DEVELOPMENT OF TIGHTENING TORQUE FOR SELF TAPPING AND THREAD ROLLING FASTENERS

摘要

Torque is the most common tightening strategy used to control clamp load when bolts are tightened, since a direct relationship between torque and clamp load exists. For unified and metric fasteners used in non-critical applications, there are torque tables available in the literature that provide a guideline as to what torque level is recommended to be used based on fastener size, thread pitch, and material class. For critical applications, the tightening specification is usually developed based on individual cases. For self tapping and thread rolling screws, the tabulated torque values available for machine threaded screws cannot be used. If the torque spec is not developed carefully for such joints then problems such as stripping the threads or torqueing out before seating the screw may be encountered. This is mainly due to the fact that these types of screws are very sensitive to many variables, such as thread type, hole size, coating, joint surface finish, tapped material thickness, and other factors. For such fasteners, the torque spec should be developed experimentally based on test data for the actual joint. This paper provides an accurate procedure for developing tightening specifications for joints that use self tapping and thread rolling screws. Additionally, it provides a criterion to determine whether the joint design is robust enough for assembly or not.