The reliability of industrial equipment depends directly on the quality of its mechanical components.

Girth gears, pinions, trunnions, shells, mill heads, mill bottoms, tyres, rollers and other large mechanical components are subject to stringent requirements regarding manufacturing quality, geometry, surface finish and metallurgical integrity.

Depending on their design, dimensions and operating conditions, these components may be manufactured by casting, forging, fabrication and welding, or machining.

Manufacturing inspection and monitoring help ensure that each stage of the process complies with the drawings, technical specifications and contractual requirements, while also enabling potential manufacturing deviations to be detected at an early stage.

With over 25 years of experience in industrial inspection and non-destructive testing (NDT), DIAG METAL provides its expertise to help clients define appropriate inspection requirements during project preparation and secure every stage of the manufacturing process through to final component acceptance.

Inspection Begins During Project Preparation

Component quality cannot be assessed solely at the end of the manufacturing process.

For large mechanical components subjected to significant operating loads, inspection requirements must be defined at the early stages of the project, taking into account the component’s function, manufacturing process, and dimensional, geometrical and metallurgical characteristics.

DIAG METAL draws on its experience to help clients establish the project’s inspection framework, including:

• Dimensional and geometrical characteristics to be inspected;
• Applicable tolerances;
• Surface finishes and roughness requirements;
• Hold points and inspection points;
• Inspections to be performed at each manufacturing stage;
• Required non-destructive testing;
• Validation of weld repairs;
• Material and heat treatment requirements;
• Documentation and records to be provided by the manufacturer.

This preparation helps align the requirements of the client and manufacturer from the outset and reduce the risk of non-conformities that could result in component rejection during final inspection.

Inspection of Cast Components

Many large mechanical components can be manufactured by casting, including trunnions, mill bottoms, mill heads, girth gears, tyres, rollers and various other cast components such as housings and frames.

Manufacturing inspection may cover material compliance, monitoring of the various manufacturing stages, dimensional and geometrical characteristics, surface finish, machining operations, non-destructive testing, any required repairs and final component compliance.

For a cast trunnion, for example, inspection is not limited to identifying potential internal or surface indications. Diameters, concentricity, runout, drilled holes and the surface roughness of bearing contact surfaces can also be checked in accordance with project requirements.

Similarly, surfaces intended for seals or sealing functions may require specific surface finish inspections.

The objective is to verify both material integrity and the dimensional, geometrical and functional conformity of the component.

Inspection of Forged Components

Forged components are used in a wide range of mechanical applications requiring high levels of strength and integrity, including pinions, gearbox planet gears, tyres, rollers and other critical mechanical parts.

Forged pinions, for example, are components for which manufacturing inspection must take into account material quality, final geometry and the various stages of metallurgical processing, including carburising and quenching.

Inspection may cover the forging process, machining operations, dimensional and geometrical characteristics, non-destructive testing, heat treatments, hardness, any required repairs and final component compliance.

Heat treatment is a particularly important stage in the manufacture of certain forged components. It can directly affect their mechanical properties and performance in service.

Inspection activities may include verifying heat treatment requirements, checking process parameters against specifications, reviewing heat treatment records, checking hardness and examining the results of post-treatment inspections.

Following heat treatment and machining, the component’s dimensions, geometry and functional characteristics can also be verified to ensure compliance with project requirements.

Inspection of Fabricated and Welded Components

Some large components can also be manufactured through fabrication and welding, including shells, girth gears and various mechanical assemblies or associated structures, such as housings and frames.

Inspection may cover joint preparation, materials used, welding operations, assembly geometry, distortion, final dimensions, non-destructive testing, heat treatments where required, any necessary repairs and final compliance with drawings and specifications.

Dimensional and geometrical inspection of fabricated assemblies is particularly important. Distortion caused by the manufacturing process must be assessed to ensure it remains within the limits permitted by the design requirements.

For fabricated shells and girth gears, controlling overall geometry, alignment, diameters and functional characteristics can be essential to ensure correct installation and proper operation of the assembly.

