Research on Checking Method of Bolt Preloading Force
The main research is to study the force form of bolts. According to different bolt connection forms, theoretically study the force between various contact surfaces. It is found that different bolts and different mounting stiffness provide different bolt friction. A new bolt calibration and selection calculation method is proposed and verified by CAE simulation. It can be seen from the analysis results that the design method is more suitable for practical application and provides a more reliable method for checking the tightening torque of the bolt.
In order to improve the manufacturing, installation, transportation, maintenance level and productivity of the machine, various connections have been widely applied in machine manufacturing.
There are two main types of mechanical connections: one is when the machine is working, the connected parts can have a relative motion connection, called a mechanical dynamic connection, such as various motion pairs; the other is connected when the machine is working. Relatively moving connections are not allowed between parts, called mechanical static connections. Mechanical static connections can be divided into three categories according to their working principle: shape-locking connection, friction-locking connection and material-locking connection.
The bolt connection studied in this paper is a friction-locking connection method. The working principle is that the friction of the connected parts is generated by the friction of the connected parts, and the relative movement of the connected parts is prevented to achieve the connection purpose.
Bolted joints are widely used due to their advantages of easy assembly, fatigue resistance, removable, integral integrity and stiffness. The bolted connection must be tightened during assembly and pre-stressed. This pre-energizing force is called pre-tightening force. The role of the preload is to create a certain amount of friction between the joint surfaces of the joints to resist lateral loads. This requires that the maximum friction generated between the joints must be greater than or equal to the lateral load.
Therefore, the selection of the pre-tightening force of the bolt is particularly important. The pre-tightening force is small, the friction generated is not enough to resist the lateral load, and the connecting member will be relatively dislocated, failing to achieve the purpose of pre-tightening. Will be sheared; the pre-tightening force is too large, the prestress of the bolt is large, and it will also affect the bolt stress.
A lot of books and literatures have been studied on the force analysis of bolts, which basically provide the same friction force for each friction surface to check roughly. In fact, due to the different rigidity of the joints of the screw inspection, the entire mechanism is not deformed by the force deformation, so the frictional force provided by each friction surface is also different. What kind of relationship between them causes uneven force, which is the need for further research.
1. How to connect the bolts
A common bolt connection is shown in Figure 1. The thickness of the upper and lower connectors in Figure 1 (a) means that the stiffness of the two connectors is different; the thickness of the upper and lower connectors in Figure 1 (b) is the same, indicating the stiffness of the two connectors; Figure 1 (c) indicates that there is only one connector.
Figure 1 Common bolted connections
2. Force analysis of bolted joints
We know that a force F N can provide the maximum friction force f max = μF N for any friction surface, but when the stiffness of the joint is not the same, the friction surfaces will not reach the maximum friction at the same time. This will be studied separately below.
(1) When the rigidity of the connecting member A and the connecting member B are different [as shown in Fig. 1 (a)], that is, K A > K B .
Since the rigidity of the connecting member A and the connecting member B are different, when a load is applied to the connecting member C, assuming that the deformation amount of A and B is ΔS, the connecting members A and B are necessarily caused to have different reaction forces of different sizes.
Since K A > K B , the contact surface between the connecting piece A and the connecting piece C first reaches a critical frictional force, that is, f 1 =f max =μF N , and at this time, the friction between the connecting piece B and the connecting piece C The critical friction force f max is not reached, so the connection will first slip on the friction surface between the connecting member A and the connecting member C, and the frictional force generated between the friction surface between the connecting member B and the connecting member C is f 2 = kμF N , k is the stiffness coefficient related to the stiffness of the two joint faces KA and KB.
f=f 1 +f 2 = (1+k)μF N
(2) When the rigidity of the connecting member A and the connecting member B is the same as [Fig. 1 (b)], that is, K A = K B .
The connector A has the same rigidity as the connector B, and when a load is applied to the connector C, the connectors A, B are subjected to the same magnitude of reaction force. At this time, the frictional force in the critical state of the bolt connection can be expressed by the following formula:
F max =f 1 +f 2 =2μF y
(3) When there is only connector A [as shown in Figure 1 (c)], that is, KB=0. The expression of the load F max in the critical state at this time is as follows:
F max = f 1 = μF y
3. CAE simulation analysis
(1) When the rigidity of the connecting member A and the connecting member B are not the same, that is, K A > K B , the simulation analysis result is as shown in Fig. 2 .
Figure 2 Connector A and connector B have different stiffness (unit: N)
It can be seen from Fig. 2 that when the friction surface 1 reaches a critical frictional force of 6 000 N, the frictional force of the friction surface 2 is only 2 000 N, which has not yet reached a critical state, and the resultant friction force is 8 000 N. .
Corresponds to the friction provided by 1.33 friction surfaces. The frictional force of the friction surface has a direct relationship with the stiffness of the joint. The greater the stiffness, the greater the friction contribution.
(2) When the rigidity of the connecting member A and the connecting member B is the same, that is, KA = KB, the simulation analysis result is shown in Fig. 3.
Figure 3 Connector A and connector B have the same stiffness (unit: N)
It can be seen from Fig. 3 that the frictional surface 1 and the friction surface 2 simultaneously reach a critical frictional force of 6 000 N, and the resultant frictional force is 12 000 N. The two friction surfaces provide the same friction at the same time.
(3) When there is only connector A, that is, KB=0, the simulation analysis result is shown in Fig. 4.
Figure 4 Only connector A (unit: N)
It can be seen from Fig. 4 that when the friction surface 1 reaches a critical frictional force of 6 000 N, the frictional force of the friction surface 2 is almost zero, and the resultant frictional force at this time is 6 000 N. Equivalent to the friction provided by only one friction surface.
4. Actual case calculation
Figure 5 Comparison of installation schemes (unit: N)
Figure 5 is a schematic view of the mounting point in front of a sub-frame. The first step is to install the bushing of the shim and the sub-frame on the girder by bolts. The support of the bushing is similar to the cantilever beam structure.
Option 2 adds a bushing reinforcement bracket structure that forms a structure similar to a simply supported beam. From the perspective of stiffness, the structure of Scheme 2 is significantly better than that of Scheme 1, and the effect of these two structures on bolt fastening can be analyzed by CEA.
Figure 6 CAE analysis model
6 is a schematic diagram of the CAE analysis model of the first scheme and the second scheme. In the model, part of the body girders is taken as the upper mounting surface of the bushing, and the mounting pipe string of the bushing is selected as the connected component, and the mounting surfaces are all contact-treated. A preload of 60 000 N is applied to the bolt to simulate the clamping of the sub-frame bushing by the bolt, and a radial displacement is applied to the bushing to extract the friction of the upper and lower mounting faces. The results of the analysis are shown in Figure 7.
Figure 7 Friction curve of the mounting surface (unit: N)
As can be seen from Fig. 7, the frictional force is basically provided by the upper mounting surface before the bushing is installed with the reinforcing bracket, and the lower mounting surface provides friction only when the upper mounting surface slides.
When the bushing is installed with the reinforcing bracket, the upper and lower mounting surfaces provide friction. When the relative sliding occurs, the total friction increases by 48.47%, which improves the fastening performance of the bolt.
5 Conclusion
It is difficult to carry out accurate calculations by using the traditional theoretical calculation method to select the pre-tightening force of the bolted joint.
This paper introduces a more realistic bolt pre-tightening method, which is more scientific and reasonable through modern CAE simulation analysis technology, and also provides a more reliable and practical theoretical basis and guidance method.
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