Tractor Design & Testing 3(2+1)

Lesson 3. Recent Trends in Tractor Design

I. Power Generation System i.e. Engines:

Average power of tractor used in India is 35 HP (26.10 kW), but trend in the tractor manufacturing is that average power of the tractor is increasing. Piston engine is not likely to be replaced immediately with other types of energy conversion systems. Most of the tractor engines in India are designed with 2 to 4 cylinders. New engines for current tractor models contain changes or modifications made by improvement in design, fuels or materials to make it more efficient, economical, more durable with low exhaust emissions. Some major principles set by the French engineer for an IC engine that combustion chamber should have smallest possible surface to volume ratio, expansion process should be rapid and compression at the start of expansion should be as high as possible must be strictly followed. Average tractor engine bore ratio in India is about 1.33. There is the tendency to further reduce this stroke bore ratio hence recent engines have a smaller stroke bore ratio which allows higher engine speeds, running in more compact size and reduced vibrations.

II. Transmission:

A modern tractor drive train is a complex arrangement of several basic components that are used to control and transmit the power delivered by the engine. Most older farm tractors use a manual transmission. They have several gear ratios, typically three to six, sometimes multiplied into two or three ranges. This arrangement provides a set of discrete ratios that, combined with the varying of the throttle, allow final-drive speeds from less than one to about 40 km/h, with the lower speeds used for working the land and the highest speed used on the road. Tractor manufacturers are giving more attention so that there should be more number of speeds available on their models. In most of the Indian tractors, now available speeds are 8 or 10 (4/5 low + 4/5 high) forward with 2 reverse speeds instead of 4 or 6 forward speeds used previously. When a limited number of speeds are available on the tractor, it will not be possible to use the maximum engine power available, especially when the load is highly variable. Hence, the tractor should be provided specific speeds and the ease with which these speeds can be selected are the most important features determining that how well the transmission meets the farmer's needs. Ideally any forward speed upto 40 kmph and 15 kmph reverse speed could be selected under any load with a simple adjustment of a control lever. Previously, Tractors were with only sliding or constant mesh type gear boxes with few speeds in their transmission systems. But, now some manufacturers have incorporated partially synchronized or fully synchronized type gear boxes with many speeds for their tractors. Consequently, the transmission is a compromise between what is designed and what can be provided with available technology at a reasonable cost. Unsynchronized transmission designs were replaced with synchronization or with continuously variable transmissions (CVTs). Either a synchronized manual transmission with enough available gear ratios (often achieved with dual ranges, high and low) or a CVT allow the engine speed to be matched to the desired final-drive speed, while keeping engine speed within the appropriate speed range for power generation.  Fully synchronized gear box technology was introduced by Bajaj Tempo in their OX-35 & 45 models. The advantage of a synchronized transmission is that gear changes can be made easily without damaging the transmission, even when the tractor is moving. In new Holland tractors there is the partially engaged synchronized gear box in which speed equalization occurs at higher speeds.  Optional four wheel drive (4 WD) technology is also provided by some local manufacturers. Net tractive coefficient and tractive efficiency are more for a 4 WD tractor than a 2 WD tractor. Hence, four wheel drive tractors are able to produce more drawbar power. Dual clutch and live PTO are now common for the Indian tractors.

III. Chassis Design

Present non-cab design of Indian tractor generally make use of the cast iron unit frame construction. Chassis of the tractor consists of the engine, transmission system, suspension system, road wheels, steering, brakes etc. suitably mounted. Application of power systems, to drastically reduce control efforts, for brakes and steering used in tractors. Hence, instead of disc brakes available in many tractors, now hydraulic brakes are introduced in HMT 7511 and oil immersed brakes are used in the OX model of Bajaj Tempo. Hydraulic braking system are simple in construction, self-lubricating type with equal braking effort at all the wheels. There are complete water sealed brakes in HMT range of tractors suitable for paddy fields. In most of the Indian tractor models, reciprocating ball and nut type steering system was provided. Optional power steering/hydrostatic steering is also now provided by many manufacturers of India.  Generally, the power steering system provides automatic hydraulic assistance to the turning effort applied to the manual steering system.

Eicher tractors introduced straight rear axle in one of their model. To increase the driver's comfort and convenience operator's seat was improved in many ways. The vehicle seat should be adjusted to provide the operator with an easy control of the steering mechanism. Deluxe and Rail Fender type seats with horizontal and vertical adjustments are now being provided by all of the tractor manufacturers. Some research studies had shown that the predominant vibrational motion of a wheel tractor is vertica1 and that the seated operator is most sensitive to vertica1 acceleration. Hence, M&M ltd. has introduced shock absorbing type suspension system to reduce the vertical vibration for tractor seat.

