The connection of screw pairs is a widely used method in many mechanical industries, such as the compressor of automobile internal combustion engine. In order to ensure the assembly quality, it is necessary to control the tightening state of the screw pair. At present, there are four main methods to control screw tightening: torque method-corner yield point method and bolt elongation method. Although the bolt elongation method is the most accurate and reliable, it has not been used in production so far because it is difficult to realize in actual assembly machinery. For a long time, the torque method has been the most commonly used method in the assembly of thread pairs because it is simple and easy to operate. However, with the continuous improvement of assembly quality requirements, the shortcomings of torque method are increasingly exposed. Therefore, in recent ten years, the tightening technology of bolt connection in important occasions has been basically replaced by torque-rotation angle method, which greatly improves the assembly quality of products. Taking the automobile engine as an example, in the engine assembly line of modern automobile factory, the tightening process of key bolt connections such as main bearing cap, connecting rod bearing cap and cylinder head adopts torque.
The essence of assembly tightening is to connect two workpieces (such as cylinder head and cylinder block) reliably through the axial pre-tightening force of bolts. Therefore, accurate control of axial preload is the basis of ensuring assembly quality. The torque method indirectly controls the pre-tightening force by controlling the tightening torque. Due to the influence of various uncertain factors such as friction coefficient, the control accuracy of axial pre-tightening force is low. In addition, for safety reasons, the maximum axial force is generally set below 70% of its yield strength in design, and the actual value is often only 30%~50%. Small and scattered axial pre-tightening force will inevitably lead to low material utilization, clumsy structure and poor reliability. The essence of torque-rotation angle method is to control the elongation of bolts. In the whole tightening range after bolt connection, the elongation is always proportional to the rotation angle, and the axial preload is proportional to the elongation. Controlling the elongation means controlling the axial force. After the bolts begin plastic deformation, although they are no longer in direct proportion, the mechanical properties of the tension members show that the axial preload can be stabilized near the yield load as long as it is kept within a certain range. In fact, the torque-rotation method mainly reaches the yield point by stretching the bolt to the superelastic limit, so as to make full use of the material strength and achieve the purpose of high-precision tightening control (as shown in Figure 3-9).
Figure 3-9