Fastener Testing

Bolted joints must function safely and reliably to avoid extra costs, recall actions or recourse claims. In this interview, Wilfried Blechmann, head of fastening technology business line at Kistler Group, explains the key points in quality assurance for fastening technology. He also goes on to describe the systems that provide effective support for users in every step of the process.

Fasteners have been around for centuries and were added to the DIN catalogue of standards back in the 1930s. Why does this tried and tested joining method still play such an important part nowadays?

“There are two good reasons for that. Firstly, the bolted joint is the only joining method that can be released without destruction – and as a general rule, the joint can be used again. Also, fasteners are becoming smaller and lighter all the time, so they are highly versatile. That’s why they also have a part to play in Industry 4.0 – in the automotive, aviation, medtech, electrical engineering, wind power technology, construction and many other areas of industry.

The second reason is the safety aspect, which is a major issue in connection with bolted joints. Just think about a container weighing many tonnes, suspended from a construction crane as it is transported above a site full of people – and it’s held in place by just a few fasteners. Or think how many safety critical bolted joints there are in a car – for the wheels, axles, the transmission, doors, brakes and even the safety belts and airbags.”

Which points require attention from manufacturers who use fasteners in the production process?

“The level of quality required is high, so manufacturers need to monitor their fastening technology from ‘end to end’ of the process. They have to carry out quality assurance on the fasteners themselves, the tools and also on the bolted joints in the assembled components. They must do this consistently and with the required frequency, depending on the application, for instance testing a tool once every shift. Manufacturers also have to document everything, in case of an investigation. For instance, after a car accident they must be able to prove that they tested the bolted joint on the safety belt according to the applicable standards. The basis for performing quality assurance is set out in DIN EN ISO 9001.

Of course, the measuring instruments and systems used for quality assurance also need to be calibrated at regular intervals. Traceability of the measurement equipment may be required or indicated so as to boost confidence in the validity of the measurement results. In these cases, calibration must take place in a laboratory accredited as per DIN EN ISO 17025. According to the risk category, these calibrations have to be repeated at intervals ranging from three months to two years.”

There are vast numbers of standards and regulations in this area. Which ones apply in which cases?

“Depending on the purpose for which the bolted joints are used, manufacturers draw up a test concept that includes the relevant standards for their application. Let’s mention some examples of important standards and guidelines derived from the ISO 9000 series: VDI 2862 sheet 2 on the ‘Minimum Requirements for Application of Fastening Systems and Tools’; VDI 2645 sheet 2 on the ‘Capability Test for Fastening Technology – Machine Capability Test – MCT’; and VDI 2230 on the ‘Systematic Calculation of Highly Stressed Bolted Joints’. As well as these Norms, customers often have their own specific requirements. VDI Guideline 2862, which I just mentioned, also includes a classification of bolted joints in three categories: ‘A’, ‘B’ or ‘C’. Category ‘A’ means danger to life and limb, ‘B’ indicates restricted function, and category ‘C’ is for bolted joints where a fault does not present a risk but would be an annoyance for customers. The testing requirements for bolted joints in categories ‘A’ and ‘B’ are naturally stricter than those for category ‘C’.”

What are the most important variables that have to be tested?

“A number of different variables such as coefficient of friction, preloading force, and torque, are relevant to the quality of a bolted joint. Preloading force – a critical factor in joint stability – is the result of interaction between torque, rotation angle and friction coefficients for the surfaces involved. These parameters have to be monitored continuously. It’s also important to document and archive all the data – so the necessary proof can be provided in case of recourse claims.” 

(The friction coefficient test stand is used to check the key measurands for the quality of the bolted joint.)
​What are the advantages of solutions from Kistler?

“One decisive advantage is that Kistler offers systems, consulting and services for quality assurance from start to finish of the fastening process. Customers benefit from support at every stage – from drafting the test concept and actually carrying out quality assurance, all the way through to documentation and archiving. Also, we often implement adaptations to meet customers’ specific requirements – if space is confined, if two systems are needed, or if a mobile measurement solution is required. 

There’s also another significant advantage: We hold DIN EN ISO 17025 accreditation both as a stationary laboratory and also for on-site deployments, so we can perform the prescribed recalibrations of test and measurement equipment on the customer’s own premises. That means there’s no need to send devices away – so costly periods when they are not available for production are eliminated.

Kistler’s portfolio of inspection systems for fastening technology includes devices to test the fasteners themselves, the tools, and the bolted joints. The systems and the matching software capture all the measured values and present the documented results in a crystal-clear format. User-friendly software modules help to manage tasks and store all the relevant data on testing assignments.

In a nutshell, when customers come to Kistler, they get everything they need for safe and reliable bolted joints – all from one single source: Inspection systems, the relevant hardware, and software, backed by high-quality service – including traceable recalibrations on-site at our customers’ premises.”