Tribological Audit Of Rolling Element Bearings
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In the current scenario, conservation of material and energy is very much important. As the principal cause of material wastage is wear, any reduction in wear leads to savings. Friction is the main cause of wear which leads to energy dissipation and damage to equipments.  Friction occurs when one surface drags upon another surface. The magnitude of frictional force depends upon the interaction of mating surfaces. All surfaces consist of minute asperities and depressions even though the surface has high degree of finish. From close observation it is clear that frictional force is produced due to the distortion of the micro asperities when one surface slides over the other. As most of the surfaces are elastic up to a limit, the original shape is regained afterwards. In extreme cases the asperities get removed due to plastic flow of materials.  This process of removal of material from one or both of two surfaces in contact is known as wear.  Lubrication is the most effective method to lower the friction and to control wear and tear. Lubrication is the process of interposing a solid liquid or gaseous lubricant in between two surfaces in relative motion in order to decrease wear and tear. 
''Tribology is the science of interacting surfaces in relative motion and of related subjects and practices''. 
Tribology is derived from the Greek word 'Tribos' which means rubbing. It deals with relative motion of the bodies, friction, wear and lubrication. The motion of the bodies includes sliding, rolling, spinning, bouncing or combination of these.  The written history of tribology is as old as about five thousand years. In olden days itself people were concerned about friction and the ways to reduce it. Assyrians used rollers made of wood to move massive stones.  Later carts are developed as rolling friction is less than sliding friction. Although the ways to reduce friction was highly concerned from the olden days itself, the word 'tribology' was introduced in 1966.  The minimum film thickness in tribological machine elements was in the order of 10µm to 100µm at the start of the century. Later advance in the tribology led to the development of components with film thickness up to 1µm. The thickness is further reduced to 0.1µm by the end of the century. In short, tribology in the important factor in design of machine elements and selection of lubricants. 
2. ROLLING BEARINGS:
Invention of wheel led to minimise the effort of moving an object as rolling friction is less than that of sliding friction. Even though rolling motion produces less friction, man was seeking different ways to decrease the rolling friction. This led to the discovery that bearing based rolling motion consumes less power. Hence it was implemented in all rolling systems including complex machine mechanisms. In earlier periods the roller bearings was not able to compete with the hydrodynamic sliding bearings as it cannot meet with the endurance characteristics of the other. The roller bearing was universally accepted in the 20th century by the development of superior bearing materials and advanced techniques. This helped to manufacture roller bearing assemblies with extreme accuracy and long life. The competition among the manufacturers increased in early 1970's which led to a sudden increase in the production of high quality products at relatively low costs.  Rolling bearing includes all types of bearings to permit minimum friction between two moving surfaces relative to one another. The main application of roller bearing is to rotate a shaft relative to some fixed or oppositely rotating structure. It also allows relative linear motion of a fixture in a fixed direction provided by a stationary shaft.  Usually a bearing consists of two steel rings each of which consists of a hardened raceway. The hardened steel balls or rollers roll in these raceways. These rolling elements are usually held in an angular spaced structure which is called separator or retainer. 
Figure 1- Ball Bearings 
There are different types of roller bearings based on their analysis of operation. A brief description on various types of bearings is explained below.
2.1 BALL BEARINGS:
2.1.1 Radial Ball Bearings:
Radial ball bearings are the most popular rolling bearings. This type of ball bearings has an inner and outer raceway grooves in between the balls are inserted. The grooves have curvature radii of about 53% of the size of the ball diameter. The inner groove is concentrically fixed inside the outer groove and the ball bearings are present in between the grooves. The bearings are separated uniformly. A cage is used to maintain the separation. Deep groove ball bearings have the capacity to carry more loads as it is having large ball diameter. It can carry radial as well as thrust loads. Seals are present to keep the lubricant in and protect from external dirt.  Figure 2- Radial Ball Bearings radial-bearings.jpg
2.1.2 Angular Contact Ball Bearings:406px-Angular-contact-ball-bearing_single-row_din628_type-b_120.png
Angular contact bearings are designed to withstand heavy thrust loads or a combination of both thrust and radial loads. The ball bearings have a contact angle which does not exceed 40 degrees. Single and double row ball bearings are usually used depending upon the nature and magnitude of force to withstand. In this type of ball bearings, the outer raceway is the part of a sphere. The load applied on the outer raceway is minimised as the balls do not conform well into it. By using large balls the load on each ball can be minimised. This type of bearings can be used in applications where the parallelism of shaft and the housing are not exact to each other. 
