How a test stand for torque tools guarantees the quality of Liebherr's mobile crane production

工場ゲートのかなり離れたところから見えてきたのは、空に向かってそびえる大きな移動式クレーンのブームです。これらのクレーンはドイツ・シュヴァーベン地方のエーインゲン（Ehingen）にある試験場で試験を受けたのち、世界中に輸出されます。Liebherr社では、自社で設けた高い基準を満たすために、製造に用いるすべてのボルト締付けツールについてキスラーの移動式試験システム「cerTest」を採用しました。

中核事業である建設機械の製造からスタートしたLiebherr社は、この数年で交通技術、機械製造、家電製品、さらには独自のホテルチェーンといった事業分野にも進出しています。2017年の売上高は約100億ユーロに達し、従業員数は43,000人以上にのぼり、業界のグローバル企業のひとつに数えられています。移動式クレーンの全事業はこのエーインゲンに集約されており、伸縮ブームを備えたオールテレーン（オンロードおよびオフロード対応）から格子ブームを備えたクローラクレーンまでが製造されています。Liebherr社がこれまでに製造した中で最大規模のものが、クローラクレーン「LR 13000」です。これはメキシコの製油工場の建設に用いられ、最大3,000トンの重量を持ち上げることができます。

エーインゲンでプラント計画責任者を務めるThomas Nüssle氏は、工場の特徴についてこう説明しています。「ここでは日々平均8台の車両を4つのラインで製造しており受注生産で、すべてジャスト・イン・タイムで進められています。出荷の前には、移動式クレーン用とクローラクレーン用に分かれた試験場で広範囲にわたる試験を行っています。Liebherr社は品質を最も重視しており、当社の卓越した製品は世界中で高く評価されています。それを維持するために、ボルト締結技術においても常にプロセス改善を図っています。」

Inhouse testing cuts costs and supplies know-how

Many different torque tools are used in the manufacture of mobile cranes. To ensure their machine capability, Liebherr deploys the cerTEST mobile test system by Kistler. Thanks to cerTEST, all the facility's power tools can be tested regularly, and all results are precisely documented with torque and rotation angle control. Heiko Springer is the project engineer in the Plant Planning Department at Liebherr Ehingen who heads this project. He explains: 'We currently have 128 torque wrenches from different manufacturers in use, and all of them have to be tested. As of 2018, we carry out our own machine capability analyses with four test points based on VDI 2645. We also carry out tests related to fastening points. We use nutrunners with different technologies for 'A', 'B' and 'C' joints – with electronic or hydraulic control, or with compressed air. Thanks to this new test stand from Kistler, we can successfully meet higher quality requirements. We base our work here on the strict standards specified by the automotive industry, which requires a machine capability index of Cm/Cmk > 1.6.'

End-to-end documentation for all tools

The cerTEST system has four simulators (two integrated and two external) to test power tools in the torque range from 250 to 6 000 Nm. This allows dynamic simulation of all joint hardnesses up to 6 000 Nm. And for tests related to fastening points, it delivers precise statements about the current performance of the tested power tools after 25 measurements. The simulator also rotates during the dynamic test. The advantage of this approach over the static method is that it allows simulation of 'hard' joints (with a small angle of rotation) as well as 'soft' ones (with a large angle of rotation). End-to-end documentation for all tools and the main joints creates an accurate picture of a machine's capability throughout its entire life expectancy.

Thanks to cerTEST, Liebherr saves about EUR 30 000 on external testing costs each year – and as an added bonus, the company can build up its own valuable know-how on fastening technology here in house. As far as Springer is concerned, the benefits are obvious: 'On the one hand, we save on costs – but that's by no means all. It's easier to ensure the quality of our internal fastening processes; also, the quality is better than when tests are outsourced. Another point is that we're building up know-how on handling power tools. And last but not least, the logistics are more efficient: there are no more costs for sending the tools out and sorting them back in again.' Because the Kistler test stand is mobile, all the power tools can be tested near the production lines. For this purpose, cerTEST is equipped with a rechargeable battery that has a lifetime of up to 16 hours.

Convincing advantages: performance scope, specialization and service

One of the key points for Liebherr was that the tool testing system must cover the entire bandwidth required. With a range of up to 6 000 Nm, the external dynamic simulator also gives Liebherr the capability of testing electrical power tools currently in use at Ehingen which operate at 4 000 Nm or more. 'That was a decisive factor for us, of course, because no other provider was able to supply dynamic simulators with a range up to 6 000 Nm. Other advantages were Kistler's independence, its specialization in measurement technology, and its expertise in fastening technology – adding up to a complete package that we found highly convincing,' Springer stresses. 'Starting in early 2016, we began to focus more attention on the machine capability of power tools; in any case, this issue has become generally more important and will become even more critical in future. So we're very well equipped to face those challenges thanks to the solution from Kistler.'

Data from all the test procedures is stored and fully documented in the CEUS software that is integrated in cerTEST. Springer notes: 'An important point here was that the data on an access point we installed on the nutrunner test stand should be sent to a server via WLAN. That enables everyone involved to access the test results in CEUS from their PC workstations.' The Plant Planning team uses handheld scanners, making it easy to set the electrically controlled tools to a test point or a specific joint at the start of the test procedure. 'This prevents any operator errors that could occur with manual input of the detailed data. That gives us accurate and reliable traceability so we know who tested which power tool, when, and to what extent,' Springer explains.

Now, about one year after the project began, he is highly satisfied with the results: 'Thanks to our excellent working relationship with Kistler, we overcame all the challenges. We've established a quality assurance process that will not only save us money in the long run, but will also offer extra security as regards product liability. And by no means least, it gives us a deeper understanding of the technology. It's highly likely that we'll be using more Kistler fastener inspection systems in the future so that we can continue to develop our expertise. At the moment, for example, we're thinking about deploying the method known as 'joint scanning'. This involves positioning a plug-on sensor between the nutrunner and the real joint. The nutrunner test stand measures the torque and rotation angle, and stores the data for the power tool. Later on, this precise joint can be simulated for this same tool on the nutrunner test stand.'