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How a Locomotive is Built (2)




FIRST of all, in building a locomotive the constructors start just as they would in building a house - with the foundation. The foundation of the engine consists of the “main frames”. These are two deep steel plates, running the whole length of the engine from the buffers back to the driver’s cab, and about one-and-a-quarter inches thick. They run parallel to each other, and throughout their length they are very strongly connected together to form a perfectly rigid framework.

The two ends of the boiler that produces the steam are mounted on this framework; the cylinders that turn the steam into work are bolted to the main frames, and with them all the complicated machinery by which the steam enters and leaves the cylinders; and then the whole of the engine, boiler, cylinders and frames, is dropped on to the wheels. In the photograph above you can clearly see the long frames of the engine, with the necessary spaces cut in them to fit over the axles.


BUILDING A “PACIFIC” ENGINE: outside cylinder and piston-valve chest bolted to main frames.

Sometimes the cylinders of the engine are arranged outside, where you can see them; sometimes they are inside, between the frames, where they are hidden under the boiler; and when an engine has more than two cylinders, you will find them both inside and outside. The trouble about inside cylinders is that you can only make them up to a certain size, because of the main frames on either side. And although you have a little more room available when the cylinders are outside, even then you are limited to a maximum of 22 inches in diameter. Bigger cylinders than that would hit the platforms and other structures that are often so close up to the line.

This is one of the problems that always cramp the British locomotive engineer so seriously in his designs. Although our wonderful railway pioneers of the early days gave the idea of the railway to the world, yet they did not see far enough ahead. The other countries that copied the idea, and especially America, profited by our experience over here, and left more room round their tracks.

The Americans, although their “gauge” (the distance between the lines) is the same as ours, have at least three feet more room vertically in which to build than we have, and proportionately in width too. So the biggest American locomotives are twice, and more than twice in some cases, the weight and power of the biggest that we can build in this country.


BUILDING AN ENGINE AT SWINDON WORKS: main frames, showing cylinders and saddle for smoke-box in position.

Bigger boilers mean more steam, and bigger cylinders mean a better opportunity of using it up; bigger boilers and bigger cylinders are therefore necessary if you want more power. So when the British locomotive designer wanted to get more tractive power than he could with a pair of 22-inch cylinders, what did he do? He simply had recourse to three or four cylinders instead of two.

Three-cylinder “simple” (I will explain the word “simple” in a moment) engines you will find principally on the London and North Eastern Railway; the famous “Pacific” engine, “Flying Scotsman”, of Wembley fame, is a 3-cylinder engine, and the North Eastern section of the same railway has a very large number of three-cylinder types. You can always tell a three-cylinder “simple” engine, because it makes six puffs every time the driving wheels go round, instead of the usual four.

On other railways four cylinders are more popular; one of the best-known four-cylinder passenger types is the “Castle” class on the Great Western Railway, which was represented at Wembley Exhibition by the well-known “Caerphilly Castle”. The “Claughton” class on the late London and North Western, and now LMS Railway, is another favourite four-cylinder express type.


EARLY 4-4-0 AND MODERN 4-6-2 CANADIAN ENGINES: notice how the big right-hand engine overhangs the track on both sides.

Quite another arrangement of cylinders is that known as “compound”. The most extensive series of compound engines in the country is found on the Midland Division of the LMS, which will soon have nearly two hundred of them. In these engines the steam is led from the boiler into one large cylinder, known as the “high pressure” cylinder, between the two main frames; and when it has done its work in that, it is led on into the two “low pressure” cylinders, which are outside the frames, in order to do some more. Compound engines are used very extensively on the Continent of Europe, but they are not so popular over here.

All locomotive engines throw a lot of unused power out of their chimneys; the force with which the “puffs” are ejected does not leave that in much doubt. Steam enters the cylinder at one end, pushes the “piston” forward and so turns the driving wheels round; then that very important mechanism known as the valve-motion suddenly allows steam to enter at the opposite end of the cylinder, driving the piston back again.



But what becomes of the steam already in the cylinder? The same valve-motion sees to it that the opening in the cylinder through which the steam came in is now connected to a passage which will lead it out into the chimney. So steam is “pushed” out of both ends of the cylinder in turn every time the driving wheels go round, and with an ordinary two-cylinder engine you get four puffs to every revolution. This steam escaping through the chimney performs another important service, of which more in a moment.

In many modern engines the valve-motion to which I have referred is built outside the engine; you will see it very clearly in the photograph on the next page. It is generally the custom on the Continent and in America to put all the machinery out-side in this way; it may not be a very pretty practice, but it is much more convenient for the engineers when they want to get at the various parts for inspection or repairs.



