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[pullquote]
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.
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:
- Design data
- Technical data
- Analysis reports
- Test reports
- Maintenance records
- Operational logs
The analyst should interview witnesses to the failure, operators, maintenance personnel, system designers and other people involved in gearbox design, operation and maintenance. The team leader should identify those responsible for operating the gearbox to provide information and resources the analyst needs.
Gear failures often attract onlookers or other curious parties. However, it’s imperative to preserve evidence and it’s in the best interest of the investigation to restrict access to the failed gearbox. If possible, the team leader should arrange to quarantine the gearbox and schedule an inspection as soon after the gear failure as possible. Failure conditions can determine when and how to conduct an analysis. It’s best to shutdown a failing gearbox as soon as possible to limit damage. To preserve evidence, carefully plan the failure investigation to include shutdown, in-situ inspections, gearbox removal, transport, storage and disassembly.
Prepare for inspection
Before visiting the failure site, the team should explain to the site contact person what is needed for the gearbox inspection, including personnel, equipment and working conditions. Ideally, the analyst should visit the site as soon as possible after failure. If an early inspection isn’t possible, someone at the site must take measures to preserve the evidence.
The analyst needs as much background information as possible, including manufacturer’s specifications, service history, load data and lubricant analyses. The analyst might send the site’s contact person a questionnaire to help expedite information gathering (Download Figure 1 using the "Download Now" button at the bottom of the page).
Before starting the inspection, the analyst should review background information and gearbox service history before interviewing those involved in the design, installation, startup, operation, maintenance and failure of the gearbox. Plant personnel should reveal all they know about the gearbox, even if some facts seem unimportant.
In some situations, the high cost of shutdown will limit the time available for inspection, in which case careful planning is required. It may require dividing tasks between two or more analysts to reduce downtime.
Keep it running?
If the gears are damaged but still functional, you may decide to continue operation and monitor damage progression. In this case, monitor the gear system under the analyst’s supervision. The analyst should ensure there are no risks to human life. For critical applications, the analyst should examine the gears with magnetic particle inspection to ensure there aren’t any cracks that prevent safe continued operation.
Other routine analysis actions the analyst should perform are a visual inspection and measurement of temperature, sound and vibration. The analyst should collect samples of lubricant for analysis and examine the oil filter for wear debris and contaminants, and inspect magnetic plugs for wear debris.
Then, it’s time to drain, flush and refill the reservoir.
Gear tooth contact patterns
The next steps to follow are important. Clean the inspection port cover and the surrounding area. Remove the cover, being careful not to contaminate the gearbox interior. Observe the condition of gears, shafts and bearings.
If there’s evidence of gear misalignment such as macropitting concentrated at ends of teeth, but no broken teeth or other failures that would prohibit rotating the gears, record the gear tooth contact patterns. The way gear teeth touch indicates how they’re aligned. Tooth contact patterns may be recorded under loaded or unloaded conditions (Figure 2). No-load patterns aren’t as reliable as loaded patterns for detecting misalignment because the marking compound is relatively thick and no-load tests don’t include misalignment caused by load, speed or temperature. Therefore, follow any no-load tests with loaded tests.
For no-load tests, paint the teeth of one gear with a soft marking compound and roll the teeth through the mesh so compound transfers to the unpainted gear. Turn the pinion by hand while applying a light load to the gear shaft by hand or brake. Use clear tape to lift the patterns from the gear and mount the tape on white paper to form a permanent record (Figure 3). The compound PT-650 Tooth Marking Grease available from Products/Techniques, Inc. (909) 877-3951, works best. Scotch No. 845 Book Tape (2-in. width) works well for lifting contact patterns.
For loaded tests, thoroughly clean the teeth with a solvent. Brush paint several teeth on one or both gears with a thin coat of machinist’s layout fluid (Dykem). Run the gears under load for sufficient time to wear off the lacquer and establish the contact pattern. Photograph patterns for a permanent record.
Record loaded contact patterns under several loads, for example, 25%, 50%, 75%, and 100%. Inspect patterns after running about one hour at each load to monitor how patterns change with load. Ideally, the patterns shouldn’t vary with load. Optimum contact patterns cover nearly 100% of the active face of gear teeth under full load, except at extremes along tooth tips, roots and ends, where contact is lighter as evidenced by traces of lacquer.
Finally, measure and record gear backlash and the endplay and radial movement of the input and output shafts.
To view part II, visit http://www.plantservices.com/articles/2006/100.html.
Robert Errichello is a gear consultant who owns Geartech in Townsend, Mont. Contact him at [email protected] and (406) 266-4624.