BR Standard Class 7 - Design Features

Design Features

The basic design of the "Britannias" owed much to LMS building practices, especially when considering Riddles' previous career with the said railway. However, in keeping with the necessity to follow best practice in creating standardised steam locomotives, they utilised a variation of both boiler and trailing wheel of the Merchant Navy Class, while weight was kept within the margins laid down by the Light Pacifics, all of which were designed by Oliver Bulleid. The firebox was also similar in having a rocking grate, which allowed the fire to be rebuilt without stopping the locomotive, removing both ash and clinker on the move. A self-cleaning smokebox was used, which enabled ash to flow into the atmosphere, reducing the workload of the engine cleaner at the end of a working day. A single chimney was placed on top of the smokebox, which was unusual for a "Pacific" type of locomotive. This was because the blastpipe was designed by S.O. Ell at Swindon Works, who claimed that "better results could be obtained from a well-designed single chimney than some of the previous double chimney arrangements". The "Britannias" had 6 ft 2 in (1.88 m) driving wheels, a compromise that took into account the intended mixed-traffic role they were designed for. This meant that they were large enough for sustained fast running with heavy passenger trains, yet small enough to allow them to undertake more mundane tasks such as freight haulage.

The design also featured raised running plates above the wheels, which allowed easy access to the inside of the frames for purposes of lubrication. The lack of wheel splashers on this running plate also reduced the risk of the bearings overheating, by allowing more air to flow around the axles when at speed. Wheel splashers were used on older locomotive designs to box-in the top portion of the driving wheels for aesthetic reasons, and to prevent water and dirt from being thrown at the cab windows. The "Britannnias'" footplate was designed around the requirements of the operating crews, with a mock-up being constructed at Crewe to test ergonomics and usability. For ease of maintenance, availability of spare parts and increased reliability, two sets of Walschaerts valve gear were used, along with the largest cylinders capable of staying within the British loading gauge. The "inside" cylinders located between the frames of a three- or four-cylindered locomotive are difficult to access for maintenance; the two-cylinder design of the Class 7 – with all the valve gear on the outside – avoided these maintenance problems. Boiler 'plumbing' was also generally exposed to maintain ease of access.

Following experience of occasional cracks appearing near the spring brackets had one or more further batches of Class 7 Standard Pacifics been built to complete the intended 91 engines the chassis' would have been rearranged to be similar to that used on the solitary Class 8 Standard Pacific and that drawn up by Derby Drawing Office for the 2nd batch of Class 6 Standard Pacifics. This would have resulted in the locomotive riding on cast steel "sub-frames" carrying the spring brackets. Unlike the smaller BR Standards the exhaust steam manifold within the smokebox saddle (along with the BR Standard Class 6 engines) was a steel casting.