"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?"
"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Chapelon's reservations about piston valves are no longer valid.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."
"[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:
"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)."
For further information on Wardale's design of the 5AT's piston valves, see notes on FDC-4 on the Calculations page.