Tightening test of threaded fasteners

Descriptors: Threaded Fasteners Tighten Tests Preface Threaded fasteners are the most common couplings in mechanical products. Bolts and nuts are the most versatile parts of threaded fasteners. The structure of the threaded fasteners is not very complicated, and the manufacturing and assembly does not seem to be surprising. But countless quality incidents continue to remind people not to look like simple threaded fasteners. Manufacturing and assembly are the two key factors that affect the quality of threaded fasteners. In a sense, the impact of assembly quality on threaded fasteners is even greater than the quality of their manufacturing. With the miniaturization of mechanical parts and the increase in connection requirements, assembly quality has attracted more and more attention. How to make the actual tightening force of the threaded fastener close to the theoretical tightening force (ie, the fastening effect) accurately or accurately is the most concerned and studied topic.

1 Tightening force of threaded fasteners The tightening force of threaded fasteners P. is generally achieved by controlling the torque M, which is based on the following relationship between P. and M: MO.OOlPMtO.ies/dM+ O+RM/dM) Table 1 Four kinds of threaded fasteners assembly method Comparison of fastening methods Assembly method Tightening force error Simple torque method Elastic region torque + Corner method Yield region torque + Corner method Elongation measurement method Yang Qi: Nanjing Automotive Group Co., Ltd. Technology Center.

f Friction coefficient Rm nut or threaded fastener head bearing surface average radius mm Obviously, it is very inaccurate to use the moment M to control Po. Because the relationship between the two contains a very large and difficult to determine the friction coefficient f. It is affected by the thread surface and seat surface roughness, lubricant, tightening speed, tightening tools, temperature changes during repeated tightening, etc. The influence of uncertain factors, the true tightening force is very scattered, the fluctuation limit is about ±40%. Analysis of the causes of various threaded fastener damage, found that the correct design, process and material qualified products, mostly due to loose threads Caused by. Looseness is caused by the insufficient tightening force of the actual fastener under various external forces (although the torque wrench has ensured the theoretical fastening force) or the relative sliding between the threaded fastener and the connected member. That is to say, the inconsistency between the actual fastening force and the theoretical fastening force of the mechanical component by the simple torque method affects the fastening effect of the threaded fastener. Therefore, this method of assembling by torque is used for the joint of general mechanical parts, and it is likely to be problematic if it is used in a mechanical connection subjected to high alternating stress. Obviously, precise control of the tightening force is the best way to improve the fastening effect of the threaded fasteners. The tightening test is an important means and prerequisite for the development of a precise tightening process (ie, optimization of the tightening process) and the precise control of the tightening force.

If Cl and c2 are respectively the rigidity of the threaded fastener and the connected member, X is the elongation when the threaded fastener is tightened, V is the compression amount when the fastener is fastened, and P. is the thread tight The tightening force of the firmware near the yield, the axial displacement of the nut (or threaded fastener) should be, then the rotation angle of the nut (or threaded fastener) is. , 2, 3 are relatively determined values ​​that do not change much, so e is also a certain value. Since the change of C| and c2 is not large, controlling the rotation angle of the nut (or threaded fastener) can ensure the consistency between the actual tightening force and the theoretical tightening force. Since the change in p near the yielding region is relatively small, the yield region torque + corner method has a smaller dispersion than the simple torque method.

It is obvious from the above analysis that if the deformation amount is used directly. To control p. is the most reliable way. This is theoretically easy to make, but due to the need for special measuring devices, the actual operation is very cumbersome, not to mention the amount of elongation that some screws cannot measure at all.

Therefore, although the accuracy of this assembly method is very high, the actual use of the table 2 two kinds of torque + angle method of comparison assembly method advantages and disadvantages elastic region torque + corner method can achieve the predetermined tightening force is not suitable for short corners Bolts (the maximum elasticity of the bolts is higher than that of the simple torque flanges.) By controlling the final torque, thread bites can be avoided, and the length of broken teeth is less than 0.05mm. Yield zone torque + corner Faro Provides accurate tightening force values ​​(according to tensile strength) to ensure that all bolts have the same locking stress requirements. The tensile strength of the bolts is less than minor. Use electronic equipment that can be accurately controlled to live on the corners of the bolts without increasing the accuracy. Short bolts for small corners (maximum elasticity of bolts, full use of bolts with a bearing capacity of less than 0.05 mm) Table 3 Preloading effect of two assembly methods Assembly method Maximum tightening force kN Minimum tightening force kN Elastic region torque + Corner method yield area torque + corner method torque (Nm) is not much.

Based on the analysis of the above tightening forces, there are several different methods of assembling the threaded fasteners.

