Steam wheel configuration questions

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Steam wheel configuration questions
Posted by zardoz on Monday, April 02, 2018 8:57 PM

I am starting to get interested in steam locomotives. I find them to be fascinating.

My question (of which I have many) is in regards to the wheel arrangement and the advantages/disadvantages of each, such as 2-6-2 vs. 2-8-2; 2-8-4 vs. 4-8-4, etc.

My limited understanding is that the front non-driving wheels were to help quide the locomotive, and the rear non-drivers were for weight distribution. If I'm way off base here, please don't laugh too hard - I'm just learning.

If the answers are too involved for posting here, perhaps someone could direct me to a knowledgable source.

Thanks.

 

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Posted by Overmod on Monday, April 02, 2018 9:54 PM

There is much more to the subject than goes in non-MEGO posts.

The four-wheel swiveling pin-guided 'Adams' lead truck or engine truck was the defined 'standard' for high-speed use; ATSF had one of the better suspension and side-bearing arrangements in the Batz truck design.  Note that what an Adams truck does is very different from a modern diesel or electric truck, as it must also effectively steer the chassis into and out of curves and, in good designs like the N&W J class, control inertial and thrust nosing/hunting.

The 2-wheel lead truck is nominally geometrically unstable (try pushing a rolling suitcase vs. towing it) but with some care in designing the lateral motion and how it couples into the equalization you can get very good speed out of these; Nickel Plate and NYC/P&LE A2a's being two examples, and the LS&MS Prairies remaining controversial to this day.

Next we come to the leading driver pair and its flange profile.  This is the great land of the lateral-motion device, which provides damped cushioning of the remaining steering moment not handled by the lateral-motion arrangements on the lead truck.  Other drivers may have lateral clearance or lateral motion for curve tolerance; the precise profile of the driver tire treads can be important (google Porta's HAWP for an example)

Some interesting stuff goes in with trailing trucks, which steer the back of the frame as well as support the heavy water-surrounded firebox, chamber, syphons, etc.  while also necessarily accommodating ashpan and radiated heat.  Once designers got beyond just providing smaller carrying wheels in rigid pedestals, some amazingly intricate linkages were developed, but the great innovation came with the second design of Delta truck, which provided easy inherent equalization, good tracking, and both support and steering almost as far back on the chassis as geometrically possible; arrangements that used the locomotive's weight rather than levers or springs for progressive restoring force could be applied easily.

You may want to study the history of Will Woodard's articulated trailing truck, which turned out troublesome.  There is a formula to calculate the axle spacing and pivot-pin placement for a proper trailing-truck design, but this may not accord with required weight distribution on the truck; Woodard used a long articulated frame which essentially acted like a long Delta truck and let the forward axle slide nearly frictionlessly from side to side on hardened rollers.  This was lovely going forward but alas! Freights often have to back consists through switches, and the articulated setup did not like doing that much at all.

You May have figured out by now all the reasons why a pin-guided truck, so good at the cylinder or leading end, is so poor as a trailing truck.  When the Germans designed a high-speed bidirectional tank engine in the '30s they had to provide actual relocation of the pin geometry with compressed air assist to make the locomotive stable.

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Posted by M636C on Tuesday, April 03, 2018 12:57 AM

You could do a lot worse than go to:

https://www.p2steam.com/design-study/

If you scroll down to the diagrams showing a "Pacific" and a "Mikado" of roughly the same size and weight on relatively sharp curves, you can see the calculated vertical and lateral forces from each axle as the loco goes around the curve.

The link-type leading truck was largely similar to that used in the USA.

That might give you some pictures to go with Overmod's text above.

I find pictures are a great help....

