Summary
Explain the common bolt loosening phenomenon in the assembly factory, and analyze several common reasons for the looseness of the assembly bolts and some anti-loose measures commonly used in the assembly factory.
Bolted connection is widely used in mechanical equipment, automobiles, trains, aerospace and other fields. It is the most basic structural form of fastener connection. For assembly plants, especially engineering machinery assembly plants, bolted connections are convenient. The most widely used applications are standardized mass production, simple structure, low cost, easy installation and easy disassembly. When the equipment is in use, due to vibration, high and low load changes, impact or long-term working condition, the bolt is prone to looseness, which directly affects the running performance of the equipment and reduces its safety and reliability. The lighter affects the normal operation of the equipment and causes economic loss. Serious people will affect personal safety. In view of this, the author takes the practical application effect of loader assembly as the starting point of the research in this paper, and discusses the reasons for loose bolting and anti-loose measures.
1. Analysis of the reasons for the loosening of the assembly bolts
The purpose of the bolted parts is to make the two parts to be tightly fitted together, and in order to withstand a certain dynamic load, to ensure the reliable connection and normal operation of the connected parts, it is necessary to have enough between the two connected parts. Clamping force F, as shown in Figure 1. Therefore, it is required that the bolt for connection has sufficient axial preload force after tightening. Based on this we analyze the reasons for the looseness.
Figure 1 Analysis of the clamping force between connecting parts
1.1 Design flaws
(1) Insufficient strength of bolt selection
The bolting connection generally adopts the yield point tightening method, that is, the pre-tightening force of the bolt should reach the yield strength close to the bolt material. Considering the safety factor, generally it should not exceed 80% of the yield limit of the material. The bolts of different materials have different yield strengths. This also means that the maximum preload of the bolts of different materials can be different.
During the tightening process of the bolt, the pre-tightening force of the bolt is continuously increased, and the clamping force between the connecting members is also rapidly increased. When the yield point is reached, the bolt begins to plastically deform, and the clamping force increases with the pre-tightening force. It becomes smaller or even unchanged, and then continues to increase the pre-tightening force, and its clamping force gradually becomes smaller until the bolt breaks.
Therefore, in the product design process, the designer needs to accurately analyze the tightening torque required by the connecting component, and fully consider the weight, volume, working state, working environment, safety standard and other factors of the connected component, and select the appropriate strength bolt and Thread.
(2) Lack of connection prevention
When the bolted parts are subjected to load changes, vibration, impact, etc., the pressing force and pre-tightening force of the connection will gradually decrease or even disappear. After repeated times, the threaded connection is loosened, and finally the bolt is broken. Loose, bolt and nut mating connections are particularly serious.
When the bolt is subjected to an axial load, the bolt is axially elongated, radially elastically contracted, and the nut is radially expanded. A slight amount of relative radial slip occurs between the contact faces of the two, and eventually the nut is loosened by the repeated action of the load. When the bolt is subjected to repeated lateral forces, the bolt will undergo elastic torsional deformation. As the deformation increases, a displacement of the torsional deformation is formed, and a force component appears in the spiral direction of the thread to cause the nut to loosen.
Therefore, effective anti-loose measures must be taken during product design to avoid loosening of bolts or nuts due to poor working conditions.
1.2 Insufficient pre-tightening of the assembly process
The pre-tightening force that the bolt can achieve during the tightening process directly determines the clamping force between the two connecting parts. The insufficient pre-tightening force will inevitably lead to the loosening of the connecting bolt and eventually the loosening of the connecting parts. The pre-tightening force of the bolt is generally obtained in the following two ways during the assembly process.
The first way is to use hand tools (such as wrenches, wrenches, hex wrenches, ratchet wrenches, etc.). The pre-tightening force of the bolt depends on the size of the operator's arm and the length of the tool arm. The general hand tools are standard tools. The arm length is fixed, and the operator's arm strength is limited. The same is true for the preload of the bolts that are controlled.
The second way is to use pneumatic tools or power tools (such as pneumatic impact wrenches, electric impact wrenches, etc.). The amount of preload of the bolt depends on the maximum output tightening torque of the tool itself and the stability of the tool operation. In addition to selecting the appropriate tool type according to the maximum preload force that can be withstood by the tightening bolts, the pneumatic tool or the power tool needs to consider the power supply, air source, and tool used by the tool in addition to the appropriate output tightening torque range. The daily maintenance status and other factors affecting the operation of the equipment, such as insufficient power supply and insufficient air supply pressure, the tightening torque of the tool output is insufficient, and finally the bolt cannot reach the required pre-tightening force.
