Part 1 of this article appeared in the March 2005 issue. It explained the makeup of the analysis team and the skills that each member should possess. It discussed getting organized, preparations for the investigation, the possibility of keeping the gearbox operating and some of the initial testing that takes place.
The analyst should perform a thorough external examination before the gearbox is removed and disassembled. This involves an inspection form to 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, if they get damaged, it will be impossible to determine when it happened.
Before cleaning the gear housing, inspect it for signs of overheating, corrosion, contamination, oil leaks and damage.
Measure shaft coupling alignment before moving the gearbox. Note the condition and loosening torque of fasteners including coupling and mounting bolts. To check for possible twist in the gear housing, install a dial indicator at each corner of the gearbox. Then, measure movement of the mounting feet as bolts are loosened. If there’s no twist, each indicator will record the same vertical movement. If not, calculate the twist from relative movements.
Unless your plant has adequate facilities for disassembling the gearbox, relocate it for disassembly at a fully-equipped gear manufacturing facility. Large gearboxes require a pair of overhead cranes for manipulating and disassembly. Special tools such as bearing pullers or hydraulic presses may be required for removing components. Most importantly, the disassembly should be done in a clean, well-lighted environment.
Take precautions if you need to ship a gearbox. Fretting corrosion on gear teeth and rolling element bearings is a common problem that may occur during transit. Ship the gearbox on an air-ride truck, and support it on vibration isolators to help avoid fretting corrosion. If possible, ship it with oil. To minimize contamination, remove the breather and seal the opening, secure labyrinth seals with silicone rubber, and cover the gearbox with a tarpaulin. It’s best to inspect the gearbox as soon as possible. If that’s not possible, store it indoors in a dry, temperature-controlled environment.
Disassemble the gearbox
A skilled technician should disassemble the equipment under the analyst’s direction. Otherwise, by disassembling, cleaning or draining oil, a well-meaning technician could inadvertently destroy evidence.
Technicians should understand that failure investigation is different from a gearbox rebuild, and the disassembly must be controlled.
The analyst should verify that gearbox drawings, disassembly tools and adequate facilities are available. Make sure you provide the analyst sufficient privacy to conduct the investigation and access to needed information.
The analyst should explain the objectives to the technician. They will review the gearbox assembly drawings together, checking for potential disassembly problems. The technician should understand that failure analysis must be done slowly and carefully. Normally they’re trained to work quickly.
After the external examination, the technician should clean the exterior of the gearbox thoroughly to avoid contaminating the interior when opening it. Once disassembled, the analyst should inspect the components, both failed and undamaged.
Other points should be obvious. Inspect components before cleaning them. Mark relative positions of components before removing them. Don’t discard any parts. Don’t touch fracture surfaces or try to fit broken pieces together. If fractures can’t be examined immediately, coat them with oil and store the parts to avoid damaging fracture surfaces. Examine functional surfaces of gear teeth and bearings and record their condition. Before cleaning the parts, look for signs of corrosion, contamination and overheating.
After this initial inspection, wash the components with solvent and re-examine them. This examination should be as thorough as possible because it’s often the most important phase of the investigation and may yield valuable clues. A low-power magnifying glass and 30X pocket microscope are helpful tools for the exam.
It’s important to inspect bearing rollers and raceways and mounting surfaces because they might provide clues about the cause of gear failure. For example:
- Wear on bearing rollers, raceways or mounting surfaces can cause excessive radial clearance or endplay that misaligns gears.
- Bearing damage may indicate corrosion, contamination, false brinelling, fretting corrosion, electrical discharge or lack of lubrication.
- Plastic deformation between rollers and raceways may indicate overloads.
- Gear failure often follows bearing failure.
Identify and mark each component (including gear teeth and bearing components) to facilitate clearly identified written descriptions, sketches and photographs. It’s especially important to mark bearings, including inboard and outboard sides, to identify their location and orientation.
Describe components consistently. An example includes always starting with the same part of a bearing and progressing through the parts in the same sequence. This helps to avoid overlooking any evidence. Important observations should be described in writing using sketches and photographs where needed. The following guidelines help maximize your chances for obtaining meaningful evidence:
- Concentrate on collecting evidence, not on determining cause of failure. Regardless of how obvious the cause may appear, don’t form conclusions until all evidence is considered.
- Document what is seen. List observations, even if some seem insignificant or if you don’t recognize the failure mode. Remember, there’s a reason for everything you see, and it might become important later when you consider the rest of the evidence.
- Document what you don’t see to help eliminate certain failure modes and causes. For example, if there’s no scuffing, you can conclude gear tooth contact temperature was less than the lubricant’s scuffing temperature.
- Search the bottom of the gearbox. Often this is where you find the best-preserved evidence, such as a fractured gear tooth that falls free without sustaining secondary damage.
- Use time efficiently. Be prepared for the inspection. Plan your work carefully to obtain as much evidence as possible. Don’t be distracted by anyone.
- Control the investigation. Watch every step of the disassembly. Don’t let the technician get ahead of you. Disassembly should stop while you inspect and document the condition of a component, and then proceed to the next component.
- Insist on privacy. Don’t let anyone distract your attention and don’t announce any conclusions until the investigation is complete.
