Download - Failure Analysis for Gears

8/6/2019 Download - Failure Analysis for Gears

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Failure Analysis for Gearingreference www.elecon.com by Gary DeLange.

Gear teeth contain evidence of failure mechanisms that include wear, surface fatigue,plastic flow and breakage.

GEAR TOOTH PROFILE TERMINOLOGY.Tooth mesh changes from sliding rolling action

at the pitch diameter and then back to slidingduring gear rotation.

As with any failure analysis, finding the root cause of damage to gearing often requires alot of detective work. You may need to review the service history and interview witnessesor employ technical tools such as vibration analysis and oil analysis. However, the causeof failure cannot be determined without a complete inspection of the condition of the gearteeth themselves. An understanding of the failure modes indicated by the condition of theteeth, when combined with knowledge of the operating conditions and maintenance

history, will permit developing methods to avoid similar failures in the future.

Getting into gearIn order to analyze and interpret gear failures, it is helpful to consider some of theterminology and practices commonly used in the gear industry. The accompanyingdrawing shows a few of the common terms used to describe gear tooth profiles.

Gear quality ratings are established by the American Gear Manufacturers Association

(AGMA). Quality levels are driven by the application requirements. In some basicapplications, AGMA 4 or 5 quality gears may suffice, while other more demandingapplications may require an AGMA 12 or 13 gear; aircraft transmissions may requireAGMA 14 or 15 accuracy. The case hardened and ground gears used in many high-

capacity gear drives today are generally at least an AGMA 11 quality level. The differencesbetween quality levels are progressive, somewhat like the Richter earthquake scale,where the difference between one level and the next is substantial. This can causeproblems if an attempt is made to reverse-engineer a replacement gear withoutknowledge of its quality level. Replacing a gear with one of lesser quality may have

disastrous effects on gear life. Service factors play an important role in selecting theproper gear drive for the application. Manufacturer catalogs list typical service factors forvarious types of applications. In a speed reducer, the ratings are applied to each gear set.A multi-stage reducer will be limited by the lowest rated gear set, which will usually bethe low-speed gear set of a typical industrial gear drive. This gear set also transmits themost torque.

Things to be aware of when reviewing an application for possible causes of failure includethe possibility of design error in specifying the original gear set. As an example, the speedreducer on a mixer might be sized adequately for operation but not for startup if the


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mixer is full and therefore requires considerably more power to overcome the inertia ofthe load. If this happens, the high speed pinion shaft could deflect, which may cause thegear teeth to run misaligned and overload them. Not only does this accelerate wear, but itcan force the oil out of the gear mesh and cause several types of failure.

The primary way to check design and manufacturing errors is to review the inspectioncharts, specifications, and other information from the manufacturer, then compare them

with the requirements determined by reviewing the actual application parameters. Theoriginal design may have been satisfactory, but subsequent changes in the applicationcould cause it to be inadequate.

Why gears fail ?One persons failure may be anothers break-in. The difference between wear and failurecan be simply a matter of time. If a gear fails in 25 years, it did its job. If it fails in 25

minutes or 25 hours, theres a serious problem.

When gears mesh, they roll only at the pitch line, as noted in the drawing. Above andbelow this line, the sliding action that occurs causes inherent wear that can lead to failure.Gear teeth also flex as they go in and out of mesh. Therefore, they have to be softenough to deflect and give without breaking. Yet a hardened gear has higher capacityratings, so most gears are heat-treated to harden them to the degree necessary for theapplication.

Gears may be either through-hardened or case-hardened. Through-hardened gears are

put through a heating and controlled cooling process as a unit, so the hardness is thesame throughout the gear. These gears are usually below 390 Brinell in hardness, abovewhich conventional machining becomes difficult or impossible. Case- hardened gears arehardened only on the surface of the gear teeth, to a predetermined depth, to about 58 to62 Rockwell C, or roughly as hard as a bearing race. The increased hardness improves thegears durability rating by providing greater resistance to pitting and greater strength, orresistance to breakage.