You will notice in this photograph the end of the “piston-rod” (numbered “3”) coming out of the cylinder (“1”), and terminating in the “cross-head” (“4”), which slides evenly to and fro between the “slide-bars” (“10”); the long “connecting-rod” (“5”) joining the cross-head to a pin on the driving-wheel, and so changing the to-and-fro motion of the piston into the circular motion of the wheel; and, last of all, the “coupling-rod” (“6”), which pins the big coupled wheels together, and so increases the grip of the driving-wheels on the rail. Above the cylinder is the “piston-valve” chamber (“2”), where is carried out the important work of admitting the steam into the cylinder, and letting it out again. The motion which operates the valves I have marked “7”, and the main steam-pipe from boiler to cylinder is numbered “9”. No. “8” is the automatic lubricator for the cylinders, valves and motion.



You will well understand that the flying round of these heavy parts, when the engine is in motion, and especially at very high speeds, would be likely to lead to very unsteady riding unless something were done to counteract these disturbing forces. So the engine is “balanced” in various ways, the most familiar of which is the balance-weight attached to the rim of the driving wheels. The remarkable machine at Swindon Works of the Great Western Railway, which you see illustrated on this page, is designed to show if this balancing has been properly done.

I well remember once seeing a big pair of driving-wheels spun round on this machine at a speed equivalent to 60 miles an hour; the machine was then stopped, and the weight in one of the balance-weights was altered by two or three pounds. The way in which the “balance” had been disturbed was simply extraordinary; in fact, the wheel concerned began to wobble so violently that the machine had to be stopped long before the full speed was reached.


WHEEL-BALANCING MACHINE AT SWINDON WORKS. Notice the pair of driving-wheels in position, and the balance-weights inside the rims.

Another matter in connection with both wheels and motion to which very careful attention must be paid is that of lubric-ation. In big modern engines the oil is forced into the cylinders and valves under pressure, and by ingenious arrangements the driver can see, while comfortably inside his shelter or “cab”, exactly how much oil each part is getting. In olden days he had often to crawl precariously round the front of the engine, while the train was travelling at speed, in order to keep some of these parts properly oiled.

And then the “axle-boxes”, which fit into those big spaces in the main frames, and through which the whole weight of the engine comes down on to the wheels, have to be most carefully designed in regard to lubrication, and lined with metal of a special wearing quality; the same applies to all the various parts where metal “rubs” metal. Otherwise, if there is too much friction - such as if small particles of the most harmless-looking grit get into the bearings - the engine will “run hot”, and have to be taken off the train.


BUILDING A “ PACIFIC ” ENGINE AT DONCASTER: the boiler in position on the frames.

And now a word or two about steam. You can see more clearly the details of boiler construction in the “undress” loco-motive pictures that follow than you can when the engine is all spick and span in its best clothes. For an engine does wear clothes, just as you do, and for the same reason - in order that it may keep warm. If you touch what appears to you to be the boiler of an ordinary locomotive, you will not burn your hand. This is because, under the beautifully-painted outside covering of the boiler, and between it and the real boiler, a layer is interposed of asbestos, or some other material which will not “conduct” heat readily. This you see clearly in the picture on this page. So the boiler is kept from wasting heat by what is called “radiation”, and, incidentally, the paint-work of the engine is protected.


ENGINES IN THEIR “WHITE JACKETS” AT SWINDON. The two engines on the left have been fitted with their asbestos “clothing” round the boiler-barrel, fire-box, cylinders, etc.; the engine on the right is waiting to be “dressed”.

Now, if you look at the big Great Western boiler, you will see that it is in two parts. At the back is the “fire-box”, as it is called; from there to the front you have the “barrel”. The barrel is designed to join at the front end on to the “smoke-box”, which is that part of the engine immediately under the chimney. It is the smoke-box and fire-box that rest on the main frames of the engine; between them they carry the barrel.

Inside the fire-box, if you could see it, is another and smaller “box” which actually contains the fire; this inner fire-box and the smoke-box are both shut off from the barrel by strong partitions, known as “tube-plates”. You see the front tube-plate in the picture of the Great Western boiler; and this gives you also a useful clue as to the way in which the boiler is constructed. A modern “inner” fire-box is illustrated on the opposite page.


STANDARD GREAT WESTERN LOCOMOTIVE BOILER. Showing fire-box, barrel, safety-valve, front tube-plate and superheater header.

You must remember, first of all, that the locomotive boiler has to produce steam at a faster rate than any other kind of boiler there is. To produce steam fast means to burn coal fast, and to burn coal fast means plenty of draught. The draught for your kitchen fire at home is obtained by having a chimney as high as the house; but this is clearly impossible on the locomotive, where we have only a total of just over 13 feet in height in which to build. This is where comes in the value of the escaping steam from the cylinders, to which I referred previously. It is a principle which was discovered long ago by the famous engineer, George Stephenson, and which was embodied in his engine, the “Rocket”, as early as 1829.

