Class 445A 4-8-4 (2007)

The ongoing quest for good balancing in the reciprocating steam locomotive gave rise to many different cylinder arrangements but not, it seems, the one shown here. The excessive overhang behind the firebox led Artur Gorote's team to abandon all the twelve-coupled giants that culminated in the extremely powerful but highly complex 675A. The Withuhn arrangement only really found its feet, as it were, in the 885A Garratt and even here, the double-cranked axles led to some well-bitten fingernails.

Without synchronised opposing outside cylinders, it only remained to revert to the time-honoured balancing of the outside cylinders by means of the inside ones. Normally this meant a double-cranked axle, which has long been discredited as not strong enough to transmit the power that a modern steam locomotive can develop. It was by a stroke of genius that Martin Porotelas, a junior draughtsman who had already shown some promise, came up with the idea of two inside cylinders one behind the other. The engine thus had two single-cranked axles of much greater stability and reliability. Furthermore, it was possible, without departing from the time-honoured quartering of the outside crankpins, for each of the inside cylinders to balance one of the outside ones. Basically, when the forward inside cylinder (medium pressure, driving the first coupled axle) is at front dead centre, the right-hand outside one is at back dead centre, the rearward inside cylinder (high pressure, driving the third coupled axle) at top dead centre and the left-hand outside one at bottom dead centre. The outside cylinders are low pressure. We say "basically" because the inside cylinders are inclined at 9 degrees to clear the front bogie and the second coupled axle and the angle of the crankpins adjusted accordingly to assure a regular torque, resulting in a very small residual unbalanced reciprocating mass. The arrangment of the inside cylinders is shown below:

Other modern RSR features include the now classical RC poppet valve gear, gas-producer firebox and intermediate superheat between stages of expansion.

The 445A is typical of more recent RSR locomotive design in that externally, it is scarcely to be distinguished from its ancestor, the 434D of 1962. Two visible innovations are the tapered boiler, which was also applied to the 434D, and a new pattern of boxpok driving wheels.

Text and graphics © Norman Clubb 2017