Tractors are the machines which change energy of the engine into tractive energy. The tractors are primarily used to pull or push loads, but they are also used for mounting many types of accessories like, bulldozer blades, rippers, front end shovels, hoes, draglines, clamshells, trenchers, winches, side booms, frontend bucket loaders, pipe layers etc.

As mentioned above, the tractor is a basic equipment and available in two types; namely (i) crawler or track type, and (ii) Rubber-tired or wheel-type. The tractor is a multi-purpose machine, light models of which are used for agricultural or short haulage works, whereas heavy models are employed for earth moving works, cranes, shovels or special rigs. Wheeled units are employed for light but speedy jobs, whereas crawler units are rugged machines and are used for heavy duty works, where more tractive power is required.

Tractor Construction

Main constituents of a tractor are: Engine, clutch, transmission system, ground drive and controls.

A diesel engine is generally used as prime mover and mounted on the tractor frame and delivers the power through a clutch to the main power shaft. The power is transmitted from the engine to the final drive through engine clutch, torque convertor, change speed gear box transmission to rear differential and then finally to the rear drive axle. The drive is also transmitted through front differential to the front axle.

Engine clutch is provided to disconnect the engine from the rest of the transmission system so that gears can be changed to change the speed of the tractor. Care should be taken while changing the gears that the engagement of the clutch should be gradual to avoid undue strain on the surfaces of the clutch plates. The clutch control in small and medium sized tractors is through pedal like that of a truck, but in large tractors it is through lever operated by hand.

Transmission system provides speed reduction and multiplication of torque of the engine shaft, since final drive needs a high torque at low speed. The reduction in speed is done by means of a change speed gear box. Now-a-days torque converters are also used for the purpose. The power is then given to the drive axle through the differential, which changes the direction of the drive to 90° by using bevel gears and also reduces the speed up to some extent. 

Differential also facilitate in steering. As tractor changes its direction, one of the rear wheels must travel faster than the other, and this is achieved by the differential. In the wheeled tractor the steering is done through the front wheel. The outer ends of the two half shafts carry pinions which mesh with larger gears of the final drive, resulting variation in speed when the tractor is taking a turn. The change in the speeds of the track is made directly by stopping power supply to one, while the other continues to rotate.

The power from drive axle is transmitted through wheels or crawler chains in wheeled tractors and crawler tractors respectively. Wheeled tractors generally have two large wheels in the rear and two smaller wheels in the front. For better traction under adverse load and ground conditions two wheels are fitted on each of the rear hubs. To make rear heavy for better stability of the tractor rear tyres are filled with water. The tread of the tyre should be selected carefully as it greatly affects its tractive efforts on a particular type of soil where it operates.

In Crawler tractors, live axles rotate large toothed wheels, known as drive sprockets, located at the rear of the track frame. In the front of each track frame, an idler is provided. An endless track chain is mounted on the sprockets and idlers. The chain is made of flat shoes pinned together end-to-end.

Crawler v/s Tyred Tractors

Each type of tractor has certain advantages in certain conditions. A comparative study of both the types of tractors is given hereunder to help in deciding that which of the two will suit more in a particular set of conditions.

(a) Advantages by using Crawler mounted tractors: 

(i) More tractive efforts, hence can also operate on loose or muddy soil.

(ii) In absence of tyres, can easily operate in rocky conditions, as there is no danger for the damage of the tyres.

(iii) Where maintenance of haul roads is difficult, it can easily travel, specially in rough terrains.

(iv) Crawler tractors are more compact and powerful and hence can handle difficult jobs as well. 

(v) Greater floatation because of the lower pressure under the tracks.

(b) Advantages by using wheel-mounted tractors: 

(i) Can travel at higher speeds during the operation and also from one job to another.

(ii) Ease in operation. Operator feels less fatigue.

(iii) Can travel on paved roads without damaging them.

(iv) Can travel long distances at its own power, whereas Crawler mounted needs trailers.

(v) When work is spread over long area, these are found to be producing more output.

(vi) Operation, maintenance and repair costs less in wheeled tractors as compared to Crawler tractors. As large number of track parts are subjected to more wear.

(vii) Initial cost of the Crawler tractor is higher than the wheeled tractors, due to expensive track system.

(viii) Easy to manoeuvre.

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