Peter

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Posted by nhrand on Tuesday, April 03, 2018 11:17 AM

Here is a simple example of why one wheel arrangement may be better for a railroad than another.  Assume the railroad needs a new freight locomotive for relatively slow speeds so decides a two wheel leading truck will suffice and there will be no need for a trailing truck because a modest firebox will produce adequate steam.  The types that may do the job would include a 2-6-0 Mogul, a 2-8-0 Consolidation or a 2-10-0 Decapod.  The locomotive with the fewest drivers will usually be preferred since it will need less maintenance and cost less.  Now consider how much starting power you need for the grades and tonnage you need to haul.  A  calculation might show that a locomotive with a starting tractive force of 45,000 pounds would be about right.  However, to provide adequate adhesion, the weight on drivers should be at least four times the tractive force or about 180,000 pounds on the drivers.  If a Mogul is chosen, the weight on each driving axle would be 60,000 pounds which would be too much for your rail size and bridge limitations.  A Decapod might be selected because only about 36,000 pounds would rest on each driving axle and consequently would not tax the bridges or light rail on the branches.  The Consolidation would place about 45,000 pounds on a driving axle and be a good choice for average road conditions.  In short, the wheel arrangement chosen would be decided by the service and limitations of the roadbed and bridges.  

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Posted by zardoz on Tuesday, April 03, 2018 11:22 AM

Gentlemen, thank you for your responses. You have given me a good start on my learning process.

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Posted by Overmod on Tuesday, April 03, 2018 12:03 PM

As a small note to add to nhrand's analysis, consider the development of boosters of various kinds. 

A booster applies effective tractive effort to a trailing axle (which would otherwise be only weight-bearing) and, at low speeds where the engine cannot utilize the full evaporative capacity of the boiler in its cylinders, gives the effect of an additional coupled axle in TE.  This permits a couple of interesting things:

You can start a longer train, or more of that train at the same time if using the slack to help start, and you can accelerate that train faster up to where your horsepower curve from 2-cylinder DA takes over.  (Note that Franklin progressively improved booster speed as the horsepower curve shifted to peak at a higher road speed with evolving design)

You can handle short grades, or critical grades that would otherwise require pushing, without the additional axle and associated complication and weight.

You can restart more easily if you are loaded close to critical weight on a ruling grade and stall.

Now, up to the late 1920s, the more coupled axles you had, the slower the engine was likely to be in service.  That is in part a consequence of the inertial mass of the required side-rods, which is one of the criteria leading to development of duplex locomotives in the 1930s.  So pure augment concerns alone might lead you to keep the engine to eight drivers with booster if you needed the general capability of a ten-coupled engine.  But the advantage does not stop there.

One significant element of steam design is to have long flame and combustion-gas travel in the firebox and chamber.  There is an enormous advantage in having an 'arch' and very low grate position in accomplishing this.  Now, a ten-coupled engine will either be a Decapod, which must have its firebox either between the drivers or completely above them, or must be longer in order to gain the same advantage as, say, a 2-8-2 with Delta truck and booster. 

Note that until you have the advantages of the Delta over the composite trucks -- most specifically, the hard pivot point with strong frame in compression, the self-steering effect at the rear, and the robust equalization -- a booster on a trailing axle will be a squirrelly thing at best.  It could be argued that even the 'first' design of Delta trailing truck was not well-suited to adaptation that way.

On the other hand, if you did not have compelling reasons to use boosters (or your crews were ham-fisted in general and didn't use them well) they wouldn't help much.  And they were not particularly economical, at least as Franklin built them.  Peter Lewty's arrangement solved most of the obvious problems with the Franklin design, but of course big mainline steam that would use boosters was pretty much dead by then.

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Posted by nhrand on Tuesday, April 03, 2018 7:03 PM

A booster was a very useful device and improved the performance of many a steam locomotive.  And, if you didn't have a Delta trailing truck to apply it to, you could add a booster to the tender  --- maybe even two if you needed a good hump engine.  A favorite of mine were the MoPac Atlantic 4-4-2s that were equipped with a booster and performed much like a Pacific.  However, as Overmond suggests some roads didn't accept the cost or maintenance requirements and there was the probability that an engineer would not be inclined to take the trouble to operate one.  From some of the instruction books I've read, I have the impression that a booster could be a headache to operate, especially when you had other things to deal with.  Some engineers might take the easy way out and not use it.

