Monthly Archives: December 2011

13: fitting it together

Not much of an update. With the major parts of the valve gear made–slide valve, rocker, link, eccentrics and rods–I am now trying to fit it all together and tweaking the components. It’s tough–add half an inch here, take 3/16 of an inch there, to get the exactly right amount of slide valve motion and lead.

Here’s how the eccentric rods look connecting to the link. Below it is the reference photo from the same angle.

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The 1-degree incline

I was wrong! If you look closely when I completed the cylinder and saddle casting, you can see the 1-degree incline known to the Rogers locomotives, of which the DLR #1 and #2 are based on. On the closer look, you can see that although the cylinder is inclined, the saddle is not. I did this intentionally because it made more sense at the time to assemble a flat-decked saddle onto the frame and the boiler on it.


I just recently came across a drawing showing that the Rogers really did have inclined saddle deck. This was a nightmare trying to incline the saddle with all the ports into, along with figuring out how to cut horizontal surfaces for the frames and the boiler.


One day later, the steam chest and valve rod are also inclined. This results in the rocker arm being slightly shorter than previously designed (6.62in).

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12: steam chest

Just a quick update today. The steam chest and its packing are in place.

I think the bolts make it look like the Pantheon, with Roman columns surrounding the perimeter.

With transparency turned on, this is a great opportunity to see the inner workings of the steam chest. Here, the 2 valve stem collars at the gland packing are highlighted in blue. The horizontal bolts are used to tighten the exterior collar into the packing, creating a seal at the valve stem.

(The actual packing is not modeled).

Another interesting finding today. Meyer suggests that the pin joining the rocker arm and the valve rod should be tapered to allow the pin to be removed later. But, the CK Holliday photos also show that the Disney engineers use a bolt-cotterpin-slotted nut assembly here, which generally are not tapered. It’s certainly possible to manufacturer’s own tapered bolt, then machine thread the end to receive the slotted nut. But, wouldn’t that make replacement and repairs time consuming?

Anyway, here is the connection. I’ve decided to go with tapered bolt, but also with cotter pin assembly.

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Knuckle investigation

In case of a locomotive with short valve rod, the traditional practice is to place a knuckle joint in the rod to allow for additional flexibility. The diagram below shows that the valve rod must be able to move in vertical direction, because it’s connected to the rocker arm which moves in an arc.

So indeed the original CK Holliday drawing shows a knuckle joint just aft of the cylinder.

But in looking at the pictures of the “contemporary” CK Holliday, we see a different connection:

It looks like the flexible knuckle joint has been replaced with a rigid, male-female thread type connection; the male valve stem is screwed into the female valve rod, with a hex nut to tighten the connection. And it appears to be adjustable. Could this be used to allow the engineers to finely tune the valve slide’s travel, by shortening or lengthening the valve rod?

And is it even possible to have a rigid connection here?

Below is a graph of the rocker’s motion through 45 degrees of travel derived through geometry. Red graph is horizontal projection of the rocker’s arm, and the blue is the vertical.

From the valve slide motion study, the maximum amount that the valve needs to travel, in one direction, to allow full exposure of the steam port is 1.13in.

Using the graph, or directly solving, the rocker will need to rotate, in one direction, 7.84 degrees to allow the valve slide to travel the distance it needs. This also results in a vertical deflection of the rod in amount of 0.077in. That’s about 6.88% of the rod’s diameter, or L/401.

So, it certainly works, as the induced stress on the rod and the rocker is very minimal. But, still, it seems like an unnecessary stress.

The computer model needs that rotation degree of freedom to function properly. The above shows that the model is rigid.

Besides, I wanted to do the CK Holliday as it was intended anyway. This will bring the model much closer to the 1955 build. So, back to the knuckle joint!

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Slide valve motion study

With the steam, exhaust ports, cylinder, and steam chest seat modeled, now we have to consider the size and motion of the slide valve, which will regulate the steam admission into the cylinder.

