Wireless, energy-autonomous remote monitoring of bolted joints

fatigue strength, smart screw connection, energy self-sufficient remote monitoring

© Fraunhofer IIS

Screws offer the possibility of designing simple, efficient and detachable connections and are used in almost all technical areas of general mechanical engineering as well as civil engineering. In this context, screw connections must ensure reliable fastening, especially for connections that help ensure safety, and must be inspected regularly. Permanent energy-autonomous and wireless remote monitoring makes it possible to detect changes in the load placed on complex screw-fastened structures that are relevant to safety at an early stage, evaluate the changes and thus significantly increase the reliability of such structures (through the digital networking of systems). Applications for this can be found in almost all technical fields, from offshore structures, wind turbines, plant and mechanical engineering through to bridges and building facades.

© Fraunhofer IIS
Wireless data connection and piezoresistive DiaForce® coating
© Fraunhofer LBF
Bearable strain of the current industrial demonstrator relative to the standard DIN screws with the same thread dimensions

Smart screw connection

The first phase of the Fraunhofer Research Center IoT-COMMs (FIOT) project, entitled “smart screw connection”, which the Fraunhofer Institute IIS, IST and AISEC were involved in, implemented a standalone solution, consisting of a combination of sensors, an energy supply and long-distance wireless communication. This makes it possible to remotely monitor individual screw connections and, in turn, the behavior of complex structures, as well as sending alerts if there are changes in the mechanical loads.

In the current phase of the project, which Fraunhofer LBF is now involved in, sensor screws are being optimized for potential target applications on the basis of a market study and user workshops. Here, Fraunhofer LBF is responsible for optimizing the mechanical reliability of the sensor screw in relation to screw tightening torque and structural durability with the help of numerical simulations, as well as validating the sensor screw, while taking into account the system components relating to the sensors, the wireless connection and the energy supply.

Optimization of the mechanical reliability

The current sensor with an M18 thread, which was used as a technology demonstrator, to demonstrate the functionality of an energy self-sufficient screw in the first project phase, has significantly reduced mechanical strength due to the electronics integrated into the screw head, as well as the energy self-sufficient solution that uses a solar cell or optional thermogenerator on the shaft. The focus of the overall sensor screw system is now on optimizing these, as well as optimizing the fatigue strength and system reliability under load conditions they will be subject to in future use, and qualifying them for industrial applications.

The optimization is carried out at Fraunhofer LBF using a numerical and experimental evaluation of the existing screw body, as well as the overall sensor screw system, in which it is compared to standard screws with the same thread dimensions. Here, potentially critical local strains are identified using FEM simulations. These are then correlated with the results of fatigue strength tests.

Here, initial investigations showed that the existing screw body of the technology generator had a significantly reduced fatigue strength relative an equivalent standard screw. This was due to there being significantly higher local strains than for commercially-available standard screws. Using the knowledge gained, solutions have been developed, whereby a sensor screw that is subject to local strains similar to those of a standard screw of the same thread size can be expected to have a similar fatigue strength. In addition, concepts to increase the mechanical reliability of the overall sensor screw system were developed, which – in addition to the fatigue strength of the screw body – also allow for the greatest screw tightening torque possible for such a screw system, without any restrictions, in relation to the integrated sensors.

The optimized and innovative sensor screw concepts are important building blocks in the continuing digitization of safety-relevant systems and offer maximum protection against the failure of connections, in comparison to conventional screw connections, while simultaneously greatly reducing inspection effort.