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Design for Reliability | Advanced Steam Traction

Design for Reliability

The following is taken from a memo written by David Wardale dated 31st March 2005 listing the principle design features incorporated into the 5AT design that would have ensured a significant improvement in reliability as compared to historic (“first generation”) designs.  Many if not most of the points could equally well be applied to any other new-build project whether of old or new design.


Design features leading to a much higher reliability of the 5AT Class steam locomotive
compared to First Generation Steam (FGS) Locomotives.

The advanced technology 5AT Class 4-6-0 steam locomotive is being designed to have a performance meeting the requirements of the modernised UK rail network. The 5AT will also be an exceptionally reliable form of steam traction. The factors which will enable the 5AT to provide much higher reliability than FGS are shown below.

A. General Points:

  1. More accurate design (i.e. replacing the empirical design used in FGS with one more closely based on whatever theory is applicable to the component concerned — as largely done in the Fundamental Design Calculations).
  2. Better materials.
  3. Better lubrication.
  4. Use of advances in Engineering Science. Engineering science has advanced considerably since the last FGS locomotives were designed and techniques such as finite element stress analysis and fatigue analysis — not available to FGS designers ¬will be employed where relevant.
  5. Replacement of bolted or riveted connections by welded ones where possible, eliminating the possibility of things becoming loose.
  6. The simplicity of the concept (two cylinder simple loco). This minimises the number of moving components compared to a multi-cylindered engine. In particular there are no inaccessible components.
  7. The use of AAR rules where appropriate. AAR (Association of American Railroads)  rules are generally considered to be the most robust design rules where empirical methods have to be used.
  8. Starting afresh with a new design, any features of FGS that gave persistent problems can be looked at again and the fault designed out.
  9. Where mandated by Railway Group Standards design will be, of necessity, “state-of–¬the art” (e.g. brake control system).

B. Specific Points:

  1. Roller Bearings. Roller bearings will be used on all major joints (e.g. axles, crankpins, connecting rod small end and valve gear). Roller bearings are more reliable than plain bearings, require “no field attention” and have effectively zero wear, and hence prevent vibration.
  2. Self adjusting wedges at all driving and coupled axleboxes — eliminate axlebox-frame gaps, hence pounding and vibration.
  3. Generally improved valve and cylinder tribology, greatly reducing wear of affected components.
  4. ‘State-of-the-art’ air sanding, reducing slip risk.
  5. Oil firing — eliminates the need for fire and ashpan cleaning, eliminates performance uncertainties caused by variable coal quality, vagaries of the firebed and variable levels of firemen’s skill and endurance.
  6. Sophisticated exhaust system — eliminates indifferent steaming caused by inadequate combustion air supply (a common problem with FGS).
  7. All welded boiler — eliminates problems caused by riveted seams and screwed stays, especially no possibility of leakage and caustic embrittlement.
  8. Superior firebox stay design, reducing incidence of fractured stays.
  9. Elimination of lineside fire-risk because of oil firing, which means the locomotive can be used in all weathers.
  10. Compensated springing which reduces the risk of spring breakage.
  11. High capacity tender and efficient locomotive give long ranges on a tenderful of fuel and water minimising the need for en-route fuelling and watering.
  12. Franklin-type engine-tender buffer and drawgear eliminates ‘stamping’ and vibrations at this point.
  13. Good riding qualities and high vehicular stability eliminate vibration caused by poor (rough) riding.
  14. Robust horn stays, minimising risk of frame cracking at top corners of horns.
  15. Use of ‘drop-type’ firebox fusible plugs which are safer than the usual lead filled plug.
  16. Use of corrosion-resistant (copper bearing) steel for such items as the tender superstructure and smokebox will minimise corrosion.
  17. Optimum boiler-frame connections (slide bearing at the barrel-frame and expansion plate at the firebox-frame connections) tie the boiler better to the frame and therefore better brace the transverse forces on the mainframe.
  18. Geared roller centring of the leading bogie eliminates the possibility of bogie guide-spring breakage.
  19. Clasp brakes eliminate axle and axle bearing loads due to breaking forces.