![]() ![]() For this reason, ensuring the proper preload across a bolted connection is especially important. Implicit in this statement is that bolts are not designed primarily to act as shear pins or in bending. This may seem obvious, but it is worth reiterating: bolts are designed to develop clamping loads between two or more components. This means that the bolts in the joint generate sufficient clamping load across the joint such that the shear through the joint is transferred through the joint member faces, not as direct shear through the fastener itself. In most cases it is best practice to design a slip-critical joint. Design for Shear Transfer Through Friction However, in our experience the following design guidance can arm engineers with a basic joint design toolkit, an understanding of what to look out for when designing, as well as orient engineers to the areas of complexity that must be investigated further (empirically, or with nonlinear contact FE models). \ĭue to this complexity, we cannot possibly cover all design cases thoroughly in this post. Even the new eastern span of the San Francisco-Oakland Bay Bridge had a complex fastener related problem ( hydrogen embrittlement) shortly after construction. General Motors recalled approximately 500,000 vehicles across seven models (Impala, Camaro, Equinox, GMC Terrain, Cadillac SRX, Buick Regal and Lacrosse) in 2014 due to fastener related problems(2). Because of the complexity and unique nature of bolted joint design, many fastener related failures occur in the field. Hand calculations can be challenging to reason through, and finite element models can be way off if not setup with proper inputs. However, for those cases where safety factors are lower due to strength, weight or other requirements, or where exact preload must be achieved, bolted joint design can be extremely difficult. Design Guidelines for Bolted Jointsīolts are ubiquitous in machine design and product engineering, and the vast majority of use cases will not require in depth analysis. The following table ( adapted from the NASA Fastener Design Manual) is a useful reference guide relating fastener materials to coatings, temperature and strength at room temperature. It is however a sacrificial material and will not last forever in highly corrosive environments. Zinc plating has the unique property that it will migrate to areas that have been scratched in a “self healing” manner. The other common coating for low carbon or alloy steel bolts is zinc plating, which helps limit corrosion in wet environments. This includes the common black oxide coating, which typically has a thin oil film that can delay oxidation for a very short period of time (on the order of days). ![]() Keep in mind that uncoated steels will eventually rust in open air. As carbon content increases above 0.25%, steels can be heat treated to significantly higher strengths at the cost of a loss in ductility. In low temperature, non corrosive environments low carbon steel is favored for its low cost. For the vast majority of non-aerospace applications bolts are typically made from carbon, alloy or stainless steels. The primary drivers of bolt material selection are strength, corrosion resistance, temperature environment and cost. ![]() Individual applications may require other considerations, but taking 30 minutes to think through the above questions can keep you working efficiently as a designer. So what are requirements for a bolted joint?Īt a minimum you should be thinking about the following: What is the purpose of the fastener in this application? What loading conditions do I expect the joint to experience? What drives the design: Static strength? Stiffness? Fatigue? What is the worst case scenario? What is the target safety factor? What materials are present? Should corrosion be considered? Should thermal stresses be considered? How will it be assembled? Does it need to be taken apart or serviced? How long does it need to last? How much will it cost? ![]() For bolted and riveted joints, this won’t take long, but will be enormously helpful. Here at Five Flute we like to make a requirements project and use issues to sketch out a quick list of design requirements. As with most problems, you can really benefit from stepping back and listing out your requirements. For machine design applications, picking the right fasteners and choosing how many to use can be challenging. As mechanical engineers, we deal with fasteners on almost every project. ![]()
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