2 Common Threaded Fastener Assembly Methods There are four common methods for assembling threaded fasteners: the torque in the elastic region + the torque in the yielding (plastic) region of the corner + the corner method. The tightening effect is shown in Table 1. The tightening process can be easily established by using the results of the tightening test (especially the tightening force-turn curve, the torque 41 angle curve and the elongation-tightening force curve). The simple torque method is relatively simple. First, the designer determines the tightening force P. required for the threaded fastener. Then, according to the tightening force P. Find the corresponding angle ctTcm on the tightening force-turn curve and finally according to the r torque. - The corner curve is found on the corresponding torque M. This torque M is the assembly torque. The determination of the torque + angle method is more complicated than the simple torque method: first, according to the tightening force required for the design, the tightening test is done. Find the corresponding corner CXtot on the Lariti angle curve; find the a1 corresponding to the pre-torque on the torque-turn curve; the effective angle aEFFTar-a of the actual assembly. When assembling, first press the threaded fastener Torque a, tighten, then turn the aEFT angle.

The difference between the elastic region torque + angle method and the yield curve torque + angle method is that the former is designed to be in the elastic region of the tensile curve of the threaded fastener, while the latter is designed to be in the yielding region.

However, the assembly results of the two methods are different from the requirements for threaded fasteners and assembly equipment. Table 2 compares the advantages and disadvantages of the two assembly methods.

Table 3 shows the tightening force and statistical analysis results of the two methods of a bolt. It can be seen from Table 3 that the tightening force of the bolt in the yield zone is very concentrated, 3 (7 is only 3.2% of the mean value. The tightening force of the elastic region is more dispersed, and its 3cr/Mean = 15.3%. Therefore, the yield region torque + The assembly accuracy of the corner method is higher than the elastic region torque + corner method.

This is a basis. =.1.|, a method of directly using the elongation of a threaded fastener to control the fastening force. Therefore, the assembly precision is extremely high, and the fastening force during assembly completely conforms to the expected force of the design. Since it is difficult to measure the elongation of a threaded fastener, it is expensive, and this assembly method cannot be used for production until a simple elongation measurement method is found.

The tightening test is also required before the assembly by the elongation measurement method, and the tightening force is applied. According to the fastening force provided by the designer, the corresponding elongation is found on the fastening force curve, and the fastening force of the threaded fastener is controlled by measuring the elongation of the threaded fastener.

3 tightening test principle of threaded fasteners The tightening test principle of threaded fasteners is as follows: the motor is used to drive the tightening device (such as the sleeve) to tighten the threaded fasteners, and the force sensor, the angle sensor and the torque sensor are used to measure the thread. Fastening force of the fastener, angle of rotation (rotation angle), torque (torque of the threaded part, torque of the head support surface and total torque), coefficient of friction (friction coefficient of the threaded portion, friction coefficient of the head support surface, and Total friction coefficient). The sensor's signal is input to the computer through AD conversion, and the computer processes the tightening force curve, the torque-turn curve and the statistical processing data of the tightening force, torque and friction coefficient by processing with appropriate software.

In addition, using the hydraulic principle, a long force curve of the fastening force is made by the force sensor and the displacement sensor.

4 threaded fastener failure analysis case An engine crankshaft pulley bolt, class 12.9, surface galvanized, assembled by torque method. U-turn occurs when in use. After changing the galvanizing treatment for DACRO. However, there has been an "extension" phenomenon.

In addition to tightening the crankshaft pulley, this bolt also requires better anti-loose ability. That is, there is a requirement for a large frictional force between the bolt head supporting surface and the crank head end surface. In other words, this bolt must take into account both the pre-tightening and anti-loose functions.

The metallographic structure, tensile strength and hardness of the bolts are in accordance with the technical requirements, but the fractures of the bolts are characterized by hydrogen embrittlement.

The bolt tightening test results show that the tightening force of the DACRO bolt is 31.7% higher than that of the galvanized bolt under the same torque. Since the assembly torque is determined according to the friction coefficient of the galvanized bolt, if the DACRO bolt is used, it will not change. As the assembly torque is reduced, the tensile force of the bolt is increased by more than 30%, and the yield strength of the bolt may be reached or exceeded, and the bolt may be plastically deformed. The bolt tightening test results also show that under the same tightening force, the friction coefficient of the galvanized bolt is 56% higher than that of the DACRO bolt. If the assembly torque of the DATRO bolt is changed, the tightening force of the galvanized bolt is achieved. Since the frictional force is proportional to the positive pressure, the friction between the bearing surface of the bolt head and the end surface of the crankshaft head does not meet the requirements for anti-looseness. It can be seen that the original design adopts galvanizing treatment because it has a large friction coefficient, which is beneficial to increase the self-locking ability.

Based on the above analysis, three solutions are proposed: Option 1: use galvanized bolts, but strengthen dehydrogenation; Option 2: Use DACRO bolts, but require friction coefficient scheme 3: use DACRO bolts, and take anti-loose measures.

Finally, using the first option, the problem is solved.

5 Conclusion The tightening test can provide a reliable basis for the assembly process of various threaded fasteners. The four methods of screw fastener assembly are based on the results of the tightening test. The four assembly methods have their own advantages. The specific operations should consider the fastening requirements, equipment conditions, the quality level of the threaded fasteners and the cost. In addition, the tightening test also contributes to failure analysis and quality improvement of threaded fasteners. The tightening test has been widely used abroad, and a few companies in the country have carried out this work. It is believed that with the continuous development of the machinery industry, the tightening test will be recognized by more people, and the tightening test will become an indispensable test in the mechanical performance test.

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