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Weight reduction in electric vehicles is paramount in ensuring extended range. Application of carbon fiber-reinforced thermoplastics is one means of achieving this end and besides adoption in bodywork, companies are investigating the use of these composites in drive train components such as double reduction gearboxes.

The challenge, however, is maintaining the required rigidity levels at the elevated operating temperatures. German vehicle engineering company ARRK Engineering and its sister companies took up the challenge in 2016 and developed a fully functional thermoplastic composite gearbox housing for electric vehicles as part of a self-financed development project, with the first half already implemented as a prototype.

The technical specifications for the composite housing to be developed were derived from the re-engineering of an existing aluminium housing.

In the first process step, the organic sheeting and reinforcing UD tapes are heated and reformed in a pressing process, creating the desired shape. The preform was cut to size using water jet cutting.

The organic sheeting was then overmolded with a short glass fiber-reinforced polyamide compound to realize the first half of the housing. A transparent second half was also created in-house. This will be replaced with a functional component during the course of the year.

A composite gearbox housing featuring a polyamide (PA) thermoplastic matrix is around 30 per cent lighter than a conventional aluminum version. The prototype developed by an interdisciplinary team focused around Technology & Innovation at ARRK Engineering is the first real component of its kind that could actually be installed in a vehicle.

The housing is molded in a two-step process. In the first step, the PA 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 back-molded using the injection molding tool, creating the final geometry including the ribs and other functional surfaces. “

“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. “The minimal investment costs and the use of tried-and-tested technologies make long-fiber-reinforced components with thermoplastic matrices very interesting from a cost perspective,” states Thomas Schneider, Head of Technology & Innovation at ARRK Engineering.

Because rigidity in particular has a significant effect on the performance of the gearbox, geometry was given a lot of thought. The development was driven forward with a heavy reliance on simulation methods, which were used for testing all approaches and trialing solution concepts in a virtual environment as to their functionality and potential.

In order to design the installation space, the force profiles in the housing first had to be determined on the back of an optimized 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 optimized 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 localized weaknesses in order to specifically minimize them and reduce the resulting strains. “In addition to FEM optimization, 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 5 mm to 4 mm, not only saving weight but also facilitating the sheet’s remodeling process.

Aluminium inserts are also employed in the housing, 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 30 µm while incurring as little reworking as possible, the corresponding process parameters and their effects on, for example, warping, were examined,” says Rademacher.

Alongside the UD tapes, ribs over-molded onto the organic sheeting through injection molding were used to ensure rigidity targets were achieved. A positive side effect of the use of injection molding technology is that it can be implemented on the final contour and it requires no additional drilling, meaning reduced reworking. Short glass fiber-reinforced injection also prevents contact between the carbon fiber and the metallic inserts: this galvanic isolation prevents the emergence of corrosion, meaning no additional coating is required.

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.

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 developers eventually hope to combine the two process steps to further lower costs production costs.

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