From one point of view, causes of gear failure may include a design error, an applicationerror, or a manufacturing error. Design errors include such factors as improper geargeometry as well as the wrong materials, quality levels, lubrication systems, or otherspecifications. Application errors can be caused by a number of problems, includingmounting and installation, vibration, cooling, lubrication, and maintenance. Manufacturingerrors may show up in the field as errors in machining or heat treating.

AGMA recognizes four main modes of gear failure, plus a fifth that covers everything else.They are wear, surface fatigue, plastic flow, breakage, and associated gear failures.

When a gear is suspected of showing signs of failure, if possible it should be examinedperiodically over time. Recording contact patterns or taking photographs at intervals willaid in comparison and help determine whether the condition is progressive. Keep in mindalso that failure never occurs as an isolated event. Two or more failure modes may occursimultaneously or in succession, and the eventual failure mode may be different from theroot cause.

WEAR FAILURE

Wear, the first failure mode category, occurs when metal is worn away from the contactareas of the gear teeth in a more or less uniform manner. Some wear is normal, but there

are several degrees of wear and many ways in which wear can occur.

Polishing is a slow process of wear in which metal-to-metal contact during operation


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causes a very smooth surface to develop on the gear teeth. It is most common duringslow-speed operation, where the lubricant film is too thin, and the gears are operatingnear the lubrication borderline. Normally, this condition does not cause a problem unlesscontinued wear prevents the gears from reaching the design life of the equipment. Oncethe gears are polished, further action can be reduced or prevented by using a higherviscosity lubricant or lowering the lubricant temperature. Other possible remedies include

reducing the transmitted load or increasing the operating speed to provide a better oilfilm.

Figure 1 Moderate Wear

Moderate wear (Fig. 1) shows up as a contact pattern in which metal removal occursfrom both the addendum and dedendum tooth surfaces, and the operating pitch lineremains as a continuous line. This may be caused by lubricant contamination but is oftenunavoidable due to limitations of lubricant viscosity, gear speed, and temperature. It may

occur normally throughout the design life of a gear set, particularly when gears operatenear boundary lubrication conditions. Increasing oil film thickness, either by cooling the

lubricant, using a higher viscosity lubricant or operating at higher speeds, can sometimesreduce normal wear. Replacing a splash-fed lubrication system with a filtered positive-spray system may improve lubrication by removing particles and delivering a moreconsistent supply of oil to the working surfaces.

Further solutions include reducing the gear loading and changing the gear geometry,materials, or hardness.

Extreme wearmay appear as the same kind of contact pattern and pitch line visibilitythat occur with moderate wear, but the progression rate is much faster. Here, aconsiderable amount of material may be removed uniformly from the gear tooth surfaces,and the pitch line may show signs of pitting. Extreme wear will cause failure to occurbefore the design life of the gear set is reached. It may cause enough damage to thetooth profile that the resulting high dynamic loads will further accelerate the wear. Causesof extreme wear include a lubricating film too thin for the tooth load, fine abrasive

particles in the lubrication system, and severe vibratory loads. Shaft seals and air-ventfilters, properly installed and maintained, may help reduce wear. Other solutions include

oil cooling, higher viscosity lubricants, higher speeds, reduced loads, and possibly reducedvibratory loads if the application permits.

Figure 2: Abrasive Wear

Abrasive wear shows up as a lapped surface, with radial scratches or grooves on the


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tooth contact surfaces. When this occurs shortly after startup of a new installation or onany open gearing, particles in the lubricating system are generally the cause. These mayinclude metal particles from the gears and bearings, weld spatter, scale, rust, and sand,dirt, or other environmental contaminants. Fig. 2 shows severe abrasion. Careful cleaningof the gearbox and lubrication system before use can minimize abrasive wear. With acirculating lubrication system, adding a filter or using a finer replacement filter will help

reduce this type of wear. Regular oil changes will help for splash-lubricated drives, andhigher viscosity oil also may help protect either type of system with a thicker oil film that

will keep the finer particles from scratching.

