Possible Modes of Failure of Cams - as well as Gears and Bearings.

Fatigue, corrosion and wear are the three major factors limiting the life and performance of a cam in an engineering system.

Failure of Machine Elements

Wear is usually defined as 'the undesirable removal of material from contacting surfaces by mechanical action'.

Machines have many moving parts. There are two movement types:

Intended movement  - the basic kinematic motion that is planned and designed.

Grounded and moving bearings, gears, cams and followers, belts with sprockets, and chains with pulleys, shafts, couplings might all be necessary in a machine. Each of these allow the parts to move relative to each other so that the machine tooling can move as intended by the designer. Each of these machine elements have at least two components that act against each other and add to the friction and resulting power losses. These parts will eventually wear, such that the motion will not be as intended, or as designed.

Unintended movement - the motion that is not planned!

There will be vibration due to each component's rigidity, stiffness and inertia.

There may be 'lost motion' from backlash. Impacts between transmission faces when backlash is traversed will also induce vibrations and wear.

Torsion Joints made between a shaft and a link, gear, cam, pulley, sprocket, or coupling should never slip. However, the joint between the two components is subject to micro-motions. These micro-motions contribute to wear at the joint so that it moves more and more, and eventually fails, frequently through Fretting Corrosion.

Types of Wear

The study of the processes of wear is part of the discipline of tribology. Some common wear mechanisms (or processes) include:

Sliding Surfaces

1.Adhesive Wear [transfer-cutting] - seizure, cold welding, galling, scuffing
2.Abrasive Wear [cutting] - scratching, ploughing, lapping and polishing
3.Fretting and Corrosion Wear

Rolling Contact Fatigue

4.Surface Fatigue [elastic contact-stress] - pitting, spalling, flaking

Plastic Deformation

5.Smearing [elastic-plastic contact-stress] - ratcheting, elastic-shakedown

Sliding Surfaces


Wear is the removal and deformation of material from a surface that is the result of mechanical action of another surface. The greatest wear occurs at the points where the wear modes are combined to the greatest effect.

Damage from wear is twofold:

Firstly, the loss of materials from the contacting surface reduces the dimension of the component. This often leads to the increased clearance between the moving parts, and consequently results in high vibration, high noise, reduced efficiency and system malfunction. If dynamic loading is involved, the reduced component dimension could promote fatigue fracture, leading to a catastrophic failure.

Secondly, the material detached from worn surface, known as wear debris, is similarly harmful. It may act as an abrasive when trapped inside the contacting surface, or it moves between the surfaces with oil circulation, causing further wear.

Of course, the intended cam-profile and motion-law is changed if this occurs between a cam-follower and cam. The motion-law will change most where wear is most deleterious.

The root-cause of wear to a cam surface may actually be the failure of another machine element. For example:

1.Debris from a bearing becomes trapped and dents the cam profile, and thus initiates pitting.
2.Bearing backlash and in misalignment and thus increased contact stress due to 'edge-effects' at the side of the cam-follower and cam.

Of course the root-cause of the bearing failure may actually be misalignment, excessive axial preload, radial pre-load due to poor fitting tolerances.

Wear rate is strongly influenced by the operating conditions. Specifically, normal loads and sliding speeds play a pivotal role in determining wear rate. In addition, tribo-chemical reaction is also important in order to understand the wear behavior. Different oxide layers develop during the sliding motion. The layers originate from complex interactions among surface, lubricants, and environmental molecules.

Stages of wear

Under normal mechanical and practical procedures, the wear-rate usually changes through three different stages:

Primary stage, or early run-in period, where surfaces adapt to each other and the wear-rate might vary between high and low.
Secondary stage, or mid-age process, where a steady rate of ageing is in motion. Most of the components operational life is comprised in this stage. The secondary stage is shortened with increasing severity of environmental conditions such as higher temperatures, strain rates, stress and sliding velocities etc.
Tertiary stage, or old-age period, where the components are subjected to rapid failure due to a high rate of ageing.

Adhesive Wear


The surface of every engineering component has asperities. When two surfaces contact, the real contact occurs only at high asperities which is a small fraction of the apparent contacting area [~1%].

