Development of a Leaf Spring U-Bolt Load Transducer: Part of an Onboard Weighing System for Off-Highway Log Trucks
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DEVELOPMENT OF A LEAF SPRING U-BOLT LOAD TRANSDUCER: PART OF AN ONBOARD WEIGHING SYSTEM FOR OFF-HIGHWAY LOG TRUCKS by MITHUN KARUNAKAR SHETTY B.E., The University of Mumbai, 2002 A THESIS SUMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Forestry) THE UNIVERSITY OF BRITISH COLUMBIA May 2006 © Mithun Karunakar Shetty, 2006 ABSTRACT This thesis was motivated by the current concern of brake failure in off-highway log trucks descending steep grades. In order to utilise a guideline being developed for the prediction of safe maximum grades for descent under a range of truck payloads, it is necessary to measure axle weights during loading. A background review found that there are no commercially available on-board weighing systems that can be retrofitted to the drive axles of an off-highway tractor. Therefore, an investigation into the development of an on-board weighing system for the off-highway log trucks was initiated. This research was divided into two stages: preliminary strain measurement with a loaded off-highway tractor, and finite element modelling of a U-bolt from the tractor's leaf spring suspension. A preliminary measurement test was carried out to identify potential suspension components that could act as load transducers for measuring axle weight. The preliminary results showed that incremental strain at two locations on the U-bolt varied linearly with payload, for an incremental load of 22.5 kN. Finite element modelling of the U-bolt was carried out to predict the maximum incremental strain occurring on the U-bolt surface. The model was calibrated with the measured data and a sensitivity analysis was done on key modelling parameters to determine the most suitable level of leaf spring block length, preload and U-bolt-to-leaf spring friction coefficient. Incremental strain on the top of the curved portion of the U-bolt was found to be relatively consistent and close to the maximum level of incremental strain and is recommended as a preferred position for strain gauging. ii TABLE OF CONTENTS ABSTRACT : ii TABLE OF CONTENTS iii LIST OF TABLES '. iv LIST OF FIGURES v ACKNOWLEDGEMENTS vii CHAPTER 1 INTRODUCTION 1 1.1 Need for investigation 1 1.2 Background review 4 1.3 Objectives 10 1.4 Organisation of the thesis 11 CHAPTER 2 PRELIMINARY STRAIN MEASUREMENTS 12 2.1 Sensor location on the HDX suspension 12 2.2 Instruments used 13 2.2 Methodology 14 2.3 Results and Discussion • 15 CHAPTER 3 FINITE ELEMENT MODEL CONSTRUCTION 21 3.1 Overview 21 3.2 Background of U-bolt modelling 22 3.3 FEM description 25 3.4 Meshing of the FEM 29 3.5 Boundary conditions 30 3.6 Preload modelling 31 3.7 External load 32 3.8 Contact modelling 32 3.9 Postprocessing 34 CHAPTER 4 FINITE ELEMENT MODELLING RESULTS 35 4.1 Overview 35 4.2 Sensitivity Analysis 35 4.2.1 Leaf spring block length 36 4.2.2 Preload 38 4.2.3 Coefficient of friction 41 4.3 Analysis of incremental strain .42 4.4 Possible causes of bending in the U-bolt shank 47 4.5 Limitations of the empirical measurements and finite element modelling 52 CHAPTER 5 SUMMARY, CONCLUSION AND RECOMMENDATION 54 REFERENCES 56 APPENDIX I .' — 58 APPENDIX II 60 iii LIST OF TABLES Table 2-1: Estimated weight of water added and resulting drive axle group weights 16 Table 3-1: FEM component physical properties 29 Table 4-1: Run sequence inputs to evaluate leaf spring block length.... 37 Table 4-2: Run scenario for the different levels of preload and coefficient of friction between the curved portion of U-bolt and leaf spring 39 iv LIST OF FIGURES Figure 1.1: Typical off-highway log truck 1 Figure 1.2: Off-highway log truck axle weights used in forest road bridge design 2 Figure 1.3: Walking beam load transducer 4 Figure 1.4: Log bunk assembly on a highway log truck 5 Figure 1.5: Beam load cell mounted on a highway log truck 5 Figure 1.6: Trailer suspension assembly with SI beam load cell 6 Figure 1.7: Bunk assembly on an off-highway log truck 6 Figure 1.8: Leaf spring suspension of an off-highway tractor 7 Figure 1.9: Load cells mounted between a drive axle and its leaf spring suspension 9 Figure 2.1: Strain gauge locations 13 Figure 2.2: Measuring drive axle weights with portable pad scales 15 Figure 2.3: Incremental leaf spring strain response to unloading the tractor with strain gauges applied when tractor was loaded 17 Figure 2.4: Incremental axial strain measured in two locations on a leaf spring U-bolt during unloading with strain gauges applied when tractor was loaded 18 Figure 2.5: Incremental axial strain at location 1 on the suspension U-bolt in response to an averaged drive axle group weight 20 Figure 3.1: FE modelling procedure for the U-bolt analysis 22 Figure 3.2: Finite element modelling of U-bolted assembly 23 Figure 3.3: FEM of a U-bolted leaf spring assembly... 