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Bolt tightening methods and torque analysis are widely used in engineering applications. The torque method involves applying a specific torque value to control the pre-load force of the bolted joint, making it simple and intuitive for operators. The relationship between torque (T), pre-load force (Q0), and bolt diameter (D) is typically expressed as T = K * Q0 * D, where K is a coefficient that depends on friction, surface condition, lubrication, and other factors. The value of K usually ranges between 0.10 and 0.30, which leads to significant variations in the pre-load force even with small changes in torque. Factors such as thread smoothness, presence of lubricant, tightening speed, tool accuracy, and temperature fluctuations during repeated tightening can cause up to 40% variation in the actual pre-load. Therefore, when designing bolts using the torque method, engineers must account for this variability by increasing the bolt size, number of bolts, or material strength, which may complicate the design, increase costs, and add weight.
An alternative approach is to tighten the bolt until it reaches the yield point, which significantly enhances the clamping resistance of the joint. However, the torque method lacks control over the plastic elongation of the bolt, potentially leading to over-tightening or failure. Another method, the angle-tightening technique, allows the bolt to be tightened just before yielding. This method relies on measuring the rotation angle rather than torque, enabling more precise control over the pre-load. When the tightening process is divided into three stages—initial, tightening, and yield—the torque gradient changes accordingly. In the initial stage, the torque increases gradually, while in the tightening stage, the slope stabilizes until yielding begins. Once the bolt enters the yield zone, the torque increases slightly, but the slope drops sharply, indicating the onset of plastic deformation. By monitoring the torque-angle curve, engineers can accurately identify the yield point, improving the reliability of the bolted connection.
In the case of tower cranes, proper bolt tightening is critical. These large machines are frequently assembled and disassembled, so the bolts must be reusable. Each tightening should be controlled to ensure the bolt is stretched just beyond its yield point without permanent deformation. To maintain reusability, the deformation should be minimized, requiring precise control of both torque and angle. Improving the accuracy of the thread pair and reducing torque and angle errors are essential for reliable and safe operations. Proper tightening procedures help prevent failures that could lead to serious accidents during construction.