Design and optimization of the tightening procedure used by the M10X60 hexalobular flanged screws

Nowadays, a high accurate clamping load can be applied easily to a bolted joint, thanks to the proper tightening method and high sensitive and accurate driver unit. But the precise clamping force, actually, is not always guaranteed during the bolt’s service life, especially when a dynamic load is applied to the joint or the bolt is employed for a high working temperature condition. The aim of this study is to design a proper tightening procedure able to guarantee an accurate initial clamping load and minimize the clamping load lost due to the working conditions, thus optimizing the joint performance. The mutual influence between bolt and clamped parts is somewhat of a complex situation and still lacking of study. Therefore the basic theory and practical experience are taken as the basics of this thesis to describe how and why some new tightening procedures have been designed. Then experimental method has been defined for performing lab tests. And thanks to the ultrasonic method, the clamping force in the assembled bolt has been measured precisely. In tests the M10X60 Agrati zinc flake coated bolt used for fixation of calipers in car’s brake system is taken as a reference test-piece. The new tightening procedures are performed firstly on the dummy joints to measure the trends of clamping load over time at room temperature and the fall in clamping load during a 5 steps repeated heating and cooling process. Secondly, the same tests are performed on the real calipers to enhance the reliability of the previous measurements. At last, based on the test results the new tightening procedures are judged by three aspects: the resistance to embedding, accuracy and joint security. By this way the optimal tightening procedure is suggested.