Steam & Excursion > Steam Question - Backward Eccentric Crank

Doing some reading in William Edson and H.L.Vail, Jr.'s book, "Steam Locomotives of the New York Central Lines Vol.2", Page 403.

I have always liked the K Class Pacifics, especially early ones, and came across this photo of NYC 4833, a K-2a Class of 1907.
The caption says, "Note backard eccentric crank." 

So my question is, looking at any particular steam locomotive image, what is the clue or visual tell-tale of whether the crank is forward style or backward style?



Edited 1 time(s). Last edit at 11/18/25 12:04 by train1275.

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Usually inside and outside admission determines the position of the crank.

Trains Magazine May 1984 Page 34 has an excellant article with pictures and diagrams on this subject. Also, some good info if you Google your subject

Well I have googled it and have a small technical library on steam locomotives. I do not have the Trains issue mentioned.

With that said, I can't wrap my head around how by looking at the photo I can determine inside vs outside admission and the crank being backwards or not.

Just looking for a few pointers.

Posted from Android

train1275 Wrote:
-------------------------------------------------------
> Well I have googled it and have a small technical
> library on steam locomotives. I do not have the
> Trains issue mentioned.
>
> With that said, I can't wrap my head around how by
> looking at the photo I can determine inside vs
> outside admission and the crank being backwards or
> not.
>
> Just looking for a few pointers.

Just my opinion but, since the main rod & piston are in the full forward position, i.e. full stroke, the eccentric crank is obviously not in the "normal" position as when the rods are down and the main rod & piston is in the centered position. Sort of a misleading photo & description, in my opinion.

Thanks HotWater

Maybe you have something there with the caption being slightly misleading. I see what you mean.
If that's the case it's kind of a strange thing to point out I guess.

Posted from Android

Here is a prior thread that may help . https://www.trainorders.com/discussion/read.php?10,1812445

The picture is not misleading as the eccentric crank is reversed from common practice. This is an outside admission locomotive as opposed to the common inside admission. Inside admission with direct motion has the eccentric crank following the main crankpin by 90 degrees. It’s easiest to see when rods are down the crank would lean 45 degrees towards the cylinder. On this engine it would lean 45 degrees back.

Posted from iPhone



Edited 1 time(s). Last edit at 11/18/25 17:32 by bmoore765.

Don't hunt too hard for the 1984 Trains. The article isn't that great.

You asked about determining inside-outside admission.
That's easy, long as you have a good side view of the engine.
Is the valve stem connected to the top of the combination lever?
If so, outside admission, like a slide valve. (Or some piston valves.)
So, 4833 has inside admission, as you'd expect.

(If you want the explanation of the above, speak up.)

4833's piston is all the way forward. Since the engine is inside admission,
where will the valve be, if the engine is in full forward gear?
Forward of center -- you understand that, right? In the pic,
the valve isn't forward -- it's a bit back of center.
Which means the engineer has it in reverse gear.

Usually, full forward gear on Walschaerts means the back
end of the radius rod is in the bottom of the link. Advantage
of that is, if you're rolling down the track and something breaks,
the radius rod drops into full forward gear. On the 4833,
it drops into backward gear.

For a decade or two, ALCo thought valve events were better
(somehow) with gear arranged like this, and many? ALCo
engines had "backward" eccentrics. Think they quit that
by 1930.

Okay, so we need the explanation about inside/outside
admission. What's the purpose of the combination lever,
the verticalish rod just back of the cylinder? To move the valve
off center by (lap plus lead) when the piston is all the way
forward, so the valve opening then equals the lead -- 1/4 inch
or whatever. With inside admission, the valve has to be
ahead of center. When the piston is all the way forward,
the bottom of the combination lever is all the way forward,
and on 4833 the valve stem is forward, because the valve
stem is below the top of the combination lever. If the engine
were outside admission, the valve stem would connect to
the top of the combination lever, so the valve would be rear
of center when the piston was full forward. As you would
want, with a slide valve.

Find a pic of a slide-valve engine with Walschaerts -- see
how the valve stem connects to the top of the combination lever?



Edited 2 time(s). Last edit at 11/18/25 17:58 by timz.

