A locomotive must be designed so that its tractive effort is not so high that it "loses its feet" on starting. Equally importantly, its tractive effort should be limited so that it does not slip at speed. This latter becomes a much more important consideration for Second Generation Steam locomotives like the 5AT which maintain a high tractive effort at high speed (see Tractive Effort page and T.E./Adhesion diagram below).
In the past, locomotives were commonly designed to operate with a minimum frictional coefficient of 0.20 (or 20%) on starting. Through careful specification of its sanding system, Wardale was able to increase this to 25% in the case of the 5AT.
In the Fundamental Design Calculations for the 5AT, Wardale took pains to evaluate the available adhesion not only under average tractive forces, but over the full range of tractive forces that apply between wheels and rail over each revolution of the driving wheels. As explained on the Tractive Effort page of this website, traction forces vary by significant amounts over each wheel revolution, most particularly on 2-cylinder machines, but also on multi-cylinder locomotives. The calculated variation in tractive force delivered by the 5AT over a 360o driving wheel rotation at various speeds, including at the maximum speed of 200 km/h, should be compared to available adhesion, as illustrated below (taken from FDC 1.4):
In consequence of this tractive force variation "self-controlled" (i.e. momentary) slipping is likely occur, especially in conditions of low adhesion, four times during each revolution resulting in uneven wheel-rim wear, the greatest wear coinciding with these peak loads.
Furthermore as such wear develops, it is likely to promote slipping through the momentary reduction in reaction force between rail and wheel at the points of greatest wear, thereby exacerbating the rate of wear. In consequence, Porta always contended that adhesion would be improved AND wheel-rim wear reduced by regular machining of the rims to maintain their circularity, preferrably by regular in-situ reprofiling. Porta also recommended maintaining the outside rim diameter of the driven wheels some 4mm larger than the coupled wheels in order to even out their wear rate. For these purposes, Porta set up a wheel rim profiling machine in the workshop at Rio Gallegos at the eastern end of the Rio Turbio Railway for routine in-situ maintenance of the driving wheel tyres of the railway's Santa Fe locomotives.
For optimising adhesion L.D. Porta "High Adhesion Wheel Profile" for use on steam locomotives, further information can be found on Martyn Bane's website at http://www.martynbane.co.uk/modernsteam/ldp/adhesion/adhesion.htm. This wheel profile incorporates a drip-groove around its outer edge, designed to prevent oil drips from the wheel hubs from contaminating the rail surface.