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Analysis of the causes of loose assembly bolts and solutions

May 15, 2024

1. Analysis of the causes of loose assembly bolts

The purpose of using bolt connections for components is to ensure a tight fit between the two connected parts. At the same time, in order to withstand a certain dynamic load, ensure the reliable connection and normal operation of the connected parts, sufficient clamping force F is also required between the two connected parts, as shown in Figure 1. Therefore, it is required that the bolts used for connection have sufficient axial pre tightening force after tightening. Based on this, we will analyze the cause of looseness.

 

1.1 Design defects

(1) Insufficient strength of bolt selection

Bolted connections generally use the yield point tightening method, which means that the pre tightening force of the bolt should reach a yield strength close to that of the bolt material. Considering the safety factor, it should generally not exceed 80% of its material yield limit. The yield strength of bolts of different materials is different, which also means that the maximum pre tightening force that bolts of different materials can withstand is not the same. During the bolt tightening process, the pre tightening force of the bolt continuously increases, while the clamping force between the connecting components also rapidly increases. When the yield point is reached, the bolt begins to undergo plastic deformation. As the pre tightening force increases, the clamping force increment decreases or even remains unchanged. Further increasing the pre tightening force gradually reduces the clamping force until the bolt fractures. Therefore, in the product design process, designers need to accurately analyze the required tightening torque of the connecting components, fully consider factors such as the weight, volume, working condition, working environment, safety standards, etc. of the connecting components, and choose appropriate strength bolts and thread patterns.

(2) Lack of anti loosening connection

The components connected by bolts will experience a gradual decrease or even disappearance of the clamping and pre tightening forces when subjected to changes in load, vibration, impact, etc. After repeated cycles, it will cause the threaded connection to loosen and eventually fail, resulting in bolt loosening. The form of bolt nut joint is particularly severe. When the bolt is subjected to axial load, the bolt elongates axially, contracts radially, and the nut expands radially. A slight relative radial sliding occurs between the contact surfaces of the two, ultimately leading to the loosening of the nut during rotation under repeated loading. When the bolt is subjected to repeated lateral forces, it will undergo elastic torsional deformation. As the deformation increases, a displacement of torsional deformation is formed, and a force component will appear in the spiral direction of the thread, causing the nut to rotate and loosen. Therefore, effective anti loosening measures must be taken in the product design process to avoid the loosening and detachment of bolts or nuts caused by harsh working conditions.

1.2 Insufficient preload during assembly process

The pre tightening force that a bolt can achieve during the tightening process directly determines the clamping force between two connecting parts. Insufficient pre tightening force will inevitably lead to loosening of the connecting bolts and ultimately cause loosening of the connecting components. During the assembly process, the following two methods are generally used to obtain the pre tightening force of the bolts.

The first method is to use manual tools (such as solid wrenches, ring wrenches, hex wrenches, ratchet wrenches, etc.). The magnitude of the pre tightening force obtained by the bolt depends on the size of the operator's arm force and the length of the tool's arm force. Generally, manual tools are standard tools with a fixed arm length, while the operator's arm force is limited, unstable, and uncontrollable. The pre tightening force obtained by the bolt is also the same.

The second method is to use pneumatic or electric tools (such as pneumatic impact wrenches, electric impact wrenches, etc.). The magnitude of 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's operation. Pneumatic or electric tools should not only choose the appropriate tool model based on the maximum pre tightening force that can be borne by tightening the bolts, that is, choose the appropriate range of output tightening torque, but also consider factors that affect equipment operation, such as the power and air sources used by the tool, as well as the daily maintenance status of the tool. Insufficient power and air pressure can cause insufficient tightening torque output by the tool, and ultimately the bolt cannot meet the required pre tightening force.

1.3 Inappropriate assembly methods

During the assembly process, there are situations where a single bolt and multiple bolts are tightened. For situations where multiple group bolts are tightened, especially those with regular distribution, the method and method of tightening the bolts are extremely important, which directly affects the actual amount of pre tightening force obtained for each bolt. During the assembly process, it is common to install bolts with a square distribution. When the four bolts are tightened randomly, due to the uneven tension and force distribution, the pre tightening force of the bolts tightened earlier will decrease as the bolts are tightened later, and even the bolts tightened for the first time will become loose directly. In this case, diagonal cross tightening is generally used to tighten the bolts, ensuring consistent tightness and balanced force distribution. So when assembling group bolts, it is necessary to tighten them in a certain order, distribution, symmetry, and step by step, otherwise it will cause inconsistent tightness of the installation bolts and even deformation of the connecting components due to uneven force.

1.4 Quality of Connection Installation Hole Processing

The size of the threaded hole or installation bolt hole is particularly important when connecting two connectors, and the thread size of the threaded hole directly affects the pre tightening force obtained by the 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 installation bolt hole is too large, plastic annular compression will occur on the surface of the connected part at the contact with the bolt or nut support surface. Severe compression will reduce or lose the pre tightening force of the bolt, leading to loosening of the nut or bolt.

2. Common ways to prevent loosening of bolts

2.1 Physical device anti loosening measures

(1) Anti loosening device with additional friction force

There are mainly three types of anti loosening devices 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 nut. After tightening the two nuts against each other, the contact surfaces between the upper and lower nuts and the bolt threads are opposite, so that the screw threads are always subjected to additional pressure and friction, as shown in Figure 2. When the working load changes, the frictional force still exists. Its structure is simple and suitable for connecting fixed devices that are stable, low-speed, and heavy-duty.

