High Strength Bolts

Detailed installation steps of high-strength bolts

In the fields of modern construction, bridge engineering, and heavy machinery and equipment manufacturing, high-strength bolts have become key components for achieving reliable structural connections due to their high tensile strength and good connection performance. However, the installation process of high-strength bolts is not a simple tightening operation, but requires strict steps and specifications to ensure the quality of the connection and the safe operation of the project and equipment. Next, we will introduce the installation steps of high-strength bolts and the technical points behind them in detail.

1. Preparation before installation

1. Material inspection and acceptance

Before installing high-strength bolts, the first task is to conduct strict quality inspection and acceptance of bolts, nuts, washers and other components. The inspection content includes whether the specifications and models of the bolts meet the design requirements, whether the threads are complete and undamaged, and whether there are cracks, rust and other defects on the surface. At the same time, the quality certification documents of the products should be checked to ensure that the various mechanical performance indicators of high-strength bolts, such as tensile strength and yield strength, meet national standards and engineering design regulations. Bolts that do not meet the requirements must not be used to avoid safety hazards. In addition, the matching of nuts and bolts must be checked to ensure that the threads can be screwed together smoothly without sticking.

2.Connection surface treatment

The reliability of high-strength bolt connections is closely related to the surface condition of the connected parts. Before installation, the connection surface must be cleaned and treated. First, remove impurities such as oil, rust, welding slag, burrs, etc. on the surface. Sandpaper polishing, sandblasting, chemical cleaning and other methods can be used. The polished surface should show a metallic luster, and the roughness should meet the design requirements, generally controlled between Ra25 - Ra63μm, to increase friction and improve the anti-slip coefficient of the connection. For the connection of some important structures, an anti-slip coefficient test is required after surface treatment to ensure that it meets the design standards. In addition, during the installation process, it is necessary to avoid the connection surface from being contaminated or damaged again.

3.Tool preparation

Appropriate installation tools are an important condition for ensuring the installation quality of high-strength bolts. Commonly used installation tools include torque wrenches, electric torque wrenches, hydraulic tensioners, etc. Before use, the tools must be calibrated and debugged to ensure that their torque output is accurate and reliable. The accuracy of the torque wrench should meet the requirements of ±3%, and it must be calibrated regularly to prevent insufficient or excessive bolt preload due to tool errors. At the same time, prepare necessary auxiliary tools, such as wrenches, hammers, vernier calipers, etc., to check the installation size and tightening condition of the bolts.

4.Technical disclosure

Before the installation work begins, the construction technicians should provide detailed technical instructions to the installers. The instructions include the installation process, quality standards, safety precautions, etc. of high-strength bolts. Let the installers be familiar with the key points and technical requirements of each installation step, clarify the quality inspection methods and acceptance standards, and ensure that the installation work can be carried out smoothly in accordance with the specifications.

2. Installation process of high-strength bolts

1. Bolt insertion

When inserting high-strength bolts into the bolt holes of the connected parts, attention should be paid to the consistent direction. Generally, it is required to insert from the side of the bolt head to the side of the nut. If it is impossible to insert from one side due to structural reasons, other reasonable insertion directions can also be used, but the bolt insertion directions on the same node must be consistent to ensure beauty and easy inspection. During the insertion process, it is strictly forbidden to use a hammer to force the bolts in to avoid damaging the threads and bolt holes. If there is a slight deviation in the bolt hole, it can be trimmed with a reamer, but the hole diameter after trimming shall not exceed 1.2 times the original hole diameter. For bolt holes with large deviations, welding rods of the same material as the parent material should be used for repair welding and then re-drilling. It is strictly forbidden to connect by filling with debris.

(2) Temporary bolt fixing

Before formally tightening the high-strength bolts, it is necessary to use temporary bolts to preliminarily fix the connected parts to ensure the stability and position accuracy of the connection. The number of temporary bolts should be determined according to the size and stress conditions of the connection node, generally not less than 1/3 of the total number of node bolts, and not less than 2. Temporary bolts should be evenly distributed around the node, and the tightening degree should be moderate to ensure that the connected parts fit tightly, but not affect the subsequent installation of high-strength bolts. When installing temporary bolts, be careful not to damage the bolt holes and threads.

(3) Tightening of high-strength bolts

The tightening of high-strength bolts is a key link in the installation process, which is usually divided into three steps: initial tightening, re-tightening and final tightening.

Initial tightening: The purpose of initial tightening is to make the connected parts fit tightly, eliminate the gap between the connection surfaces, and make the bolts initially stressed. The initial tightening torque is generally about 50% of the final tightening torque. The specific value should be determined according to the design requirements and bolt specifications. When initially tightening, it should be done symmetrically from the center of the node to the surroundings to ensure uniform force. After the initial tightening is completed, mark the bolt head and nut to distinguish whether it has been initially tightened.

Re-tightening: The torque value of re-tightening is the same as the initial tightening torque. Its function is to further evenly distribute the pre-tightening force of the bolts and ensure that the force of each bolt is consistent. Re-tightening should also be carried out symmetrically from the center of the node to the surroundings. For large nodes, group symmetrical tightening can be adopted. After re-tightening is completed, it should also be marked.

Final tightening: Final tightening is the key step to make high-strength bolts reach the designed preload. The final tightening torque should be calculated based on the preload and torque coefficient specified in the design. The calculation formula is: T = K×P×d, where T is the final tightening torque, K is the torque coefficient, P is the designed preload, and d is the nominal diameter of the bolt. The torque coefficient will be provided when the bolt leaves the factory and should be within a certain range, generally 0.110 - 0.150. When final tightening, it should also be carried out symmetrically from the center of the node to the surrounding area. For torsion shear type high-strength bolts, a special electric wrench is used to twist off the plum head at the tail of the bolt, which means that the final tightening is completed; for large hexagonal high-strength bolts, a torque wrench is used to reach the specified final tightening torque. After the final tightening is completed, each bolt should be checked to ensure that it reaches the designed preload.

3. Inspection and acceptance after installation

1. Appearance inspection

After installation, the high-strength bolts must first be inspected for appearance. Check whether the surface of the bolts, nuts, and washers is damaged or rusted, whether the markings on the bolt heads and nuts are clear, whether the connection surfaces fit tightly and whether there are any gaps, etc. Any parts that do not meet the requirements must be dealt with in a timely manner.

(2) Torque inspection

Torque inspection is an important means to check whether the preload of high-strength bolts meets the design requirements. For large hexagon head high-strength bolts, use a torque wrench to check 1 hour and 48 hours after the final tightening. During the inspection, first measure the actual torque value of the tightened bolts, and then compare it with the designed final tightening torque. The actual torque value should be within ±10% of the designed final tightening torque. For torsion shear type high-strength bolts, mainly check whether the plum head is completely screwed off, and the bolts that are not screwed off should be tightened or replaced.

(3) Anti-slip coefficient test

For some important connection nodes, anti-slip coefficient test is also required. The anti-slip coefficient test should be made on the construction site. The material, surface treatment method, bolt specifications and installation process of the test piece should be the same as the actual project. The anti-slip coefficient is measured by tensile test on the test piece, and the test result should meet the design requirements.

The installation of high-strength bolts is a job with high technical requirements and rigorous operation. Each step is related to the quality of the connection and the safety of the structure. Only by strictly following the installation steps and specifications and strengthening the quality control and inspection during the installation process can we ensure that high-strength bolts play their due role and provide reliable guarantee for the stable operation of various projects and equipment.