Draughts And Power Requirements Of Subsoilers
Abstract
Draughts, power requirements and soil disruption of tillage tools are important parameters useful for their effective design, fabrication and performance during operation for effective agricultural mechanisation. Subsoilers have gained much ground in their application for alleviating soil compaction; and are attracting awareness in their utilization for conservative tillage practices. Subsoiler is a tractor mounted implement used to loosen and break up soil hard-pan at depths up to 60 cm and above the level of a traditional disk plough, mouldboard plough, chisel plough or rotary plough.
Development and performance evaluation of subsoilers and their energy requirements during operation has been of great concern to engineers and farmers as these have direct and indirect effects on the efficiency of tillage operations. Draughts reduction, minimal power utilisation and increased soil disruption and pulverisation are some of the main performance indicators of subsoilers. Hence several researchers have studied various subsoilers and parameters to minimize draught force and total power requirements with considerable increase in soil pulverisation.
Consideration should be given to the design of shanks shape of subsoiler, as they are very important to the efficiency and effectiveness of subsoiling. Thus, variation in power requirements depends on subsoiling depth, soil water conditions and the amount of compaction. In order to achieve better soil disturbance, reduced draught force and energy requirements, and less traction resistance, the application of vibratory (oscillatory) and rotary subsoilers in modern day design and development of subsoilers are preferred for lower overall demand on engine power. Key Words: Draughts, Power Requirements, Soil Disruption, Subsoilers, Deep Tillage
Introduction
Draughts, power requirements and soil disruption of tillage tools are important parameters useful for their effective design, fabrication and performance during operation for effective agricultural mechanisation. Development and performance evaluation of tillage tools and their energy requirements during operation has been of great concern to engineers and farmers as these have direct and indirect effects on the efficiency of tillage operations.
Tillage tools are mechanical devices used for applying forces to the soil to cause one or more of cutting, movement, fracturing, loosening, overturning and pulverization of the soil to prepare a seed bed. Subsoiler is a tractor mounted implement used to loosen and break up soil at depths below the level of a traditional disk plough, mouldboard plough, chisel plough or rotary plough. Most tractor mounted cultivation tools will break up and turn over surface soil to a depth of 15-20 cm, while a subsoiler will break up and loosen soil to twice those depths.
Typically a subsoiler mounted to a Compact Utility Tractor will reach depths of about 30 cm and above. The subsoiler is a tillage tool which will improve growth in all crops where soil compaction is a problem. The design provides deep tillage, loosening soil deeper than a tiller or plough. Agricultural subsoilers has the ability to disrupt hardpan down to 60 cm depth and more. Draft reduction, optimum power utilisation and increased soil disruption and pulverisation are some of the main performance indicators of subsoilers. Hence several researchers have studied various parameters to minimize draft force and total power requirements and considerable soil loosening. This attempt is therefore made to review the draught, power requirements and soil disruption of subsoilers.
Requirements For Subsoilers
Draught is an important parameter for measurement and evaluation of implement performance. The specific draught of agricultural tools and implements varies widely under different conditions, being affected by such factors as the soil type and condition, ploughing speed, plough type, shape, friction characteristics of the soil-engaging surfaces, share sharpness, and shape, depth of ploughing, width of furrow slice, type of attachments, and adjustment of the tool and attachments. A great deal of work has been done in evaluating these various factors and investigating possible means for reducing draught.
Rational design must be based on knowledge of tool performance and soil parameters. For efficient tillage, both must be considered with the aim of minimizing specific resistance, which is draught per unit area of soil disturbance . Quantification of force response relations for the soil cutting process can be used by the equipment designer for improving cutting element design, and for mathematically simulating whole vehicle performance. Traditional tools have been designed in the light of empirical experimentation based on low speed tests and quasi-static theory of soil cutting. Experimental results cannot be directly extrapolated for use with high speed tools because the results would be unrealistic. The developed concepts in soil dynamics depend on controlled experiments. Soil-bin facilities are usually employed for such controlled studies. The use of microcomputer based data acquisition and control system has greatly enhanced data collection and processing and ensured better monitoring of the parameters varied during the experiments in the soil-bins.
A high-energy input is required to disrupt hardpan layer to promote improved root development and increased draught tolerance. Significant savings in tillage energy could be achieved by site-specific management of soil compaction. Site-specific variable-depth tillage system can be defined as any tillage system which modifies the physical properties of soil only where the tillage is needed for crop growth objectives. It was revealed that the energy cost of subsoiling can be decreased by as much as 34% with site-specific tillage as compared to the uniform-depth tillage technique currently employed by farmers.
