Analysis on the causes of assembly bolt looseness and anti looseness measures

Bolt connection is widely used in mechanical equipment, automobile, train, aerospace and other fields. It is the most basic structural form of fastener connection. For assembly plant, especially for construction machinery assembly plant, bolt connection has the advantages of convenient standardization, mass production, simple structure, low cost, convenient installation, easy disassembly and so on, and is the most widely used Application. When the equipment is in use due to vibration, high and low load change, impact or long-term working state, the bolt is easy to loose, which directly affects the operation performance of the equipment and reduces its safety and reliability. The light one will affect the normal operation of the equipment and cause economic losses, while the heavy one will affect personal safety. In view of this, the author takes the practical application effect of loader assembly as the research starting point, and discusses the causes of loose bolt connection and anti loose measures.

Analysis on the reason of assembly bolt looseness

The purpose of bolt connection of parts is to closely fit the two parts connected. At the same time, in order to bear a certain dynamic load and ensure the reliable connection and normal operation of the connected parts, sufficient clamping force F between the two connected parts is also required, as shown in Figure 1. Therefore, it is required that the bolts for connection should have enough axial preload after tightening. According to this, we will analyze the cause of looseness.

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Figure.1 Analysis of clamping force between connecting parts

Design defects

Insufficient strength of bolt selection

Generally, the bolt connection adopts the yield point tightening method, that is, the pre tightening force of the bolt shall reach the yield strength of the bolt material. Considering the safety factor, the pre tightening force shall not exceed 80% of the yield limit of the bolt material. The yield strength of the bolt of different materials is different, which means that the maximum pre tightening force that the bolt of different materials can bear is different.
In the process of bolt tightening, the pre tightening force of bolt is increasing continuously, while the clamping force between connecting parts is also increasing rapidly. When the yield point is reached, the plastic deformation of bolt begins. The increment of clamping force decreases or even does not change with the increase of pre tightening force, and then the clamping force decreases gradually until the bolt breaks.
Therefore, in the process of product design, designers need to accurately analyze the required tightening torque of the connecting parts, and fully consider the weight, volume, working state, working environment, safety standards and other factors of the connecting parts, so as to select the appropriate strength bolts and threads.

Missing connection lock

Under the condition of bearing load change, vibration, impact, etc., parts connected by bolts will gradually reduce or even disappear the tightening force and pre tightening force. After repeated times, the screw connection will be loose, and finally the bolt will be loose due to failure. The form of bolt and nut matching connection is particularly serious.
When the bolt is subjected to axial load, the bolt extends axially, shrinks radially and the nut expands radially. A small amount of relative radial sliding occurs between the contact surfaces of the two, which eventually causes the nut to turn loose under the repeated load. When the bolt is subjected to repeated transverse force, the bolt will produce elastic torsional deformation. With the increase of deformation, the displacement of torsional deformation will be formed, and a force component will appear in the screw direction of the thread, which will cause the nut to rotate and loosen.
Therefore, effective anti loose measures must be taken in the process of product design to avoid the loosening and falling off of bolts or nuts caused by bad working conditions.

Insufficient preload during assembly

The pre tightening force that the bolt can achieve in the process of tightening directly determines the clamping force between the two connecting parts. The lack of pre tightening force will inevitably lead to the loosening of the connecting bolt and eventually the loosening of the connecting parts. In the process of assembly, the following two methods are generally used to obtain the pre tightening force of bolts.
The first way is to use manual tools (such as solid wrench, ring wrench, Allen wrench, ratchet wrench, etc.). The pre tightening force obtained by the bolt depends on the size of the operator’s arm and the length of the tool’s arm. Generally, the hand tools are standard tools, the arm length is certain, while the operator’s arm strength is limited, unstable and uncontrollable, so is the pre tightening force obtained by the bolt.
The second way is to use pneumatic tools or electric tools (such as pneumatic impact wrench, electric impact wrench, etc.). The pre tightening force obtained by the bolt depends on the maximum output tightening torque of the tool itself and the stability of the tool operation. Pneumatic tools or electric tools should not only select the appropriate tool model according to the maximum pre tightening force that the bolts can bear, i.e. select the appropriate range of tightening torque output, but also consider the factors that affect the operation of the equipment, such as the power supply used by the tool, the air source, and the daily maintenance status of the tool, such as the lack of power supply, the lack of air source pressure will cause the work The output tightening torque is insufficient, and the final bolt cannot meet the required pre tightening force.

