Surfacing Repair of Tapered Section on ZQDR-410kW Traction Motor Shaft

Literature Overview

This 1993 paper by Zhou Guoping from Zhuzhou Electric Locomotive Factory, published in Welding Technology, documents the surfacing repair of the tapered section of a ZQDR-410kW traction motor shaft used in Dongfeng-type diesel locomotives. The tapered section of the shaft interfaces with the drive gear through a shrink-fit (thermal expansion) assembly, transmitting a torque of 611,796 N·cm. The paper describes a practical repair strategy that avoids the high cost and complexity of replacing the entire shaft assembly.

Application Background and Requirements

The ZQDR-410kW traction motor is the primary power source for Dongfeng diesel locomotives. The shaft's tapered section is subject to severe cyclic loading and must maintain precise geometric accuracy to ensure proper gear engagement. The following requirements govern the surfacing repair:

Surfacing Repair Strategy

The paper describes a strategy of treating the armature as a complete unit and performing surfacing repair on the tapered section of the shaft. This approach was chosen because:

1. Cost-effectiveness: Replacing the entire shaft would require disassembly of the armature, risking damage to the commutator and iron core.
2. Dimensional control: The tapered geometry requires precise dimensional accuracy that is difficult to achieve through conventional machining of a new shaft and subsequent assembly.
3. Time efficiency: Surfacing repair can be completed in a fraction of the time required for shaft replacement and reassembly.

The surfacing process employed was likely SMAW or TIG welding using a filler material matched to the shaft steel composition. The following process considerations are critical:

• Preheating: The shaft should be preheated to 200–300°C to reduce thermal gradients and prevent cracking in the HAZ.
• Filler selection: A low-hydrogen electrode or matching TIG wire should be used to minimize hydrogen-induced cracking. The filler composition should match the shaft steel (likely a medium-carbon alloy steel such as 40Cr or 42CrMo).
• Multi-pass surfacing: Multiple thin passes should be applied to build up the required thickness while minimizing distortion and residual stress.
• Post-weld machining: After surfacing, the tapered section must be machined to restore the original geometry, including the taper angle, diameter, and surface finish.

Quality Control and Verification

The quality of the surfacing repair must be verified through the following inspections:

1. Visual inspection: The surfaced area must be free of surface cracks, porosity, and incomplete fusion.
2. Magnetic particle testing (MT): The entire surfaced area and the adjacent HAZ must be inspected for surface and near-surface cracks.
3. Ultrasonic testing (UT): Internal defects such as lack of fusion and subsurface cracks should be detected.
4. Hardness testing: The hardness of the surfaced area and the HAZ should be verified to ensure no excessive softening or hardening has occurred.
5. Dimensional verification: After final machining, the tapered geometry must be measured using a coordinate measuring machine (CMM) or precision gauges to confirm compliance with the original specifications.
6. Contact area verification: After assembly with the drive gear, the contact area must be verified by applying a marking compound (such as Prussian blue) and checking the coverage area.

Engineering Practice Reflection

This paper exemplifies the practical problem-solving approach that characterizes successful welding engineering in heavy industry. The decision to repair rather than replace the shaft reflects a sound engineering judgment based on cost, time, and risk considerations. However, several aspects of the repair strategy warrant careful attention:

• Residual stress management: The surfacing process introduces significant residual stresses in the shaft, which can affect the fatigue life of the component. A stress-relief heat treatment after surfacing and before final machining is strongly recommended.
• HAZ properties: The heat-affected zone adjacent to the surfaced area may have altered mechanical properties. For a shaft subjected to cyclic loading, the fatigue strength of the HAZ must be evaluated, as it may represent the weakest link in the component.
• Long-term reliability: Surfaced repairs on safety-critical components should be subject to periodic inspection during the service life of the equipment. The interface between the surfaced layer and the base material is a potential initiation site for fatigue cracks.

Summary

The surfacing repair of the ZQDR-410kW traction motor shaft tapered section is a well-documented example of practical welding engineering applied to heavy-duty rail equipment. The approach demonstrates that, with proper process control and thorough quality verification, surfacing can be a viable alternative to component replacement. Engineers should note that the success of such repairs depends on meticulous attention to filler material selection, process parameter control, residual stress management, and comprehensive post-repair inspection. The 80% contact area requirement and the substantial torque transmission demand make this application particularly demanding, and any deviation from the specified process parameters could compromise the long-term reliability of the repaired shaft.