Bulleid Chain-driven Valve Gear - Design Principles

Design Principles

Bulleid's decision to have three cylinders, all driving the middle coupled axle of his Merchant Navy and West Country / Battle of Britain classes, gave rise to several problems. As each cylinder was to have its own separate valve gear, this left very little space for the conventional inside set of motion. This prompted Bulleid to design a new miniaturised Walschaerts motion that was compact enough to enclose the whole system in a casing. All three sets of valve gear were worked from an auxiliary three-throw crankshaft. A Morse inverted-tooth drive chain ran horizontally from a triple sprocket on the driving axle to an idler sprocket, from which a second chain ran downwards to the crankshaft. By the use of two chains the valve gear was isolated from the movement of the driving axle on its springs. The idler sprocket could be moved to give both chains the correct tension, but in practice only the vertical chain was adjusted.

The auxiliary crankshaft drove both the eccentric rods and the combination levers of the valve gear. The piston valves were of the outside admission type. The valve heads were connected by a double girder arrangement, each actuated by a vertical rocking shaft located between the two girders and midway between the two heads, with a further connecting link pivoted just behind the valve head nearest the front. A sealed oscillating shaft drove the offset upper rocker arm and link, and the whole assembly worked inside the exhaust space of the steam chest. The advantage of having admission steam working on the outside faces of the valves meant that the volume under maximum pressure was completely sealed at the ends with no glands susceptible to leakage.

The valve motions and the inside connecting rod were enclosed in an oil bath consisting of a vertical steel box located between the main frame members, containing 40 gallons (182 litres) of oil. About 2 inches (50 mm) depth of oil lay in the bath; the inside big end was splash-lubricated, and two pumps sprayed oil through perforated pipes over the various valve motion pins. None of this was particularly revolutionary, being borrowed from internal-combustion engine practice; and for use with steam it was established practice for steam motors at the Sentinel Waggon Works. It was thought that the arrangement would obviate the daily need to oil all moving parts and as they were protected from the elements they should be able to run 100,000 miles (160,000 km) without attention. It was this consideration that meant the continued use of the system, albeit in modified form, on Bulleid's Leader class.