With well over 100 patents towards the end of the 19th century there are plenty of other valve gears from which to choose. A large proportion of these are really modifications of the two primary ones, some are additions with attendant complication and others either fundamentally complex or simplified beyond useful purpose. Those of the Hackworth family, most purporting to obviate the necessity for producing a curved slot or guide, are inferior despite the ingenuity employed, and Joy gear which retains the curved guide had the most popularity. It probably suffers more than the primary gears where longer valve travels are required and consequently died a death early in the 20th century.
Both the L & Y and the LNWR used Joy gear quite extensively for a period and the L & Y in particular sought diligently to remove the inherent distortions in the driveline. No current simulator can deal with these peculiarities adequately: curved guides did not reflect the radius rod (technically not so), guides did not sit square to the driveline and distortions were created through the anchor link – all in order to appease imperfections. In addition, the weighshaft position was so critical to the production of good events that the L & Y used temporary erection on each locomotive in order to assess the best permanent location, before finally bolting into position. This sensitivity, even with the short valve travels of the day, is clearly suggestive of the battle with distortions unhelpful in procuring the excellent distribution of the primary gears.
Both Gooch and Allan straight link gears are logical descendants of Stephenson’s gear. The former obviates the necessity to lift the heavy gear to reverse motion but is otherwise flawed. The principal culprit lies in the lifting arrangement, as both lifting link and lifting arm find difficulty in properly serving both forward and reverse gears.
Allan gear employs a straight link slot and can reduce any tendency to excessive lead variation but otherwise introduces more complication. No lever system to create straight line motion will bear close mathematical scrutiny and Allan gear is no exception. The short lifting arms attempting to produce this action by ratio are ill-fitted to suspend the valve gear, through two conflicting lifting links, in any decent manner. The common result of manipulation to satisfy a reasonable lead pattern hampers the ability to secure good cut off equality and a decent fore gear often leaves back gear wanting.
Baker valve gear, originally designed for traction engines, enjoyed some success in its home environment, America. It introduces angular elements of Marshall gear into Walschaerts’ in a complex yoke drive with a bell crank. Apart from the support given by the Pilliod Company any advantages are more apparent than real, though doubtless the Company would not agree. Simulation confirms its complexity in arriving at valve events predictably similar to Walschaerts’ yet more costly to produce.
Countless gear inventions merely sought to bypass patents and can be discounted in practical terms unless the student has a particular interest. Unprotected by patents, Stephenson’s gear, properly designed, can be made to produce equality second to none and Walschaerts’ can approach this performance. Their supremacy was never destined to be endangered. A most interesting learned Paper read before the Institute by T.H.Shields in 1943 has been reprinted by TEE Publishing and appears in the DOWNLOADS page. It is highly recommended to the historical student, yet is not technically a treatise in depth.
It seems odd that elements like the Marshall lever arms, Baker yokes and bell cranks, Greenly offsets and many other attempts to allow correction have not been generally perceived as what they really are. The timing distortions of rotary to linear motion and vice versa are similarly present in all swinging arms and rockers - an arc is simply part of a circle and the same principle applies to both. It is therefore not surprising to find complex mechanisms little better than Hackworth gear in its simplest form. It is wise to include the members of suspension and reversal as legitimate constituents of the mechanism with the same ability to distort distribution.
Few of the many valve gear variants saw more than the odd trial and one or two suffered total self destruction in the process. The proponents sought, presumably besides royalties, superior distribution or simplicity in manufacture. This latter is apparently evident amongst model engineers, who seem to have a hatred of curved links and it has to be admitted that manufacturing these to a fine tolerance can be taxing. The primary saviour comes in many variations of the Hackworth family of gears and near relatives like the Bremme and Greenly lever systems. Whilst the Hackworth straight slide patently introduces an unwanted error, perhaps its greatest disadvantage is the height restriction. Its simplicity becomes its greatest enemy, since it has no mechanism giving the opportunity to counter the timing errors. The lever systems suffer the same difficulties and resort to part-obliging cranked offsets. All the elements become well overtaxed as the valve travels increase. The model engineer is often perfectly satisfied as long as the wheels turn, which is hardly putting anything to test.
None of these surpass the ingenious attempts to reverse the simple eccentric (not that this is capable of distributional excellence), starting with Dodd’s slot of 1838 and wedge variant of 1839, defeated by materials and seals of the day. Perhaps its culmination is the fascinating reversible single eccentric applied to a German 4-axle outside-hung V8, a simple illustration of which leaves the mind wondering until viewing the short video. If only the valiant Mr. Dodd could see this!
Click here to view the Article
Video and Article courtesy Dr. Allan Wallace
1920 saw the birth of Caprotti valve gear, though some of the stationary engine gears of much greater age found railway trial, disadvantaged by the demands for flexibility in the railway environment. Caprotti cam-driven valves have proved just as feasible in small scale as in full size, basing claims on two important aspects: the demonstrably more immediate opening at the beginning of the stroke and the ability to control the exhaust port independently of inlet. It must be appreciated that the available space for adequate valves, and the problems of valve seat integrity and precision components in a high temperature environment gave cause for concern.
Although both the LMS and BR can be said to have ‘dabbled’ in this area there do not appear to have been concerted tests or conclusions. Typical of railway locomotive history, poppet valves are yet another example of hoping to achieve dramatic overall improvement by the inclusion of a single item, be it a water feed heater, condensing system, exhaust injector or whatever. Piecemeal tinkering with design detail, or efforts to cure a troublesome component in isolation, seem to have been at the root of expensive attempts to raise efficiency.
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