The steam, as it escapes from the cylinders, is carried into a vertical pipe in the smoke-box (called the “blast-pipe”), which narrows from the bottom to the top. The effect of this narrowing is to make the steam escape from the blast-pipe with great violence; and as the steam spreads out and fills the chimney, it creates a very strong suction. Air must be sucked from somewhere to fill the vacuum; and so it comes from under the fire-box, through the ash-pan and through the fire-grate, on which the fireman is all the time shooting his shovelsful of coal.


THE INNER FIRE-BOX OF A “PACIFIC” ENGINE, showing fire-box tube-plate.

This strong draught enables the coal to burn rapidly and fiercely ; and the heat so produced is then drawn through all those myriads of boiler-tubes from the back end of the barrel to the front, and so out into the smoke-box. This explains why you see sparks and black smoke coming out of the locomotive chimney, as well as the partially-condensed steam.

Now the barrel itself, as well as the space between the inner and outer fire-boxes, is filled with water. Naturally, the most steam is generated round the inner fire-box, where the fire is actually burning; but steam is also being produced round every one of those two or three hundred small tubes, which are filled with heat and surrounded with water.

The fiercer the blast of the escaping steam is, so much the stronger the draught, and so much the more rapidly can steam be produced if required. So the locomotive is able to adapt itself readily to very hard work, when necessary, or to steam more easily, when the load is lighter or the speed desired is not so high.

You will also see, on the front of that Great Western boiler, a curious collection of pipes, which seem to run in all dirctions from the box, or chamber, that is fixed across the upper part of the barrel. This chamber, which is called a “header”, forms with the pipes part of the “superheater”, with which every modern engine is equipped.

The boiler is never kept quite full of water up to the top, as space must be left in which the steam may “collect”. On the top of most locomotive boilers you will see an arrangement near the centre, rather like a bowler hat in shape, and known as the “dome”. This covers a chamber opening out of the boiler, in which works the regulator - that is, the appliance, under the driver’s control, whereby the steam is allowed to pass from the boiler down to the cylinders. On the Great Western Railway, however, the boiler is always tapered outwards on top in the shape of a cone, in order to make more space for the steam, which is collected in the high front corner of the fire-box, where the Great Western regulator works.



Notice myriads of stay-bolts securing outer to inner fire-box; also regulator handle and fire-hole door.

When the driver opens his regulator, the steam rushes along the main steam-pipe into the superheater “header” - the chamber fixed on the front of the boiler barrel. Between this header and the fire-box, running right through the boiler barrel, there are arranged a number of “flue-tubes”, to the tune of about a couple of dozen or so, like the ordinary boiler tubes, but much larger.

Into these flues are led the array of little steam-pipes coming out of the header, and as these steam-pipes are doubled backwards and forwards inside the flues several times, the steam is all the time getting hotter and hotter - that is, getting superheated - as it hurries through them. Then, in this superheated condition, it is passed on without delay into the cylinders, in order to do its work there. I will not weary you by explaining the technical reasons why superheating the steam is so valuable; briefly, it tends to make the engine use less coal and water, as well as to pull better; that is to say, more value is got out of the steam produced in the boiler.

The third of the fittings generally seen on top of a locomotive fulfils a very important duty. On the Great Western boiler it takes the place in the middle of the barrel usually occupied by the steam-dome; its more usual position is above the fire-box, close to the driver’s cab. I refer to the safety-valves. A boiler is designed to take a certain pressure, which, although a wide margin of safety is always left below the pressure that would burst the boiler, must not be exceeded. The safety-valves guarantee that it shall not be exceeded, because they begin to let the steam escape immediately the right figure is passed.

I need not remind you how important this is; when the engine is standing, or when steam is shut off during running, the increasing pressure of steam in the boiler has no means of escape other than these safety-valves. The safety-valves which are being used on all the latest types of engine are of the Ross “pop” type - rightly named because of the suddenness with which they begin to blow off, and also with which they shut down again.

Amongst the other external details of the engine, you will notice the coverings to the upper part of the driving wheels, usually known as “splashers”. In American engines splashers are unknown, because the “running-plate” along which the driver walks (or along which he is able to walk, if he has any occasion to do so) from the cab towards the front of the engine is raised high up, clear of the wheels altogether. In modern British engines, also, the habit of raising the running-plate high over the wheels is getting common; it is because so much of the motion is being arranged outside the engine nowadays, and these high running-plates make it readily accessible for inspection and attention when required.


THE MAIN ERECTING SHOP, SWINDON WORKS. One of the overhead cranes is lifting a 90-ton locomotive.

You can read more on “How a Locomotive is Built”, “Halls of the Giants” and “Testing a Locomotive” on this website.