UP Steam & a Question

Apparently not, as it worked well on other Challengers also. You do realize that the piston rods are hollow, don't you?

I just learned something myself. I never knew the piston rods were hollow, I always assumed they were solid.

Yea, now that you mention it that makes sense that they are hollow just like propeller shafts on ships - submarines. 

Noticed a long while ago, but it never bugged me.  Figured it was done for a reason.

 

Being hollow, makes them stronger. That is why axles on modern high horsepower steam locomotives are also hollow.

Yep they are hollow...you can see here in this pic of the 4018.

 

 

 

 

Indeed it was--to allow room for the pilot truck.

Do you mean the Engine Truck?

Believe it or not, driving wheel axles on the 844 are hollow, too. After a photo stop, I looked at her when her side was directly in front of me, some distance away. I noticed that I was looking right through all 4 driving axles to what was on the opposite side. That surprised me.

And that is how you understand binary.  Either you understand it or you don't.  

Yes; the extra length poses mechanical engineering problems that are answered by the piston rods being hollow. A hollow tube is much stronger that a solid bar made from the same material. In the case of these piston rods, hollow material was forged closed at each end to make the rods solid in the piston and crosshead attachment areas. And THANKS! to HOT WATER for keeping us straight on the "engine truck".

No, I mean pilot truck. Also called the pony truck sometimes. To differentiate it from the trailing truck.

Actually a "pony truck" is a single axle lead truck, as on NKP 765 for example. In the "modern era" of larger steam locomotives the two axle lead truck is indeed called an "Engine Truck", and NOT a "pilot truck".

The terms have been used in the past by others...Even our own Allen Miller.

 

Pilot truck

 

Pilot truck

 

Pilot truck

 

Pilot truck

 

Pilot truck

Sure. Generally all railfan publications.

 

However, the MEN who designed and worked on modern steam locomotives refer to the two axle lead truck as the "Engine Truck". Just because Allen Miller uses the "pilot truck" term when discussing toy trains, doesn't make the term correct for those that work on real, full size, steam locomotives.

"A hollow tube is much stronger that a solid bar made from the same material."

 

really?  what are the physics behind that?

Further to the discussion on engine truck vs pilot truck, Alfred W Bruce, VP Engineering and a director of steam locomotive engineering at American Locomotive Company, in his book, The Steam Locomotive in America, Its Development in the Twentieth Century, used only the term engine truck.  He did differentiate between two-wheel engine trucks and four-wheel engine trucks, see pages 246 through 250.

 

ChipR

 

It has to do with it's moment of inertia.  Because it is hollow it has a lower MOI relative to it's diameter than a solid shaft.  Because it has less mass also, it's relative strength is greater.  It's something you learn in upper division mechanical engineering classes in college.

Hollow axles are no longer used because of the difficulty of inspecting the interior surfaces for defects. When Metro North put a wheel through the side of a church the hollow axle was finished. WAMTA also had a hollow axle failure due to a lap that could not be seen or detected. Most of the hollow axles that are still being used are being gun drilled rather than pierced and drawn.

 

Sinclair, thanks for clarifying the advantage of tubular rounds. 

"WAMTA also had a hollow axle failure due to a lap that could not be seen or detected. Most of the hollow axles that are still being used are being gun drilled rather than pierced and drawn."

 

creating more questions than you answered.

 

"lap" ??

 

gun drilled?  pierced and drawn (is this a medieval torture technique)?

 

 

Gun drilled I'm guessing means they take a solid tube and drill out the middle, like you would with a gun barrel.  And I'm sure pierced and drawn is a form of extrusion.

"A hollow tube is much stronger that a solid bar made from the same material."

 

well, i posed the question to a very fine engineer i know and got this response, which makes a lot of sense to me.

 

 That claim could be misleading. Strength to weight ratio is much better for a hollow tube than a solid bar for supporting compression or cantilever loads, so you can make a larger diameter tube of the same weight with better resistance to bending or buckling. However, you cannot increase strength in any measure by hollowing out a solid bar, it would be weaker. Also, if the bar is being loaded in tension (hanging a weight, being pulled from each end) there is no advantage to a hollow tube.

