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Performance evaluation of tillage tines operating under different depths in a sandy clay loam soil
Soil & Tillage Research 103 (2009) 399–405
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Performance evaluation of tillage tines operating under different depths in a sandy clay loam soil S.I. Manuwa * Department of Agricultural Engineering, The Federal University of Technology, P.M.B. 704, Akure, Nigeria
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 June 2007 Received in revised form 30 August 2008 Accepted 11 December 2008
The study investigated the performance of three model tillage tools (tines). The experimental tillages were made from flat 8 mm plain carbon steel. They were designated T1, T5, and T20, corresponding to tine widths of 1, 5, and 20 cm respectively. Experiments were carried out in a soil bin filled with sandy clay loam soil at average moisture content 11.5% (dry basis) and 600 kPa average cone index. The plastic limit and liquid limit and plasticity index of the soil are 20%, 31% and 11% respectively. Tests were conducted at forward speeds of 0.28, 1.0, and 2.5 m/s. Depths of operation considered were 35, 70, 150, 200 and 250 mm. Draught measurements were made for the different tines and were also calculated using soil mechanics equation. There was reasonable agreement between measured and predicted draught forces. The effects of depth of operation on draught force of the tines were studied and evaluated. It was observed that draught increased at an increasing rate with depth; the relationship was a curvilinear one best fitted by exponential function. The soil disturbance created as a result was also evaluated and reported in this paper. The parameters used to define soil disturbance of a single tine were: ridge-to-ridge distance (RRD), maximum width of soil cut (WFS), maximum width of soil throw (TDW), after furrow depth (df), height of ridge (hr) and rupture distance (f). They all increased as the depth of operation of the tool increased but less proportionately. The critical depth of the tines was also estimated. The results of analysis of variance showed that tool type and operating depth significantly affected draught at 5% level of significance (p < 0.05) and that, there was interaction between the two factors. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Soil bin Draught Tines Soil disturbance Depth Specific draught Velocity
1. Introduction Accurate knowledge of draught and energy requirement of tillage implements is essential for proper design of the implements, appropriate matching of the implements with their power sources and the selection of the optimum operation conditions (Ademosun, 1990). The most convenient method to estimate a given implement’s energy requirement is to measure the draught required to pull the implement under desired operating soil conditions (Ehrhardt et al., 2001). Two mechanisms in particular affect draft required to move soil (Rosa, 1977). Inertial forces have remarkable importance for sandy soil (frictional soil). In clay soil the draft required is not too sensitive to inertial forces, but shear strength increases substantially with increasing shear rates. Also Rosa (1977) reported that inertial forces involved in continuously accelerated new masses of soil, as the tool travels are the most important mechanisms in frictional soils when operating speeds
* Tel.: +234 803 415 2976. E-mail address: [email protected]. 0167-1987/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2008.12.004
increase. Similarly, the strain rate-dependent components are the most important mechanisms for cohesive soils. Zeng and Yao (1992) developed a soil-cutting model to predict forces on wide and narrow tools. The model incorporated shear rate effects on soil shear strength, soil–metal friction and soil inertial effects. Comparison between prediction and experimental values indicated that results were acceptable. Stafford (1979) reported that strain rate effects are most responsible for changes in soil strength with speed. Zhang and Kushwaha (1999) reported three mechanisms accounting for the draft increase with increasing operating depth: soil inertial effect; soil strength rate effect and wave propagation effect. The wave propagation effect was from the work of researchers (Azyamova, 1963; Katsygin, 1969; Vetrov and Stanevski, 1972) who noticed when the speed of a tillage tool exceeds some limits; the draft of the tillage tool inversely decreases. This was attributed to the fact that as the tool speed increased faster than the wave of stress propagation, theoretically, the plastic zone of soil in front of the tool decreased or even disappeared, thus the soil-cutting resistance decreased. Swick and Perumpral (1988) reported that soil shear strength and soil–metal friction increased with increasing shear rates. The
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Notations d df e f hr w Cl D MC RRD TDW Wfs
depth of tools (m) after plough furrow depth (m) base of natural logarithm rupture distance (m) height of ridge (m) width of tool (m) cone index (kPa) draught force (kN) moisture content (% db) ridge-to-ridge distance (m) maximum width of soil throw (m) maximum width of soil cut (m) g bulk weight (kN/m3) Ng, Nc, Na, Nq dimensionless number c cohesion (kPa) soil-interface adhesion (kPa) ca q surcharge a, d tine rake angle, angle of soil-interface friction (8) F horizontal draught (kN)
maximum speed they reached was 1.2 m/s and their results indicated however that acceleration forces accounted for a large proportion of the increase in tool force. However tillage forces vary greatly due to numerous factors that influence those forces. It is also known that complicating the relationship is the large number of factors, interactions between factors, and variability of the parameters within a short distance. Hence determining which variable have the greatest influence on the energy requirement for tillage with the most common tillage tools will greatly enhance the process of matching units to tillage implements. The ASAE standard (1999) describes tillage draught as a function of implement type, implement width, depth and speed. However, depth of operation was found to be the most significant factor while speed was often significant. It was reported (Ehrhardt et al., 2001) that most work that has been done on tillage draught focused on specific draught and has concluded that tillage depth is the primary determinant of the amount of power required to pull an implement through soil, with speed often having a significant effect. Draught as an important parameter for measuring and evaluating tillage implement performance for energy requirements has been investigated by various researchers (Oni et al., 1992; Fielke, 1996; McKyes and Maswaure, 1997; Al-Suhaibani and Al-janobi, 1997; Onwualu and Watts, 1998; Manian et al., 2000; McLaughlin and Campbell, 2004; Mamman and Oni, 2005; Manuwa and Ademosun, 2007). Natsis et al. (2002) used tillage force dynamometer to measure draught of mouldboard plough in a clay soil. 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 bottom, 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. Mathematical methods and models have been developed by researchers for predicting draught (Reece, 1965; Stafford, 1984). Soil type and condition are by far the most important factors contributing to variations in specific draught.
