Tractor Design & Testing 3(2+1)

Lesson 2. Different Type of Tractors Available in India/abroad & it’sImportance in Agriculture

2. Development

The tractor evolved in the second half of the 19th century and first half of the 20th into its present, conventional, two wheel drive form and four wheel drive variation. This form owes much to history but also the fact that it is an inherently logical arrangement. Designers followed early tractor designs that were simply replacements for horses or other draught animals.  The layout takes advantage of the transfer of weight to the main driving wheels at the rear, as the drawbar pull on the tractor increases. The layout is inherently stable in the horizontal plane because the implement commonly being pulled behind the tractor tends to follow the latter and to pull it into straight line operation. Rear mounted implements offer a minimum offset loading and moment in the horizontal plane; this contrasts with side mounted implements.

I. Type of Tractors Available in India/Abroad

Farm Tractors are classified as Follows;

(A). According to use, modern farm tractors are classified in three groups:

       1. General purpose (land utility) tractors

General purpose or land utility tractors are used for major farm operations, which are common to the cultivation of most crops, such as tillage, harrowing, sowing and harvesting. These tractors are characterized by a low ground clearance, increased engine power and good traction due to their wide tyres or tracks enabling them to develop a high pull.

2. Universal row crop (row crop utility) tractors

Row-crop tractors are for the inter-cultivation operations of crops sown in rows. Inter-cultivation can take place anytime from crop germination to harvest. Several rounds of cultivation may be done over the season. A row-crop tractor essentially brings together a farm tractor and its cultivator into one machine, The earliest tractors were to mechanize agriculture to reduce the heavy efforts of plowing and harrowing before planting, which was done by humans and draft animals. Early tractors were used mainly to alleviate this drudgery, but they tended to be very big and heavy, so were not well-suited in getting into a field of already-planted row crops for weeding, spraying etc. Row-crop tractors are generally light, affordable and reliable with more ground clearance and wide wheel track that can be adjusted to suit the particular inter row distance. Modern row crop tractors have rollover protection systems in the form of a reinforced cab or a roll bar for better safety.

3. Special purpose tractors

Special purpose tractors are modification of standard land or row crop utility tractor models and are used for definite or special crops/jobs (e.g. in cotton, garden, orchard, vineyard fields) or for various jobs under certain conditions (e.g. on marshy soils, hillsides). The agricultural tractor is sometimes modified for use under special conditions where the standard type is not suitable. The narrower tractor is a basic small tractor which has been modified to pass through the rows of orchard trees and vineyards. The high clearance tractor is usually a standard tractor which has high ground clearance for spraying, fertilizer broadcasting and other operations at later stage of crop growth. High clearance tractor with air conditioner cab was developed by the PAU, Ludhiana for collection of spectral images of the crop and for intercultural operation in wide row crops (Fig. 2.1). Light, low HP, 4WD tractors work in paddy field, where the combination of light weight and 4WD is desirable. Other special purpose tractors provide engine and transmission systems for special purpose-built machines, such as for pipe laying, drainage and mechanical harvester.

Fig. 2:1 High clearance tractor in field

Garden tractors (mini tractors) are small, light weight tractors designed for use in gardens, orchards and small estates (Fig. 2.2).The engines are generally a one- or two-cylinder petrol (gasoline) or diesel engines. Typically, diesel-powered garden tractors are larger and heavier-duty than gasoline-powered unit. Tractors designed for orchards, have features suited to passing under tree branches easily. These include a lower overall profile; reduced tree-branch-snagging risk (via under slung exhaust pipes rather than smoke-stack-style exhaust, and large sheet metal cowlings and fairings that allow branches to deflect and slide off rather than catch); spark arrestors on the exhaust pipes; and often wire cages to protect the operator from snags. Recently HMT has also developed a Tranter (Transport + Tractor), which can be used for farm work as well as for the transport purposes (Fig. 2.3). It’s having seating capacity for 5 peoples to sit comfortably during transportation. 

