Why use Walschaerts valve gear and not Caprotti driven poppet valves?

Wardale strongly defends his choice of Walchaerts valve gear and piston valves, and has explained his reasons in several letters, some of which are included in the page titled “Why Walschaerts Valve Gear?”.  However a more comprehensive reply is recorded in a letter from David Wardale to John Duncan which is transcribed over 7 pages titled “Piston Valves vs. Caprotti Valves – The Final Discussion?” and in PDF format in Wardale’s letter dated 3rd Sept 2009.

In his letter to Wardale, John Duncan refers to letters by Bryan Attewell and Angus Eickhoff relating to Wardale’s article “Steaming into the Future” published in Steam Railway magazine in 2002.  These letters are referred to below.

The following answer is taken from Wardale’s response to an unpublished letter from Bryan Attewell written in 2002 in response to Wardale’s two-part article “Steaming into the Future” published in Steam Railway magazines #272 and #273:

“Caprotti valve gear is expensive specialist equipment, so piston valves and Walschaerts valve gear have been chosen because with Porta refinements they give a performance of the same standard and at much lower capital cost. Indicator diagrams from “The Red Devil” – which the 5AT will greatly improve on – have proved that. Can Mr. Attewell produce figures to show that poppet valve gear requires “considerably less” power to drive than Porta-type lightweight piston valves driven by Walschaerts gear running in needle roller bearings?”

Wardale offered further comments in response to a letter from Angus Eickhoff published in Steam Railway #276 in Oct 2002:

“The question of Walschaerts versus Caprotti valve gear has been partly dealt with in the reply to Mr. Attewell. To answer Mr. Eickhoff’s points, the contribution of Walschaerts valve gear to the balancing issue is negligible as most of its inertia forces are out of phase with those of the main reciprocating masses, and the maximum acceleration (i.e. inertia force per unit mass) of even ultra long travel valves is only some 30% of that of the pistons.

Although altering the valve events by changing the cams may be convenient on an “experimental machine”, the terms of the project mean that the design of the 5AT is in no way intended to be experimental.

Chapelon’s reservations about piston valves are no longer valid. Porta’s invention of valve liner cooling allows higher steam temperatures to be used without lubrication difficulties, and inertia forces are kept at manageable levels by the very lightweight construction made possible by designing the valves according to stress analysis rather than empirical rules. It is also worth noting that Chapelon’s final masterpiece, the 242A-1, as well as his aborted 152P design, had piston valves driven by Walschaerts gear. No recourse to poppet valves needs to be considered before the possibilities of piston valves have been fully explored – which is what the 5AT will do.”

16 Mar 2003: Wardale offers further comment on the issue in his Fundamental Design Calcs for the main crank pins for the locomotive. In FDC 3 Item 137, he states:

“Because bending stress due to inertia load at maximum speed is greater than maximum allowable crankpin fibre stress, cushioning must be provided to relieve the inertia load on the main crankpin. Note that this is one reason to reject Caprotti valve gear as finally applied to BR locomotives, as the valves were arranged to drop from their seats during drifting thus providing a full by-pass from one end of the cylinder to the other, precluding the build-up of cushioning steam pressure. The required cushioning steam pressure at dead centre is now calculated for the worst case condition, i.e. maximum speed with minimum coupled wheel tyre thickness, this pressure being therefore suitable for all lower coupled wheel rotational speeds …..”

Wardale goes on to state: “…. other reasons are:

  • cost;
  • it is specialist equipment;
  • less than 100% certainty about steam tightness and good flow coefficients past the double-beat poppet valves;
  • the fact that the piston valves for the 5AT will in all respects (e.g. steam flow, lightness, lubrication, wear, resistance to high steam temperatures, resistance to steam leakage) be greatly superior to the general level of piston valves in the past with which Caprotti valves have been compared;
  • Walschaerts gear mounted on needle roller bearings and with mechanical lubrication of the dieblock will be extremely wear-resistant and hence maintenance-free;
  • lastly (from the point of view of the project’s aim, not least), Walschaerts gear is aesthetically more attractive.”

23rd March 2003: In response to the question “Why not make the crank-pin bigger?”, Wardale responded with the following:

“Why not make the crankpin bigger? Because if we do, the mass of everything associated with it – the crankpin itself, the roller bearings, connecting rod big-end, coupling rod eye, and seals will all get bigger too. As crankpin stress is inversely proportional to the diameter cubed and mass directly proportional to diameter squared (roughly), equilibrium will always be reached at slightly lower stress as diameter is increased, but remembering that the whole lot has to be balanced and because we can keep the original diameter (FDC.3 item 17, similar to BR 5MT design) by using alloy steel, my judgement is to use that option. Alloy steel for the main crankpin is not an unusual thing – for example the modified QJ would have had to have it, as the crankpin could be made no bigger due to clearance restrictions (on small-wheeled engines there is always a problem with gauge clearance of the big-end when the crank is at the bottom, although this does not apply to the 5AT).”