Critical depth is the depth below which the amount of soil loosening generated by the tine is minimal and the lateral extent of the major soil failure planes to the side of the tine changes little with increasing depth (Spoor and Fry, 1983). According to Spoor and Godwin (1978), there is a critical working depth for all rigid tines below which compaction occurs rather than effective soil loosening. The critical depth is dependent upon the width, inclination and lift height of the tine foot and on the moisture and density status of the soil. Godwin and Spoor (1977) reported that very narrow tines have working depths far greater than their widths and the aspect ratio (depth/width) greater than 6.0. According to Payne (1956), narrow tines have working depth far greater than the width, with an aspect ratio greater than 1.0, while wide tines have working width far greater than the depth with an aspect ratio of less than 0.5. Below the critical depth, soil is not lifted upwards but moves horizontally around the tools (McKyes, 1978). According to him, critical depth is the point below which soil is moved by a tool principally along horizontal lines. Above the critical depth, soil moves horizontally and upwards. The major implement factors influencing critical depth are aspect ratio and inclination rake angle (Godwin and Spoor, 1977). For effective soil loosening crescent failure should occur and therefore the position of the critical depth influences the maximum useful working depth of a tine. Crescent failure will only occur when the shearing resistance for upward soil flow for any particular depth is less than for lateral flow, the two resistances being equal at critical depth. A slight decrease in soil disturbance as the working depth of the tool increased indicates that the tine was below critical depth. Desbiolles (2008) reported that as a rough guide for knife blades, critical depth values of 8–12 times the blade width may be expected, in hard brittle sands, and lowering as the soil becomes wetter, less compact and as the clay content increases. In wet plastic clay soils, critical depths at 1–2 times the blade width values can be encountered. Godwin and Spoor (1977) reported three specific types of soil disturbance the actual one being dependent on the initial soil conditions and the type and form of the force applied by the implement: loosening or brittle disturbance where the soil slides along a few defined planes and the overall soil density is increased; compacting or compressive disturbance where movement occurs along many planes and the soil density is increased; soil movement without any overall soil density change. Tines working above critical depth produce a loosening type of disturbance, whereas those working below cause compaction at working depth. When working at shallow depth (above critical depth), very narrow tines produce three-dimensional crescent failure pattern (loosening effect) but when working below critical depth, the failure pattern change to two-dimensional pattern moving soil both sideways and forward producing a compaction of the soil (Godwin and Spoor, 1977). Payne (1956) observed that narrow tines produce a threedimensional type of soil failure as the soil from the working depth is brought to the surface in soil wedge, that the soil moves forward, upward and sideways in a crescent, and the end effects are significant. Similarly, it was reported that wide tines (blades) cause two-dimensional type of soil failure moving the soil upward and forward with some end effects, which are normally ignored. The soil flows over the tool surface, with some cracking thereby producing loosening effect. Sharifat and Kushwaha (1999) studied soil lateral movement under different soil conditions with a sweep and a furrow opener at speed from 5 to 8 km/h, and they concluded that different tools created different geometries of soil profiles; the parameters of soil profile were also affected by speed, soil bulk density, and soil moisture content. McKyes (1985) described the results of a soil
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disturbance study, and indicated that the shape, width and rake angle of tools strongly influence transporting and mixing of soil particles; soil throwing to the sides of a tool varied with the square of tillage speed. The objectives of this paper therefore were to study the performance of model tillage tine under different operating depths and also to evaluate the soil disturbance parameters of the tines and to model the relationship between depth and draught. 2. Materials and methods 2.1. Experimental tillage tines Three tines: T1 (very narrow tine), T5 (narrow tine) and T20 (wide tine), were used in this study. The tines were made from flat 8 mm plain carbon steel. They were designated T1, T5, T20 corresponding to 1.0, 5.0, 20.0 cm widths respectively. Tines TI and T5 are very narrow tine and narrow tine respectively and each of height 50 cm. Tine T20 is a wide tine of 15 cm 20 cm supported by a shank 35 cm long in the middle. The bottom edge of each tine was beveled at an angle of 158 to provide a sharp cutting edge.
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Table 1 Physical properties of experimental soil. Classification
Sandy clay loam
Sand (%) Silt (%) Clay (%) Organic matter (%) Particle density (kg/m3) Plastic limit % (H2O) Liquid limit % (H2O)
54 21 25 – 2510 20 31
one of the prominent agricultural soils of Ondo State, Nigeria. The soil was taken from one of the fallow agricultural lands of the commercial farm of the Federal University of Technology, Akure (78150 N, 58150 E), and elevation 210 m in the forest-Savanna zone of southwestern Nigeria in 2001. The soil was Oxic paleustalf (Alfisol) or ferric Luvisol (FAO). The site was recovered from 3 years of bush fallow. Particle size analysis was determined by hydrometer method (Bouyoucos, 1962) in air-dried 2 mm sieved soil samples. Soil organic C was determined using the dichromate wet oxidation method (Nelson and Sommers, 1982). The physical properties of the experimental soil are presented in Table 1.
2.2. Soil bin facility Experiments were conducted in the Soil Dynamics Laboratory of the Department of Agricultural Engineering, The Federal University of Technology, Akure, Nigeria. The equipment consisted of an indoor soil bin of 9.0 m length, 0.85 m width and 0.45 m depth; a soil processing trolley with a leveling blade and compaction roller, a tool carriage, a power transmission system with a 3.1 kW electric motor as prime mover, a tool mounting frame, a tool vertical and angle adjustment device, a profile meter for measuring soil disturbance parameters, and a load cell (spring dynamometer) for measuring draught (horizontal force). An overview of the soil tillage dynamics equipment is shown in Fig. 1, with the full details presented in Manuwa (2002). 2.3. Soil description and properties 2.3.1. Physical properties The soil studied was an Akure sandy clay loam (54% sand, 21% silt, and 25% clay), according to the USDA textural classification of soils. The plastic limit, liquid limit and plastic index of the soil are 20%, 31%, and 11% respectively. Its organic carbon was 1.3%. It was
2.3.2. Mechanical properties Mechanical properties including cohesion, adhesion, internal and external friction angles were determined through laboratory tests. Direct shear test method was used to measure these values of soil shear strength under same moisture and density conditions as applied in the soil bin experiments. Soil penetration resistance (cone index) was measured by using a Rimik penetrometer (model CP 20 ultrasonic, Agridy Rimik Pty Ltd., Toowoomba, Australia). The penetrometer was comprised of an in-built data logger, a 500-mm long shank, a cone with a base area of 129 mm2 and an apex angle of 308. The penetrometer was pushed into the soil by hand at a speed of approximately 0–2 mm/ s according to the ASAE standards. Core soil samplers were used to measure soil bulk density and moisture content. All the above soil property determination tests were replicated three times and the means recorded including their standard deviations as shown in Table 2. 2.4. Measurement and prediction of draught forces Different theoretical models are available for calculating soilcutting force. In this study, the universal earth-moving equation of the two-dimensional analysis after Hettiaratchi et al. (1966) reported by Stafford (1979) was used to calculate the pulling force, F: F ¼ wðg z2 Ng þ czNc þ ca zNa þ qzNq Þsin ða þ dÞ
(1)
where F is the draught force (kN), w the width of tool (m), z the depth of tools (m), g the bulk weight (kN/m3), Ng, Nc, Na, and Nq are dimensionless numbers, c the cohesion (kPa), ca the soil-interface adhesion (kPa), q the surcharge; a and d are tine rake angle and angle of soil-interface friction (degree), respectively. The N-factors were estimated from the relationships established and reported by Hettiaratchi et al. (1966). 2.5. Experimental procedure Fig. 1. An overview of the soil tillage dynamics equipment used in the study. (1) Load meter; (2) tool carriage; (3) tool vertical adjustment device; (4) tool angle measuring plate; (5) tool bar; (6) profilemeter; (7) soil processing trolley frame; (8) soil leveler; (9) compaction roller; (10) roller vertical adjustment device; (11) vertical adjustment pipe; (12) winding handle.