Fig. 2.2: Mini tractor for garden/orchard               Fig. 2.3: Tranter type of tractor

(B). As per traction unit the tractors are divided into

1) Crawler (track laying) type
2) Semi-tracked type
3) Wheeled type

(a) 2 wheeled tractors (Power tillers)
(b) 4 wheeled, 2 wheel driven (2WD) tractors
(c) 4 wheeled, 4 wheel driven (4WD) tractors
(d) 8 wheeled, 8 wheel driven (8WD) tractors

Tracked tractors are equipped with tracks instead of wheels. They are generally used on farms where soils are difficult to cultivate, such as heavy clays, or where seasons are shorter and wet conditions predominate. More power can be transmitted to the drawbar than wheeled tractors. Maintenance costs are however higher than for wheeled tractors. Also, unlike wheeled tractors, they must be transported from field to field by a 'low loader'. Recently, a tractor manufacturer designed a high powered, high speed tracked tractor that runs on rubber tracks and can be driven on the road. This model may make the tracked tractor more popular in future. Tracked tractors have power ratings of 65-700 HP (48.47-522 kW).
Two wheeled tractors are often called 'walking tractors or power tillers. The small hand held units with rotary cultivators are usually driven by petrol engines. Larger units are often coupled to 2 wheeled trailers and driven by diesel engines. Engines of 5-15 horsepower are common for power tillers. Four wheeled, 2WD tractors are the most common type of tractor. The two rear wheels are given engine power. The front wheels are much smaller and are used for steering. Engine power ranges from 25 to 120 HP (18.64 to 89.48 kW). The 4WD tractor is similar to the 2WD tractors, but all 4 wheels of 4WD tractor are powered for better traction. The front wheels are half the size of the rear wheels. The 4WD tractors usually have power ratings of 70 HP (52.2 kW) and above, although some lower horsepower units are made for special purposes especially in Japan for wet fields. The 8WD tractors are the top of the range in terms of weight and power. There is lesser soil compaction and wheel slip by distributing weight and power over 8 wheels. They are articulated in the middle, and an axle is mounted to each of the jointed halves, with both axles driven. Each axle has 2 dual wheels at each end, or 4 wheels per axle. 8WD tractors usually have engines of 200 HP (149.14 kW) or more.  
(C). As to the type of undercarriage, tractors are divided into
1) Framed
2) Semi-framed
3) Frame-less
Tractors may be framed, semi-framed or frameless. The body of framed tractors is essentially a riveted or welded frame, that of semi-framed tractors is made up of two short longitudinal beams (side members) bolted or welded to the rear axle housing, while in frame less tractors, the body is formed by bolting together the casings of individual tractor mechanisms.
(D). As indicated by 3-point linkage categories
There are five different hitch sizes, called categories (Table 2.1). The higher category hitches have sturdier lift arms and larger connector pins. There is some flexibility in the tractor hp at which one category hitch ends and the next begins.
Table 2.1: Specifications of different categories of three point linkage system

Category	Tractor HP (kW)	Top Link Pin Diameter			Lift Arm Pin Diameter
		In		mm	Inch	mm
0	Up to 20 ( 10.44)	5/8	15.88		5/8	15.88
I	20 to 45 (10.44 to 33.56)	3/4	19.05		7/8	22.23
II	40 to 100 (29.82 to 74.57)	1	25.40		1⅛	28.58
III	80 to 225 (59.65 to 167.78)	1¼	31.75		1 7/16	26.99
IV	180 to 400 (134.23 to 298.28) and above	13/4	82.55		2	50.80

Category II is most widely used in india, but category I is used for many implements designed for use with small to medium tractors. Fortunately many mounted implements and machines are now designed for use with either category I or II tractors and this is point to note in selecting equipment.

3. Global Variations in Tractors:

• Rice-land tires are used in Japan.

