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The central pipe is the dry pipe from the dome which supplies live steam. For inside admission, the exhaust ports are on the ends of the piston.

@@WingsStrings wow, you learn something every day. Cheers 🍻

Look up Century of Steam

That gear reminds me of a dog with a limp on one of its back legs. But it is beautifully simple.

If you compare the movement of the top pin of the combination lever (at the radius rod) to the middle pin (at the valve stem), you will see how much farther the valve travels as well as the difference in timing. Locomotive valve gear is akin to a mechanical calculator that combines two different motions; the first is a constant lead motion that is derived from the movement of the crosshead and sets the core movement and valve timing, and the second is a direction and amplitude component that is derived from the radius rod and expansion link that sets direction of the engine and the amount of steam admitted. The combination lever, as the name implies, combines both of these inputs into the final output of the valve stem. Note that on center, the combination lever is only receiving input from the constant lead motion of the main rod. In my stephenson gear animation, the lead motion is created by offsetting the two eccentrics per side away from 90 degrees which causes the center point of the expansion link to oscillate even in neutral, but on Walschaerts gear, there is only one eccentric per side that must be at 90 degrees and the expansion link's center is fixed, which means the lead motion must then be derived from elsewhere, hence the combination lever is added.

@@WingsStrings There is NO neutral on a steam locomotive - I have no idea why people call it neutral as it isn't. The valve gear is in center when the link block is on the trunion centerline and the valve moves 2(lap + lead) You are right that the combination lever is for lead (it is not solely for lead but the moves the valve 2(lap + lead) where lead is how much the valve is open on dead center (steam is being admitted) and lap is how much the valve overhangs the steam port when the valve is centered) Since the lead and lap lever (aka combination lever) moves the valve 2(lap + lead) AND Walshaert valve gear is a constant lead valve gear, the lead (how much the valve is open on front/rear dead center) is the same no matter where the valve gear is hooked up. This means that when in center, the lead and lap lever (aka combination lever) is moving the valve and is giving the same lead in center as it does in forward/reverse (and the exhaust passageway is open the same amount on dead center no matter where the valve gear is hooked up to) Since the valve is uncovering the steam port (lead) on dead center, steam can be admitted to push on the piston in center. Since there is no motion being imparted from the radius rod, the valve is opening/closing evenly about dead center and the loco can run in forwards or in reverse (when the valve gear is properly set and minimal load) when the valve gear is in center. Running in center does not produce much power, but it is possible when properly set up.

@@WingsStrings Here are some youtube videos of miniature locos running in center - these have 1/32" lead (valve is open 1/32" on dead center and that is all the valve port opening they have when running with the valve gear in center). Full size locos can do it too - locos with Stephenson valve gear can run forwards with the valve gear set slightly behind center (to the reverse of center) when properly set up czcams.com/video/bXMHhaQu2Z0/video.html czcams.com/video/tVnhE99jnWA/video.html czcams.com/video/uj0QMC1NbFo/video.html

@@mikado1555 Edit: after digging into the old files used for the animation, the hitbox is set to show steam right when the ports are opened; if they are meant to open on center but the animation is not, then it may be that the combination lever in the diagram is a bit too long and thus not providing enough lead to reach the hitbox and uncover the ports.

For the smoothest timing, valve gear tends to be connected to the same axle as the main rod whenever possible to keep it in sync; in some cases locomotives may need to put the eccentrics on other axles if, for example, the firebox or ash pan restrict clearances around the main driving axle, but this can result in less precise timing due to slop in the connecting rods between wheelsets.

Thanks for your reply. After looking at some real videos and images of the locomotive I realized that the illustration I was looking at was innacurate and that eccentrics were indeed put on the middle axle.

The creature's called a rotary

The main advantage of Stephenson gear is that the valve lead settings are variable, compared to constant lead for Walschaerts. Conversely, the main disadvantages of Stephenson are that it is often mounted between the frames and thus is harder to access for lubrication and maintenance, and with Stephenson gear you must raise and lower the entire expansion link + eccentric assembly, which in the days before power reverse gear was physically difficult as engines grew larger and their components grew heavier. With Walschaerts, you only need to raise and lower the radius rod.