L1 – Structural Distillation
Follow the examples given in the Structual Distillation section, apply them to your own design. Spend time thinking about how the product will interact with its surroundings; consider all forces, torques, moments, bending, tensions, compressions, velocities and temperatures that may act on it, using Free-Body Diagrams and other techniques you’ve used many times before. Based on these results you may still need to change the design and update it in accordance with the results. This is perfectly normal, it does not show weakness, it shows a stronger understanding of the task and the project and a confidence in your own design. It is widely regarded that recognising and accepting a mistake (then working towards correcting it) is far better than trying to cover it up and risk embarrassment later. Additionally it’s far better to realise those mistakes now before it gets too late. Consider Figure 1.
Figure 1 – Structural Distillation decision Flowchart
At this stage the designer needs to be aware of the Factor Of Safety (FOS) they should be working to, ensuring the concept develops within the allowable limits. To calculate the FOS, use Equation 1 below.
Where “σ equivalent” is equivalent stress (generally the answer you get from your hand calc. or the pretty colours from the FE analysis) and “σ yield” is the yield stress of the material under consideration. The FOS is usually defined by the task undertaken. Motorsport applications might have a lower value (1.2-1.4) to allow for a weight saving, whereas long-distance haulage may use a higher one (2-2.5) as they generally experience higher. If in doubt, try to work to a factor of 1.6, this is generally accepted as being a suitable allowance.
Dissecting a system down into sub-systems to simplify the calculation will generally help. For instance, a steering system that contains many imakages andand mechanisms can be broken into it’s components parts; steering column, tie rod, track rod end, control arm etc. Once you’ve performed these calculations, you could consider entering them into an Excel Spreadsheet or Matlab, so the output from one calculation forms the input to another as in Figure 2.
Figure 2 – Excel Hand Calculation decision Flowchart
L2 – Structural Distillation/FEA/CFD
1 – Finite Element Analysis (FEA) software is not guaranteed to be correct.
2 – Computational Fluid Dynamics (CFD) software is not guaranteed to be correct.
3 – A computer is only as good as its operator’s inputs.
A result is only as reliable as the inputs; if the inputs have been incorrectly applied, it stands that the results are unreliable as well. Where possible, you should validate your simulations using Hand Calculations and/or physical testing. When your hand calculations match up with the result you see on screen, only then can you begin to trust the results. This is why you should concentrate on using Structural Distillation to evaluate your design before you even try running a simulation.
Very often a student can spend hours waiting for a computer simulation to calculate, only to be told the by a quick hand calculation that the result is completely invalid. This is not to say the simulation results should not be trusted every time. Today, Computer Simulations are reducing the time to market of many producs cutting the productuin process considerably, however they rely on the accuracy of the input and the person constructing the load case.
And this is all advice before any mention of learning to the use the software!
When you do learn the software, allow yourself time to experiment. Your lectures will have many tutorials and examples of proven simulations, use these to practice. Practice again. And practice again. Don’t assume that by completing a simulation once you know the software; and if it doesn’t compute, try fixing it yourself before you ask for help. This will help you to gain a greater undtanding of the software. Consider Figure 3 as a metaphor. Do not try and calculate complex geometries before you understand how to interpret results of simple ones. Again, allow yourself time to experiment. And practice.
Figure 3 – Computer Simulation Flowchart
Following this stage of refinement, your product should be fairly close to completion. You’ll define the final layout of the design in the Embodiment Design step.
Back to Embodiment Design
Back to MAE Design Model
Selected References
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