7¼ INCH GAUGE STEAM LOCOMOTIVE




Interest in steam engineering had never lingered far beneath the surface of my dreams. Things that breathed steam and the engineer’s lathe together with the odours of cylinder oil and cutting oils were as flavour enhances to good foods. The urge to build a 7¼-inch gauge steam locomotive with riding trucks and portable track converted to action and by 1961 became operational hardware that over six years enlivened social functions. The locomotive was loosely based upon a New Zealand Railways Baldwin built 4-6-0 tender engine. The chosen track gauge being close to one sixth that of our NZR standard 3’-6” became the scale for the model. The object of the project was to create a small locomotive displaying the character of a full-size engine. The necessity for portability by car trailer limited the amount of rail track that could be carried which on completion came to 210 feet (64m) consisting of thirty 7-ft (2.134m) lengths that laid out in an oval comfortably fitted around my house. The radius of the end curves was too tight to accommodate a sixth scale 4-6-0 locomotive so the design was modified to a 2-4-0 yielding the flexibility of a four-coupled engine. The leading axle was mounted in a swivelling pony truck. While the engine was built to free-lance concepts its visual features were to scale proportions. The boiler was of locomotive pattern formed from 3/16 inch (5mm) steel plate with copper fire tubes and super heater flues. The cylinders were 2¼ inches (57mm) bore by 3 inches (76mm) piston stroke with 1 inch (25.4mm) diameter by 1 1/8-inch (28.575mm) travel piston valves and the driving wheels were 7 inches (178mm) diameter. The fire grate measured 9 inches (229mm) by 4 inches (102mm). Fuel was kerosene fed by a steam atomising scent spray type burner aimed through an aperture below the normal fire door. Carbonettes were used to stabilise the fire. Boiler feed-water was by axle driven pump while travelling and injector while standing. The tender rode on two bogies and carried 8 gallons (36 litres) of water and 3 gallons (14 litres) of kerosene as well as providing the riding truck for the driver. The engine and tender empty weighed 563 pounds (255kgs) of which the engine accounted for 391 pounds (177kgs) with 317 pounds (144kgs) on the driving wheels and 74 pounds (33kgs) on the leading pony truck. The driving axle suspension was compensated so that axle weight distribution was not affected by track irregularities.

There were two 7 feet (2.13 metre) long riding trucks each weighing 196 pounds (89kgs) carried on bogies. The seating was central plank with footboards. Each length of track weighed 41 pounds. (18.5kgs). the colour scheme was hawthorn green boiler, cab and tender. The smoke-box, chimney and under frames were painted black and the headstocks red. The passenger trucks were grey seat boards and footboards with red side panels.

Raising steam from cold was achieved with a wood chip fire with draft induced by a hand-cranked extractor fan fitted on the chimney. After a few minutes when steam pressure showed 2 pounds per sq. inch the chimney fan was removed and the blower and kerosene burner were opened up. 12 minutes from light up was the normal time required to attain the working pressure of 90 PSI. When first placed in service the ability of the boiler to maintain full pressure on load was fairly poor and much heat was wasted in the smoke-box which scorched the smoke-box and chimney paint. This was rectified by fitting a feed-water-heating coil of copper tube around the inside of the smoke-box. The engine then became a very free steamer. Even under full load the boiler easily gained water level and steam pressure. The burner steam and fuel jets had long tapered needle valves that allowed fine adjustments and produced smoke-free combustion.