2.5.1. Soil preparation and measurements The experimental soil was dried to the initial moisture content and crushed to a fine uniform size before it was put into the soil bin. Experiments were conducted first in the driest state and water
S.I. Manuwa / Soil & Tillage Research 103 (2009) 399–405
402 Table 2 Mechanical property of the experimental soils. MC (% db)
6.0 11.5 17.5 20.0
(0.2) (0.1) (0.2) (0.1)
BD (kg/m3)
1500 1520 1560 1530
(11) (10) (11) (9.5)
Cohesion (kPa)
Internal friction angle (degree)
Adhesion (kPa)
12.1 13.3 24.5 22.6
30.2 29.6 36.5 34.5
0.18 0.21 0.29 0.35
(1.2) (1.1) (1.4) (1.2)
(1.1) (1.2) (1.2) (1.3)
(0.02) (0.01) (0.03) (0.02)
Soil/metal friction angle (degree)
Cone index (kPa) 75 mm depth
150 depth
22.3 23.6 24.7 23.1
575 690 785 720
580 725 800 745
(1.3) (1.2) (1.3) (1.1)
(20) (25) (15) (22)
(25) (23) (20) (24)
MC = moisture content; BD = bulk density. Standard error in parenthesis.
was added for the consequent runs until the moisture content of the soil reached an equilibrium state similar to the procedure reported by Gupta and Surendarnath (1989). The soil processing trolley was used for processing the soil mechanically in the bin in order to achieve uniform soil condition as desired for test-run throughout the soil bed. A reasonable agreement was found in the compaction level at the different locations along the length of the bin. This was achieved by rotavating the soil and passing the roller over it for a fixed number of times usually between 4 to 6. The cone penetrometer was used to monitor the uniformity of the prepared soil by comparing the cone indices of the sampled sites. After the soil processing was over, the test tool was mounted on the tool bar, and the desired depth and rake angle were adjusted appropriately. In all the tests, the mean moisture content was about 11.5% (db), the rake angle was held constant at 908, and the mean cone index was 600 kPa. Moisture content of the soil was determined by gravimetric method. The five levels of operating depth tested were 35, 70, 150, 200 and 250 mm respectively.
3. Results and discussion 3.1. Comparison between measured and predicted draught forces The measured draught forces were recorded after the procedure explained under experimentation. On the other hand, the predicted forces were calculated using Eq. (1). The values of the measured draught forces, the corresponding predicted values and the percentage over or under prediction are shown in Table 3. Eq. (1) was more appropriate to predict draught forces with narrow tines than very narrow tines or wide tines. 3.2. Effect of depth of operation on draught The effect of depth of operation on draught of tines is presented in Figs. 3–5. The best relationship between draught (D) and operating (d) was a curvilinear one from regression analysis, and is of the form: D ¼ aebd
2.5.2. Calibration of load cell To ensure the accuracy of measurements of draught, the load cell was calibrated in advance of being used for force measurement. The dead weight method was used for the calibration in the range of force experienced in the draught measurements. 2.5.3. Experimentation For all the experiments, the soil moisture content was held constant at an average of 11.5% (db). After the soil processing was over, the test tool was mounted on the tool bar for experimentation. In each of the three blocks, the forward speed and rake angle were kept constant but the depth was varied in five levels. The fixed rake angle was 908 for all the treatments while the tool carriage was towed at the fixed forward speeds V1, V2, V3 corresponding to 0.28, 1.0 and 2.5 m/s respectively for the three blocks. During the test, the load cell measured draught. Data were collected and mean values of three replicates were used for computation and analysis. A profile meter similar to that described by Spoor and Godwin (1978) and meter scale were used to measure the soil-disturbed surface after each test. Soil disturbance parameters were subsequently defined and evaluated. For the purpose of analysis, the general form of soil disturbance was quantified by the parameters shown in Fig. 2. The parameters used to describe soil disturbance include: maximum width of soil throw (TDW); maximum width of soil cut (WFs) also referred to as width of crescent; the ridge-to-ridge distance (RRD); the height of the ridge (hr); after plough furrow depth (df) and the tool width (w). Another parameter of the soil disturbance measured but not shown in Fig. 2 is rupture distance (f), defined as the distance ahead of the tine at which the distinct shear plane broke the surface (Godwin and Spoor, 1977). Some of these parameters have been used to assess soil disturbance of tillage implements by researchers (Willat and Wills, 1965; Godwin and Spoor, 1977; Spoor and Godwin, 1978; Taniguchi et al., 1999).
(2)
where a and b are coefficients of the exponential function. The regression equations and the coefficients of determination are
Fig. 2. Parameters used to define soil disturbance of a single tine: maximum width of soil throw (TDW); maximum width of soil cut (Wfs); ridge-to-ridge distance (RRD); height of ridge (hr); after plough depth (df); tool width (w). Table 3 A comparison of the measured and calculated draught force of the experimental tines. Depth (cm)
Tine
Experimental
Calculation
% prediction
3.5
T1 T5 T20
23 90 219
13 68 273
43 24 +24.6
10.0
T1 T5 T20
55 180 639
40 203 812
27 +12 +27
15.0
T1 T5 T20
100 280 950
63 315 1256
37 +12.5 +32
+, over-prediction; , under-prediction; T1, very narrow tine of width 1 cm; T5, narrow tine of width 5.0 cm; T20, wide tine of width 20 cm.
S.I. Manuwa / Soil & Tillage Research 103 (2009) 399–405
Fig. 3. Effect of depth on draught at forward speed of 0.28 m/s. DT1, draught of tine of width 1.0 cm; DT5, draught of tine of width 5.0 cm; DT20, draught of tine of width 20 cm.
Fig. 4. Effect of depth on draught at forward speed of 1.0 m/s. DT1, draught of tine of width 1.0 cm; DT5, draught of tine of width 5.0 cm; DT20, draught of tine of width 20 cm.
shown in the respective figures. In these figures, draught increased at an increasing rate with increase in operating depth. The reason being that at higher depths more soil volume is considered, soil becomes stiffer and denser (due to overburden pressure and so strength properties vary. It should be noted that the exponential modes best fitted the relationships, with R2 (coefficient of determination) values ranging between 0.9784 and 0.9975. This curvilinear relationship is similar to that reported by Al-Suhaibani and Al-janobi (1997), Grisso et al. (1996) and Nicholson et al. (1984). An increase in specific draught (draught per maximum width of soil cut), was observed with an increase in tillage depth for all the tools tested. This also agrees well with the work reported by Al-Suhaibani and Al-janobi (1997) and Ademosun (1990). Generally specific draught increased with width of tine but less proportionately. This agreed with the findings reported by McKyes
403
and Maswaure (1997). The increase became higher as the depth increased due to the increase of bulk density with depth. Also in the range of depth considered, the increase in specific draught became higher as the tine, width increased. This is because the amount of soil displaced by narrow tines is considerably lower than that disturbed by wide tines. Inertial forces are more significant for wide tines than narrow tines. The specific draught of the tines T1, T5 and T20 ranged from 5 to 6.1, 8.6 to 12.2 and 6.8 to 13.2 N/cm respectively within the range of depth considered. In the range of depth of operation tested, under this condition, the specific draught increased by about 21.2%, 42.0% and 92.18% for T2, T5 and T20 tines respectively. Although draught force increased with width, it was less than proportionately. Specific draughts of different widths of tines operating at the same depth were relatively higher per unit for narrower tines. This is due to the shear area, which is smaller per unit width for narrower tines. This is in close agreement with the findings of McKyes and Ali (1977). The numerical values of these parameters would provide valuable information in the design of tillage tines. 3.2.1. Analysis of variance tests Analysis of variance (ANOVA) was also performed on the interaction of tool type (T) and operating depth, d—the sources of variation. For all the three levels of speed the results showed that tool type and operating depth affected the draught of tines significantly at 5% level of probability (p < 0.05). The interaction between the two factors was also statistically significant at 5% level of probability (p < 0.05). Moreover, draught increased as the forward speed of a tine increased. This is mainly because of the acceleration of the soil. Greater forces provide this acceleration and since they also increase the reaction at the interface, a higher sliding resistance results. The increased sliding resistance contributes most to the increased draught force (Spoor, 1969). 3.3. Soil disturbance parameters The effect of depth of operation of the tines (T1, T5, and T20) on soil disturbance was observed for a forward speed of 1.0 m/s (3.6 km/h), 908-rake angle, average cone index of 600 kPa and moisture content of 11.5% (db) with the results presented in Table 4. As the tool moved through the soil in the soil bin, the soil was disturbed as it was cut and thrown to the sides of the tool. The soil disturbance generated was observed, assessed and analyzed. The shape of the ridge profile was very close to an isosceles triangle at the speed of 3.6 km/h. This finding was similar to that reported by Liu and Kushwaha (2006). The width of the tool strongly influenced the soil disturbance parameters as they all increased as the working depth increased but less proportionately. This was also similar to the findings reported by McKyes (1985). This is because the major factors that control the nature of soil failure or disturbance are the aspect ratio (depth/width ratio) and the rake angle. Tines with small aspect ratio causes cause a surface crescent-like failure (Payne, 1956). As the depth/width ratio increased, the nature of soil failure changed. The inertial forces influenced the distance that the sol was thrown away from the tool path. For all the three levels of speed, the furrow level was filled to an upper level mostly with pulverized soil that fell back, this influenced the after furrow depth. 3.4. Estimation of critical depth of experimental tines
Fig. 5. Effect of depth on draught at forward speed of 2.5 m/s. DT1, draught of tine of width 1.0 cm; DT5, draught of tine of width 5.0 cm; DT20, draught of tine of width 20 cm.