Let me say it this way. " A hollow tube resists deflection better than a solid bar of the same material." Let's remember that the piston rod is subject to alternating tension and compression from the piston thrusts. On a Big Boy this could be approx. 75,000 pounds of alternating tension and compression several times a second when pulling wide open at higher speeds. True a hollow tube is a better structure when considering strength to weight ratio. And it is weakest in tension because of it's reduced cross sectional area. It is GREAT in compression!

Well, it obviously works!

And don't forget - the drivers are designed so lead can be poured in there to adjust the counterweight.  The long piston rod has to be heavier than the short one, but the extra weight may be absorbed in the crosshead guide, and not borne by the main driver.

 

I am continually impressed by the achievements of engineers from the first half of the twentieth century.  The mathematical analyses of what goes on in a modern steam locomotive is probably worth an advanced degree.

 

I am with Hot - I have always seen "Engine Truck" and "Pony Truck" in real railroad literature.  Terms like lead truck and maybe steam chest to refer to the entire cylinder assembly are apparently modeler's terms.

I've heard pony truck and pilot truck, both in conversation and in literature.

To bob2 - Counterbalance formulas take into account only half of the reciprocating weight of the crosshead shoe, piston and rod, and the front half of the main rod. So the difference of weight in a longer piston rod is not that great.

These weights are worked into a cross-counterbalance design where counterweights on the drive wheel are not directly opposite the crank pin - and they cross counterbalance the recip weights from the opposite side of the engine.

It still amazes me at the engineering that went into these engines. 

Too bad most of it has been lost.  I would love to take classes on it all to learn how to design them.

 

It's been a while since my dynamics classes, but I think this gives a misleading impression.

 

The concept is the Polar, or Second, Moment of Inertia. The general idea is that, in a load-bearing member (axle, floor joist, bridge beam, lalley column, etc.), the parts of the member farther from the central axis are more efficient at carrying the load while staying within deflection specs.

 

If you have a basement, somewhere in the support structure down there you'll find a hollow steel column holding up a beam. Let's say it's 3" in diameter, and has a thickness of 1/8". A solid column that could support the same load without deflecting would have a smaller diameter - let's say 2". They are the same strength, and can support identical loads, but the hollow column contains less steel, so it's lighter, easier to work with, and costs less.

 

An I-beam contains less steel than a rectangular member that can handle the same stresses, so it costs less and is lighter. As a result, the concrete footings for a bridge made of I-beams can be smaller (because the bridge is lighter than it would be otherwise), another source of savings.

 

The engineered floor joists that have replaced 2x12s in home construction have the same advantages (in addition to being made from reclaimed materials in some cases).

 

In the case of a locomotive axle, a hollow axle (that might be slightly larger diameter than a comparable solid axle) might be used to reduce axle loadings, or it might be used to reduce the cost of the axle.

 

Minimizing cost is usually an important part of engineering and design, so engineers gravitate toward hollow cylinders and and I-beams instead of solid cylinders and rectangular beams.

 

 

i've never heard of the term "engine truck" before.

 

pilot truck would seem like the most apt designation as to 'pilot' is to "steer or control the course of" by definition; the basic function of the lead truck of a locomotive.  in fact it wouldn't surprise me to find that the pilot (aka cowcatcher[arch.]) was actually named after the function of the adjacent truck and not the other way around.

 

just a thought, but since the trailing truck sometimes contained a booster "engine" (in the "modern era") i'd think that would be the more logical assumption.

While I have certainly heard and used the term "pilot truck" often--even by people who work with steam--I have to admit that a review of my collection of nearly 50 period steam textbooks, dating from the 1890s through the 1940s, seem to use the term "engine truck" exclusively. While I haven't scrutinized every page of every book, I have yet to come across the term "pilot truck" except by later authors, such as John White.