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Posted by Firelock76 on Tuesday, April 03, 2018 7:34 PM

Getting interested in steam?  Wonderful!  This is a hobby and field of learning which will carry you all through life, and trust me, you'll never get bored.  There's some books I can recommend, starting with...

"The Steam Locomotive, A Century Of North American Classics"  by Jim Boyd.  Published in 2000 by MetroBooks, it's an easy read and fun romp through American steam history.  Lavishly illustrated in color and black and white, it's a great intro to the subject, Jim does a great job explaining the whys and wherefores and how things developed as they did.

Next is "How A Steam Locomotive Works"  by Karen Parker, published in 2008 by TLC Publishing, www.tlcrailbooks.com , and it's a good, easy to read and comprehend study of the mechanics of steam.

Ratcheting things up a bit on the seriousness scale, there's "Perfecting The American Steam Locomotive"  by J. Parker Lamb, published in 2003 by the Indiana University Press, http://iupress.indiana.edu , this gets more heavily into the design and engineering aspects, but not beyond the average person's ability to understand.  J. Parker knows his subject, he was the Chair of the Department of Mechanical Engineering at the University of Texas, Austin, and a lifelong railfan.

Top of the seriousness scale, and long out of print, but not too hard to find if you're willing to look is "The Steam Locomotive In America, It's Development In The Twentieth Century"  by Alfred W. Bruce.  Originally published by W.W. Norton and Company in 1952, it's Mr. Bruce's story of steam development, and he should know, he was there!  A long time ALCO employee, Mr. Bruce doesn't miss much.

I'd guess you can find the first three books I mentioned on Amazon, or a local bookseller may be able to get them for you.  The Bruce book turns up fairly frequently at train shows, at least the ones I get to in this part of the country.

As a matter of fact there's a train show coming to Wisconsin July 7th and 8th of this year at Eau Claire.  www.InternationalToyTrainExpo.org  I don't know how far Eau Claire is from Kenosha, but it might be worth your while to go.  The best toy train shows ain't all just toy trains!

Good luck!

 

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Posted by nhrand on Wednesday, April 04, 2018 9:14 AM

I think the best technical book of all is, THE STEAM LOCOMOTIVE, Its Theory, Operation and Economics, Including Comparisons with Diesel-Electric Locomotives, by Ralph P. Johnson, M.E., Chief Engineer, The Baldwin Locomotive Works.  Published in 1942 and revised in 1944. 564 pages.  If you finish this book you will know more than you ever wanted to know.

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Posted by nhrand on Wednesday, April 04, 2018 10:11 AM

When thinking about wheel arrangements it is useful to consider that it is the cylinders, steam pressure and ability to sustain steam production that produce the power of a steam locomotive.  The wheels are a carriage for the boiler and only transmit power, not produce it.  An 0-8-0 is not more powerful than an 0-4-0 because it has more drivers.  An 0-4-0 could easily be built to be as powerful as an 0-8-0 but it would not be practical because it would be too heavy for most railbeds.  A 4-6-4 Hudson is not a better locomotive than a 4-6-0 or 4-6-2 because it has a four-wheel trailing truck; it is better because it can produce more steam and sustain greater steam production at speed.

Sometimes there can be too much emphassis on the importance of a wheel arrangement.  In the USA an 0-6-0 or 0-8-0 would have been found switching a yard but in England with its different needs and road characteristics an 0-6-0 would probably be found on a fast local passenger train and an 0-8-0 would be found hauling freights on the main line.  In the USA a 2-8-2 would have been found mainly on freight trains but in Spain, France, and Poland a 2-8-2 was an express passenger engine.  It is true that certain wheel arrangements are more approriate for a specific operational or performance need but you can't always know what a locomotive's purpose was by simply knowing its wheel arrangement.

