What Causes Thread Damage?

In production, thread damage can occur. Today, let's discuss this topic. We have summarized the common causes of thread damage into the following aspects. Wonder if you have encountered similar situations in your work?

I. Mechanical Stress Factors

1. Over-Tightening

   • Excessive Torque: If the tightening torque exceeds the load-bearing limit of the thread design, it can cause thread deformation or fracture. For example, using pneumatic tools without torque limit settings, or applying excessive force during manual tightening.

   • Axial Force Concentration: At the thread end (e.g., a "specific location" mentioned by the customer), if there is misalignment or eccentricity during assembly, it can lead to local stress concentration, causing thread chipping or stripping.

2. Thread Fit Issues

   • Insufficient Clearance: If the nut and bolt thread profiles do not match (e.g., overly tight tolerance), increased friction during tightening can easily lead to thread wear or galling (seizing).

   • Incorrect Thread Form: Deviations in the thread angle (standard is 60°) can reduce the contact area and cause stress concentration.

3. Insufficient Material Strength

   • Poor Bolt/Nut Material: If the material has low hardness (e.g., low-carbon steel without heat treatment), it is prone to wear under repeated tightening. Or, if the material is highly brittle (e.g., cast iron), it may fracture due to stress concentration.

   • Surface Treatment Defects: An excessively thick electroplated layer or peeling plating can affect thread fit accuracy.

II. Assembly Process Issues

1. Improper Operation

   • Non-Sequential Tightening: Tightening nuts in a cross pattern can lead to uneven load distribution and local thread overload.

   • Reusing Damaged Threads: Continuing to use already damaged threads (e.g., stripped) exacerbates wear.

2. Tool Problems

   • Tool Wear: Worn wrenches, sockets, etc., can cause the application point of force to shift, increasing lateral forces on the threads.

   • Impact Tightening: Using an impact wrench can cause instantaneous overload, damaging the threads.

3. Insufficient Lubrication

   • Dry friction significantly increases the tightening torque, leading to thread overheating or wear. This is particularly noticeable in materials with strong self-locking tendencies, such as stainless steel.

III. Design Defects

1. Insufficient Thread Length

   • If the thread engagement length is too short (e.g., less than 1.5 times the diameter), the load-bearing capacity decreases, making the threads prone to damage at the end.

2. Lack of Stress Relief Features

   • Failure to design a thread relief groove or chamfer can cause stress concentration at the thread start.

3. Poor Environmental Adaptability

   • In high-temperature, corrosive, or vibrating environments, if weather-resistant materials (e.g., stainless steel, galvanized steel) are not selected, threads may fail due to creep or corrosion.

IV. Potential Impacts from Customer Usage Scenarios

1. Frequent Assembly/Disassembly

   • If the customer repeatedly assembles and disassembles the same threaded pair, metal fatigue can lead to thread embrittlement or wear.

2. Foreign Object Contamination

   • If foreign objects like sand or metal chips enter the threads, they can scratch the thread flanks during tightening.

3. Vibrational Loads

   • If vibration is present during equipment operation, threads may fail due to the cycle of loosening and re-tightening (e.g., self-loosening phenomenon).

Solution Suggestions

1. Verify Tightening Torque: Use a torque wrench to tighten according to standard values (e.g., ISO 898-1) to avoid overload.

2. Check Thread Fit: Use thread gauges to check if pitch and thread angle conform to standards (e.g., M6*1.0).

3. Use Higher-Strength Materials: Select bolts of grade 8.8 or higher, with nuts of matching hardness.

4. Optimize Assembly Process: Adopt a cross-tightening sequence and apply lubricant (e.g., molybdenum disulfide).

5. Increase Thread Length: Ensure engagement length ≥ 1.5 times the diameter and incorporate a thread relief groove in the design.

6. Environmental Protection: Use galvanized or stainless steel components in corrosive environments, and install lock washers in vibrating applications.

Case Analysis

If damage occurs during the final turns of tightening, possible causes include:

• End Stress Concentration: Insufficient effective thread length, causing the final thread to bear the full axial force.

• Tool Force Misalignment: Angular deviation of the wrench during the final stage of tightening, generating lateral forces.

• Local Material Defect: Inclusions or uneven hardness at the end of the bolt.

• It is recommended that the customer provide physical photos or samples of the damaged threads. Further analysis of the wear characteristics (e.g., extrusion, tearing, or corrosion) can help pinpoint the exact cause.