At 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?
Bolts are ubiquitous in machine design and product engineering, and the vast majority of use cases will not require in depth analysis. 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. Hand calculations can be challenging to reason through, and finite element models can be way off if not setup with proper inputs. Because of the complexity and unique nature of bolted joint design, many fastener related failures occur in the field. 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). Even the new eastern span of the San Francisco-Oakland Bay Bridge had a complex fastener related problem (hydrogen embrittlement) shortly after construction. \
flabel practical stress analysis for design engineers pdf 22
Due to this complexity, we cannot possibly cover all design cases thoroughly in this post. 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).
In most cases it is best practice to design a slip-critical joint. 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. This may seem obvious, but it is worth reiterating: bolts are designed to develop clamping loads between two or more components. Implicit in this statement is that bolts are not designed primarily to act as shear pins or in bending. For this reason, ensuring the proper preload across a bolted connection is especially important. As engineers, it is our job to understand and control the loading conditions across structures we design and assembly plays a large and often undervalued role in this.
As engineers we design bolted and riveted joints that include multiple fasteners. The fastener positions are often patterned uniformly because of the ease of developing simple linear patterns in CAD programs. In situations where installed stiffness is critical, or factors of safety are lower, more care must be put into faster layout. Always remember that the position of fasteners relative to the applied load dictates how much of that load is carried by each fastener. This is the fundamental reason we recommend maximizing member stiffness in the previous section. A stiff component being bolted down will transfer load more evenly among a given fastener group, independent of fastener position.
This leaves us with perhaps the most difficult problem of any bolted joint design; determining the optimal preload and associated torque for each bolt in the joint. We know the optimal preload for a bolt is somewhere between zero and proof load, but how do we determine it exactly? The answer lies in balancing the mean load due to preload, and the dynamic or cyclical load due to external forces across the joint. We can visualize the impact of mean load and cyclical load alongside the stress strain curve.
Determining the proper torque and preload for a fastener is the most difficult engineering problem of any bolted joint design. Usually this is an iterative process which involves creating a joint design, selecting a fastener and fastener layout you think will work (sometimes based entirely on feel), running some analysis, and iterating on both fastener selection, layout and preload. Loading conditions can impact the difficulty of the analysis significantly. A few cases worth noting are discussed below along with simple illustrations to clarify load direction relative to fasteners and joint members.
We know from basic mechanics of materials that a rod in tension is stronger than a rod in shear. But this does not extrapolate well to rivet designs. Many rivets have a domed or countersunk head on one side, and a deformed shank on the other side. The interference fit of the deformed rivet shank on the side opposite the rivet head can cause stress risers that lead to local yielding. This might be fine for static loading because the material will yield slightly and redistribute the load. But when tensile and axial loads are combined, and the loading is dynamic or cyclical, slippage can occur in the joint well below static load ratings. This can cause the rivet to work itself loose or fatigue very quickly.
Perhaps the most important takeaway from this article is that fastener design problems should not be overlooked or underestimated. The ubiquity of fastener usage, coupled with the fact that in many cases fastener strength is nowhere near the limiting factor, can lead engineers to overlook detailed joint and fastener loading analysis in cases where it absolutely must be done.
Lastly, it is equally important to validate that the correct preload was achieved to guarantee good joint performance in the real world. When factors of safety are high you can reasonably trust the numbers and lean on your analysis primarily. When factors of safety are low and failure consequence is high (ie: aerospace), empirical testing is often the only way to be confident that your joint design can cope with the anticipated loads. Thanks for reading and good luck with your designs! If you have any questions about this article please reach out to william@fiveflute.com and we can talk about it! 2ff7e9595c
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