What Determines Bolt Torque Distribution And Friction Coefficient?
Dec 17, 2025
As a core indicator for controlling bolt clamping force, the reality is that most of the tightening torque is lost through friction, with only a small portion actually converted into clamping force. So, what factors ultimately determine the bolt torque distribution and the magnitude of the friction coefficient? Today, the editor from Jiangsu Jinrui will share an empirical study based on microtopography analysis, which reveals the key factors influencing bolt torque distribution and friction coefficient, providing a strong basis for achieving high-reliability fastening.
1. Friction Coefficient and Torque Distribution
When tightening a bolt, the input torque is not entirely used to stretch the bolt and generate clamping force. In fact, the torque is distributed among three consumption paths:
Thread friction: Friction occurs in the thread contact area between the bolt and nut, consuming a large amount of torque;
Bearing surface friction: Friction also exists between the bolt head and the washer or the surface of the connected component, and the torque consumed in this part accounts for a larger proportion;
Thread lead angle effect (i.e., effective preload component): Only this part of the torque is truly used to stretch the bolt and thus form clamping force.
Studies have shown that approximately 85% to 90% of the torque is used to overcome friction, and only about 10% is converted into bolt tensile force.
This means that once the friction coefficient changes, the torque conversion efficiency will change accordingly, resulting in a possible difference of more than double in the clamping force generated under the same torque. Therefore, it is unreliable to lock the clamping force solely by torque.
2. Scheme Design
To deeply explore the core factors determining bolt torque distribution and friction coefficient, the Tribology Laboratory of École Centrale de Lyon in France designed a systematic experimental scheme. The core goal of this scheme is to combine mechanical testing with surface microtopography analysis to establish a causal relationship between friction behavior and microstructure.
The experiment was conducted in accordance with the ISO 16047 standard for torque-clamping force testing. The bolts used were of specification M10×60, made of 30MnB4 steel, which were cold-headed, thread-rolled, and then electrogalvanized. The specific values of total torque were recorded in detail, while thread torque and bearing surface torque were separated to accurately calculate the friction coefficient and analyze the torque distribution law. Three-dimensional topography scanning technology was used to extract roughness-related parameters, and the parameter changes before and after tightening were compared to explore the intrinsic correlation between friction behavior and microtopography. This design not only considers mechanical performance but also delves into the micro level, revealing the fundamental reasons for changes in bolt torque distribution and friction coefficient.
3. Test Verification Method
Based on the above scheme, a test device conforming to the ISO 16047 standard was built, which can accurately measure torque and clamping force. The test process includes the following links:
Bolt fixing and loading: Install the bolt on a standardized test bench, apply a set torque, and real-time record the values of total torque, thread torque, bearing surface torque, and clamping force;
Friction separation measurement: Separate thread friction from bearing surface friction through the special structure of the device and sensors to ensure the accuracy of friction coefficient calculation;
Topography scanning arrangement: Before and after each tightening operation, perform three-dimensional scanning on the bearing surface of the bolt head and the washer surface to capture micron-level feature information;
Parameter extraction and analysis: Extract roughness-related parameters and combine them with friction data to analyze the corresponding relationship between surface topography changes and friction behavior.
The figure below shows the structure of the test bench and the specific positions of the measurement points.
4. Analysis of Topography Results
The test data revealed several key phenomena that help to deeply understand the fundamental factors determining torque distribution and friction coefficient:
4.1 Dynamic Changes of Friction Coefficient
During the tightening process, the friction coefficient is not constant but continuously changes with the contact state. Generally, the bearing surface friction coefficient is about 44% higher than the thread friction coefficient, indicating that most of the torque is consumed on the bearing surface rather than the thread surface.
4.2 Significant Torque Dispersibility
Even when the same clamping force target is set, the difference in the required torque may be nearly double. For example, some bolts require a torque of 96.7 Nm, while others only need 54.5 Nm. This dispersibility of torque values is directly caused by the instability of the friction coefficient.
4.3 Significant Evolution of Surface Topography
The three-dimensional scanning results show that the roughness parameters of the bearing surface have undergone significant changes:
Sq (root mean square roughness) decreased from approximately 5.3 μm to 1.04 μm, and the surface became smoother;
Ssk (skewness) turned negative, indicating a change in the distribution of surface peaks and valleys, with more material concentrated in the low points (valleys) of the surface, and the pit features became more obvious;
The value of Sku (kurtosis) increased, meaning the surface bearing capacity was enhanced.
These changes indicate that during the tightening process, the surface undergoes plastic deformation, the real contact area increases, and the friction behavior changes accordingly. The figure below shows the three-dimensional topography of the bearing surface of the bolt head before and after tightening: before tightening, the surface presents an obvious rough peak-valley structure; after tightening, the rough peaks are sheared, the surface tends to be flat, and the directionality is more obvious. This shows that friction not only consumes energy but also reshapes the surface structure at the micro level.
The figure below clearly marks the friction marks and plastic deformation areas on the bearing surface through microscopic observation: there are significant scratches in some areas, and the extension direction of the scratches is consistent with the rotation direction of the bolt, indicating that friction has caused material flow and surface damage.
The figure below presents the uneven characteristics of bearing surface contact: the actual contact area is much smaller than the nominal area, and the load is concentrated in a few micro areas, leading to local high-stress states and plastic deformation. This uneven contact is the key factor causing fluctuations in the friction coefficient.
