Railcraft Associates Ltd, Consulting Design Engineers


















© Railcraft Associates Ltd 2005. All rights reserved.

Case Study - Locomotive Structural Analysis, 10 May 2006

Most of the mechanical engineering design work we do includes an element of stress analysis, since a brilliantly innovative design is no use at all if it’s going to break!

The standards against which we measure the structural strength of our designs depend on the application, since requirements often vary with industry or with type of machine. For example railway vehicles of all types intended for main line use, are covered by either Railway Group standards (for the UK), UIC standards (for Europe) or AAR standards (for USA) – which are generally very prescriptive, with “load cases” and “acceptance criteria” covering all critical components. For non-rail applications welded steel structures can be analysed against BS 7608:1993 ‘Fatigue Design and Assessment of Steel Structures’. In addition to the available standards, we test components to levels agreed on by our team of Engineers in conjunction with information from the customer and our own scientific findings.

Nowadays most of our stress analysis is computer-based using SRAC “Geostar” finite element analysis software running on one of our dual-Athlon 64 workstations; a high performance graphics based machine for pre- and post-processing and model tidying/creation; and a powerful dual core machine for resource intensive parallel solving. Brian is our structures expert and having spent time working for the Honda/BAR Formula 1 racing team during his MSc year he commands quite a lot of respect! He has developed methods and processes to streamline the Finite Element method as much as is possible and utilise the processing power available twenty four hours a day.

Here are some examples of his recent work on our Corus steelworks locomotive project:-

The loco frame design was developed from a merger of knowledge of existing very heavy duty industrial locomotives and current and future safe working practices with the health and safety of the operator as important as the structural integrity of the frame. The locomotive had to be suitable for both the steelworks/heavy industrial application as well as for use on the main line. Network Rail in the UK defines clearly the structural standards which must be met in order to gain approval for use on their network.

GMRT 2100 Structural Standards

The standard yields a list of proof load cases which must be applied to the design, including buffing and drawing loads, jacking and lifting loads, imposed loads from body-mounted equipment and any combination of the above. Fatigue load criteria are also specified. In addition to the Network Rail load cases we have our own a Steelworks load case, based on the Network Rail standard but allowing for very heavy shunts, poor quality track, regular derailments and awkward lifts during re-railing. The data for defining the Steelworks load case was collected from implicit observations of current industry practice and derailment records as well as explicit loads and accelerations collected from load cells, strain gauges and accelerometers positioned on working test vehicles.

With the load cases defined, the first stage is to build a global locomotive frame model to have an overall look at what the frame is doing under the differing load conditions. For such a large structure, made up of relatively thin sections (where the thickness of the plate is very small compared to the length of the frame 40:12500) a surface model is created to allow a shell mesh to be used to speed up solution times and efficiency.

Creating the meshed component from the solid model.

The ‘tidy’ surface geometry was created in Unigraphics ‘Solid Edge’ and exported as an IGES file to Geostar. The geometry must be created in this way from the onset to prevent analysis anomalies and allow a stable mesh to be created. The various load cases can then be run and the results analysed in order to establish further how best to tackle any problems that arise.

Stress Analysis results, brighter colours denoting higher concentrations of stress.

In the case of the locomotive frame it was decided that the shell mesh did not yield enough detailed information on particularly interesting parts of the geometry e.g. weld configurations, plate lapping etc. so a secondary analysis was carried out largely using a ‘tidied’ solid quarter and half models where symmetry allowed.


Quarter model for detailed analysis.	Half model with load points applied.	Mesh refinement.

Each of the load cases was then run to convergence and the results pooled ready for a design review in which modifications to eliminate areas of highest stress were discussed by our team of Engineers. The design review identified areas where additional strength was needed, methods of eliminating stress-raising geometry, improved plate lapping configurations as well as fabrication and machining economies such that the final design could be manufactured easily and cost effectively.

The result?

An efficient frame design, production-engineered, strong and safe. The design meets the Network Rail acceptance criteria, as well as our derived Heavy Industrial/Steelworks criteria, is economical to manufacture and easy to maintain. On top of all of this the design enables the locomotive to meet all the relevant Health & Safety at Work requirements and in particular those relating to driver/operator safety and ergonomics.