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.
[Illustration: Fig. 465, 466, 467]
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, 306
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.
[Illustration: Fig. 468, 469, 470]
[Illustration: Fig. 471, 472, 473]
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, 309
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.
[Illustration: Fig. 474, 475, 476]
[Illustration: Fig. 477, 478, 479]
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 312
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.