Three Commonly Used Pre-tightening Methods for the Installation of Large-diameter Bolts of Hydraulic Turbines
(Last Updated On: June 29, 2023)
Bolt connection is a widely used connection method in hydroelectric generator sets. It is convenient to disassemble and fasten the connection. It is mostly used in complex stress parts and important connections. Its use of environment and force characteristics indicate that bolts are often the weak link in this type of connection.
To enhance the rigidity, tightness, anti-loosening ability, and fatigue life of the connection, most bolted connections must apply a stable and reliable pre-tightening force to the bolts during assembly.
The applied pre-tightening force must not only ensure the quality requirements of the bolted connection but also meet the strength requirements of the bolts. The excessive pre-tightening force will easily make the pre-tightening force exceed the yield limit of the bolt, failing the connection. If the pre-tightening force is too small, the residual pre-tightening force may not meet the requirements, and the bolts may loosen or even fatigue fracture after being subjected to the working load.
Therefore, TorcStark will introduce three commonly used pre-tightening methods for the installation of large-diameter bolts of hydraulic turbines, and compare the operation methods and advantages and disadvantages of different pre-tightening methods.
The method of pre-tightening the bolts with hydraulic tensioners is widely used in the installation of hydraulic turbines, and is generally mainly used in the following two types of situations:
(1) Larger diameter studs above M42 require a higher level of pre-tightening force, and the operating space is limited. Such as connecting bolts between different workpieces subject to alternating loads (bearing body and top cover connecting bolts).
(2) For important bolts that require more accurate elongation values, such as water-generator coupling bolts (hydraulic shaft and generator coupling bolts).
Use a hydraulic tensioner to pre-tighten, apply tension to the bolt to make it elastically deform, and then lightly tighten the nut. After the tension is removed, a certain pre-tightening force can be generated in the connection due to the shrinkage of the bolt.
The hydraulic tensioner is not affected by friction when applying preload to the bolt, so this method is applicable to bolts of any size. Moreover, preload can be applied to a group of bolts at the same time, and the flange and gasket can be evenly pressed so that the precise control of the preload cannot be affected by inclination or warping.
This kind of pre-tightening method is easy to operate and can improve the control accuracy of the pre-tightening force. The error of pre-tightening force mainly appears in the monitoring method of pre-tightening force or elongation value.
2. Preload by electric heating
The application of electric heating (or temperature difference method) preloading in the installation of turbines is mainly in important large-diameter bolt connection structures such as coupling bolts (runner and main shaft coupling bolts).
The principle of electric heating is similar to that of a hydraulic tensioner. The difference is that the electric heating method uses a high-power heating rod to quickly heat the center of the screw to make it elongate, and then tighten the nut lightly. After the tension is removed, a certain pre-tightening force can be generated due to the cooling and contraction of the bolt.
Because the electric heating pre-tensioning method is similar to the hydraulic tensioner pre-tensioning method, the advantages, and disadvantages of the two are basically the same. The main difference and error lie in the monitoring method of the elongation value.
The hydraulic torque wrench pre-tightening method is easy to operate and has wide applicability, and is most commonly used in the installation of hydraulic turbines.
Mainly used for top cover, bottom ring, etc. split-face combination bolts (split-face combination bolts), runner blade screws (blade screws), runner piston shaft joint bolts (piston shaft joint bolts), etc., which are larger than M42 diameter studs or screws.
Since the pre-tightening force is greatly affected by the friction between the nut and the bolt and the friction pair between the nut and the flange, it is necessary to minimize the influence of the friction coefficient during the pre-tightening process.
For example, apply threaded grease on the bolt thread, and add a steel gasket with sufficient hardness between the flange surface and the nut.
At the same time, during the bolt pre-tightening process, the torque is greater, and the friction between the threads to be overcome is also greater. If the bolts are not cleaned or the bolts are not placed correctly, it is easy to cause the bolts to be damaged.
Therefore, in structural design, it should be considered that when stud bolts can be used, screws should be avoided as much as possible. The advantage of this is that when the bolt is damaged, the bolt can be taken out only by destroying the nut.
For example, the runner blade bolts can only use screw structures. Once the bolts are damaged, they can only be taken out after the bolts are damaged, which is extremely difficult.
The pre-tightening torque has a linear relationship with the bolt pre-tightening force, and the pre-tightening force value can be calculated by experimental or theoretical methods after controlling the size of the pre-tightening torque.
The advantage of this method is that there is no need to confirm the zero position and the pre-tightening process can be precisely controlled according to the pressure value of the torque wrench oil pump.
However, due to the influence of the friction coefficient and geometric parameter deviation, under a certain preload torque, the discreteness of the preload value is relatively large, and the preload accuracy is not as good as the above two methods.
However, the monitoring method of the pre-tightening force is the most intuitive. It only needs to convert the pretightening torque into the corresponding oil pump pressure value of the hydraulic torque wrench.
In the end
Through the analysis and comparison of three common pre-tightening methods of large-diameter bolts in the installation of water turbines, each pre-tightening method has its own advantages and disadvantages.
Therefore, in practical engineering applications, it is impossible to select only one type or to completely eliminate the adverse effects of various preloading methods.
Therefore, a variety of test methods are selected for the pre-tightening loading methods of bolted connection structures with different structures and different mechanical characteristics. In order to find the most reasonable pre-tightening method to avoid adverse effects on the stability or fatigue life of the connection structure due to too large or too small pre-tightening force.