Two-wheel tractors are sources of power designed to perform most field operations.
Due to the size of such tractors, they become an economic alternative for scale farming.
In addition, two-wheel tractors are much more productive than animal traction and they require less time for attendance and preparation, giving the individual farmer more independence and contact with modern technology. Also, due to the simple design, local manufacture of two-wheel tractors has been implemented in several countries successfully, increasing the employment opportunities.
Two-wheel tractors may be classified according to the engine power in three cate- gories. Table 1.14 shows the main physical characteristics for each category.
Two-wheel tractors of Category I are normally dedicated to garden work, transporta- tion or light interrow cultivation. Many of such tractors can incorporate a rotary tiller on the axle, with a working width of 500–800 mm, Fig. 1.82. Due to their low weight and small clearance, these machines are not suitable for plowing dry and hard soils.
However they may perform a number of field operations, such as lifting water, forage cutting, transport and cultivation.
Two-wheel tractors of Category II may be used for the same field operations as those of category I. In this case tiller width is of 1 m. It is possible to cultivate and plow soft soils. Some designs incorporate wing hubs with pawls to assist turning. Many tractors of this category are equipped with a rotary tiller driven from a PTO, Fig. 1.83.
Two-wheel tractors of Category III cover the same applications as those of categories I and II, but in addition some designs are sufficiently heavy to plow in dry-land agriculture.
For silty clay, the required draft for a 250 mm mouldboard plow working at 200 mm depth with a speed of 3 km/h is approximately 3.7 kN. This value is similar to the
Table 1.14. Main characteristics for two-wheel tractors2 Overall
Dimensions Track Maximum
L×W×H Width Clearance Mass Traction Speed Power
Category mm mm mm kg kN km/h kW Engine
I 1500×410 315 150 45–60 0.3–0.5 1.55 2.7–4 2 or 4
×1000 cycle
gasoline or diesel
II 830–530×1800 400–700 200 75–148 0.6–1.2 1–12.6 5–7 4 cycle
×1230 gasoline
or diesel III 1900×560 400–750 200 175–465 1.37–3.7 1–16.3 8–10.2 4 cycle
×800 gasoline
2680×960 5461 5.471 3.911 or diesel
×1250
1 Parameters for a high clearance two-wheeled tractor of special design. Lara-L´opez et al. 1982.
2 Source: catalogues from manufacturers from Europe and Asia and publications on special designs.
Figure 1.82. Two-wheel tractor of Category I. 2.5 kW (3.5 hp) with tiller on the axle. Mass of 50 kg. Meccanica Banassi.
maximum draft obtained from some of the heaviest commercial models of two-wheel tractors, although for dry soils with crop residues the draft requirements may be higher (Fig. 1.84).
Two-wheel tractors designed for wet land may be lighter than those designed for dry land, where high traction capability is more necessary than flotation. Wet-land tractors are equipped with steel wheels with lugs to provide traction. Satisfactory results in dry- land agriculture were obtained with a power-to-mass ratio of 0.014 kW/kg. Heavy-duty two-wheel tractors designed for wet land conditions show ratios of 0.022 kW/kg.
Row crops such as corn, sorghum and vegetables are cultivated several times requiring high clearance tractors for multirow cultivation. Regions such as Latin America, where such crops are basic foods, require such type of equipment. However, the production of high-clearance tractors is rather small. Figure 1.85 shows a two-wheel tractor with axle
Figure 1.83. Two-wheel tractor of Category II. 8.8 kW (12 hp) with tiller on seperate axle, four stroke gasoline engine or diesel engine. Mass of 175 kg. Goldoni.
Figure 1.84. Two-wheel tractor of Category III. 8 kW (11 hp) heavy duty machine air-cooled or water cooled diesel engine or water cooled gasoline engine. Mass of 376 kg. Mitsubishi.
Figure 1.85. A Regular two-wheel tractor with axle extensions for incorporation of high clearance feature.
extensions for providing high clearance. Figure 1.86 shows the walking and riding version of a high clearance tractor designed for dry-soil agriculture and local manufacture. [1]
and [2].
Many tractors of Category I have a rigid axle and steering is done by pulling handles sideways causing tines on the tiller to slide with respect to the ground. Other small and intermediate tillers are equipped with towing hubs with pawls to assist turning. Some of the two-wheel tractors manufactured in industrial nations are equipped with a differential gear with locking device and independent brakes. Some designs for local manufacture in- corporate steering clutches, with brakes for heavy tractors. The tractor of Fig. 1.86 steers by controlling the tension on two independent belts. Two-wheel tractors incorporate articulated sulkies to increase productivity. In this case the chassis angle is controlled.
Modern designs incorporate driven wheels for the sulky increasing, tractive capability.
Engine power is engaged by tensioning v-belts, in some designs. This is done with idler pulleys, which in some cases are supported by springs and in other cases with an over-center linkage. This device has the advantage that the idler pulley is fixed in a position independently of the variation of the torque of the engine that may take place during one cycle. Another option for tensioning belts used in some garden equipment is by sliding the engine along guides. Some designs use this principle for loosening belts and at the same time producing contact between pulleys of special design to obtain
Figure 1.86. A high clearance two-wheel tractor designed for local manufacture, developed at the University of California and at the University of Guanajuato, Mexico. a) Walking version. b) Riding version with sulky and simple hidraulic lifting device.
reverse movement of the tractor. An economic multiratio belt drive consists of changing belts to pulleys of different diameter. Most of the two-wheel tractors manufactured in industrialized countries incorporate friction clutches and transfer power from the engine to a gear box with several gear ratios, as shown for a small tiller in Fig. 1.87. Many designs for local manufacturing incorporate roller chain drives or a combination of roller chain drives and gear drives. Power for a separate rotary tiller axle is obtained from one shaft of the gear box and an engaging device is incorporated. Other power take-offs are incorporated to drive stationary equipment from the gear box or directly from the engine.
Figure 1.86. (Continued)
Figure 1.87. Mechanical transmission of Category II two-wheel tractor based on gears.
Figure 1.88. Characteristic curves for typical engines use on two-wheel tractors. A and B are power curves, and C and D are the respective
torque curves.
Two and four-cycle gasoline and four-cycle diesel engines are used for two-wheel tractors of Category I. For categories II and III, four-cycle diesel and gasoline engines are used. Diesel engines of new design incorporate an air cooling system and operation speeds up to 3600 rpm. This type of diesel engine is competing with traditional four- cycle gasoline engines and with lower-speed water-cooled diesel engines. Figure 1.88 shows typical characteristic curves for diesel and gasoline engines.