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in agriculture, vehicle used to pull such equipment as plows, cultivators, and mowers; to power stationary devices such as saws and winches; and to push snowplows and earth-moving implements. Early tractors were steam-driven; now they are generally powered by gasoline or diesel engines. The two main types are wheeled tractors and crawlers, or caterpillars, which move on treads. Advances in technology resulted in the number of tractors in the United States increasing from 600 in 1907 to 3.4 million by 1950. The power takeoff, which transmitted power from the tractor engine to an implement via a shaft, was introduced in 1918. The small general-purpose tractor followed c.1924. Pneumatic rubber tires, affording increased speed, easier operation, lower fuel consumption, and longer wear, were introduced in 1932. Four-wheel drive and diesel power increased pulling power in the 1950s and 60s. These advances led to today's very large, double-tired tractors with enclosed, air-conditioned cabs, capable of pulling several gangs of plows.


See R. Leffingwell, The American Farm Tractor (1991).



a self-propelled crawler or wheel-type machine for the performance of agricultural, road-construction, earth-moving, and transportation work as a unit with trailer-type, toolbar, or stationary machines (seeTRACTOR-IMPLEMENT UNIT).

Historical survey. The first wheel-type tractors, powered by steam engines, appeared in Great Britain and France in 1830; they were used in transportation and for military purposes. They were used in agriculture in these countries beginning in 1850, and in the USA beginning in 1890. Valuable inventions relating to the crawler drive were made in Russia by D. A. Zagriazhskii in 1837 and by A. P. Kostikov-Almazov in about 1889. In 1888 the Russian mechanical engineer F. A. Blinov built and tested a crawler tractor with two steam engines. In the period 1893–95, the self-taught Russian inventor la. V. Mamin built a self-propelled wheeled cart with an internal-combustion engine. In 1901 the Hart-Parr company in the USA produced the first wheel-type tractors with internal-combustion engines. Beginning in 1912, crawler tractors were produced in the USA by Holt; they were later produced in Germany by Wanderer-Dorner.

The first tractors were produced in the USSR in 1923 (the Fordzon-Putilovets). Mass production of tractors began in the USSR in 1930, making possible the cessation of tractor imports in 1932 (see).

Classification. A distinction is made between farm tractors and industrial tractors. When combined with the appropriate implements, general-purpose farm tractors are used for plowing, cultivating, sowing, harvesting, and other work. The most powerful farm tractors are used in the development of virgin and fallow lands, in pulling stumps, and in removing brush and plowing it under. Row-crop tractors make possible the mechanization of interrow tillage, such as cultivation, hoeing, hilling, dusting, and harvesting of such row crops as corn, sugar beets, and cotton. The particular features of row-crop tractors are their ability to operate with toolbar-mounted implements and to operate in the interrows of row crops, and their wide track (usually adjustable), high clearance, and narrow wheels or crawler tracks.

The base models of industrial tractors are characterized by higher tractive force than farm tractors. They are used for earth-moving, road-construction, and reclamation operations in a unit with various toolbar-mounted and trailer-type implements. The first group includes bulldozer or snowplow blades and excavator

Figure 1. Wheel-type tractor: (1) engine, (2) steering wheel, (3) cab, (4) fuel tank, (5) levers of toolbar assembly. (6) power takeoff shaft, (7) hitch, (8) main drive, (9) driving wheel, (10) gear box, (11) clutch, (12) front (steerable) wheel

buckets, and the second includes scrapers and graders. Various modifications of the base models are used, depending on the operating conditions. For example, farm tractors have vineyard, swamp, hillside, and garden models, and industrial tractors are available in reclamation, rafting, and skidding versions. Tractors may have wheel or crawler propulsion.

Mechanisms and equipment. The power plant of a tractor consists of an engine and auxiliary equipment (see). The transmission includes the clutch, the coupling, the gear box, and the main and final drives (seePOWER TRANSMISSION). Friction clutches are the most common, although hydrodynamic and electric clutches are sometimes used. The multispeed mechanical transmissions of farm tractors have six, eight, 15, or more speeds, and those of industrial tractors have three to six speeds. Constantmesh transmissions or transmissions with a planetary reduction gear are becoming increasingly widespread; they are used on certain foreign tractors and on the Soviet T-150, T-150K, and K-701 models. Torque is delivered to the driving sprockets of crawler tractors through a main transmission; usually a bevel reduction gear; in wheel-type tractors a differential gear is used. The final drives, which are usually spur reduction gears, are located at the driving wheels and serve to increase the overall gear ratio of the transmissions and to provide the necessary ground clearance. Hydraulic and hydromechanical transmissions have been used on some experimental models. The former consist of a hydraulic pump and hydraulic motors, and the latter, of a torque converter and mechanical gear box. To attain especially low speeds, the transmissions are equipped with additional reduction gears.

The running gear of wheel-type tractors consists of the suspension, axles, and steerable and driving wheels, with low-pressure pneumatic tires. Half-track drive, wide cage wheels, and additional grousers are sometimes used to increase off-road capability. The running gear of crawler tractors consists of a suspension, the crawler tracks, the driving sprockets, the bogie wheels, the support rollers, and the idler wheels.