Where specified, inspection may also cover post-weld heat treatment and verification that it complies with the applicable requirements.

Inspection Tailored to Each Component

Given the considerable dimensions of these mechanical components and their often one-off manufacturing, each part has its own characteristics, functions and manufacturing risks.

Their inspection requires in-depth knowledge of the relevant manufacturing processes. Drawing on its experience across these processes, DIAG METAL is an established specialist and trusted partner in this field.

Independent Expertise for Your Projects

Engaging DIAG METAL gives clients access to an independent technical perspective on the manufacturing of their components.

Our more than 25 years of experience in inspection and non-destructive testing, combined with our expertise in metallurgy, casting, forging, welding, and dimensional and geometrical inspection, enable us to assess each component as a whole.

We do more than simply determine whether a characteristic complies with requirements. We also seek to understand:

• Why a particular characteristic is critical;
• Which parameters could cause deviations;
• What impact those deviations could have on equipment performance;
• Which inspections are appropriate for detecting them;
• How potential causes can be eliminated where possible;
• How to address confirmed non-conformities.

This approach turns inspection into a valuable tool for controlling and securing the manufacturing project.

Do You Have a Manufacturing Project?

DIAG METAL can support you with manufacturing inspection and monitoring of mechanical components for rotating industrial equipment, including kilns, dryers, mills and granulators.

Our scope includes mill heads, trunnions, tyres, rollers, shells, girth gears, pinions, gearboxes and other large mechanical components.

We provide dimensional and geometrical inspection, metallurgical expertise and non-destructive testing, tailored to the requirements of each project.

Put more than 25 years of specialised experience to work on your next manufacturing project.

Faced with environmental challenges and the need to preserve industrial resources, companies are increasingly seeking solutions to extend the service life of their equipment and reduce their environmental impact.

In this context, on-site re-cutting of girth gear teeth by machining offers a high-quality, potentially more eco-friendly long-term alternative to replacing an existing girth gear with a new one. When the condition of the component and its technical characteristics allow, this operation can help extend its service life while avoiding some of the processes required to manufacture new equipment.

Whether cast or fabricated from welded steel components, girth gears require significant raw materials, energy and industrial resources. Their manufacture involves several machining, inspection, handling and transportation operations.

On-site re-cutting by machining therefore supports resource conservation and sustainable maintenance by prioritizing the refurbishment of existing equipment whenever technically feasible.

Manufacturing a New Girth Gear: Different Processes, Different Environmental Impacts

The environmental impact of manufacturing a girth gear depends on several factors, including the manufacturing process, the material used, the dimensions of the component and the processing operations required.

Cast Girth Gears

Casting generally involves several stages: melting, alloy preparation, pouring, cooling, shakeout, fettling, machining and inspection.

Alloy production and heat treatment operations can account for a significant share of energy consumption and greenhouse gas emissions. Material requirements, manufacturing yields and any additional processing operations also influence the overall environmental footprint.

Fabricated Girth Gears

Fabricated girth gears are assembled from steel components through processes such as cutting, rolling, preheating, welding, heat treatment, machining and inspection.

Their environmental impact depends on factors including the origin of the steel, the quantity of material used, the welding and machining processes, and any additional work required to achieve the specified geometrical characteristics.

In both cases, replacing an existing girth gear with a new component requires additional resources and manufacturing capacity.

On-Site Re-Cutting by Machining: An Economical and Reliable Approach to Sustainable Refurbishment

On-site re-cutting by machining involves machining the teeth of an existing girth gear to restore the required tooth geometry and meet operating requirements, provided the condition of the component and the technical constraints allow it.

By carrying out the operation directly at the industrial site, this approach can postpone the need to manufacture a completely new girth gear and reduce certain challenges associated with urgent replacement.

