Contribution from a reader
While it wasn't part of what I was looking for, I
decided to have a look anyways, owing to my
general fascination with steam locomotion. I am
aware that this was never intended as a
comprehensive answer, there are some parts of it
that I would like to expand upon.
One such remark is that yes, trains nowadays do
use gears, although not like the selectable gears
in the transmission of a car. Although they are
not the main method of transferring power from the
prime mover (the diesel engine) to the wheel
bogies/trucks, gears are used to transfer power
from the electric drive motors in the trucks to
the wheel and axle assemblies.
Also, while most steamers used main and side
rods to transfer power mechanically from the
pistons to the wheels, there were also several
families of steam locomotives that used gears and
drive shafts as the main method of power
transmission. The main varieties of these engines
were known as Shays (which had three steam
cylinders arranged vertically on the right side of
the offset boiler), Heislers (which had two
cylinders in a Vee arrangement under the boiler),
and Climaxes (which had one cylinder on each side
of the boiler, and turned cranks attached to a
large gear box under the boiler).
These engines all used gears and drive shafts
to transmit power from the cylinders to the
wheels, although the gears and drive shafts on the
Shay ran down the right side of the locomotive
while those on the Climaxes and Heislers ran down
the center of the engine. Such engines were used
in applications where raw pulling power,
maneuverability and small size were needed at the
expense of speed, such as small logging and
industrial railroads. These engines had
additional advantages due to the fact that because
there were no large wheels permanently affixed to
the chassis, they could maneuver around very tight
track curvatures, since all the wheels were on
Additionally, because there were no pilot
wheels or trailing trucks sharing the weight of
the locomotive and tender, the entire weight of
the engine was used to press the drive wheels to
the rail. This enabled comparatively small
engines to provide pulling power far superior to
any conventional engine of equal size.
There were also engines that used gear-driven
valves to control the admission of steam into the
cylinders, but these designs were evolutionary
freaks that never went anywhere.
Another remark that I have is about the post
that contended that steam locomotive speeds were
limited by the need to save wear on their rails.
While it is certainly true that steam engines were
hard on trackage, this wasn't really the main
constraint on a locomotive's speed. The main
constraint was the intended usage of the engine.
Trackage was usually
enlarged/adjusted/reinforced/improved to reflect
the operating speeds of the trains and tonnage
being moved. For an engine that was intended
purely for hauling heavy freight trains at low
speeds, small drive wheels (63" diameter or
smaller) were most advantageous, because they gave
maximum torque, you could fit a very wide, heavy
boiler above them, and you could cram more of them
under a boiler of any given length. However,
small wheels necessarily limited the size of the
counterbalances on the wheels, so running such
engines at high speed did tend to beat the
daylights out of the track, because the small
wheels and associated connecting rods and running
gear had to spin very, very quickly at such
speeds. The general rule was the faster the
engine was meant to go, the larger its wheels had
to be. Passenger engines designed for speed could
have wheels 80" in diameter or larger.
The trade off was that passenger engines made
less tractive effort (pulling force), but this was
all right because passenger trains tended to be
lighter than freight trains.
Also, regarding item #5 of the answer above,
down there at the bottom, a steamer's main rod is
the big connecting rod that runs from the piston
crosshead to the main crankpin, and the side rods
are the smaller connecting rods that run from the
main crankpin to the other drive wheels.
Thank you for your time.
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