Reprinted from the October 1969 Model Railway News
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The 2 ft. 6 in. Gauge railway which runs for about eight miles from Belgrave to Emerald in the Dandenong Ranges is the last of the narrow gauge lines still in operation in Victoria, Australia. The Victorian Railways Department closed the line in 1958, and " Puffing Billy " (by which name each of the 2-6-2 tank locomotives appears to be known locally) was put to rest in the engine shed. The "Puffing Billy Preservation Society" was then formed, and as a result of their efforts the line was reopened for week-end and holiday trips in 1962. The history of this and all the other Victorian narrow gauge lines is given in the book " Speed Limit 20," by E. A. Downs, published by W. & K. Purbrick, Melbourne. The first of the 2-6-2 tank locomotives was made in America in 1900; eventually 17 of the class were produced. Nos. 2A and 4A were Vauclain compounds, the others were simple expansion. |
Despite the narrow gauge, these locomotives are no midgets, as the following details show:
Rail Gauge, 2 ft. 6 in.;
Coupled wheels, 3 ft. dia.;
Pony truck wheels, 2 ft. dia.;
Rigid wheelbase, 8 ft.;
Total wheelbase, 21 ft. 10 in.,
Overall length, 30 ft. 9 in.,
Width over buffer beams, 8 ft. 3 in.;
Weight, 34 tons 7 cwt.;
Height to top of chimney, 10 ft. 10 in.;
Tractive effort, 12,515 lb.;
Cylinders (two outside), 13 in. bore and 18 in. stroke.
Features of the model:
Motor
| The sectional views of the mechanism (Figs. 1a & 1b) show the armature axis at 17 degrees to the horizontal, which position permits the largest possible core diameter. The maximum gear ratio obtainable with a 40 t.p.i. worm is 15:1 I, limited, of course, by the small diameter of the coupled wheels. This layout also gives ample room for a respectably sized commutator, a very important point as inadequate brush contact area causes sparking and rapid deterioration of the commutator surface. The brushes are small blocks of copper carbon attached to metal arms pivoted on knife edges. A piano wire spring provides the tension for both brushes. The right hand brush is grounded to the frame, the left hand brush is insulated and supplied with current from the insulated tyres of the left hand coupled wheels. The armature shaft is of silver steel, with hardened worm and end pivots, the armature core is solid since I have found no advantage in laminated construction in this small scale. The three coils on the armature each have 250 turns of 0.002 in. dia. wire. The large side tanks of the prototype are useful in providing room for the iron pole-pieces of the magnetic field; the magnet is a rectangular block of Alnico ground to shape from a piece of a broken speaker magnet. The motor casing consists of the two pole-pieces joined by brass distance pieces, the whole assembly being silver-soldered together. The construction is as heavy as possible; weight is essential for good electrical contact with the rails and adhesion for traction. |  |
Frames, Wheels and Axles
The
outside cranks and half axles are each machined from the solid and press
fitted to the coupled wheel assemblies, which consist of a right hand solid
wheel and an insulated left hand wheel. One advantage of outside frames is
that they hide so much of the wheels that there is little point in providing
spokes - at least in this scale. The outside frames are made of 0.03 in.
thick hard brass sheet, with stretchers at front and rear. The middle pair
of coupled wheels are flangeless in the prototype. The front and rear pony
trucks are loose pivoted with about 0.01 in. vertical play; since all the
flanges are 0.017 in. deep this prevents derailment without the loss of adhesion
which occurs with sprung pony trucks. All wheels are turned from nickel-silver
and the axles are hardened steel.
Motion Work
Coupling rods are very difficult items to make accurately in small scale, due to the very thin section between the crank pin bosses. The best solution I have found to this problem is cold pressing. The die consists of a block of hardened steel with accurately spaced holes to form the crank pin bosses connected by a trapezoidal section groove to form the connecting bars. A piece of well annealed nickel silver wire is forced into the die cavity between the jaws of a bench vice. The die also acts as a drill guide for the crank pin holes. The excess metal is filed flush with the upper surface of the die, and the coupling rod extracted in the finished condition. The front and rear outside cranks are integral with their crank pins, and the coupling rods fitted to these as closely as possible consistent with free running. The two central crank webs are drilled and tapped to take threaded crank pins in the form of bolts. The central holes in the coupling rods are loosely fitted to the crank pins, the hexagon heads of which provide the lateral constraint for the connecting and coupling rods. The slide bars, of which the prototype has four per cylinder (the system favoured by the makers of American locomotives of 1890 vintage) have been simplified in the model. Only the outer pair on each cylinder are present, as the inner pair would only be visible on very close inspection, and would greatly complicate clearance problems. The small end of the connecting rod carries a small rivet which runs in the slot provided between the two outer guide bars. The cylinders, slide valve chests, guide bars and smokebox saddle form a single assembly attached to the chassis by one screw. Due to the preponderance of weight at the back of the mechanism, it would only run successfully on its first trials with a small weight attached to the front; fortunately the outer casing is capable of supplying the extra weight required.
