The influence of tightening technology on the pre tightening force of bolts
Thread connection is widely used. Although it looks simple, its working process is extremely complex and relevant theoretical knowledge is less. At present, torque method is widely used in the process of bolt connection, which is mainly based on the control torque to ensure the axial preload . However, it is considered that there are elements such as connection material, tightening speed and continuous friction in the process of connection In addition, for places with a large number of bolts, sealing is very important. It should not only guarantee the torque value, but also make it evenly distributed. Because the torque method is affected by a large number of external factors Even if the bolt is of the same technology and different types, the difference of the pre tightening force value is large, which can not meet the conditions. Because the uniform pre tightening force of the bolt is of great significance to the assembly, this paper proposes a star tightening technology to make the pre tightening force of the bolt more uniform.
- 1. Tightening method and pre tightening force of bolts
- 1.1 Star tightening
- 1.2 Block tightening
- 1.3 Expected pre tightening force of each wheel bolt
- 2. Experiment preparation
- 2.1 Test samples
- 2.2 Preload measuring device
- 3. Experimental results
- 3.1 Experimental results of star tightening process
- 3.2 Experimental results of block tightening process
- 4. Conclusion
- Methods for controlling the pre tightening force of screws
1.1 Star tightening
The star tightening process is mainly divided into two steps: ① in the pre tightening process of the first three rounds, by increasing the pre tightening force value of diagonal bolts, gradually achieve 100% of the target value; ② by the minimum two rounds of tightening process to make the pre tightening.
The tightening force tends to be stable. The tightening process is shown in Figure 1. In Figure 2, the solid part is the bolt number, and the dotted part is the marking sequence (pre tightening is carried out according to the sequence from small to large).
Fig.1 Starting htening process
In the first stage, screw down to 20% – 30% of the preset torque value. In this paper, the value is 25% of the preset torque value. The tightening sequence of bolts is 1-3-5-2-4-6;
In the second stage, screw down to 50% – 70% of the preset torque value. In this paper, the value is 60% of the preset torque value. The tightening sequence of bolts is 1-3-5-2-4-6;
In the third stage, tighten to 100% of the preset torque value, and the bolt tightening sequence is 1-3-5-2-4-6;
In the fourth stage, screw down to 100% of the preset torque value, and operate according to the clockwise sequence. The bolt pre tightening sequence is 1-2-3-4-5-6;
In the fifth stage, tighten to 100% of the preset torque value, and operate according to the clockwise sequence. The bolt pre tightening sequence is 1-2-3-4-5-6.
1.2 Block tightening
This process is originated from ASME, characterized by simplicity and reliability. It has been widely used due to its low working strength and high assembly efficiency. The tightening method is to tighten three bolts first, and then tighten them in sequence. Here it is temporarily called block tightening process. The tightening sequence is shown in Figure 2 (from small to large).
Fig.2 Block htening process
In the first stage, screw down to 20% – 30% of the preset torque value. In this paper, the value is 25% of the preset torque value. The tightening sequence of bolts is 1-3-5;
In the second stage, screw down to 50% – 70% of the preset torque value. In this paper, the value is 60% of the preset torque value. The tightening sequence of bolts is 1-3-5;
In the third stage, tighten to 100% of the preset torque value, and the tightening sequence of bolts is 1-3-5;
In the fourth stage, tighten to 100% of the preset torque value, operate according to the clockwise sequence, and the pre tightening sequence is 1-2-3-4-5-6;
In the fifth stage, tighten to 100% of the preset torque value, operate according to the clockwise sequence, and the pre tightening sequence is 1-2-3-4-5-6.
1.3 Expected pre tightening force of each wheel bolt
Load by star tightening and block tightening respectively. It is known from the relevant data table that the torque value of M10 bolt installation selected in this experiment is 72n. M, and the required pre tightening torque value of each wheel is shown in Table 1.
Combined with the TBM prototype of Jilin water diversion project construction site, the experimental model in this paper is reduced and reconstructed by the similarity theory. The model material is Q345, and the bolt size is M10 bolt (grade 10.9). The model structure is shown in Figure 3, and the actual object is shown in Figure 4.
Fig.3 Flange full assembly model
Fig.4 Flange full assembly entity
Xite torque wrench is a bolt tightening tool in this experiment. Its accuracy can reach 1%. It mainly collects and precisely controls the sample loading data through MTO sensor (cmk1-10t). The specific test object is shown in Figure 5.
Fig.5 Schematic diagram of detection system
Before all bolts are loaded, the collection system is cleared first, and all bolts are loaded in each stage according to the method mentioned above by using xite preset torque wrench. In the process of Bolt Preloading experiment, the loading conditions of each bolt in each stage are statistically analyzed, and finally the preloading effect of a single bolt on other bolts is obtained The specific experimental results are as follows.
3.1 Experimental results of star tightening process
Under the star tightening mode, the corresponding pre tightening torque values of bolts at each stage are shown in Table 2.
According to the data in Table 2, after the bolts are pre tightened in five stages, the pre tightening torque values are all above 95% of the preset values, and the tightening effect is good. After summarizing the relevant data, the change of pre tightening torque values in the corresponding stages of the bolts is shown in Figure 6.