The critical depth of the experimental tillage tines was estimated based on the procedure described by Desbiolles (2008), which of course was referred to as a rough guide. The results are presented in Table 5.
S.I. Manuwa / Soil & Tillage Research 103 (2009) 399–405
404 Table 4 Effect of depth of operation on soil disturbance. Parameters of soil disturbance
Depth (cm) T1
RRD (cm) Wfs (cm) TDW (cm) df (cm) hr (cm) f (cm) Draught (N) Sp. draught (N/cm)
3.5 5 6 9.5 2 0.8 5.5 23 (4) 3.8
T5 10 10 10 12.5 7.5 1.5 11 55 (6) 5.5
15.0 15 16.5 17.5 10.5 1.6 15 100 (10) 6.06
3.5 12 10.5 25 1.5 2.5 9.5 90 (11) 8.57
T20 10 13.5 20 27.5 5.5 4.5 18 180 (14) 9.00
15.0 16.5 23 33 7.0 6.0 21 280 (25) 12.17
3.5 26 28 39 1.9 2.5 17.5 219 (21) 7.8
10 30 32 43.5 3.5 3.0 23 639 (59) 19.96
15.0 33 34 46 6.2 4.5 25 950 (69) 27.94
a = 908; v = 1.0 m/s; MC = 11.5% db; f = rupture distance Standard error of draught in parenthesis.
Table 5 Estimated critical depths of the experimental tillage tines. Consistence/soil condition
Tine type
Tine width (cm)
Critical depth (cm)
Moisture status
Friable
T1 T5 T20
0.01 0.05 0.2
8–12 40–60 160–240
Moist
T1 is very narrow tine of width 1 cm; T5 is narrow tine of width 5.0 cm; T20 is wide tine of width 20 cm.
4. Conclusion The performance of model tillage tines was evaluated in a soil tillage dynamics laboratory at the Federal University of Technology, Akure, Nigeria. The mechanical background and soil mechanics theory were used to explain certain aspects of the performance of the tines. It was observed that depth of operation affected draught of the tools. Draught increased with depth of operation at an increasing rate. There was reasonable agreement between measured and predicted draught forces. The relationship was a curvilinear one best fitted by an exponential function. Soil disturbance parameters were also evaluated for the sandy clay loam soil. The soil disturbance parameters all increased with depth and also with width of tool but less proportionately. The critical depths of the tines were also estimated. Acknowledgement The author is grateful to the Federal University of Technology, Akure for the University Research Grant No. URG/MINOR/98/105 which partly funded this project. References Ademosun, O.C., 1990. The design and operation of a soil tillage dynamics equipment. The Nigerian Engineer 25 (1), 51–57. Al-Suhaibani, Al-janobi, 1997. Draught requirements of tillage implements operating in sandy loam soil. Journal of Agricultural Engineering Research 66, 177– 182. Azyamova, E.N., 1963. Studies of dynamics of deformation of soil. Trudy (TSNIIMESKH) Minsk 1, 131–139 (Translated by W.R. Gill, National Tillage Machinery Laboratory, USDA, ARS, Auburn, AL). American Society of Agricultural Engineering Standards, 1999. Agricultural Machinery Management Data. ASAE, St. Joseph, MI, USA. Bouyoucos, G.J., 1962. Hydrometer method improved for making particle size analysis of soils. Agronomy Journal 54, 464–465. Desbiolles, J., 2008. http://www.google.com/search?hl=en&q=%27+soil+failure+ with+tillage+tool%27+%27pdf%27&start=10&sa=N,250708. Ehrhardt, J.P., Grisso, R.D., Kocher, M.F., Jasa, P.J., Schinstock, J.L., 2001. Using the veris electrical conductivity cart as a draft predictor. ASAE Paper No. 011012 at Sacramento Convention Center, Sacramento, CA, July, 29–August 1, 2001. Fielke, J.M., 1996. Interaction of the cutting edge of tillage implements with soil. Journal of Agricultural Engineering Research 63 (1), 61–72. Godwin, R.J., Spoor, G., 1977. Soil failure with narrow tines. Journal of Agricultural Engineering Research 22, 213–228.
Grisso, R.D., Yasin, M., Rocher, M.F., 1996. Tillage implement forces operating in silty clay loam. Transactions of the American Society of Agricultural Engineers 19 (6), 1977–1982. Gupta, C.P., Surendarnath, T., 1989. Stress field in soil owing to tillage tool interaction. Soil and Tillage Research 13, 123–149. Hettiaratchi, D.R.P., Whitney, B.D., Reece, A.R., 1966. The calculation of passive pressure in two-Dimensional soil failure. Journal of Agricultural Engineering Research 11 (2), 89–107. Katsygin, V.V., 1969. Relation of Draught of Agricultural Machines to Operating speeds. Voprosy Sel’skohozyaistvennoi, section 2, vol. XII. National Tillage Machinery Laboratory, Auburn, AL, pp. 96–119 (Translated by W.R. Gill). Liu, J., Kushwaha, R.L., 2006. Modeling of soil profile produced by a single sweep tool. Agricultural Engineering International: the CIGR Ejournal, Manuscript PM 06 008, vol. VIII, May 2006. Mamman, E., Oni, K.C., 2005. Draught performance of a range of model chisel furrowers. Agricultural Engineering International: the CIGR Ejournal, Manuscript PM 05 003, vol. VII. November 2005. Manian, R., Rao, V.R., Kathirvel, K., 2000. Influence of operating and disk parameters on performance of disk tools. Agricultural Mechanization in Asia, Africa, and Latin America 31 (2), 19–26 38. Manuwa, S.I., 2002. Development of equipment for soil tillage dynamics and evaluation of tillage parameters. Unpublished Ph.D. Thesis in the Department of Agriculture Engineering, The Federal University of Technology, Akure, Nigeria. Manuwa, S.I., Ademosun, O.C., 2007. Draught and soil disturbance of model tillage tines under varying soil parameters. Agricultural Engineering International: the CIGR Ejournal, Manuscript PM 06 016, vol. IX, March 2007. McKyes, E., 1978. The calculation of draught forces and soil failure boundaries of narrow cutting blades. Transactions of the ASAE 21 (1), 20–24. McKyes, E., 1985. Soil cutting and tillage. Elsevier Science Publishing Company Inc., New York, NY10017, USA. McKyes, I.E., Ali, O.S., 1977. The cutting of soil by narrow blades. Journal of Terramechanics 14, 43–58. McKyes, E., Maswaure, J., 1997. Effect of design parameters of flat tillage tools on loosening of a clay soil. Soil and Tillage Research 43, 195–204. McLaughlin, N.B., Campbell, A.J., 2004. Draft-speed-depth relationships for four liquid manure injectors in a fine sandy loam soil. Canadian Biosystem Engineering 46 (2), 1–25. Natsis, A., Papadakis, G., Pitsilis, I., 2002. Experimental investigation of the influence of the foreploughshare and the disk coulter on the tillage quality and the tractor fuel consumption. Agricultural Engineering International: the CIGR Journal of Scientific Research and Development. Manuscript PM 02 002, vol. IV, December 2002. Nelson, D.W., Sommers, L.E., 1982. Total carbon, organic carbon and organic matter. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis, Part 2. ASA, Madison, WI, pp. 539–580. Nicholson, R.L., Bushford, L.L., Miclke, L.N., 1984. Energy requirements for tillage from a reference implement. American Society of Agricultural Engineers (ASAE), Paper No. 84-1028, ASAE, St. Joseph, MI. Oni, K.C., Clark, S.J., Johnson, H.W., 1992. The effects of design on the draught of under-cut sweep tillage tools. Soil and Tillage Research 22, 117–130.
