Steam & Excursion > Question about eccentric rod positioning for the experts

I've always wanted to ask this question, just never actually got around to it...

So I noticed most engines on the Boston & Maine, and Delaware & Hudson always had the eccentric rod arranged differently than most other railroads.To give an example, look at how Soo 1003's valve gear moves versus R&N 425's. Why was this? Was it purely aesthetic or were there operational benefits to one configuration over the other? Can't believe I'm only getting around to asking this now since I wondered about this even when I was a small child, watching Pentrex videos with my grandfather. He had a strange dislike for that valve gear configuration lol, he always said it made the engine look "lazy".

I'm not sure I understand what you consider unusual, or "lazy", nor what valve gear(s) you are talking about.  I don't know what valve gear the R&N 425 has, but SOO 1003 has inside admission indirect action Walschaert gear.  I'm guessing you're asking about Walschaert valve gear and that you are puzzled over why some engines have the eccentric crank on the main driver leaning forward when the main pin is on bottom center, while on other engines the eccentric crank leans back under the same conditions.  Right?

If so, then, it's determined  by whether the cylinders are set up for inside admission or outside admission, and by whether the reverse mechanism is set up for direct action (forward when in lower half of reversing link) or indirect action (forward when eccentric rod is in upper half of reversing link).  The setup of the combination lever (the vertical lever immediately behind the cylinders connecting the motions of the piston rod and the valve stem) is based upon whether inside admission cylinders or outside admission.

Valve gear other than Walschaert may give different results.

I suggest you download a copy of the Dockstader simulations and study the motions of various valve gear, starting with Walschaert, to learn to understand this.
Animations of nearly every valve gear may be found at the Charles Dockstadter site. http://www.billp.org/Dockstader/ValveGear.html


-John



Edited 3 time(s). Last edit at 07/20/19 06:24 by LarryDoyle.

The vast majority of Walschaerts-gear engines
with piston valves used direct motion, where
the eccentric crank is roughly 90 degrees
behind the main crank (assuming inside
admission valves). Then the radius rod
is in the bottom of the link in forward gear.

But in the 1920s ALCo built a bunch of
indirect-motion engines -- they said
the valve action was somehow better.
The angularities cancelled better, or something.
Think they quit that by 1930.

(Looks like Soo 1003 is an example of that--
inside admission, indirect motion. So when
the right-side main crank is at 6 o'clock the
eccentric is at 9 o'clock instead of 3 o'clock.
Which means the radius rod is in the top of
the link in forward gear.)



Edited 2 time(s). Last edit at 07/20/19 14:13 by timz.

Thank you guys for the information, I understand now.