Abstract: Objective Based on the test results, the effect and mechanism of micro-particle shot peening (FPP) technology on bolt locking performance were studied. Methods Two kinds of bolts with different hardness (stainless steel bolts and galvanized bolts) were treated by micro-particle blasting. The above two types of unpeened bolts and shot peening bolts were applied to the hydraulic servo universal fatigue testing machine using the bolt lateral vibration test device. A repeated loading vibration test was performed. At the end of the test, the wear profile of the bolt thread at the threaded joint was observed using a scanning electron microscope (SEM). As a result, the galvanized bolt is about 40% lower than the stainless steel bolt without the shot peening. After the micro-particle shot peening, the anti-loosening ability of the galvanized bolt is increased by 28%, and the anti-loosening ability of the stainless steel bolt is increased by 15%, and the anti-loosening ability of the galvanized bolt of the fine particle peening is more remarkable. The lower the degree of wear on the threaded surface, the stronger the bolt's ability to resist loosening. Conclusion Microparticle shot peening improves the hardness of the screw surface and reduces the wear of the screw, thus improving the bolt's anti-loosening performance. The improvement of the bolt's anti-loosening ability by the micro-particle blasting technique increases with the increase of the surface hardness of the bolt screw.
Key words: microparticle shot peening; thread surface; wear; loose; bolt
The threaded connection has the advantages of simple operation, convenient maintenance, reliable connection, easy replacement, etc., and is widely used in mechanical structures [1-3]. However, when the connected part is subjected to a certain vibration or impact load during the service, the bolt will gradually loosen and the pre-tightening force will gradually decrease, resulting in structural failure [4-6]. Early studies have suggested that the main cause of bolt loosening is the relative slippage of the inner and outer thread surfaces [7, 8]. The slip causes the inner and outer threads to rotate relative to each other in the loose direction, that is, the relative rotation of the bolt or the nut causes the pre-tightening force to decrease. In addition, fretting wear on the threaded mating surface can also cause the preload to drop and the bolts to loosen [9, 10]. W. Eccles [9] believes that oil lubrication of internal and external screw teeth will reduce the wear of the screw surface and improve the bolt's anti-loose ability. However, studies on other ways to improve the anti-wear properties of the thread surface to improve the bolt's anti-loose ability have not been reported.
Fine Particle Peening technology as a new surface treatment process, compared with the ordinary shot peening process, the diameter of the pellet used is very small, 50 ~ 300μm, and the diameter of the pellet used in ordinary shot peening is generally It is about 0.8 to 1 mm, and the former is much smaller. The process uses compressed air to accelerate the pellets to a very high speed and then impacts the surface of the treated material to form a hard surface hardened layer, which enhances wear resistance [11-12]. In addition, since the diameter of the pellet is very small, the roughness of the surface to be treated is small, and can be 0.8 μm or less. Moreover, the surface is uniform and the surface orientation produced by machining has been removed. FPP not only does not cause damage to the thread, makes the thread pair fit more evenly, and can improve the anti-wear ability of the surface of the screw, thereby improving the locking performance of the bolt. Therefore, it is necessary to study the effect of FPP on bolt locking performance.
In this paper, the pre-tightening force drop curves of different bolts before and after the particle peening process were tested by the bolt lateral vibration test device. The surface morphology of the thread before and after the test was observed by scanning electron microscopy. The anti-loosening performance of the particle peening process was analyzed. influences.
1 Principle of micro particle blasting process
The micro-particle blasting system is basically the same as the conventional shot peening system. It is mainly composed of the injection system, the storage system, and the pellet recovery system. The main difference is that the diameter of the pellet used in the micro-peening is very small, which can produce the surface of the sample. The temperature is higher than the phase transition temperature of the material, that is, the effect of heat treatment on the surface of the sample. Compared with the conventional shot peening process, the process can greatly improve the surface finish of the sample, and at the same time, it can form a tough microstructure in the surface of the processed sample, and even nanostructure the surface structure. Figure 1 is a comparison of the surface of three different spraying processes of sandblasting, conventional shot blasting and microscopic shot peening. It can be seen from the figure that the surface quality after the particle blasting treatment is the highest, and the dimple structure eliminates the directionality of the machine tooling marks, indicating that it has good liquid lubrication characteristics.
