Development of a test rig for handheld nutrunners

Sammanfattning: As the demands for precision in nutrunners increases, the testing equipment at manufacturers of these tools also need to get better. At Atlas Copco in Tierp there are a number of test rigs that compare the torque that the nutrunners indicate, with the torque that a sensor records. In these AR-rigs a bolted joint is tightened with a nutrunner by the help of a solution containing a sensor, a telescoping component and sockets or adaptors to compensate for different torques. This is done as a finishing check for every nutrunner. In the current design of the rig, there are some known errors and trade-offs in the 20 year old base design. One of the known problems is the alignment between the nutrunner and the bolted joint. To further investigate this, and to lessen possible measuring errors due to misalignment of components, this master thesis was carried out at Rejlers AB. To start with, a number of requirements were done and five design concepts were generated that would fulfil the requirement specification. These concepts were then compared to each other in a Pugh’s evaluation matrix and after discussion with Atlas Copco, one of the concepts were further developed. This concept comprised a locking of the sensor’s rotation and an assembly of the sensor and telescoping component placed together. A detailed 3D CAD-model was done on the new design and solid mechanics calculations were performed on the bottom plate for the sensor. Also Eigen frequencies and theoretical alignment possibilities were calculated. Furthermore an experiment was carried out, were the impact from several components were investigated. This experiment was also used to compare the current design with the new concept. The solid mechanics calculations showed that the largest deformation, 0.0062 mm, had minimum impact on the alignment. Also the maximum stress of 1.53 MPa was of insignificant size, far from the yield strength of aluminium. The calculated Eigen frequencies of the concept exceeded the rotational speed of the nutrunner’s output shafts. An alignment method was proposed using a multi-dot laser, to achieve a maximum displacement of only 0.55 mm for the centerlines of the remaining subsystems. The experiment was used as a base for calculations comparing the new concept and the current design. In the end, future work, such as the building of a prototype and testing, was proposed.