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The use of 4-wheel tractors

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4-wheel tractors can be divided into 3 categories: 2-wheel drive, front-wheel assist or unequal 4-wheel drive, and equal 4-wheel drive tractors. Each one of these tractors has different tire configurations and different ballast requirements.

2-wheel drive tractors (2WD) are the most commonly used tractors in dry or upland farming situations and for transportation. They range in size from 4-150 kW and need 80% of the weight distributed over the rear axle to maximize traction. These tractors are capable of delivering 45-50% of their rated engine power at the drawbar.

The biggest advantages of this type of tractor over other 4-wheel tractors are smaller turning circle, simplicity of design, fewer mechanical parts and lower purchase price.

The biggest disadvantage is the inability to work in very wet and muddy conditions. When these tractors are used in a wet farming situation they will require cage wheels on the rear drive.

Depending on the make and model, the new purchase price of a 4-wheel tractor ranges from $US150-450 per kW or $110-330 per hp. One 60kW tractor is capable of plowing seven to eight hectares per day or one hectare per hour. Fuel consumption is dependent on engine size, make of machine, working conditions and operator skill. A general rule of thumb is that one kW of engine power will need 0.25litres of fuel/hour.

Front Wheel Assist (FWA) or Unequal 4-wheel drive are the most popular 4-wheel tractor used in rice production. These tractors range in size from 4-180 kW and are capable of delivering between 50-55% of the rated power at the drawbar. As the name suggests the front drive tires, whilst larger than 2-wheel drive tractors, are smaller than the rear tires. These tractors require 40% of the weight distributed over the front axle and 60% over the rear axle. As all four wheels turn simultaneously, the front and rear tires must travel at the same speed. Because the front tire is generally 30% smaller than the rear tire it must turn at least 30% faster. Tire pressures must also be set and maintained correctly. Front wheel drive should be used at all times when the tractor is working in the field. If not at least 10% efficiency will be lost.

The major advantage in using this type of tractor is that it can deliver 10% more power to the ground for the same fuel consumption, and has much better traction and flotation capabilities than 2-wheel tractors of the same size. These tractors normally cost about 10-15% more than the same size 2-wheel drive tractor.

A major disadvantage is that this tractor type has more mechanical failures, displays unequal tire wear front to rear and is less maneuverable than a 2-wheel drive tractor.

Equal 4-Wheel Drive Tractors (4WD) have all four tires of equal size and range in size from 25- 450kW. Irrespective of size, these tractors should be ballasted with 60% of the weight distributed over the front axle and 40% over the rear axle. This tractor type has the greatest power to weight ratio and can deliver between 55-60% of power at the drawbar. Very few of these tractors are used in rice production in Asia because of their large size, greater turning radius and purchase price.

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