Axles, axles, axles. BMW produces more than 330,000 units annually at its Regensburg plant for various models of the 1 Series (3 and 5-door), for the 3 Series Sedan and the M3 Sedan, for the BMW X1 and X2, for the 4 Series Convertible and the M4 Convertible and, last but not least, for the BMW 2 Series GranTourer. They all run along the same line, but hardly any of them are the same. Because customers can individually configure their desired vehicle, this literally ensures a lot of variety during assembly.

This becomes clear to visitors to the axle production area at first glance, as the numerous variants and possible combinations immediately catch the eye. There are around one hundred different front, rear and all-wheel drive versions for a wide range of engines, as well as right-hand drive, left-hand drive and models with specially painted brake callipers to discover. Highly qualified assembly staff work in three shifts to ensure that the brake lines are correctly tightened on every one of these hundreds of thousands of axles. They receive important support from mechatronic torque wrenches from the MWR series (MWR stands for Mechatronic WRench) and Focus 61 controllers from Atlas Copco Tools, which were specially developed for industrial series production.

How BMW uses this intelligently networked technology can be clearly seen on an axle module for a 3 Series just entering the assembly station. The conveyor section represents an assembly cell in which nimble hands immediately begin to place hydraulic hoses for the brakes in the designated positions. Once they have threaded the threaded connections, the workers tighten them precisely using the documentable MWR kink wrenches. The immediate feedback on the tool and via info screens in the cell gives the operators certainty about the assembly result and they can move on to the next component.

A glance at the display in the worker's field of vision provides comprehensive information about the assembly result. The green light and the figures confirm this: The target specifications for the brake line screw connections have been reliably met.

© Atlas Copco Tools

The battery-powered MWRs transmit the torques and angle of rotation of each individual bolted joint to the Focus 61 control system installed in the station via radio data transmission in sync with the bolting processes. The latter in turn communicates with BMW's own production management and control system and knows exactly which car the next incoming axle belongs to thanks to the pre-production data. The production managers at BMW particularly appreciate the fact that the system records all the tightening data in fractions of a second without any intervention by the workers and links it to the data records of the respective vehicle. The employees in manual assembly can therefore concentrate fully on the actual tightening process, while BMW still retains a comprehensive overview. Instead of a mere I.O. signal (I.O. = OK), the system stores the tightening data of each individual axle module with newton meter and angular degree accuracy. And these values can be clearly traced even after many years, which is extremely important in the context of any product liability issues.

Because the colored information displays in the employees' field of vision provide immediate feedback on each individual tightening result, the workers can immediately take countermeasures or make improvements if the strict BMW tolerances are exceeded. The MWR 25 TA production wrenches easily meet the accuracy specification of ±7%. And because they also weigh less than their predecessors despite their integrated radio modules, work has once again become easier thanks to improved ergonomics - although this is not the main advantage of the electronic hand wrenches in the eyes of the Regensburg-based company.

After solid comes from

High-precision production wrenches are used to screw the brake hoses to the car axles by hand at the BMW plant in Regensburg. The tightening results are traceably documented thanks to Smart Connected Assembly.

© Atlas Copco Tools

"Although standard torque wrenches confirm that a minimum torque has been reached, overtightened screw connections remain undetected," warns Christian Böhm. According to the quality assurance specialist at Atlas Copco Tools, this poses a certain risk, especially in safety-critical bolting cases, such as here on the brake system. In the worst case, structural changes in the material due to overloading when tightening could lead to the braking effect failing. Even the next generation of semi-intelligent articulated wrenches have not yet solved this basic problem. Although the latter already monitored the torque, they were not yet able to record, let alone document, the tightening angle, Böhm recalls: "A certain residual uncertainty remained and additional test steps were necessary as a result. However, this made assembly somewhat inefficient for BMW."

After the first joint tests with Atlas Copco Tools, the Regensburg-based company realized that the combination of the Focus 61 control unit and the MWR wrenches had a high optimisation value for BMW: Thanks to their compatibility, the communication-capable screwdriving and testing technology could be integrated into the existing system with minimal effort and all relevant data was transferred to BMW's data world without errors at the first attempt. Now there is no mistaking whether a bolted joint is OK or not, confirm the production managers.

Author:
Heiko Wenke, Atlas Copco Tools Central Europe.

A new BMW every 57 seconds

The car plant in Regensburg currently produces nine different models and special versions - on a single production line. Many visitors are amazed when they discover a 4 Series Convertible or a 2 Series GranTourer behind a BMW M3 on the assembly line. Thanks to consistent digitalization in production, the site with its 9,000 employees is one of the most flexible automotive plants and produced 338,259 vehicles last year. Since 1986, BMW has invested 4.7 billion euros in its Regensburg plant and has built over 6.6 million vehicles there to date.