Fig.6 The change of pre tightening torque values in the corresponding stages of the bolts
It can be seen from Fig. 6 that after pre tightening in each stage, the pre tightening values of No. 5, No. 3 and No. 1 corresponding to star pre tightening mode are higher than those of No. 6, No. 4 and No. 2. The pre tightening sequence may be 1-3-5-2-4-6. When pre tightening bolts No. 6, No. 4 and No. 2, the pre tightening torque of bolts No. 5, No. 3 and No. 1 decreases, and then the pre tightening torque of bolts No. 5, No. 3 and No. 1 decreases The torque value is smaller than that of the late pre tightening. In the pre tightening process of the first four stages, the pre tightening torque value of each bolt is quite different. However, after the pre tightening in the fifth stage, the torque value of all bolts tends to be uniform, which basically meets the requirement of above 95% of the pre-set value. Therefore, this tightening method perfectly realizes the pre tightening standard of bolts. However, considering this tightening method Each bolt needs to be loaded in 5 stages, resulting in loading up to 30 steps, which brings great workload to the staff. Therefore, it is only suitable for those places with high requirements for pre tightening strength
3.2 Experimental results of block tightening process
In the block preloading mode, the torque values in the loading process of each stage are shown in Table 3.
It can be seen from table 3 that after five stages of tightening, the torque value of all bolts is 90% – 95% of the preset value. Compared with star tightening process, the pre tightening torque value of block tightening process is slightly reduced, but the pre tightening effect also meets the requirements. After the analysis of the results, the pre tightening torque value of all bolts changes with the load of each stage as shown in Figure 7.
Figure.7 The pre tightening torque value of all bolts changes with the load of each stage
It can be seen from Figure 7 that in the block pre tightening mode, if only bolts 1, 3 and 5 are tightened in the first three stages, the torque values of adjacent bolts 2, 4 and 6 will also change slightly.
In the fourth stage of preload application process, although bolts 6, 4 and 2 are all 72n · m, the change is very large, but because in the first, second and third stages, the preload of the other three bolts, the torque value of all bolts is relatively uniform as a whole, but the torque value of the bolts is very significant in the degeneration stage, only 87% of the preset torque value is tightened in five stages After that, the torque value of all bolts tends to be uniform, 90% of the preset torque value, but all of them are below 95% of the preset torque value, which basically meets the pre tightening demand in the assembly process. Because in this tightening mode, the first, second and third stages do not need to tighten the bolts 6, 4 and 2, so the number of tightening is small, only 21, which is star Therefore, block preloading is suitable for the place where the expectation of preloading strength is not high.
In this paper, the flange is taken as the research object. According to the way of block pre tightening and star pre tightening, the bolt pre tightening experiments are carried out respectively. According to the change of bolt torque value in the process of flange assembly, the advantages and disadvantages of the two tightening methods are compared, and the suitable working occasions of the two methods are pointed out. From the results, it can be concluded that the torque value of all bolts in the star pre tightening is the predetermined value Over 95% of them, the pre tightening effect is very good, and the main application places are those places with high pre tightening demand; for the block tightening method, the pre tightening torque value of all bolts is 90% – 95% of the preset value. Although the tightening result is not as high as the star tightening method, the main advantage of the block pre tightening is that the number of operation steps is small, only 70% of the star pre tightening method. For that Some places where the expectation of preload is not strong can greatly reduce the work intensity of the staff.
Methods for controlling the pre tightening force of screws:
Method 1: control the preload by tightening torque
There is a linear relationship between the tightening force and the pre tightening force of the bolt. If the tightening torque is controlled, the pre tightening force can be obtained by experiment or theoretical calculation. However, in practice, due to the influence of friction coefficient and geometric parameter deviation, the pre tightening force changes greatly under certain tightening torque, so the accuracy of controlling bolt pre tightening force by tightening torque is not high, its error is about ± 25%, up to ± 40%. Generally speaking, the tools with high precision of tightening torque in control area are torque measuring wrench and force limiting wrench.
Method 2: control the preload through the angle of nut
The pre tightening force can be controlled by measuring the nut angle when the nut angle is tightened according to the required pre tightening force. The simple way to measure the angle of nut is to carve a zero line, and measure the angle of nut according to the number of directions that Lu Mu turns. The measurement accuracy of the angle of nut can be controlled within 10 ° – 15 °.
Method 3: control the preload by the bolt elongation
Since the elongation of bolt is only related to the stress of bolt, the influence of variable factors such as friction coefficient, contact deformation and deformation of connected parts can be excluded. Therefore, high precision can be obtained by controlling the pre tightening force through the elongation of bolt. This method is widely used in the pre tightening force control of bolt connection in important occasions.
Method 4: control the preload through the hydraulic stretcher
Use the hydraulic stretcher to apply the tension force to the bolt to extend the bolt, and then screw in the nut to unload the load. As the bolt shrinks, the pre tension force equal to the tension force can be generated in the connection. This method can improve the control accuracy of preload. There is no friction force when the hydraulic stretcher applies the preload to the bolt, so this method is suitable for any size of bolt, and it can apply the preload to a group of bolts at the same time, uniformly compress the nut and gasket, so as not to affect the precise control of the preload due to the inclination.
Method 5: using torque angle to control the preload
Using the relationship between tightening torque and angle to control the pre tightening force is to apply a certain torque to the bolt, then make the nut turn a certain angle, and check whether the torque and angle meet the due relationship to avoid insufficient or excessive pre tightening.
The torque angle method to control the pre tightening force is as follows: first, control the tightening process with the tightening torque until the tightening torque value reaches enough to ensure that the nuts, bolts and connected parts are really tight, then start to measure the nut angle, and then control the tightening process with the nut angle and tightening torque at the same time. By using the information given by the tightening torque and the angle of the nut, the pre tightening force of the bolt can be accurately controlled, and the phenomenon of insufficient or excessive tightening may be found in the installation process.
Source: China Fasteners Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(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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