Due to its small density, excellent heat dissipation, good corrosion resistance and ease of casting and weight reduction, cast aluminum is used in more and more mechanical products, especially various boxes for mobile systems such as vehicles. The body is made of cast aluminum structure. Most of the boxes are clamped between the box and the cover by bolts to achieve the fastening and sealing of the box.

Cast aluminum materials have low strength and are not suitable for direct machining of internal threads. Internal threads are usually formed by machining threaded bottom holes and then installing wire screw sleeves or threaded steel sleeves. In addition, due to the poor anti-wear performance of cast aluminum materials, the head support surface is very easy to wear when the bolt is tightened, which affects the friction state of the support surface, and friction is the main factor affecting the twist-pull relationship in the bolt connection. M. These reasons The pre-tightening force control in the cast aluminum box bolt connection becomes a problem. If the pre-tightening force of the bolt is too large, the bolt may be broken, the steel sleeve may be pulled out, and the cover may be partially warped, so that the seal is ineffective; and the insufficient pre-tightening force may easily lead to quality problems such as loose bolts, unreliable sealing or even failure.

In order to accurately control the pre-tightening force in the threaded joint, the torque-turning method has been proposed very early, and in 1964, some countries in Europe, the United States and other countries have formulated relevant specifications. H. Dalian University of Technology Song Sha and others Nassar et al. The different pressure distribution models of the friction ring of the support surface are proposed. The equivalent friction radius calculation formulas of each model are given, and the calculation accuracy of the equivalent friction radius of different models is analyzed. Nassar's analysis shows that if the parameter selection is reasonable, the exponential pressure p distribution model has the highest calculation accuracy. At this time, the bolt tightening diagram is as shown in the figure, that is, the inner pressure value of the contact annular region is higher than the outer pressure value. In this paper, the wear of all the test fixtures is severely worn on the inside, and the outer wear is light and flared. This phenomenon is caused by the high pressure on the inner side and on the other hand due to the low inner strength. It can be inferred from the situation shown in the figure that during the tightening process of the bolt, the gasket and the cast aluminum tooling block gradually reach a complete fit as the bolt is continuously tightened. For the used cast aluminum tooling block, the flat washer gradually fits from the outer side to the inner side during the tightening process of the bolt, that is, the equivalent friction diameter of the supporting surface is continuously reduced during the tightening process, causing the torque coefficient to continuously decrease during the tightening process.

During the tightening process of the bolt, the friction coefficient of the support surface and the friction factor of the thread surface /X are two constantly changing values, and their change is the main cause of the change of the torque coefficient, and there are many reasons for the change of the two friction factors. All tests in this paper did not use any lubricant or anti-loose rubber, all friction is dry friction.

During the tightening process of the bolt, the axial preload force of the bolt increases rapidly. When the torque in the test reaches 20Nm from the target torque of 70Nm, the angle of rotation of the bolt is about 80°. Under normal circumstances, the metal surface is in elastoplastic contact state. Due to the nonlinear relationship between the actual contact area and the load, the friction factor decreases with the increase of the load. 11. This paper considers that the friction surface of the support surface is in the bolt fastening process. The contact in the friction pair of the thread surface is elastoplastic contact, that is, the two friction factors decrease with the increase of the axial preload during the fastening process, so that the torque coefficient K observed in the test appears. A phenomenon that decreases as the tightening torque increases.

The adhesion theory established by Bowden et al. believes that the friction is the sum of the resistance of the adhesion effect and the furrow effect. M. In the bolt connection test of the cast aluminum box, it can be seen that the support surface of the cast aluminum tooling block and the end face of the gasket in the test And the surface of the thread after the tooth wear has obvious friction groove marks, and is formed into a ring-shaped stripe. The more the tightening times, the more the friction stripe is clearer. 2 shows the physical view of the end face of a cast aluminum tooling block after several uses.

