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Bolt fastening failure often stems from one reason—torque decay. After assembly, the preload (clamping force) of the bolt gradually decreases over time. This can lead to connection loosening and sealing failure in mild cases, and in severe cases, it can cause equipment vibration, structural separation, or even safety accidents. Understanding the causes and taking targeted measures is crucial to ensuring long-term reliable operation of equipment.

Three Core Causes of Torque Decay

1. Excessive Surface Roughness of Assembly Surfaces
   When the surfaces of connected parts are uneven, the contact pressure distribution under the bolt head or nut becomes severely uneven. Under high preload, plastic deformation is likely to occur, leading to rapid loss of clamping force. This decay is particularly significant in the initial stage.

2. Characteristics of Soft Connection Materials
   Plastics, rubbers, composite materials, or certain sealing gaskets undergo creep under continuous clamping force—materials slowly deform as if flowing. This deformation directly consumes the elastic elongation of the bolt, causing the clamping force to gradually decrease over time.

3. Issues with Tightening Process

• Excessive Speed: High tightening speeds in the final stage (e.g., over 300 rpm) generate large inertial impact forces that exceed the material's yield point, causing deformation of threads or bearing surfaces and preload loss.

• Stress Concentration: Single-bolt tightening or improper tightening sequence (e.g., tightening clockwise one by one) causes local stress superposition in the connected parts. Internal stresses that are not released in time seek balance through local yielding or creep, leading to a decrease in overall clamping force.

Systematic Solutions to Reduce Torque Decay

1. Optimize Contact Interfaces

• Precision Machining: Strictly control the flatness and surface roughness of bolt head/nut bearing surfaces and connected parts.

• Enhanced Bearing Capacity: Use hardened washers to increase contact area and evenly distribute pressure, effectively resisting plastic deformation.

2. Address Material Creep

• Material Selection: Prefer engineering plastics or composite materials with excellent anti-creep properties while meeting functional requirements.

• Structural Compensation: Design elastic elements (e.g., washers). When clamping force decreases due to creep, their elastic recovery force can automatically compensate for part of the loss, maintaining relatively stable preload.

3. Update Tightening Process

• Speed Segmented Control: Reduce speed in the final tightening stage (e.g., below 30 rpm) to avoid impact overload. Intelligent tools like Danikor tightening guns can accurately control speed, with adjustable torque and multiple tightening strategies to ensure correct tightening.

• Multi-Step Tightening: Adopt a "tighten-pause-retighten" strategy. After tightening to the target value, pause (e.g., for several minutes), then retighten to the set target. This significantly improves preload stability.

• Synchronization and Sequence Strategy: For multi-bolt connections, use a diagonal cross-tightening sequence, loading in multiple rounds evenly. For critical flanges or sealing surfaces, a multi-axis synchronous tightening system eliminates uneven loading, ensuring uniform stress distribution and achieving correct tightening.