1.3 Improper assembly method
During the assembly process, there is a state in which a single bolt and a plurality of bolts are tightened, and the state in which the plurality of sets of bolts are tightened, especially the regular distribution, the method and the manner of bolt tightening are extremely important, which directly affects the actual acquisition of each bolt. The size of the preload.
For example, the square-shaped mounting bolts that are common in the assembly process, when the four bolts are loosely tightened, the pre-tightening force of the bolts tightened before the tightening of the rear bolts becomes smaller due to the unbalanced tension between the bolts. Even the bolts that are tightened for the first time will appear loose. In this case, it is generally tightened by means of diagonal cross-tightening to ensure that the bolts are tightly balanced and balanced.
Therefore, when assembling the group bolts, it must be distributed, symmetrical, and stepwise tightened in a certain order. Otherwise, the installation bolts may be loose and inconsistent, and the connecting parts may be deformed even due to unbalanced forces.
1.4 Connector mounting hole processing quality
The size of the threaded hole or mounting bolt hole is particularly important when the two connectors are connected. The thread size of the threaded hole directly affects the preload force obtained by the bolt. If the threaded hole is too large or too small, the pre-tightening force of the bolt cannot be achieved. If the size of the mounting bolt hole is too large, the surface of the connected piece will produce a plastic annular indentation at the contact surface with the bolt or nut supporting surface, and the severe pressing will reduce the pre-tightening force of the bolt or lose the pre-tightening force. , resulting in loosening of the nut or bolt.
2, commonly used bolt anti-loose way
Bolt lock is achieved by eliminating (or limiting) the relative movement between the thread pairs, or increasing the difficulty of relative movement, preventing the connection from loosening and ensuring a safe and reliable connection. There are various measures for anti-loose. In addition to the need to take appropriate anti-loose devices according to the relevant standards and actual working conditions of the equipment during design, it is also necessary to control the tightening torque of the mounting bolts during the assembly process to avoid loose bolts before leaving the factory. Therefore, it can be considered that the bolt anti-loosening measures can be roughly divided into the following two aspects: physical device anti-loose and process method anti-loose.
2.1 Physical device anti-loose measures
(1) Anti-loose device with additional friction
The anti-loose device with additional friction mainly has three structures: a double nut tightening structure, a spring washer tightening structure and a self-locking nut tightening structure. Double nut tightening structure: first tighten the lower nut and then tighten the upper nut. After the two nuts are tightened to the top, the upper and lower nuts are opposite to the threaded contact surface of the bolt, so that the screwing thread is always subjected to additional pressure and friction. 2 is shown. This friction still exists when there is a change in the working load. Its simple structure makes it suitable for the connection of stationary, low-speed and heavy-duty fixtures.
Figure 2 Double nut tightening structure anti-loose device
Spring washer tightening structure: Spring washer is widely used in assembly site due to its simple structure, convenient use and low cost. It is generally used with bolts or nuts. After the bolt or nut is tightened, the elastic reaction force generated by the flattening of the washer causes the screwing thread to be compressed. At the same time, the tip end of the washer oblique opening also has a loosening effect against the bolt or the nut and the bearing surface of the coupled member.
However, under the vibration shock load, the tightening structure has a poor anti-loose effect and is generally used for unimportant connections. Self-locking nut tightening structure: There are generally two types of such tightening structures, one type is an all-metal lock nut. The main purpose is that after the upper end of the metal nut body is grooved, the closing process is performed or the non-circular closing process is performed, and the nut thread will be partially deformed after the treatment. When the nut is tightened, the closing mouth is opened, and the resilience of the closing mouth is used to make the rotation. Press between the threads. These nuts are simple in structure, reliable in locking, and can be loaded and unloaded multiple times without reducing the loosening performance. However, it is not suitable for high-speed operation.
The other type is a non-metallic insert lock nut, which mainly embeds a non-metallic insert (usually a nylon washer) on the upper end of the metal nut body. After the nut is tightened, the unthreaded nylon ring is screwed into the pressure. Extrusion of the threads creates a stable and high torque friction. This type of very good anti-loosing ability, especially in a multi-vibration, multi-impact environment.
(2) Mechanical anti-loose device
The main methods of mechanical anti-loose are: hexagonal slotted nut with split pin, series wire structure anti-loose and stop washer structure anti-loose.
