Consequences Of Improper Preload On Threaded Fastener Connections

The preload of a threaded fastener connection refers to the axial compression force applied to the fastener during assembly before it bears any working load. Appropriate preload is an essential prerequisite for ensuring the reliability, tightness and stability of threaded connections. Non-compliant preload will cause various connection failures. Insufficient preload tends to result in joint gaps and relative slippage between connected components under load, causing fastener loosening and even abnormal operation of the whole machine. Excessive preload will cause artificial damage to parts. For example, in O-ring sealing structures, excessive preload will crush the O-ring and lead to seal failure and leakage.

Improper preload of threaded fastener connections mainly causes the following three types of severe consequences:

1 Static Failure of Fasteners and Connected Components

Over-tightened bolts with excessive preload will exceed the static load limit of fasteners and connected parts, resulting in artificial structural damage. In mild cases, it causes thread tooth shear damage, thread stripping and thread seizure. In severe cases, it leads to bolt fracture, crushing, distortion or fracture of connected components, resulting in complete loss of connection performance.

2 Slippage, Separation of Connected Parts and Fastener Loosening Failure

For ordinary bolt connections subjected to transverse loads, preload generates normal pressure on the joint surface, and the friction force on the contact surface resists external loads. The magnitude of preload directly determines the transverse bearing capacity of the connection. Insufficient preload leads to inadequate friction on the joint surface, causing relative slippage of connected components, component dislocation, deflection and surface wrinkling. Under extreme working conditions, the bolt may be sheared off by transverse shear force.

For bolt connections bearing axial loads, preload provides compression force on the joint surface, and the effective compression force during operation is the residual preload. Insufficient preload reduces the compression force on the joint surface and causes connection loosening. Severe separation of connected parts may occur under fluctuating working conditions. Meanwhile, insufficient preload greatly reduces the vibration resistance of the structure. Intense transverse vibration generated during equipment operation will eventually cause nut loosening and connection failure.

3 Bolt Fatigue Failure

Improper preload is a major inducement for bolt fatigue failure. Although properly reduced preload can lower the average stress of cyclic load on bolts, it significantly increases the load fluctuation amplitude and intensifies alternating stress variation. Overall, this greatly reduces the fatigue life of bolts, causing fatigue cracks and fracture under long-term alternating loads and vibration, and further triggering sudden connection failures.

Therefore, the range of bolt preload and tightening torque must be strictly specified in the assembly process. In practical production, the scientific and reasonable tightening torque range shall be determined by comprehensively considering the importance level, load condition, motion mode, structural characteristics, thread specification and strength grade, material properties of connected parts and connection purposes. Assembly operations must strictly follow the specified torque standards to avoid over-tightening and under-tightening. This effectively improves the overall reliability of threaded connections and enhances the fatigue resistance and service life of fasteners and connecting structures.