Careful cleaning of the gearbox and lubrication system before use can minimize abrasivewear.

Figure 3: Corrosive Wear

Corrosive wear (Fig. 3) is visible as surface deterioration, caused by the chemical action

of active ingredients in the lubricant. These may include acid, moisture, foreign materials,and extreme-pressure additives. During operation, the oil breaks down and allowscorrosive elements present in the oil to attack the gear contact surfaces. This action mayaffect the grain boundaries and cause fine, evenly distributed pitting. Checking the oil forbreakdown and changing it at regular intervals can help minimize corrosive wear.

Lubricants with high antiscuff, antiwear additive content must be observed even morecarefully because they are chemically active. Gear units that are exposed to salt water,liquid chemicals, or other foreign materials should be sealed from their environment.

Figure 4: Scoring

Scoring may be moderate, localized, or destructive. It can be caused by failure of thelubricant film, usually from overheating in the mesh area, as well as by misalignment,deflection, and uneven temperatures or loads. The resulting metal-to-metal contactproduces alternate welding and tearing that quickly removes metal from the gearsurfaces. Moderate scoring shows up as a characteristic wear pattern, often in patches on

the addendum, dedendum, or both. Radial tear marks usually appear more prominently insofter areas. Upon closer examination, the frosty appearance shows that the rotation has

caused the metal to weld and tear apart (Fig. 4). Localized scoring is similar to moderatescoring but takes place in concentrated portions of the contact areas of the gear teeth,rather than spreading across their full face width.

EVIDENCE OF SURFACE FATIGUE FAILURE


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Destructive scoring or scuffing shows definite radial scratch and tear marks, and materialmay be displaced radially over the tips of the gear teeth. Excessive material may bemissing from above and below the pitch line, causing the pitch line itself to stand outprominently. At this stage, the gear is unfit for further service.Reducing the temperature in the mesh area can prevent moderate scoring. This can be

accomplished by reducing the load, gear speed, or inlet oil temperature. Other solutionsinclude use of a lubricant with extreme-pressure additives, plating a solid lubricant on the

contact surfaces, or honing. Localized scoring is more likely to result from misalignmentfactors than moderate scoring. A wear pattern that shows load concentration near oneend of the teeth indicates possible misalignment or helix angle error. This results in oneportion of the teeth carrying more load than the lubrication film can support. Eliminatingthe causes of uneven loading can prevent localized scoring. These may include

nonuniform gear case deflection, excessive shaft deflection, out-of-parallel bores in thecasing, or helix angle errors. Uneven temperature gradients also may cause localized

scoring and should be remedied by changing the amount of cooling oil applied to themesh or the way in which it is applied.To eliminate destructive scoring (Scuffing), it is necessary to attack the source of the

excessive heat that causes the lubricant to break down. Extreme-pressure additives areone way to help the lubricant stand up to the load, speed, and temperature conditions.Special high-viscositycompounded gear oil or synthetic fluids with anti-scuff additives alsowill help prevent scoring. In extreme cases, the gearing may have to be redesigned to

reduce surface stresses, pitch line velocity, and oil temperature of the gears.Tip and root interference is another type of scoring, usually resulting from improper

design and manufacture. Metal removal will be seen near the root of the gear tooth profilewhile other portions of the contacting face will appear undamaged. The tip of the gear orpinion may look abraded, with tear marks in the direction of rotation. With high speedgears, scoring at start-up is considered failure, and the gears should be replaced aftercorrecting the cause of scoring.

Surface fatigue can be noticed by the removal of metal and the formation of cavities.

Surface fatigue failure

Surface fatigue can be noticed by the removal of metal and the formation of cavities.These may be small or large and may grow or remain small. It occurs when the gearmaterial fails after repeated stresses that are beyond the endurance limits of the metal.Here are the main types of surface fatigue, their causes, and cures.

