Notes on FDC 13

Notes on FDC 13 – Spring Rigging

The following notes by David Wardale introduce the calculations for the locomotive’s suspension:

Since the steam era there have been changes made to railway vehicle suspension. However the steam locomotive is different in a number of ways from other railway vehicles, e.g. (i) in its chassis layout, with coupled axles running in the mainframe rather than in bogies, (ii) being subject to periodically applied vertical loads due to connecting rod angularity and the rotating masses for balancing the reciprocating parts, and (iii) in its comparatively high centre of gravity and resultant rolling tendency, which requires relatively hard springing. Given this, it is considered that the 5AT project is not the place to experiment with alternative spring arrangements, rather the type of spring and spring rigging that have proved themselves suitable for steam locomotives during millions of km of safe running and at speeds > 100 mph (160 km/h) should be retained and optimised.

The 5AT will have a 3-point suspension with compensated springing for the 3 coupled axles. The 3 suspension points are the bogie centre and the two centres of support of the coupled wheel springing, one at each side of the locomotive. The mass transfer to the bogie is to be at the bogie pivot, i.e. nominally at the locomotive’s lateral centre, rather than by mass transfer pads at each side of the bogie as on the BR 5MT, i.e. the bogie gives zero lateral support to the locomotive. The coupled wheel springs on each side of the locomotive are not cross-compensated with each other, therefore they provide (all) the lateral support. This 3-point compensated suspension system is that almost universally used with steam traction: compared to the uncompensated springing in general use in the UK, to which it is inherently superior, it gives more constant axle loads with track irregularities and consequently better riding and traction qualities and reduced spring peak dynamic loads and stresses. It should be pointed out that the spring system for a 4-6-0 is a very simple example of the 3-point principle, which all compensated spring rigging, even on large multi-wheeled engines, seeks to achieve.