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. The valve gear of a Steam engine is the mechanism that operates the inlet and exhaust Valves to admit Steam into the cylinder and allow exhaust A steam locomotive is a Locomotive powered by Steam. The term usually refers to its use on Railways but can also refer to a "road locomotive" Sir Herbert Nigel Gresley ( 19 June, 1876 &ndash 5 April, 1941) was one of Britain's most famous Steam locomotive engineers The 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.

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.

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.

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. Edward Thompson (1881-1954 was Chief Mechanical Engineer of the London and North Eastern Railway between 1941 and 1946 The valve gear of a Steam engine is the mechanism that operates the inlet and exhaust Valves to admit Steam into the cylinder and allow exhaust ^[3] As well as introducing new two-cylinder designs, he set about rebuilding Gresley locomotives with Walschaerts valve gear on all three cylinders. The Walschaerts valve gear is a type of Steam engine Valve gear invented by Belgian railway mechanical engineer Egide Walschaerts ^[4]

USA and Australia

The third cylinder and Gresley gear are visible below the smokebox of this 4-12-2.

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. The American Locomotive Company, often shortened to ALCO or Alco (or less frequently ALCo) was a builder of Railroad Locomotives in A 4-12-2, in the Whyte notation for the classification of Steam locomotives by Wheel arrangement, is a locomotive with four Leading wheels twelve 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. See also Rail transport in Victoria The Victorian Railways operated railways in the Australian state of Victoria from 1859 to 1983 The S class was an express passenger Steam locomotive that ran on Victorian Railways from 1928 to 1954 Under the Whyte notation for the classification of Steam locomotives, a 4-6-2 Locomotive has four Leading wheels (generally arranged in a The New South Wales Government Railways (NSWGR was the government department that operated the New South Wales Government's railways until the establishment of the Public Transport Under the Whyte notation for the classification of Steam locomotives a 4-8-2 Locomotive has four Leading wheels (generally arranged in a ^[6]

References

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