• Power/weight ratios are greater for tractors in Japan.

• Radial-ply tires are common in Europe for better traction.

• Tractors outside of North America have up to four PTO speeds.

• In North America 4WD is used mainly tractors of over 100 kW are mainly used but in Japan tractors of 10 KW may have front wheel assisted 4WD

• In Europe and Japan, 4WD with front wheels smaller than the rear is common but in North America all 4WD tractors have equal size wheels

• Crawler tractors are more popular in Europe and UK.

• Power tillers are common in Asia.

4. Future Engine Designs

• CI engine is well established and not likely to be replaced soon.

• Ceramics materials which have high heat resistance as well as anticorrosion and anti-water properties are likely to be incorporated into designs of the future for increased operating temperature and reduced wear.

• Look for electric drives for cooling (air and water movement) and lubrication.

5. Trends in Tractor Design:

The future tractor design greatly depends on technological and social advancement in agricultural and economic sector including energy and environmental systems. Tractors continue to have a greater power/mass ratio, which results in the tractors travelling at a faster speed, since improvements in traction have not kept pace with the increase in power. It is also clear that the power output of tractors has continued to increase. The growth however in power and weight of tractors will probably not continue at the same rate as in the past. The major reason for limiting the increase in size of future tractors is increase in soil compaction affecting soil conservation as well as plant growth. However, Gohlich (1984) presents an excellent discussion on the trends of the tractor design as:

• Total weight reduction of tractor and implements: More emphasis will be concentrated on reducing the total weight of the tractor implement system in order to minimize the ground pressure and soil compaction, particularly during seed bed preparation

• Front and rear mounted implements: It will contribute increasingly to better, more convenient and more economical field operations.

• Lighter tractors will transfer power through PTO: Instead of the standard heavy tractors of today, for most of the required field operations a lighter, second tractor power unit with low pressure tires, which would transfer the majority of its power through the power take off (PTO) rather than through the ground drive may be a desirable approach on the future farm

• Power transfer will be controlled automatically: The optimization of power transfer will be facilitated by driver information displays as well as by control circuits that may eventually over take some of the present manual functions.

• Driver comfort and safety will be improved: The roll-over protection structure (ROPS), seat belt, AC Cabin etc.  will be provided in all future tractors for safety and comfort  of the driver.

• Introduction of information and space technologies systems like GPS devices, and robust on-board computers installed as optional features on farm tractors. These technologies are used in modern, precision farming techniques. Use of light bar or auto-steer systems on tractors, the idea being to neither overlap and use more fuel nor leave streaks when performing jobs such as cultivating.

6. Importance of Tractors in Agriculture:

Power availability coupled withother advanced technologies has linear relationship with the food grain productivity (Fig. 2.4). Power availability varying from 0.60 kW/ha in Orissa state having food grain productivity of about 1000 kg/ha as compared to the 4000 kg/ha in Punjab state having a power availability of 3.5 kW/ha. Tractor is the main power source for almost all agricultural operations. The farm tractor is used for pulling or pushing agricultural machinery for plowing, tilling, disking, harrowing, planting and similar tasks, can be used for providing rotary power to implements like rotavator, strip till drill etc. and for transportation.  A farm tractor can also be used for stationary purpose like to power a pump for irrigation, to thresher etc.

Fig. 2.4: Power availability relationship with food grain productivity in India

Although it is able to undertake a multitude of specific tasks at farms, the functions of the tractor can be reduced to the following (Reece 1971):
(i) The provision of up to full power in the form of a large drawbar pull (compared to the weight of the tractor) at low speeds. The highly variable loading that occurs in agricultural operations requires consideration of tractor performance at part load, particularly with respect to fuel consumption.
(ii) The provision of power for driving and control of a range of implements and machines performing various tasks and attached in a variety of ways.
(iii) The provision of power as the basis for a transport system in both on- and off-road conditions.