Bolt Fastening Solution

At present, almost industrial use of bolt fastening is to control the 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, thus ensuring the reliability of threaded connections. As a professional fastener manufacturer, Xinchi Fastener Company has more than ten years of experience in the fastener field. We are pleased to introduce the knowledge of bolt fastening.

Bolt fastening is a very complicated physical process. The most important factors affecting bolt fastening are torque, preload, friction force and material hardness. Only when the above factors are fully considered can safe bolt fastening be ensured. However, when a more precise and secure thread fastening is required, a manual torque wrench is not suitable. Often the applied torque does not meet the requirements of the preload force and the corresponding preset value because it is not very accurate. The source of the inaccurate value is often caused by the seizing between the tightening threads and the friction between the bolt head and the plane of the fastening object.

In order to achieve higher accuracy, even in the operation of manual tightening of bolts, Angle control tightening technology is often used, especially in the current rapid development of the automobile manufacturing industry. This technique allows each bolt to achieve its maximum tightening effect. The Angle of rotation is the Angle between the initial tightening of the bolt and the final reaching of the specified torque value.

Generally speaking, the degree of Angle will vary according to the fastener and the material of the parts to be fastened. For example, for materials with high hardness, such as carbon steel, the number of degrees of tightening required will be smaller; For materials with low hardness, such as wood, the required Angle degree of fastening will be relatively large, and the force loss caused by friction will be large, so the fastening force can be achieved is relatively small.

The torque control is initially used to tighten the bolt to a fixed torque during the control Angle thread-tightening process. After this torque is reached, the subsequent fastening process is carried out under the dual control of torque and Angle until the preset fastening torque and rotation Angle are reached. The correct use of the Angle control system can avoid the bolts from entering the plastic interval of the material and prevent the safety risks caused by exceeding the acceptance yield point of the bolts. At the same time, Angle control can also significantly reduce the loss of locking force to ensure sufficient preload.

In the process of bolt fastening, the torque and Angle degrees used are different, so the bolts that have been tightened by Angle control cannot be used again.

There are two main types of bolt-tightening methods, namely elastic tightening and plastic tightening. Elastic tightening generally refers to the torque tightening method, while plastic tightening mainly includes the Angle tightening method and yield point tightening method.

Torque tightening method

The principle of the torque tightening method is that there is a certain relationship between torque and axial preload. The pretightening force of the connected part is controlled by setting the tightening tool to a certain torque value. Under the premise of stable technological process, parts quality and other factors, the tightening method is simple and intuitive and is widely used at present. As a rule of thumb, when tightening a bolt, 50% of the torque is consumed by the friction of the bolt face, 40% is consumed by the friction of the thread, and only 10% of the torque is used to produce pretightening force. Because the external unstable conditions have a lot of influence on the torque tightening method, the torque method which indirectly implements the preload control by controlling the tightening torque will lead to the low control precision of the axial preload. And there are very few bolt connection cases, the torque has reached the specified value, while the bolt head has not been completely fitted with the connected parts or the gap is sometimes very small. It is not easy to find when visually. At this point the torque value is acceptable, but the preload is little or no. So in this case, if only to ensure that the torque is qualified, then to ensure the assembly tightening quality becomes an empty phrase.

Angle tightening method

In view of the deficiency of the torque tightening method, the relationship between bolt elongation and axial force was studied in the United States in the late 1940s. The rotation Angle of the bolt is roughly proportional to the sum of the bolt elongation and the loosening amount of the tightened part, so the method can be adopted to reach the predetermined tightening force according to the specified rotation Angle. First, tighten the bolt to the starting torque, that is, stretch the bolt close to the yield point, and then, rotate it at an Angle to stretch the bolt into 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 is proportional to the amount of elongation, so controlling the amount of elongation is to control the axial force. When the bolt begins to plastic deformation, although the two are no longer proportional to each other, the mechanical properties of the bolt under tension show that the axial preload can be stabilized near the yield load as long as it is kept within a certain range. Therefore, although the final torque of two bolts with different friction coefficients differs greatly after tightening by the same tightening method, the pretightening force differs little due to the same bolt strength and size. Compared with the torque tightening method, the Angle tightening method not only completes the tightening control with high precision but also improves the utilization rate of materials fully.

Yield point tightening method

The theoretical objective of the yield point tightening method is to tighten the bolt just past the yield limit point. When the yield point is used, the bolt is first tightened to a specified initial torque. From this point, the device is used to monitor the change in the slope value of the tightening curve. If the slope falls beyond the set value, the bolt is considered to have been stretched to the yield point and 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 strength potential of the threaded parts. However, it is sensitive to interference factors and has high requirements on the performance and structural design of bolts, so it is difficult to control. So the price of tightening tools is very expensive.

Xinchi Fastener Company is a trustworthy fastener company, especially in furniture nuts. And we are the designated fastener supplier to“STAPLE”. As a trusted fastener supplier,  Xinchi Fastener Company can offer a wide range of high-quality fasteners, including tee nuts, screws, bolts, rivets, washers, etc. If you want to know more about fasteners or want to wholesale fasteners, please feel free to contact us. And then our experienced and expert service team will reply to you as soon as possible.