24 Figure 3.4: Schematic of the U-bolt assembly 26 Figure 3.5: 3-D 10-node tetrahedral structural solid... 27 Figure 3.6: Location of contact elements 28 Figure 3.7: Surface-to-surface contact elements 28 V Figure 3.8: FEM of the U-bolt assembly 30 Figure 3.9: Boundary condition 31 Figure 4.1: Incremental strain at gauge location 1 on the U-bolt for various leaf spring block lengths 37 Figure 4.2: Incremental strain at gauge location 2 on the U-bolt for various leaf spring block lengths 38 Figure 4.3: Incremental strain at gauge location 1 on the U-bolt for various preloads and coefficients of friction (u) between the U-bolt and the leaf spring 40 Figure 4.4: Incremental strain at gauge location 2 on the U-bolt for various preloads and coefficients of friction (u) between the U-bolt and the leaf spring 40 Figure 4.5: Simulated incremental strains at locations 1 and 2 on the U-bolt for different U-bolt-to-leaf spring coefficients of friction 42 Figure 4.6: Distribution of incremental strain along the outer edge of U-bolt surface 43 Figure 4.7: Incremental strain distribution along the inner and outer surfaces of the U-bolt shank 44 Figure 4.8: Bending strain distribution along the outer surface of the U-bolt shank 45 Figure 4.9: Recommended gauge location on the U-bolt 47 Figure 4.10: Bending in the U-bolt 48 Figure 4.11: Geometry of two-dimensional model 49 Figure 4.12: Contact elements in two cases 50 Figure 4.13: Deformed shape for case 1 50 Figure 4.14: Deformed shape for case 2 51 Figure 4.15: Defining U-bolt Volume in FEM 52 Figure A.I: Strain transformation along the curved portion of the U-bolt 58 Figure A.II: Slip-on water tank dimensions 60 vi ACKNOWLEDGEMENTS I would like to express my sincere gratitude and thanks to my supervisor Dr. Kevin C. Lyons for financial support and his invaluable help over the whole course of my studies. My special thanks to Mr. Allan Bradley of Forest Research Institute of Canada (FERIC) for his invaluable professional advice, time and effort spend on proofreading this report. I would also like to thanks his family for their support during my stay in Canada. My sincere thanks to Mr. Marv Clark of FERIC for providing constructive feedback, financial assistance and instruments used in the preliminary testing. I would also like to thank Dr. John Nelson for participating on the committee for this thesis. I would like to acknowledge Mr. Allan Waugh of Hayes Forest Services (Hayes) for providing a truck and shop facilities for the preliminary testing, and Mr. Jeff Layfield of Hayes for providing useful information required in this work. Thanks to the Instrumentation Lab, Dept. of Mechanical Engineering, UBC for providing instruments used in the preliminary testing. I would like to thank Mr. Rob Jokai and Mr. Seamus Parker of FERIC for their help with using FERIC-supplied instruments. I owe my parents a special gratitude for their love, understanding and support during the course of the study. M.K.S. vii CHAPTER 1 INTRODUCTION 1.1 Need for investigation Heavy-duty off-highway log trucks, as shown in Figure 1.1, have a very limited population and now are mainly confined to use on the coast of British Columbia. These trucks are no longer manufactured and most are at least 30 years old. They have been kept in service because of their robust construction and high load carrying capacity. These log trucks commonly consist of a tandem drive axle tractor and a tandem axle pole trailer. Figure 1.1: Typical off-highway log truck The typical loaded mass of these trucks is between 107 to 122 tonnes, with payloads of approximately 68 to 83 tonnes (Oakley and Marshall, 1989). These trucks were designed for off-highway operation where axle loads and vehicle dimensions are not subject to regulations applied to public roads. Figure 1.2 presents maximum axle loads for an off-highway log truck used for the design of forestry bridges in British Columbia. 1 In addition to the differences in size and weight between highway and off-highway log trucks; there are also structural differences. Two of most common off-highway truck models are considered in this study—the Hayes HDX and the Pacific PI6.