As load increases, plastic-deformation and, in the absence of surface films, intermetallic-adhesion occurs. Cold micro-welds form between the contacting asperities. As the surfaces continue to move, the micro-welds shear and then material  transfers from one surface to the other. Transfer may be permanent or temporary. If it is temporary, then it will contribute to the three-body abrasion -see below.

If permanent, it may even be so sever it may seize the machine. The surface become badly damaged and scuff. Also, high levels of friction and heat develop.

The strength of the micro-welds is a function of the surface structure, and by the mutual solubility of two contact metals. The tendency of adhesion is the lowest for a pair of metals with almost zero mutual solubility, but this is limited to very few metals. Most metallic materials appreciable show tendency of adhesion.

Adhesion is categorized as mild, moderate, or severe scuffing (described later). Specific forms of adhesive wear are scuffing, scoring. cold welding and galling.


Scuffing can happen with lubricated surfaces with a short transient overload. It is based on the contact reaching a critical contact temperature at which the lubricant film fails and micro-welding takes place.

It occurs at high speeds when adequate lubrication is not provided by the elasto-hydrodynamic action.
Lack of lubrication causes high sliding friction. High tooth loading and high sliding velocities that produce a high rate of heat in the localized contact region causes welding and tearing of surfaces apart.
Scoring can often be prevented by directing adequate flow of appropriate lubricant that maintains hydrodynamic lubrication.
Surface finish is also an important factor for scoring. Surface finish as fine as 0.5μm cla is desirable to avoid scoring.


When the surfaces are compatible metals that are smooth and clean and brought together with sufficient force they can weld with a bond that has a strength that is similar to that of the contacting metals.


This is similar to cold-welding and a result of some contamination. It can happen at low sliding velocities and may ruin a surface in a single movement.

Typically, mild adhesion occurs during run-in and subsides after it wears local imperfections from the surface. To the unaided eye, the surface appears undamaged and machining marks are still visible. Moderate adhesion removes some or all of the machining marks from the contact surface. Under certain conditions, it can lead to excessive wear.

Factors that Influence Adhesion Wear: According to this mechanism, the coefficient of friction is equal to the shear strength divided by the yield strength of the material.

Material similarity: dissimilar metals reduce adhesion compared to cohesion.
Metal hardness and surface energy: carburizing, chromizing and nitriding reduce surface energy and give a hard surface.
Hardness: high hardness and also high difference of hardness by a factor of 3 reduces adhesion / friction.
Load, Contact Stress and Speed: reduction of all will reduce adhesion.
Lubrication: Anti-scuff, Extreme-Pressure and Anti-wear additives reduce adhesion.
Surface Roughness - high surface roughness reduces are adhesion.

Abrasive Wear


The general category of abrasive wear can be characterized by a single key word: micro-cutting.

Abrasive wear is estimated to be the most common form of wear in lubricated machinery. Particle contamination and roughened surfaces cause cutting and damage to a mating surface that is in relative motion to the first.

Two-body abrasion occurs when metal asperities (surface roughness, peaks) of the hard surface cut directly into a second metal soft surface. A contaminant particle is not directly involved. The contact occurs in the boundary lubrication regime due to inadequate lubrication or excessive surface roughness which could have been caused by some other form of wear. Two-body wear may result in three-body wear when hard particles detach and thus contribute to the over-all wear.

Two-body wear is sometimes called Ploughing.

Three-body abrasion occurs when a relatively hard contaminant (particle of dirt or wear debris) of roughly the same size as the dynamic clearances (oil film thickness) becomes imbedded in one metal surface and is squeezed between the two surfaces, which are in relative motion. When the particle size is greater than the fluid film thickness, scratching, ploughing or gouging can occur. This creates parallel furrows in the direction of motion, like rough sanding. Mild abrasion by fine particles may cause polishing with a satiny, matte or lapped-in appearance. This can be prevented with improved filtration, flushing and sealing out small particles.

Lapping and Polishing deliberately use three-body abrasion to remove material in a controlled way. The hard particles in a lapping paste embed themselves in a soft metal so that the soft metal is protected, while the harder metal is polished to give a smooth finished.

