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5 ways to prevent screws from loosening that you should know at the manufacturing site
Introduction
In the manufacturing industry, the risk of a single “screw loosening” causing a serious accident or recall is one of the issues that top management and site managers should be most vigilant about. In fact, according to a cause investigation of screw fastening-related troubles (1977 domestic survey in Japan), tightening defects account for about 43% and loosening accounts for about 20%, and it is considered highly probable that these can be prevented with appropriate knowledge and management.
Especially in overseas production bases like Vietnam, environmental factors different from Japan (temperature, humidity, vibration) and quality variations in procured parts amplify the risk of loosening. Overconfidence that “it’s fine because I tightened it with the usual torque” can lead to unexpected line stoppages and increased costs.
In this article, for technical staff and procurement managers at manufacturing sites, we will explain “5 loosening countermeasures” that can be implemented immediately, while scientifically unraveling the mechanism of screw loosening. Furthermore, we will touch upon stable procurement in Vietnam and the latest axial force management utilizing IoT, presenting concrete action plans to protect factory safety and productivity.
Mechanisms and Risks of Screw Loosening
The True Nature of Initial Loosening (Non-Rotational Loosening)
Screw loosening is broadly classified into “loosening accompanied by rotation” and “loosening not accompanied by rotation.” What is frequently overlooked on site is “initial loosening (initial settling),” which is not accompanied by rotation.
Immediately after tightening a screw, minute irregularities on the bearing surface of the bolt, the screw threads, and the contact surface of the clamped material are crushed and smoothed by the external load. As a result, the contact surface sinks at a micro level (plastic deformation), and the bolt tension (axial force) decreases. Generally, it is said that the axial force immediately after tightening decreases by several percent to about 10% within a few hours to a few days, and design and retightening considering this are necessary.
Return Rotation due to Vibration (Rotational Loosening)
More serious is “return rotation” caused by vibration or impact. When the frictional force acting between the bolt and nut is momentarily lost due to lateral cyclic loading (such as Junker vibration), the screw begins to rotate in the loosening direction naturally along its own lead angle. Once rotation begins, the axial force rapidly approaches zero, leading to falling off or breakage in the worst case.
5 Loosening Countermeasures to Practice on Site
From here, we will introduce 5 specific countermeasures to consider at manufacturing sites from the viewpoints of cost, workability, and reliability.
1. Strict Implementation of “Axial Force Management”
The basic and ultimate secret of loosening countermeasures is to apply appropriate “axial force (preload).”
- Torque Method: The most common, but axial force can fluctuate by about ±30% due to variations in the friction coefficient.
- Rotation Angle Method: A method of managing by angle up to the elastic or plastic region, which can suppress axial force variation, but requires specialized tools and management.
In Vietnamese sites, since variations in how workers handle torque wrenches tend to occur, periodic tool calibration and the appropriate use of lubricants (e.g., molybdenum disulfide, etc.) to stabilize the friction coefficient are important.
2. Adoption of Mechanical Locking Mechanisms (Hard Lock, U-Nut, etc.)
“Structural locking” that does not rely on friction has extremely high reliability.
- Hard Lock Nut: Utilizes the wedge effect of an eccentric concave-convex nut to demonstrate a powerful anti-loosening effect. It is frequently used in railways and infrastructure facilities.
- U-Nut: Contains a friction ring (leaf spring) and generates prevailing torque (friction torque) by pressing down on the bolt threads.
Although the unit price of these parts increases, they can be advantageous in terms of total cost because the maintenance frequency can be reduced.
3. Utilization of Special Washers
Conventional spring washers may be limited to auxiliary effects, but high-function washers demonstrate effects on a different dimension.
- Nord-Lock Washer: Designed so that the cam angle inside the pair of washers is larger than the lead angle of the screw, physically preventing loosening rotation.
- Belleville Spring Washer (Disc Spring): Maintains high axial force and has the effect of compensating for axial force reduction due to initial settling.
4. Chemical Locking (Adhesives/Microcapsules)
By filling the gaps in the screw, physical fixation and a sealing effect (rust prevention) can be obtained simultaneously.
- Anaerobic Adhesive: A liquid resin that hardens when air is cut off. Strength can be selected from low strength to high strength.
- Pre-coated Bolt: A bolt with microencapsulated adhesive applied to the threads in advance. The capsules burst and harden during tightening.
However, it is necessary to consider the operating temperature range (generally about -40°C to 150°C) and workability during removal.
5. “Predictive Maintenance” and Monitoring Utilizing IoT
Screws are also evolving with the progress of Industry 4.0.
- Bolt with Axial Force Sensor: A sensor embedded inside the bolt measures axial force in real-time and transmits data wirelessly.
- Displacement Detection Marking: Systems that automatically detect minute rotational deviations with image recognition cameras have appeared, replacing visual confirmation of alignment marks.
Some screw suppliers are also considering proposing solutions that combine IoT technology, especially for inspections of critical safety parts or hazardous areas where human access is difficult.
Points for Procurement in Vietnam
When procuring these countermeasure parts within Vietnam, the biggest challenges are “authenticity of quality” and “material certification.” There is a risk that low-quality copy products that only mimic the appearance are circulating in the market.
- Utilization of Authorized Distributor Routes: Always purchase brand parts such as Hard Lock Industry and Nord-Lock from suppliers with official routes (such as Ohta Vietnam).
- Traceability: Confirm whether Mill Sheets (steel inspection certificates) and shipping inspection reports can be submitted.
- Environmental Compliance: Consider corrosion resistance of surface treatments (such as Geomet treatment or Zinc-Nickel alloy plating) together to match Vietnam’s high-temperature and humid climate.
Summary
Screw loosening is an inevitable phenomenon based on physical laws, but it can be turned into a “controllable risk” through appropriate technology and management.
- Axial Force Management (Torque management and friction stabilization)
- Mechanical Locking (Adoption of Hard Lock Nuts, etc.)
- High-Function Washers (Utilization of Nord-Lock, etc.)
- Chemical Fixation (Right material in the right place for adhesives)
- IoT Monitoring (Introduction of remote monitoring)
It is essential to use these 5 countermeasures properly according to cost and importance.
If you are unsure which countermeasure is best for your manufacturing site in Vietnam, or if you need to procure reliable fastening parts, please consult Ohta Vietnam. We provide total support from part selection that meets Japanese quality standards to solving on-site issues. Why not start by inspecting the “fastening part with the most trouble” currently in use?
Conclusion
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