eChapters

11.1. Hydraulic system

It is a mechanism in a tractor to raise, hold or lower the mounted or semi-mounted equipment by hydraulic means. All the present day tractors are equipped with a hydraulic control system for operating three point hitches. Some of the tractors are fitted with a hydraulic brake system and a few others are fitted with a hydraulic steering system. Tipping trailers, front mounted loaders or dozers are examples of multi use of hydraulics.

11.2. Principle

The working principle of a hydraulic system is based on Pascal’s law (Fig 11.1.) This law states that the pressure applied to an enclosed fluid is transmitted equally in all directions. A small force acting on a small area can produce higher force on a surface of larger area.

10.8.1. Function

A power take-off (PTO) is a splined drive shaft on a tractor that can be used to provide power to an attachment or a stand alone machine. PTO shaft gets the drive from hydraulic pump shaft. This in turn gets drive from engine. PTO is designed to be easily connected and disconnected (Fig 10.8.1.). It is used to operate implements, viz., rotavator, fertilizer spreader, duster and many more machines which are stationary such as threshers, water pump and portable flour mill. These shafts either run at 540 rpm with 6 splines in the shaft or at 1000 rpm with 21 splines in the shaft. But many old tractors have 540 rpm PTO. The present day equipment is designed to operate with 1000 rpm PTO shaft. The manufacturers of old tractor models provide an auxiliary gear box which increases the speed of PTO to 1000 rpm. Selection of implements, equipment and machinery should be done on the basis of available tractor PTO speed.

9.7.1. Function

The front axle is the unit on which the front wheel is mounted. This wheel is an idler wheel by which the tractor is steered in various directions. The function is to assist the steering and support the tractor. It is a rigid tubular or I-section steel construction, pivoted at the centre. The pivot pin is housed on bushings on a support bolted to the front of the engine. As the track width of a tractor is adjustable, the front axles have provision to be extended on both sides. The tubular front axles have telescopic arrangement for extension, whereas the I-section type of front axles are three-piece axles.

8.6.1. Function

It steers the tractor in the required direction. The rotation of the steering wheel moves the steering column, which in turn rotates the sector shaft. This rotation pulls the pit man arms on one side and pushes at the other side. This motion is conveyed to the wheels by the drag link and makes the front wheel turn in the requisite direction.

7.5.1. Functions

The two primary functions of tyres, especially rear ones, are to provide maximum work output while ploughing the field and for haulage.

6.4.1. Function

The brake is used to stop or slow down the motion of a tractor. It is mounted on the driving axle and operated by two independent pedals. Each pedal can be operated independently to assist turning of the tractor during field work or both the pedals can be locked together.

6.4.2. Principle of operation

The brake works on the principle of friction. When a moving element is brought into contact with a stationary element, the motion of the moving element is affected. This is due to frictional force which acts in the opposite direction of the motion and converts the kinetic energy into heat energy.

5.3.1. Main components

Shear tube: It absorbs heavy shocks and protects the components of rear transmission and main transmission from breaking.

Crown wheel and pinion: It transmits the drive from the main shaft to the rear axle shaft and inverts the direction of rotation to 90° from the main shaft. It reduces the speed and multiplies the torque.

Rear axle: It is a semi floating type. It transfers the drive from the sun gear to the rear wheel.

5.3.2. How does it work?

The drive from the main shaft of the gear box is taken to the rear drive shaft through the coupler planetary. Depending on the position of the high–low gear and lever which moves the coupler, power is transmitted directly or through epicyclic gear train. The drive from the rear drive shaft is transferred to a safety coupling also known as shear tube. The coupling is a safety device. In case the rear wheels encounter any shock loading, the coupling breaks and disconnects the gear box from the rear transmission. This prevents damage to costly parts in the transmission, clutch and engine. Power is transmitted from safety coupling to a pinion which has a spiral bevel gear at one end. The bevel pinion is supported on two tapered roller bearings carried in a housing which is mounted in the centre housing.

4.2.1. Gear boxes

It is a unit fitted after the clutch. Forms a part of transmission and provides speed and torque conversions from a rotating power source to another device using gear ratios. Its primary function is to change the speed of the tractor to cater to varying field, road and load condition. It also helps in reversing the tractor.

