Overlay Welding Repair of Gear Surface Fracture in YNS1760 Reducer Secondary Pinion

Literature Overview

This technical paper, published in Cement (2011, Issue 10), documents the overlay welding repair of severe gear surface spalling on the secondary pinion of a YNS1760-31.5-VDR main reducer serving a 4.8m × 72m rotary kiln. The failure was triggered by an extraordinary event—a landslide caused by heavy rainfall that severed the kiln's main power cable, preventing controlled slow rotation and inducing bending deformation that resulted in excessive vibration and catastrophic gear surface damage. The repair was executed by Huaihai Zhonglian Cement Group Co., Ltd., and the paper provides valuable insights into emergency repair procedures for heavy-duty industrial gear components.

Failure Analysis and Background

The rotary kiln was commissioned in March 2005 and experienced the triggering event in August 2005. The chain of causation is instructive: a landslide severed the main power cable, the kiln could not be slowly rotated (which would have relieved thermal stress), the kiln shell developed bending deformation, this deformation was transmitted to the large and small gears through the gear mesh, excessive vibration developed, and ultimately the secondary and tertiary pinion gear surfaces suffered severe spalling and pitting.

Component	Specification	Damage Description
Rotary kiln	Φ4.8m × 72m	Shell bending deformation
Main reducer	YNS1760-31.5-VDR	Secondary and tertiary pinion gear surface spalling
Trigger event	Landslide (August 2005)	Power cable severed, kiln unable to slow-rotate
Failure mode	Gear surface spalling	Severe material loss on pinion teeth

Overlay Welding Repair Methodology

The repair strategy involved overlay welding to restore the gear surface profile and add a wear-resistant layer. Key technical considerations in this repair include:

Welding Process Selection and Parameters

For gear surface repair using overlay welding, the following process considerations are critical:

Welding process: Submerged arc welding (SAW) or gas metal arc welding (GMAW) is typically preferred for large gear surface repairs due to high deposition rates and good penetration control. Shielded metal arc welding (SMAW) may be used for smaller areas or field repairs.
Filler material selection: Must match or exceed the base gear material in hardness and wear resistance. For carburized and quenched gear steel (typically 18CrNiMo7-6 or similar), a high-carbon or alloyed steel electrode/wire is selected to provide appropriate hardness after heat treatment.
Pre-heat and interpass temperature: Critical for preventing cracking in high-strength alloy steels. Typical pre-heat temperatures of 200–300°C are applied, with interpass temperatures controlled to prevent excessive grain growth.
Post-weld heat treatment: Tempering is essential to relieve welding residual stresses and achieve uniform hardness across the repaired surface.

Repair Procedure Sequence

The repair process for gear surface spalling typically follows this sequence:

Inspection and assessment: Determine the extent and depth of spalling, assess whether the damage has penetrated into the core material.
Material removal: Grind or machine the damaged surface to a smooth profile, removing all affected material.
Surface preparation: Clean the prepared surface to remove contaminants that could cause porosity or lack of fusion.
Overlay welding: Build up the gear surface profile in multiple passes, controlling heat input to prevent distortion.
Post-weld heat treatment: Tempering to relieve residual stresses.
Machining to final profile: Restore the gear geometry to specification.
Final inspection: Dimensional verification, surface hardness testing, and non-destructive examination.

Engineering Practice Considerations

This case highlights several important engineering considerations for heavy-duty gear repair:

Emergency repair planning: The availability of skilled welders and appropriate equipment for emergency gear repair should be part of any critical asset maintenance strategy. The speed of repair directly impacts production losses.
Root cause mitigation: While the overlay welding repair addresses the immediate damage, the underlying cause (kiln shell deformation) must also be addressed to prevent recurrence. This includes kiln shell straightening, gear alignment verification, and vibration monitoring implementation.
Service life expectations: Overlay-welded gear repairs typically do not achieve the same fatigue life as the original manufactured gear. The repair should be considered a temporary measure, with a planned replacement or more comprehensive overhaul scheduled within the next major maintenance window.
Quality documentation: For critical rotating equipment, all repair procedures, welding parameters, material certifications, and inspection results must be thoroughly documented for traceability and future reference.

Study Insights

This case demonstrates that overlay welding is not limited to corrosion or wear protection applications but is also a viable solution for restoring damaged functional surfaces on heavy-duty mechanical components. The key success factors are proper filler material selection matched to the gear material, careful heat input control to prevent distortion of the gear geometry, and thorough post-weld heat treatment. The extraordinary nature of the triggering event (landslide) serves as a reminder that equipment design must account for abnormal operating conditions and that maintenance strategies should include provisions for emergency repair capabilities.