S.I. Manuwa / Soil & Tillage Research 103 (2009) 399–405 Onwualu, A.P., Watts, K.C., 1998. Draught and vertical forces obtained from dynamic soil cutting by plane tillage tools. Soil and Tillage Research 48, 239–253. Payne, P.C.J., 1956. The relationship between the mechanical properties of soil to the performance of simple cultivation implements. Journal of Agricultural Engineering Research 1 (1), 23–50. Reece, A.R., 1965. The fundamental equation of earth-moving mechanics. In: Earth Moving Machinery Symposium, vol. 179 (3F), Institution of Mechanical Engineers, London. Rosa, U.A., 1977. Performance of narrow tillage tools with inertial and strain rate effects. Ph.D. Thesis. Department of Agricultural and Bioresource Engineering, University of Saskatchewan, Saskatoon, Canada. Sharifat, K., Kushwaha, R.L., 1999. Lateral soil movement by tillage tools. ASAE Paper No. 991003. ASAE, St. Joseph, MI. Spoor, G., Godwin, R.J., 1978. An experimental investigation into the deep loosening of soil by rigid tines. Journal of Agricultural Engineering Research 23, 243–258. Spoor, G., 1969. Design of soil engaging implements. Farm Machine Design Engineering 3, 22–26. Spoor, G., Fry, R.K., 1983. Soil disturbance generated by deep-working low rake angle narrow tines. Journal of Agricultural Engineering Research 28, 217–234.
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Stafford, J.V., 1979. The performance of a rigid tine in relation to soil properties and speed. Journal of Agricultural Engineering Research 24, 41–56. Stafford, J.V., 1984. Force prediction models for brittle and flow failure of soil by draught tillage tools. Journal of Agricultural Engineering Research 29, 51–60. Swick, W.C., Perumpral, J.V., 1988. A model for predicting soil–tool interaction. Journal of Terramechanics 25 (1), 43–56. Taniguchi, T., Makanga, J.T., Ohtomo, K., Kishimoto, T., 1999. Draft and soil manipulation by a moldboard plow under different forward speed and body attachments. Transactions of the ASAE 43 (6), 1517–1521. Vetrov, Y.A., Stanevski, V.P., 1972. The investigation of the factors of the speed of cutting soils. Mining Construction and Highway Machines 8, 21–26 (Translated by W.R. Gill, National Tillage Machinery Laboratory, Auburn, AL). Willat, S.T., Wills, A.H., 1965. A study of the trough formed by the passage of tines through soil. Journal of Agricultural Engineering Research 10, 109–113. Zeng, D., Yao, Y., 1992. A dynamic model for soil cutting by blade and tine. Journal of Terramechanics 29 (3), 317–327. Zhang, Z.X., Kushwaha, R.L., 1999. Operating speed effect on the advancing soil failure zone in tillage operation. Canadian Agricultural Engineering 41 (2), 87–92.
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Soil & Tillage Research journal homepage: www.elsevier.com/locate/still
Performance evaluation of tillage tines operating under different depths in a sandy clay loam soil S.I. Manuwa * Department of Agricultural Engineering, The Federal University of Technology, P.M.B. 704, Akure, Nigeria
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 June 2007 Received in revised form 30 August 2008 Accepted 11 December 2008
The study investigated the performance of three model tillage tools (tines). The experimental tillages were made from flat 8 mm plain carbon steel. They were designated T1, T5, and T20, corresponding to tine widths of 1, 5, and 20 cm respectively. Experiments were carried out in a soil bin filled with sandy clay loam soil at average moisture content 11.5% (dry basis) and 600 kPa average cone index. The plastic limit and liquid limit and plasticity index of the soil are 20%, 31% and 11% respectively. Tests were conducted at forward speeds of 0.28, 1.0, and 2.5 m/s. Depths of operation considered were 35, 70, 150, 200 and 250 mm. Draught measurements were made for the different tines and were also calculated using soil mechanics equation. There was reasonable agreement between measured and predicted draught forces. The effects of depth of operation on draught force of the tines were studied and evaluated. It was observed that draught increased at an increasing rate with depth; the relationship was a curvilinear one best fitted by exponential function. The soil disturbance created as a result was also evaluated and reported in this paper. The parameters used to define soil disturbance of a single tine were: ridge-to-ridge distance (RRD), maximum width of soil cut (WFS), maximum width of soil throw (TDW), after furrow depth (df), height of ridge (hr) and rupture distance (f). They all increased as the depth of operation of the tool increased but less proportionately. The critical depth of the tines was also estimated. The results of analysis of variance showed that tool type and operating depth significantly affected draught at 5% level of significance (p < 0.05) and that, there was interaction between the two factors. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Soil bin Draught Tines Soil disturbance Depth Specific draught Velocity
1. Introduction Accurate knowledge of draught and energy requirement of tillage implements is essential for proper design of the implements, appropriate matching of the implements with their power sources and the selection of the optimum operation conditions (Ademosun, 1990). The most convenient method to estimate a given implement’s energy requirement is to measure the draught required to pull the implement under desired operating soil conditions (Ehrhardt et al., 2001). Two mechanisms in particular affect draft required to move soil (Rosa, 1977). Inertial forces have remarkable importance for sandy soil (frictional soil). In clay soil the draft required is not too sensitive to inertial forces, but shear strength increases substantially with increasing shear rates. Also Rosa (1977) reported that inertial forces involved in continuously accelerated new masses of soil, as the tool travels are the most important mechanisms in frictional soils when operating speeds
* Tel.: +234 803 415 2976. E-mail address: [email protected]. 0167-1987/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2008.12.004
increase. Similarly, the strain rate-dependent components are the most important mechanisms for cohesive soils. Zeng and Yao (1992) developed a soil-cutting model to predict forces on wide and narrow tools. The model incorporated shear rate effects on soil shear strength, soil–metal friction and soil inertial effects. Comparison between prediction and experimental values indicated that results were acceptable. Stafford (1979) reported that strain rate effects are most responsible for changes in soil strength with speed. Zhang and Kushwaha (1999) reported three mechanisms accounting for the draft increase with increasing operating depth: soil inertial effect; soil strength rate effect and wave propagation effect. The wave propagation effect was from the work of researchers (Azyamova, 1963; Katsygin, 1969; Vetrov and Stanevski, 1972) who noticed when the speed of a tillage tool exceeds some limits; the draft of the tillage tool inversely decreases. This was attributed to the fact that as the tool speed increased faster than the wave of stress propagation, theoretically, the plastic zone of soil in front of the tool decreased or even disappeared, thus the soil-cutting resistance decreased. Swick and Perumpral (1988) reported that soil shear strength and soil–metal friction increased with increasing shear rates. The
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Notations d df e f hr w Cl D MC RRD TDW Wfs
