A unique engineThis experimental set of outside Stephenson valve gear was developed with two fly cranks rather than eccentrics to give a maximum travel of about 6½in, the same as the piston valves in the Walschaerts gear engines. The cost in drawing office time and manufacture of the components must have been considerable for a ‘one off’ and on the eve of nationalisation of the railways in 1948. To put this into context, the last main line locomotive previously to No 4767 to have outside Stephenson valve gear in Britain are believed to be the William Dean’s single drivers of the Great Western Railway in 1884! (Railway Heritage)
The LMS paper on the trials said: 'Special cost records are to be kept of the 30 locomotives to assess the relative financial benefit that accrues from the various arrangements'; but in practice with Nationalisation in 1948 this never happened, which is unfortunate as it could have influenced the design of the BR Standards. No 4767 had a reputation of being stronger on the hills than a normal ‘Black Five’, so were the Great Western right?
But weren't Black Fives' already reasonably strong on the hills anyway? Wouldn't that be "gilding the lily" a bit? Saying that, it was fairly popular in BR days, so somebody must have been getting some thing right!
ReplyDeleteYou could well be right, I simply pose the question. Of course at change over in 1948 many claims may well have been made as you say gilding the lily. As when BR took control many of the design team were of course LMS men so the obvious question to ask would be why did they not build BR Std's with Stephenson's? I would be inclined to agree that the lily was well and truly gilded by many from whichever drawing office. It is as I say unfortunate that no records were kept so we could make our own judgment today. We will never know will we. Walschaerts was the common and indeed the most popular winner.
ReplyDeleteHi,
ReplyDeleteStephenson's valve gear is more flexible than Walschaerts but it's heavier and can be difficult to mount outside the frame. Its flexibility means that there is a wider speed range of maximum power. In BR days (the loco never ran in LMS days) it was known as the 'strong' engine.