Influence of Surface Treatment on Bolt Tightening

Theoretical Analysis of Friction Coefficient's Impact on Energy Consumption

Analyzing the influence of friction coefficient on energy consumption during bolt tightening, including energy loss in bolt head friction, thread friction, and preload work. Experimental analysis was conducted on the relationship between different surface treatments, friction coefficients of threaded fasteners, and the torque-preload relationship.

The findings revealed how zinc plating thickness and various chromate treatments affect friction coefficients and torque coefficients. It was observed that reducing the friction coefficient significantly enhances axial tension, which is crucial for improving bolt efficiency.

Figures:

- Figure 1: Force Analysis of Plane and Groove Surface Friction

- Figure 2: Force Analysis of Triangular Screw Joint  

1. Theoretical Analysis of Friction Coefficient's Impact on Torque Coefficient

Torque Calculation in Bolt Connection Tightening:

Friction in triangular thread screw joints can be divided into plane friction, inclined plane friction, and groove surface friction based on contact surface shape. For simplicity, friction is calculated as if all contacts occur along a single plane (Figure 1). The unified formula is expressed as:

Where the torque coefficients are:

2. Experimental Analysis of Surface Treatment’s Impact on Torque Coefficient

Using a multifunctional bolt fastening analysis system by Germany's Schatz, the relationship between clamping force and torque is measured in real-time. This system also calculates friction coefficients for the bolt head and thread. Experimental studies were conducted on bolts with common zinc plating thickness and chromate treatments.

Experimental Parameters:

Bolts tested: , strength grade 10.9, surface treatments:  

Average values from 20 data sets per treatment are shown in Table 2.

Results and Analysis:

1. Zinc Plating Thickness and Chromate Treatment:  

  - For bolts treated with , increasing zinc thickness from 5μm to 12μm showed no significant change in bolt head friction coefficient but increased thread friction coefficient by 12.4% and torque coefficient by 4.75%.  

  - For , similar zinc thickness increase raised thread friction coefficient by 20.34% and torque coefficient by 5.23%.

2. Chromate Treatments Comparison:  

  - Bolts with treatment had significantly higher friction coefficients compared to. Reductions in bolt head friction (40%) and thread friction (50%) led to a 40% decrease in torque coefficient.

Figure 3: Friction Coefficients of Bolts with Different Surface Treatments.

3. Impact of Surface Treatment on Bolt Allowable Strength

During tightening, threaded fasteners experience combined torsional and tensile stress. Based on the third strength theory, the allowable equivalent stress is calculated as:

By lowering torsional shear stress from frictional losses, more torque converts to preload, enhancing axial stress. Analysis shows:

- Lowering friction coefficients increases the ratio of torque converting to preload by 43%.

- For bolts with and, maximum axial tension rose by 4% at 200 N·m tightening torque.

- Comparing with , reducing friction coefficient by 45% increased maximum axial force by 38.9%.

4. Conclusion

1. Friction Coefficient’s Impact:  

  Small changes in friction coefficients significantly affect energy distribution during tightening and preload force.

2. Surface Treatment Analysis:  

  Thicker zinc layers increase friction coefficients. Chromate treatment showed lower friction compared to.

3. Efficiency Improvements:  

  Bolts with treatment achieved higher axial preload with less energy, optimizing efficiency.

Additional Learning Resource:  

Explore “Metal Forging Technology” for detailed insights into fastener production techniques. This book discusses cold forging processes, offering high efficiency and resource-saving benefits. It’s an excellent reference for professionals and students alike.