Factors that Influence Abrasion Wear:

Difference in Hardness of at less than 10%. Plough is reduced when the surfaces are similarly hard. However, metals that similarly hard can be metallurgically compatible and this increase adhesive wear.
High hardness of both surfaces. Usually result in mild abrasion with less chance of adhesion.
Low roughness of the harder surface/ Load, Contact Stress and Speed
Viscosity, Lubricating Film Thickness
Surface Hardness
Particle Size and Hardness
Particle Concentration

Fretting and Corrosive Wear


Corrosive Wear needs corrosive liquids or gases, and the reaction products are formed on the surface and these influence wear. Oxidation is the most common form of corrosion.

Carbon steels: a porous oxide layer forms, and this allows oxygen to the base metal and so continuing the oxidation and corrosion. Corrosion increases with temperature.

Fretting Corrosion is a form of surface oxidation that occurs for example, between a bearing's outer-ring with a bearing housing and the inner-ring and a shaft. It also occurs at interference fits, key and key-ways.

There only needs to be a very small relative movement, of the order of microns.

Fretting corrosion refers to corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration at surfaces that are not intended to move - such as at the fit between a cam and a shaft, with a key and key-way.

Damage can occur at the interface of two highly loaded surfaces that are not designed to move against each other. The most common type of fretting is caused by vibration. The protective film on the metal surfaces is removed by the rubbing action and exposes fresh, active metal to the corrosive action of the atmosphere.

The amplitude of motion is so small that the wear particles actually contribute to the wear process through 'three-body abrasion'.

Rolling Contact Fatigue

Often when engineers consider mechanical, or traditional, fatigue they envision cyclic, reversing stresses. As examples: a spinning shaft or the flex of aircraft wings. However, 'Rolling Contact Fatigue' (RCF) and the associated mechanical wear has a distinct difference to traditional fatigue.

RCF, or wear, may be found in bearings, on gear teeth and cam surfaces. Compressive and shear stress are induced by the metal to metal contact interface. If there is sliding, the shear stresses become near to and approximately parallel to the contacting surfaces.

Typically, the shear stresses are highest just below the metal surfaces. They are magnified at internal stress concentration features of the metal such as at brittle inclusions and impurities within the metal just below the contact point.

RCF in bearings often begins as circumferential cracks just below the metal's surface. Radial cracks then grow from these cracks. As the process continues, and circumferential and radical cracks join, small surface metal plugs fall out. RCF damage often appears as a series of small cavities, or so called pits. These "pits" are not corrosion pits.

Traditional wear processes such as Abrasion or Adhesion differ from RCF. In RCF the cyclic stresses are vital. With Abrasion and Adhesion wear there is little or no cyclic stresses because sliding dominates. Even so, RCF is often grouped in with 'wear'.

Traditional fatigue is different from RCF in the way damage occurs. Traditional fatigue occurs due to crystalline slip that form a crack or cracks that eventually grow all the way through the metal cross section until there is too little metal remaining to withstand the applied load. Final fracture due to stress overload then occurs. Thus traditional fatigue often causes catastrophic failure of the metal.

On the other hand, RCF typically causes only metal loss at the surface. However, if left unattended, even RCF can lead to catastrophic failure because of the significant damage and stress risers that are generated.

RCF in steels is reduced by case hardening the contacting surfaces and thus provide greater resistance to the stresses and damage that can result. The goal is to make the surface area metal hard to a sufficient depth.

Different types of metallurgical heat treatment processes are available for case hardening. In all cases carbon, nitrogen or a mixture is diffused into the metal to create a concentration profile - greatest at the surface and decreasing in concentration with depth into the metal. All processes involve three steps.

The first is absorption and diffusion of the carbon, nitrogen or mix into the metal surface at a high temperature to create a decreasing carbon or nitrogen concentration gradient to a depth of about 0.75mm. This is followed by a rapid temperature decreasing step (quenching) that transforms most of the crystalline austenite near the surface to the very hard metal constituent - martensite. The metal is then heated to a much lower temperature than used for diffusing so as to temper or slightly soften the near-surface martensite. The specific parameters used in these treatment steps depend directly on the original carbon content of the steel being case hardened.

There is a desirable finished hardness gradient in a properly hardened case to resist RCF. It is one in which the hardness values decrease very gradually from the maximum surface value. Maintaining relatively high hardness values below the surface is important because the maximum shear stresses in service occur not at the surface but just below the surface. Specifications to achieve this should be stated carefully.

