In manufacturing sites, fastening with screws and bolts is a crucial process that supports the reliability of all machinery and equipment. However, if proper tightening is not performed, “fastening failures” such as loosening or breakage can occur, leading to quality defects and serious accidents. Such failures often arise from mistakes in the design phase, problems with tools or the working environment, or external factors like material properties, corrosion, or temperature, requiring constant attention on site.
This article will clarify the basic mechanisms and main causes of fastening failures, while also providing easy-to-understand examples of actual troubles and their countermeasures.

In manufacturing sites, fastening failures involving screws and bolts are frequently reported, and many of them lead to serious consequences such as the detachment of structural components or machine stoppage. Below are some representative issues along with their summaries.

Case of Nut Detachment Due to Creep of Resin Parts
In a certain device, a nut securing a resin part detached during operation. The cause was the creep of the resin due to axial force during fastening, and further deformation progressing due to external forces during use. As a result, the tightening force of the nut was lost, and the machine stopped.

Fatigue Fracture of Rail Fixing Bolt (Poor Heat Treatment, Insufficient Torque)
An SCM435 bolt used for rail fixing in railway vehicles experienced fatigue fracture due to repeated loads. The cause was coarse metal microstructure due to poor heat treatment, in addition to improper management of the tightening torque. Ultimately, fatigue fracture was confirmed through fracture surface observation.

Defect Outflow Due to Misaligned Tightening and Double Tightening of Small Screws
In the assembly process of electronic devices requiring minute torque tightening, misaligned tightening and double tightening that could not be visually confirmed occurred. The outflow of defective products was a challenge, but by introducing an error detection device that can precisely manage torque and angle, the outflow was completely prevented.

Trouble and Undetected Issues Due to Appearance Defects (Burrs, Cracks)
On a mass production line, burrs generated during the rolling process and minute cracks caused by forging were overlooked during inspection, leading to insufficient strength and fastening failures after assembly. This case highlighted the limitations of manual inspection.

A common lesson learned from these cases is the importance of a three-pronged approach “design, process, and inspection” to ensure fastening reliability.

Importance of Strength Confirmation and Material Selection at the Design Stage
For fastenings involving resin, it is necessary to consider the insertion of collars that account for deformation due to axial force, and the selection of bolts based on stress analysis. Furthermore, if fatigue fracture is a concern, predicting the lifespan using S-N curves and setting an appropriate safety factor are indispensable.

Optimal Setting of Initial Tightening Torque and Axial Force
To prevent ductile fracture due to excessive torque, or loosening and fatigue fracture due to insufficient torque, setting the appropriate torque using a tightening diagram is effective. To prevent nuts from loosening, auxiliary materials such as spring washers and self-locking nuts are also utilized.

Maintenance of Tools/Fastening Devices and Utilization of Detection Functions
Wear on the screw chuck part and damage to the bit can be a breeding ground for fastening abnormalities. Regular replacement and condition monitoring, along with the introduction of devices that can monitor tightening torque and angle in real-time, can prevent defects from occurring.

Strengthening Quality Control through Automation of Appearance and Dimensional Inspections
A 100% appearance inspection system utilizing image processing sensors can accurately detect appearance abnormalities such as minute cracks and dimensional deviations. From the perspective of traceability, digital management of inspection data will be an important measure for future quality assurance.

By implementing these measures, it is possible not only to prevent the recurrence of fastening failures but also to improve the overall reliability of the product.

To stabilize the quality of screw fastening, “torque management” is paramount. Especially by applying appropriate torque and axial force during initial tightening, long-term loosening and fatigue fracture can be prevented.

Design and Torque Control Using Bolt Tightening Diagrams
A bolt tightening diagram illustrates the balance between the axial force and elongation applied to the bolt, and the compression of the fastened body. It is utilized to optimize fastening conditions during design. If the tightening torque is set appropriately, the internal force of the bolt is suppressed even when external forces act, significantly reducing the risk of fatigue fracture.