The main potential environmental benefits include:

• Preserving resources: Extending the service life of an existing girth gear can avoid the immediate consumption of additional raw materials required to manufacture a replacement.
• Reducing manufacturing operations: On-site re-cutting by machining can eliminate or reduce the need for some casting or steel fabrication stages, as well as certain machining operations associated with manufacturing a new girth gear.
• Reducing transportation requirements: On-site intervention avoids transportation associated with shipping a large component away from the facility and delivering its replacement.
• Extending service life: Refurbishing the gear teeth can defer equipment replacement and make better use of the resources already invested in its manufacture.

The actual environmental benefit depends on each individual situation. On-site re-cutting by machining also requires energy, tooling, consumables and travel for the intervention team. Its environmental value should therefore be considered as part of the overall technical, economic and strategic decision-making process.

An Eco-Friendly Solution That Must Remain Technically Reliable

Reducing environmental impact must never come at the expense of industrial equipment reliability.

Before any intervention, a technical assessment must evaluate the condition of the gear teeth, the characteristics of the girth gear, the feasibility of machining and the operating requirements of the installation.

Depending on the situation, this assessment may include dimensional and geometrical inspections, non-destructive testing (NDT) and an examination of the constraints specific to the component and its operating conditions.

The objective is to determine whether on-site re-cutting by machining can be carried out under controlled conditions while meeting the required technical specifications and potentially improving certain characteristics of the gear teeth.

An eco-friendly and sustainable approach therefore relies on a comprehensive strategy combining extended equipment service life, controlled maintenance operations and the preservation — or even improvement — of industrial reliability.

DIAG METAL: Your Trusted Partner for Gear Optimization

With more than 25 years of experience in industrial gears, inspection and non-destructive testing, DIAG METAL supports industrial companies with technical expertise, maintenance and girth gear refurbishment projects.

Our expertise in on-site re-cutting of girth gear teeth by machining enables us to evaluate alternatives to girth gear reversal or replacement, subject to the technical feasibility of the proposed intervention.

Every project requires an assessment tailored to the component’s characteristics, its current condition and the operating requirements of the installation.

By prioritizing refurbishment whenever technically feasible, industrial companies can adopt a rational, technically grounded maintenance strategy, preserve the resources already invested in their equipment and reduce certain operations associated with manufacturing a new component.

Would you like to explore on-site re-cutting by machining for your girth gear? DIAG METAL can help you assess the technical feasibility of your project.

When a large girth gear shows significant wear, pitting or scuffing, several techniques may be proposed to address the gear teeth.

Among them, re-cutting by machining and manual grinding are two approaches that serve different purposes.

For a gear requiring urgent corrective work (burrs, steps at the tooth roots, etc.), manual grinding is a commonly used solution.

However, when the damage requires removing a significant amount of material and restoring accurate, functional gear-tooth geometry, manual grinding reaches its limits and is unsuitable for achieving a precise, repeatable result.

On-site re-cutting by machining provides a much more systematic approach to restoring the gear to specification: determining the amount of material to remove, reproducing the tooth profile, maintaining the gear pitch and pressure angle, and ensuring proper tooth contact during meshing.

This fundamental technical difference between controlled geometrical restoration and manual correction, together with the urgency of the intervention, should guide the choice of gear-tooth restoration method.

Re-Cutting by Machining: A True Gear-Tooth Restoration Solution

Re-cutting a girth gear by machining does not simply mean removing material from a tooth.

The objective is to restore the functional geometry of the gear teeth after removing the damaged areas.

This approach is particularly relevant when the girth gear exhibits:

• Significant profile wear;
• Pitting;
• Scuffing;
• Tooth-flank damage caused by pinion misalignment;
• Localized indentations caused by a foreign object.

The process includes, in particular:

• Identifying the damaged areas;
• Measuring the existing geometry;
• Determining the amount of material to remove;
• Defining the reference profile;
• Maintaining the gear pitch;
• Reproducing the profile tooth by tooth;
• Taking the actual runout of the girth gear into account;
• Inspecting the teeth after the operation.

Re-cutting by machining is therefore a comprehensive gear-restoration method, rather than a simple surface-correction operation.

Restoring Geometry Tooth by Tooth

A girth gear must operate as a coherent whole.

