by Norman Barber

THIS model is based on a design by John Haining, published in Model Engineer magazine during 1999. In his description of the engine Mr. Haining states that the prototype, built about 1840, spent its entire life working farm machinery in the countryside around the English-Welsh borders and may well have been unique for, although Sissons established a fine reputation for themselves making small marine engines and  some stationary engines, their name does not feature among the lists of early portable engine manufacturers. 

A number of discrepancies were noted between the details shown in the published photographs and the drawings for the model details.  Where possible these discrepancies were corrected. A typical example is the drive pulley on the crankshaft. This is shown on the drawings as a solid casting but the actual pulley was a two part fabrication, typical of pulleys used on line shafting, etc. to enable them to be fitted to shafts between bearings without major dismantling of an installation. Why such a pulley was used in this application is not clear since there would be no problem in fitting a solid pulley to the exposed end of the crankshaft.

The builder is in the habit of making his models without the use of castings as far as is practical and the motivation to build this particular model was the thought that the main frame for the engine would present an interesting fabrication challenge.  This was the first component made - see below.  Many of the remaining components
were machined from solid material rather than being fabricated.  The cylinder, for example, was machined from a chunk of phosphor bronze.
The published design specified a crosshead feed pump bore of 9/32".  Reference to the photographs suggested that this is considerably over scale. A few calculations also indicated that the offset load on the crosshead would be quite considerable when feeding at a boiler pressure of 80 psi. The bore was therefore reduced to 7/32".  Even at this size the pump body was over scale in size and made fitting of cylinder drain cocks rather impractical - hence the omission. 

The method of support of the feed pump was also vague in the published model design and, indeed, was not very clear from the photographs of the original. It was concluded from these photographs, however, that the pump body appeared to be supported from the front cylinder cover, the purpose of the two brackets to the cylinder body being uncertain. The front cylinder cover of the model was, therefore, designed to provide the necessary support and the brackets were omitted. 

Details such as the air vessel, cylinder lubricator and boiler fittings were not detailed in the published design and were scaled from the photographs of the prototype.

The boiler was built to the basic published design except that blind tapped phosphor bronze bushes were fitted from inside the barrel wherever fixings were required for engine components or the wheel support frames. This avoided tapping holes for fixing bolts in the soft copper shell with the attendant risk of stripped threads and/or leaks. 

The grate is secured to the firebox front plate and the whole assembly can be withdrawn as a unit for cleaning or to dump the fire.

The boiler support saddles were fabricated and the wheels machined from mild steel discs (see photo below)

The engine was steamed before painting and lagging the boiler and for convenience the wheels were removed so that the model could be clamped to a support table for testing. For trial purposes a hand pump was installed to fill the boiler and sure adequate means of maintaining water level. No problems were experienced during the test running however, and the crosshead feed pump proved entirely adequate in spite of the reduced bore adopted. It was found that the engine exhaust created insufficient draft to maintain the fire and it was necessary to rely on a little assistance from the blower. The blower is almost certainly not prototypical, the original engine no doubt relying on natural draft to start and maintain the fire.

Following the steam trials the model was dismantled for painting and lagging. Since that time it has only been run on compressed air.

Ed: The following additional photos were all taken at the Bristol Exhibition 2012 where there was a major display of Norman’s work.