Maintenance Analysis Of High‑Strength Bolts For Wind Turbines

High‑strength bolts are core connecting components widely used in wind turbines under complex load conditions. With the rapid development of the wind power industry, the continuous enlargement of wind turbine units and the extension of service life have raised higher requirements for the operational safety and maintenance reliability of high‑strength bolts. The traditional preventive maintenance mode currently adopted for high‑strength bolts in wind turbines has obvious limitations and can no longer fully adapt to complex on‑site operating conditions. Therefore, it is essential to summarize the existing defects in maintenance work, optimize and upgrade field‑oriented maintenance processes, and promote the transition from periodic preventive maintenance to condition‑based maintenance, so as to ensure the safe and stable operation of wind turbines.

Wind power is one of the fastest‑growing sectors in the new energy industry. Grid‑connected wind turbines serve as the major equipment for wind power generation. With the continuous commissioning of new units and the aging of existing units, the quality control of equipment maintenance has become a key focus of wind power operation and maintenance. Based on the actual operating conditions of in‑service wind turbines, this paper systematically summarizes the common problems in high‑strength bolt maintenance, analyzes the mechanical principles of bolt tightening, and proposes full‑process quality control measures, providing a reference for refined and condition‑based maintenance of high‑strength bolts in wind turbines.

 

1 Current Status and Problems of Preventive Maintenance for High‑Strength Bolts

1.1 Strength Grades and Installation Positions of High‑Strength Bolts

Three typical strength grades of high‑strength bolts are applied in wind turbines: Grade 8.8, Grade 10.9 and Grade 12.9. Different grades are matched according to the load, vibration and stress characteristics of different structural positions. Grade 8.8 bolts are generally used for connections between generators and encoders, as well as between slip rings and gearboxes of pitch‑controlled units. Grade 10.9 bolts are widely adopted for critical load‑bearing connections, including blade‑hub joints, hub‑main shaft joints, and tower segment connections. Grade 12.9 bolts are applied for heavy‑duty connections such as joints between the main shaft and compression rings.

1.2 Main Deficiencies in Current Preventive Maintenance

1.2.1 Fixed Maintenance Cycles Leading to Over‑Maintenance and Under‑Maintenance Risks

At present, the inspection and calibration of high‑strength bolts strictly follow fixed periodic maintenance cycles, without differentiated management based on service life, actual load and vibration frequency. This commonly results in either over‑maintenance or under‑maintenance, both of which pose potential safety hazards. Excessive repeated tightening causes cyclic metal fatigue, accelerates material aging, and may lead to plastic deformation, cracks or even bolt fracture. Inadequate maintenance reduces the friction force of bolt connections and weakens preload, ultimately causing bolt loosening and connection failure, which adversely affects unit stability.

1.2.2 Non‑Standard Use of Maintenance Tools and Incomplete Operating Specifications

Existing maintenance regulations fail to clearly define applicable scenarios and operating methods for different tools corresponding to bolts of different grades and installation positions, including manual torque wrenches, torque multipliers and hydraulic torque wrenches. This leads to disordered and inconsistent field operations. In addition, some torque wrenches are not calibrated before use, and unqualified or overdue tools are occasionally adopted, resulting in inaccurate torque values, non‑standard processes and uncontrollable maintenance quality.

1.2.3 Irregular Human Operations Causing Manual Quality Risks

The calibration quality of high‑strength bolts is directly determined by the responsibility, professional skills and operational standardization of maintenance personnel. In actual field work, irregular behaviors such as arbitrarily reducing calibration torque and simplifying tightening procedures have occurred, resulting in insufficient preload and substandard maintenance quality. Long‑term operation under such conditions easily causes loosening and failure of bolt connections, seriously endangering unit safety.

2 Basic Principles of High‑Strength Bolt Tightening

2.1 Stress Variation during Bolt Tightening

During tightening, the bolt is subjected to tension while the connected components bear compression. The stress state changes dynamically with the tightening angle and can be divided into four stages:

Initial tightening stage: The bolt is not fully attached to the connected surface, resulting in zero compression force. Only minor thread friction exists, and the tightening torque remains low.