Figure 5: Pitting

Pitting failures depend on surface contact stress and the number of stress cycles. Initialpitting (Fig. 5), with areas of small pits from 0.015 in. to 0.030 in. in diameter, occurs inlocalized parts of the gear teeth that are over-stressed. It is sometimes called correctivepitting because it tends to redistribute the load by progressively removing high contactspots, and often stops once the load has been redistributed. Continued operation may

polish or burnish the pitted surface and improve its appearance. Pitting can be monitoredby periodically putting some bluing on the affected area, then applying some cellophanetape to lift the pattern and put it in a notebook. Comparing the impressions over time will


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tell whether the pitting has stopped. While accurate manufacturing control of involuteprofiles is the best method of preventing pitting, a careful break-in at reduced loads andspeeds once the unit is installed also will help minimize pitting by improving gear toothcontact.

Figure 6: Destructive pitting

Destructive pitting (Fig. 6) appears as much larger pits than initial pitting, often in thededendum section of the gear teeth. These larger craters usually are caused by more

severe overload conditions that cannot be relieved by initial pitting. As stress cycles buildup, pitting will continue until the tooth profile is destroyed. To correct the cause of

destructive pitting, the load on the surface of the gear needs to be reduced below thematerials endurance limit, or the material hardness needs to be increased to raise theendurance limit to where pitting will not occur.

Figure 7: Spalling

Spalling (Fig. 7) resembles destructive pitting, except that the pits may be larger, quiteshallow, and irregularly shaped. The edges of the pits break away rapidly, forming large,irregular voids that may join together. Spalling is caused by excessively high contact

stress levels. Remedies include reducing contact stress on the gear surface or hardeningthe material to increase its surface strength.Both spalling and destructive pitting are indications that the gears do not have sufficientsurface capacity and should probably be redesigned if possible.

Micropitting is a type of contact fatigue that appears as frosting or gray staining under

thin film conditions.

Figure 8: Micropitting Figure 9: Micropitting Magnified

Micropitting is a type of contact fatigue that appears as frosting or gray staining under


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thin film conditions (Fig. 8). The surface acquires an etch-like finish, with a pattern thatsometimes follows the slightly higher ridges left by cutter marks or other surfaceirregularities. It usually shows up first on the dedendum section of the driving gear,although it may begin on the addendum section as well. When viewed under magnification(Fig. 9), the surface is seen as a field of very fine micropits under 0.0001 in. deep. Causesinclude high surface loads and heat generation, which thins the lubrication film and leads

to marginal lubrication. Improving the surface finish is an effective remedy, through eithermanufacturing techniques such as hard honing and grinding or a careful break-in cycle.

These techniques help lower heat generation by improving conformity of tooth contactand equalizing load distribution. Reducing the lubricant temperature and surface loadingwill also minimize frosting. Sometimes, frosted areas that appear initially will slowly bepolished away during subsequent operation if loads and temperatures are not excessive.

Figure 10: Case Crushing

Case crushing occurs in heavily loaded case hardened gears, including those that arecarburized, nitrided, or induction hardened. It is a subsurface fatigue failure that occurs

on material where the case is substantially harder than the core, when surface contactstress at high cycle levels exceeds the materials endurance limit. Case crushing mayappear similar to pitting, if some damage occurs on contacting surfaces. However, it oftenoccurs as longitudinal cracks on the surface of only one or two teeth, and long pieces ofthe tooth surface may break away (Fig. 10). The case material may appear to havechipped away from the core in large flakes. Case crushing occurs when cracks formbecause stresses in the subsurface area exceed the strength of the core material. High

residual stresses may contribute to this effect. The cracks move toward the case-to-coreboundary and then to the gear surface, where they may eventually cause large pieces ofmaterial to fall off. To prevent case crushing, it may be necessary to increase the depthof the case hardening and possibly the hardness of the core material. Changes in thematerial, heat treatment process, or the design itself may be necessary.

