Details and Principles for Bolt Length Considerations in Mechanical Design Industry Solutions

Details and Principles for Bolt Length Considerations in Mechanical Design Industry Solutions

During the process from the free state to the tightening of a bolt, the pre-tension, decay, and loading create axial deformation in the bolt connection. A simplified mechanical spring model can describe this force and deformation. In this model, the bolt and the connected parts can be treated as springs under tension and compression, with δS and δP stiffness values, respectively. The length of this spring depends on the thickness of the connected part, which determines the bolt's clamping length. The clamping length and the nut height determine the bolt length. In product design, the bolt length, or "bolt stud length," may seem like a small detail, but it is crucial. The choice of bolt stud length not only impacts the product's stability, durability, and aesthetics but also directly affects its quality and user experience. So, how do we reasonably determine the bolt stud length in product design?

Here's a breakdown of how to design the bolt length from the perspectives of bolt structure, blind hole size, and clamping length design principles:

1.Dimensions and Thickness of Connected Parts: The bolt length needs to be determined based on the thickness of the connected part and the clamping length. The clamping length refers to the size required to generate pre-tension in the bolt during tightening, usually dependent on the thickness of the connected part.

2.Size of the Nut and Washer: The bolt length also needs to account for the size of the nut and washer. The nut's height and thickness of the washer will affect the total bolt length.

3.Exposed Thread Length: The exposed thread length (the length at the end of the bolt) is also an essential factor. Depending on the application, the exposed thread length has different requirements. For example, in hydraulic metal structures, the exposed thread of a regular bolt should be no less than two threads, while high-strength bolts should have an exposed thread length of between 2 to 3 threads.

4.Thread Structure Details: Thread details such as the end, shoulder distance, relief groove, and chamfer all influence the bolt length design. For example, the end portion of the external thread must be considered when calculating clamping length to avoid creating false torque.

5.Materials and Manufacturing Processes: Different materials of connected parts have different requirements for bolt length. For example, soft materials like gray cast iron and aluminum require deeper threads to stabilize connection.

6.Safety Margin: When designing the bolt length, a safety margin must be considered to ensure that the clamping force is more significant than external loads, and factors such as vibration and friction variation are considered.

Impact of Thread Structure Details on Clamping Length

The diagram below shows that regular threads include end sections, shoulder distances, relief grooves, and chamfers. In actual bolt clamping length design, several points need to be considered:

a. The impact of the external thread end must account for the range where the threads are ineffective. When fitting with the nut or threaded blind hole, the length of the connected part must consider this ineffective range to avoid false torque.

b. The presence of shoulder distance and relief grooves requires the thickness of the connected part (t) to be greater than "a." If the thickness cannot be increased, washers can be used to increase thickness, or the chamfer depth at the internal thread end can be increased to avoid issues. An example of a fastening connection for a brake mudguard is shown in the diagram, where the mudguard's thickness is less than the bolt's shoulder distance, and the internal thread chamfer cannot be changed. Adding a washer solves the issue of the mudguard not being securely clamped and causing loosening or noise.

Blind Hole and Bolt Length Compatibility

The table below shows the remaining length of internal and external threads, hole depth, and exposed nut length to ensure the bolt length does not interfere with the blind hole. In actual work, it's common to have pre-drilled holes, and the bolt or stud is selected afterward, so it's critical to check the dimensions carefully to avoid false torque.

Clamping Length Design Principles

The first principle is that the bolt's clamping length should be long enough to ensure a high length-to-diameter ratio (L/D). The second principle ensures the bolt connection does not open under external forces.

Below is an explanation of the basic principles of bolt clamping length using the concepts of bolt and connected part stiffness:

1.Bolt Stiffness Calculation

The formula for bolt stiffness is:

Cb/(Cb+Cm)

Where$C_b$Cb is the bolt stiffness, which is related to the pre-tension force$F_0$F0, and$λ_b$λb is the elongation of the bolt.

2.Stiffness of Connected Parts

The stiffness of the connected part also significantly impacts the bolt's force. When the connected part's stiffness is sufficiently high, it can effectively support the bolt, reducing uneven force distribution and loosening caused by deformation. If the connected part's stiffness is insufficient, more significant deformations under external force may affect the bolt's connection and force distribution.

3. Clamping Body Deformation Region and Pressure Distribution

The clamping length ($l_k$lk) has a direct impact on the deformation of the clamping body. When the clamping length is small, the deformation is also small, resulting in a low deformation energy storage. When parts deform due to surface roughness and coating under pre-tension, and under external working loads, the reliability of the connection may decrease.

4. Threaded Connection Force Analysis

The elastic force state and deformation coordination between the bolt and the connected part are illustrated in the diagram. The force is distributed as shown: the bolt is stretched, and the connected part is compressed.

5. Torque on Threaded Connections

Considering the eccentric load and clamping, the total bending moment applied to the bolt can be calculated.

Practical Design Guidelines:

For metal-to-metal connections, the clamping length should be greater than 2 times the bolt diameter, with an optimal size of 4 times the bolt diameter.

The clamping length should exceed 5 to 8 times the bolt diameter for connections involving elastic materials.

The clamping length should be at least 6 times the thread pitch for yield-tightening connections and more significant than the bolt diameter.

For connections with a small clamping length, the best method is to calculate the stiffness of the bolt and the connected part, plot the deformation coordination curve, and validate the connection under working load conditions.

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