Twelve sheet Drawing set available from Reeves (2000), cat no 29/507.
Note : most castings are available from the Association.
The engine I am going to describe, is not a "scale model": it is a small working locomotive following as closely as possible the outlines and general dimensions of the full-sized engine, but cutting out such things as odd "thirty seconds" in measurements except in vital places such as working parts; and making other alterations as may be necessary to ensure the engine "doing the job" to the best advantage. Nature refuses to be "scaled"; if you compare a baby, human or animal, with an adult of the same species, that fact is pretty obvious; and components for a 2½-in. gauge locomotive which will work in a satisfactory manner, and be of adequate strength, cannot be produced merely by scaling down 4 ft. 8½-in. dimensions, cross sections and shapes. The success of the many engines I have built with my own hands, plus the necessary tools and equipment, is mainly due to recognition of the above fact. Therefore, if you notice on the little "Princess Royal," something which is different from the corresponding part of her big sister on the L.M.S., you will know, as the old saying has it, "there‘s a reason."
Let‘s take a general survey of the job. Though the engine is larger than the G.W. "King" she is easier to copy The "King" has a complicated bogie with both inside and outside bearings, on account of size and position of the cylinder blocks. The "Princess" has inside bearings only; and a very simple equalised bogie as on "Ayesha" (the first engine I described in this journal) will fill the bill. The main frames are mostly "straight lines," very easy to cut out, and whilst the big girders of the trailing cradle are not so very difficult to cut from sheet material, it is quite probable that some of our advertisers who do not wait for the times to move them, will be marketing a cast cradle which merely requires cleaning up with a file. The pony truck is very simple too. The main axleboxes, horns, springing, and driving wheels will be the same as on my own pacific "Fayette," which has stood the test of time, and of which hundreds of copies have been built.
Back in 1924 I designed and built a four-cylinder pacific in which the inside cylinders were set forward and drove the leading coupled axle, whilst the outside cylinders drove the middle pair of wheels. She had outside Walschærts valve gear, the valves of the inside cylinders being actuated by horizontal rocking levers operated from the outside valve crossheads. This engine is still going well, and hauls a heavy load for half-an-hour on one fire-up. I propose to use the same arrangement on the little "Princess," copying the external details of the fullsized engine as faithfully as possible, and setting the inside cylinders on a slight inclination to match that of the outside pair, so that the bogie will have as much clearance as possible in case the engine is needed to traverse fairly sharp curves. The four cylinders will be practically the 2½-in. gauge equivalent of those on the big engine, viz., 11/16-in. bore and 11/8-in. stroke. Many folk pin their faith to small bore cylinders, and work at a high boiler pressure to try and get the power. Now a small high-pressure boiler requires careful and skilled workmanship, and must be of stout material, if it is to be safe; and even then, the required pressure is not always maintained. My own personal experience is that fairly large cylinders using really hot steam at a moderate pressure, the valves having lead, early cut-off and free exhaust, give far and away the best results™greatest work for lowest steam consumption.
The amount of steam generated by a little boiler depends upon the temperature of the water in it; and in turn, this depends not so much on the number of "square inches of heating surface" so beloved by the orthodox text-book, but the amount of heat you apply to the surface available. Now the most valuable part of the heating surface of a locomotive boiler is in the firebox, and ample firebox volume is a good thing. But you have all heard of the saying "you can have too much of a good thing," and I have seen designs of locomotive boilers - with huge barn-like fireboxes, and grates so large that they defeated their own ends, in as much that the tubes could not carry away the products of combustion. There is one easy way to obtain plenty of firebox volume without a ridiculous grate, and that is by using a combustion chamber, as is almost universal in America, where long-boilered engines are in a majority. The combustion chamber is not an American invention, however; it was introduced into British practice in the middle of last century, at the time coal-burning was being experimented with (nearly all the early engines, except on the colliery lines, used coke) and many of the L.N.W.R. "McConnell" engines had these chambers. I have built many small boilers with them; and by introducing vertical or inclined watertube struts, the chamber is not only self supporting and has no need of separate stays, but in addition to the extra firebox volume, the water tubes set up a rapid circulation and so further improve the efficiency of the boiler. As an example, my engine "Fayette" has raised steam from "all cold" in less than three minutes, against the six or seven minutes usually taken by the ordinary boiler of equal size, but with plain firebox and long tubes. Therefore we shall make the outside of the little "Princess‘" boiler like her big sister‘s, but we shall fit the Belpaire firebox with a combustion chamber and short tubes, and provide an efficient superheater of the firetube "spearhead" type. Then the four cylinders will get all the good hot steam they can use, and there will be a bit to spare for working auxiliaries and blowing the whistle. The boiler will have two pumps worked off the axle, and an injector, into the top as on the big engine.
The rest of the details, top works, tender, etc., all follow regular locomotive practice, so do not need special mention; and we will next week proceed direct with the construction.
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This page was last changed on 04/05/2016