Understanding gear failures is crucial for maintaining efficient machinery operation. This guide outlines the essential steps for analyzing gear failures, from the initial inspection to identifying the failure type, and offers practical solutions for prevention and repair.

 Someone is in charge of investigating, identifying the cause, and suggesting a solution when a significant gear failure happens. An organization has the option of choosing an outside consultant, its own engineer, or both. It is best to involve a consultant as early in the process as possible if one is needed.

The specific approach can vary depending on the nature of the failure, time constraints, and the location and timing of the inspection, even though the same procedures apply to all failure analyses. In this guide from Machinery Lubrication, we’ll provide useful answers for both repair and prevention, outlining the critical processes involved in assessing gear failures, from the initial inspection to determining the kind of failure.

Initial Inspection and Preparation

Timeliness and Location

Immediately upon failure, the engineer performing the analysis should inspect the failed components. Evidence must be kept on site if an early inspection is not feasible. In case that a suitable facility is unavailable, equipment or a different place may be brought to the site.

Nature of Failure

The condition of the gears affects how gearbox problems are analyzed. The company may carry on with operations while keeping an eye on the damage rate if the gears are damaged but still usable. In these situations, it is necessary to gather lubricant samples, empty and clean the reservoir, and reapply the lubricant. Magnetic particle inspection should be performed to stop cracks if reliability is important. In addition, measures of sound and vibration as well as regular visual examination are required.

Time Constraints

In certain cases, the time allocated for inspection is constrained by the substantial cost of shutting down machinery. Planning is necessary in these situations. One way to cut down on time is to divide up the work among two or more analysts.

Preparing for Inspection

Speak with a contact person prior to visiting a failure site to go over the gearbox's required inspections. This includes personnel, equipment, and working condition. Ask an experienced technician to disassemble the equipment. Do not clean it until you arrive.

Make sure there are sufficient inspection facilities, disassembly tools, and diagrams for the gearbox. Collect background data on the lubricant type, gear and bearing runtime, service history, and manufacturer part numbers. Assemble inspection tools such as magnifying glass, measuring tools, felt tip markers, lubricant sampling equipment, and photographic equipment. Take careful consideration when creating a set of inspection forms for the gearbox, gears, and bearings.

Conducting the Failure Inspection

Before examining a gearbox, consult the contact person to evaluate the gearbox's service history and background information. Make sure to interview everyone with any knowledge pertaining to the gearbox's design, installation, operation, maintenance, and failure. Explain your objectives to the technician and review the gearbox assembly diagram to check for potential disassembly problems.

Visual Examination

Before disassembling a gearbox, perform an external inspection using an inspection form to record important data. Record seals and keyways condition to determine damage. Take gear tooth contact patterns before disassembly. Disassemble the gearbox and examine the interior parts, including the damaged and working portions, following the external inspection. Examine functional surfaces of gear teeth and bearings, and record their condition. Look for indications of overheating, pollution, and corrosion before cleaning.

Use solvents to clean the components after the first inspection and re-examine them. Since the examination is frequently the most crucial phase and could provide key hints. It should be as comprehensive as possible. 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.

Gear tooth contact patterns (Finish this step before taking the gearbox components apart for inspection). The level of alignment of mated gear teeth is shown by the way they contact one another (Figure 1). Document tooth patterns under either loaded or unloaded. Apply marking compound to one gear's teeth for no-load testing. The contact pattern is then transferred to the unpainted gear by rolling the teeth through the mesh. Using scotch tape, remove the pattern from the gear and place it on paper to create a permanent record.

Image Source: Machinery Lubrication

In order to carry out loaded tests, coat the gears' teeth with machinist's layout lacquer, subject them to load in order to wear off the lacquer, and document contact patterns in photos for a permanent record.

Document Observation

Document observation, take pictures and sketches, and properly identify all the component part. This includes bearing rollers and gear teeth. This is to provide an accurate diagnosis of a failure.

To find the location of ay bearing in the gearbox, mark both the inboard and outboard sides of each bearing. Provide consistent description of the pieces, beginning with the same parts and going through them in the same order. Make sure to gather evidence, not figuring out why something failed, and making decisions only after taking into account all available data.

Gear Geometry

It will eventually be necessary to calculate the gearset's load capability. The gears and gear housing's drawings, or the gears themselves, should provide the following geometric data for this purpose:

• Number of teeth
• Outside diameter
• Face width
• Gear housing center distance for each gearset
• Whole depth of teeth
• Tooth thickness (both span and top land measurement)

Specimens for a Laboratory Test

Specimens for laboratory testing are chosen during inspection, and theories regarding the reasons of failure are developed. Broken pieces ought to be saved for additional examination.

Oil samples are useful, but how well they represent the running lubricant determines how useful they are. To prevent stratification, samples from gearbox drain valves should be thrown away and collected at different times. Reservoir samples ought to be taken from the top, center, and close to the bottom.

It is necessary to check the oil filter and magnetic plug for impurities and wear debris. Samples taken from oil storage reservoirs or drums can show problems such as too much water from incorrect storage. Make sure you have all the materials needed for the failure analysis before you leave the location.

Identifying the Failure Type

Bending Fatigue

This frequent kind of failure is a gradual, slow-moving failure brought on by continuous loading. There are three phases to it:

• Crack Initiation: Microscopic fractures are caused by plastic deformation, which happens in regions of discontinuities or concentrated stress, like notches or inclusions.
• Crack Propagation: Growing perpendicular to the maximum tensile stress is a smooth crack.
• Fracture: A sudden fracture results from a crack that gets big enough.

