Find the root cause of gear failure

Use the same rigorous process a gear failure analyst does.

By Robert Errichello

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Successful gear failure analysis requires proper investigation, a strong team leader and a qualified gear failure analyst. These components are what it takes to determine the root cause of gear failure and maximize your chances for preventing failure recurrence.

Failure analysis is important

A properly managed failure investigation can provide valuable feedback about how a component performs. It might uncover shortcomings or weakness in design, manufacture or quality control. It can provide information for improvements that prevent future failure. In some cases, the failure investigation can assess liability and determine whether the failure was a unique event or a symptom of a wider problem. Rigorous root cause determination might lead to machinery improvements that yield:

  • Greater safety
  • Improved reliability
  • Higher performance
  • Greater efficiency
  • Easier maintenance
  • Reduced life-cycle costs
  • Reduced impact on environment

The team leader

Successful gear failure analysis requires proper investigation, a strong team leader and a qualified gear failure analyst.

– Robert Errichello

The most effective and efficient gear failure investigation is headed by a team leader who is high enough in your corporate hierarchy to be able to establish four items at the outset:

  • The investigation’s priority
  • Available resources
  • Constraints imposed
  • The investigation’s goal

The leader must be a good communicator with the ability to integrate the team and select the best expert for each role in the investigation. The leader should have a broad background and must be skilled in failure analysis techniques such as fault tree analysis (FTA), failure mode assessment (FMA) and root cause analysis (RCA).

The team leader needs a clear understanding of the investigation’s scope to organize it effectively. Time and money are always constrained. Therefore, the scope of the investigation is controlled by what you want to know and how much you’re willing to spend.

After considering all interests, the team leader should define a clearly stated goal before launching an investigation. This involves a detailed, well-documented investigation plan that makes clear to all involved what information is expected from each step of the investigation. The documentation should address:

  • What is to be done
  • Why is it to be done
  • The findings expected to be determined

The specific investigative plan can vary depending on when and where the investigation is made, the nature of the failure and time constraints. In any case, the team leader needs to ensure that everyone involved understands the priorities, the analyst has the necessary resources, the investigation stays within imposed constraints and that the investigation will achieve its goal. This is a collaborative effort.
Additional team staffing should include a gear failure analyst who answers directly to the team leader, and metallurgists and tribologists who collaborate with the analyst and report to the team leader.

In some cases, a gear failure analyst with the necessary skills can be the team leader. Otherwise, the analyst should be responsible for technical details of the analysis, but work under the team leader’s supervision. This arrangement frees the analyst to concentrate on technical detail and permits the team leader to manage resources and logistics necessary to implement the plan.

The gear failure analyst

Gear failure analysis, a subset of general failure analysis, is conducted by an investigator who specializes in it. The requisite qualifications for the analyst include experience in gear design, stress analysis, gear manufacturing plus an understanding of how gearbox components are supposed to function and how they can malfunction. Furthermore, the analyst should have a thorough knowledge of gear metallurgy and tribology, and understand the capabilities and limitations of the analytical procedures both disciplines use.

There’s no real alternative to including an analyst. If you don’t have a qualified gear failure analyst on your staff, either train someone or hire an outside consultant. A metallurgist is unlikely to be familiar with the gear’s function, modes of operation and service characteristics, and is unlikely to be acquainted with manufacturing procedures, accepted workmanship and appropriate materials for a specific gear application.

Costs depend on gearbox complexity, nature of the failure, available resources for the investigation and risks associated with recurrence. The costs details might not be readily apparent. Therefore, cost estimates might need to be revised as the investigation progresses and the team leader needs to assess whether the budget is adequate to achieve the investigation’s goal.

The usual goal is to discover the root cause of a failure and determine the best corrective actions to prevent recurrence. In some cases, the goal might be to assess gearbox performance to improve the design. In other cases, the goal might be to assign responsibility for a failure.

Often there’s pressure to repair or replace failed components quickly and return the gear system to service. Because gear failures provide valuable data that can help prevent future failures, however, you should follow a systematic inspection procedure before repair or replacement begins. This entails a complete disassembly and thorough inspection of gearbox components.

Getting organized

As the investigation proceeds, it might become apparent that other resources are needed to corroborate evidence, such as metallurgical tests or tribological analyses. It’s often the case that investigation resources and budget must be reviewed and revised continually. Unless time and budget are adequate, it might be best not to investigate at all. The gear failure analyst should have access to:

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