By F. A. Talbot
DURING the past few years the increasing demands for greater power to haul heavier and longer trains has been responsible for the display of striking ingenuity in connection with locomotive design.
Recently the attention of engineers and others concerned with the economical operation of the great railways of the world has been attracted to quite a new and novel type of steam railway engine, which has been evolved by a British inventor, Mr. H. W. Garratt, M.I.Mech.E., and the question has been discussed as to whether it does not indicate a new era in locomotive design. Although it belongs essentially to the articulated class, it has one great advantage over its prototypes—it it more flexible—and appears to meet very completely all the varying and severe requirements of the average railway, from which stretches of heavy grades and sharp curves generally are inseparable.
It is conceded generally that inventors have very little scope for further development within the recognised limits of locomotive construction, unless resort is made to the introduction of complications which are apt to counteract any benefits that may arise from the incorporation of the new feature. The boiler and the driving wheels are two vital factors, and there is a limit to their respective diameters. The boiler has to be disposed above the wheels, and the dimensions of the latter influence the former to a very appreciable degree, because the over-all height of the engine is limited by tunnels and bridges. If the size of the wheels is augmented, their axles must be brought to a higher elevation above the track, and accordingly the diameter of the boiler must be affected.
The questions of weight distribution and a large grate area of proper proportions also influence the situation very materially. Locomotive engineers have surmounted these various handicaps by lengthening the boiler and increasing the number of driving wheels, but this has given birth to another objection. It is useless to extend the length of the boiler without enlarging the grate area of the fire-box to secure the maximum steaming capacity and complete economical combustion of fuel. Lengthening the boiler in turn precipitates the possibility of eliminating all the advantages incidental to the articulated system. The bogies, which should have free and easy movement, are rendered stiff and unnatural, so that they cannot accommodate themselves readily to the curvature of the track ; flexibility, which is so keenly demanded, is imperilled gravely, if not destroyed.
In these circumstances the problem which Mr. Garratt sought to solve was necessarily of a complex and searching character. In order to achieve any measure of commercial success it was necessary to depart from conventional lines. In this quest, however, he has succeeded, and his efforts culminated in the production of a design which is distinctly novel, ingenious, effective and economical in working. When he had completed his ideas he submitted them to one of the foremost British locomotive building organisations, Messrs. Beyer, Peacock and Company, Limited, of Gorton Foundry, Manchester. They readily appreciated the outstanding features of the new system, acquired the patents, and undertook to exploit the invention. Several months were expended upon the perfection of the details of the designer's handiwork. The first opportunity to ascertain the possibilities of the system came when the chief engineer of the Tasmanian Government railways suggested that small engines of this class should be given a practical test upon the State system of the island.
This was about as severe a test as could be conceived for a new idea. The locomotives were required for working upon the North East Dundas section of the State system, where the gauge is 2 feet, with grades running up to 1 in 25, and with curves of 99 feet radius. Rigid stipulations were laid down to which the engine had to conform, so that it cannot be said that the inventor was given the opportunity to demonstrate his ideas under the (to him) most favourable conditions. Two locomotives were built and shipped to Tasmania in 1909, where they have been running continuously ever since.
The salient feature wherein the Garratt locomotive differs from its contemporaries is that the boiler is a distinct unit, and is not mounted above the driving wheels in the usual way. Fundamentally it comprises three sections : the boiler with its fire-box, and two end bogies, each of which is a driving unit, so that it may be described as a boiler and two complete motor bogies, one placed at each end, as the latter carry the cylinders, pistons, and driving gear. The boiler, including fire-box and cab, mounted upon its frame is carried between the two bogie trucks, so that the wheels are not brought beneath the boiler and fire-box. The advantage is obvious. As there are no restrictions arising from the presence of wheel axles, the diameter of the boiler may be increased very appreciably, while there is no fear of cramping the fire-grate. In fact, the whole may be placed so low as to leave only the minimum clearance between the rails and the bottom of the fire-box. This feature exercises its advantages in several ways. In the first place the centre of gravity is kept low, ensuring steadiness and safety in running at the highest speeds ; the driver has a clearer view of the road ahead and behind, owing to the large-sized cab windows that can be fitted ; while, if necessary, the boiler diameter can be enlarged to about 20 per cent. more than is possible under present conditions, even if a Garratt type comparable with the huge Mallet engines is evolved.