Sometimes it may be thought, the more wheels the better, but that is not always the case.  A 4-8-4 Northern was a great locomotive type but many roads obtained all the performance they needed from a 4-8-2 or 2-8-4.  In 1930, the Wabash purchased 25 4-8-2s and 25 4-8-4s from Baldwin -- unless you spy the trailing truck you can't tell them apart.  Why buy a bigger engine than you need since bigger means more expense ?  In some assignments the 4-8-4 was better, but in other work the 4-8-2 was more than adequate.

Sometimes you may think that locomotive buyers just bought engines with more wheels as time went by.  One June morning in 1959 I photographed a parade of steam entering Windsor Station in Montreal.  Six of the Canadian Pacific engines were 4-6-2 Pacifics and two were 4-6-4 Hudsons.  But, all the Pacifics were newer than the Hudsons.  And, sometimes a wheel arrangement that seems obsolete is better for a service than a large, modern locomotive.  I watched Boston & Maine 2-6-0 Moguls on passenger trains in the mid 1950s after the beautiful 4-8-2s were long gone.  The Moguls were not working some distant branch, they were on local trains out of busy North Station in Boston.  In short, you could write a book on wheel arrangements, and while there are general rules pertaining to their purpose and use, the subject is complex.

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Posted by zardoz on Thursday, April 05, 2018 6:41 PM

I continue to be impressed by the quality of the responses, as well as by the knowledge of 'how they work'. I had no idea that steam locomotives are so complicated, including:

1. How to build them (designing and engineering)
2. How to maintain them
3. How to fire them
4. How to operate (drive) them

It would seem that I have some reading to do.

Thanks again to all for your great replies!

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Posted by M636C on Thursday, April 05, 2018 7:12 PM

Another book worth reading is Andre Chapelon's "La Locomotive a Vapeur". This was republished in an English translation maybe ten years ago, and copies should be available.

Chapelon provides very detailed explanations of steam operation, and includes detailed descriptions of steam locomotives contemporary to the original publication (around 1950) including most later USA steam locomotives of note.

There are hundreds of illustrations, many very detailed general arrangements drawn by Chapelon himself.

The results of scientific testing of many European locomotives are listed abnd compared. Chapelon does seem to be less than complimentary to German locomotive developments, but most of the book was written during the 1940s.

This book has more information packed into it than most and provides a different view than that of most USA authors.

It should be noted that fuel economy was very important in France, much more so than was ever considered in the USA.

Peter

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Posted by nhrand on Friday, April 06, 2018 9:51 AM

Chapelon was one of the best and his work should be in a steam enthusiast's library.

Another good book to read is A Practical Evaluation of RAILROAD MOTIVE POWER by P. W. Kiefer, Chief Engineer Motive Power and Rolling Stock, New York Central System, published in June 1947.   It is short, only 65 pages, but it is a good evaluation of the New York Central's steam, diesel and electric power by a man who had experience and responsibility for development, design and construction over two important decades in motive power history.

I sometimes get the feeling that railfans think that steam locomotive design was a "seat of the pants" thing and progress came through either accident or blind trial and error.  The amount of scientific research into steam design and construction is often overlooked and the technical knowledge of designers and motive power men is not widely recognized.  However, doing something simple, such as looking through old issues of Railway Age will show how much knowledge went into steam locomotive design.

One publication that reveals how much knowledge there was even in the earliest days of steam locomotives was a reprint I have that was excerpted from a book written by Thomas Tredgold, Civil Engineer, and published in London in 1838.  It describes at great length and detail the "Patent Locomotive Engine" of Robert Stephenson.  The comments on the horsepower, tractive force, steam production and evaluation of the performance of the engine and its design read almost like they were written today.  These men were not simply tinkerers who had no idea of what they were doing.  They may not have had computers and may have been mainly self-taught but most could calculate very well and understood the scientific principles that applied to locomotion.