Figure 3- Angular Contact Ball Bearings 
2.1.3Thrust Ball Bearings: thrust-ball-bearing_din711_ex.png
Thrust bearings are those bearings having a contact angle of 900 which can withstand thrust loads. These types of bearings are meant for high speed applications. The thrust bearings are mounted on spherical seats to obtain high degree of alignment ability. These bearings cannot be used to hold any radial loads.  They are mainly used in automotive, marine and aerospace applications. These increase smoothness and help in noise reduction thereby reducing friction in the attached part. Small size, reduced friction and long life are the main advantages of thrust bearings. 
Figure 4- Thrust Ball Bearings 
2.2 ROLLER BEARINGS:
Roller bearings are used in applications where the bearings have to support very high loads. Rollers are used in these bearings instead of balls. In these roller is cylindrical in shape and so line contact is formed between rollers and raceways. This spreads the load out a large area. Hence they have more load supporting capability.  They supply more fatigue endurance than ball bearings. Also the cost of manufacturing is high. These bearings consist of two concentric raceways which consist of rollers in between the two. To increase the load carrying capacity, cylindrical bearings with two or more rows of rollers are constructed. Different types of roller bearings include needle roller bearings, tapered roller bearings, thrust roller bearings and spherical roller bearings. 
Figure 5- Roller Bearings  Figure 6- Linear Motion Bearing 
2.3 LINEAR MOTION BEARINGS:
Linear motion bearings are the recent products developed in order to provide free motion in one dimension. In machine tool slide ways, very high friction is developed. Linear motion bearing consists of balls which carry the loads laterally in a particular direction and hence wear and tear can be minimised. They provide smooth, precision, guiding surface. The rolling element of a linear bearing rides over it thereby reducing friction. 
3. TRIBOLOGICAL FACTORS:
Tribology deals with friction, wear and lubrication. The two key factors related to tribology are 'interacting surfaces' and 'relative motion'. It is clear that tribology is concerned with two surfaces in relative motion which are adequately close to one another. Usually a poor tribological interface is designed and then various lubricants are used to lower the friction between the surfaces. The most acceptable way is to design a better interface which includes the selection of a good lubricant also to minimise the friction and wear to the lowest.  Various tribological factors are to be taken into consideration while taking a situation. The important tribological factors include:
Operating Conditions 
The initial step to be taken while designing a product is the material selection. There are various factors which are taken into consideration while selecting the material. For tribological point of view the following factors are to be given importance before material selection.
Basic Mechanical Properties
3.1.1 Basic Mechanical Properties:
The basic mechanical properties of the material such as elastic modulus, Poisson's ratio, yield stress, hardness, fatigue resistance, ultimate tensile strength and thermal conductivity are to be analysed.  Also the availability and cost of the material are also taken into account. Based on these observations the best material suitable for the manufacturing of bearings is selected. The selection of material used for bearings is to be done carefully based on standard specifications. The material plays an important role in the performance of the bearings. This affects the overall performance of the machinery also. The selected material is used to manufacture the load bearing parts of the bearings like raceway grooves and balls or rollers only. Retainers and cages are made from some other soft materials with desired qualities. 
Friction is the resistance offered by two interacting surfaces in relative motion. The degree of friction is denoted by coefficient of friction µ.  Friction occurred during sliding is known as sliding friction and which occurred during rolling is known as rolling friction. The main factors which cause friction are adhesion between surfaces, surface hardness, asperities etc. 