Improper assembly method

In the process of assembly, there are single bolt and multiple bolt tightening states. For the state of multiple group bolt tightening, especially the regular distribution, the method and method of bolt tightening are extremely important, which directly affects the actual pre tightening force of each bolt.
For example, in the process of assembly, when the four bolts are tightened at will, the pre tightening force of the bolts in front of the back will be smaller, and even the bolts in the first tightening will be loose. In this case, the diagonal cross tightening method is generally adopted to tighten the bolts, so as to ensure that the bolts are tightened uniformly and the stress is balanced as far as possible.
Therefore, when assembling a group of bolts, they must be tightened in a certain order, distributed, symmetrical and step-by-step. Otherwise, the installation bolts will be loose due to inconsistent tightness, or even the connecting parts will be deformed due to unbalanced stress.

Machining quality of connecting piece mounting hole

When two connectors are connected, the size of threaded hole or mounting bolt hole is particularly important. The size of threaded hole directly affects the pre tightening force of bolt. If the threaded hole is too large or too small, the pre tightening force obtained by the bolt cannot meet the requirements. If the size of the bolt hole is too large, the surface of the connected part will produce plastic ring indentation at the contact with the bolt or the nut bearing surface. Serious indentation will reduce the pre tightening force obtained by the bolt or lose the pre tightening force, resulting in the loosening of the nut or bolt.

Common bolt locking method

Bolt anti loosening is to eliminate (or limit) the relative movement between thread pairs, or increase the difficulty of relative movement, to prevent the connection from loosening and ensure the safety and reliability of the connection. There are various measures to prevent looseness. In addition to the need to take corresponding effective anti looseness devices according to relevant standards and actual working conditions of the equipment in the design, it is also necessary to control the tightening torque of the installation bolts during the assembly process to avoid the bolt looseness before leaving the factory. Therefore, the measures for preventing bolt looseness can be roughly divided into the following two aspects: physical device looseness prevention and process method looseness prevention.

Anti loosening measures for physical devices

Locking device with additional friction

There are three main structures of anti loose device with additional friction: double nut tightening structure, spring washer tightening structure and 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 contact surface of the bolt thread, so that the screw thread is always subject to additional pressure and friction, as shown in Figure 2. When the working load changes, the friction still exists. The utility model has the advantages of simple structure and is suitable for the connection of stable, low-speed and heavy load fixed devices.
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Fig. 2 Double nut tightening structure locking device

Spring washer tightening structure: spring washer is widely used in the assembly site because of 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 caused by the squash of the backing ring makes the screw threads tightly pressed. At the same time, the tip of the bevel of the washer against the bolt or nut and the bearing surface of the connected part also have the function of locking.
However, under the action of vibration and impact load, the anti loose effect of this kind of tightening structure is poor, and it is generally used for unimportant connection. Self locking nut tightening structure: there are two types of tightening structure, one is all metal locking nut. It is mainly used to carry out closing treatment or non-circular closing treatment after slotting at the upper end of the metal nut body. After treatment, the nut thread will appear partial deformation. When the nut is tightened, the closing will expand, and the screw thread will be compressed by the return force of the closing. This kind of nut is simple in structure and reliable in anti loosing. It can be loaded and unloaded many times without reducing its anti loosing performance. But it is not suitable for high-speed operation.
The other is the non-metallic insert lock nut, which is mainly embedded with a non-metallic insert (usually nylon washer) at the upper end of the metal nut body. After the nut is tightened, the non threaded nylon ring will be screwed into the thread and extruded out of the thread, forming a stable friction force with large torque. It has very good anti loose ability, especially in the environment of multi vibration and multi impact.

