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Electric Drive & Mechanics: A Perfect Symbiosis

Min Reading Time
Frank Thoma, April 29, 2021
Frank Thoma has been corporate editor at ZF since 2011. With a degree in journalism, he has been planning, writing and editing articles for all of the company’s internal, external media.
With regard to next-generation mobility solutions, ZF also benefits from its experience in mechanics, which has grown over many decades. With the example of the e-reduction drive, we demonstrate the outstanding significance of mechanical components for electric vehicles.

By just following the reporting on the mobility of tomorrow, one can easily get the impression that the only drive components in electric cars are an electric motor, power electronics and intelligent software control. Wrong! In order for electric vehicles to bring their power onto the road as loss-free and as quietly as possible, they also rely on highly precise mechanics in the driveline. Over the past 100 years, ZF has developed – and has been continuously perfecting – this special mechanical know-how. Even competitors for electric drivelines appreciate this know-how and can therefore be supplied by ZF with, for example, components such as e-reduction drives.

The requirements concerning this mechanical component for electromobility are manifold. "As with any other transmission, the functional specification for the e-reduction drive also contains a high level of efficiency and low drag losses. For this purpose, it must present the desired ratio in combination with a high torque transmission capability at the lowest possible dead weight – all while achieving first-class acoustic behavior," explains Robert Peter, responsible for the development of the Axle Drives product line at ZF. Remember, this must all be done while maintaining competitive prices.
View of the opened e-reduction drive housing including integrated parking lock, as used by ZF in electric drivelines.

Perfect mechanics for a longer range

Perfect mechanics for a longer range
Why is an e-reduction drive required at all in the electric driveline? It increases its efficiency. Engineers have two ways to integrate it into the driveline. They can integrate it into the housing of the electric drive as a component set – consisting of gears and differential. However, a stand-alone solution is also possible. Here, the technology is located in its own housing as an e-reduction drive system. In both cases, the solutions are scalable; they can be displayed in an axially parallel or coaxial configuration.
This only ensures a high power density of the drive once all components have been optimally matched. In addition to efficiency-optimized gearing, the use of low-friction bearings and an overall optimized design with regard to oil splash losses are important for high efficiency of the e-reduction drive. This also includes the selection of the optimal transmission oil. In the case of an e-reduction drive, the components are designed and selected according to the requirements of the individual customer application. ZF relies on tried and tested modular components wherever possible.

Optimization at the system level

Optimization at the system level
However, an excellent e-reduction drive alone does not yet ensure a first-class electric drive. This only occurs when all components in the system harmonize perfectly with each other. For example, the transmission ratio must be adapted to the characteristics of the electric motor and power electronics. Only then will the system meet the requirements for driving performance and achieve the overall efficiency objectives.

Acoustic behavior also plays an important role. Annoying noises in a ZF system have always been taboo. This applies even more to silent electric cars. "All components and subsystems influence each other and must therefore be optimally coordinated at an early stage of development," says Robert Peter. However, some acoustic phenomena only become noticeable when the complete driveline is installed in the test vehicle and is driven at certain speeds or with certain loads. When optimizing the acoustic behavior, the gearing within the e-reduction drive plays an essential role. Integrating the drive into the vehicle in an acoustically perfect manner is an important prerequisite for ensuring that an electric drive is acoustically discreet at all operating ranges.
"All components and subsystems influence each other and must therefore be optimally coordinated at an early stage of development."
ZF development engineer Robert Peter

Individual configurations of the driveline possible

Individual configurations of the driveline possible
For customers, ZF system development competence is a concrete advantage As a result, the electric driveline can be individually configured for each vehicle and additional functionalities can be added – such as a parking lock ("hand brake") in the transmission. A 2-speed-transmission and the eConnect decoupling system are also available upon request. Both are used to reduce power consumption. This increases the range or allows a smaller battery to be installed.

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