Various forms of bolts and nuts can be used as threaded fasteners, and these threaded fasteners often experience some forms of failure during use, such as breakage, twisting, and slipping. Based on the knowledge of threaded fasteners - bolts and nuts, Xiaorui will share with you today the common failure modes and factors of bolts and nuts.
Bolt fracture
1. Assembly twist and tear fracture
The characteristic of twisting and pulling fracture is obvious necking and elongation at the fracture site. The common factor is that the friction coefficient of the connecting surface is too small; Excessive torque applied during tightening or pre tightening, sleeve not aligned with thread axis during torque application, and excessive speed during torque application; The performance strength of the parts themselves is insufficient, and the perpendicularity between the fastening surface and the thread centerline exceeds the tolerance.
2. Thread is twisted and broken under shear force
The fracture site that is twisted by shear force generally has a spiral shape without obvious necking, causing the thread to be twisted by shear force. Common factors include threads getting stuck during the tightening process, such as thread deformation, inconsistent tooth profiles in mutual connections, and welding slag lights on the threads; The section where the bolt is screwed in is blocked, and if the nut is a blind hole, the effective thread depth is insufficient.
3. Fracture of stress concentration area after use
The common manifestation of fracture in stress concentration areas after use is at the bolt head and at the right angle between the head and the threaded rod. The main factor is that the corner of the right angle between the head and the threaded rod is too small; There are defects in the plastic streamline at the head during cold heading of bolts. The perpendicularity between the connected surface and the bolt exceeds the tolerance.
4. Fatigue fracture
The main fracture during the use of bolted connections is fatigue fracture, which is commonly caused by insufficient preload force; Excessive attenuation of clamping force; Unqualified bolt size and performance; The mutual coordination between parts, assembly environment, and operating conditions cannot meet the design requirements.
5. Delayed fracture
The common factor for delayed fracture is hydrogen embrittlement, which is the trace amount of hydrogen that enters the interior of steel during production processes such as electroplating and welding. Under residual or external stress, the material may become brittle or even crack. Common fasteners that are prone to hydrogen embrittlement include self tapping screws, elastic washers, and bolts with electroplated surfaces above grade.
6. Component torque alarm
The torque alarm of parts commonly occurs during the bolt assembly process controlled by angle method. The main factors are: the reason is that the assembly torque control range of the parts is unreasonable, manifested as setting the control range too small, and the control range shifting upwards or downwards.
Failure to pre tighten to the preset angle, torque reaching the upper limit alarm: factors include the friction coefficient of the parts themselves exceeding the upper limit, the friction coefficient of the parts fitting exceeding the upper limit, and interference between parts, causing a sharp increase in assembly torque.
Normal assembly, torque lower limit alarm: The factor is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the part fitting exceeds the lower limit. When the part is screwed in, the fitting torque is greater than the initial torque (that is, the torque consumption is too high), which is commonly seen in the tightening of lock nuts.
7. Thread slippage
Threaded connections often experience thread slippage, mainly due to thread decarburization. The common phenomenon is that the torque cannot be applied during assembly, and after removing the bolt, it is found that all or part of the thread is ground flat, and the surface hardness of the bolt thread or nut hole is low; The size of the internal and external threads is matched, and the contact area of the matching connection pair is small. There are two situations: one is that the number of threads in the mating gear is small, and the other is that the threads are not in contact with each other within the pitch diameter (i.e., the accuracy of the fit is poor, and the contact between the bolt thread and the nut thread is insufficient).
