Lightweight garage #10 BMW 750i xDrive: Spur geared output shaft

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Lightweight garage, News

The vehicle

Today we take a 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 automatic transmission, which enables a particularly smooth start. Depending on the drive variant, this 7-series has a gross vehicle weight of 2630 kg. A special highlight, however, is its spur-toothed 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. A radial toothing is attached to the wheel-side joint of the output shaft, which is inserted into an internal radial toothing in the wheel bearing and secured by a screw. In the vehicle presented here, however, spur gearing is used. Here, the wheel bearing and output shaft are connected with two face splines and also secured with a screw. This offers several advantages. Firstly, assembly is simplified as less space is required when joining the components. Secondly, a higher torque can be transmitted due to the larger diameter of the gear rim. In contrast to conventional technology, the geometric modification of the connection means that the bearing remains backlash-free in the gearing during operation with less effort. The new connection shape saves material on the output shaft. From the lightweight design engineer's point of view, this weight saving is the biggest advantage. For a long time, the higher costs due to more complex production were a disadvantage.

©BMW, Shaft-bearing connection with face splines

The lightweight aspect

As indicated above, the output shaft with spur gearing can be designed to be lighter than comparable models. In this case, the spur gearing means that the journal with the radial gearing for power transmission can be omitted and the output shaft can therefore be made shorter. In addition, weight savings of around 10 % can be made on the wheel bearings by eliminating the internal gearing. Overall, a weight saving of around 2.8 kg can be assumed for this vehicle. This weight saving varies depending on the torque level at the respective wheels, the number of driven wheels and the manufacturer's design.

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