Mechanical locking device

The mechanical anti loosing methods mainly include: hexagon slotted nut with cotter pin, series steel wire structure anti loosing and stop washer structure anti loosing.
The hexagon slotted nut is equipped with a cotter pin: the cotter pin passes through the slot of the nut and the pin hole at the end of the bolt, and the tail of the cotter pin is broken off to close with the side of the nut, and the nut and the bolt are locked, as shown in Figure 3. This method is reliable and can be used in severe shock and vibration parts.
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Fig.3 Hexagon slotted nut with overhead locking device
Anti loosing of series steel wire structure: the nut or bolt is designed with a safety hole, through which the low carbon steel wire is penetrated, and the screws are connected in series to make them brake each other, but pay attention to the penetration direction of the steel wire, as shown in Figure 4. Although this method has excellent anti loose ability, it is inconvenient to install and disassemble. Lock washer structure: bend the single or double ear lock washer to the side of the nut and the connected part respectively, and then lock the nut, as shown in Figure 5. If two bolts need double interlocking, double stop washer can be used to make two nuts brake each other. This method has good anti loose effect and is easy to use.
Analysis on the causes of assembly bolt looseness and anti looseness measures
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Fig.4 Anti loosing of series steel wire structure
Analysis on the causes of assembly bolt looseness and anti looseness measures
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Fig.5 Lock washer structure locking device

Adhesive locking device

The anaerobic adhesive is applied between the screw threads. After the nut is tightened, the adhesive hardens and is fixed. The matching thread can be firmly bonded to prevent the relative movement of the thread pair, so as to achieve the effect of locking and loosening prevention. Anaerobic adhesives are divided into liquid and dry type. Liquid type is generally used for field application. Dry type is precoated and dried in advance for easy operation. Anaerobic adhesive is generally used for assembly parts that do not need to be disassembled or need to be disassembled for a few times.

Process method anti loose measures

Locking of fixed torque tool

In the process of assembly, in order to ensure that the bolt can obtain enough pre tightening force, and to ensure that it is less than the bolt yield point torque, it is the most effective method to use the fixed torque tool to control. The torque setting tool can set the output torque according to the standard tightening torque of the bolt. When the preset tightening torque is reached, it will send a signal or terminate the tightening by itself to ensure a stable torque output.
In order to improve efficiency, the fixed torque tool can be used in combination with small torque pneumatic impact wrench in addition to being used alone. For key parts, the pneumatic fixed torque wrench can be used for tightening first and then the manual fixed torque wrench can be used for testing and confirmation to ensure that the ideal torque value is achieved. The accuracy of the constant torque tools must be ensured by using this locking method, and all the constant torque tools must be regularly calibrated according to the measurement instrument testing regulations.

Training and training against loosening

In the process of bolt tightening, in addition to the output tightening torque of the tightening tool, which affects the pre tightening force obtained by the bolt, its assembly method is also crucial, especially for the regular installation of bolts in groups. Tightening tools and assembly methods are used by the operator, so the skill level of the operator also directly affects the effect of bolt tightening, so the skill level of the operator can be improved through training and training, and then the ability of bolt locking can be improved.
The training of operators is mainly based on theory, mainly learning the basic knowledge of assembly, including the operation method of tightening tools, the assembly method of bolt tightening, etc. The training of operators is mainly based on the operation practice, mainly combining with the content of the basic assembly knowledge to practice the actual operation, such as establishing a training platform, allowing operators, especially new employees, to carry out repeated training, mastering these basic operation methods before going to work, reducing the bolts caused by incorrect assembly methods through the training and training of basic skills Loosening.

Parts quality control anti loose

The machining dimension of the parts also directly affects the pre tightening force obtained by the bolt, especially the specification of the thread, which is directly related to the interaction of the connecting parts. Therefore, before assembly, it is necessary to check whether the parts to be installed meet the requirements of the design drawings, and check whether the specifications, models and mechanical performance grades of the threaded connections meet the design requirements by using standard testing tools, If there is any unqualified phenomenon such as disordered thread and crack, the quality problems shall be fed back to the manufacturer for rectification to ensure that the dimension processing of parts meets the design requirements and the reliable connection of bolts.


There are many reasons for bolt looseness, different equipment will have different reasons under different working conditions. This paper mainly studies and analyzes the assembly process of the equipment before leaving the factory, makes statistical analysis of several reasons, and gives some effective thread anti loose methods. These methods have also been applied and promoted in the field, with good anti loose effect. There are many traditional methods of thread locking, but when facing the specific problem of thread locking, it is necessary to find out the root cause of thread connection loosening in the process, select the reasonable connection structure according to the actual reason, take the reasonable locking measures, enhance the locking ability and improve the fastening reliability.

Source: China Bolts Manufacturer – Yaang Pipe Industry (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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