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Originally appearing in Volume V18, Page 930 of the 1911 Encyclopedia Britannica.
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REVERSING UNK! Some of the early tractors were fitted with single-cylinder - engines,. but, although this type is still supplied by several makers, the more general practice is to provide a compound engine, with a multiplying valve which admits high-pressure steam into the low-pressure cylinder, thus enabling the engine to develop considerably more than its normal power for short periods. The engine is mounted over the boiler, with its crank-shaft at right angles to the axis of the barrel. Two changes of gear ratio are usually provided; one for normal running, and a lower one for very soft ground or steep hills. The driving axle is of the differential or live type, and provided with means for locking the compensating gear and rendering it inoperative when necessary, as would be the case if one driving-wheel were on hard ground and the other one on soft or greasy ground. A winding-drum is fitted, and this may be driven by the engine without, at the same time, driving the tractor: this result is attained by making the drum free on the axle but providing means of securely locking it thereto when desired. A flywheel generally fitted to one end of the crank-shaft, and this may be used for driving external machinery. Many makers have recently given much attention to the improvement of the spring-suspension systems of their respective machines, and chief amongst these is William Foster & Co., Ltd., of Lincoln, in which company's " Wellington " tractors the effective spring base has been so vastly increased that it may safely be termed the most stable of steam tractors. The life of all the working parts of a tractor may be considerably lengthened by the elimination of road shocks, or the prevention of their transmission, through the gearing, to the engine and the boiler plates. Foster's tractor is illustrated, in diagrammatic form, in fig. 25. An ingenious machine of the tractor class is that built to the designs of Colonel Crompton, and shown in fig. 26. This machine is intended for military purposes, or for operation in undeveloped countries. Steam is generated in a " semi-flash " boiler, and is used expansively in a four-pair, diagonal-compound engine of the type shown in fig. 20. A two-speed epicyclic gear is enclosed by the flywheel casing, and the power is then transmitted, by worm gearing, to a differential countershaft, and from sprockets on the ends of this shaft the drive is finally transmitted to the 7 ft. diameter road wheels by means of side chains. In this tractor, very long bearing springs are employed, and these are situated below the axle, so that, instead of the springs resting on the axle boxes, the whole frame and the power plant is suspended from the axle boxes. When hauling a load, the winding cable is permanently secured to the drawbar, and, when the machine becomes " bogged " or is otherwise unable to haul its trailer directly by the drawbar, a single bolt may readily be removed from the drawbar, and the winding cable may then be paid out as the tractor proceeds alone. The trailer may then be hauled up by means of the cable. The average of a number of tests with this machine, made while hauling a gross load of 8 tons, showed that its burners consumed from •65 to .85 of a gallon of shale oil for each mile travelled, and that the consumption of water was at the rate of •5 gallon per mile. The gross weight of the machine, with sufficient fuel and water for well over too m. of running, is about 7 tons. Vehicles Driven by Internal-Combustion Engines.—The general principles of the working of a steam engine are better understood than are those of the gas or oil engine, owing to the wide use of the former class of prime mover since the early part of last century, but it is beginning to dawn upon the public at large that the internal-combustion engine, or the " petrol " motor, as it is more popularly termed by those who talk or write about motor vehicles, is even more simple than the steam engine. The fundamental reason for the use of the words " internal combustion " is that the fuel, in the case of the petrol engine, is burnt (or fired) inside the working cylinder, whereas it is burnt externally in the case of a steam engine, i.e. underneath the boiler or generator. The number of units of heat which can be turned into useful work is very much greater in the case of internal combustion than of external combustion, the efficiency of the petrol engine in this respect being, on the average, about three times as great in practice as is found to be the case with typical steam engines other than those where highly-superheated steam is used, and where the whole of the parts are maintained in the best condition. The amount of petroleum spirit, or of paraffin, required to propel a steam vehicle r m. would, other conditions being equal, propel a vehicle fitted with an internal-combustion engine over a distance of 3 M. The essential parts of any internal-combustion system are: the carburetter; the engine; the radiator; the clutch; the change-speed gears and the final transmission. The carburetter is a vessel in which the liquid fuel is converted into a combustible gas or vapour, for, as there is no connexion to any gas main, the ordinary petrol engine has to make its gas " on the premises." The production of the gas is automatic, and calls for practically no attention from the driver, because, once the engine is started, the necessary aspiration to draw through the correct quantities of air and fuel is provided by the action of the valves and pistons. A smart turn of the starting handle is required to set the pistons and crankshaft in motion, so that an initial supply of the combustible mixture may reach one of the cylinders. This first, charge of gas is automatically ignited by an electric spark, the current for which is furnished and controlled without the necessity for any hand regulation, and there is then nothing An internal-combustion engine would get very hot if no precaution were taken to cool it, and it is usual to surround the cylinder with water spaces. These spaces are called jackets, and the water is forced through