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> Failure Analysis for Gearing

Failure Analysis for Gearing
by Gary DeLange.

As with any failure analysis, finding the root cause of damage to gearing often requires a lot of detective work. You may need to review the service history and interview witnesses or employ technical tools such as vibration analysis and oil analysis. However, the cause of failure cannot be determined without a complete inspection of the condition of the gear teeth themselves. An understanding of the failure modes indicated by the condition of the teeth, when combined with knowledge of the operating conditions and maintenance history, will permit developing methods to avoid similar failures in the future.

Getting into gear
In order to analyze and interpret gear failures, it is helpful to consider some of the terminology and practices commonly used in the gear industry. The accompanying drawing 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 basic applications, AGMA 4 or 5 quality gears may suffice, while other more demanding applications may require an AGMA 12 or 13 gear; aircraft transmissions may require AGMA 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 differences between quality levels are progressive, somewhat like the Richter earthquake scale, where the difference between one level and the next is substantial. This can cause problems if an attempt is made to reverse-engineer a replacement gear without knowledge 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 the proper gear drive for the application. Manufacturer catalogs list typical service factors for various 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 be the low-speed gear set of a typical industrial gear drive. This gear set also transmits the most torque.

Things to be aware of when reviewing an application for possible causes of failure include the possibility of design error in specifying the original gear set. As an example, the speed reducer on a mixer might be sized adequately for operation but not for startup if the mixer is full and therefore requires considerably more power to overcome the inertia of the load. If this happens, the high speed pinion shaft could deflect, which may cause the gear teeth to run misaligned and overload them. Not only does this accelerate wear, but it can 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 inspection charts, specifications, and other information from the manufacturer, then compare them with the requirements determined by reviewing the actual application parameters. The original design may have been satisfactory, but subsequent changes in the application could cause it to be inadequate.

Why gears fail ?
One person’s failure may be another’s break-in. The difference between wear and failure can 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, there’s a serious problem.

When gears mesh, they roll only at the pitch line, as noted in the drawing. Above and below 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 “soft” enough to deflect and give without breaking. Yet a hardened gear has higher capacity ratings, so most gears are heat-treated to harden them to the degree necessary for the application.

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 the same throughout the gear. These gears are usually below 390 Brinell in hardness, above which conventional machining becomes difficult or impossible. Case- hardened gears are hardened only on the surface of the gear teeth, to a predetermined depth, to about 58 to 62 Rockwell C, or roughly as hard as a bearing race. The increased hardness improves the gear’s durability rating by providing greater resistance to pitting and greater strength, or resistance to breakage.

From one point of view, causes of gear failure may include a design error, an application error, or a manufacturing error. Design errors include such factors as improper gear geometry as well as the wrong materials, quality levels, lubrication systems, or other specifications. Application errors can be caused by a number of problems, including mounting and installation, vibration, cooling, lubrication, and maintenance. Manufacturing errors 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 examined periodically over time. Recording contact patterns or taking photographs at intervals will aid in comparison and help determine whether the condition is progressive. Keep in mind also that failure never occurs as an isolated event. Two or more failure modes may occur simultaneously or in succession, and the eventual failure mode may be different from the root cause.

WEAR FAILURE

Wear, the first failure mode category, occurs when metal is worn away from the contact areas 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 causes a very smooth surface to develop on the gear teeth. It is most common during slow-speed operation, where the lubricant film is too thin, and the gears are operating near the lubrication borderline. Normally, this condition does not cause a problem unless continued wear prevents the gears from reaching the design life of the equipment. Once the gears are polished, further action can be reduced or prevented by using a higher viscosity lubricant or lowering the lubricant temperature. Other possible remedies include reducing the transmitted load or increasing the operating speed to provide a better oil film.

 
Figure 1 Moderate Wear

Moderate wear (Fig. 1) shows up as a contact pattern in which metal removal occurs from both the addendum and dedendum tooth surfaces, and the operating pitch line remains as a continuous line. This may be caused by lubricant contamination but is often unavoidable 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 operate near boundary lubrication conditions. Increasing oil film thickness, either by cooling the lubricant, using a higher viscosity lubricant or operating at higher speeds, can sometimes reduce normal wear. Replacing a splash-fed lubrication system with a filtered positive-spray system may improve lubrication by removing particles and delivering a more consistent supply of oil to the working surfaces.

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

Extreme wear may appear as the same kind of contact pattern and pitch line visibility that occur with moderate wear, but the progression rate is much faster. Here, a considerable 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 occur before the design life of the gear set is reached. It may cause enough damage to the tooth profile that the resulting high dynamic loads will further accelerate the wear. Causes of 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-vent filters, properly installed and maintained, may help reduce wear. Other solutions include oil cooling, higher viscosity lubricants, higher speeds, reduced loads, and possibly reduced vibratory loads if the application permits.

