Rapid Repair of Broken Spiral Feeder Shaft by Overlay Welding

Literature Overview

The article by Liu Xinhua, published in China Pulp and Paper (2002, Vol. 23, No. 8, p. 51), describes a field repair solution for a broken shaft in a spiral feeder used in a paper manufacturing plant. The paper, while brief in length, addresses a common industrial maintenance challenge: the rapid restoration of critical rotating equipment through overlay welding techniques when replacement parts are unavailable or when downtime must be minimized.

Equipment Background and Failure Analysis

Spiral feeders (also known as screw conveyors or augers) are widely used in paper manufacturing to transport pulp, additives, and other materials. The shaft of a spiral feeder is subjected to combined loading from torque, bending, and cyclic stress due to material impact. Failure modes typically include:

• Fatigue cracking originating from stress concentration points such as keyway slots, step changes, or surface defects
• Wear-induced reduction in shaft diameter, particularly at bearing journals and coupling interfaces
• Corrosion fatigue in humid environments typical of paper mills
• Thermal stress cracking from uneven heating during operation

In the case described, the shaft failure was attributed to a combination of fatigue cracking and localized wear at the bearing seat. The crack initiated at a surface defect and propagated circumferentially until the remaining cross-section could no longer support the applied torque.

Repair Strategy

The repair approach involved the following steps:

1. Assessment and preparation: The broken shaft was extracted from the feeder housing, and the fracture surface was examined to determine the root cause. The shaft was cleaned of rust, scale, and residual material.
2. Welding material selection: A high-hardness overlay welding consumable was selected to restore the worn bearing seat and reinforce the crack-prone region. The consumable was chosen to provide hardness in the range of 45-55 HRC, sufficient to resist wear while maintaining adequate toughness.
3. Preheating: The shaft was preheated to 200-250°C to reduce cooling rates and minimize the risk of hydrogen-induced cracking.
4. Overlay welding: Multiple passes of overlay weld metal were deposited on the worn and cracked areas, building up the shaft to the required diameter.
5. Machining: The overlay weld deposit was machined to restore the original shaft geometry, including bearing seat diameter and tolerance.
6. Post-weld treatment: The repaired shaft was subjected to a stress-relief anneal at 550-600°C to reduce residual stresses.

Technical Parameters

Critical Success Factors

The success of this repair depended on several critical factors:

• Surface preparation: Thorough cleaning of the shaft surface to remove rust, scale, and any residual material was essential to ensure proper weld fusion and avoid porosity.
• Heat input control: Excessive heat input would lead to excessive softening of the base metal, while insufficient heat input would result in poor fusion and lack of penetration.
• Weld sequence: The overlay welds were applied in a spiral pattern around the shaft circumference to ensure uniform heat distribution and minimize angular distortion.
• Post-weld machining: The overlay deposit must be machined to precise tolerances to ensure proper bearing fit. Excessive machining allowance must be left during welding to accommodate the final machining operation.

Practical Considerations

The rapid repair approach described in this paper is particularly valuable in the following scenarios:

• When spare parts are not available and the equipment must be returned to service quickly
• When the cost of replacement exceeds the cost of repair
• When the equipment is located in a remote site where shipping replacement parts is impractical
• When the failure mode is localized and does not compromise the structural integrity of the entire component

However, engineers must exercise caution in applying this approach. The following limitations should be considered:

• Overlay welding repairs are not suitable for components subjected to high cyclic loading where fatigue life is critical
• The repair may not restore the original metallurgical properties of the shaft, particularly if the base metal is significantly different from the overlay material
• The repair should be considered a temporary or semi-permanent solution, with a planned replacement at the next scheduled maintenance interval

Lessons Learned

This case study illustrates the value of practical welding knowledge in industrial maintenance. The repair was accomplished using standard SMAW equipment and readily available consumables, demonstrating that effective solutions do not always require advanced technology. The key lies in proper assessment of the failure mode, appropriate selection of welding materials, and careful execution of the welding and post-weld operations.

The paper also highlights the importance of documentation in maintenance activities. A detailed record of the repair procedure, including welding parameters, consumable specifications, and post-weld treatment, is essential for future reference and for evaluating the long-term performance of the repair.

Summary

This brief but practical paper demonstrates the effective application of overlay welding for rapid repair of a broken spiral feeder shaft in a paper manufacturing environment. The repair approach, while straightforward, requires careful attention to surface preparation, heat input control, and post-weld treatment. The case underscores the importance of practical welding skills in industrial maintenance and the value of documented repair procedures for long-term equipment reliability.