The two-wheel tractor consists of five components: (Fig. 1.30)
°1 Engine
°2 Engine-base assembly with a front hitch and a stand
°3 Transmission gear-case assembly with a master clutch and a rear hitch
°4 Handle assembly with several control levers
°5 Farm-wheels
Engines on Two-Wheel Tractors
An industrial engine is mounted on the tractor. A forced-air-cooled gasoline engine with a single cylinder using either a two- or four-stroke cycle is mounted on the machine so as to make it light. A water-cooled, single-cylinder diesel engine is mounted on heavy two-wheel tractors. Figure 1.31 shows each of them and an example of actual engine
Figure 1.30. Main components.
Figure 1.31. Engines and actual performance curves (Sakai).
performance curves measured in the engine test room of an R&D company. Engine performance curves in catalogues are modified by the company in a much simplified expression of these basic data. (There are a few models of small and light air-cooled diesel engines. Those engines, however, need a high level of engineering technology to develop and a limited number of countries produce them.)
Output Shaft
The rotation direction of the output shaft of the engine should be counter-clockwise from the farmer facing it, according to the industrial standard.
Power Output of the Engine
The expression of engine output is recommended to be in SI units. However in general, conventional units such as horsepower, kilogram-meter, etc. are still in popular use, especially in manufacturing companies. The expression of engine output power varies in Watt units slightly from country to country. They are HP, PS and kW as follows:
1 HP: horsepower in the U.K. system of units:
550 ftãlbf/s=76.0402ã ã ãkgfãm/s
∵ 1 lb (English system of units) represents the gravitational force acting on a mass of 0.453592ã ã ãkgf,
ft (English system of units)=yard/3=0.914399ã ã ã/3=0.304799ã ã ãm W=HPãG=745.6996ã ã ã;746 Watt;0.75 kW
∵ G=9.80665ã ã ãm/sec2as an international standard.
1 PS: Pferdest¨arke, German for horsepower in Germany DIN, Japan-JIS, etc.:
called metric horsepower
75 kgfãm/s (f means gravitational force)
W=PSãG=735.4987ã ã ã ;736 Watt;0.74 kW Thus, “PS;HP” in design and test practice.
Moreover, the measuring method for the power of the engine under a prevailing indus- trial or engineering standard differs depending on each country. For example, the SAE standard in the United States calls for the measurement of two kinds of engine output,
gross power rating of a basic gasoline engine which is measured without equipment and accessories such as a muffler and an air cleaner, etc., and net power rating of a fully equipped one. (Refer to SAE J1995, J1349.) Both test data are corrected to what would be expected under standard atmospheric pressure and temperature conditions.
Some companies indicate the gross power rating in the catalogue.
DIN in Germany and JIS in Japan (JIS B 8017, 8018) call for measurements to be made with such equipment in place, and the data are corrected to German or Japanese standard atmospheric conditions. Their companies indicate the horsepower rating of the fully equipped engine in their catalogues.
Moreover, the determination manner of a catalogued horsepower obtained from many basicdata (refer to Fig. 1.31) may be different from one manufacturer to another.
Therefore, in an actual case, the engines of the same catalogue-horsepower, PS or HP, from different countries produce considerably different power ratings, more or less 20%, in actual farming work.
The measured values of the horsepower of small industrial diesel engines also have practical differences.
Force Calculation of Engine Output
The driving force P (kgf or N), produced at the effective radius, r (m), of the output- pulley and driven by the output Ne (PS;HP) of an engine, is obtained by the following equations as illustrated in Fig. 1.32, showing a historically original expression of one horsepower as an example:
P=60ã75Ne/(2πrn) [kgf] (1.15)
∴ P=716.2Ne/(nr) [kgf] (1.16)
P=7023.5Ne/(nr) [N] (1.17)
Engine-base Assembly
The engine-base is tightly bolted and supported by a folding-type front stand located under the forwardmost section of the base. It should be safe and convenient for the farmer if he or she can operate the front stand while holding and pressing down the tractor handles.
Figure 1.32. One horsepower expression (Sakai).