a) used pad support end face b) support surface partial enlargement of the face of a cast aluminum tooling block after several times of use. Since the bolt, nut and washer surface are plated with yellow zinc, the surface of the cast aluminum tooling block has Oxide film, when the bolt is first tightened, these surface coatings can reduce and stabilize the friction factor. Therefore, the torque coefficient is always small when the first tightening occurs, and the friction factor is stable throughout the tightening process. Case. After completing the first tightening and loosening, it was found that the zinc layer on the friction surface of all the coated zinc layers was peeled off, and the surface of the aluminum block also showed obvious wear marks. 3 shows the zinc layer of the thread surface after the bolt was first tightened. Physical map. When the bolts are tightened again, all the direct contact surfaces during the tightening process are protected by the zinc layer and the surface oxide layer, the friction factor is rapidly increased and the stability is relatively poor, resulting in a torque coefficient greater than the first tightening during the second tightening process. A lot, the tile will also rise.

3 After the bolt is tightened for the first time, the zinc layer of the thread surface is removed from the second tightening. The friction between the friction surface of the thread surface and the friction pair of the support surface is the dry friction of the direct contact of the base material. The friction is mainly the furging force and Adhesive force, the friction surface is worn and scratched, and has an annular groove mark. Taking the friction surface of the supporting surface as an example, the flat washer and the cast aluminum tooling block both form a circular stripe, and the wear stripe is continuously deepened as the axial preload force increases during a certain tightening process. Since the gaskets are different in position during each tightening process relative to the cast aluminum tooling blocks and the bolts, new wear streaks are formed between the friction pairs during each tightening process, or the washers are automatically dynamically adjusted during the tightening process. The position is matched to the previously formed wear groove marks. As the number of tightening increases, that is, the number of bolts used increases, the wear fringes are basically stable, and the corresponding friction factor also decreases from the second time and tends to be stable. The torque factor has a similar law to the friction factor during the tightening process.

According to the theory of adhesive friction, sliding friction is a pulsating process in which adhesion and sliding alternate. When the sliding speed increases, the change range of the adhesion time and the friction factor will decrease, and the friction factor value and the sliding process tend to be stable. 11. This pulsation phenomenon of sliding friction is also a factor of the torque coefficient change during the tightening process. And this effect is more obvious at low speeds, that is, the torque coefficient dispersion is large at low speed tightening.

For the friction pair in the general elastoplastic contact state, the friction factor increases first with the increase of the sliding speed, and then reaches a certain maximum value and then decreases with the increase of the sliding speed. The position of the maximum point corresponds to the load or the contact surface. The stiffness increases and moves toward the smaller speed. M. The surface roughness of the cast aluminum is lower, and the friction pair in the bolt connection is medium load. The friction factor of the three tightening speeds discussed in this paper increases with the tightening speed. The increased results are consistent with the results of the small torque factor corresponding to the low tightening speed in the test.

5 Conclusion In this paper, the influence of the controllable factors on the torque coefficient of the tightening process such as the number of tightening and the tightening speed of the cast aluminum box bolting process is studied. Through the test and theoretical analysis, the following conclusions are drawn.

During the tightening process of the bolt, the torque coefficient decreases with the increase of the pre-tightening force, and the torque coefficient decreases less when the first tightening, and decreases rapidly when the second tightening, and then stabilizes.

The final torque coefficient is the smallest when tightening for the first time, and increases rapidly when tightening for the second time, then tends to be stable, and the law is not obvious at 1r/min, and the increase is larger at 5r/min.

When the tightening is performed multiple times, the variance (discreteity) of the final torque coefficient decreases and tends to stabilize as the number of tightening increases, and decreases with the increase of the tightening speed when the number of tightening times is small.

Aiming at the phenomenon that the torque coefficient dispersion is large due to poor wear resistance of cast aluminum materials, combined with the experimental research results, the following tips can be given to the relevant process personnel: simultaneous control of tightening torque and torque coefficient; The target torque should not be calculated using the same torque factor for all preloading levels; the re-used bolts should be tailored to the first tightening process based on the number of uses; the tightening process should be constrained by the tightening tool and the tightening speed. To control the preload force dispersion when multiple bolts are tightened.

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