Analysis of standard failure modes for threaded fasteners

In the assembly of mechanical products, the connection between parts mainly relies on fasteners. Among them, threaded #fasteners are the most widely used, and the performance and combined effect of pins directly affect the performance and safety of products and service life. The failure of fasteners is the cause of common mechanical product quality problems, so the analysis of potential causes of the failure mode of threaded fasteners is of great significance to the quality improvement of products.

1. Common failure modes of #threadedrod fasteners

The main failure modes of threaded fasteners encountered in our work are divided into:

(1) Assembly twisting fracture;

(2) The thread is screwed off by shear force;

(3) Fracture of the stress concentration part after use;

(4) fatigue fracture;

(5) Delay fracture;

(6) Parts torque alarm;

(7) Thread sliding teeth.

2 Analysis of causes of standard failure modes

2.1 Assembly twisting breakage

The twisting fracture is characterized by obvious necking and elongation of the fracture site. The common cause of the twisting fracture is mainly due to the small friction coefficient of the joint surface. The torque applied during #tightening or pre-tensioning is too large. The sleeve and the thread are different shafts when the torque is used. And the speed is too fast when the torque is applied; The performance strength of the part itself needs to be more, and the fastening surface needs to be tolerant to the thread's centerline.

2.2 The thread is screwed off by shear force

The broken part of the thread screwed by the shear force is generally spiral-shaped, without obvious necking. The common reason for the line to be screwed by the shear force is that the thread is stuck during the tightening process, such as thread deformation, irregular tooth shapes connected, and a line has a welding slag lamp. The section into which the #bolt is screwed is pressed, such as the adequate thread depth of the nut for blind holes, is not enough.

2.3 The stress concentration part breaks after the use

The fracture of the stress concentration part after use is commonly manifested in the bolt head and the right angle of the head and the threaded rod excessively. The common reason for the fracture of the stress concentration part is that the right angle of the head and the threaded rod is too small. There is a defect in the plastic streamline of the head when the bolt is cold-headed. The perpendicularity of the joint surface and the bolt is out of tolerance.

2.4 Fatigue fracture

In the process of use after bolting, the primary fracture is fatigue fracture, and the common causes of fatigue fracture are: insufficient preload, excessive attenuation of clamping force, The size and performance of the bolt being unqualified; The cooperation between parts, assembly environment, and working conditions cannot meet the design requirements.

2.5 Delay breakage

The common cause of delayed fracture is hydrogen embrittlement, a trace amount of hydrogen entering the steel in the production process (such as electroplating, welding), resulting in embrittlement or even cracking of the material under the action of internal residual or external stress. Standard fasteners prone to hydrogen embrittlement are self-tapping nails/elastic washers/bolts with electroplated surface treatment above grade 8.

2.6 Parts torque alarm

Part torque alarms commonly occur during bolt assembly, where torque is controlled by the angle method. The failure mode of fastener torque alarm and the reasons are: after the body is completed, the final torque of the part is higher than the upper control limit or lower than the lower control limit: the reason is that the assembly torque control range of the region is unreasonable, which is manifested as the setting control range is too small, and the control range is offset up or down, as shown in Figure 1, where F above and F down are friction coefficients.

There is no preload to the preset angle, and the torque reaches the upper limit alarm: the reason is that the friction coefficient of the part itself exceeds the upper limit, the friction coefficient of the domain is too high, and the interference between the parts causes the assembly torque to rise sharply.

Ordinary assembly, a lower limit of torque alarm: the reason is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the claim exceeds the lower limit, and the proper torque when the part is screwed in is greater than the initial torque (that is, the screwing torque consumption is too large), which is common in the tightening of the lock nut.

2.7 Threaded slip teeth

Thread slippery teeth often appear in threaded connections. The leading cause of tricky threaded teeth is thread decarburization: the common phenomenon is that it cannot add the torque during assembly, and after the bolt is removed, it is found that all or part of the thread is smoothed, and the surface hardness of the bolt thread or nut hole is low; Internal and external thread size matching: the matching connection has a small contact area, and there are two situations: one is that the number of thread buckles of the tooth joint is small, and the other is that the thread and the thread are not in contact within the middle diameter (that is, the precision match is not good, and the thread contact between the bolt thread and the nut is not enough).

At the same time, if the assembly method is not the right hole, strong screwing will also cause thread slippage; The thread friction coefficient is too small: surface coating, surface roughness, and surface lubricant are unreasonable, and the bolt thread or threaded hole has foreign matter, damage to the rope and the pitch and angle variation of the bolt and nut will lead to thread slippage.

Conclusion

By analyzing the common threaded fastener connection problems in production and listing the solution examples, this paper has reference significance for the process formulation of parts production and assembly. It has a preventive effect on common quality problems, which can reduce the occurrence of quality problems.