|Apart from its weight, this machine is very similar to the 8 ton roller. The principle difference lies in the engine layout, where the positions of crank and valve gear have been transposed. As in the 8 ton roller, the exhaust pipe from the cylinder block to the chimney can be seen on the near side, but in this case, live steam enters the block by an external elbow pipe – as in the Ransomes – instead of through a hole beneath the cylinder block itself. This arrangement requires only two smaller steam tight joints instead of one large one.
Like the 8 ton roller, it is a compound engine, the exhaust steam passing on to do more work in a second – larger diameter – cylinder before escaping up the chimney. If the work done in both cylinders (and hence the effort made on the crankshaft) is to be approximately the same, the second cylinder has to be larger since the steam it uses will have expanded much of its energy in the first, and will have expanded to a greater volume and lower pressure and temperature.
Of additional interest is that both 8 and 12 ton rollers are fitted with the PICKERING governor originally devised in 1865 by Mr Thomas Pickering of New York. The aim of any governor design is to obtain the closest possible control over variations in any desired engine speed, and this pattern is one of the most successful.