BR Standard Class 8 - Design Details

Design Details

At first, Riddles wished to develop an enlarged version of his Standard Class 7 Britannias, as the design still featured a two-cylinder layout. However, the size of the cylinders in order to achieve the 8P power classification would have put the design over the British loading gauge limit and so a reluctant reversion to the three-cylinder layout ensued. This reluctance was born from experience with the Gresley Pacifics, whose conjugated valve gear was difficult to maintain due to the location of the middle cylinder between the frames. Therefore, an alternative type of valve gear had to be found.

The valve gear that was settled upon was a modified form of Caprotti valve gear, the novel rotary cam-driven British Caprotti valve gear with poppet valves. This was based on Italian locomotive practice and allowed precise control of steam admission to the cylinders while improving exhaust flow and boiler draughting characteristics when compared to the more conventional Walschaerts and Stephenson valve gear. On paper this created a free-steaming, hard-working locomotive capable of hauling heavy loads over long distances but, in practice, fundamental design errors and undetected deviations from the drawings were made during construction, which combined to prevent the locomotive from achieving its expected performance during British Railways ownership.

The main problem was known even when the locomotive was under construction, as Mr. L.T. Daniels, the representative of the British Caprotti company, recommended the use of the Kylchap blastpipe, which could have coped with the fierce exhaust blasts experienced with the Caprotti system. A standard double chimney of the Swindon type had already been fabricated in order to cut costs and it had been installed in the smokebox before Riddles could do anything about it. As a result, the locomotive suffered due to the choke area of both chimney and blastpipe being much too small for the pressure created by the exhaust, leading to poor draughting. Further problems regarding the firebox of the locomotive were only discovered during restoration, including a poorly dimensioned ashpan and dampers that were again too small, strangling the fire of air when operating at speed.

Following experience of occasional cracks appearing near the spring brackets of the Britannias and Clans, a substantial rearrangement took place in this area that resulted in the locomotive riding on three cast steel "sub-frames" carrying the ten front-most spring brackets and lengthened spring brackets behind the rear driven axle. (Perhaps remarkably, these were not integrated into a cast combined sub-frame/pony truck pivot stretcher, the pony truck pivot stretcher being a fabrication.) Had the planned batch of further smaller Pacifics been built, they would have been fitted with this arrangement.