Self-Induced Fault of a Hydraulic Servo Valve (eBook)

Dr.-Ing. Seung is an Aircraft Engineer with emphasis on Aerodynamics, Flight Mechanics, System Technology and Hydraulics. He is also a Public-Funding Manager for Aeronautics Research Programs of the German Federal Ministry for Economic Affairs and Energy.

Acknowledgements 6
Contents 7
Abbreviations 9
Introduction 10
Topics and Arrangement of This Report 12
1 Occurrence and Suspicion 13
1.1 Occasion for Investigation, Brief Description of the Occurrence 13
1.2 Appraisal and Fundamental Contemplation for a Pressure-Controlling Servo Valve 14
1.2.1 Design Shape/Working Principle of the Hydraulic Valve 14
1.2.2 Characteristic Diagram—Static and Dynamic Demanding and the Signal Answer 15
1.2.3 Significant Influence Factors and Fault Sources of a Servo Valve 18
1.2.3.1 Particle Contamination of the Fluid (—A Widespread Superstition) 18
1.2.3.2 Robustness of the Torque Motor (First Stage) 19
1.2.4 Fault Potentials and Parameters in the Spool and Sleeve Assembly (Second Stage) 19
1.2.4.1 Dormant Fault Potentials Hidden in Design Shapes 19
Dimensional Tolerances at the Spool and Sleeve Assembly 19
Importance of the Balancing Grooves for Spool’s Working Ability 21
1.2.4.2 Significant Parametric Circumstances Whilst Operating 25
Changing of Actual Friction on the Sliding Surface Area Whilst Operating 25
Internal Leakage and Gap Tolerance as a Significant Influence Factor 25
Air/Temporary Vacuum in the Hydraulic Circuit 26
1.2.5 Fault Potentials/Possible Circumstances in a Solenoid Valve (First Stage) 27
1.3 Existence of a Self-induced Fault of a Two-Stage Servo Valve—Hypothesis 29
2 Survey and Supposition 31
2.1 Experimental Investigation—Test Execution and Results 31
2.1.1 Test Set-Up and Test Method 31
2.1.2 Measurement Data Analysis and Interpretations 33
2.1.2.1 Abnormality in the Pressure Response and Interrelations 33
2.1.2.2 Stability of the Pressure Signal Answer and Mobility of the Spool 33
2.1.2.3 Switching-Off Dynamic of the Spool 36
2.1.2.4 Feedback Intensity in the Water Hammer Effect 43
2.1.2.5 Fluid Dumping Behaviour at the Control Circuit 44
2.1.2.6 Hegemony Loss of the First Stage, Completion of the Self-induction 50
2.1.2.7 Development of a New Drainage and the Loss of Command Ability 51
2.2 Reconstruction of the Entire Fault Working Mechanism—Interim Conclusion 52
3 Supplements and Inference 53
3.1 Additional Reflections/Supplementation of Contemplation 53
3.1.1 Basic Working Mechanism of a Spool and Sleeve Assembly/Homologous Model 53
3.1.1.1 Mobility of a Spool/Equilibrium at a Working Point 53
3.1.1.2 Confinement of the Spool’s Freedom Grade/Guided Sliding of the Spool 54
3.1.1.3 Equilibrium and Meta-Equilibrium in the Second Stage 55
3.1.2 Origin of the Fault/Interlock Mechanism 60
3.1.2.1 Order of the Fault Induction/Origin of the Fault 60
3.1.2.2 Self-stabilization of the Flapper/Interlock of the Entire Servo Valve 61
3.1.2.3 Danger of the Flow Separation on the Flapper Body and the Timing 61
3.1.3 Initializing, Propagation and Escalation of the Blemish/Reason for Blemish 64
3.1.3.1 Degradation/Collapsing of the Lubrication Film—‘Dry Friction’ 64
3.1.3.2 Holistic Consideration Incl. The Reaction of the Control Loop 65
3.1.4 Arbitrariness of the Fault—Russian Roulette Effect 66
3.2 Conclusion and Summary 68
3.2.1 Conclusion 68
3.2.2 Summary 70
Epilog 71
References 74