Smart Engineering: The Cure for Almost All Fastener Failures
Every engineered part, assembly, or machine has a finite service life. Even components boasting the most precise designs executed with proven manufacturing techniques eventually fail. Fasteners are no different. As any engineer will tell you, it’s when these failures are unexpected or premature that they become real problems. So, what causes premature fastener failures? More importantly, what can be done to minimize risks and maximize a fastener’s service life?
In the field, designers and manufacturers can conduct a wide range of failure analyses to uncover the root cause(s) of a fastener failure. Throughout my years working in the fastener industry, the primary causes of failures always seem to be:
Over-tightening. To much force used to install fasteners can lead to tensile failures.
Under-tightening. Improper installation can put too much stress on fasteners and lead to fatigue failures. Fasteners and joints can also come loose due to insufficient clamping force.
Improperly designed joint. Poor design sometimes means improper loads are placed on a fastener, which can cause premature failure. Joints and fasteners can loosen after installation, or the parent material the fastener attaches to can fail.
Fastener quality. The materials used to make the fasteners dictate their performance. So, poor materials mean poor performance and early failures.
Assembly equipment. Improperly using assembling equipment often leads to improperly installed fasteners and early failures.
Hydrogen embrittlement. In high-tensile steels above a certain hardness range, tensile load applied to the fastener can cause hydrogen inherently in the component, plating, or environment to flow to area of stress, gradually causing micro cracks and delayed fastener failure.
Spec sheets cannot tell designers everything. Real-world testing is the best method of ensuring fastener issues are identified before they make it to the production line.
The specific industry and type of assembly affects the risk of fastener failures. Industries typically at high risk are often less heavily regulated by standards so designers and engineers may not be fully aware of issues affecting a fastener’s suitability. Typically, more technically specialized manufacturers are aware of such fastening issues due to better education regarding fasteners’ effect on the final product. However, this depends on the standards from industry to industry.
The automotive sector serves as a good example. Manufacturers are constantly trying to reduce curb weight to improve overall fuel efficiency and performance. This has led to the use of dissimilar materials. Cars are no longer made mostly of steel. Joining these materials can lead to failures and/or corrosion if governing parameters are not assessed by auto engineers early in the design and prototyping stages.
Many fastening companies, such as ours, work closely with customers to set design rules that address material interfaces and other potential failure issues. This minimizes risk.
Reading through the list of common failure culprits, it quickly becomes apparent that misuse, rather than poor quality, is the most common cause. Rather than focus on a specific fastener, fastener makers should look at how a customer uses fasteners. From that point, they can begin to set rules that lets a company get the most from the fasteners it purchases.
One key issue we encounter is the translation of theoretical calculations into real-world applications. The specs cannot tell you everything; real-world testing is the only surefire way to ensure fastener problems are identified before those fasteners make it to the production line. Parameters such as assembly torques and friction ranges should not be assumed, so physical testing is incredibly important to back up theoretical calculations, ensure product quality, and minimize the impact of potential fastener issues on operations.