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Damage to Gears

Damage to gears is basically categorized by two types; one is the damage to the tooth surface, and the other is breakage of the gear tooth. In addition, there are other specific damages, such as the deterioration of plastic material, the rim or web breakages.
Damages occur in various ways, for example, insufficient gear strength, failure in lubrication or mounting and unexpected overloading. Therefore, it is not easy to figure out solutions to causes. Gear damage are defined by the following standards :
• JGMA 7001-01(1990) Terms of gear tooth failure modes
• JIS B 0160: 1999 Gears – Wear and damage to gear teeth – Terminology

14.1 Gear Wear and Tooth Surface Fatigue

Wear occurs on tooth surfaces in various ways. Run-in wear is a type of wear with slight asperity occurring on start-up. This wear involves no trouble in operation. Critical wear is the state of gears from which a small quantity of the material is scraped away from the tooth surface. If the wear expands until the tooth profile gets out of the shape, the gear can not be properly meshed anymore.
Tooth surface fatigue occurs when the load is applied on the tooth surface repeatedly, or when the force is applied on the tooth is larger than endurance limit of the material. As the result of the surface fatigue, the material fails and falls off the tooth surface.
The surface fatigue includes pitting, case crushing and spalling.
If a critical wear or progressive pitting occurs on the tooth surface, the following phenomena occurs:

  1. Increase of noise or vibration
  2. Excessive increase in temperature at the gear device
  3. Increase of smear by lubricant
  4. Increase of backlash

By properly removing the causes of these troubles, damage can be avoided.

The following introduces causes of tooth damage and examples of the solution.

(1) When the tooth surface strength is insufficient against the load

Solution 1 : Increasing of the strength of the tooth surface
** Change the material to a stronger material having more hardness.
S45C -> SCM440 / SCM415 etc.
Refer to the section 9. Gear Material and Heat Treatment (Page 565 – 566).
** Enlarge the gear size
Enlarge module and number of teeth.
** Enlarge the facewidth
** Exchange the gear to the stronger gear with helical gear teeth.
– Change from Spur gear to Spiral gear
– Change from Straight Bevel gear to Spiral bevel gear
(Improvement of overlap ratio)

Solution 2 : Decreasing the load
** Reduce the load by changing driving conditions

(2) Improper tooth contact caused by bad mounting
Solution:Adjusting the tooth contact
Detailed methods for this solution differ with types of gears.
For adjustment of bevel and worm gears, refer to the section 8.3 Features of Tooth Contact (Page 562 – 564).

(3) When partial contact occurs due to bad mounting
Solution : Change design of the gear, shaft and bearing to make them stronger.
By increasing stiffness, tooth contact improves.

(4) When lubrication is in a poor condition
Solution : Provide appropriate conditions for the lubricant; proper type, viscosity, and quantity.
Refer to the section 13 Lubrication of Gears (Page 608 – 611).

14.2 Gear Breakage

There are also several types of gear breakage. Overload breakage occurs if unexpected heavy loads are applied to the tooth. Fatigue breakage occurs if the load is repeatedly added on the tooth surface. The tooth breakage caused by partial contact at the tooth end, occurs on spur or bevel gears. The following introduces causes of breakage and solution examples.

(1) When the tooth is broken by the impact load
Solution 1. Increase bending strength (Gear strength)
Changing the material or enlarging the module is one of the most effective methods. The method is the same as the method of increasing surface strength.
Solution 2. Decrease or eliminate the impact load.
For example, reducing rotating speed is effective.

(2) Fatigue breakage from cyclic Loading
Solution 1. Increase gear strength
The detailed method is the same as the way of increasing tooth surface strength.
Solution 2. Reducing the load or the rotation

(3) Breakage occurs when the wear progresses and the tooth gets thinner.
In the first place, preventing wear must be performed.

14.3 Types of Damage and Breakage

There are various types of damage and breakage that can occur to gears, this section introduces some of those as defined by the JGMA 7001-01(1990) and the industrial standards set by the Japan Gear Manufacturers Association.

