We designed a mobile tightening solution to assemble the covers of electrical transformers. M8, M10 and M12 bolts are tightened with the same tool by use of a socket tray. The customers network defines all different transformer models into their database. The operator scans the transformers serial number. The software checks the model and ensures that the correct bolt size, the correct torque values and the correct number of bolts are tightened. After all tightenings are made, the results are stored back into the database.