In order to effectively solve the problem of looseness and looseness of the threaded joint, a design idea of using a ferro-based shape memory alloy to develop a new type of nut from a functional material is proposed. The anti-loose mechanism of the new nut was analyzed theoretically, and the static and dynamic anti-loose effect and repeated use performance of the nut were studied experimentally. The test results show that the new smart nut has a good static and dynamic anti-loose effect and can be reused.
Key words: anti-loose; nut; iron-based shape memory alloy
In modern machinery, the number of fasteners accounts for approximately 60% of the total number of machine parts, with threaded coupling being one of the most common forms used in mechanical coupling. Generally, the screw fastening has self-locking property, but when the load is changed, the impact, the vibration, and the working temperature changes greatly, the nut loosens and loosens. According to relevant statistics , the direct economic losses caused by the loosening and loosening of fasteners in the world are more than several billion. Therefore, how to simply and effectively prevent the loosening of the nut has become a very important and urgent issue.
In recent years, many scholars at home and abroad have conducted a lot of research on the anti-looseness of nuts and developed many new anti-loose methods. Most of these methods start from the structure, and the basic anti-loose principle is still friction anti-loose, mechanical anti-loose and so on. The researchers have some very good ideas for the application of anti-loose threaded joints in some specific occasions, and have developed many successful models, and the adopted schemes also have their own characteristics, such as ST2 type locknut , concave-convex prevention The loose nut , the key-locking nut  and the new anti-loosening nut  developed for the loosening of the threaded connection in the railway; in addition to the eccentric tapered locknut, locking and anti-loose New threaded couplings such as nuts, anti-theft bolts and tangential reed anti-theft threaded couplings. These new anti-loosening methods have received some anti-loosening effects under certain working conditions. With the advent of new functional materials, it has begun to try to solve the problem of nut loosening from materials at home and abroad, and has achieved certain results. For example, the shape memory alloy (referred to as SMA) intelligent bolt set and pipe joint used in the US F15 and F16 military fighters successfully solved the loosening problem of key coupling bolts and pipe joints in the aircraft. However, the SMA used here is an expensive NiTi-based SMA, which greatly limits its popularization and use, but proposes a good idea for solving the problem of loose thread connection.
1, the reason for the loose thread connection
Under the action of static load, the fastening threaded connection can generally be self-locking. However, under the action of load, shock and vibration, the frictional torque between the pairs of threads may be reduced or disappeared instantaneously. This phenomenon will eventually lead to loosening of the joint after repeated repetitions. The earliest Shin ji KASEI et al.  through the static test test analysis, that the reason for the automatic loosening of the threaded joint is that the elastic deformation of the bolt body is reversed when the threaded joint lateral force is repeatedly applied. He Hongzheng of South China University of Technology  conducted further research on this basis, and believed that there are three main reasons for the automatic loosening of the threaded joint: the initial deformation of the threaded joint, the effect of the axial load and the effect of the lateral load. Among them, the lateral load plays a major role. Many experiments and practical uses have also shown that the threaded joint is more likely to loosen under lateral vibration than under axial vibration. After a relatively strong axial vibration for a long time, the pre-tightening force may be reduced by 30% to 40%, but Usually, the pre-tightening force is not lost at all; while the strong lateral load tends to cause the pre-tightening force to disappear and the fasteners to fall off. Therefore, lateral dynamic loads (vibration, shock, alternating load, etc.) and lateral vibration caused by it are the main causes of loose threads.
At present, the reasons for loosening of the above nut are mainly prevented by the following three ways : (1) preventing the friction torque from being reduced below the critical value; (2) damping vibration to prevent excessive force from acting on the threaded connection; (3) Prevent relative rotation between the thread pairs. So far, the new type of locknut developed at home and abroad has started from the structure, using the mechanism of friction and anti-loose, to prevent loosening through the first way (such as ST2 nut, concave-concave nut, etc.); or to use mechanical anti-loose And the principle of permanent stop, through the second or third way to achieve the purpose of anti-loose (such as the key lock nut, etc.). Although it has achieved some results, its anti-loosening effect is still not very satisfactory (such as ST2 nut, its locking force is only increased by 10%); or it is inconvenient to use because it needs auxiliary parts and auxiliary processing. (such as concave-and-concave nuts, self-locking nuts and new type of anti-loose structure); or fundamentally change the nature of the detachable joint (such as the key locknut); or because its cost is too high, it is not easy to promote. Therefore, to date, the problem of loosening of the threaded joint has not been solved perfectly.