It is therefore not enough to correct individual teeth in isolation. The profile, positioning and pitch must remain consistent across the entire gear.

The on-site re-cutting by machining process makes it possible to define a reference tool path and reproduce it on every tooth.

The tool path is repositioned from one tooth to the next to maintain the gear pitch, while accounting for the actual position of each tooth.

This method helps achieve consistent geometry across the entire girth gear.

It combines:

Profile reconstruction + pitch accuracy + tooth-by-tooth repeatability + adaptation to actual geometry.

Control of the tool path is one of the key advantages of re-cutting by machining.

On-Site Machining: A Solution for Large Industrial Girth Gears

Large girth gears have an important characteristic: their actual geometry can change depending on how they are installed and the conditions in which they operate.

Runout and distortion can be influenced by:

• Installation conditions;
• Deformation of the supporting structure;
• Mechanical loads;
• Temperature variations;
• Thermal expansion;
• Operating conditions.

Re-cutting directly on site makes it possible to work on the girth gear in its actual configuration at the time of the intervention.

The machining tool path can be adapted to the measured geometrical variations while maintaining the reference profile and gear pitch.

This capability is particularly important when restoring a large girth gear that is already installed on its equipment.

Re-Cutting by Machining or Grinding: Why Does the Choice of Technique Matter?

Manual grinding relies on abrasion and is suitable for certain localized corrections, particularly in emergency situations.

However, when the gear teeth are significantly damaged, the requirements are different.

It must be possible to remove enough material to eliminate the damaged area, then recreate gear-tooth geometry that is accurate, repeatable and consistent.

This is where re-cutting by machining offers a clear advantage.

Comparison of Capabilities

The table highlights a key difference: re-cutting by machining treats the gear teeth as geometry to be reconstructed, rather than merely as surfaces to be corrected.

This capability makes re-cutting by machining particularly relevant for restoring large girth gears.

Removing Pitting: Eliminate Damaged Material to Restore Sound Gear Teeth

Gear pitting is a form of damage caused by contact-fatigue stresses (Hertzian contact pressure).

When the damage becomes significant, the affected material must be removed to expose sound material.

The re-cutting process therefore involves the following steps:

Assess the damage → determine the required machining depth → remove the damaged material → recreate the complete tooth profile.

Machining makes it possible to control this process and accurately manage the amount of material removed.

The reference geometry is then reproduced consistently across the gear teeth.

This approach can restore a girth gear affected by pitting, provided the condition of the teeth and the available material make the operation technically feasible.

Maintaining Gear Pitch: Restoring the Entire Tooth Set

During re-cutting, the consistency of the pitch depends on the position of all active tooth profiles.

Each active tooth profile must be positioned accurately relative to the adjacent teeth to ensure geometrical consistency across the gear and proper meshing.

Gear pitch is therefore a fundamental parameter in the process.

Re-cutting by machining makes it possible to reposition the tool path for each tooth and maintain the geometrical consistency of the entire girth gear.

This approach restores the gear as a complete system rather than applying a series of independent corrections.

Runout: A Key Parameter for Large Girth Gears

On a large industrial girth gear, the geometry measured on the equipment can change over time.

On-site re-cutting makes it possible to take these changes into account.

The machining tool path is adapted to the actual position of the girth gear so that the reference profile can be reproduced on each tooth.

The process therefore combines:

Reference profile + gear pitch + actual girth gear position + adaptation to runout.

This ability to adapt is a major advantage when a large girth gear must be restored directly on its equipment and tooth contact needs to be optimized.

Why Choose On-Site Re-Cutting by Machining?

Replacing a large girth gear can require extensive operations, including:

• Dismantling;
• Handling;
• Lifting equipment;
• Transportation;
• Replacement of components;
• Prolonged shutdown of the installation.

When the equipment configuration allows it, on-site re-cutting by machining makes it possible to work directly on the girth gear in place.

The girth gear does not necessarily need to be dismantled for restoration.

This approach reduces the constraints associated with dismantling and transportation and, above all, makes it possible to work directly on the actual geometry of the girth gear in its industrial operating environment.