There is also a 50% reduction in fuel consumption by site-specific or precision deep tillage. Tillage implement energy is directly related to working depth, tool geometry, travel speed, rake angle, width of the implement, and soil properties. Soil properties that contribute to tillage energy are moisture content, bulk density, cone index, soil cohesion and adhesion, and soil texture. It has been reported that draught on tillage tools increases significantly with speed and the relationship varies from linear to quadratic. As reported by estimated draught and soil disturbance of conventional and winged subsoilers working at depth of 0.35 m to be 20.43 kN and 0.098 m2, and 26.58 kN and 0.184 m2 respectively. He then recommended approximate practical spacing for simple and winged tines for good soil loosening as:
- 1.5 x depth of work for simple tines;
- 2.0 x depth of work for winged tines.
Further stated that variation in power requirements depends on subsoiling depth, soil water conditions and the amount of compaction. Power to pull a subsoiler will depend on the number of shanks being pulled and tractive conditions. For most soil conditions optimum tractive efficiency can be obtained in the 10 to 15 percent slip range. If slip is more than 15 percent or less than 10 percent, ballast should be added or removed, respectively.
Forces On Subsoilers
Reported that the draught requirement of any tillage implement was found to be a function of soil properties, tool geometry, working depth ,travel speed, and width of the implement. Soil properties that contribute to tillage energy are moisture content, bulk density, soil texture and soil strength. The relationship between the draught of plane tillage tools and speed, has been defined as linear, second-order polynomial, parabolic and exponential. Reported forces acting on tillage tools to include: (i) horizontal or draught force: the amount of force required to pull or push the implement through the soil, (ii) vertical force: the implement force assisting or preventing penetration into the soil, and (iii) lateral or sideways forces. In parallel to the work referred to earlier, mathematical models have been developed to predict the magnitude of the soil forces acting upon implements of different geometry. These are based upon the general soil mechanics equation and enable the draught and vertical forces to be calculated from knowledge of the tool geometry, working depth, soil physical properties and the type of the soil disturbance pattern produced by the tool. They have been integrated into a unified model described by [16] and formulated into a number of spreadsheets for the use of those who wish to estimate the effects of different implement geometry on the soil forces in a given soil and the effect of different soils on a given implement shape. The spreadsheets consider a range of implements, namely:
- single and multiple tines,
- land anchors,
- discs, and
- mouldboard ploughs.
Measurement of tillage forces using instrumentations and reported that measurement of forces on tillage tools have been an issue of great concern in soil tillage dynamics. Draught measurements are required for many studies including energy input for field equipment, matching tractor to an implement size, and tractive performance of a tractor. Vertical force affects weight transfer from implement to the tractor, and consequently, affects the tractive performance and dynamic stability of the tractor. Several side loads can affect tractor’s steering ability. However, side force is generally negligible during field operation.
Several researchers have worked on measurement of forces on tillage. Explained four different types of instrumentations utilized in the measurement of forces on tillage tools. These are transducer, dynamometer, strain gauge and extended orthogonal ring transducer. Transducer is a device that converts a signal in one form of energy to another form of energy. Energy types include (but are not limited to) electrical, mechanical, electromagnetic (including light), chemical, acoustic and thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a transducer.
Dynamometer is an instrument for determining power, usually by the independent measurement of forces, time and the distance through which the force is moved. A dynamometer must not only be able to measure the forces between itself and a tool, it must also be able to hold the tool in position so that the tool depth, width and orientation do not change during operation. Strain Gauges have replaced earlier used dynamometers with hydraulic units. With the advancement of technology, strain gauge force transducers have been developed.
A direct-connected strain gauge that senses only the draught component of the pull has been put in place. Extended octagonal ring transducer is one of the most common methods used to measure specific forces on tillage tools. This transducer allows the measurement of forces in two directions and the moment in the plane of these forces.
On the other hand, the load cell is a transducer that is used to convert a force into an electrical signal. This deforms a strain gauge. The strain gauge measures the deformation (strain) as an electrical signal, because the strain changes the effective electrical resistance of the wire. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration. Load cells of one strain gauge (Quarter Bridge) or two strain gauges (half bridge) are also available. The electrical signal output is typically in the order of a few millivolts and according to this requires amplification by an instrumentation amplifier before it can be used. The output of the transducer can be scaled to calculate the force applied to the transducer.