When the two-wheel tractor, coupled with standard tillage equipment, is put on a flat surface, the elevation angle,α, at the front of the engine base (Fig. 1.28) should be as large as possible, more than 30◦, and if possible 40◦, in order to avoid causing damage to the levees around the paddy fields or to the plants on upland fields.
Handle Assembly
The handle assembly has several levers to operate the master clutch, parking brake, gearshift mechanism, steering clutches, and engine governor, etc. and each of these levers must be installed in its proper location.
The handles should be optimum in height and width to give a comfortable operating posture to the farmer in both operations of transportation and fieldwork.
Power Transmission Mechanisms
The transmission mechanism consists of a master clutch on the input shaft, a PTO shaft, shiftable transmission mechanisms, a parking brake and a final reduction drive using gears or a chain and sprocket mechanism. The final drive includes a set of steering clutches and drive axles, which are usually hexagon shafts.
The Size of Power Transmission Mechanisms
The size of the whole mechanism is designed with the following ideas: for a given level of power transmitted, all components are subjected to torque and forces that are inversely proportional to their rotational speed (refer to Eqs. 1.16 and 1.17). In order to make their structure smaller and economical, the first half of the transmission mechanism, including the multiratio gear mechanism, should operate with small reduction ratios at high rotating speeds, but within the range of noiseless rotation speeds.
However, additional ideas are needed to distribute the total reduction ratio to all the gears in an appropriate way, because having too great a reduction ratio for the final drive may cause a problem of reduced ground clearance for the tractor, due to excessive diameters of the final gears.
Master Clutches
The master clutch may be categorized as the following types:
°1 Belt clutch: Idler tension type, engine tension type as shown in Fig. 1.33.
°2 Disk clutch: Single, double or multiple disk type (all disk clutches are usually of a dry type).
°3 Cone clutch: Large clutch capacity, easy to produce, but big and heavy due to cast-iron components.
Figure 1.33. Tension belt clutches.
°4 Centrifugal clutch: Easy to drive. In design, it is necessary to select carefully the optimum rotation speed and torque capacity at the start of clutch engage- ment.
Torque Capacity of the Master Clutch
The torque capacity of the master clutch has to be determined under the loading conditions. The torque capacity of the clutch for long life may be as follows:
maximum engine-torque×1.5–2: for traction work maximum engine-torque×2–4: for rotary tillage Multiratio Gears and Travel Speeds
The farmer requests the following two ranges of travel speeds: (Table 1.10) human walking speeds for farm work
transportation speeds in trailing
Table 1.10. Travel speeds of two-wheel tractors (cm/s) (km/h)
Rotary tillage 25–50 0.9–1.8
Miscellaneous field work∗ 50–70 1.8–2.5
Plowing 70–120 2.5–4.3
Transportation∗∗ 15 or 25 or 30
∗ Puddling, inter-row cultivation, seeding, mowing, etc.
∗∗ Nominally traffic law may determine legal speeds. Ac- tual max. speeds may be set by local customs.
Two-wheel tractors, mainly of a traction type, frequently have a multispeed gearing system for general farm work and a range-shift gearing system as follows:
Farm work shift: 2–4 forward and 1 reverse gear ratios Range shift: A farmwork range and a transport range
Two-wheel tractors of the drive type frequently have no range-shift for transportation, because of the difficulty of detaching the tillage implement.
Steering Mechanisms
There are four types of steering mechanisms as follows:
°1 Loose pin-hole type of wheel hub: The pin-hole of each wheel hub is tangentially elongated, while a round pin is attached to the axle to transmit torque to the wheel hub. If the driver pushes the tractor handle strongly to the left or right, both wheels can tolerate slight rotation differences to allow the turn. This is applied to simple hobby tractors.
°2 Dog type: This is the most common type, with one clutch for each wheel. When engine power is more than 7–8 PS (5.2–5.9 kW), some farmers may have dif- ficulty in operating the clutch lever because of a high level of torque acting on the dog-clutch. Therefore, the clutches are recommended to be installed on the upper shaft of higher rotational speed. In order to avoid this problem, there are a few alternate mechanisms such as gear clutches, planetary gear clutches, etc.
