Critical connections Every screw counts: Quality assurance in 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 planes fly at a height of 15 km, many safety-relevant screw connections are in use. Should they fail, it could have catastrophic consequences. 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 from the US Flight Safety Authority has now 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 show: if safety-relevant screw connections fail, life and limb are at risk. This can only be prevented by adhering to a comprehensive quality process that ensures that each connection is executed according to a specified procedure.

Planning is the be-all and end-all

When planning connections — whether safety-critical or not — experts rely on their experience and qualifications, technical tools, and recognized guidelines and standards. As emphasized by Kevin Küffner, screw specialist at Nord-Lock, in a talk at the regional trade 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 screw case planning is provided by the VDI guideline 2230. This is recognized worldwide as a standard work for calculating screw connections and describes different tightening methods and preload forces that are best suited for each application case.

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

Not just roughly: Determination of torque

Screws connect different materials through the applied torque. When tightening, the screw is stretched, which creates the preload force. This force pushes the screw head and the nut downwards and creates a clamping force that holds the components together. Designers also have to take into account external operational forces such as pressure, tension, and vibrations. And friction also has a major impact: Up to 90 percent of the force expended in tightening goes into overcoming frictional forces. "The precise 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," said Küffner.

"In assembly, programmable tools that are precisely set to these values are then used," explains Harald Lukosz, speaker for the Product Area Stage at Bosch Rexroth, and continues: "This is followed by tests, where the preload force is particularly checked, which must be large enough to hold the parts together. After these tests, the whole 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 intelligently guide the worker and document the fastening process. "Especially with safety-relevant connections, every step in the screwing process has to be documented to ensure traceability and compliance with the specifications. This includes recording the tools used, the torques applied and the results of the final checks," Lukosz said.

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

Additional safety

With correct calculations, suitable material, and professional assembly, the basics for a secure screw connection are laid. Nevertheless, screws can still loosen, be released or even unscrewed. But there are measures to prevent that. For example, locking elements are available to prevent screws from loosening.

In order to prevent falling fasteners from injuring people or causing consequential damage, locking elements such as all-metal lock nuts, nuts with plastic inserts and thread-cutting screws are suitable for certain applications. Here, the maintenance of the preload force is rather secondary. For cases where the preload force is paramount, locking elements that prevent automatic loosening are the better choice.

"In aviation, safety-relevant screw connections usually have to be additionally secured with screw paint," 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 with the test marking accordingly. If the screw loosens after that, the paint breaks and thus indicates a loose connection. This allows a faster visual inspection. Other aids such as anaerobic screw locks ensure from the start that the connection does not loosen even in case of vibrations.

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 the connections, their check, 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 continuing education of its employees. "Employees must not only be familiar with the latest technologies, but also develop a profound understanding of their areas of expertise. Only in this way are they able to penetrate complex interrelationships and react appropriately when needed," Lukosz says.