The various types of load cells that exist include Hydraulic load cells, Pneumatic load cells and Strain gauge load cells. Load cells are currently being utilized in measuring different forces on tillage tools. The first attempt to measure the forces between tractor and mounted implement were made by measuring the forces in links themselves. This required simultaneous recording of at least three forces which involved very complicated instrumentation. Later developed a three-point hitch dynamometer which could be used with hydraulic linkage providing position and draught control, unlike his previous design which was for un-restrained linkages.
Measuring the drawbar power of tillage tools is accomplished by apparatuses such as hydraulic and mechanical dynamometers. Drawbar dynamometer is used for pull-type implements while the three-point hitch type is employed for mounted implements. The first attempts to measure the forces between tractor and mounted implement were made by measuring the forces in links themselves. This required simultaneous recording of at least three forces which involved very complicated instrumentation.
Developed strain gauged pins for measuring the draught of a three-point link implement. These pines could only measure longitudinal component of force in each link and were only suitable for free linkage systems. Improved the system proposed by. The system used instrumented ball joints. These ball joints system had friction induced cross sensitivity problems. Reduced this effect by using self-aligning ball bearings and longer beam length. This caused the equipment heavier, displaced the implement backwards and thus increased the bending moment. Moving the implement back from its nominal position affects the tractor-implement geometry and hence it’s operating characteristics. The instrument could not fit on many tractors. Modification to the tractor was required to fit the system. The use of PTO was also obstructed.
Later developed a three-point hitch dynamometer which could be used with hydraulic linkage providing position and draught control, unlike his previous design which was for un-restrained linkages. The shape was such that it can permit PTO use accordingly. Friction was minimized by use of self-aligning ball bearings. Cross-sensitivity was 2% on horizontal draught force and 0.5% on vertical forces. Modifications were needed if the instrument was to be used with mounted implement and was not fit to category I implements. The construction was bulky which weighted 120 kg. The implement was shifted back by 23 cm from its nominal position. Used six load cells mounted at different points within an ‘A’ shaped frame to measure horizontal, vertical and lateral forces. The measurements were made with little error.
The implement moved back by 19 cm. Developed a quick attachment coupler using pins mounted as strain gauged cantilever beams. It eliminated the need for modification in either tractor or implement since it could be used with category II and III hitch dimensions. This dynamometer gave minimum sensing errors but the implement was pushed back by 21 cm.
Designed and developed a three-point hitch dynamometer for measurement of loads imposed on agricultural tractors by implement mounted on a standard three-point linkage conforming to category I, II or III. He reported that the 350 kg mass of the dynamometer limits its use with small tractors to light weight implements. This mass and the rearward displacement of the implement by 17.35 cm is slightly more than allowed by ASAE Standards S278.6. He also reported that the developed dynamometer has a force capacity of approximately 50 kN which provides adequate sensitivity at the low end of the designed tractor power range with sufficient strength for the high power range.
Another three-point hitch dynamometer was designed and manufactured by [30]. The dynamometer was capable of measuring tractor - implement forces in three dimensions, which could help in the design of tillage tools and evaluating tractor performance. They reported that the dynamometer consists of three arms, which slide in an inverted hollow T-shaped section. The sliding arrangement also facilitates attaching the dynamometer to implement without the need for quick coupler.
The end of each sliding arm has inverted U-shaped cantilever beam. To measure the draught, two strain gauges were attached on each cantilever beam, and six strain gauges together with two other dummy gauges were arranged in a Wheatstone bridge so that only the draught force is measured. The dimensions of the dynamometer components were selected to match the Category I and II hitching systems with a capacity of 35 kN draught force. Many other designs were developed. Some measured all the forces acting between the implement and tractor by using a six point dynamometer suspension system using load cells. Other systems measured longitudinal and vertical forces only, assuming lateral forces as zero Mounted strain gauges directly on the lower links of the tractor. He mounted these gauges on the linked arms to get tension and differential cantilever bridge.
This system was calibrated for horizontal and vertical forces while applying load only up to 100 kg. The test results showed a cross-sensitivity of 2% in the differential cantilever (vertical force) bridge while 12.5% in the tension (horizontal force) bridge. A bi-axial direct mounted strain gauged lower-links system for measurement of tractor-implement forces was designed by. They developed and calibrated it for coincident and perpendicular loads up to 10 kN.
The results revealed a high degree of linearity between bridge output voltage and force applied. They reported that the hysterisis effect between the calibration curves for increasing and decreasing applied coincident and perpendicular force was very small.
