In recent years, bolted joints have been widely used to effectively solve the problem of mechanical component connection. From the practical application, the bolt is extremely susceptible to fatigue damage or looseness due to the effects of cyclic load and strong vibration. If the bolt fails, it will have serious consequences. Based on this, it is necessary to do bolt anti-loose and health monitoring.
1 Construction machinery bolt
As the main component of the equipment work, the construction machinery has many types of products, and it is used in the construction industry and the power industry, and the use environment is rather harsh. From practical applications, bolt loosening problems are more common. For ordinary loosening problems, replacement of high-strength bolts, such as 10.9 bolts, can reduce the occurrence of falling off. However, some machines use this method, and it is difficult to solve the problem of loose bolts. Based on this, it is necessary to analyze the problem of loose bolts and propose effective prevention methods.
2 commonly used bolt anti-loose method
2.1 Preload
In the 1920s, German engineers proposed the concept of wedge-type locking, using clamping forces to prevent loose bolts. Taking a wedge-type locking product as an example, the outer surface is radial sawtooth, and the inner surface is wedge-shaped, and is used in pairs during installation to ensure that the wedge faces are opposite. When tightening, ensure that the outer surface of the serration and the contact surface of the component are engaged. The displacement is between the inner surfaces of the wedges, and when the preload is reached, the displacement is stopped. When the vibration causes the bolt to be connected to the spin, the wedge-type locking system can expand in the thickness direction to increase the clamping force. Tested and verified, the product can maintain 80% preload, ensuring that the bolts are not loose.
2.2 Friction and anti-loose
The loosening of the bolt can be effectively controlled by means of a nut tightly and a spring pad. Among them, the double nut and tight method is suitable for fixed connection occasions, such as heavy load and low speed, for fixing of engineering machinery working equipment, such as U-bolt. The spring pad method is used, and if the elastic force is uneven, it is easy to cause the bolt to loosen. Taking an excavator as an example, it uses M8 and M10 bolts, and uses spring pad + flat pad anti-loose system. The anti-loose effect is poor. In the application work, no more than 500h, a large number of bolt loosening occurs. After the transformation, the use of self-made shims, a good anti-loosening effect. The application of the self-locking nut anti-loose method is reliable, and even if it is disassembled many times, it will not affect the anti-loose performance, and is suitable for U-shaped handles and radio antennas.
2.3 Mechanical anti-loose and broken thread anti-loose
Normally, for engine damper bolts, a mechanical anti-loose method is usually adopted. In the specific operation, the nut is welded to the bent bracket, and the bolt is locked by the limit of the bent side of the bracket. To destroy the application of the thread secondary anti-loose method, apply the fastening glue to the thread before tightening the thread, and tighten the thread pair. When the thread glue is hardened and solidified, the thread pair can be prevented from loosening. For important parts of bolts, such as roller and engine mounts, this method can be used for anti-loose treatment.
3 bolt anti-loose technology
The bolts are subjected to anti-loose treatment based on the fastening principle, and many methods are available. Now combined with the common techniques of bolt anti-loose, the following discussion.
3.1 Using reaming bolts
In engineering machinery and equipment, ordinary bolts are generally used, that is, bolts with a diameter smaller than the installation aperture of 0.5 to 1 mm, and the pre-tightening force is used to prevent relative sliding and movement of the connected components. When the device is in use, when the sliding condition occurs, the pre-tightening force can be converted into frictional force to prevent its movement. Such as
Therefore, a large preload is required. With the reaming bolts, it can withstand lateral loads and axial loads. For lateral loads, relying on their own shear resistance to prevent their movement, the required pre-tightening force is small. For axial bearing capacity, it is the same as ordinary bolts [1]. Based on this, for machines that need to withstand vibration loads, reaming bolts can be used to avoid loosening. For example, the rotary drilling rig mechanical equipment, the high-strength fastening bolt used on the side of the power head bracket, is a structure that utilizes friction force transmission, and the working condition is relatively bad. Exceeding the pusher, the maximum torque of the power head is 400kN· m, when frequently rotating in the forward and reverse directions, it is subject to large dynamic load and large vibration load, and the problem is solved effectively by using the reaming bolt.
