Steam & Excursion > What determined angle of eccentric crank?

In looking at various steam locomotives, the angles between the eccentric and main/side rods vary. If the main driver's crank pin on the engineer's side is at it's 6 o'clock position (bottom dead center?), some cranks lean to the left (advance) or right (delay) when the wheel rolls forward. What insights can be gained into the arrangement and timing of an engine's valve cylinder by noticing the angle of the eccentric crank? Would I be correct in thinking that the various types of valve gears "required" specific angles on the eccentric crank?

Thank you,

* JB *

http://www.trainweb.org/tusp/libra.html

In Walschaert valve gear, the ahead or behind was determined by whether the vavle was inside or outside admission and the position on the link operating the radius rod.


I realize now that it's all relative...to where one calls zero, 'ahead' or 'behind', so I've edited my original post. Assume a "normal" set up is as follows:
(you're looking at the engineer's side with the front of the locomotive pointing right)
You've got piston valves (inside admission) and you want to run forward with the radius rod in the lower position in the link...If you put the driver crank pin down as close to the ground as possible, then your eccentric crank would point up and to the right (1-2 o'clockish). If you change to D valves (outside admission) OR you want to run forward with the radius rod in the upper position in the link, you've got to flip the eccentric crank 180 degrees (pointing toward 7-8 o'clock with the crank pin down closest to the ground)

The Walschaert Locomotive Valve Gear by W. W. Wood is available on Google Books
http://books.google.com/books?id=XAEpAAAAYAAJ&printsec=frontcover&dq=locomotive+valve+gear

As you maybe know, full forward gear on a
Walschaerts engine usually means the link
block (back end of the radius rod) is in
the bottom of the link-- but at one time
ALCo (at least) thought the valves would
work better if full-forward used the top
of the link, and many of their engines
from around 1920 did so. Assuming inside
admission, that requires the eccentric
crank to be "backwards" too-- i.e.,
leaning toward the rear of the engine
when the main pin is at 6 o'clock.

With the eccentric crank in the tailing position it would be about 10 o'clock not 8. Actually if I remember correctly (without looking it up) it would be about 22 dgrees off center either to the left or right depending on inside or outside emission.

If the main pin is at 6 o'clock, the other end
of the "eccentric crank" (i.e. the back end of
the eccentric rod) will be at about 9:00 or 3:00.
It would be exactly at 9:00/3:00 if the link foot
were level with the driving axles.

OK, take a look for yourself. Thr rods on 616 are almost at 6 o'clock and note that the eccentric crank is right at 10 o'clock.

---

---

These three drawings, make with the help of Dockstadters programs, show various configurations of Walschert valve gear.

1) Outside admission, little used with Walschaert gear. Note the eccentric crank at 9:00. The radius rod is in the lower half of the link for forward motion.

2) Inside admission, direct acting. Eccentric is at 3:00, and the radius rod is in the lower half of the link for forward motion.

3) Inside admission, indirect acting. Eccentric at 9:00, and radius rod in the upper half of the link for forward motion.

---

---

---

> Thr rods on 616 are
> almost at 6 o'clock and note that the eccentric
> crank is right at 10 o'clock.

Define "at 10 o'clock".

We agree the pin at the back of the
eccentric rod is not at 10 o'clock?
(Assuming the main pin is at 6:00.)

Theoretically, the crank should be at 90 degrees to the main pin, which would put it at 3:00 or 9:00 with the main pin at 6:00.
This angle may vary a very few degrees (not as much as the difference between 9:00 and 10:00) to ensure equal steam distribution in both directions, if necessary due to angularity caused by positioning of the link trunion and the offset of the link. The exact angle is figured with the main pin at dead center, setting the crank 90 degrees to a line drawn from the center of the main axle to the pin where the eccentric rod drives the link. See line x-y in the drawing below.

---

That assumes the gear is intended to give
constant lead, which it usually was.

My bad on the 180. You know, just flip it to the other side. ;)

Looks like Tom and you are talking about the same thing only different. Tom is referring to the angle of the eccentric crank, such as a line drawn from the main pin up through the center of the crank, and the other guy is talking about the position of the pin on the end of the eccentric crank where the eccentric rod is attached. Essentially you're both right.
The critical question is: If an engine with inside admission left Detroit with a train of 14 coaches and an engine with outside admission left Chicago with 75 freight cars, what was the callboy's name in Indianapolis?

If he walked with a limp, it was Harold Fudpucker!

Cut and past this link and you can see it all in action.


http://en.wikipedia.org/wiki/Walschaerts_valve_gear

Jerry

These are generally referred to as "direct" and "indirect" linkages, but without going into massive (and boring) details there were variations which put the lie to those terms.

What I'd like to know is why the builders determined it advantageous to undertake either design under any given circumstance. I have a large collection of period literature but still haven't found what I'm looking for in this sense.

I disagree the assertion that Alco decided they liked indirect gear. I don't think it was that simple. I believe it was more a collaboration of their engineering staff with those of the various railroads. As an example I will offer the Union Pacific's 0-6-0's constructed from approximately 1912-20. I don't know why, but these engines were constructed with indirect gear (radius rod in upper half of link in forward motion) at a time when Union Pacific was construction no other power with such arrangement, and these engines came from several builders.

Each type, direct and indirect, apparently had advantages and it seems to me the general use of the indirect arrangement was more road-specific than builder-specific (witness some of the Northeastern roads as well as Canadian and Canadian-controlled roads). One disadvantage of the indirect standard Walschaert gear was fact radius bar would drop into lower half of link under certain failure conditions which, of course, meant immediate jump to full reverse. At speed this could and I'm sure did prove interesting.

One thing I found intriguing while doing some research years ago was fact that Alco, during the early years of the 3-cylinder push, arranged many of these engines with indirect gear. There was clearly some reason for this, only I don't know what it could have been. I do know that Union Pacific changed their first 4-10-2 from direct to indirect in fairly short order and that all their remaining 3-cylinder engines were direct. Yet they kept the 0-6-0's indirect their entire lives.

I sure wish I could find some data on this.