MANSON’S GSWR CLASS 8 4-4-0
The Glasgow & South Western Railway traversed main lines in decidedly hilly country and locomotives of good climbing ability were a must. The renowned researcher of the GSWR, David L. Smith, was as fond of Manson’s Class 8 4-4-0s as the Company were proud. The Stranraer road was demanding both in traffic and gradients and few drivers had other than great praise for the Class 8s, as David Smith recorded.
As built, these locomotives had Stephenson’s valve gear with launch-type links driving a 180o rocker to slide valves above the main bores. This quite conventional arrangement does not immediately proclaim excellence but a closer look at the neat proportions and details begins to mark the design as one of astute understanding. Matters are aided by a substantial guide just ahead of the link so that the die connects directly to a slotted rocker end, and the residual angularity problems are easily accommodated by a trunnion offset of a mere ½″. This point is also raised vertically by the same small amount but I cannot find that this is necessary.
Good symmetry in the gear proportions is rewarded with excellent results in the events, the equality of which shows up well in the graphic display.
The valve travel in full gear is 5″, rather greater than most of her contemporaries and one can only argue about the somewhat excessive preadmission – typical of the era. Nevertheless, the engines were noted hill climbers and free-running in character.
As with many other designs, a new CME eventually carried out some rebuilding, the primary result being the complete destruction of the Class 8’s attributes. Nobody seemed to discover why. Neither the drivers nor the running people could understand why the performance was so lacking and Mr. Smith told of a shed fitter almost sacked for hacking at a slide valve in an effort to improve matters! The reasons are obvious on inspection. The rocker had been deleted and a heavy bar substituted, albeit with a guide to relieve the valve spindle glands, making the drive direct but with a massive offset. Quite apart from the dubious mechanical engineering the major transgression of directly driving outside admission valves via launch-type links is inexcusable. The angularities so neatly dealt with in the original design now added together to produce very poor valve events. In 1950 Whitelegg denied all knowledge of a problem, but who would be brave enough to tell him at the time?
Thankfully, I was able to explain the reasons to the perplexed David Smith in his 81st year, thereby exonerating his revered designer. The table portrays the excellent cut offs of the Manson gear.