CHAPTER VI.
Comparative Weights of some Types of Modern Locomotives.
Weights of Locomotive Engines.—The demand for higher speeds of passenger trains, with more conveniences, luxuries, and consequent increased weights in the carriages, has naturally led to greatly increased power and weight of the locomotives devoted to the passenger service. Although these engine weights have so largely increased during the past twenty-five years, there is nothing to indicate that they have yet reached the maximum. The tendency is still to increase, and will doubtless continue, so long as the permanent way can be made to sustain such enormous rolling loads. Locomotives for goods trains have also increased in power and size, but perhaps not in the same proportion as those for the passenger service. There is not the same disposition to expedite the transit of goods and minerals, which do not deteriorate during a long journey. Perishable articles, such as fish, fruit, and milk, are usually conveyed by passenger trains, or trains set apart specially for the purpose.
The heavier engine doubtless possesses greater tractive power, but apart from the question of tractive power is the all-important one of steadiness and safety on the rails. A locomotive passing round a curve, even at a moderate velocity, produces disturbances in proportion to the capability of the machine to adapt itself to the altered position, and if both the engine and permanent way are constructed so as to be almost unyielding, then destructive wear and tear and increased risk of derailment must ensue. The adoption of the four-wheel bogie truck to the locomotives on our home and continental lines—although very slow in coming—has contributed greatly to their improvement, enabling the weight to be distributed over a longer, yet more flexible wheel-base, affording greater facility and comparative safety in traversing curves; and rolling, or
passing over the rails, with as little injurious effect to them as possible. It is strange to find that the four-wheel bogie truck, originally designed in England in the early days of the railway era, should for so long have met with so little favour on this side of the Atlantic. The Americans, at all times prompt to recognize any appropriate mechanical arrangement, adopted the bogie truck upon its first introduction into the States. They have worked out many improvements in the details, and upon the thousands of locomotives on their vast network of railways, the bogie truck, in one form or another, has been universally adopted from the beginning.
On our home and continental lines, the modern express locomotive, with a four-wheel bogie truck in front, is a much longer vehicle than its predecessors, and its total weight is distributed over a greater wheel-base; but the actual weight placed upon the driving-wheels, or on the coupled wheels, is now very much in excess of former practice, and must be taken into consideration when working out the details of girders and cross-girders of under-line bridges. Numbers of girder bridges have had to be taken down and replaced with stronger structures, not for reasons of wear or decay, but simply because they were incapable of carrying with safety the modern heavy rolling loads. Present experience points out the expediency of providing in all new under-line bridges a liberal margin of strength to meet future developments.
[Figs. 465] to [479] are diagram sketches of a few modern types of locomotives, giving leading dimensions and weights, and may be found useful for reference when working out the necessary strengths of the various portions of bridge-work. Upon comparing some of the principal particulars with those of the earlier class, it will be noted that in many of the modern types the piston area has been doubled, the boiler-pressure doubled, and the weight of the engine doubled also.
The engines shown in [Figs. 465 and 467] have great weights placed on the single driving-wheels, and should only be used where there is a very strong permanent way. With the four-wheel coupled engines, the weight for adhesion can be distributed between the driving and trailing wheels.
[Fig. 473] represents a very excellent type of American engine which has been extensively adopted in the United States for many years. The six coupled wheels distribute the weight over
a fairly long wheel-base, retaining their united weight for adhesion. The four-wheel bogie truck in front forms a valuable path-finder to the engine, both for passing round curves or on straight line. This class of engine is very serviceable for various kinds of traffic, and is particularly suitable for lines where the rails and fastenings are comparatively light. In the example shown, the flanges are turned off the centre pair of coupled wheels; but for lines where the curves are of small radius, the flanges may be turned off the leading pair of coupled wheels, instead of the centre pair, to reduce the length of rigid wheel-base. This type of engine has latterly been introduced on various European and foreign railways, and recently on the Highland Railway of Scotland, as shown in [Fig. 470]. The writer has had engines of this class under his charge abroad, and found them to be most useful for heavy passenger and goods-train service. They run very steadily, are easy on the permanent way, and light in repairs. As they become better known they will be more appreciated, and will doubtless before long supersede in many cases the rigid six-wheel-coupled goods engine. The principal objection of any importance that can be raised against them is that on many lines the present engine turn-tables are too small for such long engines; but it would be far more economical in the long run to enlarge a few turn-tables than to continue the adoption of rigid engines which from their form and arrangement tend to unnecessary wear to themselves and the permanent way.
[Fig. 476] shows an average sample of the ordinary six-wheel-coupled goods engine in use on so many of our home railways. Where the curves are easy and the permanent way strong, the drawback of the long rigid wheel-base may not be so apparent; but for a line abounding in sharp curves, perhaps no more destructive machine could possibly be devised than the ordinary six-wheel-coupled goods engine. Without any flexibility, forced along with great power, and too often driven at unnecessary high speeds, engines of this type have too small a margin of safety when traversing the curved portions of the road. A slight unevenness in the rails, or a sharp flange on the wheel may supply all that is wanting to cause the engine to leave the track, and the probability that such risks are more common than is supposed, is far from satisfactory. The great weight of the engine doubtless tends to keep it on the track, but the rapid
wear of the tyres, and of the inside of the rail-heads clearly demonstrate the enormous amount of friction and abrasion that takes place.
[Fig. 477] represents a type of eight-wheel-coupled engine designed for hauling passenger or goods trains over long lengths of heavy mountain inclines. The engine is a large one in every way, and of great total weight, but the weight is distributed over a long wheel-base and without imposing a greater tonnage per pair of wheels than is done in some of the smaller and less powerful engines. The flanges are turned off the leading pair and third pair of coupled wheels reducing the rigid wheel-base for curves to 9 feet 8 inches. The four-wheel bogie truck in front carries only a moderate weight, being so close to the coupled wheels. Engines of this description require a strong permanent way, as there is a total weight of 60 tons on the four pairs of coupled wheels standing on a wheel-base of 15 feet 6 inches.
[Figs. 478 and 479] are types of tank-engines in use on some of the narrow-gauge (3 feet) railways. In general design they are somewhat similar to the modern class of engine on main lines of 4 feet 8½ inches gauge, with four-wheel bogie truck in front, and four wheels or six wheels coupled, but with all the parts and weights smaller, to suit the narrow gauge and lighter permanent way.
The extended use of the bogie truck is an admission of its advantage over the fixed-wheel arrangement, both for distribution of weight and facility in passing round curves; but although it is now so largely adopted for engines and carriages on our home and continental railways, it is somewhat of an anomaly to find it so very rarely used for tenders. In the United States all the locomotive tenders—and many of them of very large size and weight—are carried on two four-wheel bogie trucks, and traverse the curves as easily as the engines. On this side of the Atlantic, the prevailing custom is to mount the tender on six rigid wheels; and as many of these tenders weigh as much as from 35 to 40 tons in working order, and have a rigid wheel-base of 15 feet, it will be seen at a glance that much unnecessary friction and wear and tear would be avoided by substituting two four-wheel bogie trucks for the fixed wheels.