Specification for Bolting Connections of Commercial Large Fans

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Author : JU LAI
Update time : 2023-06-27 09:07:34

Commercial large fans come in various forms, including hanging, standing, and wall-mounted types. These fans are favored in many commercial settings due to their large size, slow speed, and low power consumption. Most of the components in these commercial fans are secured using high-strength bolts, including the motor and chassis, as well as the fan blades and chassis. However, whether the high-strength bolts are installed according to specifications directly affects the overall quality of the fan structure. Here, Industrial Fan Brand Network (https://www.julfans.com) provides an explanation and correction for the improper operations commonly seen during the installation of high-strength bolts in commercial large fans.

  1. Do not use bolts as temporary installation bolts. According to the production regulations for commercial large fans, when assembling the connections, temporary bolts should be used first before installing the high-strength bolts. The temporary bolts, which should account for at least one-third of the connector bolts, are inserted into the holes aligned with a guide pin or a pilot pin. They should be tightened using a wrench. After the holes are reamed, the high-strength bolts can be installed. Only by following this process can the bolts accurately align with the holes and evenly distribute axial force. Some installers often skip the step of using temporary bolts to save time and directly replace them with high-strength bolts for installation. This often results in incorrect hole alignment, forced hole alignment, damage to the bolt threads, changes in torque coefficients, uneven bolt axial force, the generation of internal stresses in the connecting plates, and insufficient bolt preload, thereby reducing the connection strength.

  2. Do not forcefully insert bolts into the holes. During the manufacturing process of commercial fans, the high-strength bolt holes should be drilled as perfect cylinders, with the hole walls perpendicular to the component surface. The bolts should be inserted into the holes according to regulations, and the bolt holes should be 1mm-2mm larger than the bolt diameter. If the holes are slightly smaller or have burrs, a reamer or electric drill should be used to enlarge the holes before inserting the high-strength bolts. It is important to avoid forcefully hammering the high-strength bolts into the holes. Doing so will not only damage the threads, affecting the preload effect but also compress the hole walls and subject the bolts to shearing forces, altering the force state of the high-strength bolts.

  3. Pay attention to the proper treatment of friction surfaces. The quality of the high-strength bolt connections is indicated by the coefficient of friction. Therefore, the friction surfaces should be carefully treated before installation, as this is a necessary condition for transferring stress. Rust, welding splatters, slag, dust, oil stains, burrs on the bolt holes and plate edges, etc., should be thoroughly removed from the contact surfaces. Burrs and welding nodules on the hole walls and plate surfaces should be ground with a grinding wheel and then sandblasted. The procedures should not be reversed, which could damage the friction surfaces. Some production personnel lack awareness about the importance of treating friction surfaces and may handle it carelessly or even skip the treatment before installation. This would result in the presence of interlayers between the friction surfaces or sandblasting performed after grinding burrs and welding nodules. Improper protection of friction surfaces could lead to corrosion, causing unevenness on the friction surfaces. All these factors reduce the friction coefficient and lower the connection strength. In such cases, the friction surfaces must be re-treated to ensure quality.

  4. Do not interchange bolts, nuts, and washers. The magnitude of the frictional force between the high-strength bolts connecting the joint plates depends on the bolt's preload and the coefficient of friction between the mating surfaces. To ensure the required preload, the manufacturers of high-strength bolts, nuts, and washers have conducted tests to match them before supplying them for commercial large fans. This ensures a constant torque coefficient and prohibits the mixing of parts from different packages. At the same time, they should not be repeatedly installed or discarded, which could damage the threads or be contaminated by soil or oil due to rain or humidity.

  5. Avoid insufficient tightening torque or axial force. To achieve the specified preload and obtain the required connection strength, high-strength bolts are generally tightened by applying a certain torque. However, deviations in electric or manual wrenches or large errors can occur if they are used without calibration, resulting in insufficient torque or unevenness in the contact surfaces. Improper tightening can consume part of the torque in overcoming deformation or result from poor operation, where the torque value reaches the reading on the wrench but the actual preload is not achieved. Some bolts may be missed during initial or final tightening, leading to uneven bolt forces or failure to achieve the design-required preload axial force value. All these factors affect the preload force and reduce the connection strength. Therefore, attention should be paid to preventing such issues during installation.

  6. Do not tighten all bolts at once or deviate from the specified sequence. According to the product specifications of commercial large fans, to ensure the uniform distribution of forces among the bolts and reduce axial force losses, bolt tightening should be divided into two stages. First, the initial tightening should be performed, with the bolt tightened to 60%-80% of the standard tensile force. Then, the final tightening should be carried out, reaching the standard preload with a deviation of no more than 10%. Some production personnel may be eager for quickness and directly perform the final tightening without the initial tightening step, causing some of the axial force of the bolt to be consumed in overcoming the deformation of the steel plate. When the surrounding bolts are tightened, the axial force is shared and reduced. In addition, to ensure uniform forces among all the bolts, both initial and final tightening should follow a certain sequence. Generally, tightening should start from the middle of the bolt group and proceed outward. Some operators may find it more convenient to adopt a reverse sequence, tightening from both ends toward the middle. This often causes bulging in the middle, with some of the axial force consumed in overcoming deformation, resulting in insufficient preload, reduced friction coefficient, and decreased connection strength. The connection of high-strength bolts has advantages such as simple construction, reliable force transmission, good quality, and relatively low cost, making it a mature fastening technology for commercial large fans. However, strict requirements are placed on the production process and operations. If insufficient attention is paid to production, it will not only fail to achieve a good connection but also lead to serious quality accidents. Therefore, commercial fan manufacturers need to impose strict requirements on every aspect of production and conduct 100% product quality inspections.

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