WINTER 2017

14 THE DISTRIBUTOR’S

14 THE DISTRIBUTOR’S LINK Laurence Claus Laurence Claus is the President of NNi Training and Consulting, Inc. He has 25 years of experience with a medium sized automotive fastener manufacturer, holding positions including Vice President of Engineering, General Manager, Director of Quality, Director of New Business Development and Applications Engineer. In 2012 he formed NNi offering technical and business training courses as well as technical consulting, expert witness and consultation work. He can be reached at 847-867-7363 or by email: Lclaus@NNiTraining.com. You can learn more about NNi at www.NNiTraining.com. FUNDAMENTALS OF THREAD FORMING SCREWS - PART 2: THREAD FORMING SCREWS FOR THERMOPLASTICS Review Looking back at Part 1 of this series, we explored the fundamental science of thread forming screws. We learned that although the material the screw is thread forming is important in how we view the entire system, FIGURE 1: HOW A THREAD FORMING SCREW WORKS- GRAPH OF TORQUE VERSUS TIME there are certain universal fundamentals about thread forming that are independent of the material. We learned that during installation all screws, but especially thread forming screws, experience an immediate increase in torque followed by a gradual but continuous rise in torque up until the point the head is seated against the captured material. This inflection point is normally referred to as the driving torque. As the installer continues to turn the screw, torque rises rapidly until the joint fails by either the screw breaking (less common) or the joint stripping (more common). This point is often referred to as the ultimate torque or stripping torque. It is during this time, from when the head seats to the ultimate failure, that the joint is developing the desired axial clamp load. We learned that engineers designing thread forming joints are extremely interested in the value of these two points, and particularly the range between them. The resulting drive to ultimate ratio is a strong determining factor in whether the joint can be safely and consistently assembled or not. If the band is too narrow, it is likely that normal installation equipment cannot adequately handle the deviation and parts will be prone to either not being fully installed, breaking during installation, or both. TECHNICAL ARTICLE CONTINUED ON PAGE 100