North Midland 2-4-0 Locomotives in Great_Britain

Class 70 (Locobase 10310)

Data from "The Locomotives of the Great Eastern Railway," The Locomotive Magazine, Vol VIII (2 May 1903), p. 296-297 and Steamindex's Midland Railway section at [] . For a full discussion of the question of who invented the Stephenson [sic] link motion; See James G H Warren, A Century of Locomotive Building by Robert Stephenson & Co. - 1823-1923 (Newcastle Upon Tyne: Andrew Reid & Company, Ltd, 1923), pp. 358-372. Boiler pressure is an estimate.

Steamindex notes that these locomotives were incorporated into the Midland Counties when North Midland and Birmingham & Derby Junction combined. A sketch of the engine shows a typical hemispherically domed firebox, straight "long boiler", tall stack with flared cap, two very-closely spaced coupled axles, and a leading carrying axle well forward.

Warren's careful explication of the origins of the "Stephenson link motion" considers its antecedents, other valve motion designs and their designers, and finally digs into the dispute between those who claimed that it was the Stephenson -William Howe motion (Howe was a pattern maker at the time) and those who argued for the William Williams-William Howe parentage. Page 365 of the account carefully lays out Warren's view, which conforms to the final consensus:

Essentially, it was William Williams, "a young draughtsman", who came up with a link within which a block could be raised and lowered. In his original 1841 sketches, the ends of the link were attached directly to the straps on each eccentric. It also appears that Williams' later explorations evolved the stationary link. But it was Howe's introduction of rods extending from each eccentric to meet at the ends of a stationary link that "completed the inevitable evolution of the Link Motion."

In an effort to settle the dispute in 1870, George Crow wrote to Robert Stephenson's cousin George Robert, then in charge of the company, asking for his opinion. In Locobase's opinon, Stephenson's reply answers for all: "I have refrained from answering yours of last month ...relating to the invention of the Link Motion, for the simple reason that it is not a pleasant matter to deal with."

The subsequent memorial referred to the Williams-Howe motion and that is how it has been characterized by most ever since. It's operation is relatively simple to imagine. The die block mounted at the end of the valve stem moves only horizontally. A link that connects the ends of the eccentric rods contains a slot that allows it to slide up and down and thus change the setting on the valve.

When starting a locomotive forward, the upper end of the link rests on the die block. This is full-forward. To change the valve's lead and thus how long the valve port is open, the driver raises the link through a "reverser" and reduces the port opening. As the block moves closer and closer to the middle of the link, the amount of steam admitted decreases. Move past the middle of the link, and the block will induce a reversal of valve operation and the locomotive will begin moving in reverse.

Not long after the class was delivered, the MCR sold either 71 (Locomotive Magazine) or 73 (Steamindex) to the ECR, which renumbered it 140. LM notes that the ECR engine was renumbered twice (271 in 1850, 95 some years later).