Valve Gear



Steam locomotive valve gears are required to distribute the steam as equally as possible to each port of a double-acting cylinder, via a one-piece valve. Their design professionally seems not to have ventured into print except in most basic terms, leaving those who seek knowledge of the details and procedures – both analysers and modellers – in areas of complexity not easily demystified. The sources of misinformation and unverified statements, both in print and on the internet, unfortunately far outstrip those of accurate aid to design and understanding. In fact a concerted attempt to discover anything beyond a layman’s knowledge of valve gears in the myriad of internet entries fails miserably and inexplicably. The majority of statements have been gleaned from inadequate printed sources without validation; some are nothing less than ridiculous; very few are the contribution of thoughtful and technical inquiry and none in my search revealed better than a most basic understanding.

How can it be that after more than 150 years of the widespread application of a mechanism our global knowledge of its workings, foibles and corrections is so minimal? There appear to be a number of contributive factors. In the first place history has given the responsibility of printed material to the scholastic and not to the technical. Indeed the technician would find little time to devote to the general dispensation of skills. Mathematicians of the quality of Dr. Zeuner could not be accused of a similar lack, yet the frequent typical statements “ignoring the third term, which is very small….” and “neglecting the connecting rod angularity….” are unhelpful and clearly unacceptable. The profoundly knowledgeable Willie Pearce advised against a mathematical treatment with good reason. We can’t ignore the mathematics but it must be accurate to be of any use and is best accompanied by a thorough knowledge of the kinematics in conjunction with a simulator: one is of no use without the other.

There is ample evidence to suggest that many career designers themselves lacked essential detailed knowledge, just as some early fitters misunderstood Walschaerts’ gear, and there are countless examples of Stephenson’s gear prevented from delivering their best by the ill-conceived application of end suspension and a general disregard for correct application of the reversing components. There are, however, very early designs that bear excellent scrutiny by computer simulation, designed under CMEs as part of the drawing office team in anonymity except for such well-known names as W.H.Pearce and H.Holcroft. Even the wealth of Locomotive Institute Papers failed to tackle the subject at all adequately.

The model and miniature world can be forgiven under these circumstances for long-suffering and somewhat misguided attempts to unravel the design mysteries: there is still a long way to go. More history can be read under MINIATURES.

It is not generally appreciated that the valve gear constitutes a timing mechanism that is required to consist of as few links and pins as possible, thereby ‘disqualifying’ well over 100 patents involving ‘improvements’ and modifications. Throughout a single cycle each component is not only itself distorted but its relationship with the other component is also distorted. Minimising these distortions to produce near perfection at the valve formed the essential skills of the best designers, almost lost to the present generation. Because the steam engine is so inherently flexible the wheels will still turn when the inequality of distribution exceeds 10%, at around which point the problems will begin to be heard at the chimney. The best designs achieved a perfection within 1% equality anywhere in the working range, with good ones straying less than 3% in the later cut offs and better in the shorter cut offs. Such excellence should be the guide to all designers.

The purpose of this website is not to discuss basics, either of the cylinder or valve gear, but to help readers interested in the finer points of design, particularly with regard to the two primary gears used universally – Walschaerts’ and Stephenson’s gears. It is assumed that those readers are already conversant with what the gears are expected to achieve. Some notes are given on other gears and there are downloadable files to enable design and analysis.

About Don Ashton Himself

Don Ashton, like a thousand others, found a fascination in the steam locomotive in childhood and began to query the purpose of those shiny steel rods and links just as the newly formed British Railways contemplated a series of Standard designs. He soon discovered that information was somewhat sparse and decidedly basic. Most textbooks on steam engines (discounting the huge array of picture albums and volumes of history) contain only the rudiments of valve

gears and ignore both essential geometry and detailed kinematics. Indeed it soon becomes obvious to the inquirer that even those sources having ‘valve gears’ in the book title fail to satisfy, and that educated guesswork leaves the model engineer particularly disadvantaged.

Much frustrating study gradually formed a clearer picture, aided by close correspondence with like-minded devotees – Jim Ewins and Ted Gowan, the former experimentally inclined, admitting of nothing other than strict principle, and the latter retired from the Canadian National Railway and a mine of information and good practice. Alan Gettings lent his mathematical expertise. The culmination was Don Ashton’s production of two booklets dedicated to Walschaerts’ Gear and Stephenson’s Gear, published in 1976. These volumes were combined and updated with additional information in 2000 and again in 2007, when the opportunity to include the advances in computer simulation became so useful.

Don Ashton is neither engineer nor mathematician by profession but has long been active in valve gear design and modification for small scale locomotives, seeking knowledge from the dwindling ranks of those professionally involved, and enthusiastically corresponding with those whose primary object was and is to demystify the subject. Regrettably, the few professionals intimately concerned with the details of valve gear design appear not to have considered publication of their skills and knowledge worthwhile. Many must be the discarded personal note books and scraps of calculations that would enlighten, if found.

This web site is intended to aid those interested in pursuit of clarity and is not of itself finite.

Outside the locomotive and model engineering fields Don Ashton has held a long standing interest in tugs and their operations and has written extensively on the subject for a professional specialist magazine. In fact his authorship extends to other interests: as a qualified teacher and a well respected repairer of woodwind instruments he is active as a writer, arranger and adult residential course tutor. A long time champion of the saxophone, he is responsible for a specialist chapter in the CUP’s ‘Cambridge Companion to the Saxophone’.