When an important gear failure occurs, someone becomes responsible for analyzing the failure, determining its cause, and recommending a solution. A company can select its own engineer, an outside consultant, or both. If a consultant is called in, this should be done as early in the process as possible.

Though similar procedures apply to any failure analysis, the specific approach can vary depending on when and where the inspection is made, the nature of the failure, and time constraints.

When and where. Ideally, the engineer conducting the analysis should inspect the failed components as soon after failure as possible. If an early inspection is not possible, someone at the site must preserve the evidence based on instructions from the analyst.

If a suitable facility for disassembling and inspecting the gearbox is not available on-site, it may be necessary to find an alternate location or bring the necessary equipment to the site.

Nature of failure. The failure conditions can determine when and how to conduct an analysis. For example, if the gears are damaged but still able to function, the company may decide to continue their operation and monitor the rate at which damage progresses. In this case, samples of the lubricant should be collected for analysis, the reservoir drained and flushed, and the lubricant replaced.

If gearbox reliability is crucial to the application, the gears should be examined by magnetic particle inspection to ensure that they have no cracks. The monitoring phase will consist of periodically checking the gears for damage by visual inspection and by measuring sound and vibration.

Time constraints. In some situations, the high cost of shutting down equipment limits the time available for inspection. Such cases call for careful planning. For example, dividing tasks between two or more analysts reduces the time required.

Preparing for inspection

Before visiting the failure site, interview a contact person located at the site and explain what you need to inspect the gearbox including personnel, equipment, and working conditions.

Request a skilled technician to disassemble the equipment under your direction. But, make sure that no work is done on the gearbox until you arrive. This means no disassembly or cleaning. Otherwise, a wellmeaning technician could inadvertently destroy evidence.

Verify that the gearbox drawings, disassembly tools, and adequate inspection facilities are available.

Ask for as much background information as possible, including manufacturer’s part numbers, gear and bearing runtime (hr), service history, and lubricant type.

Now, it’s time to assemble your inspection equipment, including items such as a magnifying glass, measuring tools, felt tip markers, lubricant sampling equipment, and photographic equipment. A well-designed set of inspection forms for the gearbox, gears, and bearings should be at the top of your priority list.

Failure inspection

Before starting the inspection, review the background information and service history with the contact person. Then interview those involved in the design, installation, operation, maintenance, and failure of the gearbox. Encourage them to tell everything they know about the gearbox even if they feel it is not important.

After completing the interviews, explain your objectives to the technician who will be working with you. Review the gearbox assembly drawings with the technician, checking for potential disassembly problems.

Visual examination. Before disassembling the gearbox, thoroughly inspect its exterior. Use an inspection form as a guide to ensure that you record important data that would otherwise be lost once disassembly begins. For example, the condition of seals and keyways must be recorded before disassembly. Otherwise, it will be impossible to determine when any damage may have occurred to these parts. Gear tooth contact patterns should be taken before completely disassembling the gearbox (see next section).

After the external examination, disassemble the gearbox and inspect all internal components, both failed and undamaged. Examine closely the functional surfaces of gear teeth and bearings and record their condition. Before cleaning the parts, look for signs of corrosion, contamination, and overheating.

After the initial inspection, wash the components with solvents and re-examine them. This examination should be as thorough as possible because it is often the most important phase of the investigation and may yield valuable clues. A low power magnifying glass and pocket microscope are helpful tools for this examination.

It is important to inspect the bearings because they often provide clues as to the cause of gear failure. For example:

• Bearing wear can cause excessive radial clearance or end play that misaligns the gears.
• Bearing damage may indicate corrosion, contamination, electrical discharge, or lack of lubrication.
• Plastic deformation between rollers and raceways may indicate overloads.
• Gear failure often follows bearing failure.

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