EVIDENCE OF PLASTIC FLOW

Plastic flow failurePlastic flow is a surface deformation that occurs when high contact stresses combine withthe rolling and sliding action of the meshing gear teeth to cause cold working of the toothsurfaces. Although usually associated with softer materials, it also can occur in heavilyloaded case hardened and through-hardened gears. Plastic flow generally takes one ofthree distinct forms.

Cold flow, rolling, and peeningcan be identified through evidence of metal flow in thesurface and subsurface material. The surface material may have been worked over thetips and ends of the gear teeth, resulting in a finned appearance. Tips of the gear teethmay be heavily rounded over, and a matching depression may appear on the toothsurface. Cold flow occurs under heavy loads and high contact stresses, as the rolling andpeening action of the meshing gear teeth cold-works the surface and subsurface material,pushing or pulling it in the direction of sliding. Continued operation during thisdeterioration increases dynamic loading and results in a dented, battered appearance onthe surface, much as if it had been hit with a ball peen hammer. To eliminate the problem


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it is necessary to reduce contact stress and increase hardness of the contacting surfaceand subsurface materials. Increasing the accuracy of both tooth spacing and profiles willhelp reduce dynamic loads, and any mounting deflections or helix angle errors should alsobe corrected.

Figure 11: Rippling

Rippling is a regular, wave-like formation that occurs at right angles to the direction ofmotion and has a fish scale appearance (Fig. 11). It is most common on hardened gear

surfaces and is generally considered a surface failure only when it has progressed to anadvanced stage. It usually occurs in slow speed operation with an inadequate oil filmthickness. High contact stresses during repeated cycles may then roll and knead thesurface, causing it to ripple. Rippling can be prevented by case hardening the toothsurface, reducing the contact stress, increasing oil viscosity, and using an extreme-

pressure oil additive.

Figure 12: Ridging

Ridging is a definite series of peaks and valleys that occur across the tooth surface in thedirection of sliding (Fig. 12). It occurs when high contact compressive stresses and lowsliding velocities cause plastic flow of the surface and subsurface material. It is frequentlyfound on heavily loaded worm gear drives, as well as on hypoid pinion and gear drives.Remedies for ridging include reducing contact stress, increasing material hardness, and

using a more viscous lubricating oil with extreme-pressure additives.

Breakage failureBreakage is the fracture of a whole tooth or substantial part of a tooth. Common causesinclude overload and cyclic stressing of the gear tooth material beyond its endurancelimit.

Bending fatigue breakage starts with a crack in the root section and progresses until

the tooth or part of it breaks off. It can be recognized by a fatigue eye or focal point ofthe break. The break area itself usually shows signs of fretting corrosion and smooth

beach marks that resemble patterns in the sand on a beach. A small area will probablyhave a rough, jagged look where the last portion of the tooth broke away. Most suchfailures result from excessive tooth loads, which cause repeated root stresses thateventually exceed the endurance limits of the material. Stress risers, such as notches inthe root fillet, hob tears, inclusions, small heat treating cracks or grinding burns, mayaggravate this condition. To remedy this condition, root fillets can be polished and shot-


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peened. Material should be properly heat-treated to minimize residual stresses. Ifredesign is necessary, use a full-fillet radius tooth, which is less prone to breakage andhas greater capacity than a tooth with too small a fillet radius.

Overload breakage appears as a stringy, fibrous break that has been rapidly pulled ortorn apart. In harder materials, the break will have a finer stringy appearance. The eyeand beach markings found in fatigue breakage will be missing. This type of breakage is

caused by an overload that exceeds the tensile strength of the gear material. Typicaloverloads that lead to such breakage include a bearing seizure, failure of drivenequipment, foreign material passing through the gear mesh, or a sudden misalignment.Since the failure is usually the result of some unpredictable occurrence, it is difficult orimpossible to prevent. If possible overloads are anticipated, torque-limiting couplings mayprovide some protection.

