Production of the drive unit: modern methods digitally documented

  • Precisely defined and fully digitised processes
  • Drive unit consists of 250 components
  • Quality checks are integrated into the assembly process

Precisely defined and digitised processes, but only fully automated where this protects people and the product: this is the philosophy behind the production line for the drive unit of the GLC F-CELL pre-series models in Kirchheim-Nabern around 40 kilometres to the south-east of Stuttgart. Production-friendly product design was already the focus from its inception in 2014. Another important factor was to configure the geometry of the components so that all testing points, e.g. threaded line connections, are easily accessible.

The new fuel cell system is around 30 percent smaller than its predecessor, and thanks to its compact dimensions it fits into a conventional engine compartment for the first time. This also has advantages during assembly of the drive unit, which consists of 250 components including the fuel cell stack and the cooling, fuel and exhaust systems plus the electronics. The production process uses a suspended conveyor system which is also used for engine assembly at the Untertürkheim plant. The fuel cell systems are assembled in the Kirchheim-Nabern location, an annex of the Mercedes-Benz plant in Untertürkheim, which has been producing the drive systems for the pre-series vehicles since 2017. The conveyor system only needed minor modifications for the alternative drive unit, mainly involving an adapter to hold the assembly in place.

The assembly worker is able to rotate the drive unit in the conveyor system so that the respective assembly points are ergonomically accessible. The worker receives digital, visual instructions for every working stage via a screen at the relevant workstation. This is updated in real time, as tools linked by WLAN are used in the assembly process. For example, when a bolted connection has been made to the prescribed torque, the on-screen marking of this bolt changes to green. At the same time the assembly stage is documented and archived.

The same applies to the quality checks integrated into the assembly process. These include leak-testing of the hydrogen, air and coolant circuits. Here too, attention was paid to a test-friendly product design, e.g. eliminating unnecessary interfaces with integrated wiring and lines. Testing is carried out with a hydrogen mix, so that even leaks in the ppm (parts per million) range can be reliably detected.

Safety first

The test procedure for the electrical insulation and resistances, designed for an operating voltage of 400 volts, is a high- voltage test with 2150 volts. To protect the personnel, the high voltage is only switched on after a laser scanner has definitely confirmed that no person is in the vicinity of the drive unit. To protect the components from electrostatic discharge (ESD), the entire working area in the assembly shop is shielded against electrostatic discharge by a floor coating.

At the end of the production process, a test run is carried out over the entire dynamic performance range of the drive unit. This too is precisely electronically documented – and the power generated is fed into the plant's network. The fuel cell systems are then dispatched to Bremen by truck, where final vehicle assembly takes place with the batteries from Kamenz (Saxony) and the hydrogen tanks from Mannheim (Baden-Württemberg).

Education and further training for the drive systems of tomorrow

The electrification of drive systems also presents the workforce with new challenges. Thorough training and corresponding experience are very important for product quality. Further training for additional qualifications in e.g. product-specific know-how from assembly trials and prototype production, handling safety-related features, handling high-voltage components or handling of electrostatically sensitive components is also available.

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