Know-how: Optimisation of the ROTWILD S240 stem for selective laser melting

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ROTWILD employee Martin Zimmermann worked on the weight optimisation of the existing ROTWILD S240 stem as part of his master's thesis. This was the first time that selective laser melting was used as a manufacturing technique for the stem. Here we explain the process and the advantages it offers for the development of lightweight components.

Die Anforderungen an einen Vorbau sind hoch. Um dessen Gewicht zu reduzieren, setzt Engineering Mitarbeiter Martin Zimmermann in seinem Masterarbeit-Projekt auf ein innovatives Herstellungsverfahren, das selektive Laserschmelzen.

Selective laser melting (SLM) as an innovative manufacturing process

Martin Zimmermann, a student on the Mechanical and Process Engineering Master’s course at the Technical University of Darmstadt, and a working student in engineering at ROTWILD since 2017, dedicated himself to optimising the ROTWILD S240 aluminium stem as part of his master's thesis. The subject of his Master's thesis came about through a partnership with the company Sauer Product and was carried out in collaboration with the department of Structural Lightweight Design and Construction Methods (KLuB) at TU Darmstadt, which is headed by Prof. Dr.-Ing. habil. Christian MittelstedtDr.-Ing. Alexander Großmann from TU Darmstadt’s KLuB department and, on the company side, our product manager Johannes Matschos were on hand to supervise Martin’s work.

The aim of this work was to optimise the weight of the bicycle stem and to manufacture it using selective laser melting (SLM) at Sauer Product.

Selective laser melting (SLM) as an additive manufacturing process offers several advantages over the conventional production of aluminium components:

  • shorter product development times
  • resource-efficient manufacturing
  • great creative freedom
  • the ability to integrate functions within a component

By using computer-aided topology optimisation in the component’s development for additive manufacturing, it was possible to create a weight-reduced and load-bearing lightweight construction that would have been impossible with conventional manufacturing processes. To explain, computer-aided topology optimisation is a development tool that amasses material only at those points where it is needed for the existing load situation.

The development process in seven steps

1. Define requirements

First, the optimisation task’s requirements were defined. The available assembly space was largely predetermined by the geometry of the ROTWILD S240 stem and its interfaces. In addition, the loads to which a stem is subjected were determined. These are based on the loads defined in the DIN EN ISO 4210 standard for the counter-phase fatigue test (load during stand-up pedalling), the inphase fatigue test (load during braking) and the lateral bending test (overload test). The requirements definition also specified the extent of the weight reduction (at least 25% compared to the initial component) and the material of the optimised component (aluminium alloy AlSi10Mg).

2. Baseline analysis

At the beginning of the optimisation task, an FEM simulation of the ROTWILD S240 bicycle stem was carried out in order to gain an understanding of the stresses in the component under the loads defined in the requirements.

3. Carrying out computer-aided topology optimisation

To carry out the computer-aided topology optimisation, the maximum available assembly space was first constructed. Then the computer-aided topology optimisation took place, taking into account the calculation task’s boundary conditions and the defined optimisation goal. The result was an optimal stem design for the given load cases. However, it was not possible to create the exact calculation result in reality. Rather, it was a design proposal that served as a template for the remodelling.

4. Remodelling and adaptation of the design concept

The design proposal obtained through computer-aided topology optimisation was reconstructed using CAD software.

5. Strength verification through FEM simulation

After the remodelling, an FEM simulation was carried out to identify critical component areas and to mitigate or eliminate any potential weaknesses through appropriate design measures.

6. Production of the optimised stem using the selective laser melting process

The optimised stem as approved for production was manufactured at Sauer Product using the selective laser melting process. The optimised stem prototypes produced are approx. 42% lighter than the conventional ROTWILD S240 bicycle stem.

7. Test bench testing at EFBE Prüftechnik GmbH

To check the FEM simulation’s calculation results, the prototypes were subjected to test bench testing at EFBE Prüftechnik GmbH. The loads defined according to the DIN EN ISO 4210 standard requirements were used as a basis. The optimised stem passed both the inphase fatigue test and the lateral bending test. Only the loads of the counter-phase fatigue test could not be withstood by the new optimised stem manufactured using selective laser melting.

Conclusion:

The master's thesis offers ROTWILD's engineering department the opportunity to get to grips with a comparatively young and emerging manufacturing technology, to gather new knowledge for further product developments and to apply this knowledge throughout the entire engineering team. The project is thus consistent with the ROTWILD product philosophy: to develop innovations and implement them independently using the latest technologies.

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