Theoretically, with a super-large wheel design like the Hurricane, couldn’t you hang the boiler under the axle?
And to the torque question, doesn’t that also depend on how far out from the center the piston rods connect, too? It seems to me that, for any given piston rate, the speed would be determined by the ratio of those two radii.
Cool locomotive of the Great Western Railway, the Iron Duke class with 8 ft driver wheels. Of course they ran on the Brunel broad gauge of 7 ft 1/4 inch.
You’re right about the ratios. But the stroke of the piston in the cylinder is equal to 2x the radius where the piston rod connects to the wheel.
So the optimization problem trades off your “gear” ratio versus top speed versus piston stroke. Ideal bore vs. stroke ratios depend on intended cycle rate, metallurgy, and the steam’s expansion ratio which in turn is driven by the boiler output pressure.
As we were discussing in another thread, the distance between the rails is usually much less than the width of the locomotive. The space between the drive wheels is smaller still.
If you click on the 2nd of the links Bert Nobbins posted, you’ll see another solution: the boiler is on a separate chassis from the pistons and drive wheels. This solves the issue of the locomotive being too tall, but you end up with much less weight on the drive wheels, which means inadequate traction.
One of the earliest designs of locomotive actually did have the boiler under the axles. The Novelty was one of the locomotives that competed at the Rainhill Trials, and had a boiler underneath the chassis, and the ‘footplate’ and firebox were both above the wheels. A neat little loco, but woefully underpowered. Novelty
One of the builder/designers of Novelty went on to design the USS Monitor, one of the first steam-driven ironclads.
Yeah, I saw the crazy design on that link. I’m just trying to brainstorm other crazy designs.
Genuine, everyday steam working on India’s railways came to an end – after many years’ gradual fading-out – approximately in the year 2000. In a few restricted locations in India, steam locos continue to be used; but consciously and deliberately as tourist- and railfan-bait – which takes most of the shine off it for me.
Regrettably for folk such as myself, there is now almost nowhere left on earth where steam locos are in everyday, commercial use; as opposed to in “entertainment” roles of one or another kind.