Table 14.1 Damages to Gears

Term No. Damage Description
1 Deterioration of tooth surface
11 Wear (Abrasion) Gradual loss of material on the tooth surface from various causes.
111 Normal Wear Not really identified as damage. After initial use, the irregularity of the tooth surfaces is kept in good balance.
1111 Medium Wear Wear on tooth surface identified by checking tooth contact.
1112 Polishing The state of the tooth surface becomes smooth like a mirror as the asperity of the surface is removed gradually.
112 Abrasive Wear Linear scratches run irregularly on the tooth surface in the slipping direction.
113 Excessive Wear Excessively worn over the lifetime of the product.
114 Interference Wear Wear of the tooth root, occurred by interference between the corner of the gear and the tooth root of the mating gear.
115 Scratching Type of abrasive wear. Linear scratches occur on the surface.
116 Scoring Surface deterioration caused by alternate deposition and tearing of tooth surface.
1161 Medium Scoring Type of light damage on the tooth surface. Slightly scratched in slipping direction.
1162 Destructive Scoring Visible scratching and tooth profile destroyed.
1163 Local Scoring Medium scoring occurred locally.
12 Corrosion
121 Chemical Corrosion Brownish-red rust or pitting corrosion occurred on surface.
122 Fretting Corrosion Surface damage occurs on the part where two of the tooth surfaces are in contact and involve relative reciprocal motion with fine vibration.
123 Scaling A prominent area of the tooth surface was oxidized when heat treatment was applied. The prominent area gets glossy.
13 Over Heating Excessive heat temperature on tooth surface. Temper color appears.
14 Cavitation Erosion Local erosion caused by forced oil-jet lubrication and its impact.
15 Electric Corrosion Small pitting on tooth surface that occurs due to electric discharge between the meshed gear teeth.
16 Tooth Surface Fatigue Damages on teeth, involving a fall-off of material.
161 Pitting Pits occurred on the tooth surface. Pitting often occurs at the pitch line or under.
1611 Initial Pitting A wearing phenomenon occurs in initial usage. It stops its progression when the tooth surface is broken-in.
1612 Progressive Pitting A wearing phenomenon occurs and does not stop its progression even when the tooth surface becomes engaged.
1613 Frosting Slight pitting occurs when only a thin oil film is generated and when heavy loading is applied.
162 Flake Pitting A kind of spalling. Thin steel pieces fall off from the rather large area of the tooth.
163 Spalling Material fatigue occurs under the surface, and quite large pieces of steel fall off.
164 Case Crushing Abrasion occurs on the surface layer. The layer is damaged in broad areas.
17 Permanent Deformation
171 Indentation Dent in the tooth occurs by involving an object enmeshed in the teeth while working.
1721 Plastic Deformation A typical state of permanent deformation. The deformation is not recovered after removing the load.
1721 Rolling Indented streaks occur around the pitch line.
1722 Deformation by Gear Rattle Deformation occurs when excessive vibration load is added and the meshed teeth engage with each other.
173 Rippling Ripples periodically occur on the tooth surface in the rolling and normal direction.
174 Ridging Ridges or crests occurs from plastic flow of the material right under the tooth surface.
175 Burr A plastic deformation similar to rolling. The state of the material at the tooth tip or edge is evident.
176 Dent A small plastic deformation occurs on the tooth surface or at the corner of the tip. This deformation involves concaving and prongs.
18 Crack A type of fracture. There are two types of cracks, one occurs in the production process, and the other occurs from usage.
181 Quenching Crack Cracks occurred by quenching.
182 Grinding Crack Slight cracks occurred when grinding the teeth.
183 Fatigue Crack Cracks at the tooth root or fillet, occur under reserved alternating stress and variable stress.
2 Tooth Breakage
21 Overload Breakage Breakage occurs on the tooth when unexpected heavy loads are applied to the tooth.
22 Breakage on Tooth Ends Often occurs on spur or spiral gears. This brakeage is caused from partial contacts of meshed teeth in the width-direction.
23 Tooth Shearing The state of teeth sheared from the body, occurred by a one time excessive load.
24 Smear Breakage Marked and deformed tooth profile, caused from intolerable heavy loads on the material of the tooth.
25 Fatigue Breakage A breakage caused from running cracks occurring at the tooth root fillet.
3 Rim and Web Breakage
4 Deterioration of Plastic Gears
41 Swelling Volume expansion occurs when solid substance absorbs fluids without changing the structure.

Additional Explanation
(1) Pit:
Tiny holes like pockmarks occur on the tooth surface.
(2) Beach mark fractured surface :
Patterns occurred from fatigue breakage, similar to the striping patterns on sandy beaches, which occur from the waves of the sea or ocean.

Appendix : Gear Damage Analysis Models

Gear damage belongs to complicated and compound phenomenon, and it is very difficult to analyze the reason. The basic models in this data are the essence accumulated in many years by KHK Company, which are very valuable. In practice, the damage is often the complicated phenomenon including certain model as the principle thing which is accompanied with other models.

