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Influence of bolt assembly location on bolt connection performance: Shanghai giant Bearing Co., Ltd. 2020-04-20 17:18:01

Bolt connection is widely used in modern machinery industry, especially in pressure vessel industry. Its advantages are reliable connection, easy disassembly and maintenance.
The initial pretightening force on the bolt not only ensures the sealing performance of the connection [1], but also makes the connecting parts do not move relative to [2] under the action of external loads. The common failure modes of bolts include fatigue, shearing and loosening of bolts. [3]
The failure of bolts will not only increase the maintenance cost and unnecessary downtime of industrial machines, but also cause serious industrial accidents. [4].  
Good bolt connection performance depends not only on the initial preload force [5G7], the contact surface friction coefficient [8G9] and material [10], but also on the bolt diameter [[11]] and the assembly position between bolt and bolt hole.
Many scholars have done a lot of research on bolt assembly.
Lawlor and McCarthy. [12G13] has studied the influence of bolt hole clearance on bolt connection. The research shows that the assembly gap between bolt and bolt hole will not only affect the load distribution, but also affect the fatigue life of bolts.  
Ma Chuanbao et al. [14] studied the deviation between the axis and the center of the hole, considered the possibility of successfully assembling bolts and bolt holes, and considered the effect of position on the double hole bolted connections.
Mccarthy and Egan. [15G16] studied the effect of assembly clearance on bolted connections. The research shows that the assembly gap will affect the strength, stiffness and fatigue life of bolted joints, but the effect of location is not considered in the study.
At present, the research on the influence of bolt assembly position on connection performance is not perfect. There is no theoretical or experimental quantitative research on the influence of assembly location on bolt connection performance.
The influence of bolt assembly position on the performance of bolted connections is studied, and the influence of relative position of bolts and bolt holes on bolt reliability and connection life is determined.  
1 experience and experience
The bolt connection can be simplified, as shown in Fig. 1, assuming that the external load is transverse shear load, which can be applied to any connection. In order to study the influence of bolt and bolt hole location on bolt connection, an experimental device is designed, which is shown in Figure 2. Three
In experiments, M12 bolts and nuts were used, the pitch was 1.75mm, and the material was 304 stainless steel. The connection material is 304 stainless steel, the thickness is 18mm, the bolt hole diameter is 12.7mm, the maximum clearance between bolt and bolt hole is 0.7mm.
In order to protect the connector's contact surface, copper strips and gaskets are arranged between the connectors and between nuts and connectors.
Among them, the copper strip is coated with lubricating oil.
The gasket material is 304 stainless steel.
Before the experiment, all contact surfaces should be scrubbed with absolute alcohol to remove oil stains and impurities on the surface.
In the experiment, the bolt clamping force is collected by the load sensor with a maximum range of 50kN. The experimental device is placed on Instron 8872, and its maximum tensile load is 25kN, which can meet the experimental requirements. The experiment was conducted at room temperature.
One end of the connector is fixed, and one end exerts periodic dynamic loads.
Relative rotation of bolted connections through nuts
Rotation is used to generate prestress to achieve connection.
In order to facilitate disassembly and assembly and replacement of parts, clearance fit is used between bolts and bolt holes.
In actual assembly, the minimum fit gap between bolt holes can not be controlled as a certain value, depending on the processing method and assembly technology. Theoretically, the most reasonable position of bolt and bolt hole assembly is to ensure that the two axes coincide, so that there is a good alignment between bolt and bolt hole.
In the actual assembly process, due to the uncertainty of assembly tools, processes and methods, the bolt assembly position can not be completely in a theoretical and reasonable position, and there is a certain relative distance between the two axes.
When the bolts are assembled near the surface of the bolt hole wall, one is far away from the lateral load and the other is near the lateral load.
In the experiment, the external load control mode uses displacement control. The displacement of external load is sinusoidal, the frequency is 1 Hz, and the relationship between displacement and time is
Delta \ \ u003d0.45sin2 PI T1
Among them: Delta is tensile displacement, mm; t is loading time s.  
In the experiment, use the torque wrench to tighten the bolts. The torque applied to the load sensor is 7.5kN, and the initial preload of the bolt is 7.5kN. In this experiment, the clamping force of bolts is collected by load sensors to avoid torque wrench.
Errors caused by readings.
When clamping connectors, check alignment between bolt holes.
In order to ensure the reliability of the experimental data collection, the load sensor is calibrated before the experiment to determine whether the relative error of the load sensor data collection meets the experimental requirements.
