Critical connections Every screw counts: Quality assurance for screw connections

Related Vendors

NORD-LOCK GmbH

Nord-Lock GmbH

Bosch Rexroth AG

Taiwan Machine Tool & Accessory Builders' Association (TMBA)

Wuxi InfiMotion Technology Co., Ltd.

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SIMTOS (Seoul International Machine Tool Show)

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When heavy loads are moved by cranes, trains travel at 200 km/h, or airplanes fly at 15 km altitude, many safety-relevant screw connections are in use. If they fail, the consequences could be catastrophic. Therefore, the correct execution of screw connections is a critical manufacturing aspect that begins in the planning phase.

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 by the US aviation safety authority has since identified the cause: Four bolts, essential for securing a door cover, were missing. Investigations revealed that these bolts may not have been installed due to a maintenance error.

Incidents like these clearly demonstrate: if safety-relevant screw connections fail, life and limb are at risk. This can only be prevented if a comprehensive quality process is adhered to, ensuring that each connection is carried out according to a prescribed 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 technician of Nord-Lock, emphasized in a lecture at the regional fair Schraubtec in Sindelfingen 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 VDI directive 2230. It is recognized worldwide as a standard work for the calculation of screw connections and describes different tightening methods and preloading forces that are best suited for the respective application.

The first step of planning is to define exactly how a connection is made, including the selection of the appropriate type of screw connection, such as angular or torque screw connections, based on the load analysis and the required strength.

Not just 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 and creates a clamping force that holds the components together. External operating forces such as pressure, tension, and vibrations must also be calculated by the designers. And also, friction has a great influence: Up to 90 percent of the force applied during tightening goes into overcoming frictional forces. "The exact calculation of the friction is crucial for reliably determining the preload force. If I know the exact friction window, I can apply the torque more precisely," says Küffner.

"In assembly, programmable tools are then used that are precisely set to these values," explains Harald Lukosz, speaker for the Product Area Stage at Bosch Rexroth, and continues: "This is followed by tests in which the preload force is particularly checked, which must be large enough to hold the parts together. After these tests, the entire process is checked again, and adjustments are made if necessary."

Modern tools can not only correctly screw in the screw according to the technical specifications but also guide the worker intelligently and document the fastening process. "Especially with safety-relevant 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," says Lukosz.

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

Additional safety

With correct calculations, suitable material, and professional assembly, the foundations for a secure screw connection are laid. However, screws can still loosen, unwind, or even unscrew. But there are measures to prevent this. For instance, securing elements are available to prevent screws from loosening.

To prevent falling fasteners from injuring people or causing subsequent damage, securing elements such as all-metal lock nuts, nuts with plastic inserts, 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 the focus, securing elements that prevent unscrewing by themselves 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 technician can easily lose track. After the screw has been properly tightened, it is marked accordingly with the test mark. If the screw comes loose afterwards, the lacquer breaks, indicating a loose connection. This allows for faster visual inspection. Other aids such as anaerobic screw locks ensure from the start that the connection does not loosen even under shocks.

Incidents like the one at Air Alaska in January can only be avoided if all steps of quality assurance are observed: from the calculation of the connections, their review, 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 their employees. "The employees not only need to 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 the complex relationships and respond appropriately if necessary," says Lukosz.