Damage phenomenon: Breakage at gear flank of gears

Explanation :

  1. The breakage often happens at the gear flank, and it is total breakage.
  2. The breakage surface is kept with overall metal crystal graininess, with no trace of plastic breakage.
  3. The breakage surface appears in sharp edge around.
  4. The breakage surface is uniform in color, with no trace of color layers.
  5. It belongs to acute damage situations.
Reasons :
  1. Fault in design, and there is great distance between gear strength and practical load.
  2. Unstable load, and there is extreme sudden over stress.
  3. There is excessive impact load during operation process.
  4. Other accidents on operation.
Countermeasures :
  1. Confirm the design again, especially, if this breakage happens not long after usage, it illustrates that the gear strength is not sufficient, and verification on design shall be conducted.
  2. Verify the using conditions again, and pay special attention to the starting load, inertia problem and vibration shock problems.
  3. Eliminate accident reasons.

Damage phenomenon: Breakage at gear edge of gears

Explanation :

  1. The breakage often happens at the end of the single edge of the gear.
  2. The breakage surface is kept with metal crystal graininess, with no trace of plastic breakage.
  3. The breakage surface appears in sharp edge.
  4. The breakage surface is uniform in color, with no trace of color layers.
  5. It belongs to acute damage situations.
Reasons :
  1. The load is not distributed on the tooth width, but concentrated on the end, which leads to the generation of extreme large stress concentration.
  2. The main reason leading to stress concentration is the error on gear shaft alignment.
  3. It is caused by insufficient rigidity of gear box or support frame.
Countermeasures :
  1. Improve the parallel precision between gear shafts.
  2. Strengthen the rigidity of gear box and support frame in reinforcement method.
  3. Adopt crowing gear teeth.
  4. Shorten the distance between gear shaft bearings.
  5. Overstrike the size of gear shaft.

Damage phenomenon: Breakage at gear face of gears

Explanation :

  1. The breakage happens at the gear face, and there are different colors or color layers on the breakage surface.
  2. The breakage surface is kept with metal crystal, with trace of plastic bearing deformation.
  3. The breakage surface edge is not sharp enough.
  4. There are striped irregular debris and foreign matters.
  5. It can be regarded as acute damage situation.
Reasons :
  1. The gear is inserted with striped foreign matter with high rigidity, and bearing breakage happens.
  2. The centre distance of the gear is wrong (much larger than the correct value), and there is sudden change in load or aggressive load during operation.
  3. Others.
Countermeasures :
  1. Check and verify that the adjacent parts are fixed and locked firmly, without phenomenon of dropping on the gear set.
  2. Confirm the correct centre distance, with full depth meshing operation.
  3. Eliminate possibilities of aggressive loads and sudden change in load.
  4. Eliminate all the accident reasons.

Damage phenomenon: Fatigue failure of gears

Explanation :

  1. There is no metal crystal graininess on the breakage surface, which appears with bearing smooth blocks, and there are directional fiberous metal flows.
  2. The breakage surface does not appear as sharp edge.
  3. The breakage does not generate at a moment, but develops along with certain direction from an edge. There are color layers band on the breakage layer.
  4. It is obviously the chronic aging damage.
Reasons :
  1. It is obviously the damage caused by material fatigue under long term high speed operation.
  2. The gear surface is not smooth, with tiny concave-conves, scratch and craquelure, which will generate stress concentration in long term high speed operation, and accelerates the fatigue failure.
  3. There is not sufficient consideration on endurance limit during design, which leads to the poor endurance tension of materials and the failure.
  4. It is easy for rusted and erosive gear to generate fatigue failure.
Countermeasures :
  1. Re-considerate the endurance limit factor and set the allowable stress, and conduct design again.
  2. Adopt quenching hardening; gear with smooth surface such as gear grinding. Can greatly improve fatigue resistance effect, and rust resistance measures have good effect on anti-fatigue.
  3. Thoroughly check, and do not adopt gears with rough surface, tiny veins, crazing and scratch.
  4. Change the material of the gear, and adopt high rigidity rolled steel such as SNCM series of JIS.

Damage phenomenon: Aggressive wear of gears

Explanation :

  1. It is inevitable for the wear of the gear with sliding contact surface. However, normal wear has a very slow speed, with slight degree, and the hard and fine surface with bright friction surface will not invasively and rapidly deteriorate.
  2. The invasive friction will deteriorate rapidly and invasively, and the friction surface is not fine enough, and not bright enough, without forming hard surface. In condition of further deterioration, there is irregularly raised edge on the top of the gear, and the thickness of the gear becomes thinner rapidly, which is called as peening phenomenon.
Reasons :
  1. The stress born by the gear exceeds than the elastic limit and achieves around the yield point. It appears as over stress.
  2. The material of the gear is over soft, without sufficient hardness, or there is no heat treatment at all.
  3. The gear is exposed under dusty environment which accelerates wear.
  4. The poor matching of other materials accelerates the wear.
  5. There is metal powder impurity in the lubricating oil which accelerates wear.
Countermeasures :
  1. Recheck the design, and adopt modulus one grade larger to avoid over stress.
  2. Verify the heat treatment method, to ensure the hardness of the surface of the gear.
  3. Change the matching of materials and the coordination between hardness.
  4. Avoid dust, and isolate dust, and change the environment.
  5. Use senior grade lubricating oil with good nature.
  6. Pay attention to the filtration and purification of lubricating oil.