depth of tools (m) after plough furrow depth (m) base of natural logarithm rupture distance (m) height of ridge (m) width of tool (m) cone index (kPa) draught force (kN) moisture content (% db) ridge-to-ridge distance (m) maximum width of soil throw (m) maximum width of soil cut (m) g bulk weight (kN/m3) Ng, Nc, Na, Nq dimensionless number c cohesion (kPa) soil-interface adhesion (kPa) ca q surcharge a, d tine rake angle, angle of soil-interface friction (8) F horizontal draught (kN)
maximum speed they reached was 1.2 m/s and their results indicated however that acceleration forces accounted for a large proportion of the increase in tool force. However tillage forces vary greatly due to numerous factors that influence those forces. It is also known that complicating the relationship is the large number of factors, interactions between factors, and variability of the parameters within a short distance. Hence determining which variable have the greatest influence on the energy requirement for tillage with the most common tillage tools will greatly enhance the process of matching units to tillage implements. The ASAE standard (1999) describes tillage draught as a function of implement type, implement width, depth and speed. However, depth of operation was found to be the most significant factor while speed was often significant. It was reported (Ehrhardt et al., 2001) that most work that has been done on tillage draught focused on specific draught and has concluded that tillage depth is the primary determinant of the amount of power required to pull an implement through soil, with speed often having a significant effect. Draught as an important parameter for measuring and evaluating tillage implement performance for energy requirements has been investigated by various researchers (Oni et al., 1992; Fielke, 1996; McKyes and Maswaure, 1997; Al-Suhaibani and Al-janobi, 1997; Onwualu and Watts, 1998; Manian et al., 2000; McLaughlin and Campbell, 2004; Mamman and Oni, 2005; Manuwa and Ademosun, 2007). Natsis et al. (2002) used tillage force dynamometer to measure draught of mouldboard plough in a clay soil. 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 bottom, 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. Mathematical methods and models have been developed by researchers for predicting draught (Reece, 1965; Stafford, 1984). Soil type and condition are by far the most important factors contributing to variations in specific draught.
Critical depth is the depth below which the amount of soil loosening generated by the tine is minimal and the lateral extent of the major soil failure planes to the side of the tine changes little with increasing depth (Spoor and Fry, 1983). According to Spoor and Godwin (1978), there is a critical working depth for all rigid tines below which compaction occurs rather than effective soil loosening. The critical depth is dependent upon the width, inclination and lift height of the tine foot and on the moisture and density status of the soil. Godwin and Spoor (1977) reported that very narrow tines have working depths far greater than their widths and the aspect ratio (depth/width) greater than 6.0. According to Payne (1956), narrow tines have working depth far greater than the width, with an aspect ratio greater than 1.0, while wide tines have working width far greater than the depth with an aspect ratio of less than 0.5. Below the critical depth, soil is not lifted upwards but moves horizontally around the tools (McKyes, 1978). According to him, critical depth is the point below which soil is moved by a tool principally along horizontal lines. Above the critical depth, soil moves horizontally and upwards. The major implement factors influencing critical depth are aspect ratio and inclination rake angle (Godwin and Spoor, 1977). For effective soil loosening crescent failure should occur and therefore the position of the critical depth influences the maximum useful working depth of a tine. Crescent failure will only occur when the shearing resistance for upward soil flow for any particular depth is less than for lateral flow, the two resistances being equal at critical depth. A slight decrease in soil disturbance as the working depth of the tool increased indicates that the tine was below critical depth. Desbiolles (2008) reported that as a rough guide for knife blades, critical depth values of 8–12 times the blade width may be expected, in hard brittle sands, and lowering as the soil becomes wetter, less compact and as the clay content increases. In wet plastic clay soils, critical depths at 1–2 times the blade width values can be encountered. Godwin and Spoor (1977) reported three specific types of soil disturbance the actual one being dependent on the initial soil conditions and the type and form of the force applied by the implement: loosening or brittle disturbance where the soil slides along a few defined planes and the overall soil density is increased; compacting or compressive disturbance where movement occurs along many planes and the soil density is increased; soil movement without any overall soil density change. Tines working above critical depth produce a loosening type of disturbance, whereas those working below cause compaction at working depth. When working at shallow depth (above critical depth), very narrow tines produce three-dimensional crescent failure pattern (loosening effect) but when working below critical depth, the failure pattern change to two-dimensional pattern moving soil both sideways and forward producing a compaction of the soil (Godwin and Spoor, 1977). Payne (1956) observed that narrow tines produce a threedimensional type of soil failure as the soil from the working depth is brought to the surface in soil wedge, that the soil moves forward, upward and sideways in a crescent, and the end effects are significant. Similarly, it was reported that wide tines (blades) cause two-dimensional type of soil failure moving the soil upward and forward with some end effects, which are normally ignored. The soil flows over the tool surface, with some cracking thereby producing loosening effect. Sharifat and Kushwaha (1999) studied soil lateral movement under different soil conditions with a sweep and a furrow opener at speed from 5 to 8 km/h, and they concluded that different tools created different geometries of soil profiles; the parameters of soil profile were also affected by speed, soil bulk density, and soil moisture content. McKyes (1985) described the results of a soil
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disturbance study, and indicated that the shape, width and rake angle of tools strongly influence transporting and mixing of soil particles; soil throwing to the sides of a tool varied with the square of tillage speed. The objectives of this paper therefore were to study the performance of model tillage tine under different operating depths and also to evaluate the soil disturbance parameters of the tines and to model the relationship between depth and draught. 2. Materials and methods 2.1. Experimental tillage tines Three tines: T1 (very narrow tine), T5 (narrow tine) and T20 (wide tine), were used in this study. The tines were made from flat 8 mm plain carbon steel. They were designated T1, T5, T20 corresponding to 1.0, 5.0, 20.0 cm widths respectively. Tines TI and T5 are very narrow tine and narrow tine respectively and each of height 50 cm. Tine T20 is a wide tine of 15 cm 20 cm supported by a shank 35 cm long in the middle. The bottom edge of each tine was beveled at an angle of 158 to provide a sharp cutting edge.
