Design and Analysis of Bolted Joints (DABJ)

Just about everyone involved in developing hardware for space missions (or any other purpose, for that matter) has been affected by problems with joints using threaded fasteners. Common problems include rupture, fatigue, detrimental yielding or joint slip, galling, inadequate preload, loss of preload, low or nonlinear stiffness, excess weight, procurement cost and lead time, incompatibility with the space environment, and time-consuming assembly.

The main objective of this three-day course is to build understanding of how bolted joints behave, how they fail, and how to design them to be dependable and cost effective.

Participants will work multiple problems in class.

Click here for a free download of the course materials.

Mechanical design engineers, structural analysts, and others interested in or involved with bolted joints.

1. Overview. Common problems with bolted joints. A process for designing a bolted joint. General design guidelines. The importance of preload. Introduction to NASA-STD-5020 and 5020B. Key definitions. High-level requirements from NASA-STD-5020A. Margin of safety. Establishing internal standards and criteria.
2. Screw Threads:       Evolution and Important Characteristics. Brief history of screw threads and thread forms. Rolled vs. cut threads. Thread-form features and compatibility. Tensile stress area. Fine threads vs. coarse threads.
3. Developing a Concept for the Joint. General types of joints and fasteners. Configuring the joint. Designing a stiff joint. Shear clips and tension clips. Avoiding problems with fixed fasteners.
4. Calculating Bolt Loads when Ignoring Preload. How a preloaded joint carries load. Temporarily ignoring preload. What about friction as a load path? Common assumptions and their limitations. Finite element modeling of a bolted joint. An effective process for calculating bolt loads in a compact joint. Examples.
5. Failure Modes & Assessment Methods. Understanding stress analysis from the engineer’s perspective. An effective process for strength analysis. Bolt tension and shear. Tension joints. Shear joints. Class exercise:       identifying potential failure modes. Bolted joints with composite materials.
6. Thread Stripping & Pull-out Strength. How threads fail. Computing theoretical shear engagement areas. Including a knock-down factor. Test results.
7. Selecting Hardware & Detailing the Design. Selecting compatible materials. Nuts and threaded inserts. Use of washers. Bolt features and geometry. Selecting fastener length and grip. Recommended fastener hole sizes. Guidelines for simplifying assembly. Establishing preload. Torque-preload relationship. Locking features and NASA-STD-5020B.  Maintaining preload.
8. Mechanics of a Preloaded Joint. Mechanics of a preloaded joint under applied tensile load. Estimating bolt stiffness and clamp stiffness. Understanding the load-introduction factor. Worst case for steel bolts and aluminum fittings. Key conclusions regarding load sharing. Effects of bolt ductility. How temperature change affects preload.
9. Fastening System Analysis per NASA-STD-5020B. Objectives and summary. Calculating maximum and minimum preloads. Tensile loading: ultimate-strength analysis. Separation analysis. Tensile loading: yield-strength analysis. Shear loading. Interaction of tension, shear, and bending. Joint-slip analysis. Fatigue. Justification for low likelihood of fatigue failure.
10. Special Topics. Finite element modeling of bolted joints. Design tables: preliminary bolt sizing, based on NASA-STD-5020B analysis criteria.

• “It was a fantastic course—one of the most useful short courses I have ever taken.”
• “Interaction between instructor and experienced designers (in the class) was priceless.”
• “(The) examples (and) stories from industry were invaluable.”
• “A must course for structural/mechanical engineers and anyone who has ever questioned the assumptions in bolt analysis”

Tom Sarafin is President and Chief Engineer of Instar Engineering and Consulting, Inc. He has worked full time in the space industry since 1979 as a structural engineer, a mechanical systems engineer, a project manager, and a consultant.  Since founding Instar in 1993, he’s consulted for NASA, DARPA, the DOD Space Test Program, Lockheed Martin, DigitalGlobe (Maxar), Sierra Nevada Corp, Spaceflight Industries, Millennium Space Systems (Boeing), and other organizations.  He was a key member of the team that developed NASA-STD-5020, “Requirements for Threaded Fastening Systems in Spaceflight Hardware” (March 2012).  He is the editor and principal author of Spacecraft Structures and Mechanisms:  From Concept to Launch and is a contributing author to Space Mission Analysis and Design.  He’s also the principal author of a series of papers titled “Vibration Testing of Small Satellites.”  Since 1995, he has taught over 300 courses to more than 6000 engineers and managers in the aerospace industry.