Gresley conjugated valve gear
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The Gresley conjugated valve gear was a valve gear for steam locomotives designed by Sir Nigel Gresley, chief mechanical engineer of the LNER, assisted by Harold Holcroft. It enables a three-cylinder locomotive to operate with only the two sets of valve gear for the outside cylinders, and derives the valve motion for the inside cylinder from them by means of levers.[1] The gear is sometimes known as the Gresley-Holcroft gear, acknowledging Holcroft's major contributions to its development.
Although the conjugated design functioned well in a peacetime environment with regular maintenance and inspections, it proved to be poorly suited to the rigours of heavy running and low maintenance levels of World War 2. Gresley's successor at the LNER, Edward Thompson, was critical of this particular valve gear.[2] As well as introducing new two cylinder designs, he set about rebuilding Gresley locomotives with Walschaert valve gear on all three cylinders.[3]
Gresley conjugated valve gear was used by the American Locomotive Company under license and the 4-12-2 locomotives they built for the Union Pacific Railroad were the largest locomotives to use this valve gear. It was also used in Australia for the Victorian Railways S class 4-6-2[4] and New South Wales Government Railways D57 class 4-8-2.[5]
Locomotives with Gresley valve gear are sometimes noted for having an unusual exhaust note. In order for the inside crank to clear the leading coupled axle, the inside cylinder of a locomotive with Gresley valve gear is typically positioned higher than the outside cylinders and angled downward.[6] To maintain a smooth flow of torque, the crank angles are offset from equal 120 degree spacing to compensate for the angle of the inside cylinder (eg. 120/113/127 degrees). The resultant uneven timing of the blast from steam exiting the cylinders gives these three cylinder locomotives an unusual, "off-beat" exhaust.
[edit] References
- ^ Restoration of Gresley A4 #60019 Bittern Diagram of Gresley and Walschaert valve gear arrangement on LNER A4 locomotive] - retrieved 4th October 2006
- ^ (LNER) Encyclopedia Edward Thompson pageretrieved 1st October 2006
- ^ (LNER) Encyclopedia A1/1 pageretrieved 1st October 2006
- ^ AHRS Railway Museum History: 1900 - 1950 retrieved 1st October 2006
- ^ australiansteam.com NSW page retrieved 1st October 2006
- ^ Public Record Office Victoria photograph of cylinder castings for VR S class -retrieved 4th October 2006. Note incline of centre cylinder.
[edit] External Links
Southern California Chapter of the Railway and Locomotive Historical Society Information on Union Pacific ALCo Built three cylinder 4-12-2 UP9000, including sound recordings and photographs