 
Figure 2: Abrasive Wear

Abrasive wear shows up as a lapped surface, with radial scratches or grooves on the tooth contact surfaces. When this occurs shortly after startup of a new installation or on any open gearing, particles in the lubricating system are generally the cause. These may include metal particles from the gears and bearings, weld spatter, scale, rust, and sand, dirt, or other environmental contaminants. Fig. 2 shows severe abrasion. Careful cleaning of the gearbox and lubrication system before use can minimize abrasive wear. With a circulating 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, and higher viscosity oil also may help protect either type of system with a thicker oil film that will keep the finer particles from scratching.

 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 allows corrosive elements present in the oil to attack the gear contact surfaces. This action may affect the grain boundaries and cause fine, evenly distributed pitting. Checking the oil for breakdown and changing it at regular intervals can help minimize corrosive wear. Lubricants with high antiscuff, antiwear additive content must be observed even more carefully 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 the lubricant film, usually from overheating in the mesh area, as well as by misalignment, deflection, and uneven temperatures or loads. The resulting metal-to-metal contact produces alternate welding and tearing that quickly removes metal from the gear surfaces. 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 in softer 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 moderate scoring 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
 
Destructive scoring or scuffing shows definite radial scratch and tear marks, and material may be displaced radially over the tips of the gear teeth. Excessive material may be missing from above and below the pitch line, causing the pitch line itself to stand out prominently. 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 solutions include 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 misalignment factors than moderate scoring. A wear pattern that shows load concentration near one end of the teeth indicates possible misalignment or helix angle error. This results in one portion of the teeth carrying more load than the lubrication film can support. Eliminating the causes of uneven loading can prevent localized scoring. These may include nonuniform gear case deflection, excessive shaft deflection, out-of-parallel bores in the casing, 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 the mesh 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 are one 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 also will 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 profile while other portions of the contacting face will appear undamaged. The tip of the gear or pinion may look abraded, with tear marks in the direction of rotation. With high speed gears, scoring at start-up is considered failure, and the gears should be replaced after correcting the cause of scoring.

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 gear material 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. Initial pitting (Fig. 5), with areas of small pits from 0.015 in. to 0.030 in. in diameter, occurs in localized parts of the gear teeth that are over-stressed. It is sometimes called corrective pitting because it tends to redistribute the load by progressively removing high contact spots, 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 monitored by periodically putting some bluing on the affected area, then applying some cellophane tape to lift the pattern and put it in a notebook. Comparing the impressions over time will tell whether the pitting has stopped. While accurate manufacturing control of involute profiles is the best method of preventing pitting, a careful break-in at reduced loads and speeds once the unit is installed also will help minimize pitting by improving gear tooth contact.

 
Figure 6: Destructive pitting

Destructive pitting (Fig. 6) appears as much larger pits than initial pitting, often in the dedendum 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 build up, 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 the material’s endurance limit, or the material hardness needs to be increased to raise the endurance limit to where pitting will not occur.

 
Figure 7: Spalling

Spalling (Fig. 7) resembles destructive pitting, except that the pits may be larger, quite shallow, 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 hardening the material to increase its surface strength.
Both spalling and destructive pitting are indications that the gears do not have sufficient surface capacity and should probably be redesigned if possible.

Figure 8: Micropitting

Figure 9: Micropitting Magnified

Micropitting is a type of contact fatigue that appears as frosting or gray staining under thin film conditions (Fig. 8). The surface acquires an etch-like finish, with a pattern that sometimes follows the slightly higher ridges left by cutter marks or other surface irregularities. 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. Causes include 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 either manufacturing techniques such as hard honing and grinding or a careful break-in cycle. These techniques help lower heat generation by improving conformity of tooth contact and equalizing load distribution. Reducing the lubricant temperature and surface loading will also minimize frosting. Sometimes, frosted areas that appear initially will slowly be polished 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 are carburized, 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 contact stress at high cycle levels exceeds the material’s endurance limit. Case crushing may appear similar to pitting, if some damage occurs on contacting surfaces. However, it often occurs as longitudinal cracks on the surface of only one or two teeth, and long pieces of the tooth surface may break away (Fig. 10). The case material may appear to have chipped away from the core in large flakes. Case crushing occurs when cracks form because 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-core boundary and then to the gear surface, where they may eventually cause large pieces of material to fall off. To prevent case crushing, it may be necessary to in- crease the depth of the case hardening and possibly the hardness of the core material. Changes in the material, heat treatment process, or the design itself may be necessary.