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Posted by nhrand on Friday, April 06, 2018 11:02 AM

Zardoz -- you mention the issue of operating or driving a steam locomotive.  One thing to remember is that the throttle of a steam locomotive is not like the gas pedal on a car.  The gas pedal feeds fuel to the car engine but it is the fireman who feeds the fuel on a steam locomotive.  The throttle allows steam in the boiler to flow to the cylinders of a steam locomotive and it is best to operate the locomotive with a fully opened throttle when possible.  The reverse lever on a steam locomotive determines how long the steam will be admitted during the piston stroke (the reverse lever or "Johnson Bar" may be thought of as the stick of a car's manual transmission -- the reverse lever controls the timing of the valves, however, not the gears).

It is most economical to allow expansion of the steam in the cylinder to do the work.  Operating with a partly opened throttle "wire draws" the steam, that is, passing the steam through a reduced opening reduces its pressure.  It is best to allow the steam to enter the cylinders at full pressure and "cut it off" after the piston begins to move  -- in that way the steam expands and moves the piston without steam from the boiler being used over the full stroke.  Generally an engineer opens the throttle a bit to get started and when underway opens the throttle even more.  At some point the engineer will begin to move the reverse lever to reduce the "cut off", that is shorten the period of steam admission.  The engine probably would not gain speed without reducing the cut off because as speed builds the spent steam could not escape fast enough and would create "back pressure" on the other side of the piston and act as a brake.  At high speeds the locomotive is probably operating with a wide open throttle and a reverse lever "in the center" meaning steam admission is cut off after the cylinder moves only a very short distance.  While operating with a fully open throttle is best, when speed needs to be changed frequently the throttle is primarily used.

The power and therefore the speed of a steam locomotive is determined by the mean effective steam pressure in the cylinders.  You can conrol that pressure by using only the throttle which simply wire draws the steam and reduces its pressure in a less than optimum way or you can control the mean effective pressure by adjusting the cut off which saves steam.  Obviously, operating a steam locomotive is complicated and requires skillful use of both throttle and reverse lever.  However, its is not easily explained in a few words.

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Posted by zardoz on Saturday, April 07, 2018 9:39 PM

nhrand

Zardoz -- you mention the issue of operating or driving a steam locomotive......  Obviously, operating a steam locomotive is complicated and requires skillful use of both throttle and reverse lever.  However, it is not easily explained in a few words. 

Perhaps, but you are certainly doing a very good job of it so far!

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Posted by SD70Dude on Sunday, April 08, 2018 1:25 AM

Regarding wheel arrangements, one must also consider the diameter of the drive wheels.  In general, larger drivers will allow a higher top speed at the expense of starting power, while smaller ones allow more power at low speeds, but limit your top speed.

I like to explain this by comparing a steam locomotive to a car that can only have one gear: when designing it you pick the gear you think it should be in most of the time.  This is why passenger engines tended to have larger wheels, and yard goats had small ones.

Firing is also difficult to explain in few words, and coal is very different (and much more difficult) to use than oil.  An oil flame can be turned up and down much like the burner on a gas stove, but the boiler's reaction is not instantaneous due to the large mass of metal, water and firebrick that must be heated or allowed to cool down.

Fuel quality is also a big consideration when firing.  Diesels get the exact same grade of fuel pretty much anywhere (the only difference is summer vs winter diesel in colder regions), but fuel oil or coal can come in a seemingly infinite number of forms.  Coal-fired engines were normally designed to burn a particular grade, and many would not steam properly if given a different kind.  Hard coal in small to medium-sized lumps is best, as they will sit on the grates and burn evenly while not falling into the ashpan or being sucked away by the draft.  Soft, chalky coal with a lot of fine powder in it is harder to work with, and lower-quality coals also tend to contain more ash, which make them more prone to forming clinker on the grates (a clinker is a solid plug of unburnable material), which blocks airflow.

