Lightweight Garage #10 BMW 750i xDrive: Spur toothed drive shaft

The Vehicle

Today we look at the BMW 750i xDrive from 2016. This car is powered by a 4,4 l V8 engine with a torque of 650 Nm, with an output of 445 hp and a top speed of up to 250 km/h. The gears change automatically through an 8-speed torque converter automatic, which allows for a particularly smooth start. This 7-series has a permissible total weight of 2630 kg. A special highlight, however, is its spur-geared output shaft.

©Kārlis Dambrāns, 2016 BMW 750i xDrive, CC BY 2.0

The Technology

In most vehicles, the power is transmitted from the output shaft to the wheel hub by means of radial gearing. In the vehicle presented here, however, a spur toothing is used, which offers the following advantages. On the one hand, assembly is simplified because less space is required when connecting the components. On the other hand, a higher torque can be transmitted due to the larger diameter of the gear rim. Due to the geometric change of the connection, the bearing remains backlash-free in the gearing during operation with less effort, in contrast to the usual technology. However, from the lightweight engineer’s point of view, the biggest advantage is the weight saving. The higher costs could be a disadvantage.

Shaft-bearing connection with face splines

The Lightweighting Effect

As indicated above, the output shaft with spur gearing can be produced significantly lighter than comparable models. In addition, significant weight savings can also be made on the wheel bearing side. Overall, a weight saving of 2.8kg can be assumed for this vehicle. This weight saving varies depending on the amount of torque at the respective wheels, the number of driven wheels and the manufacturer’s design.

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