Damage phenomenon: Gear failure by pitting

Explanation :

  1. There are many shrinkage pools on surface of the gear, especially around the pitch cerdes, similar to the condition of eroded.
  2. With the continuity of operation time, the little pools continue to extend; at the same time, the pitting is larger and larger. A lot of falling off blocks distributing in gear box, until the gear is totally damaged.
  3. It obviously belongs to chronic damage, and surface fatigue of gear surface.
Reasons :
  1. Because of the improper consideration of texture, hardness and load, the actually born load exceeds than the surface endurance limit of the gear.
  2. The harden layer on surface of the gear is too tiny and thin, and cannot bear the surface pressure, and the crack generates which invokes to surface peeling.
  3. The harden layer on the surface is crispy, and it is easier for the occurrence of hole erosion when the hardness of the core is too soft.
Countermeasures :
  1. Adopt surface pressure strength, i.e., Herz surface pressure calculation formula to re-verify the design.
  2. Take sufficient consideration on the depth of the hardening layer during heat treatment, to ensure sufficient hardening layer, to keep the gear surface strength.
  3. Especially, as for the carburized gear surface, in consideration that the core material is low-carbon soft tissue, which cannot bear the surface pressure from the hard surface of the gear surface and leads to surface crazing and hole erosion; therefore, it is necessary to pay special attention to the depth of the carburized layer, and the insufficient carburized layer gives rise to the insufficient surface pressure strength which leads to hole erosion.

Damage phenomenon: Gear failure by scoring

Explanation :

  1. There are lines of cutting traces from the gear flank to the gear edge along with the gear surface, and there are traces of burning and melting.
  2. There are oil shortage and dry friction type surface roughening traces at first, and then there are burning, melting and tearing traces.
  3. There seems to be the meshing of the other gear surface material on the gear surface, which is obviously scoring phenomenon. Besides, this damage belongs to chronic and gradual deterioration.
Reasons :
  1. The lubrication oil quality is poor, with improper viscosity, and it loses lubricating effect because of the cracking of oil film when during the gear meshing contact.
  2. The poor lubricating results in the oil shortage state and losing of lubricating effect.
  3. The poor matching of material; there may be scoring phenomenon between materials with advertency, for example, it is very easy to generate scoring with the matching between soft copper.
  4. The load is over large, and the oil temperature is too high, and there is abnormal temperature rise, and incomplete cooling.
Countermeasures :
  1. Re-consider the specification of lubricating oil, and adopt oil sample with high scoring index.
  2. Strengthen the circulation volume of lubricating oil and adopt forced lubrication.
  3. Take the cooling system of lubricating oil into consideration, to reduce the temperature rise of lubricating oil.
  4. It may happen in conditions that the load is over large, and the oil temperature is too high, and there is abnormal temperature rise, and incomplete cooling.

Damage phenomenon: Gouging damage on gear flank of gears

Explanation :

  1. The damage situation is similar to that of scoring; however, it happens on the gear flank at firstly, and extends to the gear edge.
  2. It firstly happens at the gear flank of driving gear, and not at the passive gear at the same time.
  3. There is only bearing phenomenon at the sharp corner on the passive gear at first.
  4. It belongs to chronic damage, which gradually deteriorates and spreads to all the passive and driving gears.
Reasons :
  1. The sharp corner of the gear edge is too sharp, which leads to the damage during the meshing moment.
  2. The hardness of passive gear greatly exceeds than the driving gears. According to the hardness matching principle, it is required for the driving gear to be harder than the passive gear; there is error in the shape of the pair of gears.
  3. The lubricating oil is not properly used, and the improper matching of texture gives rise to the scoring.
Countermeasures :
  1. Conduct semi-topping on the edge of the top of the passive gear, to relieve the striking at the contact moment.
  2. The most complete method is: cutting at the addendum of the gear, to make it the trimmed gear shape.
  3. Topping has better effect.
  4. It is very important to adopt the same value for the standard pressure angle of the hobber cutter or sharpener of a pair of gears.

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