2, iron-based shape memory alloy intelligent lock nut
In order to effectively solve the problem of looseness of threaded joints, this paper develops a new type of nut with anti-loosening function from the new functional materials, using the special mechanical and physical properties of iron-based shape memory alloy. The nut is characterized in that it does not change the original structure of the nut, only changes the material, and uses the shape memory property of the material itself to make the threaded connection firm without loosening. This nut is considered for its three ways of preventing loosening due to its own properties:
For the first route, due to the shape memory effect (SME) and the recovery effect of the iron-based SMA, the nut will generate a large restoring force after the heat treatment is resumed, and this restoring force can be converted into a friction self-locking moment, and in the vibration It always exists during the impact process, so it can effectively prevent the friction torque from falling below the critical value.
For the second route, the iron-based SMA has a pseudo-elastic effect in addition to a good SME. This property allows the nut made of the alloy to have good damping and vibration damping, and can absorb part of the energy under the action of impact, vibration and dynamic load, not only to prevent loosening, but also to improve the threaded connection. Fatigue life.
For the third route, due to the existence of restoring force, and the vibration and impact are reduced under the pseudoelastic action of the alloy, the rotation of the nut relative to the bolt is more difficult than that of the ordinary nut, so that the looseness can be effectively prevented.
2.1 Design concept
The new locknut is made of a low-cost, high-performance third-generation shape memory alloy, an iron-based shape memory alloy. Using the shape memory effect of the alloy and its recovery effect in the constrained state, the internal thread of the nut is machined to a size slightly smaller than the external thread of the bolt, and then the hole is deformed to the size of the internal thread of the standard nut, and tightened according to the specified torque. When the nut is heated, the nut will contract to generate a radial restoring force, and the restoring force can be converted into a certain self-locking friction torque to prevent relative rotation between the thread pairs, thereby achieving the purpose of preventing looseness; Under the action of the tightening force, the thread will cause bending deformation and stress-induced martensite. When the nut is heated to the point of As (the starting temperature of martensite to austenite transformation), the stress induced martensite occurs. In the reverse phase transformation, the bending deformation will be restored in whole or in part, but due to the constraint of the bolt thread, an axial restoring force will be generated, and the restoring force can also be converted into a self-locking friction torque to play a certain anti-loose effect. In addition, the iron-based SMA has a relatively small elastic modulus and a large deformation amount, which can effectively adjust the difference between the bolt pitch and the nut pitch, and reduce the stress of the first and second coils where the nut stress is most concentrated. The stress value improves the stress state of the bolt thread, so that the probability of fatigue fracture of the threaded joint is greatly reduced, and the looseness, looseness and fatigue fracture of the threaded fastener which has not been solved by the conventional techniques and methods for many years are effectively effectively solved. problem.
2.2 Anti-loose principle
Compared with ordinary nuts, due to the special properties of the new nut itself, the new iron-based shape memory alloy smart nut is subjected to the following three frictional moments after preloading and restoring heating:
(1) The friction torque T1 generated by the preload. Like the ordinary nut, the iron-based alloy nut is subjected to the self-locking friction torque due to the pre-tightening force Fα (including the torque generated by the self-locking action between the thread pair and the friction between the nut, the bolt head support surface and the coupled member). The role of the frictional torque, by the knowledge of thread mechanics, which can be calculated by equation (1) : excerpted from the Department of Statistics, China Machinery Industry Federation
(2) Self-locking friction torque T2 due to axial restoring force. This part of the friction torque is caused by the bending stress of the thread teeth and the stress-induced martensite due to the compressive stress of the nut end, and the axial restoring force Fα' is generated due to the constraint during the subsequent recovery heating (Fig. 1), Fα' also produces a certain self-locking friction torque, the size of which can be obtained by formula (2):
(3) Self-locking friction torque T3 due to radial restoring force. Due to the radial pre-deformation, the nut made of the iron-based alloy undergoes radial contraction during the recovery heating process, and a large radial recovery stress σr is generated due to the constraint of the bolt during the shrinking process. figure 2). According to the material mechanical stress analysis and related knowledge of thread mechanics, the friction torque generated by the restoring force can be expressed as follows:
In the formula (3), H is the thread height, H1 is the height of the nut, P is the thread pitch, α' is the tooth half angle of the vertical section of the thread, α is the tooth half angle of the axial section, and μs is the friction of the thread surface. The coefficient, d2 is the diameter of the thread, and d is the nominal diameter of the thread.