Say what?
I think my brain just broke. Steam locos have just too many thing a majigs and whirly parts.
At least I have a full understanding of internal combustion engines!
Gonut

Gonut1 Wrote:
-------------------------------------------------------
> Say what?
> I think my brain just broke. Steam locos have just
> too many thing a majigs and whirly parts.
> At least I have a full understanding of internal
> combustion engines!
> Gonut

What?????

bmoore765 Wrote:
-------------------------------------------------------
> The picture is not misleading as the eccentric
> crank is reversed from common practice. This is an
> outside admission locomotive as opposed to the
> common inside admission. Inside admission with
> direct motion has the eccentric crank following
> the main crankpin by 90 degrees. It’s easiest to
> see when rods are down the crank would lean 45
> degrees towards the cylinder. On this engine it
> would lean 45 degrees back.
>
This thread has taught me something new and my comment above is not accurate. A backwards eccentric crank is not necesarilly indicitive of an outside admission locomotive. It seems what we have here is an example of inside admission with indirect motion. The orientation of the eccentric crank is still reversed from common practice as the photo caption calls out.

I'm learning, a bit slow, but learning, thanks timz.

So, so far we can establish this is inside admission. I took what timz posted and referenced "Basic Steam Locomotive Maintenance by D.C. Buell and Railway Locomotive Maintenance, Steam Locomotive Valve Gear by F.E. Kardes, M.E. 1942 reprinted by the Tourist Railway Association in 1995.

The radius rod is connected to the combination lever above the valve stem.

Now the part I am still struggling with; the piston is forward and the combination lever is angled inward towards the bottom, and angles outward vertically as it goes up towards the valve stem. 

In a forward eccentric crank style, the radius rod would be at the bottom of the Walschaerts link when in full forward motion and the eccentric crank will follow the main crank pin.

In a backwards eccentric crank, the radius rod will be at the top of the Walschaerts link in full forward gear and the main crank pin will follow the eccentric crank.

Do I have that correct ?

So in the above example as timz has previously stated the locomotive is hooked up in reverse.

Now .... if I have assimilated and understood this so far .... I get it .... except for the fact I know this to be true of this locomotive. But how do I tell by looking at it that it is set up backwards so that when the radius rod is in the position seen in the image that it is in reverse, not forward and the crank is actually backwards. That is my hang up. Now contemplating that the connection of the radius rod and combination lever, above or below the valve stem, does not neccessarily define whether the gear itself is set up for forward or backwards eccentric. 

So that is where I'm at now with this puzzle.

As a follow up, refer to images 2 & 3 which show an NYO&W Class E ten wheeler. Note the combination lever and radius rod connect below the valve stem. So can I then infer that this is an outside admission engine, converted from slide valves (it was indeed built by Alco-Brooks in 1911 with slide valves) to piston valves ?

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Direct ("normal crank") vs Indirect ("backward crank") variations of the valve gear.

Mary McPherson
Dongola, IL
Diverging Clear Productions

> In a backwards eccentric crank, the radius rod
> will be at the top of the Walschaerts link in
> full forward gear and the main crank pin will
> follow the eccentric crank.

Assuming the engine is inside admission, yes.

By the way -- looking quickly at the pic of 4833,
the eccentric crank looks to be at 8 o'clock, but
it isn't. It has to be close to 90 degrees from the
main pin, and the main pin is close to 3 o'clock.

> how do I tell by looking at it that it is set up
> backwards so that when the radius rod is in
> the position seen in the image that it is in reverse,
> not forward and the crank is actually backwards.

Okay, you know how to tell it's inside admission.
Maybe you're not clear on what "inside admission" is.

You can envision a slide valve, and you know that's
outside admission. With such an engine, when the
piston is all the way forward, and you want to admit
steam to the front of the cylinder, you need to move
the slide valve rearward, to uncover the front steam
port beneath the slide valve. An outside-admission
piston valve ditto -- it has to move rearward, since
the steam is ahead of the valve.

Maybe you know all that, and you know that an
inside-admission valve has to move forward in
that situation. So, how to move it forward?
On the 4833, the engineer has to lift the back
end of the radius rod toward the top of the link.

(As it happens, in the pic, with the eccentric crank
near 6 o'clock, the link is "aimed" just about at the
top of the combination lever, so as long as the engine
stays stationary you could move the back of the radius
rod to the top or bottom of the link without moving the valve.
In other words, the radius of the link's curve always equals
the length of the radius rod. You've heard Walschaerts
described as a "constant lead" gear? That's what that means.
When the piston is at one end of the cylinder, the valve
position doesn't change when you change from forward gear
to reverse gear. With an inside admission engine, the valve
is forward-of-center a distance equal to the lap plus the lead,
so the forward steam port is open 1/4 inch or whatever
the constant lead is.)