 

Spring washer tightening structure: Spring washers are widely used in assembly sites due to their simple structure, convenient use, and low cost, and are generally used in conjunction with bolts or nuts. After tightening the bolt or nut, the elastic reaction force generated by the cushion ring flattening causes the threads to be tightened. At the same time, the tip of the washer's slanted mouth also has an anti loosening effect on the supporting surface between the bolt or nut and the connected component. However, this tightening structure has poor anti loosening effect under vibration and impact loads, and is generally used for unimportant connections.

Self locking nut tightening structure: There are generally two types of tightening structures, one is all metal locking nuts. It mainly involves slotting the upper end of the metal nut body and performing closure treatment or non circular closure treatment. After treatment, the nut thread will undergo local deformation. When the nut is tightened, the closure will expand and use the rebound force of the closure to compress the screw threads. This type of nut has a simple structure, reliable anti loosening, and can be loaded and unloaded multiple times without reducing its anti loosening performance. But it is not suitable for high-speed running parts. Another type is non-metallic insert locking nuts, which mainly embed a non-metallic insert (usually a nylon washer) at the upper end of the metal nut body. After tightening the nut, the unthreaded nylon ring will be screwed in and squeezed out of the thread, forming a stable and high torque friction force. This type of material has excellent anti loosening ability, especially in environments with multiple vibrations and impacts.

(2) Mechanical anti loosening device

The main mechanical anti loosening methods include: hexagonal slotted nuts with split pins, series steel wire structure anti loosening, and stop washer structure anti loosening.

Hexagonal slotted nut with split pin: The split pin passes through the groove of the nut and the pin hole at the end of the bolt, and the tail of the split pin is broken open and tightly attached to the side of the nut, locking the nut and bolt, as shown in Figure 3. This method is reliable in preventing loosening and can be used in areas with intense impact and vibration.

 

Series steel wire structure anti loosening: The nut or bolt is designed with a safety hole to thread low-carbon steel wire into it, and the screws are connected in series to brake each other, but attention should be paid to the direction of wire penetration, as shown in Figure 4. Although this method has excellent anti loosening ability, installation and disassembly are very inconvenient. Anti loosening structure of stop washer: Bend the single or double ear stop washer towards the side of the nut and the connected part respectively to lock the nut, as shown in Figure 5. If two bolts require double interlocking, a double locking washer can be used to brake the two nuts against each other. This method has good anti loosening effect and is convenient to use.

 

(3) Adhesive anti loosening device

Apply anaerobic adhesive between the screw threads, tighten the nut, and the adhesive hardens and solidifies, firmly bonding the matching threads to prevent relative movement of the threaded pair, achieving the effect of locking and preventing loosening. Anaerobic adhesives are divided into two types: liquid and dry. Liquid type is generally used for on-site application. Dry type is pre coated and dried in advance for easy operation. Anaerobic adhesives are generally used for assembly parts that do not require disassembly or require minimal disassembly.

2.2 Process methods and anti loosening measures

(1) Fixed torque tool anti loosening

During the assembly process, in order to ensure that the bolts obtain sufficient pre tightening force and are less than the yield point torque of the bolts, using a fixed torque tool for control is the most effective method. The fixed torque tool can set the output torque based on the standard tightening torque of the bolt. When the pre-set tightening torque is reached, it sends a signal or terminates the tightening on its own to ensure stable torque output. In order to improve efficiency, fixed torque tools can be used separately and can also be used in conjunction with small torque pneumatic impact wrenches. For key parts, pneumatic fixed torque wrenches can be used for tightening first, and then manual fixed torque wrenches can be used for testing and confirmation to ensure that the ideal torque value is achieved. The use of this anti loosening method must ensure the accuracy of fixed torque tools, and all fixed torque tools must be regularly calibrated according to the testing regulations of measuring instruments.

(2) Training and anti loosening measures

In the process of tightening bolts, in addition to the output tightening torque of the tightening tool affecting the pre tightening force obtained by the bolts, the assembly method is also crucial, especially for installing bolts with multiple groups and regular distribution. The tightening tools and assembly methods are both used by the operator, and the skill level of the operator directly affects the effectiveness of bolt tightening. Therefore, training and enhancement of the operator's skill level can enhance the ability of bolt anti loosening. The training for operators is mainly based on theory, mainly learning the basic knowledge of assembly, including the operation methods of tightening tools, assembly methods of bolt tightening, etc. The training of operators is mainly based on practical operation, mainly combining the basic knowledge of assembly in the training to practice practical operations. For example, establishing a training platform for operators, especially new employees, to undergo repeated training and master these basic operation methods before they can start work. Through training and training in basic skills, the loosening of bolts caused by incorrect assembly methods can be reduced.

(3) Component quality control anti loosening

The machining dimensions of the components also directly affect the pre tightening force obtained by the bolts, especially the specifications of the threads, which are directly related to the interaction of the connecting components. Therefore, before assembly, it is necessary to check whether the installed components meet the requirements of the design drawings. Standard inspection tools should be used to check whether the specifications, models, and mechanical performance levels of the threaded connectors used meet the design requirements, and whether there are any unqualified phenomena such as loose threads and cracks. Any quality problems that occur should be reported to the manufacturing party for rectification, ensuring that the machining of the component dimensions meets the design requirements and ensuring the reliable connection of the bolts.