Gears are usually made of alloy steel. Since the tractor has to transmit heavy torque all the time, the best quality lubricants, free from sediments, grit, alkali and moisture, are used for lubrication. SAE 90 oil is generally recommended for gear box.

The description about gear boxes stated below pertains to major tractors manufactured in India. The number of teeth in each of the gear and the engagement between gears of the main shaft and the corresponding gear on the counter/lay shaft also change in different tractors evolved over a period of time. Hence, information hosted in this section is only indicative.

3.1.1. Introduction

Clutch is a device which is used to connect and disconnect the tractor engine from the transmission gears and drive wheels. It is fitted in between the engine and gear box. The clutch transmits power by means of friction between driving members and the driven members.

2.1. Introduction

The engine is the source that generates power. It transforms chemical energy in fuel into mechanical energy for power. Diesel internal combustion engines are invariably used in all the tractors. In these engines, very high compression of the air ignites the fuel as compared to spark plug that ignites fuel in petrol engines. The air in the diesel engine is compressed with a compression ratio typically between 15 and 22 resulting in a 40 bar (about 600 psi) pressure as compared to 8 to 14 bar (about 200 psi) in the petrol engine. This high compression heats the air to 550°C (1022°F). At about this moment, small droplets of fuel are sprayed in an atomized form directly into the compressed air in the combustion chamber.

2.2. How does it work?

Any engine will work with four strokes, namely suction, compression, power and exhaust (Fig 2.1). It will also complete two revolutions of the crankshaft. In the suction stroke, pure air is drawn into the cylinder through the inlet valve. The exhaust valve gets closed and the crankshaft completes half a revolution. In the compression stroke, the piston moves up while the valves remain closed. The air which is drawn into the cylinder during the suction stroke is compressed. There is a rise in temperature. The hot air ignites the fuel injected into the cylinder. The crankshaft again completes half a revolution. In the power stroke, the hot products of combustion, viz., carbon dioxide and nitrogen expand. The piston is forced down. The valves remain closed and the crankshaft completes half a revolution. In the exhaust stroke, the exhaust valve opens while the piston reaches the lowest point of travel. Then the burnt gases are swept out.

1.1. Definition

A tractor is a specific vehicle designed to deliver a high tractive torque at slow speeds, for the purposes of haulage or for pulling an agricultural implement or equipment or machinery. This vehicle is also used as a source of power for stand-alone agricultural machineries. The word tractor is taken from Latin being the agent noun of trahere “to pull”.

1.2. History of evolution

The first engine-powered farm tractors used steam and were introduced in 1868. These engines resembled small road locomotives. They were used for general road haulage and in particular for timber trade. The most popular steam tractor was the Garrett 4CD.

In 1887, gasoline-fuelled engines were tried as an alternative to steam engines. Later, Henry Ford produced his first experimental gasoline powered tractor in 1907, under the direction of his chief engineer Joseph Galamb. It was referred to as an “automobile plow”. While unpopular at first, these gasoline powered machines began to catch on. In the 1910, the size of the tractor was smaller than the earlier introductions and also became more affordable. In 1917, Henry Ford introduced the first mass-produced tractor. These were built in the U.S.A., Ireland, England and Russia. The unique feature of these tractor consisted of the strength of the engine holding the entire machine together. By 1920, tractors with a gasoline powered internal combustions engine had become a norm.

1.3. History in India

The level of mechanization in Agriculture was very low at the time of independence in 1947. But, the initial few five-year plans implemented by the Government of India gave an impetus to the improvement in the production and productivity of major crops including mechanization via joint ventures and tie-ups between local industrialists and international tractor manufacturers. It was three decades after independence that the domestic production of 4-wheel tractors grew slowly. In 1990, the tractor production was nearly 2,70,000 units per year and by the 2013, the volume approached to about 6,96,000 units per year. India has presently overtaken the United States as the world’s largest producer of four-wheel tractors with over 16 national and 4 multinational corporations engaged in its production. But still the Food and Agriculture Organization estimates that from out of the total agricultural area, less than 50% is under-mechanized land preparation. This indicates that potential for large tractorization still exists. Credit facilities for farmers have been improving steadily over the years. This is one of the prime drivers of the sale of tractor in India.