Tractor Design & Testing

Lesson 1. Hierarchical Development in Tractor Design

I. Engines:

Experiments with engines were conducted date back many centuries, but James Watt is credited with patenting the first practical steam engine in 1769 (Gray 1954b). A steam engine for agricultural work was in use by 1849 (Norbeck, 1976). It was pulled from place to place using draft animals. Development of power trains and crude steering systems transformed the steam engines into tractors that could be used for heavy drawbar work.  By 1858, J. W. Fawkes had produced a steam tractor that can pull eight plows at 4.8 km/h in virgin sod (Gray, 1954b).

By 1907, tractors with internal combustion (IC) engines were beginning to appear. Competition between the two types of tractors was fierce and climaxed in the tractor trials that were held in Winnipeg, Manitoba, Canada in 1908-1911 (Goering et. al. 2003), where the limitations of the steam tractors became apparent. Steam tractors required an operating crew, including two men to run the tractor and two to haul coal and water. The firebox of steam tractors was usually fired with coal, and the open-cycle engine required periodic replenishment of the boiler water. A tractor with an IC engine required only one person to operate.

After the Winnipeg trials, steam tractors rapidly gave way to tractors with IC engines. The transition helped transform agriculture. Steam tractors were large and clumsy, lacking the versatility that tractors with IC engines provided. Steam tractors could not be used until the fire was started in the fire box and the water was brought to a boil. When the job was finished, the energy stored' in the heated water was wasted. Conversely, IC engines could be started and stopped quickly and, not having a firebox or boiler, could be made much -smaller.

II. Pneumatic Tires/Traction:

Rubber tires designed for agricultural use came along in 1933, making it much easier for tractors to function even on the roughest, muddiest ground. An Allis-Chalmers Model U tractor belonging to Albert Schroeder of Waukesha, Wisconsin, was outfitted with a pair of Firestone 48X12 airplane tires in place of lugged steel wheels. Tests by the University of Nebraska Tractor Test Laboratory found that rubber wheels resulted in a 25 percent improvement in fuel economy. Rubber wheels also mean smoother, faster driving with less wear and tear on tractor parts and the driver. Minneapolis Marine Power Implement Company even markets a "Comfort Tractor" with road speeds up to 40 mph, making it usable on public roads or hauling grain or transporting equipment and ever mindful of the power plant.

As manufacturers seek better productivity through improvements in transmissions, tires, and engine power, some tractors have become heavy again. To avoid soil compaction problems, modern tractors are sometimes '4 wheel driven' (4WD), with the weight evenly distributed over the four wheels. Dual or twin wheels are sometimes fitted to further reduce ground pressure. Four-wheel drive tractors began to appear in the 1960s. Some four-wheel drive tractors have the standard "two large, two small" configuration typical of smaller tractors, while some have four large, powered wheels. The larger tractors are typically an articulated, center-hinged design steered by hydraulic cylinders that move the forward power unit while the trailing unit is not steered separately.

III. Power Take Off (PTO):

Experimental power take-offs (PTOs) were tried as early as 1878, but in 1918, IRC was first to install a PTO on a production tractor (Goering, 2004). This option was on their Mode115-30 tractor in 1920, when it was the first tractor with a PTO to undergo a Nebraska tractor test. However, that PTO was not tested; rather, belt pulley power was measured in those days. Another early innovation, introduced by International Harvester in 1922, was the so-called power takeoff. This device consisted of a metal shaft that transmitted the engine power directly to a towed implement such as a reaper through a universal joint or similar mechanism; in other words, the implement "took off" power from the tractor engine.

 An early question was whether the PTO speed should be keyed to ground speed or engine speed. In 1925, experience in rice-producing states demonstrated the wisdom of linking PTO speed, to engine speed. The rice crop was heavy and traction was poor, but the grain binders could run at full speed while travel speed was reduced to accommodate the heavy crop. In 1926, ASAE adopted the first PTO standard that specified the direction, speed, size, shape and location of the PTO shaft. The first standard speed was 536 rpm, which got rounded-off to 540 rpm. Later, when power demands rose, the 1000 rpm PTO was developed. The John Deere Company followed in 1927 with a power lift that raised and lowered hitched implements at the end of each row—a time- and labor-saving breakthrough. Engineers in the 1930s came up with diesel engines, which provided more power at a lower cost.

IV. Hydraulic System:

Irish mechanic Harry Ferguson developed a tractor that incorporated an innovative hydraulic draft control system, which raised and lowered attached implements—such as tillers, mowers, post-hole diggers, and plows are automatically adjust their needed depth.In 1917, Henry Ford had formed a company (Ford and Son) separate from his automobile business to manufacture farm tractors (Wendell, 1979). Their Fordson tractor was manufactured and sold until 1928, at which point Ford & Son was merged back into the Ford Motor Company.  The David Brown Company in England was the first to build the tractor, but Ferguson also claim it to Henry Ford in the United States. With a handshake agreement, Ford manufactures Ferguson’s tractor and implements from 1939 to 1948. The three point hitch was developed by Harry Ferguson by 1935, after 17 years of experimentation. In 1936; he began selling a light tractor equipped with the hitch in the British Isles and Norway. The Ferguson system included an automatic draft control system that was very effective. In 1938, Ferguson demonstrated his tractor to Henry Ford in the U.S. (Gray 1954b). A few years later Ferguson’s company merged with Canadian company Massey-Harris to form Massey-Ferguson.

Tractor sales continued to climb, peaking in 1951, when about 0.8 million tractors were sold in the United States.Pulled and powered by tractors, an increasingly wide variety of farm implements were mechanizing just about every step in the crop-growing process, from seed preparation, planting of seed to the harvesting of the final seed/fruit.

