I. Common Signs of Failure
1. Loosening: The nut and bolt rotate relative to each other, resulting in decreased preload.
2. Damaged Anti-loosening Devices: Broken cotter pin, faulty locking washer, flattened spring washer.
3. Thread Damage: Stripped threads, scratches or defects at the thread root.
4. Body Damage: Cracks, corrosion, deformation.
5. Functional Failure: Unable to reach the specified torque value, or unable to maintain tightness after tightening.
II. Judgment Steps and Methods
Step 1: Visual Inspection
1. Check Anti-loosening Devices:
Cotter pin/locking washer: Are they intact, not detached, or displaced?
Spring washer: Are they completely flattened or broken?
Double nuts: Check for gaps between the upper and lower nuts.
2. Check the nut body:
Cracks: Focus on checking stress concentration areas such as the closing part and the thread root. **Rust:** Surface rust accelerates wear and corrosion.
Thread Damage: Check for scratches or defects at the thread root.
Step Two: Functional and Performance Testing
1. Torque Verification:
Method: Use a torque wrench to loosen the nut and record the required torque value.
Judgment: If the torque value is significantly lower than the initial installation torque or standard value, it indicates insufficient preload or failure.
2. Vibration Test:
Method: Install the nut in an environment simulating working vibration and observe for a period of time.
Judgment: Loosening or falling off the nut is considered a failure.
3. Professional Testing (Critical or Batch Products):
Tensile/Shear Test: Test the nut's ultimate load-bearing capacity under axial or lateral loads.
Hardness Test: Check the hardness of critical parts such as the thread root; abnormalities may indicate a risk of brittle or ductile failure.
Thread Accuracy Inspection: Use a thread gauge to check the pitch diameter, thread pitch, etc. Excessive deviations will affect engagement and preload. Step 3: Comprehensive Assessment and Decision-Making
1. Minor Failure (e.g., minor corrosion, minor thread damage):
Treatment: After cleaning and repair, consider replacing with a more corrosion-resistant material (e.g., stainless steel) or strengthening anti-loosening measures.
2. Severe Failure (e.g., cracks, severe deformation, stripped threads):
Treatment: Must be replaced; cannot be repaired.
3. Uncertain Situation:
Treatment: Replacement is recommended, or send to a professional institution for non-destructive testing (e.g., magnetic particle testing, ultrasonic testing).
III. Preventive Measures
1. Selection and Matching: Select the anti-loosening type based on vibration intensity, disassembly requirements, and environment.
2. Standardized Installation: Follow torque specifications and correctly install the anti-loosening device.
3. Regular Maintenance: Develop an inspection plan, focusing on connections in vibration environments.
4. Environmental Adaptability: In special environments, select corrosion-resistant and high-temperature-resistant materials.
IV. Replacement Procedure
1. Preparation: Disconnect power and release pressure. Prepare tools and new nuts (specifications and strength grades must match the original nuts).
2. Disassembly:
Rust Removal and Lubrication: Spray lubricant on rusted nuts and let stand for 15-30 minutes before attempting to loosen.
Correct Disassembly: For single nuts, loosen counterclockwise with a wrench; for double nuts/locking nuts, first secure the main nut, then loosen the secondary nut.
3. Installation:
Pre-assembly: Hand-tighten the new nut onto the bolt, ensuring thread alignment.
Initial Tightening: Tighten with a wrench until the parts fit snugly.
Final Tightening: Tighten to the torque value specified in the vehicle/equipment manual. For multi-bolt connections, use a symmetrical, cross-tightening method to gradually tighten.
4. Post-Installation Inspection:
Visual Inspection: Confirm that the nuts fit snugly and are not misaligned, and that anti-loosening devices (such as cotter pins and locking washers) are installed correctly.
Functional Testing: Use a torque wrench to randomly check if the tightening torque meets the requirements, and simulate vibration in the working environment to check for looseness.
V. Precautions
1. Inspection Cycle: For important connections (such as engines and bridges), it is recommended to inspect monthly or quarterly; increase the frequency in vibration environments.
2. Records and Maintenance: Keep records after each inspection, and deal with any problems promptly (such as replacing cotter pins or retightening).
3. Environmental Factors: High temperatures and corrosive environments will accelerate failure; increase the inspection frequency and use corrosion-resistant materials (such as stainless steel).