220.127.116.11 Laws of Friction:
There are different laws of friction which are applicable to dry friction between interacting surfaces. The first and second laws were put forward by Leonardo da Vinci and third law was introduced by Coulomb. 
''The first law of friction states that force required to initiate or sustain sliding FT is proportional to FN, the normal force.'' 
Thus FT α FN or FT = µ.FN 
Where µ = Coefficient of friction
''The second law of friction states that the friction force FT is independent of the apparent area of contact Aa'' 
''The third law of friction states that friction is independent of sliding speed.'' 
It was found that the first two laws of friction are obeyed in almost all experimental conditions. Lubrication systems can be used to lower the friction developed within the bearings. Friction in bearings depends upon the viscosity of lubricant and shear rate. Self lubricated bearings are widely used as it offers only very low friction. 
Wear can be termed as the material loss or volume loss from the surface of the materials in contact.  The main types of wear occurring on material surfaces are adhesive wear, abrasive wear, fatigue wear, corrosive wear. The local pressure at the asperities becomes extremely high when the normal pressure is acted upon the mating surfaces. When the pressure exceeds a certain limit, the asperities deform plastically until the area of contact increased sufficiently to withstand the load.
The wear produced due to adhesion process has been explained by Archard equation.
Wad = K 
Where Wad = wear rate
K = wear coefficient
F = normal load
H = hardness of softer material
When two dissimilar materials slide over one another, the softer material gets scratched away. Thias type of wear is called as abrasive wear. Fatigue wear occurs when a rotating shaft is subjected to reversal of bending stresses. Corrosive wear is due to the reaction of the material surface with the environment or an external agent.  Wear occurs in ball and roller bearings due to corrosion, shock loading of fatigue. Atmospheric agents like dirt and girt are the other external factors which are responsible for wear. 
Alloys are usually selected as bearing materials. Alloy is a solid composition consisting of two or more metals and elements in fixed proportions. The two alloys which are used to manufacture the bearings are Chrome Steel and Stainless Steel. Steel is an alloy of iron and carbon.  Various other elements like silicon, manganese, phosphorous, sulphur, chromium and molybdenum are added in chrome and stainless steels in order to increase the strength, hardness and corrosion resistance of the alloys. 
Carbon is used in the alloy to form cementite structure and to formation of pearlite, spheroidite, bainite, and iron-carbon martensite. This improves hardenability of the alloys up to 0.65%. Wear resistance is increased up to 1.5%. Toughness of the alloy reduces and the brittleness increases when carbon is added. Manganese increases the hardness penetration of steel. Chromium is added to increase hardness, toughness and wear resistance to steel. Silicon acts as the deoxidiser during the manufacturing process. 
18.104.22.168 Chrome Steel
Chrome steel is one of the widely used alloys to manufacture bearings. It is used mainly in high load bearing applications as this material is capable of withstanding very high loads. The machinability of the material is as high as highly finished, less noisy bearings are obtained. Also the life of the bearings is also very high. Under test conditions, the Rockwell hardness of the material is observed as '60 - 64 C'. This material is used in applications where corrosion is not considered.  The constituent elements in the chrome steel are given in the table 1.
22.214.171.124 Stainless Steel
The standard material used for the bearings is 400 series Martensitic Stainless Steel. This type of steel is highly resistant to corrosion and hence stainless steel bearings are used in applications where corrosion is to be taken into account. It is having less load withstanding capacity when compared to chrome steel. There are three types of 400 series Stainless Steel suitable for making bearings. These materials are developed by manufacturers to be used for certain specific applications. 
126.96.36.199.1 DR Stainless steel
This material is highly resistant to corrosion and is used in applications which deal with highly corrosive medium. The material can be precisely machined so as to obtain high degree of finish to the surface of bearing. This helps in smooth, low noise operation of the bearings. The Rockwell hardness of the material is observed as '58 - 60 C'.  The chemical composition of the material is given in table 2.
