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Checklist to minimize screw-related problems at manufacturing sites
Introduction
In a manufacturing setting, screws are a critical component that determines the strength and reliability of a product. However, when problems such as loosening, falling out, or thread damage occur, they can lead to product defects, increased accident risks, and higher rework costs, significantly reducing production efficiency on-site. This article introduces a practical checklist to minimize such “screw troubles.”
The intended audience includes site managers, assembly workers, and quality assurance personnel. We have organized the key points for fastening management compliant with JIS and ISO standards into a “checklist format” that can be immediately used in daily assembly and inspection tasks. We have specifically focused on the keywords “screws,” “troubles,” and “checklist,” comprehensively summarizing the check items for each stage.
First, let’s understand the typical trouble cases that frequently occur on-site and their backgrounds, and then grasp the essential points in each phase from design and selection to procurement and storage, fastening work, and inspection and maintenance. Finally, we will also propose how to incorporate this into the PDCA cycle and a preventive flow utilizing the latest tools such as IoT torque wrenches. This will enable you to simultaneously achieve quality improvement, cost reduction, and worker safety in the manufacturing workplace.
Types of Screw Troubles Frequently Occurring On-Site
Loosening and Falling Out
This is a phenomenon where screws gradually loosen due to vibration, impact, or thermal expansion/contraction, eventually falling out.
- Cause: Insufficient preload, not using lock washers or thread-locking adhesives.
- Standard Example: JIS B 1050 “Tools and techniques for screw fastening”
Stripping (Crushing) of Screw Heads and Thread Damage
This is a defect where the screw head or threads are crushed due to excessive torque or misalignment of the screwdriver.
- Cause: Inadequate torque management by workers, wear and tear of tools.
- Countermeasure: Selection of strength classes recommended in ISO 898-1 “Mechanical properties of fasteners”.
Breakage (Fracture/Rupture)
This is a phenomenon where screws crack and break due to excessive load or repeated stress.
- Cause: Insufficient consideration of impact load, use of materials with poor fatigue strength.
- Countermeasure: Material selection based on fatigue test data, improving fatigue resistance through surface treatment.
Misalignment and Cross-threading
Due to insufficient positioning of the workpiece, screws are fastened at an angle, causing poor contact and stress concentration.
- Cause: Lack of jigs/positioning pins, incorrect assembly sequence.
- Countermeasure: Introduction of dedicated jigs, clarification in Standard Operating Procedures (SOP).
Rust and Seizing
This is a problem where the screw surface rusts due to humidity or corrosive gases, causing it to seize and become difficult to loosen.
- Cause: Insufficient anti-rust treatment, poor humidity control during long-term storage.
- Countermeasure: Surface treatments like zinc plating or parkerizing, storage rules using desiccants.
Checkpoints at the Design and Selection Stage
Confirmation of Material and Strength Class Suitable for the Operating Environment
First, clarify the operating environment, including the temperature, humidity, and presence of chemicals where the product will be used.
- In high-temperature environments, select materials with excellent heat resistance such as stainless steel (e.g., SUS316) or heat-resistant alloys.
- In environments with salt damage or chemical atmospheres, combine with surface treatments focused on corrosion resistance (zinc plating, nickel plating).
- If not only static loads but also vibration and fatigue loads are expected, apply the screw strength classes specified in ISO 898-1 (e.g., 8.8, 10.9) to ensure sufficient tensile and fatigue strength.
Neglecting these selections makes breakage and loosening due to material incompatibility more likely in later processes or during product use.
Presence of Anti-Loosening Measures (Washers, Thread-locking Adhesives, etc.)
To prevent loosening due to vibration or thermal fluctuations, always incorporate anti-loosening measures at the design stage.
- Use spring washers or toothed lock washers to suppress the decrease in preload.
- When using adhesive thread-locking agents (e.g., Loctite 243), incorporate it into the torque management procedure and ensure sufficient curing time.
- Adopt metal nylon insert nuts (self-locking nuts) for products that are frequently disassembled and reassembled.
These measures require implementation in accordance with JIS B 1050 “Tools and techniques for screw fastening.” Incorporating appropriate anti-loosening measures leads to reduced maintenance frequency and improved worker safety.
Checkpoints at the Procurement and Storage Stage
Thorough Implementation of First-In, First-Out (FIFO)
At the time of procurement and receiving, record the receiving date for each lot and thoroughly implement a system to use the oldest items first.
- Set recommended usage deadlines in the lot management system.
- Affix lot display labels to the receiving shelves so that workers can identify them at a glance.
- During inventory checks, periodically check for defect rates or expired screws.
This prevents material degradation and surface oxidation due to long-term storage, allowing you to avoid on-site troubles before they occur.
Implementation of Anti-Rust Packaging and Humidity Control
To prevent screws from rusting, consistent anti-rust measures are necessary from receiving to storage.