EVIDENCE OF PLASTIC FLOW

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

Cold flow, rolling, and peening can be identified through evidence of metal flow in the surface and subsurface material. The surface material may have been worked over the tips and ends of the gear teeth, resulting in a finned appearance. Tips of the gear teeth may be heavily rounded over, and a matching depression may appear on the tooth surface. Cold flow occurs under heavy loads and high contact stresses, as the rolling and peening 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 this deterioration increases dynamic loading and results in a dented, battered appearance on the surface, much as if it had been hit with a ball peen hammer. To eliminate the problem it is necessary to reduce contact stress and increase hardness of the contacting surface and subsurface materials. Increasing the accuracy of both tooth spacing and profiles will help reduce dynamic loads, and any mounting deflections or helix angle errors should also be corrected.

 
Figure 11: Rippling

Rippling is a regular, wave-like formation that occurs at right angles to the direction of motion 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 an advanced stage. It usually occurs in slow speed operation with an inadequate oil film thickness. High contact stresses during repeated cycles may then roll and knead the surface, causing it to ripple. Rippling can be prevented by case hardening the tooth surface, 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 the direction of sliding (Fig. 12). It occurs when high contact compressive stresses and low sliding velocities cause plastic flow of the surface and subsurface material. It is frequently found 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 failure
Breakage is the fracture of a whole tooth or substantial part of a tooth. Common causes include overload and cyclic stressing of the gear tooth material beyond its endurance limit.

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 of the 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 probably have a rough, jagged look where the last portion of the tooth broke away. Most such failures result from excessive tooth loads, which cause repeated root stresses that eventually exceed the endurance limits of the material. Stress risers, such as notches in the root fillet, hob tears, inclusions, small heat treating cracks or grinding burns, may aggravate this condition. To remedy this condition, root fillets can be polished and shot-peened. Material should be properly heat-treated to minimize residual stresses. If redesign is necessary, use a full-fillet radius tooth, which is less prone to breakage and has greater capacity than a tooth with too small a fillet radius.

Overload breakage appears as a stringy, fibrous break that has been rapidly pulled or torn apart. In harder materials, the break will have a finer stringy appearance. The eye and 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. Typical overloads that lead to such breakage include a bearing seizure, failure of driven equipment, 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 or impossible to prevent. If possible overloads are anticipated, torque-limiting couplings may provide some protection.

Random fracture can occur in areas such as the top or the end of a tooth, rather than the 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, or improper heat treating. Little can be done to prevent random fracture, except at the design and manufacturing stages. However, maintaining cleanliness of the lubricant can help prevent one cause.

Associated gear failures Associated gear failures usually are caused by improper processing, environmental conditions, or possibly by accidents. To minimize many of these failures, any gear that is repaired and heat treated should be checked by magnetic particle inspection before being put back into service to be sure no cracks have developed. 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 have been 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 of quenching cracks calls for a thorough review of heat treating procedures, as well as an inspection of the equipment used.

 
Figure 14: Grinding cracks

Grinding cracks (Fig. 14) usually show up as a definite pattern, either as a series of short 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 being deeper than the chicken wire pattern. Causes include improper heat treatment or a metallurgical 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 the heat developed during grinding. The metallurgy of the gear material also should be examined to choose an alloy and heat treatment that will not tend to crack during grinding.

 
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 web sections. Causes include stress risers from holes in the web as well as from web vibrations. Remedies include increasing rim or web thickness, depending on failure mode, and eliminating stress risers such as grinding marks, tool marks, and sharp fillets. Rim and web failures also may be caused by vibrations, which can be minimized by damping or 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 is distributed 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 results from electric current passing through two lightly contacting surfaces, either from arc welding or from electric equipment such as motors or electrically actuated clutches. The remedy is to insulate the electrical equipment or relocate the grounding wires properly. Welders and maintenance workers should be made aware of proper grounding procedures.

Determining the real cause
A 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. All available information on operating conditions, performance history, and maintenance details will help to point to the specific cause and to develop solutions to prevent future failures. The purpose of this article is provide a basic knowledge of the terms used in gear failure analysis and to promote accurate communication when determining the cause of failure and how to prevent future problems. In the majority of cases a single failure mode is not evident. The initial failure damage may be obscured by subsequent damage. To determine the specific mode and cause of the initial failure, the assistance of an experienced 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, the American Gear Manufacturers Association, 1500 King Street, Suite 201 Alexandria, VA 22314.