Fatigue, corrosion and wear are the three major factors limiting the life and performance of a cam in an engineering system.
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 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 elements allow parts to move relative to each other so that the machine tooling can move as intended by the designer. Each of these machine elements has at least two components that act against each other and add to the friction and resulting power loss. 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 not slip. However, the two components the make the joint are subject to micro-motions. The micro-motions contribute to wear so that the two components at the joint slip relative to each other more and more.
The study of the processes of wear is part of the discipline of tribology. Some common wear mechanisms (or processes) include:
|1.||Adhesive Wear [transfer-cutting] - seizure, cold welding, galling, scuffing|
|2.||Abrasive Wear [cutting] - scratching, ploughing, lapping and polishing|
|3.||Fretting and Corrosion Wear|
|4.||Surface Fatigue [elastic contact-stress] - pitting, spalling, flaking|
|5.||Smearing [elastic-plastic contact-stress] - ratcheting, elastic-shakedown|
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.|
Fretting and Corrosive Wear
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.
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
Macro-pitting is divided into specific modes or degrees, including initial pitting, progressive pitting, flaking, and spalling.
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 1.0mm in 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.
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-roller 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:
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.
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.