The body of a tractor is usually made in the form of frames of varying designs. The control equipment consists of the steering system and band or disk brakes. For wheel-type tractors, a change in direction is usually accomplished by the front wheels. To improve maneuverability, tractors are sometimes designed with four-wheel turning, adjustment of torque on the driving wheels, or relative rotation of the front and rear parts of the tractor (articulated-frame design). Crawler tractors are turned by

Figure 2. Crawler tractor: (1) engine, (2) cab, (3) fuel tank, (4) levers of tool-bar assembly, (5) power takeoff shaft, (6) hitch, (7) driving sprocket, (8) main drive, (9) crawler track, (10) gear box, (11) bogie wheel, (12) clutch, (13) idler wheel

changing the speed of the driving sprockets for the right or left tracks using clutches and brakes; sometimes a single-stage planetary mechanism is used with two pairs of brakes. Cabs are used on all Soviet tractors and most foreign tractors to create comfortable working conditions for the operator. The electrical equipment of a tractor consists of sources of electric current (a storage battery and a generator mounted on the engine) and equipment for starting the engine, for illuminating the path and working machinery (implements), for ventilating the cab, and for providing sound and light signals. Longitudinal sections of wheel-type and crawler tractors are shown in Figures 1 and 2.

Performance indicators. The main performance indicators of tractors are divided into technical-economic, technical, and agro-technical. Among the technical-economic indicators are unit productivity, the traction qualities, labor-intensiveness of maintenance and servicing, and consumption of metal; among the technical indicators are the longitudinal and transverse stability, ease of control, and comfort and convenience for the operator (the presence of a cab, control devices and instruments, and the number of seats); among the agrotechnical indicators are the specific pressure on the soil, ability to operate in interrows (ground clearance, outlines, and protective zones), maneuverability in a unit, smoothness of operation, and accuracy in following a set direction.

Types. The number of tractor models produced in the USSR, as well as the indicators of each model, is determined by the standardization—that is, by a system of machines built according to a previously developed technical plan based on a rational combination of the tractive indicators of the various types of tractors with a minimum number of base models. A class in the standardization is the aggregate of tractor models, or specific designs, having uniform classification parameters. The base model is the most common type of tractor in a given class. A modification is a model belonging to the same class as the base but having a different configuration or additional equipment. The greatest tractive force developed by a tractor with limited skidding has been adopted as the basic classification parameter in the USSR and member countries of the Council for Mutual Economic Assistance (COMECON).

The first standardization was proposed in 1923 but was not developed and realized until 1946. A second tractor standardization, for 1961–65, was made up in 1956. Provision was made for an increase in operating speeds to 5–6 km/hr and a rise in service life to 2,500–3,000 hr for engines and up to 5,000–6,000 hr for transmissions. Tractors corresponding to the world level of technology were produced. They included the T-40, MTZ-50, T-16, DT-14, and K-700 wheel-type tractors and the T-74, DT-75, and DET-250 crawler tractors. The third tractor standardization, for 1965–70, consisted of 13 base models with tractive forces from 6 to 250 kilonewtons (0.6–25 tons-force) and a series of modifications.

For a further improvement in the performance indicators, a fourth tractor standardization was developed for 1971–80 (Figure 3). The base models to be produced under this standardization

Figure 3. Tractor standardization for 1971–80: (P) tractive force (tons-force), (W) effective engine power (hp), (G) weight (tons)

Table 1. Base models of tractors produced in the USSR according to fourth standardization (1975)
0.2. . . . . . . . . . . . . . . . . . . . . . . .Rioni
0.6. . . . . . . . . . . . . . . . . . . . . . . .T-25; T-16M
09. . . . . . . . . . . . . . . . . . . . . . . .T-50
1.4. . . . . . . . . . . . . . . . . . . . . . . .MTZ-80
2. . . . . . . . . . . . . . . . . . . . . . . .T.CJW
3. . . . . . . . . . . . . . . . . . . . . . . .T-74-DT-75M; T-150; T-150K
4. . . . . . . . . . . . . . . . . . . . . . . .T-4A
5. . . . . . . . . . . . . . . . . . . . . . . .K-701
6. . . . . . . . . . . . . . . . . . . . . . . .T-130
15. . . . . . . . . . . . . . . . . . . . . . . .DET-250

are shown in Table 1. In the fourth standardization, operating speeds are to rise to 35 km/hr, service life before a major overhaul has been increased, and labor-intensiveness of maintenance has been reduced. Because of the increased speed of the tractors, additional elastic elements have been introduced in the suspension; the cabs are equipped with springs, are airtight, and are equipped with ventilating and heating devices and air conditioners. Broad standardization of assemblies and parts is to be introduced within each class and between classes. Tractors with electric and hydraulic drive are being developed. Equipment is to be introduced for the automation of tractor loading and the operation of a tractor-implement unit and for protection against emergencies.


Konstruktsiia, osnovy teorii i raschet traktorov. Moscow, 1971.
Gurevich, A. M., and E. M. Sorokin. Traktory i avtomobili, 2nd ed. Moscow, 1974.
Kolesnye traktory. Moscow, 1974.



(mechanical engineering)
An automotive vehicle having four wheels or a caterpillar tread used for pulling agricultural or construction implements.
The front pulling section of a semitrailer. Also known as truck-tractor.


A powerful engine-driven vehicle, on wheels or on tracks, used for pushing or pulling attachments or tools.


1. a motor vehicle used to pull heavy loads, esp farm machinery such as a plough or harvester. It usually has two large rear wheels with deeply treaded tyres
2. a short motor vehicle with a powerful engine and a driver's cab, used to pull a trailer, as in an articulated lorry
3. an aircraft with its propeller or propellers mounted in front of the engine
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