Bolt tightening requires control of the force, and the three methods commonly used to resolve bolt tightening

At present, the bolt fastening used in almost all industries requires control strength, that is, the so-called torque control torque refers to industrial fastening with a predetermined torque or a predetermined torque and angle to ensure sufficient clamping force. Ensure the reliability of the threaded connection.

Bolt fastening is a very complicated physical process. The most important factors affecting bolt fastening are torque, preload, friction, and material hardness. As long as the above factors are fully considered to ensure safe bolt fastening. Torque wrenches can control the force applied to a threaded fastening, no less and no more. In most cases, traditional torque wrenches have been able to provide sufficient precision bolts. However, when a more precise and safe thread fastening is required, the manual torque wrench is not suitable because the applied torque does not reach the pre-tightening force requirement and the corresponding preset value because it is not accurate. Sources that produce inaccurate values ​​are often caused by the bite between the tightening threads and the friction between the bolt head and the plane of the fastening object. The so-called pre-tightening force or clamping force is common in the screw connection, and a contact pressure is generated by the contact of the workpiece. The pressure causes the friction between the workpieces to become large, and the friction makes the torque not fully pre-tightened, 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, angle control tightening techniques are often used even in the manual tightening of bolts, especially in the rapidly growing automotive industry. This technology allows each bolt to achieve its maximum fastening effect. The angle of rotation is the angle between the original tightening of the bolt and the final achievement of the specified torque value.

In general, the number of turns will vary depending on the material of the fastener and the part being fastened. For example, a material with a high hardness such as carbon steel will have a smaller number of rotation angles for fastening; a material with a lower hardness such as wood will have a larger number of rotation angles for fastening, and the force due to friction. The loss will also be hit, and the tightening force that can be achieved is relatively small.

In the thread tightening process of the control angle, the torque control is initially used to tighten the bolt to a fixed torque value. After this torque is reached, the subsequent tightening process is carried out under the dual control of torque and angle until the pre-up is reached. Set tightening torque and angle of rotation. Proper use of the corner control system can prevent the bolt from entering the plastic zone of the material and prevent the acceptance of the yield point of the bolt, resulting in a safety hazard. At the same time, the corner control can also significantly reduce the loss of the locking force, and ensure that sufficient pre-tightening force is achieved.

During the tightening process, the torque and the angle of the rotation are different, so the bolts that have been tightened using the 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 the torque tightening method. Plastic tightening mainly includes corner tightening method and yield point tightening method.

1. Torque tightening method

The principle of the torque tightening method is that there is a certain relationship between the torque magnitude and the axial preload. The pre-tensioning force of the coupled member is controlled by setting the tightening tool to a certain torque value. Under the premise of stable process and component quality, the tightening method is simple and intuitive, and is widely used. As a rule of thumb, when tightening the bolts, 50% of the torque is consumed on the friction of the bolt end faces, 40% is consumed by the friction of the threads, and only 10% of the torque is used to generate the preload. Since the unstable external conditions have a great influence on the torque tightening method, the torque method in which the preload force control is indirectly implemented by controlling the tightening torque will result in low control accuracy against the axial preload force. Moreover, there are a very small number of bolted joints, the torque has reached the specified value, and the bolt head has not been completely attached to the coupled member or the gap is sometimes small, which is not easy to visually detect. At this time, the torque value is acceptable, but the pre-tightening force is small or not, so in this case, if only the guaranteed torque is qualified, then ensuring the assembly tightening quality becomes an empty talk. MORCATO torque wrenches do 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 angle of rotation when the bolt is tightened is approximately proportional to the sum of the bolt elongation and the looseness of the tightened member, so that a predetermined rotation angle can be adopted to achieve the predetermined tightening force. First tighten the bolt to the starting torque, ie, the bolt is stretched to near the yield point, and then rotated a certain angle to stretch the bolt to the plastic zone. The essence of the rotation angle tightening method is to control the elongation of the bolt. In the elastic range, the axial preload force is proportional to the elongation. The control of the elongation is to control the axial force. After the bolt begins to plastically deform, although both have It is no longer proportional, but the mechanical properties of the bolt when it is stretched indicate that the axial preload can be stabilized near the yield load as long as it remains within a certain range. Therefore, the bolts with different friction coefficients have a large difference in final torque after tightening by the same tightening method. However, since the bolt strength and size are the same, the pre-tightening force is not much different. Compared with the torque tightening method, the tightening control is not only performed with high precision, but also the material utilization rate is sufficiently improved. MORCATO's corner torque wrenches are well handled and can be achieved when working.

3. Yield point tightening method

The theoretical goal of the yield point tightening method is to tighten the bolts just past the yield limit. When tightening with the yield point, first tighten the bolt to a specified starting torque. From this point on, the device monitors the change in the slope value of the tightening curve. If the slope drops beyond the set value, then the bolt is considered to be pulled. Stretched to the yield point and the tool stopped running. The biggest advantage of the yield point tightening method is that the bolts with different friction coefficients are tightened to the yield point, which maximizes the potential of the thread strength, but it is sensitive to the interference factor, and the bolt performance and structural design requirements are extremely high. High, difficult to control. Therefore, the price of the tightening tool is very expensive. And MORCATO's torque wrench series, in the same quality brand, the price still has certain advantages, good quality and low price.