Farm Machinery - Tractors - A Collection of Articles on the Operation, Mechanics and Maintenance of Tractors

Various Authors

Description

This book contains a collection of classic articles on the subject of tractors. They primarily deal with the operation, mechanics, repair, and maintenance of tractors, offering simple instructions and invaluable tips for those with a practical interest in the subject. “Farm Machinery – Tractors” will appeal fans of vintage tractors and those looking to find out more about the history of agricultural machinery. Contents include: “Agricultural Tools And Machinery”, “The Tractor As A Source Of Power”, “Overturning Of Tractors”, “Hints When Buying A Tractor”, “Tractor Engines (1): Spark-Ignition”, “Tractor Engines (2): Compression-Ignition (Diesel)”, “Tractor Overhaul And Maintenance”, “Tractors: Development And Principles Of Operation”, “Tractor Types: Constructional”, “Features”, “Tractors”, “Working Principles Of Tractor Engines”, etc. Many vintage books such as this are increasingly scarce and expensive. It is with this in mind that we are republishing this volume now in an affordable, modern, high-quality edition complete with a specially-commissioned new introduction on agricultural tools and machinery.

Sujets

• Machinery
• Sciences et techniques
• Technology
• Engineering

Informations

• Publié par: Read Books Ltd.
• Date de parution: 29 janvier 2013
• Nombre de lectures: 1
• EAN13: 9781447483755
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Farm Machinery - Tractors
A Collection of Articles on the Operation, Mechanics and Maintenance of Tractors
By
Various Authors
Contents
Farm Machinery and Tractors . R. H. Cochrane
Farm Machinery . Claude Culpin
The Operation, Care, and Repair of Farm Machinery . Anon
Tractors on the Farm - Their Choice, Use and Maintenance . H. J. Hine
The Tractor as a source of power
T HE APPLICATION of power to the tasks involved in farming has been a main factor influencing the history of farm machinery. So long as the only sources of power were human labour or that of the ox, the mule and the horse, development of farm machinery was necessarily restricted. Even in favourable conditions, draught animals could not produce sufficient power unless used in unwieldy groups. The steam engine, which 100 years ago seemed to hold out great promise as a source of power for most purposes, proved to have limitations when applied to farming; steam tackle in its day was too ponderous and the provision of fuel for steam engines was always difficult. The perfecting of the internal combustion engine, however, made possible an efficient and economical farm tractor, and this in turn has led to such developments as the power take-off and the hydraulic lift, with consequent changes in the design of many farm implements.
Excluding the early experiments in the application of steam power to the land, tractor history began about 1890, when one of the first tractors powered with an internal combustion engine was made and used in the United States. A British oil-engined tractor followed in 1897. Built by Ruston-Hornsby, this was awarded the Silver Medal at the Agricultural Show of that year. Since then there has been a continuous development.
There were, of course, many problems to be solved in develpping a tractor that would work efficiently and economically under farm conditions. As a self-propelling power and traction unit it needed an engine sufficiently robust to stand up to the most difficult conditions and its general construction had to be suited to travel over very uneven ground. This last factor was one which gave considerable trouble when tractors first began to be used in the field, for it was found that the rigid construction originally used gave rise to what is known as frame whip . The remedy adopted was to do away with the frame and to bolt the engine to the transmission, making a single unit. This was known as unit construction and together with the practice of suspending the unit at three points, overcame the difficulties of travelling over uneven ground. This design is now almost universal.
To-day the range of tractors is very wide. Petrol, kerosene and diesel oil engines are available, and a wide choice exists between high-powered tracklayers at one end of the scale and four-wheeled tractors, small tracklayers and two-wheeled walking tractors at the other end of the scale. The most significant development during the last few years has been the steady advance in the use of implements mounted on the tractor as opposed to implements merely trailed behind it.
The modern tractor is a versatile power unit. In addition to direct pulling from the drawbar, it is capable of delivering power at the belt pulley, at the power take-off and through the hydraulic mechanism used to lift tractor-mounted implements. In addition, it may be fitted with a winch. If it is mounted on wheels, its capacity for heavy work can be increased by exchanging the wheels for half-track or rotaped equipment. Starter motors and lighting equipment are becoming standard, and a wide range of gears is regarded as essential. Gear ratios range from very low, for row-crop tractors, to high ratios for transport work. Special types have been developed for horticultural work, with the engine placed behind to gain improved visibility for the operator.
The Engine
A tractor engine derives its power from an internal combustion engine. This engine may be:-
(1) A spark-ignition engine, using as fuel:
(a) petrol;
(b) kerosene.
(2) A compression-ignition (diesel) engine, using heavier fuels.
The operation and maintenance of these various types of engine are discussed in detail in chapters 18, 19 and 20.
The power produced by the engine is used in various ways-either to move the tractor, to drive the belt pulley, the power take-off shaft or the hydraulic mechanism.
The Meaning of Power
Before discussing the problems involved in the transmission of power, it is well to define what power means, and how it is measured. Work is expressed as the product of a force times the distance through which it moves. Power is the rate of doing work. It is the amount of work done in a given time. The unit measure is so many foot-pounds of work done per minute.. This unit is too small when large measurements are to be made and so the unit of horsepower is used. The horsepower is a unit devised by James Watt and is equivalent to 33,000 foot-pounds of work per minute.
Power from the Belt Pulley
The first point at which power from the engine of a tractor becomes available is at the belt pulley. The output can be estimated by means of an electrical absorption dynamometer, a prony brake, a transmission dynamometer, or an equivalent power-measuring device. When planning work for the belt pulley consult the manufacturer s specification to make sure the tractor has the capacity for the job. Also make sure that the proper operating speed for the driven machine will be obtained. *