Surface Fatigue

Adhesion, Abrasion and Corrosion occur between Sliding Surfaces. In rolling contact, possibly combined with some slip, surface fatigue is generally the predominant wear mechanism.

Surface fatigue is a process by which the surface of a material is weakened by cyclic loading, which is one type of general material fatigue. Surface Fatigue wear is produced when the wear particles are detached by cyclic crack growth of micro-cracks on the surface. These micro-cracks are either surface cracks or sub-surface cracks.

Sub-surface Origin 

Sub-surface Origin

Subsurface-Origin Fatigue.

Subsurface-origin fatigue occurs during pure rolling motion of one element across or around another. This is most common in anti-friction, or rolling-element, bearings such as ball and roller bearings, needle bearings, roller and cams, and on gear teeth. Because the maximum shear stress is located a relatively short distance below the surface, this is the normal location for fatigue fracture to originate.

It begins with inclusions or faults in the metal below the surface. Subsurface micro-cracks form due to long-term repeated load cycles and stress, causing elastic deformation (flexing) of the metal. The contact stress is concentrated at a point below the metal surface.

These micro-cracks usually propagate to the surface, which eventually results in a piece of the surface material being removed or de-laminated. They appear as surface damage or wear (large pits) referred to as spalling. Other terms for subsurface fatigue include flaking, peeling and mechanical pitting. A full oil film exists and no metal-to-metal contact or surface damage is needed. Subsurface fatigue is not a common issue if better quality metals are used in bearing manufacture. Most bearings will fail by another mechanism first.

Sub-surface fatigue failure is the result of a bearing living out its normal life span based on the load, speed and lubricant film thickness that it is exposed to. The L10 fatigue life of a bearing is the average time (in hours or cycles) to fail 10 percent of a set of identical bearings under certain conditions. An estimate of the L10 life can be calculated, providing a rating life of a bearing.


Surface Origin

Surface Origin

Surface-initiated Fatigue

This begins with reduced lubrication regime and a loss of the normal lubricant film. The oil film is reduced to boundary or a mixed regime. Some metal-to-metal contact and sliding motion occurs. Surface damage occurs. The high points of the metal surface asperities are removed, which initially appear as a matted or frosted surface. This is not smearing, as in adhesion. This type of surface damage is usually visible with a magnification of three to five times.

The surface damage is coupled with the cyclic loading of the follower rolling over the race. This creates asperity micro-cracks and micro-spalling. The cracks start at the surface and migrate down into the metal. An edge of metal is created at the surface which flexes at the edge of the surface crack. This creates a cold worked edge which is lighter in colour. The cracks propagate and may intersect within the metal, and a piece of surface material is then removed. Flaking, mechanical pitting and micropitting are other names used to describe spalling.

Surface fatigue can also occur as a result of plastic deformation - see image to the left. Contaminant particles in the oil enter the high-load rolling contact area between rollers and the race, or between gear teeth, and cause some form of surface damage - a dent. Improper handling of bearings can cause similar surface damage.

These round-bottomed dents often have a raised berm around their edges. The raised berm of metal acts as a point of increased load or stress, or creates a reduced lubrication regime (mixed or boundary), and leads to a lower surface fatigue life. Improved filtration reduces plastic deformation, and therefore indirectly reduces the occurrence of surface fatigue.

Notice that the term "contact fatigue" is not used by ISO. This is a vague term sometimes used to describe both forms of fatigue. It does not specify whether metal flexing damage started in the subsurface or from some initial surface damage. It encompasses any change in the metal structure caused by repeated stresses concentrated at a microscopic scale in the contact zone between the rolling elements.



Failure Criteria associated with Cam and Cam-Followers.

A cam can be considered to have failed when the payload's motion does not repeat reliably due to the effects of wear of the cam at the contact surface between the cam and the cam-follower.

Cam and cam-followers can fail in a number of ways. The main modes of failure are:


Cams with roller followers will usually fail by pitting. Cams with sliding followers can fail in any of the above modes.

Each failure mode has a 'failure mechanism'. There are desirable material properties to prevent each failure mode.

Pitting Failure and Material Strength.

