It is vital that the tractive forces exerted by a locomotive's wheels do not cause the available wheel/rail friction coefficient to be exceeded. For this reason, a locomotive's "tractive effort" (TE) is limited by the adhesive weight on its driving wheels.
Most steam locomotives were designed so that their TE was no more than 20% of their adhesive weight - meaning that they should not slip if the available friction coefficient is greater than 0.20, however the 5AT's relatively high TE demands a minimum friction coefficient of approximately 0.25. This will be achieved through appropriate use of sanding equipment (see below).
It is not always appreciated that the tractive force exerted by steam locomotives is very far from even. The variation in force exerted by the pistons over the lengths of their stroke is most marked on a 2-cylinder "simple" locomotive like the 5AT, and occurs even at the highest speeds to the extent that it can cause momentary slipping if the available friction coefficient is exceeded. The diagram at right (taken from FDC 1.4) shows the variation in tractive force applied by the 5AT at starting, at maximum drawbar power (113 km/h) and at maximum speed (200 km/h). [Click on the diagram for an enlarged image.]
The adhesion requirements for the 5AT are covered in the FDCs that are available for download from this website.
FDC 1.1 (Line 51) confirms that the average tractive frictional force available to the locomotive under all rail conditions over the locomotive's full speed range exceeds the tractive force available over that range.
FDC 1.4 (lines 33) determine the instantaneous wheel rim forces at 15 degree increments over the full (360o) cycle at starting, maximum power and at maximum speed, and verifies that they remain below the available adhesion limits. Furthermore, in line 78 Wardale checks that the effects of dynamic augment (hammer blow) do not result in momentary loss of adhesion.
Wardale summarizes his findings as follows:
The relationship between maximum wheel rim tractive effort and adhesive weight is generally well-balanced throughout the Class 5AT's speed range, and especially at the high speeds at which most miles will be run.
Although it will be a modest-size locomotive, it is worth noting that the 4-6-0 type has the same level of adhesive weight as much larger 4-6-2's and 4-6-4's (the only European locomotives of these two wheel arrangements which exceeded the adhesive weight of the GWR 'King' Class 4-6-0's were the Belgian Class 1 4-6-2's, the LMS Turbomotive and de Caso's SNCF 4-6-4s 232R & 232S).
An analysis of the principal latter-day standard gauge express passenger steam locomotives in fifteen European countries (including the UK), covering 27 classes, gave wheel arrangements of 4-6-0, 4-6-2, 4-6-4, 4-8-0, 4-8-2, and 2-8-4. 74% of the classes concerned were 6-coupled types, and the 8-coupled locomotives were often where axle load was restricted or gradients severe. The average adhesive weight of these 27 classes was 63.4 tons, only 5.7% more than that of the Class 5AT. Therefore the proposed 4-6-0 has the same order of adhesive weight as that generally found on the last of the steam-era express passenger locomotives running on standard gauge European railways, an era when train weights were generally heavier than they are now.
In order to guarantee that the wheel-rail friction is not exceeded in the worst conditions, the 5AT specification includes the provision that:
All powered wheels shall be sanded for forward running and the trailing coupled and driving wheels for reverse running. In addition an extra independent sand supply shall be fitted for semi-continuous light sanding ahead of the leading bogie front wheels when long stretches of bad rail are encountered.