The bolt connection is achieved by the clamping force provided. The friction coefficient at the screw end and the end face is an important factor affecting the clamping force.
In this paper, the relationship between the friction coefficient and the clamping force is firstly theoretically derived, and then the tightening test is performed for the bolt nut of the ordinary non-lubricated state and the bolt nut of the lubrication state. Finally, the friction coefficient of the bolt and nut in the lubrication state is relatively stable, and the reliability and stability of the bolt connection are high.
1 Introduction
Bolted joints are a typical mechanical coupling that is ubiquitous in various industrial fields. Bolt connection is an important way to ensure the normal operation of the equipment and safe driving of the vehicle. The reliability and stability of the bolt connection have attracted more and more attention.
Accidents caused by bolt joint failures are frequent, and the resulting property losses are immeasurable. How to ensure the safety and effectiveness of bolt joints is an important research topic, and important factors affecting bolt joints require continuous analysis and research.
Bolting is achieved by applying torque to the bolt or nut and converting the torque into the clamping force of the bolt to achieve the coupling of the parts. After theoretical analysis and experimental comparison, it is found that the friction coefficient is one of the important factors affecting the clamping force of the bolt, which affects the reliability and stability of the bolt connection.
2. Theoretical analysis and research
According to the standard ISO 16047, the torque of the bolt during the tightening process is:
In the middle
Ma - total torque, Nm;
Mg - thread torque, Nm;
Mk - end face torque, Nm;
F——clamping force, N;
Gg——thread friction coefficient;
Kk——end coefficient of friction;
Ρ——pitch, mm;
D2 - thread diameter, mm;
Dkm - the average diameter of the end face, mm.
During the tightening process of the bolt, the total torque applied to the bolt is finally converted into the clamping force by overcoming the frictional force of the bolt end face and the friction of the thread pair. The end face friction coefficient μk and the thread friction coefficient μg are the main factors affecting the end face friction torque and the thread friction torque, see Figure 1.
figure 1
Considering the end face friction coefficient and the thread friction coefficient, the total friction coefficient μTot during bolt tightening can be obtained.
then:
It can be seen that for the bolt connection with the specified specification, when the total torque is applied, the total friction coefficient μTot is an important factor affecting the magnitude of the friction force F.
The bolt tightening process simultaneously satisfies:
Where K is the torque factor;
d - nominal diameter of the thread, mm.
According to (2) and (4):
It can be seen that the total friction coefficient μTot during bolt tightening depends on the torque coefficient K value.
3, test analysis
Through testing the tightening process of different series of bolts, the torque coefficient K and the total friction coefficient μTot of the bolted joints under different conditions are analyzed and analyzed, and the influence of the friction coefficient on the clamping force in the bolt connection is analyzed.
According to formula (4), during the tightening process of the bolt, the total torque Ma and the clamping force F can be measured, and then the torque coefficient K can be obtained. According to the formula (5), the total friction coefficient μTot can be expressed by the change of the torque coefficient K. Variety.
The test equipment uses the German Schatz portable bolt analyzer and high-precision digital display torque measuring instrument to measure the clamping force F and the total torque Ma during bolt tightening, respectively. See Figure 2.
figure 2
In order to study the influence of the friction coefficient on the bolt clamping force, the bolting test and the lubrication state bolt nut of the ordinary non-lubricated state were respectively tested for tightening.
The test selected a bolt and nut commonly used by a car company. The bolt size is M14×1.5, grade 10.9. The test clamp sample material is 45 steel, and the hardness after quenching and tempering is 28-33HRC.
The key point test data is shown in Table 1.
Table 1
Figure 3 is a graph of the torque-clamping force during tightening of the bolt and nut in the lubricated state and the normal non-lubricated state, with a maximum torque of 240 Nm. The curve of the lubrication state is above the normal non-lubricated state, indicating that under the same moment Ma, the clamping force of the bolted state of the lubrication state is much larger than the clamping force of the bolt connection of the ordinary non-lubricated state.
image 3
For example, for Ma=200Nm, the lubrication state bolt connection clamping force F1=108k N, the normal bolt connection clamping force F2=41.5k N, the clamping force is less than 40% of the lubrication state. According to the formulas (4) and (5), the torque coefficient K and the total friction coefficient μTot can be obtained in two states.
