Definitions

chain driven

Bulleid chain-driven valve gear

The Bulleid chain-driven valve gear is a design of steam locomotive valve gear designed by Oliver Bulleid for use on his Pacific (4-6-2) designs during the Second World War. They were peculiar to the Southern Railway in Britain, and were based upon motor-vehicle practice in an attempt to create a compact, but efficient design with a minimum of service requirements.

Design principles

Oliver Bulleid's decision to have three cylinders, all driving the middle coupled axle of his Merchant Navy, West Country and 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 Oliver Bulleid's attempt to design a new miniaturised Walschaerts motion layout 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 driven from the driving axle by triple chains through an idler gear. The auxiliary crankshaft drove both eccentric rods and combination levers displacing the piston valves; these 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 that 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.

Valve motion and the inside connecting rod were enclosed in an oil bath consisting of a vertical steel box located between the main frame members. About 2 inches (50 mm) depth of oil lay in the bath, the inside big end was splash-lubricated and a pump sprayed 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 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.

Problems

In practice, though, the arrangement had a number of disadvantages. Cracks developed in the oil bath casing due to incorrect welding procedure. Condensation caused corrosion, and oil leaked out through inadequate seals, causing wheelslip and fire hazards. Valve timing was highly unpredictable; this has been attributed to chain stretch, although Bulleid claimed that it was allowed for. A more likely cause may have been geometrical due to the sequential proportions of levers, especially the final rockers transmitting the limited movement of the "miniaturised" valve gear to the union link driving the long travel piston valve; this "anti-lever" alone would have made accurate valve timing difficult to achieve and given rise to stress in the drive mechanism. Wear developing in the pin joints could only have exacerbated this lack of precision making chain wear more of a consequence than the cause of the problem. A further complication was the unpredictable behaviour of the Eastleigh type of steam reverser employed. If this reverser misbehaved under the fluctuating pressures present in the steam chest, the locomotive seemed to have a will of its own. For example, if the reverser dropped into full gear, the slackness in the motion would make the valves overrun their designed full travel inside the cylinders. This would result in the locomotive taking off like an unleashed race horse, high coal consumption, throwing the fire out of the chimney and running the risk of a violent high-speed slipping.

These problems combined contributed to the gear's eventual replacement by three sets of Walschaerts gear mounted in the conventional manner.

Footnotes

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