them, either by a pump or by thermo-siphon (natural circulation) action. It is expedient to keep down the weight of water, and for that reason pipes, tubes or small boxes are built up in such a manner that a large cooling surface is exposed to the air. A fan, which is driven from the crankshaft of the engine by gear or a belt, is employed to aid this cooling by reason of the increased volume of air that passes round the outside of the components of the radiator members. The general scheme is the same, both for heavy and light motor-cars. It is very important that the driver should have a convenient means of separating the engine from the driving mechanism, and of putting the two in connexion again, whenever it becomes necessary, without jar or shock. The common practice is to use a leather-faced, circular member with a coned face, and to control the amount of " grip " between this member and a corresponding enclosing member attached to the engine fly-wheel by means of a pedal and springs. When the driver wishes to disengage the two members, he has merely to depress the foot lever. It will be clear that a clutch of this description can be made to engage without any difficulty, there being no fixed positions or steps such as one associates with the ordinary jaw-clutch, and this gradual application of the load can only be accomplished by the aid of two or more surfaces in frictional contact, and by the holding together of these surfaces by the pressure of one or more strong springs. The Hele-Shaw multiple-disk clutch gives very good results, and is easy for drivers to use in traffic. An internal-combustion engine cannot develop power unlessthe crank-shaft can rotate at a relatively high number of revolutions, and the rate of doing work is lowest when the angular velocity is at its minimum. It is, therefore, necessary to intro-duce a system of levers between the engine and the road wheels, in order to permit the number of revolutions of the crankshaft to be maintained when hill climbing, or when the vehicle is carrying a heavy load, and the common practice is to introduce The most common form of final drive is, perhaps, that in which two "roller" or "silent" chains transmit the power from sprockets on the ends of the differential shaft to chain rings which are bolted to the rear road wheels. Figs. 27, 28 and 29 show typical vehicles, ranging in load capacity from 30 cwt. to 6 tons, on which the side-chain method of final drive is adopted. One of the chief advantages of the side-chain drive lies in the fact that there is, with it, less weight below the springs than with any other form of final drive. The only parts below the springs are: the fixed back axle; the chain rings (bolted to the road wheels); the road wheels themselves; the road-wheel brakes and part of the weight of the chains. The differential gear and chain sprockets are carried in a countershaft casing, which is securely bolted to the main frame. Cars, Ltd., Luton. In a number of very successful vehicles the final drive is transmitted by means of spur pinions. These are mounted on the ends of bevel-driven differential shaft, and mesh with internally toothed or externally-toothed gear rings on the road wheels. Milnes-Daimler and De Dion commercial vehicles are amongst the machines on which the internally-toothed form of gear is employed, whilst Ryknield is the most representative vehicle embodying the externally-toothed form of final drive. The direct drive, from the ends of the differential shaft, as is shown in fig. 30, is another type of final transmission that has met with a considerable amount of success, particularly on the Leyland machines of five-ton and six-ton capacity. The differential gear and the bevel-drive reducing gear are both enclosed. within a casing that is bolted to a fixed back axle; the ends of the driving shaft pass through tunnels in the axle body; and claw pieces on the outer ends of the differential shaft engage with similar claws on the road-wheel hubs. The two last-named forms of gear are highly efficient, provided the pitch and shape of the teeth are carefully considered and the designs provide for the encasing of all the pinions and gear rings. Reducing Gear $S Differential Gear, The only other type of final drive which is used to any great extent for commercial motors is that which employs a hardened and ground steel worm meshing with a machine-cut phosphor-bronze worm wheel which is bolted to the differential-gear cage of a live back axle. The employment of this type of gear for the final transmission on commercial motors generally leads toincreased efficiency, on account of the ease with which all the parts can be enclosed in an oil-tight casing. It also gives silence of running. The strongest advocate of the worm drive for heavy vehicles is the Guildford manufacturer, Dennis Bros., Ltd., one of which company's machines is illustrated in fig. 31. Although there are many difficulties in the matter of the manufacture of worm gearing, they are not insurmountable, and, given proper attention at the hands of the designer, followed by accurate workmanship, probably no other mechanical means of transmitting power can approach it for smooth and silent operation. Both thrust bearings on the worm shaft should be on one side of the worm, to avoid lack of truth in meshing if any heating occurs between the worm and the wheel. There are many examples of the worm drive to be found in London on public-service passenger vehicles, and also on delivery vans. One of the great charms of this type of transmission is that a very large gear reduction may be obtained without making the worm wheel unduly large in diameter; this is an important factor in the design of a back axle, as every inch of road clearance is of value for operating on rough country roads. As a large gear reduction is thus rendered possible on the back axle, it will readily be understood that the change-speed gear-box may be made considerably smaller than would be necessary for a bevel-driven live axle, where a large gear reduction is not permissible, both on account of its size and because such a gear would be very noisy in its working. Although the use of tooth wheels is still the only practical method of obtaining variable transmission for motor vehicles, the fundamental defects