A BRIEF HISTORY OF VALVE GEARS IN MINIATURE GAUGES
THE WALSCHAERTS’ MECHANISM IN PERSPECTIVE
Possibly due in part to an almost nonexistent liaison between locomotive designers and those concerned with small scale steam engines, and a similar lack of verifiable published information, the model world was destined to attach too much importance to ‘backset’ of the expansion link drive in Walschaerts’ gear. The singular published attempt to enlighten the fraternity was Henry Greenly’s book and faith was generally accorded since Greenly was esteemed as the designer of the fine miniature locomotives of the Romney, Hythe & Dymchurch Railway. The minimal equations therein require little attention to find them wanting.
In the 1960s and 70s those interested in the design of the gear were in good regular touch with each other in an effort to trace the efficacy and precision of backsetting, with but little intimate knowledge or confirmation of the results. Perhaps the single fact to emerge was that at some definitive point the practical application of straightforward formulae could result in precisely equal angles of swing of the expansion link, in effect replacing the exercise with trammels on paper. This was perceived as a great step forwards, satisfying the principle of lead equality at dead centres and rationalising the return crank drive system.
In some ways this was unfortunately taken as a solution to a perceived problem rather than as a datum easily calculated, from which to derive a tuning of the mechanism in order to satisfy equal steam distribution at the two ports. It should be obvious that the provision of equal curved expansion link swings cannot of itself deliver equality of dieblock movements, particularly for long valve travels, and that its provision does nothing to alleviate either the effects of crank/piston relationships or the small foibles encountered in the return crank’s excursions. There is a mitigating circumstance in the wanton way in which many earlier full size designers and most textbook authors treated these foibles as irremediable or simply not present. This hardly produces the best historical background to encourage better logic from those amateurs who boldly sought to crack a proverbial nut – Don Ashton, Jim Ewins, Ted Gowan and Alan Gettings. The ‘solution’ so found was subsequently discovered to be a reinvention of formulae printed in a German locomotive journal in 1924!
Further mathematical analysis, although by then more satisfying and applicable to the new hand-held calculators, becomes much more complex once the ‘necessity’ for equiangular swings is removed. This was tackled by Dr. F.M.Burrows, a Whitworth Fellow at Bangor University in the 1970s, who provided a Table from which straightforward interpolation could bypass the complexities adjoining unequal swings to provide solutions in these cases. The lead equality principle remained, unless one required to compromise this slightly without ill effect at the cylinder.
Unfortunately, whilst being a mathematical triumph, this evaluation remained centred around the precise backset calculation, without consideration of the valve events other than those of lead. The Burrows discussion to remove the ‘primary timing errors’ appeared not to include the piston positions other than at the dead centres, nor for that matter those of the valve. One or two practical items were also ignored: the valve setters’ primary act is to verify the equal lead principle and adjust the eccentric rod length to conform. The consequences of subsequently setting the valve to equalise leads when the gear design cannot support this were not discussed. There was praise for the King gear of 1927 as a thorough development from earlier classes without knowing the resultant events other than from the unreliable testimony of a drawing office Table: many are the cases of rudimentary or massaged figures now put to task by computer analysis. The King design is good, but not beyond improvement by simulation. Suffice to say that this example confirms that the ‘backset’ is modified as the inclination of the eccentric rod drive becomes greater, reaching zero and then becoming positive in the King.
Computer simulation reveals considerably more. In particular it shows that the two components comprising the valve movement cannot be altered individually without affecting each other. The variables are legion and do not readily submit to formulaic construction. The simple fact that the principle of equal leads depends so heavily on the return crank derived motion should warn any student of the gear that attempted separation of the component functions is doomed. It is still advantageous in the initial stages of design to regard geometric symmetry throughout the gear layout as good practice, creating a sensible datum from which to deviate only with knowledge of the consequences. By several variations, some very sensitive and others less so, the two inputs can be made to render the final output considerably improved. The chief of these concerns the anchor or union link and the backset. Alteration of the heights of weighshaft and expansion link fulcrum gain some control over a bias shift in equality between back gear and fore gear, but the method of suspension of the radius rod rear end requires very careful design if it is to avoid imparting the opposite of equality.
The progress or otherwise of equality throughout the range can be represented by two curves to show the differences between each port activity. Perfect equality would produce complete coincidence of these curves, but as this condition is almost impossible the two curves form a typical relationship irrespective of the gear used. Alteration in the simulator then becomes an exercise in achieving what is known to be possible independent of the particular layout under scrutiny - a self-evident measure of design efficacy and distributive performance. It highlights the problems faced before the advent of the simulator dramatically.
Another aspect also becomes clear: our locomotive engineers, who had no such technological aid, had amongst their number from the earliest times many of distinction, able to appreciate and attend to all the finer points of Walschaerts’ design. There are, naturally, examples displaying a more rudimentary understanding, a description that unfortunately encompasses all the smaller scale designers. Jim Ewins had a saying ‘they don’t know what they don’t know’. How telling! Simulation will lead to a better understanding and eventually enable the present trial and error to be replaced by a more logical approach.
STEPHENSON’S MECHANISM IN PERSPECTIVE
The Stephenson’s mechanism is perhaps more readily assimilated, though essential details did not begin to appear to the miniature world before G.S.Willoughby’s rather vague explanations in The Model Engineer in 1937. Apparently his information came from Swindon Works, but the precise details were either misconstrued or misrepresented, even though precise details of the mechanism were elucidated by Auchincloss as early as 1867. These were not presented to modellers until Ted Gowan’s revelations to the SMEE nearly a century later, which laid down the principles and the methods of carrying out the necessary design work graphically. Although this was an enormous step forwards few designers appear to have realised its significance and mystery continued unabated.
With Ted Gowan’s ready permission this information was incorporated in Don Ashton’s self-published booklets on Stephenson’s Valve Gear and Walschaerts’ Gear, 6000 copies of which went all over the world. The graphical exposition was later augmented by precise calculations for the correct design of Stephenson’s gear to near perfection in Excel spreadsheet form (see the DOWNLOADS page).
There is plenty of evidence to suggest that this neat suspension, known almost since the invention of the Howe link, was a mystery to many designers – if no offset is incorporated in a design it is in error. If the wrong link type is used in the inappropriate driveline all the angularity errors combine and a large offset and less perfect distribution result, made abundantly clear when a modeller substitutes an outside admission valve in place of a prototype’s inside admission one or misguidedly employs a launch-type link to keep eccentric sizes moderate. Such a mistake was rarely perpetrated unknowingly in full size practice even where the offsetting exercise seems absent.
A simulator proves beyond doubt the veracity of correctly offsetting the trunnions, allowing steam distribution to each port in the order of 1% equality, an achievement rarely obtainable in Walschaerts’ gear designs, where a good layout can typically produce 2.5% in full gear lessening towards the shorter running cut offs. In such cases it is the front port which has the longer cut off, yet displays the least port opening, indicating that the valve is travelling more slowly here. There is a measure of compensation between the front and rear ports in spite of the graphic disclosure, and it is the primary reason why old slide valve ports were made as wide as possible.