The two end bogies, in addition to carrying the driving mechanism, are also utilised for the bunkering of the coal and water, so that a tender in the usual sense of the word is rendered unnecessary. The weight thus imposed not only increases the adhesion of the wheels, but when the locomotive is running at high speed, they effectively assist to prevent oscillation of the bogies, so that the wear and tear upon the flanges of the wheels, and also the rails, is reduced to a minimum. The capacity of the tanks may be varied according to requirements, this factor being governed entirely by the number of wheels to the bogie and the axle-loads permitted.
In the case of simple expansion working, the steam from the boiler is taken from the dome by means of two regulators with piping, one of which extends to the smoke-box end of the central unit, and thence to the front bogie through a flexible joint ; the other runs to the fire-box end, and in a similar manner to the rear bogie, a Y pipe in each instance delivering the steam to the cylinders on either side.
By dividing the locomotive into three parts in this manner the full effects of articulation are obtained. Owing to the rigid section, that of the boiler and its frame, being kept as short as possible, and the two bogies having free play, the sharpest curves and inequalities in the track may be negotiated with extreme ease, and the flexibility is such that the whole engine conforms with natural freedom to the curve. In rounding a curve, the rigid central section forms a true chord of the arc, while the sharper the curve the more the centre of gravity is brought inwards, so that exceptional stability is secured. The extreme flexibility of this type of engine is demonstrated most convincingly, possibly, on sharp reverse curves, which may be rounded at high speed with far greater safety than is possible with the ordinary locomotive, there being an entire absence of stiffness or grinding of the flanges against the rails. There is none of that climbing tendency of the engine which often is experienced under such conditions, and which has been responsible for many derailments. Another point which cannot fail to be observed is that there is no overhang of the boiler frame when rounding a curve, as the articulating centres are fixed at the extreme ends of this frame. This contrasts very vividly with the overhang of the boiler and frame upon the general type of semi-rigid articulated locomotive, which is now so much in vogue.
It might be thought that difficulty would arise in distributing the loads uniformly over the axles, especially as the weight of the fuel and water is fluctuating constantly ; but with efficient designing this is not so. The combined weight of the fuel and water represent such a small proportion of the total weight of the bogies that even if the whole of these commodities were consumed—in actual running it is very improbable that their weight would fall below 20 per cent. of the full load—there is still ample weight upon the wheels ; the variation in the loads upon the axles certainly would not be more than in the ordinary type of tank engine which is in daily use.
In the first engines of this type, built for Tasmania, there is a small bogie at either end, fitted with two coupled axles ; compounding was stipulated, although it is not essential to the Garratt system, and its incorporation involved somewhat heavy and unavoidable complications. The high-pressure cylinders are mounted on the trailing, and the low-pressure cylinders upon the leading, bogie. In this instance the steam is led from the dome to the rear truck, and distributed by a Y pipe to the cylinders on either side. The exhaust steam is received through a second Y pipe and carried to a flexible coupling or ball-joint, then under the frame of the boiler to the front bogie, where it is distributed by a similarly bifurcated pipe to the low-pressure cylinders. The exhaust from the latter is taken back by another Y pipe and ball-joint coupling to the chimney, to be discharged into the air. For simple expansion working the cab is fitted with devices to intercept the return of the exhaust steam from the high-pressure cylinders, while live steam is admitted direct into the cylinders of the leading bogie. The high-pressure cylinders have a diameter of 11 inches, while the low-pressure cylinders have a diameter of 17 inches, with a common stroke of 16 inches. The boiler barrel is 7 feet in length by 3 ft. 11-1/8 in. diameter outside, and has 170 tubes of 1-3/4 inches external diameter. The over-all length of the locomotive is 33 ft. 10-1/2 in. by 7 feet wide, and the total weight in working order is 33 tons 10-3/4 cwt.
When the Tasmanian engines appeared they aroused considerable interest, but were regarded in many railway quarters as a novelty, comparable with the Fontaine and other unusual designs which have appeared from time to time. But the experience gained on the Tasmanian railways tends to indicate that the engines are eminently adapted to peculiar conditions. While inquiries concerning the adaptability of the idea to other countries commenced to roll in, the system received its complete vindication when the Tasmanian Government, which first had submitted the idea to practical trial, ordered larger and more powerful types for their main lines. Here again a variety of difficult and rigid requirements had to be fulfilled, for which Mr. W. R. Deeble, the chief mechanical engineer to the Tasmanian Government, concluded that the Garratt system offered the only solution.
In Tasmania the railway situation has developed, as it has in other countries. Increased weights had to be handled by the engines. The adoption of corridor coaches in the express passenger service doubled the weight of the train to be handled by the existing locomotives, and, in combination with high speed, the ordinary type of engine was ruled out of court upon the 3 feet 6 inch gauge with grades of 1 in 40 and curves of 330 feet radius. The axle-loads and length of the fixed wheel base were restricted by the physical characteristics of the road, while speed imposed special conditions concerning the size and distribution of the wheels, as well as the balancing of the reciprocating forces, so as to prevent side movement.
