Detailed Methodology for Determining Torque Limits to Maximize Preload for High-Strength Threaded Fasteners
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DETAILED METHODOLOGY FOR DETERMINING TORQUE LIMITS TO MAXIMIZE PRELOAD FOR HIGH-STRENGTH THREADED FASTENERS by D. ANDY HISSAM A DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in The Department of Mechanical and Aerospace Engineering to The School of Graduate Studies of The University of Alabama in Huntsville HUNTSVILLE, ALABAMA 2013 ii iii ABSTRACT The School of Graduate Studies The University of Alabama in Huntsville Degree Doctor of Philosophy College/Dept. Engineering/Mechanical and Aerospace Engineering Name of Candidate D. Andy Hissam Title Detailed Methodology for Determining Torque Limits to Maximize Preload for High- Strength Threaded Fasteners High-strength threaded fasteners are an amazing product of engineering, and one of the most common methods of making structural attachments. They are frequently used in structural joints because of two important features: such fasteners allow disassembly and reassembly of the joint, and they can generate high clamping loads called preload. To achieve the full benefit of threaded fasteners, the initial preload must be maximized. The benefits of high preload are well documented and include improved fatigue resistance, increased joint stiffness, and better resistance to vibration loosening. For joints in shear, preload resists relative motion, or slip, of the clamped members. Many factors, like elastic interactions and embedment, tend to lower the initial preload placed on the fastener. These factors provide additional motivation to maximize the initial preload. Also, in aerospace applications, maximized preloads help to achieve the full structural capacity from the fasteners, thereby minimizing weight. Of the available methods for controlling fastener preload, "torque control" is the most common. Unfortunately, determining the torque to maximize preload is problematic and greatly complicated by the large preload scatter generally seen with torque control. This dissertation presents a detailed methodology for generating torque limits to maximize preload for high-strength threaded fasteners. The methodology accounts for the large scatter in preload found with torque control, and therefore, addresses the statistical challenges of dealing with preload uncertainty. In developing this methodology, the probabilities of yielding the bolt during installation were calculated. To calculate these probabilities, the expected scatter iv v ACKNOWLEDGMENTS In all of life's great endeavors, we require the support of others. I could easily list one hundred people that have helped me during the course of my continued education and this research. Without their help, this advanced degree and research would not have been possible. I want to thank everyone that has made a contribution, regardless of how big or small. However, there are a few people that I would like to give special thanks. These people played a major role in helping me successfully reach this point. First, I want to thank Dr. Mark Bower for encouraging me to continue my academic studies. Without his support and encouragement, I would have never pursued this degree. He is truly missed. I also want to thank Dr. John Gilbert for accepting me as a graduate student and serving as my doctoral advisor. My first graduate class was with Dr. Gilbert back in 1998, and I still remember how much I enjoyed his class. Therefore, how appropriate that I complete my graduate studies working with him. I want to thank Neill Myers for acting as a sounding board for many of my ideas. Over the past five years, we have had many technical and philosophical discussions on fastener design and bolt preload. He always provided a fresh perspective when I was struggling with a problem. I also want to thank John Forbes for his many years of support, and most recently, for proofreading my dissertation. He performed a very thorough technical review and provided many excellent editorial comments. I want to thank James Hodo and Bruce Bice in the MSFC Materials Lab for helping me conduct fastener tests. Their background in fastener testing and knowledge of the test equipment helped me tremendously. I want to thank my parents, Frank and Rosemary, who from my early days as an undergraduate, encouraged my education and supported me financially. Their moral support and encouragement have always been fundamental to my success. I want to thank my wife and daughter, Deborah and Melaina, for their faithful support. For the past several years, they have had to tolerate stacks of books in the bedroom, spreadsheets taped to the walls, and a dominated vi computer. I could not have done it without them. Finally, I need to recognize my employer, NASA, for providing me full financial support and the resources to do much of this research. Ronald McDonald in the training office has been especially patient throughout my many years of study. I also want to thank David Whitten and Richard Stroud for their support and for recognizing the importance of this research -- which should ultimately benefit many projects and programs. Thanks again to all. Your support and friendship are truly appreciated. vii TABLE OF CONTENTS Page List of Figures .......................................................................................................................... xi List of Tables ........................................................................................................................... xiii List of Symbols ........................................................................................................................ xiv List of Acronyms ..................................................................................................................... xvii Chapter I. INTRODUCTION ...................................................................................................... 1 A. Purpose of the Study ...................................................................................... 1 B. Statement of the Problem ............................................................................... 2 C. Bolt Preload ................................................................................................... 3 D. Torque Control ............................................................................................... 4 E. Prevailing Torque .......................................................................................... 6 F. Objectives/Motivation ................................................................................... 7 G. Scope .............................................................................................................. 8 H. Outline of the Dissertation ............................................................................. 9 II. BACKGROUND ........................................................................................................ 11 A. The Screw Thread and Preload ...................................................................... 11 B. Torque-Tension Relationships ....................................................................... 12 1. Short-Form Equation ........................................................................ 12 2. Long-Form Equation ........................................................................ 13 3. Long-Form Equation with Prevailing Torque ................................... 17 C. Preload Scatter ............................................................................................... 18 1. Friction .............................................................................................. 20 2. Preload Distribution .......................................................................... 22 viii D. Literature Review .......................................................................................... 23 III. APPROACH ............................................................................................................... 27 A. Torque Limits for Specific Fastener Combinations ....................................... 27 B. Criteria for Determining Torque Limits ........................................................ 28 1. Distortion Energy Criterion .............................................................. 28 2. Tensile Stress Area and Bolt Stress .................................................. 29 3. Fastener Yield Strength .................................................................... 30 C. Analytical Studies .......................................................................................... 31 1. Torque-Preload-Stress Relationships ................................................ 31 2 Statistical Interference of Bolt Stress and Strength .......................... 34 3. Tolerance Intervals ........................................................................... 38 D. Testing ........................................................................................................... 39 1. Preload-Torsion Tests ....................................................................... 40 2. Prevailing Torque Tests .................................................................... 43 IV. RESULTS .................................................................................................................. 47 A. Torque-Preload-Stress Relationships ............................................................. 47 B. Statistical Interference of Bolt Stress and Strength ....................................... 57 C. Tolerance Intervals .......................................................................................