Meyer talks about this extensively (mostly about relating the motion of slide valve to the eccentrics), but the subject can be compressed into the following three “rules”:

1: “Steam must be admitted into the cylinder at one of its ends only
at one time.”

Below is a cross section through the cylinder and the slide valve. The blue solid is the slide valve. Its arc in the middle is the exhaust cavity. The two tubes curving out left and right are the steam ports, terminating at each end of the cylinder. The trapezoidal cavity in the middle is the exhaust.

Ok! Let’s take a look at the slide valve motion:

The first picture shows the 2 “legs” of the valve covering up both steam ports. The second shows the slide valve at its extreme travel (exact distance is not known yet, since I haven’t designed the eccentrics), exposing only 1 steam port (left) while covering the other. Looks like the slide valve meets rule 1.

2: “The valve must allow the steam to escape from one end of the
cylinder, at least as soon as it is admitted into the other end of the cylinder.”

To satisfy this rule, the distance between the inner face of the valve’s legs must satisfy the following equation:

distance = 2 x Width(bridge) + Width(exhaust)

Plugging in the “as-virtually-built” measurements:

2*(0.625) + 1.25 = 2.5in

The picture below shows the measurement of this distance satisfying the 2.5in rule.

What this means visually is that the valve should allow the steam at one end of the cylinder be evacuated (via the exhaust cavity and port) before allowing the steam to enter the other end of the cylinder. This is important to eliminate backpressuring the piston.

The picture below shows that the left steam port would be exhausted before the steam is admitted into the right side. Looks like this rule is satisfied!

3: “The valve must cover the steam ports so as not to allow the
steam to escape from the steam chest into the exhaust port.”

This rule is satisfied when the exhaust cavity is not greater than the length given in the above equation. As you can see, it is not! So, this rule too is satisfied.

Visually, in any of the pictures above we can see that at no time can the steam from the chest escape into the exhaust.

So, indeed, the slide valve satisfies all three rules!


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11: saddle assembly

With the left-side of the saddle made, it then gets copied over to the right side. There’s a bit of tweaking involved since the cylinder has a 1 degree tilt, so both sides are not complete “mirror” of each other. Like everything so far in this project, that would make it too easy!

As shown above, the saddles are bolted together at the centerline, completing the “cradle” that will receive the boiler later. The saddles are bolted to the frames just inboard of the cylinder. They can be seen in the see-through section below, along with the piping in the casting.

Here’s comparing the layout to Lingenfleter’s:

Pretty close!

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10: saddled up

The half-saddle is fairly complete now. From the outside it doesn’t look like much. There is a half-cradle for the boiler (smokebox), a bolt-tab at fore and aft to receive the other half of the saddle, and steam/exhaust ports.

With the x-ray vision, changes are more obvious.

The cast connecting the boiler to the cylinder has 3 ports running through it: the exhaust port in the middle, straddled by 2 steam pipe on either side.

The front view above shows the 3 ports running from the inboard end, curving down and running outside, and up again terminating at the cylinder.

The side view above shows the ports lay out. What looks like 2 “eyes” are the steam pipe, and the “nose” in the middle is the exhaust.

For comparison purposes, here’s a cross-section plan I referenced, taken from Meyer.

In Fig 13, label “J” marks the steam pipe, and “K” the exhaust.

Much of the time I spent making this part was just trying to visualize what is happening inside the cylinder and casting. Before CAD was invented, these old plans try to compress as much information as possible into few drawings. The practice these days is to have many drawings to communicate the idea, since producing drawings is much easier now.

And of course, they did not make 3d sections either.

But when everything is drafted correctly, it’s quite obvious! Here’s my 3d section through the middle of the saddle. In this section, the exhaust port is shown. Compare its similarity to Meyer’s Fig 13 above.

And below is a section through the 3 ports at the cradle. The middle is the exhaust. The long pipe above it splits the incoming steam pipe into 2 branches, terminating at fore and aft of the cylinder, supplying the steam to the steam chest.

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