The ubiquitous Black Five provides as popular a model in all gauges as it performed in full size.
One of their number was designed with Stephenson’s outside gear and little appears to be known as to the purpose of this exercise. It would be instructive to know what the official assessment of this gear was – the only comments of which I am aware are those of the drivers who liked the engine. The resultant figures in the official table of events must have taken a lot of time to evaluate. They are good results and it must have been clear even before she took to the road that these exceeded the event equality of the standard version.
One can perceive in the gear the (then) current LMS thinking, so it was not an attempt to equip a Class 5 with a Western gear and this sort of idea probably survives only as an over-enthusiastic locospotters’ rumour. There appears to be some attempt to equate with the lead and exhaust characteristics of the Walschaerts’ equivalent, maybe for comparison – LMS penchant for 200-300 thou lead and some exhaust clearance at this time shows clearly enough. It would require a new generation to supplant this sort of thinking, whatever was happening on other railways.
Unusually, the drawing displays a lot of dimensioning. The eccentric throw and advance angle lay out precisely the centres, yet these are confirmed by some superfluous trigonometry in four decimal places! CAD indeed proves it. Beyond that, the neat gear is perfectly conventional, employing a double return crank. The weighshaft position is exactly that of the other Class Fives so the gear size has been built around this(!) and its nominal sizes therefore reflect the GW layouts but without the rocker arm differences, hence the whole travel is derived from the gear itself. That is reflected in the 27.25o angle of the full gear lifting arms, where 25o suffices on the Western. The swing of the lifting link is not symmetrical around a vertical (1 in 24) meridian and lays open to question the suspension offset of 5/8ths, though this is not critical – more does it point to a shorter than optimum lifting arm length, as the lifting link should ideally be vertical at the half stroke for the offset to take proper effect.
The two hangers which carry the motion from dieblock to valve rod transfer the travel to the upper plane in similar fashion to the GW arrangement and have perceptively little effect even though the first hanger is ahead, and not in line with dieblock – it occasions a very small die slip of little consequence in practice. It does, however, require a little interpretation in drawing out. If the extension bar is started at the dieblock centre and produced through the first hanger point it does not reach the elevation of the valve rod. Aligning the rod between the two hanger arcs places the dieblock a fraction below its central position to equalise the die slip about that centre through a cycle of the gear. The draughtsman casually ignores this (frequently done on motion drawings but impossible using CAD) perhaps wisely, as one can tie oneself in unwarranted knots with this sort of thing – unwarranted because the cause and effect are not of import. Oddly, one of the hanger pivots is dimensioned partly through the 1 in 24 gear inclination and partly through the frame horizontal.
Placing the valve centrally over the ports reveals a lap of 1.4375″ and an exhaust clearance of 1/16th. From the eccentric advance there is a nominal positive lead of 0.1075″, increasing to just over 3/8ths in mid gear. On simulation this reveals very heavy preadmission in comparison to the GWR gears. End clearance of the piston, at 5/16ths, is little different to the GWR standard and one wonders whether the cushioning is rather overdone. Judging from indicator diagrams the GWR gauged the preadmission pressures rather well by applying negative lead in full gear. With the simulator valve-set to favour the best events the leads appear a little more disparate than the official table results and both the events and cut off tables for this simulation are illustrated herewith. The events graph clearly shows clean events, but with exhaust symmetry slightly to the left, and the disparate leads and overbearing preadmission are prominent in the left hand corner. The good basic quality in the design of the gear is apparent from the almost overlapping event curves and this is a sure sign of intuitive design.