Contact Fatigue

A failure mode known as contact or Hertzian fatigue occurs when surface fissures and the separation of metal pieces from the tooth contact surface are brought on by repetitive pressures. Macroscopic and micropitting surface fatigue are common forms.

Macroscopic and micropitting surface fatigue are common forms. When cracks begin at or below the surface, a fragment of the surface material breaks out and forms a pit with sharp edges. This phenomenon is known as macropitting.

There are four types of macropitting: flake, progressive, spall, and nonprogressive. Progressive pitting covers a large amount of the tooth surface, whereas nonprogressive pitting is less than 1 mm diam in small regions. Over a wide area, spelling and flake macropitting create uneven craters. Under magnification, micropitting appears as frosted, matte, or gray stained, with minuscule pits all over it.

Wear

The three main types of gear tooth surface wear are adhesion, abrasion, and polishing. It can be brought on by mechanical, chemical, or electrical activity. Adhesion is limited to oxide layers on the surface of teeth and entails the transfer of material from one tooth to another as a result of welding and ripping.

It is divided into two categories: mild and moderate. Severe adherence is known as scuffing. When the gearset is running in, mild adhesion develops, but it goes away as local flaws wear out. In some situations, moderate adhesion causes the contact surface to become excessively worn by removing some or all of the machining marks. Abrasion is the result of lubricant impurities and manifests as parallel, smooth scratches or gouges.

There are different levels of abrasion on tooth surfaces: light abrasion leaves fine scratches, whereas moderate abrasion removes most machining marks. All machining markings are eliminated by severe abrasion, which can also result in wear steps at the contact surface and dedendum. Chemically active lubricants tainted with fine abrasive encourage polishing, a fine-scale abrasion that yields a mirror-like sheen on gear teeth.

Scuffing

Metal can be scratched off teeth by moving it from one surface to another. This can happen in bands in the sliding direction in the addendum or dedendum. Load concentrations allow it to be localized. Surfaces appear ripped and plastically deformed with a rough or matte appearance.

There are several levels of scratching; mild scratches are nonprogressive and only affect tiny areas. If the operating environment doesn't alter, moderate scuffing could worsen and cover large areas of teeth in patches. On large sections of a gear tooth, there is severe scuffing. The surface material may become plastically distorted and shifted across the tooth root or tip.

Laboratory Tests and Calculations

Failed components and inspection data might not always provide sufficient details to identify the root cause of a failure. To create and validate a hypothesis for the likely reason, lab testing and gear design computations are frequently required. Based on transmitted loads, gear geometry data is useful in estimating lubricant film thickness, tooth contact temperature, bending stress, and tooth contact stress in gears.

Gear geometry information is useful in estimating the temperature of the gear tooth contact, lubricant film thickness, and tooth contact stress depending on transmitted loads. One can ascertain the risk of macropitting, bending fatigue, and scuffing by comparing these values to the AGMA allowed values. Fatigue crack origins can be determined or the failure mode confirmed by laboratory testing and investigations using instruments like light and scanning electron microscopes (SEM).

If the geometry of the gear is likely to have an impact on the primary failure mode, look for any metallurgical or geometric flaws that might have aided in the failure. For example, use gear inspection machines to check the gear for correctness if tooth contact patterns show misalignment or interference. On the other hand, look for metallurgical flaws in the teeth if contact patterns show satisfactory alignment and the computed loads fall within the rated gear capacity.

Before doing any destructive testing, perform nondestructive testing. These nondestructive tests, which offer rating data and help identify manufacturing or material flaws, consist of:

• Surface hardness and roughness.
• Magnetic particle inspection.
• Acid etch inspection.
• Gear tooth accuracy inspection.

Then conduct destructive tests to evaluate material and heat treatment. These tests include:

• Microhardness survey.
• Microstructural determination using various acid etches.
• Determination of grain size.
• Determination of nonmetallic inclusions.
• SEM microscopy to study fracture surfaces.

Formulating Conclusions

Create hypotheses regarding the likely cause of failure and assess the evidence after all computations and testing are finished. This comprises written descriptions, witness statements, drawings, photographs, gear geometry, contact patterns, gear design calculations, and lab data in addition to the documentation. The assessment could result in the revision or rejection of original theories or new research directions. Determine the most likely cause and secondary factors leading to the failure after comparing the hypothesis with the available data.

Reporting Results

All results, tests, inspections, evidence evaluations, conclusions, and suggestions should be included in a failure analysis report. It should be presented briefly, ideally using tables or figures, and include high-quality images to emphasize the aspects of the failure. To stop failures in the future, recommendations include fixing equipment or altering its design or functionality.

Understanding gear failures is just the beginning. Implementing effective solutions is crucial to preventing future issues. At Le Price International, we offer advanced gearbox solutions designed to enhance reliability and performance. Our products are engineered to address common failure modes, ensuring prolonged operational efficiency. Our gearbox solutions offer flexibility and adaptability for a wide range of applications across many industries, and they are entirely adaptable to fit your individual demands.

Contact us today to learn more about how our innovative solutions can benefit your machinery. Let us help you achieve optimal performance and prevent gear failures with our expert solutions.