CONJUGATING

NSW D 57 Class Steam Loco -The Conjugating Gear for the centre valves may be seen below the Smokebox door.

"Conjugated" is not a word that we hear often. This is doubly so in these times, in this post-classical world, because it is a word heavy with Latin association. Literally," it means " joined together with". So, how does it relate to the massive, brooding, almost brutish image above? The clue is right in front of us, right at the front of the locomotive itself. Right there under the "chin" of the smokebox is  "Gresley's conjugating valve gear".

The locomotive shown above is one of the D57 Class freight locomotives of the New South Wales Government Railways. When they were introduced in 1927 they were the only three-cylinder steam locomotives on the N.S.W. Railways. They retained that exclusive status until 1957 when the nearly identical, but strangely less successful D 58 Class came into service. 

Three-cylinder steam locomotives were developed where the outside cylinders and valve gear on either side were complemented by a central third cylinder between the frames and driving a cranked axle, this was most commonly made practical using Gresley's conjugating valve gear. This valve gear drew its action from the outer valve gear via a rod moving on a pivot under the front of the smokebox.

Three-cylinder steam locomotives were already common in British railway practice and had been so for several decades. They were the result of the restrictions of the British loading gauges and the need for greater power to draw heavier and heavier trains at greater and greater speeds.This arose from the intense competition between the various railway companies prior to the disastrous railway nationalisation undertaken by the Socialist Labor government in 1948.

In New South Wales the need for additional power had historically been met by increasing the size of the locomotive - principally its boiler.This process can readily be traced through the development of the C32, C34, C35, C36 classes of passenger locomotives culminating in the C38 Class. In the C38 Class, we see the absolute limit in many respects, of what was possible. The locomotive and its tender filled the limits of the loading gauge much as a bodybuilder might cause his shirt to bulge and strain. They were as long as the principal turntables in place could handle. Freight locomotives were a different set of requirements, and the D57 Class was also built to the limits of the loading gauge. Moreover, its weight, especially the load on the driving axles, restricted its employment to certain main lines only.If extra power was to be gained, the third cylinder was the way to go.  It is interesting to note that in the USA there were very few three-cylinder locomotives. The reason was simple, the American loading gauges were so large that it was possible to easily envisage and build larger locomotives.

Three-cylinder locomotives had, by and large, been avoided in N,S,W, practice due to the desire to keep mechanical servicing simple since servicing facilities were relatively few and far between in N.S.W. as they are throughout Australia.

The D57 Class locomotives were colloquially known to railway staff as "the big engines".

This classic photo by the great Leon Oberg shows the Gresley conjugating valve gear

Anyone who has heard the triple-beat of the exhaust of a D57 or D58 Class locomotive, and felt the trembling of the earth as they passed at the head of a heavy freight train, will never forget the experience.