BELT PULLEY
Fig. 15.
Excessive slippage of the belt may be due to too narrow, or too loose, a belt. The belt should ride on the crown of the pulleys and be no wider than either pulley. Operate in a direction that will keep the top side of the belt slack. With a rubber-tyred machine static electricity generated by the belt should be grounded by means of a chain or wire from the frame of the tractor to the ground. Belt drives are discussed in detail in chapter 14.
Power from the Take-off
The power take-off shaft is not assessed in terms of horsepower, but in terms of the number of revolutions per minute. * Some tractors are equipped with a separate power take-off clutch. This is a definite advantage. If the machine or implement driven from the power take-off becomes overloaded, the forward travel of the tractor and the coupled machine can be stopped by disengaging the transmission clutch. The power take-off goes on working and clears the machine. Also, of course, the forward speed of the tractor can be varied by means of the normal gears without stopping the operation of the power take-off shaft. If a separate clutch is not fitted the power take-off shaft ceases to turn when the transmission clutch is disengaged.

POWER TAKE OFF
Fig. 16.
Machine-cut gears and anti-friction bearings provide an efficient means of transmitting power. There is very little loss therefore between the maximum engine power as developed at the flywheel and what is available at the belt pulley and the power take-off. Any loss is due to friction and the churning of oil in the transmission.
Power at the Drawbar
Most of the work of a tractor, however, is done at the drawbar, and it is here that tractors are least efficient.

DRAWBAR
Fig. 17.
Tractor performance at the drawbar begins with the contact the wheels or final drive assembly make with the ground. The cable tackle of last century, with a stationary engine and wire ropes to draw a plough or other implement, was the most efficient means of changing engine power into a pull. The problem of getting a grip on the soil has still not been solved satisfactorily. Suppose a tractor to be equipped with rubber-tyred wheels. The tread of the tyre imprints its pattern on the surface of the soil so as to form a rack. As the wheels turn, it is the resistance to shear (or breaking) of the soil imprisoned between the tread bars, plus friction between the tyre and the soil, that prevents the wheel from slipping. To every force there is an equal and opposite force. As the wheels turn, the force exerted by the tread bars against the sides of the rack, plus friction between the tyre and the soil, causes an equal force to act in the opposite direction at the axle. This force is the drawbar pull. A better grip is obtained on the soil if more weight is added to the rear wheels, and in tractor design allowance is made for weight to be transferred to the rear wheels from the front wheels when the tractor exerts a drawbar pull. This weight is transferred from the front wheels, which have a tendency to be lifted owing to the force representing the drawbar pull having a turning effect (a moment ) about the point of contact of the rear wheels and the ground.
When a hitch is inclined upwards from an implement to the tractor the vertical component of the resistance to drawbar pull offered by the implement acts downwards, thus adding to the weight already on the rear wheels. Mounted implements also provide some downward force on the rear wheels, plus of course their own weight. Provision is sometimes made for wheel weights to be carried on the rear wheels and it is common practice for water ballast to be used with rubber tyres.
To gain more efficient use of engine power various final drive assemblies have been developed. There are all manner of strakes (wheel grips) and chains to be fitted over rubber tyres to increase their grip on the soil. There are steel wheels with lugs, rotaped equipment, half-tracks and full-tracks. There are advantages and disadvantages with each type.
Wheels
The grip of wheels and the rolling resistance they offer in their passage over the soil both vary with the size, the shape, and number of lugs or bars on the tread, with the overall diameter of the wheels and with the weight upon them. Still more does the grip vary with the structure, moisture content and type of soil. Pneumatic tyres are useful for light work. Steel wheels with lugs have a high rolling resis