Hexagon slotted nut with split pin: The split pin passes through the slot of the nut and the pin hole at the end of the bolt, and the tail of the split pin is pryed open to the side of the nut to lock the nut and bolt, as shown in Figure 3. This method is reliable and can be used for severe impact and vibration parts.
Figure 3 hexagonal slotted nut with overhead anti-loose device
The steel wire structure of the series is anti-loose: the nut or the bolt is designed with a safety hole, the low-carbon steel wire is inserted into it, and the screws are connected in series to be integrated to brake each other, but attention should be paid to the penetration direction of the steel wire, as shown in Fig. 4. Show. Although this method has excellent anti-loose ability, it is very inconvenient to install and disassemble. The locking washer structure is loosened: the single or double-eared retaining washers are respectively bent toward the side of the nut and the coupled member to lock the nut, as shown in FIG. If the two bolts need to be double-locked, a double-stop washer can be used to brake the two nuts. This method has good anti-loose effect and is convenient to use.
Figure 4 Series wire structure anti-loose
Figure 5 Stop washer structure anti-loose device
(3) Bonding anti-loose device
The anaerobic adhesive is applied between the screwing threads, and after the nut is tightened, the adhesive is hardened and fixed, and the matched thread can be firmly bonded to prevent the relative movement of the thread pair, thereby achieving the effect of locking and locking. Anaerobic adhesives are available in both liquid and dry types. Liquid type is generally used for on-site application. The dry type is pre-coated and dried in advance for easy operation. Anaerobic adhesives are generally used in assembly parts that do not require disassembly or require less disassembly.
2.2 Process method anti-loose measures
(1) Fixed torque tool anti-loose
In the assembly process, in order to ensure that the bolt obtains sufficient pre-tightening force, and also to ensure that it is less than the bolt yield point torque, it is the most effective method to control with a constant torque tool. The fixed torque tool can set the output torque according to the standard tightening torque of the bolt. When the preset tightening torque is reached, the signal is issued or the screw is terminated by itself to ensure stable torque output.
In order to improve the efficiency, the twisting tool can be used in combination with a small torque pneumatic impact wrench. For key parts, it can be tightened with a pneumatic torque wrench and then checked with a manual torque wrench to ensure that it is achieved. The ideal torque value. The use of this anti-loose method must ensure the accuracy of the fixed torque tool, and all fixed torque tools must be periodically verified according to the measurement requirements of the measuring instrument.
(2) Training and training to prevent loosening
In the process of bolt tightening, in addition to the tightening torque of the tightening tool affecting the pre-tightening force of the bolt, the assembly method is also crucial, especially for a plurality of groups of regularly distributed mounting bolts. The tightening tool and the assembly method are all used by the operator, and the skill level of the operator also directly affects the bolt tightening effect, so the skill level of the operator can be improved through training and training, thereby improving the bolt locking ability.
The operator's training is based on theory, mainly learning the basic knowledge of assembly, including the operation method of tightening tools, the assembly method of bolt tightening. The operator's training is based on operational practice. It mainly combines the training basic knowledge of the training to carry out practical exercises, such as establishing a training platform, allowing operators, especially new recruits, to conduct repeated training and master these basic operating methods. Only after the job can be put on the job, through the training and training of the basic skills, the bolt loosening caused by the incorrect assembly method can be reduced.
(3) Quality control of parts and components
The machining dimensions of the components also directly affect the pre-tightening force of the bolts. In particular, the specifications of the threads are directly related to the interaction of the connecting parts. Therefore, it is necessary to check whether the required components are in compliance with the design drawings before assembly. The inspection tool checks whether the specifications, model and mechanical performance grade of the threaded joints used meet the design requirements, and whether the threads are unacceptable, cracks, etc., and the quality problems that occur should be corrected by the manufacturer to ensure the size of the parts. Meet the design requirements and ensure the reliable connection of the bolts.
3, the conclusion
There are various reasons for loose bolts. Different equipments have different working conditions. This paper mainly studies and analyzes the assembly process before leaving the factory, statistical analysis of several reasons, and gives some effective The thread anti-loose method, these methods have also been applied and promoted in the field, and the anti-loose effect is good. There are many traditional thread anti-loosening methods, but in the face of specific anti-loose problems, it is necessary to find out the root cause of loose thread connection from the process, and select a reasonable connection structure according to actual reasons, take reasonable anti-loose measures, and strengthen anti-loose Ability to improve fastening reliability.
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