Hydropneumatic Suspension Systems

Transcription

1 Hydropneumatic Suspension Systems

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3 Wolfgang Bauer Hydropneumatic Suspension Systems 123

4 Dr. Wolfgang Bauer Peter-Nickel-Str Weinheim Germany dr.w.bauer-de@web.de ISBN e-isbn DOI / Springer Heidelberg Dordrecht London New York Library of Congress Control Number: Springer-Verlag Berlin Heidelberg 2011 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Cover design: WMXDesign GmbH, Heidelberg Printed on acid-free paper Springer is part of Springer Science+Business Media (

5 To Jingbo and Linda for their support and patience

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7 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 advantage 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 characteristic 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 element 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

8 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 facilities (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 discussions. Last but not least I thank Dr. Alastair McDonald who polished the linguistic roughness out of my English translation. Weinheim, Germany April 2010 Wolfgang Bauer

9 Contents 1 Suspension Systems Basics Requirements for Suspension Systems Minimize Accelerations on the Isolated Side Equalize Variations of Vertical Wheel Forces General Setup of a Suspension System Hydropneumatic Suspensions Compared to Other Suspension Methods ComparisonofSpringCharacteristics ComparisonofDampingCharacteristics LevelControl Non-functional Requirements Applications for Hydropneumatic Suspensions Spring and Damping Characteristics of Hydropneumatic Suspension Systems General Setup and Working Principle SpringCharacteristics Thermodynamic Background Calculation Predeterminations Non-preloaded Hydropneumatic Suspensions Systems with Mechanical Preload SystemswithConstantHydraulicPreload SystemswithVariableHydraulicPreload DampingCharacteristics Boundary Friction Damping FluidFrictionDamping End-of-StrokeDamping CombinedOperationofSpringandDamper Dimensioning of the Hydropneumatic Suspension Hardware Dimensioning of the Hydraulic Spring Components Cylinder Accumulator Gas Precharge Detailed Calculation of p 0 and V ix

10 x Contents 3.2 DimensioningoftheHydraulicDampingElements Single-Acting Cylinder in a System Without HydraulicPreload Double-Acting Cylinder in a System Without HydraulicPreload Double-Acting Cylinder in a System with HydraulicPreload End-of-StrokeDamping Hydraulic Components Design Cylinders Function and Requirements Types of Cylinders SealingElements End-of-StrokeDamping Types of Support Elements Accumulators Function and Requirements Types of Accumulators Methods to Reduce Diffusion Pressure Loss Integration into Available Design Space FlowResistors Non adjustable Orifices and Throttles Flow Direction Depending Resistors AdjustableFlowResistors Hydraulic Lines and Fittings Function and Requirements Required Flow Cross Section Tubes Hoses Fittings Level Control Self-Pumping Suspension Elements Mechanical Level Control with External Hydraulic Power Supply Electronic Level Control with External Hydraulic Power Supply Function HydraulicCircuits ControlAlgorithms Special Functions of Hydropneumatic Suspension Systems Suspension Lockout Lockout by Blocking the Hydraulic Circuit LockoutattheCompressionEndStop Quasi-Lockout Through High Spring Stiffness

11 Contents xi 6.2 Adjustment of the Zero Position AlterationofRollandPitchBehavior Coupling Cylinders on Corresponding Sides Decoupling Cylinders Coupling Double-Action Cylinders on Opposite Sides Spring Rate Adjustment by Selective Connection of Accumulators Design Examples Tractor Front Axle Suspension TLS by John Deere Passenger Car Axle Suspension by Citroen Citroens First Hydropneumatic Suspension Hydractiv Suspension Activa Suspension Important Patents Improvement of Suspension Characteristics DE DE DE DE DE DE DE US DE US DE Roll Stabilization and Slope Compensation GB DE DE US US Suspension Lockout US DE DE Looking into the Future Index of Symbols and Abbreviations References Index