Contact conditions give high surface stresses. As we have seen, the stress must be kept below a critical value to avoid yield or the onset of plasticity. However, since the cam is continually loaded and unloaded, we should expect that the steel will fatigue at a level of stress that is less that the yield stress. The fatigue limiting contact stress (Endurance Limit) is a function of, mainly, the hardness of the steel. The stresses must be kept below a critical value called the Permissible Contact Stress.

If these conditions are not met, then fatigue failure will occur in the form of pitting. The pitting produces a surface roughness due to the braking out of flakes and spalls. A material with a high fatigue strength will have a good resistance to pitting.

Pitting occurs only after a large a number of repeated loading where the oil film breaks down because of zero sliding velocity - if a sliding cam.

Pitting is classified as contact fatigue which is subdivided into three general modes: pitting (macropitting), micropitting, and sub-case fatigue.

Further definitions for the specific mode or degree of pitting can provide clues as to the actual failure mode


Macropitting is divided into specific modes or degrees, including initial pitting, progressive pitting, flaking, and spalling.

Initial Pitting
Small pits less than 1 μm in diameter. They occur in localized areas and tend to redistribute the load by removing high asperities. When the load becomes more evenly distributed, the Macro-pitting stops.
Progressive Pitting
Characterized by pits significantly larger than 1mm in diameter. Pitting of this type may continue at an increased rate until a significant portion of the cam surface has pits of various shapes and sizes.
Flake Pitting
Triangular pits that are relatively shallow but large in area. The fatigue crack extends from an origin at the surface of the tooth in a fan shaped manner until thin flakes of material break out and form a triangular crater.
Progressive pitting where pits coalesce and form irregular craters that cover a significant area of the cam surface.

Surface fatigue (micro- and macro-pitting)

Surface fatigue, commonly referred to as pitting or spalling, is a wear mode that results in loss of material as a result of repeated stress cycles acting on the surface. There are two major sub--groups

under surface fatigue known as micro- and macro-pitting. As their names imply, the type of pitting is related to the size of the pit. Macro-pits usually can be seen with the naked eye as irregular shaped cavities in the surface of the tooth. Damage beginning on the order of 0.5 to 1.0mmin diameter is considered to be a macro-pit.

The number of stress cycles occurring before failure is referred to as the fatigue life of the component. The surface fatigue life of a gear is inversely proportional to the contact stress applied. Although contact stress is probably the major factor governing life, there are many others that influence life. These include design factors such as tip relief and crowning, surface roughness, physical and chemical properties of the lubricant and its additive system, and external contaminants such as water and hard particulate matter.


Micro-pitting is a fatigue phenomenon that occurs in Hertzian contacts that operate in elastohydrodynamic or boundary lubrication regimes and have combined rolling and sliding. Besides operating conditions such as load, speed, sliding, temperature and specific film thickness, the chemical composition of a lubricant strongly influences micropitting.

Damage can start during the first 105 to 106 stress cycles with generation of numerous surface cracks. The cracks grow at a shallow angle to the surface forming micro-pits that are about 10 – 20 μm deep by about 25 - 100 μm long and 10 – 20 μm wide. The micro-pits coalesce to produce a continuous fractured surface which appears as a dull, matte surface to the observer.

Micropitting is the preferred name for this mode of damage, but it has also been referred to as grey staining, grey flecking, frosting, and peeling. Although micropitting generally occurs with heavily loaded, carburized gears, it also occurs with nitrided, induction hardened and through--hardened gears. Micropitting may arrest after running-in. If micropitting continues to progress, however, it may result in reduced gear tooth accuracy, increased dynamic loads and noise. Eventually, it can progress to macropitting and gear failure.


Macropitting is also a fatigue phenomenon. Cracks can initiate either at or near the surface of a gear tooth. The crack usually propagates for a short distance at a shallow angle to the surface before turning or branching back to the surface. Eventually, material will dislodge from the surface forming a pit, an irregular shaped cavity in the surface of the material. With gears the origin of the crack is more likely surface initiated because lubricant film thickness is low resulting in a high amount of asperity or metal-to-metal contact. For high--speed gears with smooth surface finishes, film thickness is larger and sub--surface initiated crack formation may dominate. In these cases an inclusion or small void in the material is a source for stress concentration. Laboratory testing commonly uses a 1% limit on tooth surface area damage as a criteria to stop a test. However, for field service applications one should always abide by the equipment manufacturer’s recommendations or guidelines for acceptable limits of damage to any gear or supporting component.


