The perfect (rotor) shaft

Without a rotor shaft, there cannot be an electric car:

As the heart of the electric machine, it converts electrical energy into kinetic energy and transmits this into the drive train. Its design determines the speeds and torques at which an electric motor can be operated. In the development of the assembled rotor shaft from thyssenkrupp Dynamic Components, the Group was able to draw on valuable expertise from a completely different drive technology: experience with assembled camshafts from the internal combustion engine.

Work on rotor shaft development already began about a decade ago: At thyssenkrupp Dynamic Components, the assembled camshaft had already established itself as an alternative to the cast or forged shaft. Its main advantage: Within this complex component, different materials can be combined in a way that is suitable for the applied load. This means that the material of the entire shaft does not have to be oriented to the component with the highest load. In assembled camshafts, only the highly stressed components such as the cams are made of high-alloy steel. For the other functional areas, the material properties can focus on completely different criteria and the flexibility in material selection increases. This results in cost and weight advantages as well as enabling more flexibility in production. However, in order to master this technology – especially joining the various materials and components together – special expertise is required: The engineers were able to draw on the Group's own experience and the manufacturing processes of thyssenkrupp Dynamic Components.

From the camshaft to the rotor shaft

The camshaft specialists from Eschen, Liechtenstein decided early on to transfer their skills across to rotor shafts for electric motors. Ten years ago, it was hardly possible to foresee the dynamic trend that would develop in the electric mobility market from the 2020s onwards. But – as with camshafts – the rotor shaft was all about weight optimization and maximum cost efficiency. However, the torques and speeds are much higher: a camshaft rotates at speeds of around 3,500 rpm on average – only half as fast as a crankshaft – whereas a speed of 20,000 rpm was already emerging for rotor shafts at that time. This represents an enormous load, but it is by no means the end of the development goals: the trend today is already pointing in the direction of 25,000 rpm.

The rotor shaft, which ten years ago was predominantly a solid component, transmits the kinetic energy generated by the alternating electromagnetic field between the rotor and stator to the transmission. At the aforementioned speeds of up to 25,000 rpm, torques are transmitted that are 3 – 5 times greater than those in conventional automobile camshafts. In order to cope with this, load-compliant design and low manufacturing tolerances are the top priority – for example, in the concentricity of the splined gearing. Martial Danthois, Head of Advanced Engineering at thyssenkrupp Dynamic Components: "We quickly came to the conclusion that we could effectively meet the requirements for a rotor shaft with an assembled design, and we expected to derive benefits from it. Our experience with the assembled camshaft proved worth its weight in gold when it came to developing an assembled rotor shaft for series production in the first place: because in both cases, the crucial expertise concerns the connections between the various materials and components."

The use of different materials also offers plenty of potential for the rotor shaft. Since higher torques occur on the output end of the shaft in the area of the gearing, these can be perfectly balanced by a targeted selection of materials with higher alloys. Materials with a lower alloy grade are used for the tube and the bearing flange on the other end.

Hollow design for additional functions

Another key advantage of the assembled design is that the shaft is hollow. In the case of particularly powerful electric motors, the cavity can be used for additional functions. The thyssenkrupp engineers use it for cooling by injecting coolant into the shaft and having it distributed over the inside wall thanks to the centrifugal force of rotation. The convective heat transfer dissipates heat from the rotor shaft and thus cools the electric motor. The coolant is then led out of the shaft and cooled down in a circuit with a heat exchanger.

The cooling function via centrifugal force is also accomplished by one-piece hollow rotor shafts, but the same applies to this design as to one-piece camshafts in the internal combustion engine: their design is inevitably based on the area subject to the heaviest loading. The material quality of the entire shaft is thus predefined, and a flexible and demand-oriented material selection is not possible. For this reason, assembled shafts with flexible material selection offer cost advantages.

From the pilot project to series production

In close cooperation, the series development was pushed ahead at the locations of Eschen (LIE), Ilsenburg (D) and Chemnitz (D) – together with the component and system suppliers and taking into account the specifications of the OEM. After delivery of the required prototypes and qualification of the product, the production lines were set up and series production started, initially at the Chemnitz site in Germany. Follow-up projects have been started in Ilsenburg, Germany. In 2021 the Group was renamed thyssenkrupp Dynamic Components due to the new, expanded product portfolio.

The successful market launch of the first assembled rotor shaft using the thyssenkrupp Presta process has triggered two further initiatives. On the one hand, an increased number of inquiries from OEMs has led to new rotor shaft projects, by means of which expertise and experience could be further expanded, and innovative manufacturing technologies investigated and introduced.

Expansion of focus: from the shaft to the rotor

In addition, the thyssenkrupp Dynamic Components research and development team has broadened its focus from optimizing the rotor shaft to the assembled rotor. Special challenges are, for example, fastening the lamination stack on the rotor shaft as well as the magnets in the lamination stack.

An additional challenge in the production of assembled rotors arises from customer requirements for permissible residual imbalance or balancing quality. On the one hand, these requirements can be derived from the high rotor speeds; they influence the bearing service life and consequently the service life of the electric motor. On the other hand, the dynamic overall system of the electric motor is excited to oscillate by imbalances. The characteristics of these vibrations and their propagation (airborne and structure-borne noise) have a significant influence on drive comfort. Having dispensed with the internal combustion engine, it can be assumed that the limit values with regard to airborne and structure-borne noise will be further reduced.

Martial Danthois: "Our ambition is to further optimize the existing series production methods. But we are also working on completely new manufacturing techniques that will allow us to further increase productivity and reduce scrap and rework."

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