At present, almost all the bolt fastenings used in industry need to control the strength, that is, the so-called torque control torque refers to the use of predetermined torque or predetermined torque and angle for industrial fastening to ensure sufficient clamping force. Ensure the reliability of threaded connections.
Bolt fastening is a very complex physical process, and the most important factors affecting bolt fastening are torque, preload, friction, and material hardness. As long as the above factors are fully considered, safe bolt fastening can be ensured. A torque wrench can control the force applied to a thread fastening, neither less nor more. In most cases, the traditional torque wrench has been able to provide sufficient precision to tighten the bolt.
However, when a more accurate and safer thread fastening is required, a manual torque wrench is not suitable, because the applied torque often does not meet the pre-tightening force requirement and the corresponding preset value, because it is not very accurate. The source of inaccurate values is often caused by the seizure between the tightening threads and the friction between the bolt head and the flat surface of the fastened object.
The so-called pre-tightening force or clamping force is a contact pressure generated by the contact of the workpiece in the screw connection, which is ubiquitous. The pressure makes the friction between the workpieces larger, and the friction makes the torque not fully preloaded, so only about 10% of the torque we apply can be converted into the tightening force of the bolt.
In order to achieve higher precision, even in the operation of manually tightening bolts, angle control tightening technology is often used by people, especially in the current rapid development of automobile manufacturing industry. Through this technology, each bolt can achieve its maximum tightening effect. Rotation angle refers to the angle value between the original tightening of the bolt and the final specified torque value.
Generally speaking, the number of rotation angles will vary depending on the material of the fastener and the fastened part. For example, for materials with high hardness such as carbon steel, the number of rotation angles required for fastening will be relatively small; for materials with low hardness such as wood, the number of rotation angles required for fastening will be relatively large, and at the same time, the force caused by friction The loss will also be hit, and the fastening force that can be achieved is relatively small.
In the thread tightening process of controlling the angle, the bolt is tightened to a fixed torque value at the beginning using torque control, after reaching this torque, the subsequent tightening process is carried out under the dual control of torque and angle until the predetermined value is reached. Set tightening torque and rotation angle. The correct use of the rotation angle control system can prevent the bolt from entering the plastic zone of the material, and prevent the bolt from exceeding the acceptable yield point of the bolt, causing safety hazards. At the same time, the corner control can also significantly reduce the loss of locking force and ensure sufficient pre-tightening force.
During the bolt tightening process, the torque used and the degree of rotation angle are different, so the bolts tightened by the rotation angle control cannot be used again.
There are two main types of bolt tightening methods, elastic tightening and plastic tightening. Elastic tightening generally refers to torque tightening method, plastic tightening mainly includes corner tightening method, yield point tightening method, etc.
1. Torque tightening method
The principle of the torque tightening method is that there is a certain relationship between the torque and the axial preload. Control the preload of the coupled parts by setting the tightening tool to a certain torque value. Under the premise of stable process, parts quality and other factors, this tightening method is simple and intuitive to operate, and is currently widely used.
According to experience, when the bolt is tightened, 50% of the torque is consumed on the friction of the bolt end face, 40% is consumed on the friction of the thread, and only 10% of the torque is used to generate pre-tightening force. Because the external unstable conditions have a lot of influence on the torque tightening method, the torque method which indirectly implements the pretightening force control by controlling the tightening torque will lead to low control accuracy of the axial pretightening force.
Moreover, there are a very small number of bolt connections, the torque has reached the specified value, but the bolt head has not completely fit with the connected parts or the gap is sometimes small, which is not easy to find visually. At this time, the torque value is qualified, but the pretightening force is very small or even non-existent, so in this case, if it is only proposed to ensure that the torque is qualified, then it becomes empty talk to ensure the quality of assembly tightening. Morcato's torque wrench does this very well.
2. Corner tightening method
In view of the shortcomings of the torque tightening method, the United States began to study the relationship between bolt elongation and axial force in the late 1940s. The rotation angle when the bolt is tightened is roughly proportional to the sum of the bolt elongation and the looseness of the tightened part, so the method of achieving the predetermined tightening force can be achieved according to the specified rotation angle.
First tighten the bolt to the initial torque, that is, stretch the bolt to close to the yield point, and then rotate a certain angle to stretch the bolt to the plastic region. The essence of the rotation angle tightening method is to control the elongation of the bolt. The axial preload is proportional to the elongation within the elastic range. To control the elongation is to control the axial force. It is no longer directly proportional, but the mechanical properties of the bolt when it is stretched show that as long as it is kept within a certain range, the axial preload can be stabilized near the yield load.
Therefore, two bolts with different friction coefficients, although the final torque after tightening by the same tightening method is very different, but because the strength and size of the bolts are the same, the pretightening force is not much different. Compared with the torque tightening method, it not only completes the tightening control with high precision, but also fully improves the utilization rate of materials. MORCATO's corner torque wrench can be handled very well, and this effect can be achieved when working.
3. Yield point tightening method
The theoretical goal of the yield point tightening method is to tighten the bolt to just past the yield point. When the yield point is used for tightening, the bolt is first tightened to a specified initial torque. From this point, the equipment monitors the change of the slope value of the tightening curve. If the slope drops to more than the set value, then the bolt is considered to be pulled. When the yield point is reached, the tool stops.
The biggest advantage of the yield point tightening method is that the bolts with different friction coefficients are tightened to their yield points, which maximizes the potential of the strength of the threaded parts, but it is sensitive to interference factors and has extremely high requirements on the performance and structural design of the bolts. high, it is more difficult to control. Therefore, the price of tightening tools is very expensive. The torque wrench series of MORCATO, among the brands of the same quality, still has certain advantages in price, high quality and low price.
