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DEVELOPMENTS IN TERRAIN-VEHICLE MECHANICS
For a long period, one of the challenges in the design of an off-road vehicle was to equip it with a traction device that can develop high traction efficiently with the minimum soil degradation. The aim of terrain-vehicle mechanics is to provide guiding principles to obtain a better understanding of the interaction of the soil-vehicle system. The studies of terrain-vehicle mechanics are generallydirected toward the problems most frequently encountered in the categories of (Yong, 1984): excessive soil compaction induced by vehicle traffic; excessive wheel or track sinkage due to the imposed ground pressure and physical characteristics of both the soil and the vehicle; and excessive wheel or track slippage and insufficient traction caused by internal soil shear or surface friction failure.
OPTIMIZATION OF NEW TRACTION SYSTEMS
In the past, the choice of conventional tractive elements used for off-road vehicles to generate tractive effort was mainly restricted to either pneumatic tyres or steel tracks. It is commonly recognized that tracked vehicles are better draught tractors because they are capable of producing high drawbar pull at a lower slip value and high tractive efficiency, even under difficult conditions such as on very soft surfaces. The large ground contact areas of the tracks result in low ground pressure and good stability on steep slopes. However, steel tracks have adverse characteristics when compared to pneumatic tyres from the point of view of steerability, manoeuvrability, noise, driver fatigue, maintenance and limited speeds. Additionally, travel on public roads is restricted in most areas due to road surface damage from penetration by the steel track grousers.
THE PREDICTION AND EVALUATION OF TRACTIVE PERFORMANCE
M. G. Bekker (1956, 1960, 1969) pioneered the theoretical investigation into the tractive mechanism for off-road vehicles. Although numerous attempts and considerable progress has been made in the past few decades to quantify the soil-machine interaction, understanding of this phenomenon is still far from satisfactory. Generally, models for prediction and evaluation of traction performance can be currently categorized as: empirical models; semi-empirical models; and analytical models.
SOIL CHARACTERIZATION FOR TRACTION MODELLING
For off-road vehicle engineering the measurement of the soil properties is one of the fundamental tasks for the prediction and evaluation of tractive performance. Performance evaluation of terrain-vehicle systems involves both the design parameters for the vehicle and the measurement and evaluation of the physical environment within which the vehicle operates. The soil mechanical properties can be categorized as soil physical properties and soil strength parameters.
ACKNOWLEDGEMENTS
CHAPTER I INTRODUCTION
1.1 Developments in terrain-vehicle mechanics
1.2 Optimization of new traction systems
1.3 The prediction and evaluation of tractive performance
1.4 The development of a prototype track and the motivation for the research
CHAPTER II LITERATURE REVIEW
2.1 Soil characterization for traction modelling
2.1.1 The cone penetrometer technique for soil characterization
2.1.2 The bevameter technique for soil characterization
2.1.2.1 Measurement of pressure-sinkage relationships
2.1.2.2 Measurement of soil shear characteristics
2.1.3 Friction and adhesion characterization for the soil-rubber contact surface
2.2 Traction performance modelling for wheeled vehicles
2.2.1 Empirical methods for traction performance modelling
2.2.2 Analytical methods for traction performance modelling
2.3 Traction performance modelling for tracked vehicles
2.3.1 Empirical methods for traction performance modelling
2.3.2 Analytical methods for traction performance modelling
2.4 Development of and traction characteristics for rubber tracks
2.5 Measurement of the distribution of contact and tangential stresses below a track
2.5.1 Track link dynamometer by Wills (1963)
2.5.2 Applications of extended octagonal ring transducers for measuring two perpendicular forces
2.6 Development of the prototype traction system based on soil-rubber friction
2.7 Justification for conducting this study
2.8 Objectives
CHAPTER III CONSTRUCTION OF THE PROTOTYPE RUBBER-FRICTION TRACTION SYSTEM
3.1 Introduction
3.2 The prototype track
3.2.1 The fundamental construction and layout
3.2.2 The centre ground wheels
3.2.3 Track mounting, tensioning and driving friction at interface
3.2.4 The beam effect
3.3 The drive train, steering control and automatic differential lock
3.4 Dimensions of the prototype track
3.5 Preliminary tests and assesment of tractive performance
3.6 Summary and remarks
CHAPTER IV DEVELOPMENT OF THE TRACTION MODEL FOR THE PROTOTYPE TRACK
4.1. Introduction
4.2 Characterization of rubber-soil friction and soil shear with displacement
4.3 Characterization of the relationship between contact pressure and sinkage
4.4 Analysis of the distribution of track-soil contact pressure
4.4.1 Tractive effort for uniform and trapezoidal pressure distribution
4.4.2 Tractive effort for a rigid track model with a tilt angle
4.4.3 Tractive effort for the flexible track model
4.5 The prediction of motion resistance
4.6 Internal resistance and the friction drive between the wheel and the track
4.7 Total drawbar pull of the prototype track
4.8 The coefficient of traction and tractive efficiency
4.9 Modelling procedure
CHAPTER V INSTRUMENTATION, CALIBRATION AND EXPERIMENTAL PROCEDURE
5.1 Introduction
5.2 Apparatus for soil characterization.
5.3 The extended octagonal ring transducers for measuring the distribution of contact pressure and tangential stress
5.3.1 Design of the transducer
5.3.2 Calibration and installation of the transducers
5.4 Instrumentation for measuring torque, slip and drawbar pull
5.4.1 Instrumentation for measuring the side shaft torque.
5.4.2 Instrumentation for measuring speed and slip.
5.4.3 Instrumentation for measuring drawbar pull
5.5 The computerized data logging system
CHAPTER VI FIELD EXPERIMENTS AND DATA COLLECTION
6.1 Measurement of soil properties
6.1.1 Soil classification
6.1.2 Soil density, soil water content and cone index
6.2 Experimental procedure for soil characterization
6.2.1 Pressure-sinkage characterization for the test plot.
6.2.2 Soil-rubber frictional and soil shear characterization6
6.3 Drawbar pull tests and data collection
CHAPTER VII RESULTS, ANALYSIS AND MODEL VALIDATION
7.1 Introduction
7.2 The distribution of contact pressure
7.2.1 The contact pressure distribution and frictional stress on a hard surface
7.2.2 The effect of the ground wheels on the pressure distribution and frictional stress for a hard surface
7.2.3 The contact pressure distribution and frictional stress
on a soft surface with zero drawbar pull
7.2.4 The effect of the ground wheels on the contact pressure distribution and frictional stress for a soft surface
7.2.5 The influence of the soil water content and the drawbar pull
on the contact pressure distribution
7.3 The relationships of traction coefficient and total slip
7.4 The tractive efficiency
7.5 Analysis of the factors affecting the tractive performance
7.5.1 Soil water content
7.5.2 Track tension
7.5.3 Motion resistance and internal friction
CHAPTER VIII SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
8.1 Summary
8.2 Conclusions
8.3 Recommendations
