Fastener Testing

By Kay Dierecks, product manager fastening technology, Kistler Group

Bolted joints need to be highly resilient as they have to withstand enormous forces, adverse conditions, high currents and much more. Therefore, the quality of bolted components is essential to make sure they function reliably in spite of these influences. This is especially true if electric current flows through the components – as is often the case in the automotive industry and, more specifically, in the electromobility sector.

For these reasons, manufacturers are well advised to determine key parameters – such as the friction coefficient, preload force and also electrical resistance in the product development phase – and then monitor these values as part of their quality control procedures. Test and inspection solutions, such as the extended ANALYSE system from Kistler, are now coming onto the market and enable users to measure accurately and efficiently the electrical resistance along with other parameters of the bolted joint – in one integrated system.

Moving a car window up or down conveniently at the touch of a button; easily switching the seat heating on; or quickly charging a mobile phone battery – functions such as these used to require manipulations such as manual winding, or didn’t even exist, but now we take it for granted that they can be performed electrically. However, there’s another side to the story; reports about batteries bursting into flames or smoldering fires with unknown causes make us sit up and take notice. 

One possible cause of fire in many of these cases is excessive contact resistance in the current-carrying bolted components. Electrical resistance is increased by factors such as high surface roughness –
leading to gaps between the contact surfaces of the connected parts. Although many of these irregularities are only on a microscopic scale, they can still impede the flow of current. Other effects, such as insufficient preload due to sub-optimal bolting parameters, can equally contribute to heat being generated in the component. They may also result in defects or even lead to fires. Hence, manufacturers are right to fear technical problems of this sort, as well as the risks of recalls and recourse costs that they can entail.

Quality control must meet  complex requirements

However, measuring the quality relevant characteristics of bolted electrical components is a complex undertaking. On the one hand, the sheer numbers of parts built into modern cars, and the many technical requirements to be met, present challenges for development and quality control. On the other hand, bolted electrical components still have to function safely and reliably – even though the voltages they must withstand are constantly increasing, because traction batteries (for example) are becoming more and more powerful. What makes measurements particularly complex is the need to measure different parameters, depending on the location and purpose of use. 

Making measurements simpler and easier

For many measurands, suitable testing solutions are already available on the market to perform combined measurements of various parameters in one test stand. Yet until now, no integrated solution for electrical resistance has been available, which is why the company is launching new add-on modules for its tried and tested ANALYSE system in the summer of 2023. This includes the test module for electrical resistance, which can easily be integrated into the ANALYSE system without any complications. Developers and designers will now be able to perform, document and visualise a comprehensive and varied range of tests – even before the parts are actually installed. To take one example, they can test how high the electrical resistance will be for any given tightening torque. Thanks to these combined tests, they can ultimately determine the correct balance between tightening torque and electrical contact resistance for their products and define the optimal tightening specification of the bolted joint on this basis.

Highly sensitive – and ideal for the microohm range

The new module for Kistler’s ANALYSE system uses the Kelvin method to measure electrical resistance. With this method, leads connected to the Unit Under Test (UUT) impress a constant current into the bolted joint. A connected measuring device registers the voltage loss caused by the resistance. Because the measuring module is highly sensitive, the smallest changes in resistance – even down to the microohm range – can be measured as the bolted joint is gradually tightened and loosened.

For accurate results, it is important that only the resistance in the UUT is registered. To meet this requirement, the test set-up from Kistler isolates the bolted joint under test so that current flows exclusively through the UUT, and the instruments only measure the actual resistance of the joint. The choice of measuring points during the test is of overriding importance here. The closer these highly sensitive measuring tips are to the bolted components, the more precise the results will be. As an additional option, a camera can be installed to record the entire test procedure. The corresponding module of the evaluation software then synchronises the video and the recording of the measured values, so any unusual or suspicious values can be checked quickly and easily. 

Combined testing of quality characteristics

Exact measurements and detailed visualisations of the results provide manufacturers with a sound basis for decisions on whether the current-carrying parts of the joint meet the requirements for durability, safety, and reliability, as well as enabling them to compare different materials. Providing one solution that combines different measurements – such as friction coefficient, tightening torque, preload or electrical contact resistance – means manufacturers can use Kistler’s ANALYSE system to test components and their behaviour in one single test procedure. Users can now determine optimum conditions and tightening specification for each bolted joint – so the products they bring to market are safer and more reliable.