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SUMMARY
The continued increase in the price of fossil‐based fuels and lubricants has resulted in tremendous increase in the cost of land preparation. This has resulted in considerable increase in the cost of food. The situation is worsened by the prevalent use of the conventional tillage system in the preparation of seedbeds; particularly for deep tillage. This system of tillage escalates land preparation costs because it requires a series of operations using passive tillage tools to realise an acceptable tilth quality. It also ties down capital in the form of additional machinery and tillage tools; thus increasing significantly the cost of land preparation. Therefore, it is necessary to design better tillage tools that are capable of reducing the number of tillage operations required for the realization of seedbeds of acceptable tilth quality.
The rotavator is one of the tillage tools with the capability for realizing the desired soil tilth quality with significantly reduced number of tillage passes. In comparison to passive tools, the rotavator has a superior soil mixing and pulverisation capability. When rotated in the down‐cut direction, it generates a forward thrust that aids traction under difficult field conditions. However, no documented analytical models capable of predicting the performance of rotavators fitted with commercially available blades was found in literature.
In addition, there is dearth of information on the behaviour of the magnitude of the horizontal thrust forces generated for a down‐cut rotavator for different set tillage depths. This study was undertaken to develop an analytical model that is capable of predicting the torque requirements of a rotavator fitted with commercially available L‐shaped blades. In developing the proposed model, an analytical approach based on the limit equilibrium analysis was used. An interactive computer program was developed, in MATLAB (Version 7, Mathworks Inc., USA), to solve the proposed model. The proposed model was verified by comparing the model and measured torque requirement at predetermined rotavator blade angular positions from the horizontal for a down‐cut rotavator.
Field experiments were conducted in a sandy loam soil, using two instrumented research equipment. The research equipment were calibrated in a laboratory and field‐tested prior to conducting the field experiments. A torsional shearing apparatus was used to characterize the soil by determining the soil shear strength and soil‐metal friction parameters. The rotavator operational parameters, necessary for analyzing its performance, were recorded using an instrumented tool‐frame carrier. The experiments were conducted in the down‐cut direction of rotation, in the 200 mm – 500 mm set tillage depth range.
The study findings indicated that there was an optimum set tillage depth for each rotavator configuration and operational conditions at which the resultant horizontal thrust generated was greatest. This unique depth was influenced by the bite length. The validation of the proposed model showed that the predicted and measured torque requirements, at different angular blade positions from the horizontal, correlated reasonably well for all the set tillage depths. As the depth of tillage increased, however, the curve for the measured torque requirements exhibited a cyclic behaviour after the peak torque requirements value had been recorded. The cyclic behaviour was probably due to the re‐tilling and the instability of the tool‐frame carrier, which increased with the set tillage depth.
The knowledge contributed by this research will afford the designers of active tillage tools a better understanding of the operations of the rotavator, particularly in deep tillage. The modelling approach, and instrumentation technique used in this research, can be extended to analyze the performance of rotavators fitted with other types of commercial blades.
Key terms: rotavator, deep‐tilling, soil‐failure modelling, tillage performance, soil shear strength, soil‐metal friction, bite length, kinematic parameter, down‐cut rotavator, power, specific energy
CHAPTER 1: INTRODUCTION
1.1 Background
1.2 General hypothesis and model
CHAPTER 2: LITERATURE REVIEW
2.1 Soil parameters .
2.1.1 Soil physical properties
2.1.2 Soil shear strength and soil failure
2.1.3 Soil‐metal friction.
2.1.4 Dynamic soil strength components
2.2 Tillage tool parameters .
2.2.1 Blade configuration
2.2.2 Direction of rotation
2.2.3 Depth of tillage
2.2.4 The rotavator kinematic parameter, λ
2.3 Modeling energy requirements of tillage tools
2.4 Analytical soil failure models
2.5 Rotavator performance prediction models
2.5.1 Empirical models
2.5.2 Analytical models
2.6 Summary and conclusion from the reviewed literature
2.7 Justification
2.8 Hypotheses
2.9 Objectives
CHAPTER 3: MODELING ROTAVATOR TORQUE AND POWER REQUIREMENTS
3.1 Introduction
3.2 Rotavator kinematics
3.2.1 Equation of motion and the cutting trajectory of the tiller blade
3.2.2 The bite length
3.2.3 Furrow bottoms produced by rotavators
3.2.4 Side surface area of the soil chip
3.2.5 Chip thickness
3.2.6 Volume of the soil chip
3.2.7 Length of the tilling route
3.3 Identification of torque requirement sources
3.4 Proposed analytical model for rotavator torque requirements
3.5 Performance of the experimental tiller
3.6 Chapter summary
CHAPTER 4: RESEARCH EQUIPMENT, INSTRUMENTATION AND CALIBRATION
4.1 The instrumented experimental deep tilling rotavator
4.2 Soil characterisation apparatus
4.3 The data acquisition system
4.4 Calibration
4.5 Preliminary field testing of the apparatus
CHAPTER 5: RESEARCH METHODOLOGY
CHAPTER 6: RESULTS, ANALYSIS AND DISCUSSION
CHAPTER 7: SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
