Valvegears
Distribution and timing of steam admission and exhaust in an engine is accomplished by the valves, these have to be driven and the collective name for the mechanisms which achieve this is Valvegear.
There have been a huge variety of valve gears developed during the history of the steam engine, of which only a percentage are relevant to the stationary steam engine.
As the operation of valves and valvegear is dynamic, static illustrations are not much help in showing their operation, we are fortunate therefore that two individuals have written valvegear simulation programs which run under MSDOS and can be downloaded as shareware or freeware (see the conditions if you download)
I cannot recommend these programs too highly as, while primarily for the Locomotive fraternity, the animations show far more easily the operation of valves and valvegear than any amount of text and still pictures would achieve.
There are two you can download :-
Charlie Dockstader's valvegear simulation program
and
Allen Wallace's valvegear simulation program
Here is a Valvegear and Slide Valve Demo from Charlie Dockstaders program which run under MSDOS -
CAUTION - only click 'open' on these last two if you KNOW you will not upset your computer by running a MSDOS executable file at this time - otherwise save to a file and run later in an MSDOS window.
Enjoy !