The situation has been met completely by a Garratt simple locomotive, having two groups of four-coupled wheels, with four-cylinder balanced engines, each having an inner pair of carrying-wheels and an outer four-wheeled bogie. Virtually it is an Atlantic type of engine adapted to the Garratt system, with coupled wheels of 5 feet diameter. The weight upon the driving axles varies between 11-1/2 to 12 tons per axle, with a rigid wheel base of 6 feet, and capable of attaining speeds up to 50 miles per hour on the straight, and 30 miles per hour round reverse curves of 330 feet radius. This is probably the most powerful articulated locomotive yet built for passenger service upon a 3-1/2 feet gauge.
So far as the goods locomotives are concerned, the same governing factors, except high speed and axle-load, which was limited to 9-1/2 tons, had to be taken into consideration. For this work a Garratt simple goods locomotive of the 2-6-2+2-6-2 class was adopted, there being two groups of six coupled wheels, with two cylinder engines, each having an inner pair of carrying wheels provided with side play, and an outer two-wheeled radial bogie, the coupled wheels being 3 feet 6 inches in diameter, with a rigid wheel base of 8 feet. This arrangement of the wheels affords the maximum-powered engine on the specified axle-load.
The absence of side tanks and of wheels below the boiler—the characteristic features of the Garratt locomotive—has facilitated the provision of a large boiler of simple and well-proportioned design, with a wide and deep fire-box of the Belpaire pattern. The one design of boiler in this instance is common to both passenger and goods engines, and provision is made for using oil fuel if desired.
In the Garratt locomotive the power or tractive effort is governed solely by the permissible load per axle, and the number of coupled axles, since in this type the boiler can be made so large as to be capable of supplying sufficient steam for cylinders of such proportions as may be required to make full use of the adhesive weight. For instance, for standard gauge working, a Garratt engine, with two six-wheel coupled bogies—0-6-0+0-6-0 type—with a load distribution of 18 tons per axle, has a tractive effort of 50,000 lb. Such an engine on the level could haul 3,000 tons, or 850 tons up a grade of 1 in 50, at a speed of 10 miles per hour. The total weight of the locomotive—no tender—would be about 108 tons, and the total length about 62 feet. Similarly, another engine, with two eight-wheel coupled bogies—0-8-0+0-8-0 type—having a load of 20 tons per axle, has a tractive effort of 72,000 lb. This would be sufficient to pull 4,500 tons on the level, or 1,200 tons up a bank of 1 in 50, at 10 miles per hour. In this case, while the engine would weigh 160 tons complete, and have an over-all length of about 67 feet, the longest rigid portion would only be some 30 feet.
Up to the present engines of this design have not been adopted for working upon standard gauge railways, but owing to the success of the engine upon narrower gauges, combined with its great possibilities, the day doubtless is approaching when it will be taken up for such work. It may not be seen for some time in this country, since the problem of the railway locomotive is not so acute as in the United States, Canada, and India, where heavy banks, sharp curves, and mammoth train loads are more common.
Railway operators cannot fail to appreciate other advantages which the system. offers, and which tend towards highly economical working. The design and arrangement conduce to easy riding, so that the track is given a longer lease of life, while the engine itself is spared those severe racking strains and stresses inseparable from the conventional articulated locomotive. It forms a perfect double-ender, and can be driven in either direction. This facility affects another question. Turning for every trip is dispensed with, so that. turntables are not required. This in itself represents a distinct advantage, seeing that the monster locomotives used for handling heavy loads demand turntables ranging up to 90 and 100 feet in lengthy with massive foundations. If the development of the Mallet engine offers any criterion of the limits to which locomotive dimensions and weight may be carried, it is not impossible to assume that the Garratt engine will undergo development to the same degree, giving greater power with smaller dimensions. This possibility has been anticipated, since, if the boiler is brought up to the height of the largest Mallets, the outlook from the cab will be reduced. This disability will be met by placing another cab forward of the smoke-stack, to be used for forward running, the existing cab being employed for driving in the reverse direction.
The ease with which overhaul and cleaning operations can be carried out must not be overlooked. Owing to the fire-box being free from the presence of wheels and tanks in close proximity, the wash-out plugs, etc., are quite accessible, as is also the ash-pan for the rapid clearing out of ashes. By lifting the boiler unit and making a few disconnections, the two bogies can be drawn quite clear, the three units being thus easily accessible for overhauling.