Manufacturing technology Anti-loose treatment of threaded fasteners during assembly of electric locomotives 佘 Dan (Zhuzhou Electric Locomotive Works, Zhuzhou 412001, Hunan) Measures for loosening of fasteners.
With the continuous improvement of railway locomotive speed requirements for electric locomotives, the quality of threaded connections is increasingly important for the safe operation of trains. In the assembly work of electric locomotives, the threaded connection is an important process, and the anti-loose treatment of the threaded fasteners is also very necessary.
1 threaded connection features and requires threaded connection to obtain a large connection force, easy to assemble and disassemble, low cost, and interchangeable, widely used in electric locomotives. With the development of electric locomotives in the direction of high speed, high power, precision, small size and light weight, higher requirements are placed on the performance and fastening work of threaded fasteners: the performance of threaded fasteners is predictable and the dispersion is small. The relationship between tightening torque and bolt elongation is accurate and stable; the nut will not loosen after tightening, will not be killed when disassembled, and can be reused.
2 Threaded fasteners pre-tightening 2.1 Determination of pre-tightening force value Most threaded joints are used to clamp two or more connected parts together with threaded fasteners to resist various external loads, so that they are connected The parts do not separate, do not slip, or the joint surface does not leak. To this end, before applying the external load, the threaded fastener is tightened to clamp the coupled piece, which is called pre-tightening, which is said to be the pre-tightening force (F).
Preloading increases the reliability of the bolted connection and enhances the tightness and rigidity of the joint. A higher preload is beneficial to the reliability of the connection and the life of the connected parts, especially for the connection required for the seal. The greater the initial preload, the more the number of vibrations required to reduce the frictional force (the helical pair and the bearing surface) to the critical value and the looseness of the nut, the greater the vibration intensity (the greater the power). In fact, in some cases, the nut will never loosen as long as the initial preload is large enough. Generally, the preload force F is as follows: the above is the yield limit of the bolt material, MPa; As is the nominal stress cross-sectional area of ​​the thread, mm2. 2.2 The bolts of the pre-tightening bolt set of the bolt group will affect each other when the bolts are tightened. The characteristics of the interaction depend on the structure of the joint (thickness, shape, bolt spacing, etc.) and the tightening side. The tightening sequence for minimizing the mutual influence between the bolts is to first tighten the bolts closest to the centroid of the joint surface and then screw them out from the center line in a spiral shape; the flange joints should be tightened symmetrically with respect to the center of the flange. . When the bolt set is tightened, it cannot be screwed to the predetermined pre-tightening force once, but should be tightened several times. Multiple tightening, pre-tightening force can be more consistent, usually 2 to 4 times to the specified pre-tightening force is appropriate.
2.3 The consequences of improper pre-tightening force The static damage of the threaded connection parts. Under excessive preload, if the control is improper or accidental overload, the threaded fastener will be statically damaged, causing the thread to be sheared (tripped) or the bolt broken. The coupled part may be crushed, bitten and twisted. Or break, causing the connection to fail. Usually the maximum preload stress F0mai is less than 73% of the yield limit. The joint is slipped, separated or the fastener is loose. For ordinary bolted joints subjected to lateral loads, if the pre-tightening force is insufficient, the connected parts will slip, causing the joints to be misaligned, skewed, wrinkled, and even the fasteners to be cut; for bolted joints subjected to axial loads, Insufficient preload will cause the joint to leak and even the two joints to separate. In addition, insufficient preload will also cause strong lateral vibrations, causing the nut to loosen.
Bolt fatigue damage. Most bolts fail due to fatigue. Although reducing the preload force can reduce the average value of the total load of the cyclic change on the bolt, it increases the load amplitude, so the total effect is mostly to reduce the fatigue life of the bolt.
Increase product quality and cost. If the pre-tightening force is too small, more and larger fasteners are needed, and a larger connected part is often required, thereby increasing the product quality. At the same time, the cost of many products is proportional to the number of parts that need to be assembled, so too little pre-tightening will result in increased manufacturing costs and maintenance costs.
