Overlay Welding Repair Process for 55kW Motor Shaft

Literature Overview

This 1993 paper by Wang Zhengjian, published in Machinery, documents the overlay welding repair of a motor shaft in a 55 kW pump motor assembly at the Yongchuan Phosphate and Cement Factory. The shaft, having suffered severe wear at the bearing seat after prolonged service, was restored using a systematic overlay welding procedure. The paper reports that the repaired shaft operated reliably for over two years, demonstrating the economic viability and technical soundness of the approach.

Repair Specifications and Requirements

The shaft specifications and repair requirements are clearly defined in the paper and form the basis for the process design. The shaft has a total length of 1200 mm and a diameter of 110 mm. The wear zone requiring overlay is 120 mm long, with a minimum required overlay thickness of 8 mm. These parameters are critical in determining the welding procedure, consumable selection, and post-weld machining allowance.

The 8 mm minimum thickness requirement is particularly significant. It implies that the overlay must provide not only wear resistance but also sufficient material for post-weld machining to achieve the final dimensional tolerance and surface finish required for bearing installation. A typical bearing seat for a 110 mm shaft would require a surface roughness of Ra 0.8 μm or better, which necessitates a machining allowance of at least 1.0 to 1.5 mm on the diameter.

Process Design and Key Technical Points

The repair process involves several critical steps that must be carefully executed to ensure a sound overlay. The following process flow was adopted:

1. Surface preparation: The worn surface was thoroughly cleaned to remove oil, rust, and loose material. Any cracks or deep pits were inspected and addressed before welding.
2. Consumable selection: The choice of welding consumable is critical for ensuring compatibility with the shaft material and achieving the required hardness and wear resistance. For motor shafts, a hard-facing consumable with adequate toughness is typically selected to resist both abrasive wear and the dynamic loading encountered during operation.
3. Welding procedure: Multiple layers were deposited to achieve the required thickness, with each layer carefully controlled to minimize dilution and ensure full fusion. The welding sequence was designed to minimize residual stress and distortion.
4. Post-weld machining: After the overlay was complete, the surface was machined to the final dimensional tolerance and surface finish requirements.
5. Quality verification: The repaired shaft was inspected for dimensional accuracy, surface integrity, and hardness before reassembly.

The two-year service life achieved with this repair is a strong validation of the process. In the context of pump motor applications, where shafts are subjected to continuous rotational loading, bearing friction, and potential misalignment, the overlay must maintain its integrity under cyclic stress. The success of this repair suggests that the consumable selection and process parameters were well matched to the service conditions.

Engineering Practice Considerations

Several lessons can be drawn from this repair case for engineers dealing with similar shaft restoration challenges:

• Economic analysis: Replacing the entire shaft assembly, including the commutator and iron core, would be significantly more expensive than overlay welding repair. The economic threshold for repair versus replacement should be carefully evaluated based on the cost of the consumables, labor, and downtime versus the cost of a new shaft.
• Stress management: Motor shafts are subjected to significant bending and torsional stresses during operation. The overlay layer must be designed to accommodate these stresses without cracking or spalling. Preheating and controlled cooling rates are essential to minimize thermal stresses in the weld zone.
• Dimensional control: The overlay must be deposited with sufficient excess to allow for machining to the final tolerance. Over-deposition is preferable to under-deposition, as the latter would require additional welding passes and increase the risk of defects.
• Quality documentation: Maintaining detailed records of the welding procedure, consumable batch numbers, and inspection results is essential for traceability and future reference.

Study Insights and Reflections

This paper, though brief, exemplifies the practical engineering approach to component repair. The focus on measurable outcomes—service life, dimensional accuracy, and economic benefit—reflects the pragmatic mindset required in industrial maintenance. The two-year reliability record is particularly noteworthy, as it demonstrates that overlay welding repair can restore a component to a condition comparable to a new part, provided the process is properly designed and executed.

For modern engineering practice, this case study reinforces the importance of systematic repair procedures. The 5W2H framework—What, Why, Where, When, Who, How, and How much—applies naturally to repair planning: What is the defect and its extent? Why did the failure occur? Where on the shaft is the damage located? When should the repair be performed? Who is responsible for execution? How will the repair be carried out? And how much will it cost? Applying this framework ensures that all aspects of the repair are addressed before welding begins.

In conclusion, the overlay welding repair of the 55 kW motor shaft demonstrates that systematic, well-documented repair procedures can extend component life significantly while reducing maintenance costs. Engineers should adopt this approach as a model for shaft restoration, ensuring that process parameters, consumable selection, and quality verification are rigorously controlled to achieve reliable long-term service performance.