In general a Fireman would try to maintain a even coal fire, and also keep planned departure and stopping times in mind so he could build it up and have a roaring fire going at the right time, and not have too big of a fire when stopped, which due to the lack of draft and steam demand would likely result in black smoke and/or the safety valves lifting, wasting steam.

Oil firing is much easier, and fuel grade does not matter as much as with coal.  Most engines had steam coils in the tender tank to pre-warm the fuel and keep the heavier stuff liquid in the winter, Bunker C and lower grades like to solidify at anything less than room temperature.  Many preserved engines now burn re-refined used motor oil, which adds another complication: the presence of water and antifreeze in the fuel, neither of which burn very well.  There is nothing like a slug of antifreeze coming out of the atomizer and putting the fire out while you are trying to light up, oil starts spraying everywhere and not only do you get a nice explosion when it lights up again, you also get some oil spilled on the ground (hopefully you have put a nice big drip tray down there!).

My organization used to get free fuel in the form of used oil straight from Edmonton Transit's bus garages, and at the time most of the city buses were older GM's with "green leaker" Detroit engines.  Much frustration and colourful language ensued when lighting up.  Even though we no longer burn bus oil we still try to fill the tender a couple days before operating to let it settle, and then drain the water out from the bottom of the tank.  We then blow air back through the atomizer fuel line to mix up whatever is left in the tank, which seems to work pretty well.  

In addition to all this oil firing is harder on the firebox and flues than coal, and the lighter the oil the harsher the flame.  But oil firing means there is no ash to dispose of, and good-quality locomotive coal is becoming quite difficult to obtain in these modern times.  As an example Manitoba's Prairie Dog Central Railway has to order theirs from a supplier in Pennsylvania, and they have to buy several years' worth (2 carloads) at a time to make it worth everyone's while.

An additional problem with coal-fired is hot cinders and sparks being ejected out the stack and starting fires.  This is why UP converted 3985 to oil firing, after a Utah excursion on a particularly dry day...

Whew!, now on to water.  This is even more important to the Fireman than the fire, because if the boiler's water level drops too low the top of the firebox (crown sheet) will overheat, weaken and rupture, and then you get a boiler explosion.  The water level should not be too high either, or liquid water will be sucked into the throttle valve (referred to as "working water") and make its way to the cylinders, which can potentially blow a cylinder head off (just like hydrolocking a internal combustion engine).  A Fireman must also know his territory, as the sight glass level will fluctuate depending on whether the locomotive is facing uphill or downhill.  Adding water will cool the boiler down and cause steam pressure to drop, temporarily of course. 

 Not all water is created equally either, and it is never pure, which means treatment chemicals must be added.  The idea is to remove dissolved oxygen from the water and make dissolved solids precipitate out instead of remaining dissolved and rising to the top of the boiler, where they will cause foaming, which also leads to liquid water being sucked into the throttle valve.  The solids should sink to the bottom of the boiler, and the Engineer will periodically open the blowdown valve to try and remove them (this creates a great roar and show of steam, but is not to be confused with opening the cylinder cocks when starting or working water).  But this never gets all the mud out, so the boiler must be washed out by shop employees on a regular basis.  Engines working in bad water districts had to be washed out after almost every other trip.

Santa Fe actually gave up on using some of the local water in New Mexico and Arizona, and resorted to hauling in millions of gallons a day in tank cars.  No wonder they were so eager to get diesels!

Hope you can make some sense of my ramblings, and best of luck with your learning, there really is a huge amount of good reading material out there.

Greetings from Alberta

-an Articulate Malcontent

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Posted by zardoz on Thursday, April 19, 2018 9:51 PM

Thanks again guys for those wonderful responses - such a wealth of information.
And I have yet to check out the books mentioned or the links provided (too busy digesting all the posts' information).

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