188.8.131.52.2 AISI 440C Stainless Steel
AISI 440C Stainless Steel is highly resistant to corrosion and is easily available. But it is not widely used now a day when more modern materials are developed. The Rockwell hardness of the material is '58-60 C'.  The chemical composition is given in table 3.
184.108.40.206.3 ES1 Stainless Steel:
This material is highly resistant to corrosion and has high machinability. Thus highly finished surfaces can be produced. The material is having a Rockwell hardness number of 58 - 60 C.  The composition of the material is given in table 4.
Surface quality of the materials is one of the major tribological factors which determine the efficiency of the product.  Surface finish decides the amount of friction and wear of the material. Surface texture and conformity are the terms which are to be given importance.
3.2.1 Surface texture:
The surface of the material is the part which interacts with the environment. The surface may not be highly finished due to several reasons. It may be due to the imperfections caused during manufacturing, due to external agents or due to loads acting on the surface. The deviations observed on the surface can be mainly classified into three which includes roughness, waviness and error of form.  Roughness is the surface irregularities which consist of rises and valleys. This creates an uneven surface pattern. Waviness is the small ups and downs on the surface which is having less amplitude that of roughness. These may be caused due to work piece deflections, vibration or heat treatment. Error of form is the slight deviations from the nominal shape.  The surface profiles can be traced by using different apparatus like Abbott profilometer, Tomilson surface finish recorder and Talysurf surface finish recorder. 
Conformity is the degree of agreement between the surfaces.  An interface consisting of two plates is said to have high conformity. Journal bearing is an example for high conformity material. An interface consisting of a plate and a ball is said to be having low conformity. Ball bearing is an example for low conformity material. Conformity is an important factor which is to be considered as it has an important role in determining the pressure, shear stress and temperature in the interface. 
Figure 7- Surface Texture 
Lubrication is the technique employed to reduce wear by the interposition of a solid, liquid or gas between the two surfaces which come into contact with one another. It was a persistent problem for man for centuries from the day he used to move materials. Friction consumes and wastes energy whereas wear leads to the damage of the equipment which are undesirable and it has to be avoided at any cost. Every moving part of machinery is subjected to friction leading to wear and tear which has to be avoided. Lubrication plays a vital role in avoiding the friction between mating surfaces. The history of lubricant begins thousands of years before. It was found that the ancient Egyptians used lubricants to avoid friction in the wheels of chariots.  In ancient times lubricants were of animal, plant and marine origin. Later mineral oils were started to use as lubricant. The new science of lubrication based on hydrodynamics came in 1880s.  Lubricants are of solids, liquids and gases but liquid lubricants are widely used. These include mineral oil, synthetic oil, grease, vegetable oil and water. Lubrication is done by applying a thin film of lubricant between the surfaces in contact.  Thus undesirable problems like friction, wear, noise and vibrations can be reduced to a very high extent.
Viscosity of a lubricant is the main characteristic which is taken into account before selecting it for a specific application. Friction, heat generation, film thickness and load carrying capacity depend upon the viscosity of lubricant used. Generally viscosity can be stated as the hindrance of the fluid to flow. But based on lubrication, viscosity is the measure of ability of fluid to maintain lubrication at specific operating conditions. 
3.3.2 Lubrication Regimes:
Lubrication can generally be classified into four different regimes based on the thickness of the fluid film in between the surfaces in contact. They include:
220.127.116.11 Hydrodynamic Lubrication:
Figure 7- Hydrodynamic Lubrication 
In hydrodynamic lubrication, a fluid film is formed in between the two surfaces in contact. Thus one surface floats on the other and there occurs no direct contact between the surfaces. In microscopic level the surfaces of the materials consist of asperities. In normal condition without lubrication, the asperities are in direct contact with each other. When one surface slides over the other, the asperities get deformed producing friction and hence wear. Hydrodynamic lubrication helps avoid the friction as there is no direct contact between the surfaces. The viscosity of the lubricant helps to increase the fluid pressure in between the surfaces. Thus the lubricant layer holds the surfaces apart. As the lubricant layer formed is thicker than the surface roughness of the material, one material slides over the other without damaging the surface. It is observed that the thickness of the film is more than 1µm. [24, 25] The surface is separated apart by hydrostatic lift. A fluid flowing over a surface immediately takes the speed of the surface. Similarly when the fluid moves in a converging gap, the pressure increases and the surface is lifted. 
