Hi
Forgive me as I'm a beginner on understanding steam locomotives. I saw a footplate video of the Duke of Gloucester 71000 and how the driver regulated the speed and power by turning the valve gear from the cab rather than just using the regulator. This looked a really good efficient way of regulating the power.
We're glad to share the understanding. Someone has to take all the knowledge on for the future.
All conventional steam locos have both a regulator and valve gear, and are controlled by a combination of the two. It's notable that other forms of transport have a similar approach, cars with accelerator and gearbox, and propeller aircraft with throttle and propeller pitch control. All allow different proportions of the power of the throttle to be transmitted, are adjusted as speed increases, and are used to reverse the direction of travel - and have different drivers handle them in different ways they feel are "best".
The main valve control in a steam loco is to switch the steam supply to the cylinders on and off, in a way that combines the functions of the inlet/exhaust valves in a car engine and the car's gearbox for changing ratios as speed rises. Unlike a car with say five fixed gears, the steam loco is infinitely variable, normally measured as a "cut-off", that is how much of the piston travel the steam is opened before it is stopped and expansion allowed to do the rest. Typical figures are 75% when you start off, reduced to 25% or even less at high speed. You can advance things if you wish, say to 30% or 35% at speed, and you will get a bit more power, but will also be disproportionately wasteful of steam, which of course the fireman won't thank you for. A bit like driving a car on the motorway in 3rd gear.
In the 19th century the control in the cab was a large lever (the "pole" as a GWR man would say) which went forward and backwards, fully forward to start, fully back to go backwards, with neutral ("mid gear") in the middle. This got progressively harder to use, especially at speed with a full steam flow, as locos became more powerful, so screw gear, the wheel you noticed, became standard on larger locomotives (smaller ones kept lever reverse to the end). Beyond that, a few locos in Britain, and more overseas where they were even larger such as in the USA, had power reversers, generally by a small steam cylinder, but some US oddballs used compressed air from the braking supply. Power reverse had a nuisance the others did not, that it would tend to "creep" towards Full Gear (that 75%) and have to be regularly adjusted as you went along. The Bulleid Pacifics of the Southern were notorious for this.
The actual mechanical arrangement of the gear mechanisms were of various types. The GWR pretty much always stuck with Stephenson gear, where the connecting rods look straightforward. The other three companies, and BR Standards, moved on to Walschaerts, with many fiddly outside links to it, while the Duke of Gloucester you noticed was one of the few British locos to use Caprotti gear. each type had its own pluses and minuses, and supporters. Bulleid inevitably did something completely different on his Pacifics and used a series of oversized bicycle chains to connect the mechanism, sealed (supposedly) inside an oil filled box. There, now that's been said. Stand by for incoming from the Southern supporters.
hock:
I meant the 15XX panniers not, as I wrote, 95XX. The 15XX had outside Walschaerts valve gear and only 10 were built. One survived and is at, if I remember correctly, the Severn Valley Railway.