Analysis of the Duchess valve gear starts with the reconstruction of the gear from the official drawing. In this case the dimension of the return crank is not given, but the associated pitch circle and angular setting provide the input. The front end of the radius rod is located from the cylinder centreline, and the weighshaft and expansion link trunnions are located both vertically and horizontally, though square to the frame line and not that of the 1:50 motion inclination. Thus placed, the given expansion link cannot support the eccentric rod of 66.25″ without some adjustment to the 92.5o angle and length of the return crank.
Even when this adjustment is made at back dead centre a half revolution does not allow the expansion link to attain an identical position, causing the valve to move using the reverser with the crank on front dead centre. The effect is to render reasonable valve events at the expense of equal leads, with back gear proving the better set of the two modes. Since it is the practice of valve setters to adjust the eccentric rod length to attain primary equalisation any CAD analysis must allow a slight variation in this component, so mental account of this is taken – an angle of 92.5o produces an eccentric rod of 66.208″, with inferior lead equality. The upper pins measurement of the combination lever is obscured but this figure can be gained on simulation to conform to the lap/lead geometry.
If we therefore allow the principle (no valve movement at dead centres), achieved in practice at valve setting by correcting the length of the eccentric rod, then the 92.5o return crank setting on the drawing has to be corrected first and the resultant CAD reappraisal fed into the simulation. All the simulated results must then bear in mind the alteration made.
Simulation proceeds initially with the stated eccentric rod and associated return crank, without any correcting influence of a reduced backset dimension, since this amounts to far more than production tolerances might incur in making the expansion links. The immediate results are not expected to be correct, since their sensitivity to valve setting is so critical and this is the next stage in simulation. Initial valve setting in the simulator differs from that pertaining in practice because, effectively, the position of the cylinder in relation to the gear (or the relative length of valve spindle) is not made in the same way.
Regrettably, most locomotives have no means of adjusting the valve on its spindle, with a heavy reliance on manufacturing tolerances. The initial aim, however, in both cases attempts to produce equality of leads, but in the case of simulation establishes the gear’s ability to support such equality. This is important in gear design analysis and reveals directly what happens in reality when a valve setting exercise equalises leads at the valve heads when the gear design has no means of complying. It must be understood that equalising leads at the valve when a gear cannot physically support this condition plays havoc with all other events. All gears exhibit events very sensitive to very small changes in the valve setting. Since the leads display variations of power and economy for given cut offs any acceptable difference in lead from front to rear ports is probably ascertained more clearly by computer calculation for a specific locomotive.
The Duchess gear responds as frequently expected – attempts to gain better coincidence of the exhaust and cut off curves, in this case necessarily taking into account the reversal of events against a much shorter connecting rod in the rocker drive to the inner cylinder, results in lead asymmetry. An equal lead of 0.25″ produces too much event inequality and the Duchess requires something like 0.178″ at the front port and 0.278″ at the rear in order to give events that satisfy forward running both for the outside cylinders and to pass on to the inside. This leaves events portraying a typical LMS pattern, with a full gear cut off approaching 3% difference. Note that the dimensioned return crank angle has been corrected – one must hope that the angle was corrected at manufacture – and that the lead discrepancy is fairly typical of Walschaerts’ designs.
The stated full gear cut off of 75% nominal is not obtainable at a 29o lifting arm angle. It barely stretches above 71%. The nominal figure is only 70%. As so often happens, the reverse events are marginally better and one wonders why some design effort to shift the favour to forward at the expense of back gear seems not to have been applied to an express engine. The age-old problems of layout associated with large-wheeled engines – finding a suitable place to accommodate the weighshaft and to produce a stiff enough mounting for the expansion link – have affected the Duchess, though with less trouble than her forebears, the Princess Royals.
Comparison is best made with the contemporary Class Five gear, which portrays marginally better events. The Duchess has ¼″ more lap, slightly greater depth-in-gear and 1″ greater return crank circle to give 6.98″ travel, whereas the Five manages 6.47″ travel. The net result is that the Class Five is comfortably within its design remit where the Duchess struggles to make its nominal parameters.