401
Table 1 Physical properties of experimental soil. Classification
Sandy clay loam
Sand (%) Silt (%) Clay (%) Organic matter (%) Particle density (kg/m3) Plastic limit % (H2O) Liquid limit % (H2O)
54 21 25 – 2510 20 31
one of the prominent agricultural soils of Ondo State, Nigeria. The soil was taken from one of the fallow agricultural lands of the commercial farm of the Federal University of Technology, Akure (78150 N, 58150 E), and elevation 210 m in the forest-Savanna zone of southwestern Nigeria in 2001. The soil was Oxic paleustalf (Alfisol) or ferric Luvisol (FAO). The site was recovered from 3 years of bush fallow. Particle size analysis was determined by hydrometer method (Bouyoucos, 1962) in air-dried 2 mm sieved soil samples. Soil organic C was determined using the dichromate wet oxidation method (Nelson and Sommers, 1982). The physical properties of the experimental soil are presented in Table 1.
2.2. Soil bin facility Experiments were conducted in the Soil Dynamics Laboratory of the Department of Agricultural Engineering, The Federal University of Technology, Akure, Nigeria. The equipment consisted of an indoor soil bin of 9.0 m length, 0.85 m width and 0.45 m depth; a soil processing trolley with a leveling blade and compaction roller, a tool carriage, a power transmission system with a 3.1 kW electric motor as prime mover, a tool mounting frame, a tool vertical and angle adjustment device, a profile meter for measuring soil disturbance parameters, and a load cell (spring dynamometer) for measuring draught (horizontal force). An overview of the soil tillage dynamics equipment is shown in Fig. 1, with the full details presented in Manuwa (2002). 2.3. Soil description and properties 2.3.1. Physical properties The soil studied was an Akure sandy clay loam (54% sand, 21% silt, and 25% clay), according to the USDA textural classification of soils. The plastic limit, liquid limit and plastic index of the soil are 20%, 31%, and 11% respectively. Its organic carbon was 1.3%. It was
2.3.2. Mechanical properties Mechanical properties including cohesion, adhesion, internal and external friction angles were determined through laboratory tests. Direct shear test method was used to measure these values of soil shear strength under same moisture and density conditions as applied in the soil bin experiments. Soil penetration resistance (cone index) was measured by using a Rimik penetrometer (model CP 20 ultrasonic, Agridy Rimik Pty Ltd., Toowoomba, Australia). The penetrometer was comprised of an in-built data logger, a 500-mm long shank, a cone with a base area of 129 mm2 and an apex angle of 308. The penetrometer was pushed into the soil by hand at a speed of approximately 0–2 mm/ s according to the ASAE standards. Core soil samplers were used to measure soil bulk density and moisture content. All the above soil property determination tests were replicated three times and the means recorded including their standard deviations as shown in Table 2. 2.4. Measurement and prediction of draught forces Different theoretical models are available for calculating soilcutting force. In this study, the universal earth-moving equation of the two-dimensional analysis after Hettiaratchi et al. (1966) reported by Stafford (1979) was used to calculate the pulling force, F: F ¼ wðg z2 Ng þ czNc þ ca zNa þ qzNq Þsin ða þ dÞ
(1)
where F is the draught force (kN), w the width of tool (m), z the depth of tools (m), g the bulk weight (kN/m3), Ng, Nc, Na, and Nq are dimensionless numbers, c the cohesion (kPa), ca the soil-interface adhesion (kPa), q the surcharge; a and d are tine rake angle and angle of soil-interface friction (degree), respectively. The N-factors were estimated from the relationships established and reported by Hettiaratchi et al. (1966). 2.5. Experimental procedure Fig. 1. An overview of the soil tillage dynamics equipment used in the study. (1) Load meter; (2) tool carriage; (3) tool vertical adjustment device; (4) tool angle measuring plate; (5) tool bar; (6) profilemeter; (7) soil processing trolley frame; (8) soil leveler; (9) compaction roller; (10) roller vertical adjustment device; (11) vertical adjustment pipe; (12) winding handle.
2.5.1. Soil preparation and measurements The experimental soil was dried to the initial moisture content and crushed to a fine uniform size before it was put into the soil bin. Experiments were conducted first in the driest state and water
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402 Table 2 Mechanical property of the experimental soils. MC (% db)
6.0 11.5 17.5 20.0
(0.2) (0.1) (0.2) (0.1)
BD (kg/m3)
1500 1520 1560 1530
(11) (10) (11) (9.5)
Cohesion (kPa)
Internal friction angle (degree)
Adhesion (kPa)
12.1 13.3 24.5 22.6
30.2 29.6 36.5 34.5
0.18 0.21 0.29 0.35
(1.2) (1.1) (1.4) (1.2)
(1.1) (1.2) (1.2) (1.3)
(0.02) (0.01) (0.03) (0.02)
Soil/metal friction angle (degree)
Cone index (kPa) 75 mm depth
150 depth
22.3 23.6 24.7 23.1
575 690 785 720
580 725 800 745
(1.3) (1.2) (1.3) (1.1)
(20) (25) (15) (22)
(25) (23) (20) (24)
MC = moisture content; BD = bulk density. Standard error in parenthesis.
was added for the consequent runs until the moisture content of the soil reached an equilibrium state similar to the procedure reported by Gupta and Surendarnath (1989). The soil processing trolley was used for processing the soil mechanically in the bin in order to achieve uniform soil condition as desired for test-run throughout the soil bed. A reasonable agreement was found in the compaction level at the different locations along the length of the bin. This was achieved by rotavating the soil and passing the roller over it for a fixed number of times usually between 4 to 6. The cone penetrometer was used to monitor the uniformity of the prepared soil by comparing the cone indices of the sampled sites. After the soil processing was over, the test tool was mounted on the tool bar, and the desired depth and rake angle were adjusted appropriately. In all the tests, the mean moisture content was about 11.5% (db), the rake angle was held constant at 908, and the mean cone index was 600 kPa. Moisture content of the soil was determined by gravimetric method. The five levels of operating depth tested were 35, 70, 150, 200 and 250 mm respectively.
3. Results and discussion 3.1. Comparison between measured and predicted draught forces The measured draught forces were recorded after the procedure explained under experimentation. On the other hand, the predicted forces were calculated using Eq. (1). The values of the measured draught forces, the corresponding predicted values and the percentage over or under prediction are shown in Table 3. Eq. (1) was more appropriate to predict draught forces with narrow tines than very narrow tines or wide tines. 3.2. Effect of depth of operation on draught The effect of depth of operation on draught of tines is presented in Figs. 3–5. The best relationship between draught (D) and operating (d) was a curvilinear one from regression analysis, and is of the form: D ¼ aebd
2.5.2. Calibration of load cell To ensure the accuracy of measurements of draught, the load cell was calibrated in advance of being used for force measurement. The dead weight method was used for the calibration in the range of force experienced in the draught measurements. 2.5.3. Experimentation For all the experiments, the soil moisture content was held constant at an average of 11.5% (db). After the soil processing was over, the test tool was mounted on the tool bar for experimentation. In each of the three blocks, the forward speed and rake angle were kept constant but the depth was varied in five levels. The fixed rake angle was 908 for all the treatments while the tool carriage was towed at the fixed forward speeds V1, V2, V3 corresponding to 0.28, 1.0 and 2.5 m/s respectively for the three blocks. During the test, the load cell measured draught. Data were collected and mean values of three replicates were used for computation and analysis. A profile meter similar to that described by Spoor and Godwin (1978) and meter scale were used to measure the soil-disturbed surface after each test. Soil disturbance parameters were subsequently defined and evaluated. For the purpose of analysis, the general form of soil disturbance was quantified by the parameters shown in Fig. 2. The parameters used to describe soil disturbance include: maximum width of soil throw (TDW); maximum width of soil cut (WFs) also referred to as width of crescent; the ridge-to-ridge distance (RRD); the height of the ridge (hr); after plough furrow depth (df) and the tool width (w). Another parameter of the soil disturbance measured but not shown in Fig. 2 is rupture distance (f), defined as the distance ahead of the tine at which the distinct shear plane broke the surface (Godwin and Spoor, 1977). Some of these parameters have been used to assess soil disturbance of tillage implements by researchers (Willat and Wills, 1965; Godwin and Spoor, 1977; Spoor and Godwin, 1978; Taniguchi et al., 1999).