It can be seen from Table 2 that the torque coefficient and the total friction coefficient of the ordinary non-lubricated state are nearly three times that of the lubrication state, which is an important factor affecting the clamping force of the bolt.
Table 2
In order to study the stability of the torque coefficient and the friction coefficient, 10 sets of test pieces were selected for the lubrication state and the bolt and nut of the ordinary non-lubricated state, and the comparison test was carried out.
Figure 4 shows the K-value curve of the torque coefficient. The results show that the K-factors of the 10 sets of samples in the lubrication state are distributed between 0.131 and 0.138, and the values ​​are relatively stable. The K-factors of the 10 sets of samples in the normal non-lubricated state are distributed at 0.220. Between ~0.440, the value is large and extremely unstable.
Figure 4
Figure 5 shows the total friction coefficient μTot curve. The results show that the μTot distribution of the 10 samples in the lubrication state is between 0.100 and 0.106, and the value is relatively stable. The μTot distribution of the 10 samples in the normal non-lubricated state is 0.185~0.371. Between, the value is large and extremely unstable.
Figure 5
In the ordinary bolt, the torque coefficient K (1) of the test piece 1 is 0.440, and the torque coefficient K (10) of the sample 10 is 0.220. According to formula (4), when the same torque Ma is applied, the clamping force of the sample 10 is twice as large as that of the sample 1: F (10) = 2F (1). In contrast, for bolted joints in the lubricated state, the clamping force differs by only 5%.
Figure 6 shows the state of the test when the ordinary bolt reaches a large clamping force. The torque is applied to make the bolt clamping force reach 75%~80% of the theoretical yielding load of the bolt. It is found that the bolt and nut thread damage is serious, and the sliding phenomenon occurs. .
Image 6
Figure 7 shows the normal bolt and lubrication bolt test results after the bolt clamping force reaches 75%~80% of the bolt's theoretical yield load. The results show that the common bolt thread damage is serious, and the lubrication bolt has no signs of damage.
Figure 7
As explained above, the bolting of the lubrication treatment has a small torque coefficient and a total friction coefficient, and is relatively stable, while the common non-lubricated bolt coupling torque coefficient and the total friction coefficient are large and extremely unstable. When a large bolt is applied with a large torque, the total friction coefficient is large, and the thread damage is likely to occur, and the total friction coefficient of the bolt in the lubrication state is small, and the thread is not easily damaged.
4. Industry technology status
The industry is paying more and more attention to the friction problem in bolt joints. A well-known commercial vehicle company controls the total friction coefficient of 0.08~0.14 by lubricating the surface of bolts and nuts, and strives to average 0.11; a well-known passenger car company requires total The coefficient of friction is between 0.09 and 0.15; some are even between 0.06 and 0.09.
In short, each company has effective control of the friction coefficient of bolts and nuts according to its own technology and process level, and the lubrication treatment methods are also different, such as oiling, lubricating wax and so on.
5, the conclusion
Based on the above theoretical analysis and experimental research, we found that:
(1) The torque coefficient and total friction coefficient of common bolt connection are large and extremely unstable; lubrication treatment can reduce the torque coefficient and total friction coefficient of bolt connection;
(2) Due to the extreme instability of the total friction coefficient of ordinary bolts, the clamping force in the bolt connection is extremely unstable, and the clamping force under the same torque can be up to 2 times, in contrast to the lubrication bolts. The clamping force is relatively stable and the fluctuation range is small;
(3) Due to the large friction coefficient of ordinary bolts, when the increased torque is applied and the clamping force is reached, the threaded part of the bolt and nut is seriously damaged, and the lubricating bolt nut is often not damaged.
In short, the bolt and nut in the bolt connection are lubricated, the friction coefficient of the bolt and nut itself is reduced, and the torque coefficient in the bolt connection is reduced and stabilized, thereby effectively improving and controlling the clamping force, thereby improving the reliability and stability of the bolt connection. The top priority.
Double J Hook,Double J Hook Straps,Double J Wire Hooks,Double Wire Display Hooks
Jiangsu Zhongyi Work Rigging Co., Ltd. , https://www.zy-rigging.com