Effective tightening stage: After the bolt fully contacts the connected surface, formal tightening begins. Both compression force and tightening torque rise rapidly with the increase of rotation angle.

Plastic deformation stage: When the bolt stress reaches the yield limit, plastic deformation occurs. The rotation angle continues to increase, while the compression force and torque grow slowly and tend to stabilize.

Overload failure stage: Continuous over‑tightening causes excessive stress, leading to a rapid drop in compression force and torque and eventually resulting in bolt tensile fracture.

2.2 Influence of Friction Coefficient on Bolt Compression Force

There is no fixed correspondence between tightening torque and preload. The friction coefficient greatly affects the final compression force. Under the same tightening torque, differences in friction at thread contact surfaces and bolt bearing surfaces can lead to significant deviations in preload. Thread corrosion, oil contamination, impurities, uneven lubrication and coating damage will change the friction state, causing preload deviation and connection failure, which are major causes of bolt loosening and uneven stress in field operation.

3 Quality Control Measures for High‑Strength Bolt Maintenance

3.1 Core Influencing Factors of Maintenance Quality

The quality of bolt maintenance can be analyzed through a hierarchical pyramid model. In descending order of importance, the key influencing factors are personnel, operation methods, tools and equipment, and material properties (including bolt and connected component materials). These factors interact and jointly determine the maintenance effect and long‑term operational reliability of bolt connections.

3.2 Full‑Process Quality Control Measures

3.2.1 Strengthen Personnel Training and Standardize Operation Behavior

Specialized training on high‑strength bolt maintenance shall be conducted regularly to improve staff responsibility and professional competence. Training contents include bolt stress principles, grade matching criteria, standardized tightening processes and failure identification methods. Operators shall be capable of identifying defects such as corrosion, deformation, cracks and loosening, and shall strictly follow standard procedures to avoid manual errors and arbitrary torque adjustment.

3.2.2 Implement Pre‑maintenance Quality Inspection of Bolts

Comprehensive visual and performance inspection shall be carried out before maintenance, covering dimensional accuracy, coating integrity, thread condition and lubrication status. Defects including corrosion, burrs, deformation and surface damage shall be fully checked. The friction state of threads shall be evaluated according to operating conditions to predict potential torque deviation risks. Unqualified bolts are prohibited from reuse to eliminate hidden dangers at the source.

3.2.3 Standardize Tool Management and Ensure Detection Accuracy

A regular inspection, calibration and maintenance system shall be established for maintenance tools. Dynamic and static accuracy tests shall be performed on torque wrenches, torque multipliers and hydraulic wrenches before operation. Overdue, inaccurate or damaged tools are forbidden. Corresponding tools and tightening methods shall be selected according to bolt strength grade, installation position and load characteristics to realize classified and standardized operation. Tools shall be cleaned after use, and manual torque wrenches shall be reset to the minimum torque value to maintain long‑term measurement accuracy.

3.2.4 Unify Technical Standards and Realize Full‑Process Consistent Control

Unify technical specifications, measurement standards and calibration criteria throughout bolt inspection, tool verification and torque testing processes to eliminate inconsistent operation quality caused by different operators or batches. Establish complete maintenance records including bolt position, calibrated torque, operator information and test data, realizing full‑process traceability and stable maintenance quality control.

4 Conclusion

As critical load‑bearing connecting components, high‑strength bolts directly determine the operational safety and stability of wind turbines. The traditional periodic maintenance mode has shortcomings such as rigid cycles, non‑standard tools and frequent manual errors, which cannot meet the requirements of refined wind power operation. Comprehensive quality improvement can be achieved through standardized personnel management, optimized operation processes, strict tool control and complete pre‑inspection mechanisms. With the continuous improvement of operator professionalism, upgrading of maintenance processes and popularization of intelligent testing equipment, the maintenance of high‑strength bolts will gradually realize condition‑based, accurate and controllable management, effectively reducing bolt failure risks and ensuring long‑term safe and stable operation of wind turbines.