458
STEPHENSON’S PATENT
rail, and the driving wheels could not be case-hardened, as the others were, from its /
diminishing the adhesion upon the rails. Wheels with wooden spokes and rims and
wrought iron tires, were tried on the Liverpool and Manchester Railway, and found
to wear better, being more elastic; and flat wrought iron spokes were then tried.
The wheels with tubular spokes, and cast iron rims with wrought tires, are now
very generally used, and wear very well, lasting two or three years ; the driving
wheels being subject to the most wear, in consequence of the slipping to wdiich they
are liable. The tires squeeze out at the sides as they wear, and when worn out are
replaced by new ones; they are now made wider, the flanch tires being six inches,
and those of the driving wheels seven inches, in order to prevent squeezing out at the
sides, which is the greatest cause of their wearing out. The cast iron rims are rather
objectionable from their brittleness, as they have to run with so great a velocity; and to
obviate this, some engines have wheels with wrought iron rims, to which the spokes
are fixed by rivets, having the tires shrunk upon them ; this construction is con
siderably more expensive, though very durable.
All the earlier locomotives on the Liverpool Railway, and many of the present
ones, have been made with only four wheels, D' L'; the third pair of wheels, M',
placed behind the fire box, has been added but lately; but six-wheeled engines are now
coming into more general use, and on several railways none others are used. In the
earlier engines the fire-box was considerably smaller than the present size, and that
end of the engine behind the crank axle was but little heavier than the other end
before the front axle, so that the engine was nearly balanced upon the axles and ran
steadily along. But the weight of the hind end of the engine has been so much in
creased, by increasing the fire-box, that it has a considerable preponderance, and the
present engines are far from balanced; in the engine shown in the engravings, the
weights at the wheels, D' L', supposing the hind wheels, M', removed, are six tons at
the large wheels, D', and only four tons at the front wheels, L', including the weights
of the wheels. This excess of weight behind the wheels causes in the four-wheeled
engines a pitching motion, which makes them rise on the springs of the front axle,
and is considered dangerous when running very fast. The pitching of the engine
causes also great injury to the rails, as the wheels are made continually to strike upon
them with very great force.
The hind wheels in the six-wheeled engines support the fire-box, and prevent this
action ; the springs over their axle are hung very light, so that in the ordinary state of
the engine they only just bear against the frame, and take scarcely any weight away
from the driving wheels; but they serve to catch the weight in the oscillations of
the engine, and prevent that overbalancing which causes the pitching motion. The
weight on the rails at these wheels is therefore only that of the wheels and axle, or