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.

1 Operation
2 Crank angles
3 Problems
4 USA and Australia
5 References
6 External Links

[edit] Operation

The Gresley conjugated gear is effectively an adding machine, where the position of the valve for the inside cylinder is the sum of the positions of the two outside cylinders, but reversed in direction. It can also be thought of as a rocking lever between one outside cylinder and the inside cylinder, as is common on 4-cylinder steam locomotives, but with the pivot point being moved back and forth by a lever from the other outside cylinder.

[edit] Crank angles

Locomotives with Gresley valve gear must have the three pistons operating at precisely 120 degree intervals. 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.^[2] 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 timing of the blast from steam exiting the cylinders still gives these three-cylinder locomotives a regular exhaust beat.

[edit] Problems

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. This gave rise to big-end problems on the center cylinder connecting rod on the famous A4 class of streamlined pacifics, and many of these locomotives were fitted with a reduced diameter piston and had the inside cylinder lined up as a temporary measure. Gresley's successor at the LNER, Edward Thompson, was critical of this particular valve gear.^[3] As well as introducing new two-cylinder designs, he set about rebuilding Gresley locomotives with Walschaert valve gear on all three cylinders.^[4]

[edit] USA and Australia

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^[5] and New South Wales Government Railways D57 class 4-8-2.^[6]