Experimental Study on Torsional Friction of Stringing Blocks on Conductor in Tension Stringing Construction

An experimental study was conducted to analyze the torsional friction effect of stringing blocks on the conductor during tension stringing construction. By analyzing the torsional friction force of stringing blocks on the conductor, the torsional friction experimental device of the stringing blocks on the conductor was designed. The torsional friction torque of the JL/G1A-630/45-45/7 Aluminum Conductor Steel Reinforced (ACSR) under the different tensions and wrap angles passing through the stringing blocks was measured, and then the friction coefficient was obtained. The experimental results show that the torsional friction coefficient of the stringing blocks to the conductor μ decreases with the increase of the tension of the conductor and the wrap angle passing through the stringing blocks. The relationship between the coefficient of friction and the wrap angle and tension of the conductor passing through the stringing blocks is obtained by fitting, and the formula for calculating the torsional frictional torque is obtained.


Introduction
The conductors in overhead transmission lines usually use concentric bare strands, that is, multiple aluminum strands are spirally twisted on the steel core (or other core wires) layer by layer in a concentric circle manner, and the strands of adjacent layers are twisted in the opposite direction [1][2].In the process of tension stringing, the torsional friction of the stringing blocks on the conductor leads to the accumulation of torque on the conductor, which makes the defects such as loose strand and broken strand of the conductor occur from time to time in tension stringing construction, and seriously affects the quality, safety, progress and cost of tension stringing construction [3][4][5].
The traditional research mainly focuses on the friction coefficient of the stringing blocks in the direction of conductor traction, that is, the tension difference between the conductors on both sides of the stringing blocks, and specifies the friction coefficient [6][7].At present, there is a lack of research on the impact of stringing blocks on the torsional friction of conductors, and also a lack of relevant experimental equipment.As a result, it is impossible to determine the magnitude of the effect of stringing blocks on the axial torque of conductors, and the research into the cumulative effect of the stringing blocks on the torque of the conductor and the defect of loose strands lacks theoretical and experimental support.
In view of the above problems, the torsional friction experimental of the stringing blocks on the conductor was carried out.The torsional friction experimental device of the stringing blocks to the conductor is designed, which can measure the torque difference between the two sides of the stringing blocks under different conductor tension and the wrap angle between the conductor and the stringing blocks, that is, the torsional friction torque of the stringing blocks to the conductor, and then obtain the torsional friction coefficient between the conductor and the stringing blocks.

Calculation Method of Torsional Friction Torque of Stringing Block to Conductor
After simplifying the force on the conductor passing through the stringing block, the calculation method for the torsional friction torque of the conductor passing through the stringing block is obtained as follows.Assuming that the contact pressure of the stringing block is uniformly distributed throughout the entire range of contact with the conductor, as shown in figure 1(a), F is the axial tension acting on the conductor, α is the wrap angle, and R is the radius of the stringing block.Without considering the rolling friction force of the stringing block, the force analysis of the infinitesimal conductor under axial tension F is shown in figure 1 Where, dP is the resultant force of tension at both ends of the infinitesimal conductor, and dN is the support force of the stringing block on the conductor.
For the approximate treatment of the infinitesimal dθ, then dd sin 22 Further considering the influence of the axial torque of conductor, the force analysis of the infinitesimal conductor in the groove of block is shown in the figure 1(c).
The force analysis of the infinitesimal conductor in the groove of block is carried out, as shown in figure 1(c).List the equilibrium equation: Where, dFS is the friction force between the infinitesimal conductor and the groove of block; dFN is the support force of the block groove on the infinitesimal conductor; dT is the axial torque of the infinitesimal conductor (the difference of torque at both ends of the infinitesimal conductor); d is the diameter of conductor, β is the angle between dFN and the positive direction of Y-axis.
The friction between the infinitesimal conductor and the block groove is Where, μs is sliding friction coefficient.
It can be seen from equation (2), equation (3) and equation ( 4), ( ) The friction torque Tf generated by the transverse friction of the stringing block on the conductor is Set the torsional friction coefficient between conductor and block μ = μscosβ.Due to cosβ≤1, so μ ≤ μs.That is, the torsional friction coefficient of the conductor is not greater than the sliding friction coefficient between the conductor and the block, thus During the experiment, the friction torque Tf generated by the stringing block on the conductor can be obtained by measuring the difference of torque (T1-T2) of the conductor at both ends of the stringing block under different working conditions.Then the friction coefficient can be obtained according to equation (8).12 22 Where, T1 is the torque applied to the conductor at one side of the block, T2 is the torque of conductor measured at the other end of the block.