Calculate gear load capacity using geometric data from either the drawings or the gears and housing -- number of teeth, outside diameter, face width, gear housing center distance, whole depth of teeth and tooth thickness (both span and topland thickness).
During inspection, begin to formulate hypotheses regarding the cause of failure and use them as a basis for selecting specimens for laboratory testing. Take broken parts for laboratory evaluation or, if this isn’t possible, preserve them for later analysis. After completing the inspection, coat parts with oil and store them properly to avoid corrosion or damage.
Effective analysis also depends on how well oil samples represent the operating lubricant. To take samples through the gearbox drain valve, first discard stagnant oil from the valve. Then, take a sample at the start, middle and end of the drain period to mitigate stratification effects. To sample from a storage drum or reservoir, draw samples from the top, middle and near the bottom. Oil samples can uncover problems such as excessive water content.
Compare failed parts with new, unused components, if available. Compare a sample of fresh lubricant to the used lubricant.
Do you have it all?
Gather everything needed, including completed inspection forms, written descriptions and sketches, photos and test specimens. Devote at least two days for the failure inspection, even if the gearbox is small and simple. This affords time after the first day’s inspection to collect your thoughts and analyze data. Often the first day’s inspection discloses a need for additional data, which you can be gathered on the second day.
Determine failure modes
Now is time to examine the information and determine how the gears failed. Several failure modes may be present and you need to distinguish between the primary mode responsible for the failure and the secondary modes that are consequences of the primary mode or which might have contributed to the failure. An understanding of these modes helps to identify the root cause of failure. The six general classes of gear failure modes that describe the characteristics of physical damage are:
- Bending fatigue
- Hertzian fatigue
In many cases, failed parts and inspection data don’t yield enough information to determine the root cause of failure. You may need gear design calculations and laboratory tests to develop and confirm a hypothesis about the probable cause.
Gear geometry data aids in estimating tooth bending stress, contact stress, lubricant film thickness and gear tooth contact temperature based on transmitted loads. Calculate values according to American Gear Manufacturers Association (AGMA) standards such as ANSI/AGMA 2001 and AGMA 925. Compare calculated values with AGMA’s allowable values to help determine risks of bending fatigue, micropitting, macropitting, wear and scuffing.
Light microscopes are useful for confirming the failure mode, identifying the origin of a fatigue crack, or disclosing a material flaw such as a nonmetallic inclusion. Start the examination with a stereo-zoom microscope at 5X to 40X magnification. Later, if necessary, use a scanning electron microscopes (SEM). A SEM with energy dispersive X-ray is especially useful for identifying corrosion, contamination and inclusions.
If gear geometry or metallurgical properties are likely to influence the primary failure mode, check for geometric or metallurgical defects that may have contributed to the failure. For example, if gear tooth contact patterns indicate misalignment or interference, inspect the gear teeth for accuracy with a gear inspection machine. Conversely, where contact patterns indicate good alignment and loads are within rated gear capacity, check gear teeth for metallurgical defects.
Conduct nondestructive tests before starting any destructive tests. Nondestructive tests aid in detecting material or manufacturing defects and provide information needed for gear rating calculations. These tests include:
- Surface hardness and roughness
- Magnetic particle inspection
- Acid etch inspection
- Gear tooth accuracy inspection
Before proceeding with destructive tests to evaluate material and heat treatment, verify that all parties involved approve the tests. Carefully document positions of cuts and relative locations of pieces with sketches and photographs, and mark specimens with unique identifiers. Destructive tests include:
- SEM microscopy to study fracture surfaces
- Determination of nonmetallic inclusions
- Microhardness survey
- Microstructural determination using acid etches
- Determination of grain size
Form and test conclusions
By the time calculations and tests are completed, you should already have several hypotheses for the probable cause of failure. Now, determine if the evidence supports or disproves the hypotheses. Here you need to evaluate gathered evidence:
- Documentary evidence and service history
- Statements from witnesses
- Written descriptions, sketches, and photos
- Gear geometry and contact patterns
- Gear rating calculations
- Laboratory data for materials and lubricant
Results of this evaluation might make it necessary to modify or abandon initial hypotheses or pursue new lines of investigation. Speculations about failure modes, failure mechanisms and root causes are an essential part of a failure investigation. Ask yourself how the failure occurred; it’s the key to understanding why it happened.
Failure analysis is an iterative process that narrows down possible explanations for failure by eliminating explanations that don’t fit the evidence. It’s helpful to remember Sherlock Holmes’ rule: When you’ve eliminated the impossible, whatever remains, however improbable, must be the truth. Also helpful is Occam’s Razor: When two or more explanations exist for a sequence of events, the simplest explanation more likely will be the correct one.
Finally, after thoroughly testing the hypotheses against the evidence, and evaluating the consistency of results from different tests and analytical methods, you should reach a conclusion about the most probable root cause of failure. In addition, you might identify secondary factors that contributed to the failure.
The failure analysis report should describe relevant facts found during analysis, inspections and tests, and weighing of evidence. Present data succinctly, preferably in tables or figures. Good photographs are especially helpful for portraying failure characteristics. Summarize the facts in conclusions and recommendations. If possible, include recommendations for repairing equipment or making changes in equipment design or operation to prevent future failures.
Robert Errichello is a gear consultant who owns Geartech in Townsend, Mont. Contact him at email@example.com and (406) 266-4624.