I. Common Manifestations of Failure
Failure typically manifests as loosening, stripping, breakage, or jamming. Specific phenomena include:
1. Loosening: The nut and bolt rotate relative to each other, causing a decrease in preload.
2. Stripping: The threads fail to engage, preventing torque transmission.
3. Breakage: Cracks or breakage appear in the nut body.
4. Jamming: The nut and bolt cannot rotate relative to each other, but the preload may be abnormal.
II. Steps and Methods for Determining Failure
Step 1: Visual Inspection (Initial Screening)
1. Check Anti-loosening Marks: Check if the marks on the nut and bolt are aligned; misalignment indicates possible loosening.
2. Check Anti-loosening Devices:
Cotter Pins/Locking Washers: Are they intact, not detached, or displaced?
Spring Washers: Are they completely flattened or broken?
Double Nuts: Check for gaps between the upper and lower nuts.
3. Inspect the Nut Body:
Cracks: Focus on stress concentration areas such as the closing section and 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 (In-depth Verification)
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 to determine if it meets strength requirements.
Hardness Testing: Tests the hardness of critical components such as the thread root. Abnormalities (too high or too low) may indicate a risk of brittle or ductile failure.
Thread Accuracy Inspection: Uses thread gauges to check the pitch diameter, thread length, etc. Excessive deviations will affect engagement and preload.
Step 3: In-depth Analysis of Failure Causes (for Failed Parts) If the nut has failed, the following analyses can be used to pinpoint the root cause:
1. Macroscopic Morphology Analysis: Observes the fracture morphology to determine whether it is brittle or ductile fracture.
2. Microscopic Analysis (SEM/EDS): Inspects the thread surface for pear-groove scratches, microcracks, oxidation marks, etc., to analyze the failure mechanism.
3. Chemical Composition Analysis: Verifies whether the material meets the standard (e.g., 42CrMo).
4. Hardness Testing: Compares the hardness distribution of the failed part with that of a new part to determine if abnormal hardness caused the failure.
III. Key Precautions
1. Inspection Cycle: For critical connections (such as engines and bridges), monthly or quarterly inspections are recommended; increased frequency is necessary in vibration environments.
2. Record Keeping and Maintenance: Record all inspections carefully and address any issues promptly (e.g., replace cotter pins, retighten).
3. Environmental Factors: High temperatures and corrosive environments accelerate failure; therefore, increased inspection frequency and the use of corrosion-resistant materials (such as stainless steel) are necessary.