Random fracture can occur in areas such as the top or the end of a tooth, rather thanthe usual root fillet section. These failures are typically caused by stress concentrations

from such things as minute grinding cracks, foreign materials in the gear mesh, orimproper heat treating. Little can be done to prevent random fracture, except at the

design and manufacturing stages. However, maintaining cleanliness of the lubricant canhelp prevent one cause.

Little can be done to prevent random fracture, except at the design and manufacturing

stages.

Associated gear failures Associated gear failures usually are caused by improper

processing, environmental conditions, or possibly by accidents. To minimize many ofthese failures, any gear that is repaired and heat treated should be checked by magneticparticle inspection before being put back into service to be sure no cracks havedeveloped. Whenever repairs are made to any gearing, at the very least, a dye penetrant

inspection should be performed to check for cracks.

EVIDENCE OF ASSOCIATED GEAR FAILURE

Figure 13: Quenching cracks

Quenching cracks may appear across the top land of a tooth, in the fillet area, or

randomly at the tooth ends, although they may not become visible until after they havebeen used for a short time (Fig. 13). They are caused by improper quenching or uneven

cooling during heat treatment, which causes excessive internal stresses. Prevention ofquenching cracks calls for a thorough review of heat treating procedures, as well as aninspection of the equipment used.


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Figure 14: Grinding cracks

Grinding cracks(Fig. 14) usually show up as a definite pattern, either as a series ofshort cracks that are parallel to each other or with the appearance of chicken wire mesh.Usually, they are between 0.003 in. and 0.005 in. deep, with the parallel type beingdeeper than the chicken wire pattern. Causes include improper heat treatment or ametallurgical structure that is prone to cracking. To prevent this cracking, the grinding

procedure should be reviewed. Feeds and speeds may have to be reduced to lower theheat developed during grinding. The metallurgy of the gear material also should beexamined to choose an alloy and heat treatment that will not tend to crack duringgrinding.

Figure 15: Rim and Web failures

Rim and Web failures tend to start between two teeth and propagate through the rim

and into the web (Fig. 15). These failures are common on highly loaded thin rim and websections. Causes include stress risers from holes in the web as well as from webvibrations. Remedies include increasing rim or web thickness, depending on failure mode,and eliminating stress risers such as grinding marks, tool marks, and sharp fillets. Rimand web failures also may be caused by vibrations, which can be minimized by dampingor by redesign to change the natural frequencies of the gear.

Figure 16: Electric current damage

Electric current damage shows up as tiny pits occurring in a well-defined pattern that isdistributed uniformly along the gear surfaces (Fig. 16). They can be further identified by

their smooth, molten appearance and lack of any fibrous appearance. This damage resultsfrom electric current passing through two lightly contacting surfaces, either from arcwelding or from electric equipment such as motors or electrically actuated clutches. Theremedy is to insulate the electrical equipment or relocate the grounding wires properly.Welders and maintenance workers should be made aware of proper groundingprocedures.

Determining the real causeA complete and accurate assessment of the cause of any gear failure requires a

knowledge of the basic gear failure modes, their causes, and possible remedies. Allavailable information on operating conditions, performance history, and maintenance

details will help to point to the specific cause and to develop solutions to prevent futurefailures. The purpose of this article is provide a basic knowledge of the terms used in gearfailure analysis and to promote accurate communication when determining the cause of


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failure and how to prevent future problems. In the majority of cases a single failure modeis not evident. The initial failure damage may be obscured by subsequent damage. Todetermine the specific mode and cause of the initial failure, the assistance of anexperienced gear failure analyst may be required.

All figures in this article extracted from ANSI/AGMA 1010-E95, Appearance of Gear Teeth-

Terminology of Wear and Failure 1995, used with permission of the publisher, theAmerican Gear Manufacturers Association, 1500 King Street, Suite 201 Alexandria, VA

22314.


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