Bolted electrical components: Electrical contact resistance is key

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