This paper establishes a threaded connection failure model based on the problem of threaded connection failure encountered during vehicle development.

Through theoretical analysis of the key factors causing torque attenuation, wear and crush, thread tripping and loosening failure, it is proposed to verify the improvement measures such as structural design optimization, torque optimization, part quality control and tightening process optimization, and finally effective. Guarantee the reliability of the whole thread connection.

1 Introduction

Due to its simple structure, convenient assembly and reliable connection, the threaded connection is the most commonly used connection method in the assembly of the whole vehicle, accounting for 70% of the assembly work of the assembly shop.

The same problem of thread connection failure is a large part of the maintenance record. According to statistics, 23% of the maintenance problems are caused by fastener failure, and 12% of the new cars have incorrect fastener tightening.

The connection failure mainly manifests as bolt breakage, thread sliding, bolt and nut rotation, but the torque is obviously reduced and the joint is crushed or plastically deformed.

This paper mainly based on the common thread assembly failure problem in the actual production process, analyzes the cause of the failure problem and proposes effective improvement measures.

2. Threaded connection failure model

The thread assembly uses torque to apply torque. The torque is divided into two parts, which overcome the friction torque of the thread pair and the friction torque between the flange surface of the fastener and the bearing surface of the connected member to elastically stretch the bolt to obtain proper axial clamping. force:

T—total torque; T 1 —thread friction torque; T 2 — support surface friction torque; K—torque coefficient; F—axial preload force; d—thread nominal diameter;

According to the relevant literature, under the tightening torque, the bolt is subjected to both the tensile stress generated by the axial clamping force and the torsional stress generated by the torsion of the thread friction torque. In the analysis of whether the bolt connection fails, it is necessary to judge whether the composite stress is less than its Yield stress:

The torsional stress is:

σ red, B — composite stress; σ max — tensile stress; τ max — torsional stress; R p0.2 min — yield stress; d s — equivalent stress diameter; w p — bending section coefficient.

At the same time, when performing the connection failure analysis, it is necessary to connect the pressure bearing surface according to the actual judgment. There are common connection forms such as plane and cone surface, as shown in Fig. 1, to ensure that the pressure bearing surface of the connection does not cause crushing or plastic deformation, that is, the maximum The surface pressure does not exceed the surface pressure of the material to be joined:

P max - compressive stress per unit area; P G - critical stress; A - stress cross-sectional area.

3. Discussion on the cause of connection failure

The final cause of the failure of the threaded connection is that the axial clamping force is insufficient. Combined with the connection failure problems encountered in the actual production assembly process, the main performances are torque attenuation, crushing of the connected parts, thread sliding, loose bolts, etc. The causes are analyzed for each failure form and effective improvement measures are proposed.

3.1 Torque attenuation caused by soft connection

It is generally believed that there is no torque attenuation in the hard and neutral connections, and the soft link torque attenuation is more serious.

However, in actual production and use, for any connection, there will be a certain degree of torque attenuation over time, and the torque attenuation in the soft connection is particularly serious. The torque attenuation cannot be completely avoided, and only through the control of various influencing factors. And optimization to improve the attenuation, to ensure that the clamping force after torque reduction is not lower than the minimum requirements for design clamping force.

For example, the bolting of a certain type of luggage rack (with anti-shock cotton in the middle) is invalid. The bolt size is M6x1-8.8, the target torque is 8N*m, and after tightening to the set torque with the electric gun, the left and right sides are tested after the bump test. The connection point torque data is shown in Figure 2. The torque attenuation is severe and even loose.

The torque attenuation control measures caused by the soft connection include speed control and step-by-step tightening. The speed reduction and step-by-step tightening effectively alleviate the stress concentration caused by the high speed and the stress cannot be released in a short time.

Tightening the speed control to reduce the speed, replace the hand-held straight gun with a pulse pneumatic elbow gun, the speed is changed from 1500r/min to 430r/min; the step-by-step tightening is two-step tightening, and the tightening is continued at intervals of 0.5s.

The above two methods are used for assembly, and the detected torque is maintained in the range of 6N*m-8N*m, and the attenuation ratio is 25%, which satisfies the performance requirements.

3.2 The worn parts are worn and crushed

3.2.1 Wear phenomenon

The main cause of wear and tear is surface embedding. The surface is embedded mainly because the threaded contact surface, the fastener supporting surface and the contact surface of the connected part have a certain degree of roughness, verticality and flatness during the screw tightening process. After the tightening, the rough surface is embedded with each other, and the uneven surface may be generated very much. Large local plastic deformation, resulting in torque attenuation and axial clamping force reduction;

For this failure mode, the clamping force loss caused by the connection embedding is considered in the initial design, the axial clamping force is increased correspondingly, and the axial clamping force is compensated by the deformation of the embedded amount, as shown in Fig. 3, namely:

Where: f z — the amount of embedding (plastic deformation caused by embedding); δ s — the amount of springback of the bolt; δ p — the amount of springback of the connected piece

3.2.2 Crushing phenomenon

The main reason for the connection is that the torque design is too large or the pressure surface is too small, so that the pressure of the connected part is greater than its own limit pressure, the plastic deformation of the connected parts, the internal stress is reduced, and the torque is attenuated; The torque control axial clamping force needs to be correctly designed; the contact area between the bolt nut and the connected member can also be increased by increasing the flange surface, increasing the flat washer and reducing the diameter of the through hole.