IV. Hydraulic Systems:

The agricultural tractor is only one half of a machine combination and cannot start to solve its purpose until the other half the implement added. Implement can be added with three point linkage systems operated hydraulically. The mounted implement, 3-point linkage and hydraulic draught controls pioneered by Ferguson are the fundamental to most of the world's tractors.The ford 9N tractor with the Ferguson three-point hitch, introduced in 1939, was equipped with a hydraulic system. Harold Brock, at the age of 93 reported on his experiences in working with Henry Ford, to develop the hydraulic system on the Ford 9N at the ASABE centennial celebration in Minneapolis.There is a continuous improvement in the hydraulic system of a tractor. Now a day, Automatic Depth and Draft Control (ADDC) system is available on all the recent models introduced by Indian manufacturers of a tractor. Bosch type control valve has been used in the hydraulic system of the OX-35 &45 models of the Bajaj Tempo.

V. Traction Improvements:

Because the drawbar is one of the most used but least efficient methods of using tractor power, traction has been the subject of much research, resulting in traction improvements. Pneumatic tires improved traction by allowing drawbar power to be developed with more speed and less pull. Added ballast reduces slip in drive wheels. When pneumatic tires were introduced in the 1930s, B.F. Goodrich discovered that use of water in the tires helped improve traction (Gray, 1954b) and calcium chloride was added to prevent freezing. Radial introduced on tractors in 1980s, tolerate lower inflation pressure and provide a bigger soil-tire footprint to improve traction. In addition, use of mounted implements rather than pull-type implements helps increase the loading on the drive wheels, thus improving traction.

Traction limits the amount of drawbar power that can be delivered by tractors driven by only rear wheels. In 1958, the Steiger tractor with four-wheel drive (4WD) and 175 kW of engine power was introduced (Larsen, 1981), and soon other companies also began producing 4WD tractors. These were unsuitable for row crop work, but in 1979, IH announced their 2+2 tractor that was a 4WD tractor capable of row crop work (Larsen, 1981). While production of 4WD tractors continues, a movement has begun toward rear wheel drive tractors supplemented with front wheel assist (FWA). The Europeans were offering FWA in the 1950s, with FWA in the American market coming later. Initially, the front wheels were driven hydrostatically, but this quickly gave way to mechanical FWA.

Crawler tractors, i.e. tractors with steel tracks, have excellent traction but have had limited success in agriculture because of their limited mobility and travel speed. They cannot be driven on highways. In 1987, Caterpillar introduced their Challenger tractor. Its rubber tracks allowed the high, ground contact area of a crawler tractor with the mobility of a wheeled tractor. In 1997, John Deere introduced a similar rubber-tracked tractor. Caterpillar sold their Challenger line of tractor to AGCO in 2002.

The steel wheels and lugs of early tractors greatly limited travel speed. Because drawbar power is the product of pull and speed, these tractors needed a lot of mass to support high pull and appreciable drawbar power. The first reported use of rubber tires on tractors was in Florida orange grooves; steel lugs were damaging tree roots, so tire casings were attached to the wheels to protect the roots. These tires had no tubes i.e., were not pneumatic (Gray, 1954b).

The higher speeds permitted by pneumatic tires helped to transform agriculture, in that higher speed allowed the average farm size to grow much larger. Thus, when farmers wanted to increase the size of their farming operations, they could include land parcels so widely separated that reaching them with steel lugged tractors would have been impractical with average farm size thus increasing, fewer people remained in farming operations, a trend that was accelerated by the use of pneumatic tires on farm tractors.

VI. Comfort and Safety:

Early tractors were not known for comfort and safety. For example, starting one of these tractors usually required turning a hand crank at the front of the tractor or spinning a flywheel at the side of the tractor. If the tractor was inadvertently left in gear and started easily, the-person starting the engine could suddenly find himself standing directly in front of a moving, tractor, with his life on the line. Tractor seats were typically of the steel pan type, with no cushioning. The logic of tractor controls varied from manufacturer to manufacturer. Some tractors had a. foot-operated clutch, while other clutches were hand-operated. Even hand-operated clutches differed; some were engaged by pushing them forward, while others were engaged by pulling them backward. An operator, moving from one tractor to another, could easily become confused and get into life-threatening situations.

The tractor PTO, if unshielded, was also dangerous. There have been numerous press reports of injury or death when people became entangled in unshielded PTO shafts. The danger was recognized early on when, in 1926, PTO Standard S203 was developed. It included a section on shielding the PTO shaft. Unfortunately, the metal cover shielding the PTO line between the tractor and implement was often discarded by farmers. It was not until decades later that the driveline safety shield was made an integral part of the driveline: and could not be discarded.

In US, tractor-related injuries account for approximately 32% of the fatalities and 6% of the nonfatal injuries in agriculture. Over 50% is attributed to tractor overturns. Early tractor owners were not enthusiastic about comfort and safety features. As Larsen (1981) pointed out, "If a farmer placed a pad on the seat or raised an umbrella to protect himself from the sun, he was generally viewed as a sissy." Consequently, tractor manufacturers gave little thought to operator comfort. The Minneapolis-Moline Comfort tractor was likely the first to cater to operator comfort. Its enclosed cab was designed to protect the operator from rain, snow, and cold weather. When that tractor was introduced in 1938, few farmers were willing to pay for the added cost of the enclosed cab, and not many Comfort tractors were sold.

