Bolt fastening solution

Original title: Bolt fastening Solutions

At present, almost all bolts used in industry need to be controlled, which is the so-called torque control torque refers to the use of predetermined torque or predetermined torque and Angle, to carry out industrial fastening, in order to ensure enough clamping force, to ensure the reliability of thread connection.

Bolt fastening is a very complicated physical process. The most important factors affecting bolt fastening are torque, pretightening force, friction force and material hardness. As long as the above factors are fully considered to ensure the safety of bolt fastening. Torque wrenches can control neither less nor more of the force applied to a thread fastening. In most cases, conventional torque wrenches have been able to provide a bolt tightening effect with sufficient precision. However, when a more accurate and safer thread tightening is required, manual torque wrenches are not suitable because the torque applied is often not up to the pre-tightening requirements and corresponding preset values because it is not accurate. Sources of imprecision are often caused by the bite between tightening threads and friction between the bolt head and the plane of the fastening object. The so-called preload force or clamping force is in the screw connection, through the workpiece contact to produce a contact pressure, is universal. The pressure increases the friction between the workpiece, and friction prevents the torque from fully pretightening, so only about 10% of the torque we apply can be converted into bolt tightening force.

In order to achieve higher accuracy, even in the operation of manually tightening bolts, Angle control tightening technology is often used, especially in the current rapidly developing automotive industry. This technique allows each bolt to achieve its maximum tightening effect. The rotation Angle is the Angle between the original tightening of the bolt and the final achievement of the specified torque value.

Generally speaking, the Angle degree will be different according to the material of fasteners and fastening parts. For example, high hardness material such as carbon steel, fastening the required Angle degree will be relatively small; The material with low hardness is like wood, the Angle degree required for fastening will be larger, and the force loss caused by friction will also be hit, so the fastening force can be achieved is smaller.

In the screw-tightening process of the control Angle, the bolt is screwed 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 preset tightening torque and rotation Angle are achieved. The correct use of the rotation control system can avoid the bolt into the plastic zone of the material, prevent the acceptance of the bolt yield point, causing safety hazards. At the same time, the Angle control can obviously reduce the loss of the locking force and ensure enough pretightening force.

In the process of bolt fastening, the torque used and the degree of Angle are different, so the bolt fastening after using Angle control can not be used again.

There are two main tightening methods for bolts, namely elastic tightening and plastic tightening. Elastic tightening generally refers to torque tightening method, and plastic tightening mainly includes corner tightening method, yield point tightening method, etc.

1. Torque tightening method

The principle of torque tightening method is that there is a certain relationship between torque and axial pretightening force. The pretightening force of the connected parts is controlled by setting the tightening tool to a certain torque value. Under the premise of stable factors such as technological process and parts quality, 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 spent on the friction of the bolt face, 40% on the friction of the thread, and only 10% of the torque is used to generate pretightening force. Because the external unstable conditions have a lot of influence on the torque tightening method, the torque method that indirectly implements the pretightening force control by controlling the tightening torque will lead to the low accuracy of the axial pretightening force control. And there are very few bolt connection, the torque has reached the specified value, and the bolt head is not completely fitted with the connected parts or the gap is sometimes very small, it is not easy to find visually. At this time, the torque value is qualified, but the pretightening force is very small or even no, so in this case, if only to guarantee the qualified torque, then to ensure the assembly tightening quality becomes an empty word. MORCATO's torque wrench can do this very well.

2. Corner tightening method

In view of the shortcomings of torque tightening method, the United States began to study the relationship between bolt elongation and axial force in the late 1940s. The rotation Angle of the bolt during tightening is roughly proportional to the sum of the bolt elongation and the loosening amount of the tightening piece, so the predetermined tightening force can be achieved 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 region. Tighten rotation Angle method is the essence of control the elongation of bolt axial pre-tightening force in elastic range is proportional to the amount of elongation, elongation control is to control the axial force and bolt began after plastic deformation, although is no longer proportional to the relationship between, but the bolt by the tensile mechanical properties show that when just keep within a certain range, the axial pre-tightening force can stabilize near the yield load. Therefore, although the final torques of two bolts with different friction coefficients differ greatly after being tightened by the same tightening method, the pretightening forces differ little due to the same bolt strength and size. Compared with the torque tightening method, not only the tightening control is completed with high precision, but also the material utilization rate is fully improved. MORCATO's corner torque wrench can be well handled, when working, you can achieve the effect.

3. Yield point tightening method

The theoretical goal of the yield point tightening method is to tighten bolts to just past the yield limit point. When the yield point tightening is adopted, the bolt is first tightened to a specified initial torque. From this point on, the equipment monitors the change of slope value of tightening curve. If the slope drops to more than the set value, it is considered that the bolt has been stretched to the yield point and the tool stops running. The biggest advantage of the yield point tightening method is that bolts with different friction coefficients are tightened to their yield points, which maximizes the strength potential of threaded parts. However, it is sensitive to interference factors and requires high performance and structural design of bolts, making it difficult to control. Therefore, the price of tightening tools is very expensive. MORCATO torque wrench system, in the same quality of the brand, the price is still a certain advantage, inexpensive.