of transmission in this way are inherent to the system and must always be present; they are now less apparent, thanks to the remarkable improvement which has taken place in the use of suitable materials and improved design. It is still the hope of some manufacturers that a form of infinitely variable change-speed device will be produced, which will Centrifugal Pump. replace the step-by-step movement of toothed gearing; the two chief directions in which this has been attempted are electrical and hydraulic. Of these two, electrical devices are really fugal Pump. step by step, and the hydraulic method is apparently the only one that permits of infinite variation. Enormous sums of money have been spent in the search for an effective hydraulic S F.Lus a gear; the work of Hall, Pittler, Jannay, Hele-Shaw, Renault and others is, perhaps, the best known. It must be confessed, however, that in 1910 none of these gears could be said to be on the market for motor vehicles, although hydraulic gears were being successfully applied in connexion with other problems, such as the steering of ships, the movement of turrets, &c. Electrical transmission systems, too, have been tried, and appear to have been attended with more success than those of the hydraulic type. Such systems include vehicles which carry heavy batteries of accumulators, the current from which is utilized for the driving of the vehicle by means of electric motors. Other variations include the Hallford-Stevens system, shown in fig. 32, in which it may be seen the petrol engine drives a dynamo, and the current is then caused to drive an electric motor at each side of the chassis. Each motor drives one of the back road wheels, through a worm and worm wheel. The changes of vehicle speed are effected by altering the method of grouping the electrical windings of the dynamo and motor field-magnets and armatures. This system of control is known as the series-parallel, and is effected by a single lever, which actuates a mechanical switch, or " controller." In one of the most-recently-introduced petrol-electric systems —the " K.P.L." system, as worked by the Daimler Co., of Coventry—each of the rear road wheels is provided with a separate power unit, consisting of a four-cylinder petrol engine; which is direct coupled to a dynamotor, the armature of the latter being coupled to a worm which meshes with a worm wheel attached to the road wheel. A small electrical storage battery forms part of the system, and this receives the excess of current from the dynamotors when the whole power of the engine is not required for the propulsion of the vehicle. When the machine is being driven up a steep incline, or when it is required to travel in a reverse direction, the battery may be called upon to supply current to the dynamotor, and, in this manner, the power of the engine is augmented by the dynamotors' working as electric motors. tI Still another petrol-electric system is that invented by Mr Thomas. In this system, which is at the same time the simplest and most practical form of petrol-electric transmission, two dynamotors and an epicyclic gear are employed so that the electrical load is at no time greater than one half of the total load; consequently, the risk of a " burn out " of the windings, as the result of sudden and unforeseen periods of overload, is materially minimized. Special Applications.—Amongst the special applications of the petrol-engined commercial vehicle is the motor fire-engine, which has brought to the front in this branch several enter-prising motor manufacturers, amongst whom Dennis Bros., of Guildford, Halley, of Glasgow, and Leyland, of Preston, are prominent. The general construction of the chassis closely follows the lines of other petrol vehicles of equal load capacity, but the gear-box tail-shaft is prolonged to the after-end of the machine, and is direct-coupled to the rotary member of a multi-stage centrifugal pump. In the Dennis fire-engine, the vacuum which is necessary in order to lift the water in the suction hose is obtained by means of priming the pump chamber, further assisted by a water ejector and a small water tank which is carried on the machine. This machine is shown in fig. 33, whilst fig. 34 shows the successful motor fire-engine built by Halley's Industrial Motors, Ltd., of Yoker, Glasgow. In the latter machine, as also in the Leyland and other fire-engines which employ centrifugal pumps, the vacuum is created by means of reciprocating air pumps. These machines have given very satisfactory results in the hands of practical firemasters, in various parts of the United Kingdom and abroad. Merry-weather and Shand-Mason, who were formerly builders of steam fire-engines, now also build petrol-engined machines, these makers favouring reciprocating water pumps. Other special applications of the internal-combustion motor are for grass mowing and rolling, and for road mending and rolling gravel paths, &c. One of the latter type of machines is shown in fig. 35. In this machine a petrol or paraffin engine drives a water-ballast roller through the medium of a clutch, a simple form of change-speed gear-box, and a single roller chain. The leading roller, by which steering is effected, is also filled with water, in order to obtain the dead weight necessary for rolling. Marshall, Sons & Co., Ltd., of Gainsborough, Thorny-croft, of Basingstoke, and Broom & Wade, of High Wycombe, have also produced special machines for agricultural and military purposes, and one of the smallest tractors built by the first-named maker is shown in fig. 36. The engine is one of the two-cylinder type, consuming paraffin fuel, and driving the live back axle through a substantial gear-box and a final drive of the externally-toothed type. Such a machine is well below the weight limit for heavy motor-cars. Fig. 37 shows a more powerful oil tractor by Thornycroft. This machine is the same type which was so successful in the tractor trials promoted by the British War Department in March 19o9. It is capable of hauling a gross load of seven tons practically anywhere, and even of lifting that load vertically by means of its winding cable. The engine has four cylinders, and the fuel may be paraffin, alcohol or crude oil. (E. S.. S.)
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