page_title: SR Merchant Navy class
section_title: Cylinders, valve gear, wheels and brakes
hierarchical_section_title: SR Merchant Navy class / Design / Cylinders, valve gear, wheels and brakes
caption_attribution_description: Bulleid Firth Brown (not a Boxpok) wheel from SR 4-6-2 Merchant Navy Class no. 35010 "Blue Star". on Static display awaiting restoration. at the Colne Valley Railway
context_page_description: The SR Merchant Navy class is a class of air-smoothed 4-6-2 Pacific steam locomotives designed for the Southern Railway by Oliver Bulleid. The Pacific design was chosen in preference to several others proposed by Bulleid. The first members of the class were constructed during the Second World War, and the last of the 30 locomotives in 1949.
  Incorporating a number of new developments in British steam locomotive technology, the design of the Merchant Navy class was among the first to use welding in the construction process; this enabled easier fabrication of components during the austerity of the war and post-war economies. In addition the locomotives featured thermic syphons in their boilers and the controversial Bulleid chain-driven valve gear. The class members were named after the Merchant Navy shipping lines involved in the Battle of the Atlantic, and latterly those which used Southampton Docks, a publicity masterstroke by the Southern Railway, which operated Southampton Docks during the period.
context_section_description: Three 18 inches (46 cm) diameter cylinders drove the centre coupled axle. The inside cylinder was steeply inclined at 1:7.5 but the outside cylinders were horizontal.
  It was originally intended to use a gear-driven valve gear, but space restrictions within the frames and wartime material shortages led Bulleid to design his novel chain-driven valve gear. This component was unique amongst British locomotive design practices. It later gained a bad reputation, because it could cause highly irregular valve events, a problem compounded by the fast-moving Bulleid steam reverser. The entire system was located in a sealed oil bath, another unique design, providing constant lubrication to the moving parts.
  The locomotives were equipped with the unusual 6 ft 2 in (1.88 m) Bulleid Firth Brown (BFB) driving wheels which were lighter, yet stronger than the spoked equivalent. These proved to be successful and were later used on other Bulleid classes. The leading bogie was based upon that of the SR Lord Nelson class, although it had a 6 ft 3 in (1.90 m) wheelbase as opposed to Maunsell's 7 ft 6 in (2.28 m) design, and featured 3 ft 1 in (0.94 m) BFB wheels. A long coupled driving wheelbase was incorporated into the design, to keep the locomotives within the lineal loading of the Southern Railway's narrower bridges. The supporting rear trailing truck was a one-piece steel casting that gave the smoothest of rides; the design was utilised in the future BR Standard Class 7.
  The spaces between driving wheels housed steam-powered clasp brakes, that gripped the wheels by way of a "scissor" action. The two middle brake hangers held two brake blocks each, whilst the two outside hanger on the leading and rear driving wheels held one block each. These were connected together by outside rodding for ease of access, and the whole system was operated from the footplate.
caption: A driving wheel of the distinctive Bulleid Firth Brown design, seen here on 35010 Blue Star.  Note the balance weight on the lower right, a feature not present on the Merchant Navy locomotives as-built.
exif: {}