Handling Minute Torque with Nut Runners with Error Detection
For small screws often used in electronic devices, minute torque management of less than 0.1 N·m is required. For such fastenings, a nut runner that can monitor torque and angle in real-time and automatically detect abnormalities like misaligned tightening or double tightening is effective. Its introduction can prevent the outflow of defective products, enabling both productivity and reliability.

Utilization of Jigs and Collars to Ensure Stable Axial Force
Especially for materials where the fastening surface is prone to deformation, such as resin parts, inserting a metal collar (spacer) can stabilize the axial force. This is an effective countermeasure to prevent the reduction of axial force due to part creep and the loosening of nuts.

Source_Japanese：Screw Tightening Test System｜Osaka Research Institute of Industrial Science and Technology

To prevent fastening failures, not only preliminary design and construction but also post-process inspection and quality assurance systems are indispensable.

Introduction of 100% Visual Inspection by Image Processing System
In recent years, image processing inspection combining high-precision cameras and AI has become widespread, allowing for high-speed and high-precision inline detection of appearance defects such as burrs, cracks, and deformation. This ensures that even minute abnormalities that are often overlooked in manual inspection are reliably captured, preventing the outflow of defective products.

Dimensional Control by Pitch Gauge, Limit Gauge, and Torque Test
For checking screw thread shapes, pitch gauges, limit gauges, and screw-in torque inspection using a torque wrench are effective. By utilizing these devices, quality control compliant with JIS and ISO dimensional tolerances can be performed. Standardization and recording of measurement data are also important management points.

Enhancement of Quality Control, Traceability, and Training
To minimize the occurrence of defects, improving the skills of on-site workers is essential along with the inspection system. The development of training programs is required so that they can understand the meaning of tightening torque and signs of abnormalities. Furthermore, digital management of inspection records and defect history leads to securing traceability and improving processes.

By comprehensively adopting these measures, it is possible to prevent fastening failures and maintain stable manufacturing quality.

A1. The most frequently reported troubles are due to “loosening” or “fatigue fracture.” In particular, cases where bolts fracture due to repeated stress caused by a decrease in axial force resulting from nut loosening are often observed.

A2. Torque management is fundamental, but depending on the condition of the fastened surface and the properties of the fastened material, the fastening force may be insufficient even with the correct torque. It is necessary to consider axial force management and the use of auxiliary jigs (e.g., metal collars) in conjunction.

A3. Defects such as misaligned tightening or double tightening can be detected using devices like micro nut runners that can monitor torque and angle simultaneously. They can also handle the management of minute torque, which is difficult with visual inspection.

A4. 100% inspection is recommended for industries where product safety is required. Especially for minute defects such as burrs and cracks, automated inspection using image processing systems is effective. Inline inspection can prevent the outflow of defective products.

Source_Japanese：Newly created test fixture and test method for investigating the influence of tightening torque in bolt-nut fastening｜Research Institute for Applied Mechanics, Kyushu University

Screw fastening failures manifest in various forms, including loosening, excessive torque, fatigue fracture, and material abnormalities. Many of these occur due to oversights in the design phase, incorrect tightening condition settings, or inadequate inspection. The real-world examples introduced in this article have clarified the importance of practical countermeasures such as torque management, appropriate tool/equipment selection, and automation of visual inspection. To enhance fastening quality, a three-pronged approach of process design, equipment selection, and operation management is required. By establishing these at the manufacturing site, product reliability and safety can be further enhanced.

Conclusion

For inquiries regarding samples or materials, please respond to this email or contact our sales representative!

At Ohta Vietnam, we not only provide ready-made products such as screws and bolts, but also support activities in the manufacturing field such as cutting, inspection, assembly, packaging…
Our equipment list is here

As a “specialized support” for the manufacturing industry in Asia. If you have any related questions, feel free to contact Ohta Vietnam for detailed support!
URL: https://ohtavn.com/en/contact/