Critical connections Every screw counts: Quality assurance in screw connections

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When heavy loads are moved by cranes, trains travel at 125 mph, or planes fly at an altitude of 15 kilometers (approx. 10 miles), many safety-critical bolted joints are in use. If they fail, the consequences could be catastrophic. Therefore, the correct execution of bolted joints is a critical manufacturing aspect that begins in the planning stage.

In January 2024, Alaska Airlines Flight 1282 made headlines when the Boeing 737-9 lost an exit door shortly after takeoff in Portland and had to make an emergency landing. Fortunately, none of the 171 passengers and 6 crew members suffered serious injuries. A preliminary report from the US aviation safety authority has since identified the cause: four bolts, essential for securing a door cover, were missing. The investigation found that these bolts may not have been installed due to a maintenance error.

Incidents like these clearly show: if safety-relevant screw connections fail, life and limb are in danger. This can only be prevented if a comprehensive quality process is followed, which ensures that each connection is carried out according to a defined procedure.

Planning is key

When planning connections—whether safety critical or not—experts rely on their experience and qualifications, technical tools, and recognized guidelines and standards. As Kevin Küffner, screw specialist from Nord-Lock, emphasized in a lecture at the regional fair Schraubtec in Sindelfingen, Germany, in April 2024, the quality of the connection primarily depends on the planning and calculation in advance.

An important basis for the screw case planning is provided by the German VDI Guideline 2230. It is considered a standard work for the calculation of screw connections worldwide and describes different tightening methods and preload forces that are best suited for the respective application case.

The first step in planning is to precisely define how a connection is made, including the selection of the appropriate type of screwing, such as angle or torque screwing, based on the load analysis and the required strength.

Not just by guesswork: determination of the torque

Screws connect different materials through the applied torque. When tightening, the screw is stretched, creating the preload force. This force presses the screw head and the nut downwards, generating a clamping force that holds the components together. Designers must also calculate external operating forces such as pressure, tension, and vibrations. Friction is also a major factor: up to 90 percent of the effort used when tightening goes into overcoming frictional forces. "Accurate calculation of friction is crucial to reliably determine the preload force. If I know the exact friction window, I can apply the torque more precisely," Küffner says.

"In assembly, programmable tools are used that are precisely set to these values," explains Harald Lukosz, Product Area Stage consultant at Bosch Rexroth, and further elaborates: "Tests follow, where the preload force, which must be large enough to hold the parts together, is checked in particular. After these tests, the entire process is reviewed again, and adjustments are made if necessary."

Modern tools can not only screw in the screw correctly according to technical specifications, but also guide the worker intelligently and document the fastening process. "Especially with safety-related connections, every step in the screwing process must be documented to ensure traceability and compliance with specifications. This includes recording the tools used, the torques applied and the results of the final checks," said Lukosz.

Digitalization and intelligent tools make it possible to optimize such processes and reduce costs without neglecting necessary testing steps or safety measures. "However, even these technological solutions cannot completely replace human expertise," emphasizes Lukosz.

Additional Security

Despite correct calculations, suitable material, and professional assembly, the basics for a secure screw connection are established. However, screws can still loosen, unscrew, or even spin out. But there are measures to prevent this. For instance, securing elements are available to avoid the unscrewing of screws. These include spring-loaded securing elements such as disc springs and tension washers. According to Nord Lock expert Küffner, they are suitable for screws of low strength classes or at low preload forces.

To prevent falling connecting elements from injuring people or causing subsequent damage, securing elements such as all-metal lock nuts, nuts with a plastic insert, and thread-forming screws are suitable for certain applications. Here, the maintenance of the preload force is rather subordinate. For cases where the preload force is paramount, securing elements that prevent automatic unscrewing are the better choice.

"In aviation, safety-relevant screw connections usually have to be additionally secured with screw lacquer," adds Moritz Julian Storch, Managing Director of Lackfabrik Bäder. Because with many screw connections, the fitter can easily lose track. After the screw has been properly tightened, it is marked accordingly with the test mark. If the screw loosens afterwards, the lacquer breaks and thus indicates a loose connection. This allows for faster visual inspection. Other aids like anaerobic screw locks ensure from the start that the connection does not loosen even when subjected to shocks.

Incidents like the one with Air Alaska in January can only be avoided if all steps of quality assurance are taken into account: from the calculation of connections, their verification and the corresponding tools, to the qualification of the personnel. In summary, Harald Lukosz emphasizes that the elements of this safety chain usually offer little potential for savings. Therefore, it is crucial that the industry invests in the training and ongoing education of its employees. "The employees must not only be familiar with the latest technologies, but also develop a deep understanding of their fields of expertise. Only in this way are they able to penetrate complex relationships and react appropriately when necessary," says Lukosz.