Figure 1 Comparison of different spray process surfaces
Fig.1 Comparison of glasses treated with different shoot peening process
2 test materials
This test tests the pre-tightening force of four different bolts under the lateral vibration load. The samples were shot blasted and unpeened galvanized bolts (Zinc, Zinc-FPP), shot peened and unpeened stainless steel bolts (Stainless, Stainless-FPP). Among them, the galvanized bolts and the stainless steel bolts have the same particle blasting process, and the process parameters are shown in Table 1. The average diameter of the pellet corresponding to the 300# sieve number in the table is about 50 μm.
Table 1 Microparticle shot peening process parameters
Tab.1 Process parameters of FPP
Table 2 Test bolt material performance parameters
Tab.2 Material property parameters of tested bolts
Table 2 shows the performance parameters of the four bolt materials. It can be seen from the table that the hardness and yield strength of the galvanized bolts are lower than that of the stainless steel bolts before and after shot peening. After shot peening, the surface hardness of the galvanized bolts is increased more, and the surface roughness of both bolts is increased. The dimensions of the four bolts are M5×25mm, and the actual objects are shown in Figure 2.
Figure 2 Test bolt physical map
Fig.2 Pictures of tested bolts
3 test methods
The bolt loosening test was carried out using the bolt lateral vibration test device shown in FIG. During the test, the exciter and the mounting base are bolted to the T-slot of the base. The lower steel plate is fixedly connected with the mounting base, and the upper steel plate is fixedly connected with the vibration exciter. The upper and lower steel plates are made of 45 steel and have a thickness of 6.5 mm. Fix the acceleration sensor on the upper steel plate with AB glue.
Figure 3 Schematic diagram of bolt lateral vibration test device
Fig.3 Schematic diagram of transverse vibration test device
During the test, the shaker loading amplitude and loading frequency were constant. For different pre-tightening conditions, when the pre-tightening force drops to 20% of the initial value, the bolt structure is considered to be loose [14], and the number of lateral vibrations at this time is the loosening life of the bolt under the working condition. . When the number of vibrations exceeds 5 × 105 times, and the bolt does not loosen, it is considered that the bolt does not loosen and the test is stopped.
For the four different bolt tests, the pre-tightening force of the first specimen is set to 50%~70% of the bolt yield strength, and then the pre-tightening force of the bolt is determined according to the test result measured by the previous sample. If the bolt is loose, increase the applied preload; if the bolt does not loosen, reduce the applied preload. The test is gradually carried out, and finally the minimum preload of the bolt without loosening is obtained. Each bolt is subjected to a looseness test with a preload force level of 4 to 5, and the preload force level of each stage is repeated 3 times, and the average value is taken as the loosening life of the bolt under the preload force level.
4 test results
4.1 Relationship between vibration acceleration and bolt preload
Figure 4 shows the relationship between the vibration acceleration of the four bolts and the bolt preload. As can be seen from the figure, as the preload force is gradually increased, the vibration acceleration is gradually reduced. When the preload force is the same, the vibration accelerations of the four bolts are substantially equal. Therefore, it can be assumed that the corresponding vibrational acceleration of the four bolts is consistent with the bolt preload. The four sets of test data are combined and fitted to obtain the functional relationship (2).
A = 4.397e-0.001226F(2)
Where: A is the vibration acceleration, F is the pre-tightening force, and the function fit is 0.9625. According to the aforementioned looseness criterion, when the pre-tightening force is reduced to 80% of the initial value, it is considered that the bolt is loosened, and it is substituted into the formula (2), that is, the vibration acceleration when the bolt is loosened is obtained.
The preload force and the preload torque applied to the bolt are converted by the formula (1).
T=K×F×d (1)
Where: T is the tightening torque, K is the tightening factor (value is 0.2) [13], F is the bolt pre-tightening force, and d is the bolt outer diameter. The bolted connection is pre-tensioned using a digital torque wrench.
The vibration is applied by the vibration exciter of model HEV-500, and the vibration acceleration of the upper steel plate is monitored to determine whether the test bolt is loose or not. Before the test, the relationship between the pre-tightening force of the bolt and the vibration acceleration of the upper steel plate is tested to judge the degree of decrease of the pre-tightening force by monitoring the vibration acceleration of the upper steel plate in the loosening test. Different pre-tightening forces are applied to the bolts. Under the loose test conditions (exciter loading amplitude 250 N, frequency 20 Hz), the vibration acceleration of the upper steel plate is collected, so that the upper steel plates corresponding to different pre-tightening forces are obtained. Vibration acceleration. Considering the dispersion of the test results, three samples were tested under each set of preload, and the average value was taken.