2.4 Pre-tightening control method The pre-tightening force should be determined by the following factors: the force form of the bolt, whether the bolt is subjected to the variable load, whether there is a sealing requirement for the connection, the accuracy of the installation tool and the installation method, and whether the connection location is Easy to install, etc.
The pre-tightening control method and its error are shown in Table 1. Table 1 Pre-tightening force control method and its error control method Error range/% tightening torque method Nut twist angle method Bolt extension method Special indication gasket method Use hydraulic tension The control method has no accurate value. According to the tightening torque and the nut rotation angle control method, the nut rotation time control method has no accurate value. The resistance strain gauge method is the main method used to control the pre-tightening force at home and abroad. Generally, the torque wrench is used to control the tightening torque. To achieve pre-tightening control.
3 The reason for the looseness of the threaded connection and the proof of the experiment and the actual use prove that the loosening of the nut occurs only in the threaded connection subjected to vibration, and it is easier to loosen under the vibration in the lateral direction (vertical bolt axis direction) than in the axial vibration. Stronger axial vibration, after a long time, may reduce the pre-tightening force by 30%~40%, but usually does not lose all the pre-tightening force and cause the fastener to fall off. The strong lateral vibration can make the pre-tightening force disappear and the fasteners fall off, because the lateral vibration can eliminate the friction between the screw pair and the supporting surface.
Insufficient thread connection is caused by load, vibration and shock, and when the working temperature changes greatly, it may loosen, which can not guarantee the tightness of the connection, or even cause loose connection or premature fatigue damage of each part in the connection. Eventually, a serious accident of the locomotive occurred.
In summary, there are three main ways to avoid the screw connection under the vibration. The nut is automatically retracted. There are three ways to prevent the friction from decreasing below the critical value. Damping the vibration to prevent the excessive vibration from acting on the threaded connection; preventing the nut from rotating relative to the bolt. .
For a specific bolt connection on the locomotive, the looseness may be caused by various factors. Therefore, to prevent loose bolt connection, measures should be taken for each factor, which is to prevent the thread pair from rotating in the loose direction.
The anti-loose measures widely used in the production of electric locomotives can be summarized into the following five types: increasing friction. Spring washers are used as anti-loose components. Common types of washers are: light spring washers, wave spring washers, outer (inner) tooth lock washers, conical spring washers, etc. The counter nut (back cap) is another way to increase the friction to prevent loosening. It is a much more effective method than the spring washer, but this method will lead to an increase in the bolt length and an increase in the number of nuts, thus the joint Increase in size and quality.
Increase the effective torque. With the structural measures, the threaded connection, after the tightening of the nut, generates an additional torque, called the effective torque, in addition to the threading moment and the friction on the bearing surface, to prevent the nut from loosening. With effective torque protection, it is extremely effective, and there are standardized metal lock nuts and nylon insert lock nuts.
Prevent the joint from slipping relative to each other. Take measures on the structure of the joint to minimize relative slip, such as adding pins or keys between the joints.
Use the lock (stop) attachment. With the metal attachment, the position of the nut relative to the bolt by mechanical action, or the position where the nut and the bolt are coupled to each other, can prevent the nut from rotating relative to the bolt, thereby preventing the loosening. This type of attachment is called a locking (stop) attachment. It includes a pin, an elastic locking ring, a stop platen, a clamping plate, a split pin (with a slotted nut), a series wire, a single (double) ear stop washer, and an inner (outer) tongue stop washer.
A meshing force is formed at the fastener contact surface. For example, coating the surface of the bolt head and the nut support surface, forming a tooth pattern on the end face of the spring washer, the nut or the bolt head, and applying an adhesive on the surface of the thread.
The transverse vibration tester is usually used to evaluate the anti-loose ability and reliability of the threaded connection anti-loose device.
4 Conclusion In addition to the screw connection loosening reasons and anti-loose measures mentioned above, we should also pay attention to unpredictable conditions such as human error and tool use error. With the continuous development of electric locomotive technology, the importance of threaded fasteners in the design, manufacture and assembly of locomotives will continue to strengthen. The working environment of electric locomotives will put forward higher requirements for threaded connections, which requires us to further explore.
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