It can also postpone turning the girth gear over, thereby extending its service life over several operating cycles.

A Repair Process Based on Measurement and Diagnosis

A girth gear cannot be re-cut correctly without first assessing its condition.

At DIAG METAL, the intervention follows a structured process.

1. Inspection

Identify wear on the working and non-working tooth flanks, pitting, scuffing and other forms of damage.

2. Measurement

Analyze the existing geometry and characterize the gear teeth.

Check pinion-to-girth-gear alignment.

3. Diagnosis

Identify the areas that can be machined and assess the available material, including verification through calculations where necessary.

4. Defining the Profile

Use calculations to determine the reference geometry to be restored.

5. Defining the Tool Path

Evaluate the machining tool paths, taking the tooth profile and gear pitch into account.

6. Accounting for Runout

Adapt the tool path to the actual geometry of the girth gear.

7. Re-Cutting

Machine the teeth in a controlled manner, one tooth at a time.

8. Final Inspection

Verify the resulting geometry after the operation, including the profile, pitch, pressure angle and operating contact pattern.

This approach transforms a repair operation into a controlled gear tooth restoration process. Depending on the size of the girth gear, the on-site operation, including re-cutting and associated inspections, typically takes between 5 and 15 days, without requiring the girth gear to be dismantled.

Restore or Replace?

When a large girth gear is damaged by normal wear, turning it over or replacing it is not necessarily the only option.

Depending on the extent of the damage, the overall condition of the girth gear and the amount of material available, on-site re-cutting by machining may make it possible to restore the existing gear.

The process can be summarized as follows:

Measure → diagnose → define → machine → inspect.

The benefits are twofold: restoring the functional geometry of the gear teeth and, where technically feasible, retaining the existing girth gear.

When replacement becomes unavoidable (due to cracking, excessive wear, etc.), on-site re-cutting may help keep the equipment in service while a new girth gear is being supplied, while limiting the impact on production requirements. This remains subject to technical feasibility and operating conditions.

The Advantages of On-Site Re-Cutting by Machining

Genuine Restoration Capability

The process removes damaged material and recreates the functional tooth profile while optimizing tooth contact during meshing.

A Controlled Tool Path

Each tooth is machined using a defined and controlled tool path.

Maintaining Gear Pitch

The positioning of each tool path helps preserve the geometrical consistency of the gear.

Adapting to Runout

The machining strategy can account for the actual geometrical variations of the girth gear.

Work Directly on the Equipment

Re-cutting is carried out without dismantling the girth gear.

An Alternative to Turning Over or Replacing the Girth Gear

Re-cutting can postpone turning the girth gear over and extend the service life of the existing component.

Conclusion: Re-Cutting by Machining or Grinding — Two Approaches for Different Needs and Requirements

The choice of restoration technique should not be based solely on its ability to remove material.

When a large girth gear is damaged, the real question is how to remove the affected material while correctly restoring the gear-tooth geometry.

Grinding offers the advantage of rapid intervention for urgent work, but its capabilities become very limited when significant defects need to be corrected.

Re-cutting by machining, by contrast, makes it possible to determine precisely how much material must be removed, recreate the original tooth profile, reproduce that geometry tooth by tooth, maintain the gear pitch and account for the actual runout of the girth gear.

For large industrial girth gears, the ability to accurately restore the gear-tooth geometry directly on the equipment is a decisive advantage.

On-site re-cutting by machining is therefore much more than a material-removal operation.

It is a systematic, engineering-based solution for restoring industrial gear teeth.

Before considering turning over or replacing your girth gear, a feasibility study for on-site re-cutting by machining can determine whether this solution is technically viable for your gear.

Is your girth gear showing significant wear or abnormal vibration?

DIAG METAL can analyze the existing geometry, characterize the damaged areas and define a restoration strategy suited to your equipment.

Contact us to discuss your requirements and assess whether on-site re-cutting by machining your girth gear is a suitable alternative to turning it over or replacing it.