SYN] Magazine
#1 | 2019

Flexible optimization of brake line assembly for BMW

Despite all the progress made with automation, extremely wide variations in production continue to make manual assembly steps indispensable in the automotive industry.

To reliably avoid any errors, BMW uses active process monitoring systems for axle production. The solution delivered by Atlas Copco Tools already monitors the key parameters during tightening, informs the worker and generates seamless documentation of all tightening operations within the BMW production system.

Axles, axles and even more axles. Each year, BMW’s Regensburg plant produces more than 330,000 units for a variety of models. These models include the 1-series (3-door and 5-door), the 3-series and M3 saloons, the BMW X1 und X2, the 4-series and M4 convertibles and the BMW 2-series Grand Tourer. All these models are produced on the same line but scarcely two of them are alike. The fact that customers can specify their own individual dream cars means that there is considerable variation in assembly.

Visitors to the axle production unit can see this at first glance. In fact the large number of variants and combination possibilities are among the first things that a visitor notices. There are about 100 different front, rear and all wheel drive versions for a variety of engine options as well as right-hand and left-hand drive variants, models with specially painted brake calipers and many more different options.

Working in three shifts, highly qualified assembly operatives ensure that the brake lines on every single one of these many hundreds of thousands of axles are properly tightened. In their efforts, they are assisted by mechatronic torque wrenches from the MWR series (MWR = Mechatronic WRench) and Focus 61 controllers developed by Atlas Copco Tools for industrial serial production.

Worker information during continuous assembly

The use of this smart connected assembly technology by BMW can be illustrated by the axle module for a 3-series vehicle that is just entering the assembly station. This section of the production line constitutes an assembly cell. Nimble hands have already started to install the hydraulic hoses for the brakes in the required positions.

After they have threaded in all the connections, the workers tighten them precisely to the final torque values using MWR click-type torque wrenches with documentation capabilities. “Immediate feedback on the tool and via information screens in the cell gives the operators reassuring certainty concerning the assembly results. They can then turn to the next component,” says Christian Böhm. The 33-year-old quality assurance specialist from Atlas Copco Tools goes on to provide further information on new digital tightening systems, which are highly advanced and flexible.

Seamless continuous data transfer for secure documentation

“As the joints are tightened, the battery-fed MWR wrenches transmit the torque and angle values of each individual joint to the Focus 61 controller installed on the station via a radio link.” The controller communicates with BMW’s own production control system and has already received information on the next axle arriving at the station.

According to Böhm, the BMW production managers highly appreciate the fact that the Atlas Copco system records all the tightening data in a fraction of a second without any intervention by the worker and merges them with the data records of the individual vehicle. “This reduces the workload of the production line operators, allowing them to concentrate on the tightening operation itself. At the same time, BMW has a comprehensive overview of all the information.”

Instead of a simple OK signal, the new system stores the tightening data of each individual axle module with torque values recorded down to the last newton-metre and angles down to the last degree. This way, the values remain traceable after many years, which is extremely important in view of potential product liability issues.

Ergonomic production tool and measurement instrument combined 

The colour-coded info displays with a traffic light configuration in the field of vision of the production line operators provides them with immediate feedback on the results of each individual tightening operation. This means that the workers can take immediate corrective action in the event that BMW’s strict tolerances are exceeded. The MWR 25 TA wrenches comply with the accuracy limits of ±7 % without any problems. The positive experiences at the Regensburg plant indicate that practically no errors now occur. As the handy MWR production wrenches weigh less than their predecessors despite the integrated radio modules, ergonomics have been improved and work has become even easier. However, this is not the decisive advantage of the hand-held electronic torque wrenches in the opinion of Böhm’s customer.

Tight, but not too tight

“Conventional torque wrenches can confirm that a minimum torque value has been reached but they cannot identify joints that have been over-tightened,” the quality expert warns. In his opinion, this may represent a problem for safety-critical joints, as for example on these brake systems. In the worst possible case, changes in the material structure caused by overloading during tightening may result in brake failure. He says that the semi-intelligent click-type torque wrenches of the last generation had not yet solved this problem. These monitored torque but they could not record or document a tightening angle. Looking back, Böhm says: “There was still a certain amount of uncertainty and additional tests were needed. For BMW, assembly was therefore inefficient to a certain extent.”

Smart Connected Assembly
The team in Regensburg already recognized that the combination of Focus 61 controller and MWR wrenches offered considerable optimization potential for BMW after the first joint tests with Atlas Copco Tools. Thanks to their groundbreaking compatibility, it was possible to integrate the tightening and measurement systems, with their communication capabilities, into the existing system with a minimum of effort. All the relevant data were immediately transmitted to the BMW data world without any errors. The production managers confirm that it is possible to say precisely whether a joint has been tightened properly or not. They say that the solution meets BMW’s requirements and tolerances, allowing continuous production without any interruptions.

Green lights everywhere! A glance at the large display in the operators’ field of vision provides comprehensive information on the results of tightening. Green lights and the figures confirm that the brake line joints were reliably tightened in accordance with the specified requirements– 17 newton-meters of torque and tightening angles between 10 and 100 degrees.

All the tightening results of the battery-powered MWR torque wrenches are transferred to the Focus 61 controller via radio link and then on to BMW’s production control system. Via the vehicle identification number (VIN) these data are linked to the appropriate vehicle without any possibility of confusion, with seamless documentation.