Pre-tightening bolts are a commonly used method of tightening bolts in the field of engineering construction, especially in steel structures and concrete structures. So, do you know about preload bolts? Such as definition, main uses, how to preload bolts, its advantages and disadvantages. In this article, TorcStark will introduce in detail the issues related to preloading bolts, hoping to be helpful to you.

Now, let’s get started!

What is Pre Tensioning of Bolts(Preloaded Bolts)?

Pre-tightening bolts(Preloaded Bolts) mean that when bolts are used for connection, a certain torque will be applied during the tightening process so that a pre-tightening force along the axis of the bolt is generated between the bolt and the connected parts. The purpose of this pre-tightening force is to enhance the reliability and tightness of the connection, prevent gaps or relative slippage after loading, and ensure that the fasteners can be reliably connected.

The purpose of bolt pretension is to maintain the mechanical properties and sealing of the connection.

1. Maintain mechanical properties

The pre-tightening force can firmly fix the connecting parts together and limit the rotation and deformation between the connecting parts, thus ensuring the stability and safety of the connection. In addition, pre-tightening can also reduce the vibration and impact suffered by the connection during use and extend the service life of the connection.

2. Ensure sealing

The pre-tightening force can better compress the gasket or gasket in the connection, thus ensuring the sealing of the connection. Without sufficient preload, the gaps between the connections may become larger, causing leaks or loosening.

Pre Tensioning of Bolted Joint is Used to…

The pretension of bolted connections is used to ensure the reliability, safety and longevity of mechanical and structural systems, etc.

The preloading of bolt joints is used in the following important aspects:

1. Improved load distribution: By applying a predetermined tension to the bolt before external loads are applied, pretensioning helps ensure that the load on the joint is evenly distributed across the connected components. This reduces the potential for localized stress concentrations and helps prevent premature joint failure.

2. Prevents Loosening: Preloading helps prevent bolts from loosening over time due to factors such as vibration, thermal expansion, and contraction. The initial tension applied to the bolt creates a clamping force that keeps the joint securely tight, even under dynamic or harsh operating conditions.

3. Increased fatigue strength: Bolts subjected to pretension will experience compressive stresses, which can offset the tensile stresses caused by external loads. This increases the fatigue strength of the joint, allowing it to withstand repeated load cycles without failure.

4. Enhanced Structural Stability: Pre-stretching is often used in structural applications such as bridges and buildings to enhance overall stability and performance. Preloading helps maintain structural integrity and minimizes the risk of structural failure by ensuring bolted connections remain tight and secure.

Preloaded bolts are used in the following areas of application:

1. The connection is subject to other loads: When the bolted connection is subject to other loads, the pretightening force can increase the stability of the connection and prevent the bolts from loosening or being damaged due to the load.

In construction projects, bolted connections are usually used to connect steel structures and concrete structures. The operating conditions of these parts are also very harsh and need to withstand a large amount of load and environmental effects.

2. High temperature conditions: In high temperature environments, due to the influence of the expansion coefficient, bolts and connectors will deform to a certain extent, thus affecting the tightness of the connection. At this time, applying pre-tightening force can eliminate deformation to a certain extent and ensure the reliability of the connection.

In aerospace products such as aircraft and rockets, bolted connections are used in important modules such as engines. Bolts are often affected by high temperatures and high vibrations.

3. High vibration environment: In a high vibration environment, the bolt pretightening force can limit the displacement of the bolt and prevent the bolt from loosening during vibration.

In the automotive industry, bolted connections are usually used in the engine, suspension system, chassis and other parts of the car. The working conditions of these parts are often very harsh and they are often in high vibration environments.

How to Pre Tension Bolts and Bolt Pre Tensioning Method

The pre-tightening methods of bolts mainly include two common methods, namely the torque control method and the angle control method.

Torque control method

The torque control method controls the pretightening force of the bolt by giving a preset torque value. The specific steps are as follows:

1. Determine the two parts that need to be connected, and prepare bolts and nuts.
2. Use the appropriate tool to insert the bolt into a part and tighten it through the nut to the specified pre-set torque value.
3. If a second part needs to be connected, place it on the other side of the bolt and use an appropriate tool to tighten the nut to the specified pretightening torque value.
4. In the case of multiple bolts, tighten the bolts in the correct order until all bolts reach the preset torque value.
5. Using a torque wrench or torque control device, check the torque of each bolt in turn until all bolts reach the preset torque value.