Cam surface appears frosted, matted, or gray stained. Under magnification, the surface appears to be covered by very fine pits, less than 20microns deep. Metallurgical sections through the micro-pits show fatigue cracks that are inclined to the surface at an angle of less than 45 degrees. The cracks may extend deeper than the visible micro-pits. Micro-pits occur most frequently on surface hardened cams although it may also occur on through hardened cams.

Micro-pitting is a Hertzian contact fatigue phenomenon and it is a form of localised material surface damage that occurs under rolling and sliding contact when the parts in contact are operating in elastohydrodynamic (EHL) or boundary lubrication regimes. See Lubrication and Surface Finish. The pitting of a cam is considered to be a fatigue phenomenon that results from the contact stress at the surface and or the maximum shear stress below the surface.

The stress field is complex because it is a function of the Hertzian contact stress, stress at roughness asperities, stress because of metal inclusion stress raisers, and also residual stresses with the cam from heat treatment and or machining.

'Initial micro-pitting' can often start by the removal of 'high-spots' of surface roughness asperities or machining errors, to form cavities.   Micro-pitting can often stop once the high spots are removed. In most industrial application, 'initial-pitting' that does not progress beyond a matt and grey finish, is deemed to be not that serious. Micro-pitting can can also start at small, surface or subsurface cracks that are very small compared to the contact zone.

The small micro-pits are approximately 10 – 20μm deep, 25 – 100μm length and 10 – 20μm width.

When pits merge, and the pit becomes larger, they are more often called a spall. If it continues it results in reduced cam accuracy, increased dynamic loads and noise.

The removed of cam material can itself also damage the cam-follower bearing unless it can be eliminated with lubricant filtration.

Micro-pits appear in case-hardened, nitrided, induction-hardened, through-hardened, and of course, cams that are not heat treated.

Sub-case Fatigue

Origin of the fatigue crack is below the surface of the cam in the transition zone between the case and core. Fatigue beach marks may be evident on the crater bottom formed by propagation of the main crack.

Scuffing and Polishing Failures.

There should be lubricant to separate the cam and cam-follower. The film thickness that is needed depends on the surface roughness of the cam and the cam-follower.

With sliding contact, there may not be sufficient film thickness for part of the machine cycle when the relative velocity between the cam and cam follower reduces to zero or reverses. If this occurs, then materials must be chosen carefully in order to withstand the scuffing.

Sliding contact with a thin film can produce different wears patterns:

It occurs at high speeds when adequate lubrication is not provided by the elasto-hydrodynamic action.
Lack of lubrication causes high sliding friction. High tooth loading and high sliding velocities that produce a high rate of heat in the localized contact region causes welding and tearing of surfaces apart.
Scoring can often be prevented by directing adequate flow of appropriate lubricant that maintains hydrodynamic lubrication.
Surface finish is also an important factor for scoring. Surface finish as fine as 0.5μm cla is desirable to avoid scoring.

Scuffing is an 'Adhesion Phenomenon'. This is severe wear. Small portions at the surfaces of the cam and cam-follower weld together at asperities. The resulting surface is roughened. The scuffed area appears to have a rough or matte texture. Under magnification, the scuffed surface appears rough, torn and plastically deformed. Scuffing is not a fatigue phenomenon and it may occur instantly.

Certain materials combinations can resist scuffing better than others. This might be because they are chemically dissimilar or one or both of the materials contains a lubricant - for example graphite flakes. Frequently, when either the cam or the cam-follower is harder by approximately 2-3HRC (Rocker Hardness C scale), then scuffing is reduced.


This is less critical wear. There is contact only between peaks of the surface roughness. This can result in the surfaces becoming smoother so that wear might stop. The wear might continue indefinitely so that a large amount of material is removed from the cam or follower.

Plastic Flow

Is cold working of cam-follower and cam surfaces caused by high contact stresses because of the rolling and sliding action. It is a surface deformation that results from the yielding of the surface and subsurface material and is usually associated with softer cam materials, although it also occurs in heavily loaded case-hardened and through hardened cams.

Cold Flow failure, where the surface and sub-surface material shows evidence of metal flow. Surface material has worked over the sides of the cam giving a winged appearance.

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