As the most common and critical type of detachable connection in mechanical structures, threaded connections are widely used in equipment such as railway gearboxes, reducers, and transmissions. During major overhauls of gearboxes, bolts are typically replaced, but the screw holes on the housing need to be reused multiple times due to structural reasons. Scientifically assessing the failure risk of threaded pairs during repeated assembly and disassembly processes and establishing reasonable standards for inspecting and replacing screw holes is a crucial technical challenge for ensuring equipment reliability.

1Composition and working characteristics of threaded connection pair A typical threaded connection pair consists of the following two parts: bolt or screw (external thread) nut or screw hole (internal thread) In the process of assembly, the bolt and screw hole realize the transmission of preload through the threaded pair to form a reliable mechanical connection.

The threaded connection pair has the advantages of repeated disassembly and assembly, simple structure and convenient installation. However, in the process of multiple disassembly and reassembly, the threaded surface will inevitably produce wear, fatigue and damage, which will affect its service life and connection strength.

2.Typical failure modes of threaded connections during repeated use. In the context of repairing the threaded holes in the gearbox housing, the common failure modes of threaded connections include: 1. Thread wear During repeated tightening and disassembly, the thread threads gradually wear out, the chamfer becomes rounded, and the effective engagement length decreases; this leads to a decrease in the actual preload force of the connection, increasing the risk of loosening.

2. Creep and plastic deformation of screw hole material Especially in aluminum alloy and cast iron box, the screw hole is subjected to repeated force, and the local material will undergo micro flow; the inner diameter of the screw hole increases, the tooth deformation is weakened, and the strength of the thread is weakened.

3. Thread tensile fatigue Repeated tightening causes the root of the thread to repeatedly subjected to tensile stress; micro cracks gradually appear, and finally lead to the fracture or slip of the screw hole.

4. Pre-tension force reduction: The degradation of thread tooth profiles and changes in friction coefficients result in insufficient actual pre-tension force under the same tightening torque, affecting sealing performance and structural stability. 3⃣ Factors influencing the repeated service life of thread pairs: The actual repeated service life of threaded connections is influenced by a combination of multiple factors:

Limit of repeated use of thread pair and standard recommendation Based on industry standards and practical maintenance experience, the following suggestions are proposed:

If any of the following conditions are found, it is strictly prohibited to continue to use: obvious wear of the thread in the screw hole, missing teeth, tooth deformation; the torque of loosening after tightening the bolt is obviously low; the thread gauge test is not up to standard.

5⃣ On-site Inspection and Replacement Criteria for Threaded Joints To ensure the reliability of threaded connections during gearbox maintenance, on-site operations are recommended as follows: �� Inspection Tools Use a thread gauge (thread stop gauge) to check the accuracy of the thread hole dimensions; use an internal diameter dial gauge to measure the variation in the major diameter of the thread hole; perform a bolt tightening test to confirm the actual pre-tightening force. �� Replacement and Repair Methods For minor wear, use a dedicated thread repair tool (thread tap) to clean the thread hole; for severe damage, use a steel wire thread sleeve (Heli-Coil) or replace with a dedicated thread protector; if repair is not possible, consider replacing the entire unit or having it professionally repaired.

6.Technical Summary and Assembly Recommendations: Scientific and standardized management of thread pairs is crucial for ensuring the quality of gearbox maintenance and extending the equipment's lifespan. Always replace bolts with new ones to ensure reliable preload. Strictly control the material and usage frequency of the housing screw holes. Regularly inspect the condition of the screw holes and promptly repair or replace them as needed. The thread surface can be treated with a thread locking agent to enhance anti-loosening and sealing performance. On-site operations must avoid violent disassembly and incorrect assembly.