Angle control method

The angle control method controls the pretightening force of the bolt through a given rotation angle. Specific steps are as follows:

1. Determine the two parts that need to be connected, and prepare bolts and nuts.
2. Use appropriate tools to insert the bolt onto a part and tighten through the nut to the appropriate initial tightening force.
3. Use an angle wrench or angle control device to rotate the bolt to the preset rotation angle.
4. If a second part needs to be attached, place it on the other side of the bolt and use the appropriate tool to tighten the nut to the appropriate initial tightening force.
5. Use an angle wrench or angle control device to rotate the bolt to the preset rotation angle.
6. In the case of multiple bolts, tighten the bolts in the correct order until all bolts reach the preset rotation angle.

The torque control method is suitable for bolted connections that require higher pre-tightening force, while the angle control method is suitable for connections that require higher angle control. For example, the angle control method is commonly used in vehicle engine connections and wind turbine connections.

Pre Tensioning Bolt Installation

The installation quality of preload bolts directly affects the stable operation and service life of the equipment. Therefore, the installation method of preload bolts needs to be strictly controlled, which mainly includes the following steps:

1. Select bolts designed for preload applications and suitable for the expected loads and environmental conditions.

2. Make sure the surfaces of the bolt components are clean, flat, and free of any contaminants or irregularities that could affect clamping force or joint integrity.

3. Assemble the components to be joined together according to design specifications, including any gaskets or other hardware.

4. Use a calibrated torque wrench or hydraulic tensioning device to tighten the bolts to the specified torque or initial tension. This initial tightening is typically performed to snug the bolt and ensure proper alignment of the components.

5. Apply the required pre-tightening force to the bolts using a hydraulic tensioner, torque wrench or other special equipment.

6. Use a load indicating device or other method to verify that the correct preload force is applied to each bolt.

7. Once the required preload is achieved, secure the bolt in place using a locking mechanism such as a locking nut, cotter pin, or thread locking compound to prevent the bolt from loosening over time.

8. Perform a final inspection of the bolt joints to ensure that all bolts are tightened and secured correctly. Check for any signs of damage or irregularities that could compromise the integrity of the joint.

9. Keep detailed records of the preloading process, including the torque or tension applied to each bolt, and any measurements or inspections made during installation.

Advantages and Disadvantages of Preloading Bolts

Preloaded bolts have many advantages and disadvantages, mainly including the following aspects.

Advantages of preloaded bolts:

1. Enhance the reliability and tightness of the connection: Before the bolt connection bears the working load, through the action of pre-tightening force, the connecting parts can maintain a tight connection and prevent loosening or deformation when they are impacted by external forces or vibrations.

2. Control the loosening of connections: In high vibration environments, bolt pretightening force can limit the displacement of bolts, prevent bolts from loosening during vibration, and improve the reliability of connections.

3. Increase contact stress: After the bolts are tightened, as the contact pressure between the stress-bearing surfaces increases, mutually dependent contact areas may be formed, thereby increasing the contact stress and making the connection tighter.

4. Generation of friction: Since the friction between the bolt and the base material increases with the increase of the pretightening force, the tightness of the bolt also increases.

5. When the connection is subject to other loads, the pretightening force can increase the stability of the connection and prevent the bolts from loosening or being damaged due to the load.

6. In high-temperature environments, due to the influence of the expansion coefficient, bolts and connectors will undergo certain deformation. At this time, applying pre-tightening force can eliminate the deformation to a certain extent and ensure the reliability of the connection.

Points for pre-tightening bolts:

1. Complex installation: Preloading bolts requires careful control of the applied tension to accurately achieve the required preload. This may require specialized equipment and trained personnel, making the installation process more complex and time-consuming.

2. Possibility of overload: If the preload force is applied incorrectly or exceeds the design limit of the joint, it may cause overload and damage the connected components. Proper engineering analysis and calculations are required to ensure that preload forces are within acceptable limits.

3. Limited reusability: Once a preloaded bolt is installed and tightened, it may not be easily adjustable or reusable, especially if a locking mechanism like threadlocking compound is used to prevent loosening. Over time, this increases maintenance and replacement costs.

Finally

Through the above content, I believe you have a certain understanding of preload bolts. If you have any other questions about pretensioned bolts, please leave a message or contact us by email.