Due to space constraints, many connection structures cannot be tightened using the torque method to achieve the required performance requirements.
The joint shown in Fig. 1 consists of two steel pieces and an intermediate crosshead, clamped together by an M12 hex flange bolt and nut.
Fig. 1 Based on the analysis of the forces acting on the joint, the minimum clamping force requirement for the joint is 50 kn, taking into account the loss of embedding and the influence of axial forces. After a series of tests, it was determined that the achievable clamping force scattering by the torque method was between 25 kN and 40 kN. This is shown in Figure 2 below.
Fig. 2 Due to the large dispersion of the torque method, it is difficult to meet the influence of the required axial force. Based on experience, decide whether the tightening of the investigation angle method meets the requirements?
Angle tightening is a two-stage process, with the first step specifying the initial tightening torque, also known as the tightening torque (the purpose of which is to "tighten" the joint's connectors into metal-to-metal contact). The angle of rotation of the bolt nut is then defined, which causes the bolt to be stretched to the elastic limit to the yield point, a process that eliminates the scattering of clamping forces due to friction.
In order to define the initial torque-what should the angle be? A torque-angle data is usually required. A typical plot of this data is shown in Figure 3.
The point in Figure 3 that deviates from the straight line is an indicator of the beginning of yielding. In the case shown in the figure, yielding occurs after rotation to approximately 200°.
Because the unevenness of the connector affects the angle of rotation, the right torque is required to fit the connector together.
According to the information provided in the graph, an initial torque of 15 Nm and a rotation angle of 180° are defined.
A series of tests were carried out using a set of 5 bolts, and the results are shown in Figure 4, the axial force at the yield point is about 65kN, and the minimum clamping force to meet the requirements is 50kN, so the tightening process of 15nm+180° is recommended.
Figure 4: The fitting may need to be tightened again due to the need for subsequent repairs. The effect of re-tightening on the performance of the joint is investigated.
Figure 5 shows the axial force curve of a bolt that is tightened twice following the same angle process.
Figure 5 To illustrate the situation, the second curve has moved along the x-axis. The diagram shows an interesting feature of re-tightening the bolt to the plastic area.
The yield strength at the second tightening is higher than the yield strength at the first tightening. Stressing a ductile material beyond its yield point will increase the yield strength for the next cycle.
In order to further investigate, the same bolt was tightened several times according to 15nm+180° to observe the test situation. Figure 6 illustrates the results of this test.
Fig. 6 At the end of the 10th tightening application, the bolt fails. In this graph, in order for each curve to be comparable to its predecessor, the curves have been continuously moved along the x-axis, and it can be seen that the ten curves are similar in shape to the torque-turn curve shown in Figure 3.
Since there must be at least 50 kn in the bolt in this application, it is obviously not desirable to retighten the bolt 10 times.
As the number of tightening times increases, the process has exceeded the highest point, so it is recommended to reuse it no more than twice.
In practical application, when the torque method cannot meet the design requirements, the angle method can maximize the utilization of bolts, which is another advanced means to solve engineering problems.
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