18.104.22.168 Boundary Lubrication:
Boundary lubrication is a type of lubrication in which the surface roughness of the material is same as that of the fluid layer thickness. Thus when one surface moves upon the other, the asperities comes into contact. When load is applied the asperities gets plastically deformed which leads to friction and wear. As friction is not desirable different methods are also adopted to reduce friction in this type of lubrication. This includes the use of various additives to the lubricants. The additives are classified according to the surface action exhibited on the material surface. They are mainly of three types.
Physically absorbed layers
Chemically absorbed layers
Chemical reaction layers
22.214.171.124.1 Physically adsorbed layers:
When the lubricant is applied, it forms a layer on the material surface. The weak intermolecular force called van der Wall's force exerted by the layer of the lubricant helps to attach to the surface. This layer lubricates and allows the other surface to slide over the other without causing friction. As the lubricant is not undergoing any chemical reaction, the process is reversible. The bonds formed by van der Wall's force are weak and it can be removed. The main disadvantage of the physically adsorbed lubricants is that they have a temperature limit above which the lubricant melts.
126.96.36.199.2 Chemically adsorbed layers:
Chemically adsorbed layers are those formed due to the chemical reaction with the surface material. Chemisorption is a type of adsorption in which the molecules are hold tight by the valence force of the molecules of the lubricant. The bonds created are stronger than the physically adsorbed layers. Usually chemically adsorbed lubricants form long chain fatty acid molecules, which has high affinity to metal surfaces. Fatty acid additives like stearic and oleic acid forms chemically adsorbed layers. The typical thickness of the boundary film is about 3µm. As chemical reaction takes place in the interface, the adsorbed layer can be removed only under extreme conditions like high temperature, vacuum or by using some chemical treatments. 
188.8.131.52.3 Chemical reaction layers:
Chemical reaction layers are formed by the reaction of additives in the lubricant with the surface of the material on which it is applied. In this adsorption is not taking place, instead a chemical reaction that bonds the lubricant and the material occurs. Thus the layers formed are permanent. The main disadvantage of physisorption and chemisorptions is the temperature limit above which the lubricant gets ineffective. In this type of reaction the working temperature is not a factor. Hence this type of lubrication is used in high speed and high load operating systems as high temperature is developed in these cases. Zinc dialkyl dithiophosphate (ZDDP) is widely used as an additive to produce effective chemical reaction films. 
184.108.40.206 Mixed Lubrication:
Mixed lubrication has the features of both hydrodynamic lubrication and boundary lubrication. In hydrodynamic lubrication the contact surfaces are separated apart by the pressure of the lubricant interposed between the surfaces. Hence there will not be any contact between the two surfaces in motion. But in boundary lubrication, one surface rests on the other. When the surface moves the asperities gets plastically deformed which causes friction and wear. Mixed lubrication acts between the two extremes of hydrodynamic and boundary lubrication. In mixed lubrication regimes a fluid film layer is developed where contact takes place between the asperities of the surfaces. Thus a slight deformation is formed while sliding occurs which is highly desirable. The film thickness ranges from 0.05µm to 0.1µm. 
