Steam Locomotive Anatomy 101

[Devon_Sinsley]

Now time for some practical application to put the parts we have so far into a working order. A steam locomotive works off steam being at high pressure. High pressure moves things. We put water in a boiler. We light a fire in the firebox. That creates hot gas (smoke) which is drawn by draft through the fire tubes up to the smoke box and out the chimney. In doing so those fire tubes heat the water in the boiler to temps above boiling and steam is produced. That steam is then piped to various things to “move” them in such a way that we accomplish the needed task. In the case of the locomotive cylinders (the primary reason for the steam) it pushes a piston back and forth as it fills a chamber and then exhausts that chamber. We will go into more detail about steam chests and cylinders later. As this work is accomplished water/steam is consumed and exhausted. A boiler needs to have the right amount of water in it to operate efficiently. We have already discussed this. As water is consumed it must be replaced. So as the train crew monitors the sight glass and tri-cocks, they are aware that it is time to fill the boiler. When that time comes, they grab the lever on the injector give it a pull and fill the boiler until it is back where they want it.

I said I would come back to the Crown Sheet and why it must be kept in water. The Crown Sheet is simply the top (crown) sheet of metal that makes up the fire box. It must remain submerged in water so that it can not super heat and unexpectedly over pressurize the boiler. So the purpose of everything we have mentioned so far is to prevent this from happening. Another note here is on small mountain railroads, locomotives encountered steep grades. One issue with this was if the locomotive was pointed nose down the grade the water would slosh to the front and expose the crown sheet and boom. So it was not at all uncommon on steep mountain railroads to see locomotives backing down the grade to keep their nose pointed up hill and keep the water sloshed to the back and over the crown sheet.

So now we have pretty well covered the parts and pieces that keep the boiler full of water and producing steam. Next up will start talking about the main reason to have steam, driving the cylinders which make the locomotive go.

[Jim_Rowson]

Fantastic trove of info Devon! Thanks! I’m looking forward to the grand climax where we find out that the engineer did it in the tender with a Johnson bar!

[Devon_Sinsley]

@Jim_Rowson

Fantastic trove of info Devon

I’m doing it for you Jim. Thank you for the complement. I know when I started asking these questions people stepped up and explained it to me. One thing I hate is when people say “Google it”. First off whats the purpose of a discussion forum if we won’t discuss it. Second, how does it end up on a Google search, if someone doesn’t write it down and put it on the web to be searched. So I offer this because you are my friend and you asked for the information. And I would never tell my friend to google something, but rather I would discuss it with them. So this is me discussing it with you. And for anyone else, who like me, wanted to know but didn’t want to just google it.

[ctown2 (Craig Townsend)]

No wonder steam was replaced. Although there was a study in the 1970’s by the BN to make a nuclear powered steam engine/turbine. Never made it past the conceptual stage but there’s a report about it floating around the internet.

[Bruce_Chandler]

Devon,
PERFECT attitude - I REALLY like this. You are right - too many folks tell you to look it up. I remember folks telling me that when I asked how to spell a word! Hey, if YOU don’t know, please don’t tell me to ‘look it up’.

THIS is what a forum is for - discussion!

[Devon_Sinsley]

@ctown2

No wonder steam was replaced.

Yeah diesels are easy, get in turn the ignition key put her in drive and step on the gas pedal.

[Devon_Sinsley]

I want to throw out this resource here. I mentioned David Fletcher (Australian G gauge modeler) who did a series of “Master Classes”. One of them in particular is his 2-6-0 Mogul build. Chapter six of that class, found here is a great resource to locomotive plumbing and how to model it.

I am relying on his description in that class for this next part. A final piece of plumbing coming off the turret is what he refers to as the blower pipe. What the blower pipe does is act like a super charger. Some back ground; we talked about the fire being pulled through the boiler by draft created when the fire is made in the fire box and smoke exhausted out the stack. A way of super charging this draft is to exhaust the steam from the steam chests/cylinders into the smoke box. This is one of the “other” things I said the smoke box does. Steam enters the bottom of the smoke box through the saddle. I will talk about this more when I talk about the cylinders. But for now that steam enters the bottom and rushes out the top of the chimney. It is white the “smoke” from a locomotive often times is more white/grey than black. Remember our friend the venturi? Well that steam rushing by the fire tubes creates a venturi and increases the draft by sucking the fire through the tubes, in effect we can think of this as a turbo charger. But how do we create this venturi effect when the throttle is off and no steam is going to the cylinders? We still want to rev up the engine but we don’t want to go anywhere. We want to do this so we can build up steam prior to taking off? Enter the “blower valve and piping”.

A valve is fixed either to the turret or the side of the boiler with a pipe that runs to the smoke box. and into the bottom of the chimney. At idle the valve is opened and steam is allowed to rush down this pipe and out the chimney. As it rushes out the chimney we again get a venturi that pulls the hot gas through the fire tubes and out the chimney. It increases the draft of the locomotive and in turn raises the temp in the boiler which in turn increases the pressure. So at idle the locomotive can be “given a little gas” and be “reved up” prior to actually opening the throttle and driving the locomotive. Prior to the blower valve a locomotive had to get under way to start the turbo charger effect. This valve and piping most typically was on the fireman’s side and ran under the walks and it is a reason its hard to find a picture of. That and not all locomotives employed a blower system.

