Mechanical products are often used in modern life. In the use of mechanical products, quality accidents occur, such as loosening, breaking, and tightening of the connecting fasteners. These problems are common in life, but there is no optimal solution. Method.
This paper combines a large number of data on the cause of fastener failure in the detection of the coefficient of friction of the thread. At the same time, it analyzes the production status of the heavy-duty vehicle and the actual failure of the test loader. The impact is discussed, and how to solve the problem of fastener failure, improve the quality of mechanical products, and propose countermeasures and suggestions.
At present, heavy-duty vehicles often have bolt connection failures in the assembly of the whole vehicle. Although some experts have raised some problems, there are no effective measures. Experts believe that because of the small tightening force, the bolt connection is loose and the tightening force is attenuated. The conclusion is that the friction coefficient of the threaded fastener is unstable during the assembly process, and the tightening force does not reach the anti-loose force value.
On this issue, the author has reviewed a large number of information on the influence of the coefficient of friction of the thread on the fastener, combined with the problems that may occur in the actual use process, and analyzed and discussed the failure form, cause and significance of the fastener.
1. Reasons for loose fasteners
1.1 Failure form of threaded fasteners in use
In actual use, there are five failure modes of threaded fasteners, namely, screw breakage, screw thread tripping, nut thread tripping, screw nut thread simultaneous tripping, bolts and nuts eventually lose their working ability due to looseness.
These five conditions are likely to occur during the actual use and test conditions of the test vehicles for production and special test loading, and the last one can be guided by the previous cases.
What is common is that there are a large number of loose fasteners in the threaded fasteners, and the proportion of looseness is much larger than the fracture.
The difference is that the threaded fasteners produced normally have more fractures than the threaded fasteners that are tested under test conditions, and the fasteners that work under actual working conditions are more likely to break.
1.2 Reasons for loose threads
In actual work, the thread often appears loose, and the looseness of the thread means that the bolt connection loses all or part of the axial preload force. Since the thread has the advantage of self-locking in production, and can be locked under the condition of friction, why is the car loose during use?
Scientific research shows that under static load conditions, the bolt has only one effect, that is, it bears the axial load, but the running angle of the thread will play its own role. The torque of loosening the nut and tightening the nut is different. Generally, the loose torque is the tightening torque. Eighty percent of the value, if there is a connection friction at this time, the connection will not be loose.
Variable load, vibration and impact are the main factors affecting the looseness of threaded fasteners. Most of the fasteners used in automobiles are in a working environment of vibration or impact, and because of the inertia of the parts and the interaction of other connected parts, the friction coefficient of the threads is gradually reduced, and even the frictional resistance may disappear instantaneously. In this case, once this situation occurs, the balance of the original force will be destroyed. At this time, the thread cannot be self-locking, and a small sliding force will be generated, resulting in looseness.
According to the above analysis, it is necessary to overcome the greater resistance in the direction of tightening the nut than in the direction of loosening the nut. Therefore, the possibility of sliding in the direction of the tightening nut is not large, and if a small range of relative sliding occurs multiple times in use, the pre-prediction may result. The tightening force is reduced and the connection is loose.
Under the action of great stress, the fastener and the connected parts will be plastically deformed. The mutual contact surface in the threaded connection causes local plastic deformation due to surface roughness, shape and position error, etc., and the local plastic deformation is very good during the use of the automobile. The possibility will continue to occur, and the final result is that the pre-tightening force of the connection is reduced, which leads to a decrease in the positive pressure of the connecting surface, which causes the self-locking ability to decrease. Once the external force is generated at this time, the loosening phenomenon is accelerated.
In addition to the above two main reasons, there are other reasons for the failure of the fastener in practice, such as whether the selected fastener is suitable or not, whether there is an error in the tightening machine or the wrench, and how the operator uses the tightening tool. Whether the specification, the angle is correct, the quality of the bolt is acceptable, etc., these factors may cause the fastener to fail.
2. The concept of thread friction coefficient and its significance
2.1 Friction coefficient concept
The concept of the coefficient of friction is to measure the various friction coefficients without considering the shape and size of the fastener under basic friction conditions. The coefficient of friction is the infinite tempering data, which can be derived by measuring the physical properties. Its final value is determined by the surface type and geometry.
Fv———Clamping force (according to 75% of the load of the test piece or test piece, the smaller of the two), the unit is kN;
M———tightening torque, the unit is Nm;
d———The nominal diameter of the fastener, in mm.
In the formula, μ total—the total friction coefficient;
K———torque coefficient;
D———the nominal diameter of the fastener, the unit is mm;
P———pitch, the unit is mm;
D2————The diameter of the thread, the unit is mm;
Db————The friction diameter of the support surface tightened by the threaded fastener, the unit is Mm
Db=(D0+Dh)/2
D0———The outer diameter of the support surface under the threaded fastener (tightening end); the unit is mm;
Dh———Supported by the clamped hole or the inner hole of the gasket or the threaded fastener (tightening end), the unit is mm, the three are the largest.
2.2 Definition of moment coefficient
The measurement method of the moment coefficient K is not difficult and can be calculated by the formula K=T/(Fd). However, in this calculation method, only the value of d is related, which means that the k index is determined by only one size. Under the same friction conditions, for the same size d and the same area of ​​the fastener, the K index can be calculated by the torque-clamping force.