Figure 4 Relationship between bolt pre-tightening force and vibration acceleration of upper steel plate
Fig.4 Relation between bolt pretightening force and vibration acceleration of upper steel plate
4.2 The relationship between bolt preload and loose life
Figure 5 shows the relationship between the loosening life of the bolt and the preload force. The power function model is used to fit the relationship between the preload force and the loose life of the four bolts. It can be seen from the figure that in the double logarithmic coordinate, the preload force has a linear relationship with the loose life and is similar to the fatigue SN curve. The loose life and its dispersion increase with the increase of the pre-tightening force, and there is a minimum pre-tightening force to make the sample have an infinite loosening life, which is the loose endurance limit of the bolt. The size of the loose endurance limit can be used to characterize the bolt's anti-loosening properties. The smaller the value, the better the anti-loose performance. Before shot peening, the loose endurance limit of galvanized bolts and stainless steel bolts is 2800, 2000 N, respectively. The anti-loose performance of stainless steel bolts is better than that of galvanized bolts. After shot peening, the loose endurance limit of galvanized bolts and stainless steel bolts is 2000 and 1700 N, respectively, which is 800, 300 N lower than that before shot peening, and the corresponding pre-tightening force is reduced by 28% and 15%, indicating bolt loosening performance. Upgrade.
Figure 5 Relationship between bolt loosening life and preload force
Fig. 5 Relation between pretightening force and looseness life of bolts
From the above results, it is understood that the bolt having a higher hardness has a stronger anti-loose ability. The microparticle peening treatment improves the hardness of the surface of the screw, thereby improving the locking performance of the bolt, and the degree of improvement increases as the surface hardness increases.
4.3 Wear surface of the thread surface
In order to study the change of the surface state of the screw during the test, the wear morphology of the bolt thread surface was observed by scanning electron microscopy. The results are shown in Fig. 6. As can be seen from the figure, when the pre-tightening force is 2300 N, the number of vibrations of the unpeened stainless steel bolts is much higher than that of the unshot galvanized bolts, but the former is much less worn than the latter. When the pre-tightening force is 2000 N, the degree of thread wear of the galvanized bolt after shot peening is reduced. When the pre-tightening force is 1700 N, the wear profile of the stainless steel bolts before and after shot peening is small, and relatively slight wear occurs.
Figure 6 Wear profile of various bolt threads
Fig.6 Worn morphology of various bolt threads
5 Analysis and discussion
After the bolt is pre-tensioned, a positive pressure is formed between the inner and outer threads. When a lateral cyclic load is applied to the bolt, relative sliding between the inner and outer threads is caused, and wear occurs. The wear of the thread surface will reduce the degree of mating of the thread pair, which will lead to a decrease in the bolt pre-tightening force and cause the bolt to loose [10].
Since the stainless steel bolt material is harder than the galvanized bolt, and the high hardness material has strong abrasion resistance, the wear profiles of the two bolt thread faces are different, and this difference is the cause of the difference in the locking ability of the two bolts. The galvanized bolt material is soft, and the surface hardening degree after shot peening is high, so that the surface anti-wear ability of the galvanized bolt after shot peening is obviously improved. In addition, the impact of the pellet on the surface of the material results in an increase in surface roughness, which in turn causes an increase in the coefficient of friction. The increase of the bolt friction coefficient will inhibit the relative slip between the thread faces, which is beneficial to the resistance of the bolt loose [15]. After shot peening, the hardened layer formed on the surface of the galvanized bolt and the increase in surface roughness are the reasons for the improvement in the anti-loosening ability.
The surface hardness of stainless steel bolts did not change significantly before and after shot peening, and the wear resistance did not show significant differences, but the surface roughness became significantly larger. Therefore, the improvement of the anti-loosening ability of the shot peening stainless steel bolt is affected by the roughness of the thread surface. However, since the anti-wear ability is not improved obviously, the improvement of the anti-loose ability of the shot peening on the stainless steel bolt is limited.
6 Conclusion
1) Galvanized bolts are 40% less resistant to loosening than stainless steel bolts. After the micro-particle blasting treatment, the galvanized bolt has a 28% improvement in the anti-loosening ability, and the anti-loose ability of the stainless steel bolt is increased by 15%.
2) The bolt with high hardness has strong anti-loose ability, and the micro-particle shot peening improves the hardness of the surface of the screw, thereby improving the anti-loosening performance of the bolt, and the degree of improvement increases with the increase of the surface hardness.
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