Outer Casing
This item is constructed mainly from 0.007 in. thick nickel silver sheet, the components being shaped over steel formers and the main structural joints silver-soldered. The boiler is solid brass, the smokebox providing most of the extra weight needed at the front, the large domes also make a useful contribution. The casing is held to the mechanism by one screw which passes through the rear sand dome. Detail work includes:
All handrails (0.01 in. dia. brass wire - over scale, but true scale looks too small and is too flimsy to stay straight).
Large headlight and small footplate headlamp.
Four brackets on the rear of the coal bunker, with small lamps on three of them.
Two safety valves and whistle on the central dome.
Air pump on the front left hand side tank.
Stays from smokebox to front buffer beam, with inclined concave plate between them.
Hinges and central handle on smokebox door.
Guard irons and brake hoses on front and rear buffer beams.
Roof ventilator on cab.
Toolbox on right-hand front of footplate.
Coal in bunker - actually white sugar crystals glued on and painted black.
The towing eyes on the front and rear buffer beams are reasonably close in external dimensions to the buckeye type couplers of the prototype, but of course greatly simplified. The detail work is mostly soft soldered with a small iron, and the excess solder removed with dental burrs and small scrapers. The casing is finished in semi-matt black enamel, except for the buffer beams and footplate edges, which are in the traditional red. The lamps were given a touch of silver paint where the glass should be and the chimney top and handrails scraped clean to expose the naked brass. The number 7A was put on the side tanks with white enamel and a fine pen, but no attempt was made at lining out. From the latest photographs of the prototype, the lining out was also omitted the last time it was painted, although the pictures taken of No. 7A taken after manufacture in 1905 show a neat job of lining out.
Coaches and Track
The
coaches are all modelled on the N.B.H. class excursion coach. The coach bodies
are made from 0.01 in. sheet brass grooved to represent the wooden planking
of the prototypes. Each coach is carried on two four wheel bogies of 0.162
in. wheelbase and 0.084 in. diameter wheels. Each axle assembly consists
of a steel axle with 0.015 in dia. end pivots; the central part of the axle
carries a plastic sleeve on to which the nickel silver wheels are press fitted.
The tow hooks are made from 0.01 in. dia. brass wire. The coaches are finished
in red enamel on the sides and ends, with grey roofs and black underframes,
the brass handrails being left bare as in the locomotive.
The track was built in a hurry for the October 1968 Sydney Model Railway Exhibition, with no thought of scenic effects. The baseboard is about 12 in. square and 3 in. thick. A circular groove 0.16 in. wide and 0.25 in. deep was turned in the baseboard in a 9 in. centre lathe to receive two brass rings formed from 0.03 in. x 0.3 in. strip and silver soldered at the butt joints. Araldite adhesive was poured into the groove to form a firm bond between the rings and the baseboard. When the adhesive had set hard the baseboard was set up again in the 9 in. centre lathe and the projecting edges of the rings machined to form a track of 0.125 in. gauge with 0.005 in. superelevation on the outer rail. No attempt was made to represent sleepers. From a functional point of view this method of production could hardly be improved upon it gives a real precision track with no rail joints. A transparent cover was made from perspex sheet to keep the dust out during the operation of the train.
Performance
As usual with very small models quite a few adjustments were necessary before satisfactory running was achieved. The tension on the commutator brushes and the wire brushes on the left-hand coupled wheels is fairly critical. The application of lubricating oil must be very sparing or some will get on the commutator or the wheel treads. When properly adjusted it is just possible to get consistent running at a scale 20 m.p.h. which corresponds to one lap of the track in 24 seconds, taking about 100 milliamps in the process. The adhesion of the locomotive is sufficient to take the six coaches up a 1 in 24 incline without slipping. The exhibition was open for 12 hours on the first day, 7 hours on the second and 10 hours on the third; during the whole of this time the model ran continuously at about a scale 20 m.p.h. despite frequent questions from the audience: "Won't it go any faster ?"
[Copyright Note: The copyright for this article must still reside with the estate of the late A.A. Sherwood. The current owner or owners are unknown, so the article has, therefore, been reproduced without permission but with full acknowledgement of copyright.]