3.3 Influence of friction coefficient on clamping force consistency

Residual torque measurement of the joint after assembly and tightening is an important step to judge the connection reliability. Generally, the torque measurement is performed within 5 minutes after assembly.

Taking the sub-frame and the body connecting bolt of a certain model as an example, the detection torque tolerance is ±30%; after the investigation and analysis, the main reason is that the friction coefficient of the part has large dispersion, that is, the friction coefficient of the bearing surface fluctuates greatly (the friction coefficient of the fastener is 0.13±0.03, but the friction coefficient between the bearing surfaces is the empirical value). When the tightening test overcomes the frictional torque of the bearing surface and the design state is different, the torque overshoot phenomenon will inevitably occur.

Substituting the bolt specification (M12x1.25-10.9) parameters into the formula (6) shows the distribution of the clamping force with different friction coefficients:

Torque conversion ratio:

K 1 - pre-tightening torque coefficient; k 2 - thread friction torque coefficient; K 3 - support surface friction torque coefficient; η - conversion ratio;

It can be seen from Table 1 that the friction coefficient is 0.15 to 0.1 and the torque conversion ratio is reduced by 40%. Therefore, it is known that controlling the friction coefficient stability is a necessary condition for ensuring the consistency of the clamping force. An improvement can be made to the problem by adding a flat washer method, see Figure 4.

Because the flat washer has the characteristics of increasing contact area and stable friction coefficient, it is widely used in the important connection point of the chassis. Finally, the torque attenuation of the sub-frame and the body connection point is controlled within ±15% by adding the flat washer measure, which is normal attenuation.

3.4 thread tripping

3.4.1 Analysis of the cause of failure

The aluminum alloy sub-frame of a certain model adopts the bolt and aluminum internal thread connection structure. The bolt specifications are M12x1.25-10.9, the meshing length is 20mm, as shown in Figure 5, the torque is 145±10N*m, and the endurance test finds that the bolt is loose. As a result, the auxiliary frame is abnormally sounded, and the thread slippery teeth are found to be tripped. For details, see Figure 6. The main reason for the analysis is that the meshing length does not meet the requirements.

When the meshing length is too short, the external thread strength (1040Mpa) is greater than the internal thread strength (220Mpa), then the shearing damage will occur at the root of the internal thread. Otherwise, the shear failure will occur at the root of the external thread; The length is generally selected from 2d to 2.5d. Therefore, the connection reliability is optimized by the internal thread engagement length of the sub-frame.

3.4.2 Thread trip model establishment

Through the analysis of the failure case, the thread tripping model is established to prevent the thread from tripping. It is necessary to ensure that the external thread breaking stress is less than the internal thread shear stress. The thread pair contact model is shown in Figure 7, namely:

among them:

σ BB — bolt tensile strength; τ BN — internal thread shear strength; α—fang angle; ρ—friction angle; P—pitch; L—thread engagement length; D 2 — internal thread diameter;

Substituting formula (8) into (7), the minimum thread length required for the thread not to trip

It can be seen that in order to ensure a reliable connection, it is necessary to check the strength of the internal and external threads when selecting the thread engagement length.

3.5 Loose bolts caused by lateral vibration

3.5.1 Analysis of the cause of failure

The tightening torque of the stabilizer bar of a certain type of vehicle is seriously attenuated, and the torque of the initial nut (M10x1.25-10.9, friction coefficient 0.13±0.03) is reduced to 10N*m by 50N*m, and the bracket hole is deformed and the contact surface is concave, as shown in Fig. 8. This will eventually lead to serious abnormal noise.

Analysis of this failure problem can result in two reasons:

a. Insufficient axial clamping force, the horizontal alternating load of the fastening point will cause the axis of the bolt to tilt, as shown in Figure 9, resulting in uneven force on the thread. The friction torque at one end is less than the external torque. The force produces a slight slip, and the static friction becomes dynamic friction; as time goes by, the slip between the thread pairs becomes more and more obvious, eventually causing the screw hole to be deformed, loosened or even broken.

b. Structural defects: There is a gap between the two plates of the bracket, which causes an important cause of torque verification attenuation.

3.5.2 Control measures

Firstly, the process ensures that the double-layered plate of the bracket has no gaps and eliminates the risk of torque attenuation caused by the gap between the plates;

The second step is to verify and verify the tightening torque.

This connection point is subjected to forces Fx (0.6 KN), Fy (0.4 KN), and Fz (4.5 KN) in three directions. It is transformed into the axial and lateral forces of the fastening point, that is, the axial load is combined by Fy, and the lateral load is composed of the combined force of Fx and Fz.

After analysis, when the anti-slip safety factor is 1.1, the minimum anti-sliding clamping force is 33.3KN, and the clamping force is substituted into the torque formula (11):

Where: F—axial preload; d 2 — medium diameter; μ smin — thread friction coefficient; μ bmin — friction coefficient of the support surface; D w — equivalent support surface diameter

It is calculated that the minimum torque needs 66.6N*m, and the design torque is only 50N*m, which can not guarantee reliable connection. Therefore, by increasing the torque to 70N*m, sufficient axial clamping force is ensured to effectively solve the problem of loose connection.

4. Summary

Aiming at the failure of threaded connection encountered during the development of the model, a thread failure model is established, and common phenomena such as torque attenuation, wear and crush, slipping and loosening of the sliding teeth are listed; the key influencing factors are investigated and theoretically The analysis proposes measures such as structural design optimization, torque design optimization, part quality control and tightening process optimization to effectively solve the connection failure problem, thus ensuring the reliability of the thread connection.

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