Equally important developments were occurring on the other side of the hitch system. Until the three-point hitch standard was developed by ASAE in 1959, each tractor producer had a different method of attaching implements to their tractors. Typically, it was difficult or impossible to mount one manufacturer's implements on another manufacturer's tractor. A decade later, Henry Ford was ready to re-enter the farm tractor business. He liked Ferguson's three-point hitch and entered into a verbal agreement to manufacture tractors equipped with it. These Model 9N tractors entered the market in 1939 when a dispute ended the verbal agreement in 1946, Ford formed the Dearborn Motors Company to produce Ford tractors. Harry Ferguson began producing his Ferguson tractor at a new Ferguson Park plant in Detroit. In 1953, Ferguson’s company merged with Massey Harris and production of the Massey Ferguson tractor began (Wendell, 1979). In the early 1950, Oliver introduced a three point hitch without draft control.

After ASAE developed their three point hitch standard in 1959, most other tractor manufacturers adopted it, as a result, it became possible to use the implements produced by any manufacturers on nearly any tractor. Development of the three point hitch also spurred the trend to larger average farm sizes. Implements fully mounted over roadways at higher speeds on road than would have been possible with pull type implements.

Open-center hydraulic systems include a fixed-displacement pump supplying an actuator through a tandem-center directional-control valve. Such systems are inexpensive and function well with a single actuator. When more than one actuator is used, the system pressure rises to that of actuator with the lowest pressure demand, which could create problems. International Harvester Company (IHC) in 1960, developed a sickle bar mower powered by a hydraulic motor. In a field test, the mower worked well in cutting heavy grass until the operator raised the hitch, at which point the mower stalled. The hitch lift cylinder had a lower pressure demand than the hydraulic motor powering the mower. Thus, the hydraulic fluid took the path of least resistance and stopped flowing to the hydraulic motor. The phenomenon is called sequencing, i.e., when two actuators have different pressure demands, the one with the heavier demand has to wait until the one with the lighter demand stops receiving hydraulic fluid. Over the years, improved hydraulic systems were developed. When John Deere introduced their "New Generation of Power” in 1960, they introduced a constant-pressure system featuring a 'variable-displacement pump that adjusted its stroke to maintain constant system pressure at a level designed to handle the heaviest load. The constant pressure systems were later succeeded by load-sensing systems. These systems include a variable-displacement pump in which a stroke control valve senses the highest pressure demand in the system and increases the pump stroke enough to meet that demand. Constant pressure and load-sensing system work, if the total fluid demand can be supplied by the pump operating at part stroke. With the touch of a lever, the operator can now control the movement of very heavy loads. When coupled with electronic system, hydraulic systems also enable automatic control of steering and other functions, control that would have been very difficult to provide without hydraulics.

V. Fuels:

One advantage of using draft animals was that farmers could grow their own "fuel," i.e., feed for the animals. With tractors, it was necessary to buy the fuel. Between 1900 and 1960, gasoline was the predominant fuel, with kerosene (the Rumely Oil Pull was the most notable of this kind) and ethanol being common alternatives. Generally, one engine could burn any of those, although cold starting was easiest on gasoline. Often, a small auxiliary fuel tank was available to hold gasoline for cold starting and warm-up, while the main fuel tank held whatever fuel was most convenient or least expensive for the particular farmer. Dieselisation gained momentum starting in the 1960s, and modern farm tractors usually employ diesel engines, which range in power output from 18 to 575 horsepower (15 to 480 kW). Size and output are dependent on application, with smaller tractors used for lawn mowing, landscaping, orchard work, and truck farming, and larger tractors for vast fields of rice, wheat, maize, soybean and other crops.

Diesel engines made an early appearance about 1930, but the required precision-made fuel metering parts made them too expensive for most farm tractors and starting was also a problem. Diesel powered tractors usually included a small, gasoline-powered "pony" motor to crank the diesel engine.  Minneapolis-Moline introduced their Model U diesel tractor in 1952. The International Harvester Company (IHC) Model MD tractor used a unique starting system. A spark plug for each cylinder was in a small auxiliary chamber linked through a third valve to the main chamber.

Opening this cam-operated valve reduced the compression ratio and allowed the engine tn be started on gasoline supplied via a carburettor: the valve was closed and the carburettor bypassed to switch to diesel operation. Diesels steadily gained market share in the 1950s, and virtually all new tractors since 1976 have had diesel engines. Electrical systems had developed enough by that time to allow electric starters to replace pony motors. Diesel engines provide much higher fuel efficiency than gasoline engines. The switch to diesels had a letter effect on biofuels development, when farmers again began to grow fuel. Ethanol is not a suitable fuel for typical, unmodified compression-ignition (Cl) engines. The biofuel developed for Cl engines are calledbiodiesel, the subject of a recent ASABE lecture (Van Gerpcn et al., 2007).

VI. Safety Features:

Around 1985, it became mandatory to fit tractors with 'roll over protection structures' to lessen the high risk of fatal injury if the tractor rolled over.This basic design has remained unchanged for a number of years, but enclosed cabs are being fitted now on modern models, for reasons of operator safety and comfort.

VII. Recent Advances:

After 1994, space technology has been incorporated into agriculture in the form of GPS devices, and robust on-board computers installed as optional features on farm tractors. These technologies are used in modern, precision farming techniques. The spin-offs from the space race have actually facilitated automation in plowing and the use of light bar or autosteer systems on tractors,  the idea being to neither overlap and use more fuel nor any missing when performing jobs such as cultivating. Global Positioning System (GPS) receivers on tractors to record precise locations on their farms to determine which areas need particular quantities of water, fertilizer, and pesticides.