Loosening: A phenomenon where the screw gradually rotates and the fastening force weakens due to vibration, shock, or temperature changes after tightening. This is especially critical in machines that operate under constant vibration.

Breakage: When the applied torque (the force used to turn the screw) is too high, the screw or the material being fastened cannot withstand the force and breaks. On the other hand, insufficient torque can lead to loosening or rattling.

Stripping (Damaged Screw Head): This occurs when the tool doesn’t properly engage with the screw head’s slot or corners and still rotates, resulting in the slot or corners being rounded off, making it impossible to turn the screw. This is common with small screws or screws made of soft materials.

Misalignment / Angled Fastening: This happens when the screw is not inserted straight or in the correct position before being tightened. This can lead to improper assembly, malfunction, or insufficient strength.

Improper Screw Selection: Choosing screws that do not match the operating environment (temperature, humidity, vibration), required strength, or the type of material being fastened (e.g., incorrect material, strength grade, or type) can easily lead to corrosion, insufficient strength, or loosening.

Poor Torque Management: Fastening torque is not properly set or controlled, tools like torque wrenches are not calibrated, or tightening is done by feel. These factors contribute to over-tightening or under-tightening.

Operator Error or Lack of Skill: Human errors such as forgetting to tighten, following incorrect procedures, or using tools improperly are also major causes of defects.

Worn or Poorly Maintained Tools: Worn driver bits or wrenches can lead to stripped screw heads. Inadequate maintenance of automatic fastening machines can also result in torque inconsistencies or misalignment.

Improper Storage Conditions: Rust, dirt, or deformation of screws due to poor storage can reduce strength, change the friction coefficient, and interfere with proper fastening.

Working Environment: Excessive vibration, tight working spaces, and insufficient lighting can all increase the likelihood of mistakes and negatively impact quality..

Checkpoint 1: Are the screws selected appropriate for the usage environment and load?

• Material: Choose materials such as stainless steel or appropriately surface-treated steel according to environmental conditions (e.g., humidity, corrosiveness). Also, be mindful of galvanic corrosion that may occur when different metals come into contact.

• Strength Grade: Select screws with sufficient strength grade (e.g., 4.8, 8.8, 10.9) to meet the required clamping force (axial force) as per design. Consider safety factors as well.

• Type: Depending on the application, choose the proper type of screw such as machine screws, bolts, or tapping screws.

• Head shape (e.g., pan, flat, hex) and recess shape (e.g., Phillips, hex socket) should be selected based on the fastening position, the tools used, and appearance requirements. Choose ones that offer good workability and proper engagement with tools.

• In areas with vibration or temperature shifts, consider spring washers, nylon nuts, locking adhesives, or double nuts, as appropriate.

• Setting the correct fastening torque: To generate the appropriate axial force (the force that holds components together), determine the required torque value based on the target axial force and screw specifications, and clearly communicate it to operators.

• Proper use and calibration of torque measurement tools: Use tools such as torque wrenches, torque screwdrivers, and nut runners to achieve accurate tightening. These tools must be calibrated regularly to maintain measurement precision.

• Recording and monitoring torque values: For critical fastening points, it’s also effective to record the torque values and monitor variation.

• Establishing standardized procedures: To ensure “consistent quality regardless of who performs the work,” define clear work procedures and share them through standardized work instruction documents.

• Accurate positioning and vertical tightening: Screws should be inserted perpendicular to the hole to avoid angled fastening. In automated systems, regularly check positioning accuracy.

• Appropriate tightening speed: Excessively fast tightening may cause galling or disrupt torque control. Tightening should be performed smoothly and at a proper speed.

• Training in knowledge and skills: Regularly train workers on topics such as types of screws, the importance of torque control, and proper tool usage to improve their skills and understanding.

• Promoting understanding of the reasons behind procedures: By explaining not only what to do but also why it’s done, operators’ awareness and commitment to quality can be enhanced.

• Inspection and replacement of bits and sockets: Since driver bits and sockets wear out over time, check their condition regularly and replace them promptly when worn. It is helpful to establish clear replacement criteria.

• Maintenance of fastening tools: Power screwdrivers and nut runners should be inspected and maintained periodically according to manufacturer instructions to maintain performance

• Torque verification: Confirm that the screws are tightened to the specified torque using a torque wrench, applying methods such as the tightening confirmation torque method or the loosening torque method. Depending on the importance of the product, this can be done for all units or by sampling.

• Visually check the screws and surrounding areas after fastening. Look for signs such as protruding or tilted screws, damage to the head or seating surface, stripping, or cracking/deformation of the fastened materials. If necessary, use gauges and measuring tools for more precise inspection.

• Especially for parts exposed to vibration or those critical to safety, it is recommended to establish and carry out regular inspections after operation (e.g., looseness checks, re-tightening). Record the results and use them for future improvement.

• Humidity and temperature control: To prevent rust, avoid storing screws in hot and humid environments; keep them in dry conditions. In humid climates, consider using dehumidifiers, sealed containers, or desiccants.

• Rust prevention measures: If screws are coated with anti-rust oil or packed in rust-preventive packaging, be sure to use them promptly after opening or repackage them properly to maintain their protective effect.

• To avoid mixing different types of products, clearly sort screws by type, size, and material, and ensure all containers are properly labeled. This also improves work efficiency.

• To reduce the risk of deterioration during long-term storage, establish and follow a FIFO system where older stock is used before newer stock.

• Regularly inspect the storage conditions and check for rust, dirt, and deformation. For screws with special coatings, be sure to follow the manufacturer’s recommended storage conditions and observe any specified usage period.

Proper Selection: Choose screws that suit the usage conditions, and apply anti-loosening measures if necessary.

Strict Torque Management: Set accurate torque values, use calibrated tools, and monitor torque consistently.

Standardized Work Processes: Establish clear procedures, provide operator training, and ensure stable quality.

Reliable Tool Management: Check for tool wear and conduct regular maintenance.

Thorough Inspections: Detect abnormalities early through post-fastening torque checks and visual inspections.

Appropriate Storage: Store screws in an environment that prevents rust and deterioration, and implement FIFO (First In, First Out) practices.