(Fig. 1.34).
°3 Planetary gear type: The steering clutch-lever of this mechanism is easily op- erated. This mechanism can reduce the total number of transmission shafts, and give a larger reduction ratio to the final drive.
°4 Differential gear type: This type is useful for easy operation of the steering drive for trailer transport. A differential-lock mechanism is not always necessary. The brake mechanism with a waterproof structure should be installed to drive wheels.
This type has the disadvantage of high cost and is rarely used for two-wheel tractors.
Front and Rear Hitches and Hitch-pins
The two-wheel tractor has a hitching mechanism at the rear and sometimes at the front of the tractor as shown in Fig. 1.29. They are usually of the same dimensions.
Figure 1.35 shows their main hitch and pin dimensions of the JIS B 9209 and TIS 781-2531. Figure 1.36 shows types of the hitch for swinging functions of attachments (refer to p. 75). JIS recommends rolled steel for general structures, such as SS41, as their materials.
Figure 1.34. Steering clutches.
Figure 1.35. Hitches and pins.
Figure 1.36. Types of swinging functions (Sakai).
Figure 1.37. Rubber tired wheels and steel wheels.
Figure 1.38. Cross sections of rubber tires.
Wheels and New Wheel Dynamics
In this section, minimizing the conventional principles of wheel structures, only new principles of wheels for two-wheel tractors on paddy soil will be explained. In order to avoid disorder, the process of finding and setting terminology will be summarized herein.
Types of Wheels and Lugs
They are grouped into two kinds, rubber tired wheels or steel wheels, and upland-use wheels or paddy-use wheels. Those with unique lugs are called lugged wheels (Fig. 1.37).
a. Rubber-Tired Wheels. Paddy-use tires are called high-lug tires, wide-lug tires and paddy-lug tires, developed in the 1960s in Japan (Fig. 1.38), of which the lug size is about two times higher, the lug pitch is longer and the lug width is less than those of the
Figure 1.39. Minimum number of lugs (Sakai).
upland-use tire. The overall width of the total lug pattern is wider than the tire section (refer to the terminology in ASAE S296.4 DEC95).
b. Steel Wheels. These are classified into two types. In general, the upland wheel has many lugs on its plate-rim. The paddy wheel has fewer and larger lugs on its pipe-rim than the upland one. The usual number of lugs of a paddy wheel for two-wheel tractors is 6–12 as shown in Fig. 1.37. The reason is that the wider lug spacing and smaller number of lugs on a pipe rim are effective in preventing the wheel from trapping adhesive soil clods between lugs.
To minimize the number of lugs NLin planning design is one of the important tasks for the design engineer. This is calculable by the following radian equations, with the consideration that the lugs should be given a maximum lug spacing to move only in the downward and backward directions (from A to B in Fig. 1.39) in the soft paddy soil [2]:
NL≥2π/cos−1{30v/(πnr1)} (1.18)
or NL≥2π/cos−1(1−S) (1.19)
where v : expected travel speed of the tractor (cm/s) n : rotational speed of the wheel (rpm) r1: outside radius of the wheel (cm)
S : expected travel reduction. The practical value is 0.10–0.20 in paddy fields in general.
Loading Pressure and Mobility
If the machine has better mobility than that of the farmer, the farmer cannot follow the machine. Approximate human foot-pressure is 0.4 kgf/cm2(39 kPa). In order to have machine mobility similar to human mobility and to maintain the plow pan surface, the loading pressure of the lug is recommended to be 0.2–0.3 kgf/cm2(20–30 kPa).
The Shape of a Model Lug and Classifications
Figure 1.40 shows a new expression of model lugs from the design point of view.
Namely, a basic lug shape is formed with five points (A, B, C, D and E). Lug geometry is described by the following four straight or curved surfaces:
°1 AB surface: called a lug side, trailing.
°2 BC surface: called a lug face.
°3 CD surface: called a lug side, leading.
°4 DE surface: called an undertread face.
Figure 1.40. Model lug-shapes (Kishimoto).
Figure 1.41. An example of rigid-lug motions (Kishimoto).