During the calibration process, the pressure load is applied to the weighing sensor. Calibration data are shown in Figure 4.
In order to better compare the test data of load sensors and test machines, the absolute value of test data will be obtained during data processing.
The calibration data show that the maximum relative error of the sensor is 1.91, and the data follow well, which can meet the experimental requirements and ensure the reliability and rationality of the collected data.  
2 Analysis
2.1 life analysis of bolted connections
Under the action of periodic external loads, the bolt bears the reciprocating shear load, which weakens the clamping force and loosens the bolt.
In the process of bolt self loosening, the relative rotation between bolts and nuts will occur, and the nut will release [16] axially. The pretightening force P0 and clamping force P are very important for measuring the self loosening and connecting life of bolts.
Parameter, the change of bolt clamping force in three assembly positions.
When compared with bolt assembly, when the load is close to the load and the load is far away from the load, the fatigue life of the bolt in the middle assembly process is higher and the connection is more reliable.
Comparing the change of bolt clamping force in different assembly positions, it can be concluded that under the action of external force, when the bolt axis deviates from the bolt hole axis assembly, the fatigue life of the bolt connection is small, and the bolt connection is more likely to loose, which is not conducive to the tight structure.
2.2 working load analysis
As an important parameter to analyze bolt looseness, the working load can reflect the difficulty of overcoming the friction on the connection surface. Greater friction requires greater work load to overcome.
At the same time, the direction of the load will affect the direction of friction on the connection surface.
Workload at different locations
F, cycle number N and load displacement Delta.
The relationship between hysteresis loops and the shape of hysteresis lines changed significantly in the cyclic experiments: the envelope area formed by hysteresis lines at early stage is significantly lagging behind, and the envelope area of the line is larger than that of the lag line at the later stage.
It is explained that as the circulation proceeds, the friction force between the contact surfaces is reduced due to the decrease of clamping force.
If it decreases, the friction work will decrease with the increase of friction force. Cycle number. If the external load displacement is set in the experiment, the external load will be smaller.  
3 results and discussion
Because of the randomness of bolt assembly, the position of bolt in bolt hole is also random, so the axis of the bolt can not be completely coincided.
Ideally, when the bolt is assembled, the axis of the bolt and the axis of the bolt hole coincide with each other to ensure that the bolt holes and bolts are completely aligned.
Full alignment of bolts and bolt holes, as shown in Figure 9a, offset components
DB is the diameter of the bolt; DN is the diameter of the bolt hole; Cmin is the minimum clearance of assembly; Cmax is the largest gap of assembly; Cd is the distance between the bolt axis after assembly and the bolt hole axis; O1 is the center of the bolt; O2 is the bolt hole.
Center of a circle.
The definition is called centering degree to represent the bolt assembly position.
* \ u003d Cmin / delta / 22
In the formula, the value of The is [0,1]; the difference between the bolt radius and the bolt hole radius is the delta.
When bolted connections are subjected to lateral periodic external loads and the load amplitude is small, the larger the bolt is, there is enough space between bolts and bolt holes to meet the relative movement of bolts during cyclic stretching.
There will be shear contact; if the load amplitude is large and the relative sliding between the bolt head and the connector is obvious, after the consumption of Cmin, the bolt will be cut by the connector, and the bolt will contact with the bolt hole.
The smaller the smaller, the worse alignment of bolts and bolt holes. The relative distance between the bolt axis and the center line of the bolt hole is very small. The smaller load amplitude will make the bolt contact with the bolt hole.
The smaller the Cmin, the worse alignment of bolts and bolt holes. Under the same load, the bolt and bolt hole are easier to shear contact, which is not conducive to structural fastening.
Controlling the size of Cmin during assembly can effectively extend the service life of bolted connections and ensure the reliability of connections.  
4 Summary
In order to study the influence of bolt assembly position on loosening of bolted connections, experiments were carried out to design bolted connections.
The experimental results show that when the bolt and bolt hole are completely aligned, under the action of lateral load, the bolt hole wall and bolt are not easy to cut, the connection life is long, and the connection is reliable; the bolt is not aligned with the bolt hole.
At this time, the shear contact between bolts and bolt hole walls is easy to occur, the clamping force loss of bolts is greater, and looseness is easy to occur, and the fatigue life of connections is low.
Therefore, when assembling bolts, by controlling the alignment between bolts and bolts, it can effectively improve the loosening resistance and connection life of bolted connections, and ensure the reliability of bolted connections. To talk about