220.127.116.11 Elastohydrodynamic Lubrication:
Elastohydrodynamic lubrication is a type of lubrication which brings together hydrodynamic lubrication, hertzian contact and change in viscosity with pressure. In this method of lubrication, the surface is modified to minimise friction. Elastohydrodynamic lubrication is used in applications where low conformity and highly loaded tribological interfaces are present. Roughness of the wearing surface is taken into consideration in this lubrication. Roughness is taken as the average of high and low points of the surface which is called as the centreline average.  A non conformal contact can produce pressure to a very high level. When the pressure increases it directly affects the viscosity of the lubricant. In normal conditions the viscosity of the lubricant is less noticeable but during elastohydrodynamic lubrication the viscosity increases which makes the lubricant act as a semi solid. This creates a thin film of oil in between the mating surfaces. 
3.4 OPERATING CONDITIONS:
Operating conditions are the main aspect to be taken into account while designing a mechanical component including bearings. There are different factors which comprises the operating conditions.
The load applied on each ball and roller is to be determined to manufacture the bearings which can operate at the desired applications without failure. In order to calculate the load deflection relationship is to be developed for roller element contacting raceways. In almost all the applications the outer or inner raceways is in steady state rotation. The speed of the rotation may not be large enough to affect significantly the distribution of the load equally on each roller or ball elements. The rigidly supported bearing is subjected to radial load which may lead to deflection. The radial deflection at any rolling element is given by
δψ = δr cos ψ - ½ Pd 
Where δψ = radial deflection
r = ring radial shift
Ψ = angular location
Pd = diametral clearance
Thrust load and roller bearings are also subjected to thrust loads which are distributed equally among the rolling elements. The thrust load can be calculated using
Q = Fa / Z Sin α 
Where Q = thrust load
Fa = applied load
Z = number of rollers per row
α = contact angle
3.4.2 Relative Motion:
Relative motion is the motion of either one or both surfaces with respect to each other. There are different forms of relative motion depending upon the style of motion. They include sliding, rolling, spinning and bouncing. Sliding refers to the smooth movement over a surface maintaining the contact between each other. Rolling is the movement by revolving within the axis. Spinning is the rotation of a material within its axis. Bouncing is the rebounding due to an impact. Chance for sliding occurs in the rolling bearings due to thrust loads. Bouncing may occur due to sudden fluctuation in loads which may lead to destruction of bearing material.
Operating temperature is a factor which is to be taken into consideration for the smooth functioning of the bearings at load conditions. Temperature rise may occur in the bearings during operation which may be due to different reasons. One factor which causes the rise in temperature is the application of excess load. If excess load is applied, the material may not be designed to withstand that much high load. So it may lead to failure. Another reason for the temperature rise is the absence of lubricant. The lubricant should be interposed to the surfaces in contact. If it fails to continuously supply the lubricant to the surface, friction is produced which results in gradual rise in temperature. Various coatings are also used to overcome temperature extremes.
Nature of environment is the feature which can influence the efficiency and life of the bearings. There are different agents which affect the operating conditions of the bearings which may be harmful. These include contaminations like dust, wear particles, air, water, process fluid etc. Insulations are to be provided to avoid exposure to these external agents. This can be done by using seals or coatings.
The various tribological factors are to be considered while designing rolling bearings. This tribological audit looks for the different factors which affect the performance and life of the bearings. The major tribological factors found include material, surface, lubrication and operating conditions. Different solutions are available which can be applied to each factor to lower the friction. Material should be wisely selected which can be easily machined to obtain highly finished surfaces and should possess the strength to withstand the loads applying on it. The material should be selected according to the application. Surface texture and conformity of the material is to be carefully observed as the surface finish is one of the major factors which cause friction. Lubrication plays a major role in the performance of the equipment. Right method of lubrication is selected. Elastohydrodynamic lubrication forms the film with minimum thickness which is the most desirable in bearings. Operating conditions affects the performance and life of the bearings which includes contaminations like dirt, wear particles, water, corrosive process fluid etc. Proper measures are to be taken to avoid exposure to these agents. From this audit it is clear that we have to take care of the tribological factors and select the most suitable ones to obtain maximum performance and life to the rolling bearings.
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