The Motion of Lugs and the Lift Reduction
Lug motion is conventionally described with the idea of the travel reduction SHin the X-axis direction only. A new reduction, SV, in the Y-axis direction was proposed by T.
Kishimoto, Japan [3]. This value, SV%, has been named Jyosho-teika-ritsu ( ) in Japanese, and lift reduction in English, and has been defined as follows:
SV≡100(HV0−HV)/HV0 (1.20)
where HV0: max. lift of the axle on a rigid level surface.
HV: actual lift due to sinking of the lug into soft soil.
The lift reduction of zero% means lug motion on the rigid surface, and that of 100%
means smooth straight motion of the wheel center on the soft soil. Figure 1.41 shows the motion equations with travel and lift reductions, SHand SV, of the rigid lug make it possible to simulate rigid-lug motions on/in soft soil as well as on a rigid surface.
External Forces and Lift Resistance
The conventional theory shows that there are two kinds of basic external forces acting on the towed wheel as follows: (Fig. 1.42)
°1 Soil reaction force RNin the upward direction, to the dynamic load WNin the downward direction as an action force. Their values should be equal: RN=WN.
Figure 1.42. Conventional expression of forces.
°2 Motion resistance f in the backward direction.
f≡àRN (àis a motion resistance ratio) (1.21) There is a gross traction PGin the direction of travel on a driven wheel, and a net traction PNis:
PG−f≡PN (because if PN⇒zero,PG⇒f) (1.22) It was noticed in 1988 by Sakai that when lug wheels operate on compressed adhesive soil, a considerable external force, RL, in the downward direction, resisting the upward motion of the lug at the bottom of trochoidal motion, is generated on the lug. This was reported in 1991 [5] after getting the experimental proof measured with two kinds of new sensors [4, 6]. This force has been named Jyosho-teikoh-ryoku ( ) in Japanese, Sangseung-jeohang-ryeok in Korean, and lift resistance. [5, 7]:
The principal factor contributing to lift resistance is a downward resistance, AP, acting on the lug face on the soil surface when the lug starts to move up. This force APhas been called contra-retractive adhesion [5] or perpendicular adhesion [7].
There are other factors affecting the lift resistance on the leading or the trailing lug side. An experiment showed these were small, representing several to 20% of the total lift resistance at high levels of travel reduction [11].
Conventional slow-speed wheel dynamics shows that the sum of dynamic loads, 6WN, acting on all the supporting soil surface on average is the same as the total weight WTof the car. There was, however, a new finding in 1993 by J. S. Choe, [7, 9, 10] that on the plow pan surface, the actual dynamic load WGacting on the lug to the soil is greater than the dynamic load, WN. Namely, the average sum of the actual dynamic loads,6WG, acting from all the wheels to the supporting soil surface of a tractor is greater than the total weight WTof the tractor.6WG≥ WTFig. 1.43 shows the principle of how a 60 kgf person produces 80 kgf load and soil reaction on his left foot [7].
This phenomenon has been called a load transfer phenomenon [8]. So as to harmo- nize with the conventional definitions and data of the dynamic load, motion resistance ratio, traction efficiency, etc., the lift resistance ratio CLRhas been defined as follows [9, 10]:
CLR≡RL/RN ∴ CLR≡RL/(RG−RL) (1.23)
Figure 1.43. Actual forces on the human foot (Sakai).
Figure 1.44. Lift resistance ratios (Choe and Kishimoto).
RN: soil reaction force without the lift resistance. WN, has been named the net dynamic load [8]. RNcan be called a net dynamic-load reaction.
RG: (=WG), gross dynamic-load reaction with the lift resistance:
RG= |RN| + |RL| (1.24) Experimental data [11] showed that the lift resistance ratio CLR for light clay soil is in the range of 0.05–0.35 (Fig. 1.44), and proved that the higher the travel speed of the lug, the larger the value of the ratio becomes [7].