(2)
where a and b are coefficients of the exponential function. The regression equations and the coefficients of determination are
Fig. 2. Parameters used to define soil disturbance of a single tine: maximum width of soil throw (TDW); maximum width of soil cut (Wfs); ridge-to-ridge distance (RRD); height of ridge (hr); after plough depth (df); tool width (w). Table 3 A comparison of the measured and calculated draught force of the experimental tines. Depth (cm)
Tine
Experimental
Calculation
% prediction
3.5
T1 T5 T20
23 90 219
13 68 273
43 24 +24.6
10.0
T1 T5 T20
55 180 639
40 203 812
27 +12 +27
15.0
T1 T5 T20
100 280 950
63 315 1256
37 +12.5 +32
+, over-prediction; , under-prediction; T1, very narrow tine of width 1 cm; T5, narrow tine of width 5.0 cm; T20, wide tine of width 20 cm.
S.I. Manuwa / Soil & Tillage Research 103 (2009) 399–405
Fig. 3. Effect of depth on draught at forward speed of 0.28 m/s. DT1, draught of tine of width 1.0 cm; DT5, draught of tine of width 5.0 cm; DT20, draught of tine of width 20 cm.
Fig. 4. Effect of depth on draught at forward speed of 1.0 m/s. DT1, draught of tine of width 1.0 cm; DT5, draught of tine of width 5.0 cm; DT20, draught of tine of width 20 cm.
shown in the respective figures. In these figures, draught increased at an increasing rate with increase in operating depth. The reason being that at higher depths more soil volume is considered, soil becomes stiffer and denser (due to overburden pressure and so strength properties vary. It should be noted that the exponential modes best fitted the relationships, with R2 (coefficient of determination) values ranging between 0.9784 and 0.9975. This curvilinear relationship is similar to that reported by Al-Suhaibani and Al-janobi (1997), Grisso et al. (1996) and Nicholson et al. (1984). An increase in specific draught (draught per maximum width of soil cut), was observed with an increase in tillage depth for all the tools tested. This also agrees well with the work reported by Al-Suhaibani and Al-janobi (1997) and Ademosun (1990). Generally specific draught increased with width of tine but less proportionately. This agreed with the findings reported by McKyes
403
and Maswaure (1997). The increase became higher as the depth increased due to the increase of bulk density with depth. Also in the range of depth considered, the increase in specific draught became higher as the tine, width increased. This is because the amount of soil displaced by narrow tines is considerably lower than that disturbed by wide tines. Inertial forces are more significant for wide tines than narrow tines. The specific draught of the tines T1, T5 and T20 ranged from 5 to 6.1, 8.6 to 12.2 and 6.8 to 13.2 N/cm respectively within the range of depth considered. In the range of depth of operation tested, under this condition, the specific draught increased by about 21.2%, 42.0% and 92.18% for T2, T5 and T20 tines respectively. Although draught force increased with width, it was less than proportionately. Specific draughts of different widths of tines operating at the same depth were relatively higher per unit for narrower tines. This is due to the shear area, which is smaller per unit width for narrower tines. This is in close agreement with the findings of McKyes and Ali (1977). The numerical values of these parameters would provide valuable information in the design of tillage tines. 3.2.1. Analysis of variance tests Analysis of variance (ANOVA) was also performed on the interaction of tool type (T) and operating depth, d—the sources of variation. For all the three levels of speed the results showed that tool type and operating depth affected the draught of tines significantly at 5% level of probability (p < 0.05). The interaction between the two factors was also statistically significant at 5% level of probability (p < 0.05). Moreover, draught increased as the forward speed of a tine increased. This is mainly because of the acceleration of the soil. Greater forces provide this acceleration and since they also increase the reaction at the interface, a higher sliding resistance results. The increased sliding resistance contributes most to the increased draught force (Spoor, 1969). 3.3. Soil disturbance parameters The effect of depth of operation of the tines (T1, T5, and T20) on soil disturbance was observed for a forward speed of 1.0 m/s (3.6 km/h), 908-rake angle, average cone index of 600 kPa and moisture content of 11.5% (db) with the results presented in Table 4. As the tool moved through the soil in the soil bin, the soil was disturbed as it was cut and thrown to the sides of the tool. The soil disturbance generated was observed, assessed and analyzed. The shape of the ridge profile was very close to an isosceles triangle at the speed of 3.6 km/h. This finding was similar to that reported by Liu and Kushwaha (2006). The width of the tool strongly influenced the soil disturbance parameters as they all increased as the working depth increased but less proportionately. This was also similar to the findings reported by McKyes (1985). This is because the major factors that control the nature of soil failure or disturbance are the aspect ratio (depth/width ratio) and the rake angle. Tines with small aspect ratio causes cause a surface crescent-like failure (Payne, 1956). As the depth/width ratio increased, the nature of soil failure changed. The inertial forces influenced the distance that the sol was thrown away from the tool path. For all the three levels of speed, the furrow level was filled to an upper level mostly with pulverized soil that fell back, this influenced the after furrow depth. 3.4. Estimation of critical depth of experimental tines
Fig. 5. Effect of depth on draught at forward speed of 2.5 m/s. DT1, draught of tine of width 1.0 cm; DT5, draught of tine of width 5.0 cm; DT20, draught of tine of width 20 cm.
The critical depth of the experimental tillage tines was estimated based on the procedure described by Desbiolles (2008), which of course was referred to as a rough guide. The results are presented in Table 5.
S.I. Manuwa / Soil & Tillage Research 103 (2009) 399–405
404 Table 4 Effect of depth of operation on soil disturbance. Parameters of soil disturbance
Depth (cm) T1
RRD (cm) Wfs (cm) TDW (cm) df (cm) hr (cm) f (cm) Draught (N) Sp. draught (N/cm)
3.5 5 6 9.5 2 0.8 5.5 23 (4) 3.8
T5 10 10 10 12.5 7.5 1.5 11 55 (6) 5.5
15.0 15 16.5 17.5 10.5 1.6 15 100 (10) 6.06
3.5 12 10.5 25 1.5 2.5 9.5 90 (11) 8.57
T20 10 13.5 20 27.5 5.5 4.5 18 180 (14) 9.00
15.0 16.5 23 33 7.0 6.0 21 280 (25) 12.17
3.5 26 28 39 1.9 2.5 17.5 219 (21) 7.8
10 30 32 43.5 3.5 3.0 23 639 (59) 19.96
15.0 33 34 46 6.2 4.5 25 950 (69) 27.94
a = 908; v = 1.0 m/s; MC = 11.5% db; f = rupture distance Standard error of draught in parenthesis.
Table 5 Estimated critical depths of the experimental tillage tines. Consistence/soil condition
Tine type
Tine width (cm)
Critical depth (cm)
Moisture status
Friable
T1 T5 T20
0.01 0.05 0.2
8–12 40–60 160–240
Moist
T1 is very narrow tine of width 1 cm; T5 is narrow tine of width 5.0 cm; T20 is wide tine of width 20 cm.
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