That should do it for the basic stuff coming off the back head.

[PeterT]

Nice work Devon.

I do recall there is a very good offline resource in the Model Railroader Cyclopedia - the first few sections describe all the plumbing and controls on a typical locomotive. That book apparently was started in 1936 and continuously updated. I think I got mine in the 1980s.

[Devon_Sinsley]

Now we can get going on the cylinders and steam chests. If the boiler is the heart and steam dome the lungs, we could say the fire box is the belly, and the plumbing is the arteries and veins. To keep up the analogy, the cylinders and the steam chests are the muscle. Included in this discussion we will include the boiler/cylinder saddle. First a couple of pictures of the pieces.

Picture for the San Diego Railroad Museum
The saddle does a few things. One of its most important jobs is a support for the boiler. It is a heavy cast iron affair that is bolted to the locomotive frame and supports the weight of the boiler at the front. It also acts as a hanger for the cylinders and steam chests. But it is not a solid bit of iron. It is rather a set of “pipes”. In the line drawing you can see that one set of pipes connects to the steam chest and the other set of pipes connects to the cylinders. The piping for the inlet of the saddle comes from the steam dome and runs through the boiler. When the throttle is applied the steam leaves the dome, runs through the pipe and into the saddle to the steam chests.

The steam chests is a fancy valving mechanism. When the locomotive is shifted between forward, neutral, and reverse in the cab it is translated to the steam chest via some linkages attached to the Johnson bar.

Picture from Highball Sim
In this picture we see the linkage from the Johnson bar entering into the side of the steam chest. It moves forward and backward. When the johnson bar is in the forward position, steam from the saddle enters the steam chest and is applied to one side of the cylinder piston. Switch it in neutral and the steam from the saddle enters the chest and is directed immediately out the exhaust. Switch it in reverse and it applies steam to the opposite side of cylinder piston reversing the locomotive. As the locomotive moves this rod moves in and out in time with the drivers on a elliptical linkage on the axle. So it is alternating applying steam to one side then the other of the cylinder piston. So the piston is powered in both directions, one way then the other. During what we will call the opposite stroke, where the steam is applying force to one side of the piston the other side is opened up to the saddle to exhaust the steam from the previous cycle. This is the chuffing we get as the steam is exhausted on side then the other then back again, chuff chuff chuff.

Oh, I better add this now or we will be confused. So I said that rod moves back and forth. and I said it is attached to an elliptical linkage on the axle. Well that elliptical linkage is what is actually attached to the Johnson bar. So what happens to reverse the direction the drivers is the Johnson bar changes the relationship of that elliptical linkage and the bar sliding in and out. I realize this is about as clear as mud, Heck I am confusing myself just talking about it. It was also very hard to wrap my brain around until I watched video animations of it. So I am going to leave it here and just say that its magic and it works.

Once the steam leaves the cylinders it travels back through the saddle and into the smoke box where it is ejected out the stack. Remember here that not only is this just a good way of getting rid of it, but it is also acting like a super charger to draw more draft through the fire tubes.

[Devon_Sinsley]

Phew, I think I gave about as basic an understanding of the main operation of the boiler and its relationship to the cylinders and steam chests as I can without getting too deep and making it totally incomprehensible. There are a ton of things I am leaving out. especially how that whole Johnson bar thing works and all the weirdness going on between the frame rails of the chassis on the axles.

Next, I will move on to some of the other gizmos and gadgets we find on our locomotive. I’ll work my way back to the cab picking up some of the other dohickies we see.

[Devon_Sinsley]

The Dynamo/Generator.
Prior to the wide spread use of electricity on a locomotive lighting was done with lamps that burned oil or some other flammable. In the early days of electricity it was either fixed to poles or provided by huge generation plants. Even before the wide spread use of electric locomotives some smart person figured out how to make the dynamo or steam powered generator for steam locomotive use. This gave the locomotive the ability to use them fancy 'lectric lights. I am not sure if Pyle invented it, but the folks at Pyle-National surely perfected it.

The generator or dynamo is really nothing more than a steam turbine generator. Steam comes in from pipe that starts at a valve usually on the turret. It spins the turbine and is exhausted out a vertical pipe. That turbine is connected to the other side of the device to a generator. It spins the generator and produces electricity. That is.

[Devon_Sinsley]

The Westinghouse Brake Compressor.

I got this photo from Mike’s Railway History in an article he wrote called “Stopping the Train” Go there and visit that page and you learn as much as you likely need to know about steam locomotive brakes. So much so I will spend no time explaining it I will just point out the pieces and give a very basic description of its use.

Westinghouse and others developed a positive air pressure brake system. The compressor has a turbine on the bottom where steam enters, spins a turbine and then is exhausted. That in turn spins a drive shaft, that in turn drives the air compressor on the bottom. This picture from the same website says it all as far as the plumbing.

