Multi-axis Force Sensor for Sensor-integrating Bolts.

We present a multi-axis force sensor capable of measuring both axial forces and bending moments that is integrable into a cavity within a M20 bolt. This cavity, which reduces the resilience of the bolt by 23.5 %, and, thus, corresponds to a reduction by only one strength class, is designed by conducting a finite element analysis. The force sensor featuring a diameter of 13.9mm and a length of 19.5mm is based on a deformation body that consists of three beams. The manufactured deformation body is applied with one strain gauge on each of the inner and outer beam surface providing a half-bridge configuration for each beam. Measurements when loading the force sensor solely with a bending load of 75 Ncm and an axial load of 100N show low linearity errors of +/- 0.4% and +/- 1.1% for those load cases. Furthermore, decoupling of various loads from different load cases is achieved with a maximum error of 3.1 %. These results confirm that the force sensor developed is suitable for integration in bolts to detect faults in bolt connections due to multi-axis loads at an early stage, and, thus, increase safety of machinery and plants.