- Individually package them in anti-rust bags containing a desiccant such as silica gel.
- In high-humidity environments, introduce storage cabinets with air dryers or dehumidifying functions.
- Periodically check the sealed state of the anti-rust bags and regenerate (or replace) the desiccant.
- Install a simple hygrometer near the site and operate a system that sends an alert if the humidity exceeds 60%.
By turning these management practices into rules and visualizing them in manuals and on bulletin boards, you can significantly reduce troubles caused by seizing due to rust or a decrease in strength.
Checkpoints at the Fastening Work Stage
Torque Value Setting and Record Management
- Predefine recommended torque values for each screw size and strength class (e.g., M6 8.8 = 10 N·m, M8 10.9 = 25 N·m).
- Utilize the digital management function of torque wrenches or torque drivers to record the fastening torque for each task.
- Link fastening logs with QR codes or barcodes to ensure traceability.
- To prevent loosening or over-tightening, standardize the retightening/check torque method (double torque method).
Tool Wear Inspection and Calibration Schedule
- Set the calibration cycle for torque wrenches and electric drivers based on usage time or number of operations (e.g., every 3 months or 3,000 times).
- Register calibration certificates in a tool management ledger and automatically notify the next calibration date.
- Visually inspect bits and sockets for wear and deformation monthly, and replace them immediately if they exceed the wear limit.
- Affix calibration labels to all tools to prevent the use of uncalibrated tools on-site.
Standard Operating Procedures (SOP) and Worker Training
- Make the SOP for the fastening process into a poster for on-site display, visualizing key points (how to use tools, torque operation, check methods).
- For new employees and transferees, conduct hands-on training during their initial education and utilize video manuals for fastening work.
- Hold regular meetings on fastening quality to share trouble cases and improvement measures.
- Manage training records in a digital ledger and list individuals who require re-training.
Checkpoints at the Inspection and Maintenance Stage
Post-Fastening Torque Check (Retightening/Check Torque Method)
- After the initial fastening, retighten to 1.2 times the specified torque, then measure the check torque to confirm that the preload is securely maintained.
- Automatically record the check torque value with a digital torque wrench and determine pass/fail based on whether it is within the standard range (e.g., 90-110% of the initial torque).
- Save torque check logs linked to individual part numbers or assembly line IDs and use them for future trend analysis and defect analysis.
Periodic Inspection Plan and Record Keeping
- Set the periodic inspection cycle on a risk basis (e.g., monthly for high-vibration equipment, quarterly for normal equipment).
- Digitize the inspection checklist and set it up so that inspectors can enter and sign it on a smartphone or tablet.
- Centrally manage inspection results in cloud storage and notify relevant parties by alert email when an abnormality occurs.
- Comply with the storage period (minimum 5 years) for inspection records in accordance with the requirements of JIS Q 9001 (Quality Management System).
Practical Trouble Prevention Flow
Proposal for On-Site Poster
- List major troubles (loosening, stripping, misalignment, etc.) with icons.
- Color-code the checkpoints for each phase so that the next process can be confirmed at a glance.
- Also include operational rules such as “Contact information in case of abnormality” and “Tool calibration schedule.”
- Recommended size is A2 to A1. Posting it at the entrance of the work line or near the tool rack prevents oversights.
Real-time Monitoring Case Study with IoT Torque Wrench
- Introduce digital IoT torque wrenches on the assembly line to collect torque values during fastening in the cloud.
- Instantly grasp “below/above specified value” on a real-time dashboard and issue an immediate alert.
- For example, an IoT torque wrench with a torque waveform analysis function contributes to improving the accuracy of abnormality detection.
- Conduct trend analysis from the collected data and use it for tool life prediction and accuracy comparison for each worker.
Support for Small-Lot, High-Mix Screw Procurement in Special Environments such as Overseas Bases
- Register multiple local suppliers on a procurement platform and automatically check for the shortest delivery time and optimal price.
- For custom-made screws in small lots, achieve a shorter lead time from the initial estimate to sample delivery.
- Conduct quality inspections at local testing institutions to guarantee hardness, thread accuracy, and surface finish.
- Share KPIs such as “delivery delays” and “defect rates” in regular review meetings and continuously follow up on process improvements.
Summary
Screw troubles in a manufacturing setting can be significantly suppressed by operating a checklist that covers each process from design and selection to procurement and storage, fastening work, and inspection and maintenance. By quantitatively recording recommended torque values, tool calibration, humidity control, etc., and visualizing them with SOPs, on-site displays, and IoT tools, problems can be discovered and improved early. Furthermore, as seen in the case of procurement support at overseas bases, it is also important to utilize resources according to the local situation and build a rapid response system. Let’s incorporate this into the PDCA cycle, continuously make improvements based on performance data and on-site feedback, and simultaneously achieve safety, quality improvement, and cost reduction.
Conclusion
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