Proving performance in EV powertrains
simulation-driven development replaces aluminium with thermoplastic composite in gearbox housing
When it comes to electric vehicles, keeping weight down is paramount in ensuring long range. In addition to the bodywork, drive components such as the housings typically used in double reduction gearboxes are of interest in terms of materials substitution. Thanks to their mechanical properties and the well-established production possibilities, carbon fibre-reinforced thermoplastics have plenty to offer. The challenge, however, is maintaining the required rigidity levels at prevailing operating temperatures. Together with its sister companies, ARRK Engineering has taken up this highly promising challenge: in 2016, a fully functional thermoplastic composite gearbox housing for electric vehicles was developed as part of a self-financed development project, with the first half already implemented as a prototype.
A composite gearbox housing featuring a thermoplastic matrix is around 30 per cent lighter than a conventional aluminium model – as was confirmed in studies carried out prior to development. Until now, the potential for optimisation offered in this area had only been highlighted through various technology demonstrations. The prototype, which was developed by an interdisciplinary team focused around Technology & Innovation at ARRK Engineering, is therefore the first real component of its kind that could actually be installed in a vehicle. “We were drawn to the project due to its mix of complex nature and feasibility,” explains Monika Kreutzmann, Head of the Center of Competence (CoC) Composite at ARRK Engineering. Those involved aspired to match the profitability of the conventionally used materials, at a sufficient order quantity. “The minimal investment costs and the use of tried-and-tested technologies make long-fibre-reinforced components with thermoplastic matrices very interesting from a cost perspective,” states Dr Thomas Schneider, Head of Technology & Innovation at ARRK Engineering.
Re-engineering achieves target values for composite housing
The project time was divided into three phases: the rough concept was set out in the first, the second dealt with the final draft and the third phase looked at the fine details. In order to be able to determine the “technical specifications”, as it were, the target values – which at the minimum had to be met – were first calculated by re-engineering an existing aluminium housing. As rigidity in particular has a significant effect on the performance of the gearbox, geometry was given a lot of thought here. The development was driven forward with a heavy reliance on simulation methods, which were used for testing all approaches and trialling solution concepts in a virtual environment as to their functionality and potential.
“Due to the short cycle times and high quantities involved, we opted for a thermoplastic material which was to be extrusion-coated using short-fibre-reinforced plastic,” explains Kreutzmann, describing the basic approach. The various scenarios and their feasibility were each evaluated and optimised in pressure casting and injection moulding simulations. “Our experiences of working with thermoplastics and their combination with other materials also contributed to the development. Our knowledge of the relevant failure models and our expertise in injection moulding processes also proved useful,” says Kreutzmann. Recasting the organic sheeting, however, was a different matter entirely: the team possessed little prior knowledge of cycle times or temperatures. The limited availability of the desired fibre and matrix material also presented a challenge for implementation, meaning that a provider first had to be found who also recognised the future potential of carbon-fibre materials in the automotive industry had to be found – a task that was eventually successful.
Topology optimisation: UD tapes provide localised reinforcement
In order to design the installation space, the force profiles in the housing first had to be determined on the back of an optimised topology – taking the traction and tension areas into account. The resulting concept served as an indicator of where the material was to be positioned and how the layers of the organic sheeting had to be optimised in order to achieve the required rigidity. Furthermore, the deformations that could potentially arise when placed under load were examined in extensive simulations, allowing the torsion of the housing to be derived as a dimensioned variable, which was countered with 45° layers. Additionally, it was necessary to identify localised weaknesses in order to specifically minimise them and reduce the resulting strains. “In addition to FEM optimisation, we also manually sought out specific strengthening methods which involved as little additional weight as possible,” explains Raik Rademacher, Engineering Co-Project Leader, describing the developer’s methods. Crossed unidirectional (UD) tapes proved to have a particularly positive impact here: the thickness of the actual organic sheets was subsequently able to be reduced from 5mm to 4mm, not only saving weight but also facilitating the sheet’s remodelling process.
The detailed draft still included the use of aluminium inserts, which transmit the loads discharged into the bearing onto the organic sheeting. These inserts allowed shaft tilt to be significantly reduced. “As the bearing seats have to be precisely adjusted to 30µm while incurring as little reworking as possible, the corresponding process parameters and their effects on, for example, warping, were examined,” reports Rademacher. Alongside the UD tapes, injection moulding ribs on the organic sheeting were used to ensure rigidity targets were achieved. A positive side effect of the use of injection moulding technology is that it can be implemented on the final contour and it requires no additional drilling, meaning reduced reworking. Short glass fibre-reinforced injection also prevents contact between the carbon fibre and the metallic inserts: this galvanic isolation prevents the emergence of corrosion, meaning no additional coating is required.
The production process – from the preform to the finished component
Manufacturability was ensured by the close involvement of Shapers, the tool manufacturing specialists within the ARRK group, and through simulations of the pressing process using the software of cooperative partner ESI. As the prototypes for the first half of the housing were created in a two-step process, the new development served as a tool for the pressing process and was also required for the subsequent injection. In the first phase, the organic sheeting and reinforcing UD tapes are heated and reformed in a pressing process in such a way that the matrix materials are combined and the desired preform is created. The preform is then cut to size using water jet cutting. In the second phase, the preform is heated again and moulded with the injection moulding tool, creating the final geometry including the ribs and other functional surfaces. “As expected, this proved to be difficult due to the high temperatures and mechanical strain before and during the pressing process for the organic sheeting. Our efforts eventually resulted in an appropriate method,” remarks Kreutzmann, offering an insight into the work of the Technology & Innovation team.
The weight reduction was as hoped: the gearbox housing was made up to 30 per cent lighter through the use of fibre-reinforced thermoplastic. The prototype is to be subjected to comprehensive hardware tests later this year for functional control purposes while, at the same time, the second half of the housing is to be created. A transparent PMMA plastic prototype has already been implemented to better illustrate the technology. The assembled version was unveiled to the public for the very first time as part of the Composites Europe event in December 2016. “The excited interest from a wide range of different industries is a testament to the development’s potential. We believe that the cost-efficiency of this process will increase as the use of its automation potential rises. If, for example, the two process steps could be combined, the production costs would sink significantly,” predicts Kreutzmann.