If the piston's at the front end of the cylinder of the cylinder,
and you want the engine to move forward ... like I said, the
front steam port is already open a bit, and you want it to
open wider as the engine starts to move. So you want the
inside-admission valve to move forwarder; on the 4833, that
means the engineer needs to lift the back of the radius rod.

> an NYO&W Class E ten wheeler. Note the combination lever
> and radius rod connect below the valve stem. So can I then
> infer that this is an outside admission engine

Right. There's a question -- when slide-valve engines got
converted to piston-valve, did they always stay outside
admission? Or just usually? Dunno.

 

Thanks timz.

I am getting there. Yes, I do understand outside and inside admission.
And I understand your scenario of the radius rod and the valve.

But I am still wondering if I am walking through an engine terminal, or more realistically looking at photos, how do I look at a locomotive and say, "hey that one has a backwards crank!" I understand what you are saying about the subject locomotive 4833, but what I understand comes from knowing that the locomotive is backwards. I need to get to the next step which is to determine which style I am looking at. Basically the difference between a backwards crank loco hooked up in reverse vs. a forward crank loco hooked up in forward .... or more to the point maybe if both are set at center.  Does that make sense ?  Once I know which way a locomotive is configered I can go from there. It is the initial knowing which I am looking at (aside from the outside vs. inside admission).

So initially looking at the photo of the 4833, knowing nothing about it but a visual glance, and being quizzed will it go forward or reverse. Is the clue with the combination lever?
Relative position of the crank related to the piston?

Several images attached:

1. Slide valve, outside admission - Basic Steam Locomotive Maintenance - Buell, Page 156

2. Piston Valve, inside admission - Basic Steam Locomotive Maintenance - Buell, Page 157






 



Edited 1 time(s). Last edit at 11/19/25 15:06 by train1275.

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Forward vs. Backwards Eccentric Crank

Which is which and why .... which way will it go ? (wasn't that sort of what that dumb cartoon dog Willoughby used to say?  I can't recall the cartoon. Fox and hound or something). "Which way did he go?"

 

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One more.....

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When Edson and Vail said the eccentric was backwards,
they just meant it was leading the main crank -- the 4833's
right-side crankpin was at 3 o'clock and the eccentric crank
was at 6 o'clock. Far as I know that's all that "backwards" ever
means. You've noticed that most engines weren't "backwards".

If ALCo is building an engine with inside admission valves
circa 1920, and they like to  have the radius rod in the top
of the link when the engine's in forward gear, then you see
how the eccentric crank has to be "backwards"?

You heard about "direct motion", which was the usual --
when the eccentric rod is pushing forward, the radius rod
is pushing forward too (when the engine is in forward gear)
since it's in the bottom of the link. Note one disadvantage
of indirect motion: more wear on the pivot of the link. When
the eccentric rod is pushing the bottom of the link forward,
the radius rod is pulling the top of the link forward. In direct
motion, no such problem.

Guess you know PRR 5495 is as straight-vanilla as you can get:
inside admission and direct motion. Ditto Frisco 184.

You're asking is 3340 in forward or reverse? It looks to be
inside admission -- agreed? And the eccentric crank trails
the main crank -- if the engine moves forward a couple feet,
the crankpin will be at 12 o'clock and the eccentric will be at 3.
So, more straight vanilla.

The valve on 3340 is forward of center, so it's admitting
steam to the front of the cylinder, so the piston will turn
the drivers backward.

Not 100% certain, but the SR 4-6-2 looks straight vanilla too.

It occurred to me to wonder -- you do know that the link
rotates around a pivot that's fixed on the engine? That's
not that obvious, just looking at pics.

No help with these pics, but often you can see the
linkage between the rod from the cab and the radius rod --
you can see that when the engineer pulls his rod back,
the radius rod rises to the top of the link. Far as I know,
when the engineer's rod moves rearward, that always
means he's putting the engine in reverse. Think that
goes for power reverse too -- if the power reverse rod
goes forward, the engine's going into forward gear.
(Do any engines have backward power reverse cylinders,
ahead of the thing they're connected to?)



Edited 2 time(s). Last edit at 11/19/25 17:12 by timz.

Ok, timz, I think it has finally sunk into my thick skull. Thanks for the valve gear tutorial.

I take my hat off to the likes of Egide Walschaerts, Abner D. Baker, Otis William Young, William Sherman Brown and all the others who pioneered and developed this technology.