Acting Locations of the Lift Resistance
The acting location of lift resistance changes delicately. However, reasonable approx- imation can be made similar to the method used for a soil reaction force. The equation to calculate the distance e of R (=RN) with a motion resistance f (≡àR) on the wheel is: (Fig. 1.42)
e;àr1 (1.25)
∵ Re=f(r1−δ) δ→ very small where à: motion resistance ratio.à≡f/R=f/RN
δ: height of the point of action of the motion resistance from the compressed soil surface: approximately less than a half of wheel sinking depth or tire deformation.
Figure 1.45. New expression of external forces (Sakai).
Practical equations for the location distanceεof the lift resistance might be: (Fig. 1.45) [9]:°1 Rubber tire wheel ε;(2r1D−D2)1/2 ∵ r21−ε2=(r1−D)2 (1.26)
°2 Rigid lug wheel
ε;(rR+HL) sin(180/N) ∵ θ=360/N (1.27) where rR: radius of the rigid wheel-rim (cm)
D : deformation of tire in radius direction (cm) N : number of lugs
θ: lug pitch angle (◦)
These principles for tractor dynamics on paddy soil are available to those on dry soil by substituting zero for RL.
Two-Wheel Tractors with the Plow and Plowing
In order to be accepted satisfactorily by farmers, plowing performance of two-wheel tractors should be definitely better than that of the existing animal draft plowing with native plows to which they are accustomed. Agricultural engineers need to master the native plow and plowing technology in order to develop such a locally-made tiller.
Plows attached behind two-wheel tractors may be classified into two types. They are originally a European type and a Japanese type (Fig. 1.46).
Figure 1.46. The two-wheel tractor with a plow [16].
Figure 1.47. Animal draft native plows [12, 13, 16, etc.].
Figure 1.48. Japanese plows for two-wheel tractors (Sakai).
Figure 1.49. Plowing methods on fields.
Differences between European Plows and Asian Plows
Figure 1.47 shows some examples of animal-draft native plows. Figure 1.48 shows illustrations of Japanese plows for the two-wheel tractor.
Historically, European plows have been developed basically for upland farming, while Asian plows have been developed for paddy farming as follows.
a. Plowing Technology on Small Fields. There are two kinds of basic plowing methods and their modifications in large farm fields (Fig. 1.49). One is the return plowing method (a) and the other is the continuously-circuitous plowing method (b). These plowing methods are carefully followed by the farmer with consideration given to the gathering and casting of the furrow slices.
However, in small fields, especially in paddy fields, these plowing methods are not so practical. The method in which the plow is raised on the pass inside the levee causes the headland to be compacted by driving the tractor repeatedly, and continuous plowing involves busy turning in the center of the field. The problem of both methods is poorly leveled field surface after plowing, so the farmer is required to do additional work in puddling to make the soil surface flat. Therefore, if there is a practical reversible plow, a continuous return plowing method ((c) in Fig. 1.49) is preferred in narrow fields [13].
In museums in Asian countries, there are ancient plows whose shares and moldboards of symmetrical shape are set at the center front tip of the plow handle bottom (refer to (c) and (d) in Fig. 1.47). It is supposed that they were used as two-way plows by changing the tilt angle of the plow beam-handle to the left or right, and their small fields were plowed with a continuous return method. Those Asian native plows were originally very simple but convenient on small scale fields.
b. Plowshares, Moldboards and Furrow Slices. The shape of a European plowshare is like a narrow trapezoid, while an Asian plowshare is a spherical triangle or a half- oval, and has two equal cutting edges. As shown in Fig. 1.50, the moldboard of the European plow has a shape to impart a strong inverting action to furrow slices. Usu- ally surface residues, stubble and weeds are buried well under the disturbed topsoil layer.
The Asian plow, however, does not need to overturn furrow slices so much because, in paddy farming, plant residues and weeds are mixed into and buried under the topsoil through puddling before transplanting.
c. Supporting Principles of the Plow Bottom. Figure 1.50 shows that the European plow is supported by the landside whose cross-sectional shape is vertical in order to counteract the strong side-force produced by the plow, and to minimize the formation of a plow pan. The Asian native plow has a sole, a wide bottom surface, to form and maintain a plow pan (Table 1.9).
Figure 1.50. Plows, furrows and machines.