Extension agricultural engineers developed scale-model tractors that could be used to show how tractors could upset when used improperly (Larsen, 1981). When John Deere introduced their New Generation of Power in 1960, the tractors included numerous comfort and safety features. Dr. Janet Travell, personal physician to President John Kennedy, helped Deere design the tractor seat. In addition, the location and operation of tractor controls were designed to reduce operator error. The control logic to achieve that result is stated in ASABE Standard S335, Operator Controls on Agricultural Equipment, i.e., "This standard is based on the principle that a given direction of movement of any control should provide a consistent and expected result." Thus, for example, moving a throttle lever forward should increase, not decrease, the engine speed. ASABE has two other related documents that help- tractor safety. These are S304, Graphical Symbols for Operator Controls and Displays on Agricultural Equipment (first adopted in 1967), and EP443, Color Coding Hand Controls (first adopted in 1984). The symbols are designed to be widely understood. For example, the turtle symbol is used to indicate SLOW, while the rabbit symbol indicates FAST. In color coding, for example, red is used only for single function engine stop controls. While John Deere was cited as an introducer of numerous comfort and safety features in 1960, other tractor manufacturers were also adding such features about the same time.

Row crop tractors necessarily have a high center of gravity, so overturns can easily occur. In the late 1960s, work was underway in Sweden on what became Roll Over Protective Structures (ROPS), (Larsen, 1981). ROPS was highly effective in preventing fatal overturn accidents. Initially, ROPs was offered as an option, but when John Deere introduced ROPS as standard equipment 'on their tractors, farmers went looking elsewhere for new tractors. The situation changed when John Deere offered their technology to other companies in exchange for an agreement to install ROPS on their tractors, and farmers began to accept tractors with ROPS (LeffingweIl, 1994): While ROPS began as a roll bar, ROPS strength was later incorporated into tractor cabs. The independent left and right wheel-braking augments the steering of the tractor when only the two rear wheels are driven. This is usually done when it is necessary to make a sharp turn. The split brake pedal is also used in mud or soft soil to control a tire spinning due to loss of traction. The operator presses both pedals together to stop the tractor. For tractors with additional front-wheel drive, this operation often engages the 4-wheel locking differential (diff-lock) to help stop the tractor when traveling at road speeds.

Modern tractors have a ROPS to prevent an operator from being crushed if the tractor turns over. The ROPS does not prevent tractor overturns; rather, it prevents the operator from being crushed during an overturn. This is especially important in open-air tractors, where the ROPS is a steel beam that extends above the operator's seat. For tractors with operator cabs, the ROPS is part of the frame of the cab. A ROPS with enclosed cab further reduces the likelihood of serious injury because the operator is protected by the sides and windows of the cab.

For the ROPS to work as designed, the operator must stay within its protective frame. This means the operator must wear the seat belt; not wearing it may defeat the primary purpose of the ROPS.

A fifth pedal is traditionally included just in front of the driver's seat to operate the rear differential lock (diff-lock), which prevents wheel slip. The differential normally allows the outside wheel to travel faster than the inside wheel during a turn. However, in low-traction conditions on a soft surface, the same mechanism could allow one wheel to slip, further reducing traction. The diff-lock overrides this, forcing both wheels to turn at the same speed, reducing wheel slip and improving traction. Care must be taken to unlock the differential before turning, usually by hitting the pedal a second time, since the tractor with good traction cannot perform a turn with the diff-lock engaged. In modern tractors, this pedal is replaced with an electrical switch.

VII. Levers and switches:

Many functions once controlled with levers have been replaced with some model of electrical switch with the rise of indirect computer controlling of functions in modern tractors.

The three-point hitch was controlled with a lever for adjusting the position, or as with the earliest ones, just the function for raising or lowering the hitch. With modern electrical systems, it is often replaced with a potentiometer for the lower bound position and another one for the upper bound, and a switch allowing automatic adjustment of the hitch between these settings.

VIII. GPS/Navigation:

Space does not permit inclusion of many other parts of tractor history. Among the important omissions are lubrication systems, air cleaner, and numerous drive train variation, Other tractor improvements loom on the horizon, and only two will be mentioned. Automatic tractor guidance based on the Global Positioning System (GPS) and/or machine vision, is nearing commercial reality. In Europe, work is underway on tractors with spring-mounted cabs and/or axles to protect the operator when the tractor is used at high speeds, and such tractors are also entering the U. S. market.

The modernfarm tractor (Fig. 3.1) has become a marvel of engineering with features that would be beyond the imaginations of early tractor designers.

(Source:http://images4.wikia.nocookie.net/__cb20120621203660/tractors/images/2/2e/VTZ_504_MFWD_ (Foton) _-_2012.jpg)
Fig. 3.1 Modern farm tractor

(Source: http://daddystractor.com/category/social-studies/page/2/)

How could they have imagined a future operator sitting in an air conditioned cab, using an internet-connected computer to check crop prices while a GPS signal guided the tractor across the field? Further, the tractor has transformed agriculture. It has so improved worker productivity that now less than 2% of the U. S. population are needed to be farmers.