[Devon_Sinsley]

Now we will move back into the cab this will be another quick description. as we already discussed most of what is going on in there. But we did leave a couple things out. The Johnson bar

From Discover Live Steam.

Shifts the locomotive into forward/neutral/reverse.

The brake actuator

Picture from “Stopping the train”
An air pipe in and an air pipe out attached to a valve. Operate the valve to control how much air pressure you are delivering to the brakes and thereby determining how much brake is being applied.

[Devon_Sinsley]

Just a few last bits of stuff to finish the article off. Most locomotives have a bell. Bells can be either hand rung with a pull cord, or mechanically operated with air pressure.

Inside the cab you will have some means of actuating that bell so either a cord or a valve handle. If a locomotive has gravity fed sand domes there will be a lever that attached to the base of the sand dome and extends back into the cab through the front wall. You will see a cord for the whistle that either goes through the front wall out to the whistle on the steam dome or some whistles are on the cab roof if the steam dome is inside the cab and the chord will come through the roof.

On the floor in the cab is sometimes a lever that you pull up or push down that is connected to the ash pan. The ash pan is nothing more really than the floor of the fire box. There are doors on it that drop down and dump the ash into a lined pit between the rails.

On a vacuum brake locomotive you will see a little different set up than the positive pressure brake systems like the Westinghouse. The brake handle on these almost looks like a hybrid between the Westinghouse brake lever and an injector. It has an exhaust pipe that exits through the roof of the cab. That device is called the “ejector” and looks like this

Amazon book on Eames Vacuum Brakes

i have a fondness for the Eames vacuum brakes. One because the Coeur d’Alene Railways and Navigation company used them exclusively for their short existence and because they are just different. Unlike the Westinghouse system they are steam actuated and use a venturi like an injector. You pull on handle that is horizontal and this “turns on” the brakes by applying steam to the venturi creating negative pressure. With that on then you can pull on the vertical lever which regulates how much vacuum is applied to the brakes and thereby determining how much brake is applied.

I think that’s about it. I hope it helps. If you are going to want to actually model this stuff I highly recommend downloading and reading and following David Fletcher’s build article. There is also a lot of diagrams that show the various stuff. And I will always try and answer questions.

I hope you enjoyed this and found it useful.

[Jim_Rowson]

This is awesome, Devon. Both the descriptions and the pointers to more info. Mucho thanks!

I do have one nagging question: it seems that there is a throttle but there are also multiple “stops” on the Johnson bar. Is the throttle just on/off? Or is it a fine tuning lever given a Johnson bar setting which is kind of the equivalent of gears in a car (without the gears of course)?

Or do I not have a clue what the relationship is between those two controls?

[Devon_Sinsley]

While the throttle and the Johnson bar work in concert to make the locomotive go, they are independent systems. Really no different than a car with an automatic transmission. You can push on the gas all day long and it will rev the motor or slow it down. Doesn’t mean the car will go anywhere. The throttle controls the engine not the transmission. Conversely, the transmission can be moved from forward to neutral to reverse and if the engine is off it wont go anywhere. The Johnson Bar runs the transmission. The notches on the Johnson bar just hold the whole system of linkages out to the steam chest in a particular configurations. You sorta can control the speed with it but not because you are giving it gas but because you are riding the clutch letting the transmission slip. The Johnson Bar can be positioned to either fully engage the steam chests or some variation in between where the valving system is actually a little bit out of time with the drivers and lets the steam exhaust a little bit causing a sorta slipping action which does effectively control the speed. Just like letting the clutch out slowly you don’t just slam the Johnson Bar “into gear” or you will get quite a jolt. So you “notch it (my term)” more and more as the locomotive starts to move until its fully engaged.

now once it is fully engaged comes the throttle. It has no notching. Think of the lever on a standard regular old ball valve. The throttle lever is connected to that handle ( this is way not what it really looks like) and you move it back and forth to go from fully closed to fully open or anywhere in between.

Does that answer your question

[Devon_Sinsley]

This is what it looks like inside a steam dome

Mid Continent Railway Museum

wikipedia

That lower picture of the cut away shows it pretty good. The throttle lever is connected to that rod heading off to the left, which in turn is opening and closing the valve inside the pipe. You can see where steam would enter that pipe at the top and then be “throttled” by that valve and then it travels down the pipe heading off to the right and into the saddle.

In this picture you can see the hole where steam exits the steam dome and enters the throttle valve mechanism.

Mid Continent Railway Museum.
If I have it right the throttle lever is attached to the bottom of that linkage. as it is pulled on it must hen pull down on a plunger closing off the valve. Push on it and it pushed the plunger up and opens the valve.

[Rick_Marty]

Devon,
Great Steam Locomotive 101course your posting, good job. Isn’t it interesting where a curious mind will lead a person?

[Rooster]

Is that Dave Taylor cause all the old guys look the same to me?

[Devon_Sinsley]

It was kind of refreshing to write it. It’s been awhile now since I was asking these same questions and I realized I started taking it for granted. Forced to explain it made me reexamine what I thought I knew.