It can be seen that the pusher with similar operating conditions of the rotary drilling machine can use a high-strength bolt to obtain a good anti-loosening effect.
3.2 Using steel spacers
When the working condition of mechanical equipment is relatively bad, it needs to bear a great impact. The problem of loose bolts is related to the structural design of mechanical equipment. It can absorb or buffer the dynamic load and directly determine the performance of bolts. If the structural design cannot be changed, bolting can be done by adding a steel spacer to improve the bolt's ability to withstand impact. By lengthening the bolts, the total amount of elastic deformation is increased, and the amount of deformation is used to offset the impact of the load, thereby ensuring the pre-tightening force and avoiding loosening of the bolt. In general, steel spacers made of 20 steel or 45 steel can achieve good results. Because of the steel spacers of this type, 40Cr and heat treatment are used. The method of use is as follows: First, use unilaterally. If used unilaterally, the bolt head side and the connector surface are in contact with each other; the second is used on both sides. When used on both sides, the length of the spacer should be designed according to the shorter requirements, and the surface of the connector should be in contact with each other, or the mats can be used together to facilitate the pre-tightening of the bolts. This can be done at the front hood and wing mounting position of the pusher machinery.
3.3 positioning pin anti-loose
In mechanical equipment, locating pins are more commonly used, which are characterized by convenient installation and high assembly quality, and are widely used [2]. From the perspective of the loosening mechanism of the bolt, the positioning pin can function to limit the movement of the connecting member and avoid the breakage of the bolt. Take the locating pin to prevent the bolt from loosening. In order to better make the locating pin to limit the micro-sliding of the connecting piece, the locating pin hole cannot be processed in place before assembly, and a reservation of 0.3 to 0.5 mm should be taken.
When the balance is assembled, the hinge method is adopted to improve the positioning accuracy. In the mechanical equipment such as the pusher and the bulldozer, the positioning pin is installed, and a good anti-loosening effect is obtained.
3.4 Squirrel fastening technology
The quality of mechanical equipment is greatly affected by loose bolts. In order to find effective prevention and treatment measures to solve the problem of loose bolts, people combined with the actual situation of mechanical equipment loosening, from the perspective of wear mechanism analysis, analyze the reasons for bolt loosening, and put forward effective control measures for bolt loosening. The use of the Spruce fasteners to take advantage of its structural advantages can solve the problem of fastener loosening. Because the thread bottom has a 30° wedge-shaped bevel, when the bolt and nut are tightened, the cusp can be placed on the wedge-shaped slope, resulting in a large locking force [3]. Because of the change in the angle of the teeth, the normal force generated by the contact between the threads is at an angle of 60° to the bolt axis. From the actual situation of the application, it plays a positive role. At present, the application of anti-loose technology to prevent loose mechanical equipment can achieve good results.
3.5 Online monitoring technology
Bolted joints are widely used and are subject to vibration and load shocks, which are prone to fatigue damage and looseness. If a failure occurs, it will cause a major loss. Based on this, doing a good job of bolt online monitoring has a positive effect. Under different loads and vibrations, the mechanism characteristics of the bolts are different, so online monitoring can be achieved. At present, bolt load online monitoring instruments have been applied to mechanical equipment. By constructing a distributed load collection system, the actual situation of the bolts on different flange faces can be monitored, and then the problem of loose bolts can be found, and measures can be taken in time to avoid major accidents [4].
4 Conclusion
In this paper, bolt bolting technology is used to analyze the causes of bolt loosening based on the bolt wear mechanism, and corresponding measures are proposed. Mechanical equipment bolts can be used to prevent loosening. In order to ensure the health of the bolts, people should do a good job of monitoring, find problems in time, take anti-loose measures to ensure that the bolts can be in a normal state.
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