Experimental Device
According to the purpose of the experiment, the design and processing of the experiment device for the torsional friction coefficient of the stringing block to the conductor are completed, as shown in figure 2. The device can apply axial tension and torque (or elongation and torsion angle) to the conductor, and adjust the wrap angle between the conductor and the stringing block.By measuring the difference of torque at both ends of the conductor, the friction torque of the stringing block to the conductor is obtained.Furthermore, the torsional friction coefficient is obtained.

The Samples of Experiment
JL/G1A-630/45-45/7 ACSR commonly used in UHV AC transmission line are used in the experiment.The work-cases of experiment are shown in table 1.The effective length of the experimental conductor is about 6m, as shown in figure 3. The two ends of the conductor are connected by an assembled traction device, which can be reused, as shown in figure 4.

Experimental Process
The specific experimental process is shown in figure 5.

Analysis of Experimental Results
The experimental data of JL/G1A-630/45-45/7 conductor are analyzed respectively according to the wrap angle of 20° and 30° when the conductor passes through the stringing block.
(1) Analysis on experimental results of conductor passing through the stringing block The average friction torque of the conductor passing through the stringing block under different working-cases was measured in the experiment, as shown in figure 6.With the increase of the conductor tension and wrap angles, the torsional friction torque of the stringing block to the conductor also gradually increases.(2) Experimental results of torsional friction coefficient of conductor Set load equivalent P = Fα.Then according to equation ( 8), the torsional friction coefficient μ of the stringing block to the conductor can be written as: The friction coefficient μ is calculated from the experimental results of the friction torque Tf in figure 6, and the variation curve of friction coefficient μ with load equivalent P is shown in figure 7. The fitting relationship ( 10) is obtained by shielding the three data points with large residuals as outliers, and the coefficient of determination R 2 = 0.99, indicating that the fitting effect is good.

Conclusion
The torsional friction moment of JL/G1A-630/45-45/7 conductor under different tension and wrap angle between conductor and stringing block is obtained through the experiment, and then the torsional friction coefficient is obtained.The experiment results show that: (1) Torsional friction coefficient μ of the stringing block to the conductor decrease with the increase of the tension of conductor and the wrap angle passing through the stringing blocks.
(2) By fitting the torsional friction coefficient μ obtained under different working-cases, the relationship between the torsional friction coefficient μ expressed by the tension F and wrap angle α is obtained, and then the calculation formula of the torsional friction torque Tf is obtained.

Figure 1 .
Figure 1.Diagram of the force acting on the conductor passing through the stringing block.
10) into equation (7), the torsional friction torque Tf of the stringing block to the conductor can be obtained, i.e This research was supported by Science and Technology Research Project of State Grid Corporation of China (Research on Simulation of Conductor Damage Based on Tension-Torsion Coupling Effect and Control Technology).The number of the project is 5200-202216090A-1-1-ZN.References [1] Li Y P, Song F L and He G Y 2008 Calculation of tension coupling effect of ACSR Journal of Nature Science Edition (04): 78-81.[2] Lin J H, Zeng W 2015 Research on aluminium conductor steel reinforced tension layered characteristics of overhead conductors Electric Wire & Cable (4): 4. [3] Wan J C, Qin J, Qiao L, Wang S L, et al. 2023 Influence of tension stringing construction process factors onconductor loose strands and its risk assessment Guangdong Electric Power 36(8):113-123.[4] Qin J, Qiao L, et al. 2024 Analysis, simulation and experimental research on the mechanisms of lantern-shaped strand defects in the conductor construction of transmission line Structures 59. [5] Wan J C, Qin J, Qiao L, et al. 2023 Research on physical model and criterion of loose strands in tension stringing construction Journal of Physics: Conference Series, AAME 2023 2512： 12003 [6] DL/T 371-2019 2019 Stringing Blocks of Overhead Transmission Line.[7] Jiang P H 2009 Effects of slot bottom size on stringing pulley friction coefficient and conductor abrasion Electric Power Construction (1): 8-11.