Talk:Geared steam locomotive

Confusion

"European engineers preferred cogs to adhesion when force is an issue, possibly because the locomotives can be much lighter. Cog locomotives can also employ down-gearing." This statement is misleadingly naive.

I think the writer is trying to say that you can not start or pull a heavy load if the wheels slip. Sand is often applied to reduce slip when starting, sometimes a heavier locomotive can be used. In critical applications (gradients between, say 1 in 6 and around 1 in 25) a rack railway system can be used (what the writer calls 'cogs'). Whether or not the locomotive is geared is a separate matter although gearing is usually needed on rack locomotives.:These considerations should not be confused 'geared locomotives' and 'rack railways' each deserve a separate article. 82.38.97.206 09:49, 19 December 2005 (UTC)mikeL

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Whyte Notation?

To assuage the curiousity of some UK steam buffs, would Whyte notation be able to cope with these? 86.141.118.151 (talk) 13:54, 27 December 2008 (UTC)

ratios

the idea of a compromise between torque and speed isn't entirely accurate and gearboxes for any locomotive that was steam powered to change gear whilst moving would be rather heavy, and the 'variable gear ratio' is performed by the radius link,when in 'full gear' steam is admitted for most of the stroke, which is why steam locos 'chuff' loudly when setting off, and the link gets moved further to the centre the faster / under less load, so steam may only be admitted for 10% of the stroke or less, and only produce a little wisp, and use less steam, ratio gearboxes are used in internal combustion engines as they don't go with radius links, and not all mountain locos use gear reductions, most use indirect drive, and center the pivot to give the cylinder more leverage. — Preceding unsigned comment added by 2.221.131.231 (talk) 05:55, 26 October 2012 (UTC)

The above is correct up to a point, but moving the die block in the expansion link to alter the action of the radius rod only modifies the degree of admission of steam into the cylinders. This can be used to maximize tractive effort when starting or ascending gradients, or to economise on steam consumption when running fast under light or moderate loads. It does not change the necessarily fixed relationship between the movement of the piston and the rotation of the wheel, which limits the range of the ratio of torque/tractive effort per wheel revolution. This ratio can be changed by mechanical, electric or hydraulic transmission systems as used in diesel locomotives, ensuring variable torque/tractive effort over a wide range of train speeds (= number of wheel revolutions per second). In particular high torque/tractive effort is available at low speeds, thus greatly improving acceleration and performance on gradients. Do not confuse the meaning of the word `gear' in valve-gear and gear-wheel. Barney Bruchstein (talk) 18:42, 18 October 2013 (UTC)

A gearbox for a steam locomotive is no heavier than that for a diesel-hydraulic locomotive. There's a 30 year period where such things would have been possible. A look at steam motor might suggest some of the approaches, especially for railcars.

Steam locomotives don't need final drives because they develop maximum torque from zero speed, unlike internal combustion engines with compression, that need to be running within their power band. Where final drives were used (see steam motor), this was to increase power to weight ratio, using a small steam engine run fast rather than a larger engine.

They don't need gearboxes because the torque curve over speed is relatively flat over a wide range, unlike IC engines. Because they isolate boiler and engine, the boiler's evaporative power is decoupled from the engine speed. An IC engine can't run faster than its peak speed (not necessarily its mechanical redline) because it can no longer burn its fuel fast enough in the decreasing time of each stroke. In a steam engine, the in-cylinder process is purely thermodynamic (and largely adiabatic) rather than chemical, so it runs as fast as the machinery does. The limits are the steam supply pipework, the boiler capacity (for which a bigger boiler can be used) and eventually the mechanical breakage limit.

It's misleading to think of expansion gear as "gear ratio"; much better to see it as a throttle, and the regulator as a stop valve. British drivers weren't well trained on this – unlike the French, who had quite serious driver training schools. Many British drivers, right up to today, use a partial regulator for light loads rather than opening the regulator throttle wide and using the expansion ratio via the reverser as the main power control once under way.

Note also that expansion ratio acts as an overdrive, rather than having a simple monotonic or linear relationship with speed. A locomotive isn't notched up because it's going faster, it's notched up because it's no longer needing to accelerate. As the locomotive's top speed is approached, the reverser needs to be notched back again to give less expansion and thus marginally more power. At this point, Stephenson help the poor fireman! The useful top speed of steam locomotives is this point, beyond which they start to use steam inefficiently (and then empty the boiler). The achievable top speed of any express passenger locomotive, except the A4, is pretty much unknown because the only way to reach it is by working the engine increasingly inefficiently, so soon exceeding the boiler capacity. It's not merely that "speed requires power" but that achieving the most power possible from an engine also requires setting it from an efficient setting to an inefficient one, thus making the boiler demands greater out of proportion. Andy Dingley (talk) 12:03, 21 October 2013 (UTC)