High Strength Bolt: Mechanism of Loss of Clamping Force

he Mechanism of Clamping Force Loss in a High Strength Bolt: An Inevitable Challenge

The axial clamping force in a bolted connection is crucial to ensuring a secure joint after the bolt is tightened. However, over time and under the influence of external factors, this clamping force inevitably decreases, which is an unavoidable challenge in connection design. This article delves into the mechanisms behind the loss of clamping force in a high strength bolt, analyzes the contributing factors, and discusses optimization strategies.

Mechanisms of Clamping Force Loss in a High Strength Bolt

Surface Embedding

Micro-surface irregularities can cause surface embedding, where protrusions on the surface are compressed during the tightening of the high strength bolt, further deforming the surface and bringing it closer together. This phenomenon, known as "embedding" or "insertion," is one of the primary mechanisms for clamping force loss.

Material Creep

Material creep refers to the slow, irreversible plastic deformation that occurs under constant temperature and load over time. After pre-tightening, the bolt remains under tension, while the connected components are in compression, leading to creep. This causes a reduction in the pre-tightening force, which is a significant cause of clamping force loss.

Thermal Expansion

When the thermal expansion coefficients of the high strength bolt and the connected components differ, temperature changes can result in variations in the axial force of the bolt, thus affecting the clamping force. Selecting materials with similar thermal expansion coefficients is an effective strategy to mitigate this issue.

Contact Surface Wear

Micro-movement wear caused by vibration loads gradually flattens the protrusions on the contact surface, leading to changes in the thickness of the joint components and subsequently affecting the clamping force. Using wear-resistant materials and increasing the contact area can help reduce clamping force loss due to surface wear.

External Load Effects

Different types of external loads have varying impacts on clamping force. Shear loads have the greatest effect, followed by torsional loads, while tensile loads have a relatively minor impact on clamping force. Therefore, structural optimization should be considered to reduce shear loads, and anti-loosening measures should be applied to connections subjected to shear loads.

Strategies to Mitigate Clamping Force Loss in a High Strength Bolt

To minimize clamping force loss, the following strategies can be employed:

Surface Treatment

Improving the flatness and smoothness of the contact surfaces helps reduce the embedding phenomenon, thereby slowing the rate of clamping force loss.

Material Selection Optimization

Choosing materials with low creep rates helps reduce the impact of creep on clamping force.

Temperature Control

Selecting materials with similar thermal expansion coefficients reduces the impact of temperature changes on clamping force.

Minimizing the Impact of External Loads

Through structural optimization, shear loads can be minimized, and anti-loosening measures can be applied to improve the resistance of the joint to torsional forces.

The mechanisms behind clamping force loss in a high strength bolt are complex and diverse, requiring comprehensive consideration of various factors during design and usage. By selecting appropriate materials, applying surface treatments, and effectively mitigating the impact of external factors, the decay of clamping force can be delayed, ensuring the long-term stability of the bolted connection. In practical engineering, considering the mechanisms of clamping force loss and implementing effective optimization strategies will provide strong support for the reliability and safety of bolted connections.