Therefore, the relaxation torque of the iron-based alloy intelligent loosening nut is
It can be seen from the above analysis that the locking torque of the new threaded coupling is increased by 0.8T2 and T3 compared with the conventional threaded joint, so that a good anti-loosening effect can be achieved.
Here, an M10 mmX1.5 mm threaded joint is taken as an example for calculation. According to the experimental results of the research on the iron-based shape memory alloy pipe joints of Tianjin University , assuming that the radial restoring force (maximum) is 150 MPa, the self-locking friction torque generated is:
Since the axial restoring force is theoretically difficult to determine, the axial preload force Fα'=577. 2676 N (corresponding recovery processing temperature is 400 °C) is determined according to the static test result of the M10mmX1.5mm nut. The self-locking friction torque obtained by the formula (2) is:
According to the yield point tightening method , it can be determined that the preload force of the nut is 6 100N, and the corresponding self-locking friction torque can be obtained by the formula (1):
The formula (4) can obtain the anti-loosening torque T's of the iron-based alloy nut T's = 18.283 0N?m, and under the same pre-tightening force, the common nut is 1.892 2 times, which shows that the new loose nut has good static relaxation performance. .
2.3 Experimental research
In order to verify the anti-loosening effect of the iron-based shape memory alloy nut, the inner diameter of the base metal (solid solution treated at 950 °C) is pre-deformed by 5% by the expansion rod, and then tapped to form an iron-based shape of M10 mmX1.5 mm. The memory alloy locknut is tightened under the pre-tightening force of 6 100 N with the bolt, and then the appropriate temperature is restored and heat-treated to achieve the anti-loosening effect. The locknut made according to the process is compared with a conventional nut.
The test results show that: (1) Under static load, under the same pre-tightening force, the anti-loose nut is annealed at 200 °C, compared with the ordinary nut, the loosening torque of the locknut is increased by 21.48%; when the recovery temperature When it is 400 °C, its loose torque can be increased by 20.08%, which is basically similar to the above theoretical calculation results. (2) With the same pre-tightening force and vibration condition (with a static load of 8. 00 kN and a load width of 4.00kN, the impact coefficient is tested by means of a fatigue testing machine (operating frequency of 80 to 250 Hz) and a bolt group test bench) O.5) The anti-loose test of the lower locknut and the ordinary nut shows that the locknut has a high anti-loosening life, the ordinary nut loosens after 10,000 vibrations, and the vibration life of the locknut reaches 50,000 times. (corresponding to a recovery temperature of 200 ° C). The theory proves that if the recovery annealing temperature is increased, the anti-loosening life of the iron-based shape memory alloy nut will be improved.
One of the advantages of threaded coupling over other coupling methods is that it is easy to disassemble and has good reusability. The reusability of this new type of nut can be achieved by controlling the alloy to restore the annealing temperature. In order to verify the reusability of the new smart nut, the M10 mm nut processed according to the above process was used to restore the annealing of the nut at different temperatures (200, 250, 300, 350, 400, 450, 500 ° C, respectively). After annealing, the loose torque is tested, and then the nut is pre-tensioned with the same pre-tightening force, and then the nut is restored and annealed. The test results obtained by repeating the above process are shown in Table 1. The results show that the recovery temperature gradually increases. The increase of the self-locking friction torque (compared with the ordinary nut) is also gradually increased; when the recovery temperature reaches a certain temperature, the increase of the self-locking friction torque begins to gradually decrease. This phenomenon can be explained as follows: the anti-loosening property of the locknut is mainly determined by the recoverable deformation amount of the nut after heating (that is, the stress-induced martensite content can be recovered). With the gradual increase of the recovery annealing temperature, the recoverable stress-induced martensite content gradually increases, but the stress-induced martensite content is constant, which is induced by the pre-deformation as the recovery annealing process is repeated. The amount of martensite (the amount of martensite that can be recovered) is gradually reduced, and as a result, the amount of increase in the self-locking friction torque is gradually decreased. In short, by controlling the annealing temperature, the large self-locking friction torque can be added to the threaded joint multiple times, which can make the nut can be reused and ensure the convenience of nut removal.
3, the conclusion
Both theory and experiment have shown that the iron-based shape memory alloy smart nut can be matched with the bolt without changing the size of the original nut structure, which can not only greatly improve the self-locking friction torque, but also achieve the purpose of anti-loose and anti-break, and easy to process. , can be reused. Its successful development can not only effectively ensure the safety of railway transportation, but also can be widely applied to various occasions such as automobiles, airplanes, engines, etc. with high vibration and high connection requirements, which can bring greater social and economic benefits. .