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EspañolBULLEID 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 of valve gear to power a steam locomotive.
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, initiated 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. Bulleid, H. A. V.: ''Bulleid of the Southern'' (Hinckley: Ian Allan Publishing, 1977) ISBN 071100689X This prompted Oliver Bulleid's attempt to design a new 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 crankshaft.; this in turn was 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 which controlled admission of the steam into the cylinders and exhaust, thereby driving the pistons. Each valve head had its own steam chest, opposite pairs being connected by a double girder arrangement actuated by vertical rocking shafts located midway between the two.
Valve motion and the inside connecting rod were contained in an oil bath which consisted of a vertical steel box located between the main frames. About 2in depth of oil lay in the bath, the inside big end was splash lubricated and pumps sprayed the various valve motion joints. None of this was particularly revolutionary, being borrowed from internal-combustion engine practice, and in principle it was superb. It was thought 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 without attention. It was this consideration that meant the continued use of the system, albeit in modified form on Bulleid's Leader Class.
In practice, though, it proved unable to successfully undertake its job, for two main reasons. First, cracks developed in the oil bath the box due to incorrect welding procedure. Condensation caused corrosion, and oil leaked out through inadequate seals, causing wheelslip and fire hazards. Second, the play which could develop in the myriad pin joints made the valve movements highly unpredictable.
This was further complicated by the Eastleigh type of steam reverser used. If this reverser decided to misbehave 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 over-run their nominal full travel within 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.
★ Image of chain-driven gear as applied to the 'West Country' class locomotives
| Contents |
| Design principles |
| Problems |
| Footnotes |
| External links |
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, initiated 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. Bulleid, H. A. V.: ''Bulleid of the Southern'' (Hinckley: Ian Allan Publishing, 1977) ISBN 071100689X This prompted Oliver Bulleid's attempt to design a new 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 crankshaft.; this in turn was 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 which controlled admission of the steam into the cylinders and exhaust, thereby driving the pistons. Each valve head had its own steam chest, opposite pairs being connected by a double girder arrangement actuated by vertical rocking shafts located midway between the two.
Valve motion and the inside connecting rod were contained in an oil bath which consisted of a vertical steel box located between the main frames. About 2in depth of oil lay in the bath, the inside big end was splash lubricated and pumps sprayed the various valve motion joints. None of this was particularly revolutionary, being borrowed from internal-combustion engine practice, and in principle it was superb. It was thought 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 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, it proved unable to successfully undertake its job, for two main reasons. First, cracks developed in the oil bath the box due to incorrect welding procedure. Condensation caused corrosion, and oil leaked out through inadequate seals, causing wheelslip and fire hazards. Second, the play which could develop in the myriad pin joints made the valve movements highly unpredictable.
This was further complicated by the Eastleigh type of steam reverser used. If this reverser decided to misbehave 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 over-run their nominal full travel within 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.
Footnotes
External links
★ Image of chain-driven gear as applied to the 'West Country' class locomotives
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