2.3 Factors affecting the coefficient of friction of the thread
The thread friction coefficient μ total (corresponding to the moment coefficient K value) is a dimensionless data, which can be calculated from the measurable physical properties. It is the surface type and geometry connected, and the friction range is constant. - The clamping force is calculated.
When the ideal state is reached, it can be calculated from the torque-clamping force that the friction coefficient μ of the threaded fastener (corresponding to the torque coefficient K value) should be a constant value.
These values ​​can be calculated by calculation in practice. In the actual production process, the discrete types of the above parameters are relatively large, resulting in a discrete type of friction coefficient of the threaded fasteners. If the parameters of all the above data are within the specified range. Then, the coefficient of the threaded fastener is discrete and controllable.
2.4 Thread friction coefficient meaning
According to the above data analysis, the friction coefficient of the threaded fastener only reflects the relationship between the torque and the tightening force of the threaded fasteners of different specifications. In the research and development process of the enterprise, the theoretical research on the failure of the threaded fasteners is more, often on the thread. Failure analysis continues, and can also be calculated by measuring the physical dimensions, the clamped aperture or the inner diameter of the support surface under the threaded fastener.
In the actual design process and production of the automobile, the specifications of the fasteners can be selected according to the numerical values, and the assembly torque can also be corrected by the friction coefficient. Therefore, in theory, the friction coefficient of the threaded fastener should be a fixed value.
3. Effect of friction coefficient stabilizer on thread friction coefficient and bolt fastener failure rate
3.1 Effect of friction coefficient stabilizer on thread friction coefficient
The friction coefficient stabilizer is an effective means to improve the friction coefficient of the thread. In a broad sense, the friction coefficient stabilizer acts as a lubricating coating for reducing and stabilizing the friction coefficient. In the application range of the fastener, the friction coefficient stabilizer has the speed of promoting the tapping of the fastener, and can achieve the reduction of the locking torque and the looseness of the fastener in the swinging environment.
The study found that the variation between the friction coefficient values ​​between the fasteners can be reduced, as long as a reasonable and effective way can be quickly reduced, which can greatly improve the overall quality of the fasteners and assembly of the parts. For future safety, the maximum possible value of the preset torque and automatic assembly settings is achieved.
The friction coefficient stabilizer is an aqueous colloidal dispersion containing a high molecular polymer. It is applied by dipping in practical use, and after drying and solidifying, a thin and lubricious film is formed to adhere to the outer layer. The film has a function of lowering the coefficient of friction and reducing the difference in friction coefficient in use. Moreover, since the lubricating film is extremely thin, it does not actually affect the tolerance.
3.2 Effect of friction coefficient stabilizer on fastener failure
The friction coefficient stabilizer can be used to stabilize the friction coefficient within a fixed value range within the allowable range of the friction coefficient design of the fastener, so as to meet the requirements of the automated assembly for the quality stability of the threaded fastener assembly, and thus the manufacturing friction defect coefficient The problem can't be solved, so there is no problem that affects tolerance. That is to say, although the friction coefficient stabilizer can improve the friction coefficient, it does not have a substantial effect on the failure control of the fastener.
4. Threaded fastener failure control recommendations
The reason for the failure of the threaded fastener is that the friction coefficient of the threaded fastener is too large. In the use, the friction coefficient stabilizer can be used to help reduce the small friction coefficient, but it is unreasonable to solve the quality problem of the threaded fastener failure.
By looking up a lot of information about the threaded fasteners, combined with the failure of threaded fasteners in actual vehicle production, the following recommendations can be made:
(1) Enterprises need to do more detailed research and analysis on the causes and discrete laws of the friction coefficient.
(2) Whether the quality of the threaded fastener meets the requirements of the specification, in terms of material requirements, part size, whether it meets the standard when entering the factory quality inspection. In fact, the diameter of the thread, the heat treatment of the thread material, the surface roughness, hardness, lubrication, fatigue resistance, internal chamfering and other factors are related to the thread assembly torque. It can be seen that the numerical influence factor of the thread assembly torque is not only The coefficient of friction is related. However, in the materials provided by the quality department in practice, there has not been a strong emphasis on this aspect. In the future research, many power companies should strengthen research and further refine this aspect.
(3) It is necessary to pay attention to the rationalization of the assembly process of threaded fasteners, and the on-site management should be gradually improved to improve the quality and efficiency of on-site work.
(4) Suppliers are advised to have appropriate inspection methods to test the problems and improve them in a timely manner.
5. Conclusion
In summary, the coefficient of friction of the thread is the number of infinite steel calculated by the enterprise in actual activities, which can reflect certain product quality problems, but the international standard is to use this value as experimental data, and the nature of the dimensionless number itself is also It does not affect the root cause of the reaction friction coefficient.
At present, the problem of threaded fasteners of the whole vehicle is more serious, and there are many reasons for this problem. The design process is complicated, such as design links, assembly links, product manufacturing and manufacturing, etc., if it affects the quality of the fastener itself. Reasons are not considered, in-depth study of the causes of failure of threaded fasteners and to achieve maximum avoidance of failures, from product design analysis to later development, experiment, research, purchase and other aspects need to do a more complete treatment.
Although there is a certain relationship between the thread friction coefficient and the fastener failure mode, the conventional physical side face parameter method can achieve the purpose of controlling the thread friction coefficient. In addition, this type of equipment is more expensive, so it is used in actual production. The means of quality control of the entire vehicle is unreasonable, and companies need to constantly seek more effective ways.
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