even the slightest

unevenness. A ride behavior which is unsafe and uncomfortable. In this case the

spring rate of the suspension system (i.e. the tire) was too low and the available

suspension travel was too small. Therefore the suspension reached the limit of its

stroke and ran heavily into the end stop – rim and road surface with the rubber of

the tire in between. On the other hand, a too high tire pressure and an accordingly

too high spring rate can also lead to discomfort on the bike. Without sufficient tire

elasticity the roughness of the road is transferred directly into the bike frame and

furthermore into the rider. This again has a negative effect on the comfort of the

rider. It is clear that it is necessary to find a suitable level of tire pressure and thus

spring rate which fits in particular to the weight of the rider.

This brings us to the first basic objective of a suspension system: it has to pro￾tect the components of its isolated side (for example, chassis and driver) from the

movements and accelerations of its input side (for example, road or wheel). This

isolation of the vibration ensures comfort and health for the driver and prevents

components on the isolated side from damage from inertial forces. If the suspen￾sion system fulfills these requirements for vehicles, another important advantage is

achieved: compared to a vehicle without a suspension system it can be driven faster

at equal or even lower vibration loads on the isolated side.

Particularly for wheel suspension systems there is at least one more tremendously

important objective: the time history of the vertical wheel forces on the road should

be as smooth as possible in order to ensure that a high level of lateral and lon￾gitudinal wheel force can be transferred to the road surface at any time. Strong

peaks in the vertical wheel force vs. time curve can lead to a situation where the

W. Bauer, Hydropneumatic Suspension Systems, DOI 10.1007/978-3-642-15147-7_1, 1

C Springer-Verlag Berlin Heidelberg 2011

2 1 Suspension Systems Basics

Safety Easier operation of operators controls

Better driver’s fitness

Better road holding

Comfort Minimize accelerations on the

isolated side

Equalize variations of vertical

wheel forces

Health

Damage prevention

Increased productivity Higher speeds possible

Increased pulling force

Increased efficiency

Fig. 1.1 Tasks and functional requirements for a wheel suspension system

normal force is lower than the necessary level to create a sufficient friction force

for the transfer of lateral and longitudinal forces. This then causes a transition from

static to sliding friction resulting in unexpected and unsafe ride behavior. But not

only is road holding better with a smooth vertical wheel force transfer; a better

transfer of pulling forces with lower wheel slip results in higher efficiency and

productivity especially for pulling working machines like tractors or other off-road

equipment.

Further objectives especially for wheel suspension systems are, for example, the

prevention of road damage (by high wheel forces) and an acceptable roll and pitch

behavior of the chassis. For passenger cars it is also especially important to create

a subjective ride behavior that fits to the type of vehicle – from supersports car to

luxury sedan.

Figure 1.1 explains the relationship between several tasks and the two deduced

functional requirements “minimize accelerations on isolated side” and “equalize

variations of vertical wheel forces” for a wheel suspension system. These two

requirements will be explained in more detail on the following pages of this

section.

1.1.1 Minimize Accelerations on the Isolated Side

Mechanical components on the isolated side can often be designed to withstand the

prevailing vibration level. Yet in many cases it is the human as the “living compo￾nent” of the isolated side who is the limiting factor: he too must not be subjected to

excessive vibrations. Vibrations are perceived by humans on different parts of the

body and in different frequency ranges. From 1 to 100 Hz they are sensed to be

accelerations and displacements, in the frequency range of 20 Hz–10 kHz they are

perceived acoustically (noise). Reimpell points out that the range from 1 to 4 Hz