Design Details
The arrangement consisted of a modified Standard Class 7 boiler, with smaller steel cylinders and other modifications to save weight and hence increase route availability. The wider firebox, designed for use with cheaper imported coal, was also utilised to spread its weight evenly over the axles, whilst the standard smokebox completed the boiler, which at 225 lbf/in² was rated at a lower working pressure than that of the Britannias. A single chimney was incorporated into the design, although this was to create problems later on, due to its small diameter, which reduced the choke area that allowed the fierce exhaust blast to escape, reducing the overall efficiency of the locomotive. Similarities with the Britannias rested with the frames, tenders and running gear, allowing easy standardisation of parts common with other classes. The design was fitted with a standard set of two Walschaerts valve gear systems, and all members of the class were equipped with 4,200 gallon BR 1 tenders.
Following experience of occasional cracks appearing in the frame plates near the spring brackets, had the second batch of Class 6 Standard Pacifics been built, the chassis' would have been rearranged to be similar to that used on the solitary Class 8 Pacific. This would have resulted in the locomotive riding on three cast steel Combined Frame Stretcher & Spring Brackets carrying the 10 front-most spring brackets and lengthened spring brackets behind the rear driven axle. These Combined Frame Stretcher & Spring Brackets are often referred to as "sub-frames". (Perhaps remarkably, the rearmost spring brackets were not to be integrated into a single cast combined sub-frame/pony truck pivot stretcher. The pony truck pivot stretcher being a separate fabrication).
Unlike the smaller BR Standards the exhaust steam manifold within the smokebox saddle (along with the BR Standard Class 7 engines) was a steel casting that was welded into the saddle. Other original drawings confirm the exhaust steam manifold was a steel fabrication in the smaller BR standards.
Read more about this topic: BR Standard Class 6
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