Updated Tribology Lecture Notes [PDF]

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Tribology Introduction  Tribos = rubbing (Greek word)  A literal translation is the “Science of rubbing”. A more reasonable definition is the “science of lubrication, friction and wear”.  Friction and wear are related; however, it is a misconception to say high friction means high wear rate. Table 1 below illustrates some metals with their friction and wear values. Table 1 Friction and wear values for some metals Materials

Friction Coefficient (μ)

Wear Rate X 10-12 cm3/cm

Mild steel on mild steel

0.62

157, 000

PTFE on PTFE

0.18

2000

Tungsten Carbide on itself

0.35

2

Polyethylene on itself

0.65

30

Economy and Tribology An early report published by the Lubrication, Education, and Research Center in the U.K (Jost Report) has shown the effect of tribological problems on the British economy. Given as a guide only.

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Pounds 10 millions/annum in manpower savings

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Pounds 10 millions/annum in lubrication

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Pounds 22 millions/annum investment

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Pounds 28 millions/annum in less frictional dissipation

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Pounds 100 millions/annum in longer life of machines

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Pounds 115 millions/annum in fewer breakdowns

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Pounds 230 millions/annum in less maintenance and replacement parts

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Solutions to Tribological Problems (a) Contacting materials are chosen because they have intrinsically low friction and/ or wear characteristics (b) Chemical films chosen to reduce actual contact between the surfaces (c) Solid coatings such as graphite, molybdenum disulphide have low resistance to shear (d) A lubricant can be a liquid like oil or gas as in air systems (dentist drill). Used for hydrodynamic and hydrostatic lubrication (e) & (f) Elastomers and flexible strips bonded to the contacting surfaces are used to separate them. Used in small transverse movements and electrical isolation systems. (g) Use of roller anti-friction bearings between the contacting surfaces (h) Magnetic force fields are used to separate surfaces as in domestic electricity supply meters, and bullet trains moving on railway tracks at very high speeds. From the list of tribological solutions for various contacts shown in Fig.1 the designer of an engineering system or machinery can select the best solution for his application. In this selection, the designer has to consider such factors as the load to be carried in the contact, the speed, the nature of the working environment and any limitations on friction and wear in arriving at the most appropriate answer to his design problem. Figure 2 highlights this concept. The shaded areas on the figure indicate where tribological contacts have to be considered between various engine parts. Figure 3 indicates that tribology has been with us for more than 4000 years as indicated by the movement of the Egyptian Colossus Statue in 1900 B.C.

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Fig. 1

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Fig.2

Fig.3

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Surface Properties and Measurements Fig. 4 shows the nature of surface features. The Bielby layer is a layer of molecules formed during the melting and subsequent quenching and machining processes.

Fig.4

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Topographic features (geometry of surface texture) The characteristic of the surface texture is controlled by the finishing process by which they are produced. See Fig. 5, a combination of two processes gives the resultant surface finish shown below. Micro-roughness (short wavelengths of hills or asperities and valleys as in finishing processes) + Macro-roughness (long wavelengths) as in rough finishing process

Resultant surface finish Fig.5 Surface Texture Measurement Many methods are available for surface finish assessments; of which some are shown in Table 2. Method Resolution (μm) Lateral

Vertical

Optical microscope

0.25 to 0.35

0.18 to 0.35

Light profile

0.25

0.25

Oblique section

0.25

0.025

Interference microscope

0.25

0.025

Multiple beam interference

5

0.005

Reflection beam microscope

0.03 to 0.04

0.02 to 0.008

Electron microscope

0.005

0.0025

Profilometers

1.3 to 2.5

0.005 to 0.25

Table 2 Surface texture assessment (you can measure or assess the surface up to these values)

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The most common method used is the Profile-meter (used in the Department laboratory), in which a fine diamond stylus traverses on the surface and its vertical movements are recorded. Surface parameters Two important parameters are used to identify a surface texture as the one shown in Fig.6 below.

Fig.6

1. Center-line average (roughness average): this is the arithmetic average value of the vertical deviations of the profile from the center line. Abbreviated as C.L.A C.L.A = [1/n

i=1nZi] = RA = [1/x  0x Z dx]

2. Root mean square value. This is the square root of C.L.A. Also called the standard deviation, where  =  1/n i=1nZi2  1/2= [1/x  0x (Z2) dx]1/2 Where; n = number of points on the center line at which the profile deviation Z i is measured. The center line is taken as a line which divides the profile into equal areas above and below it. Assume n is the number of contacts between contact surfaces asperities on a light loading situation, the contact width is approximately given by; Contact width  0.2/N and where N  100/mm, contact width = 0.2/100 = 2 um---This is not valid for highly loaded contacts under high pressures. It is worth stating that   1.11 RA. Equivalent surface roughness The previous surface parameters RA and  don’t take into account the size, shape, and slope of the asperities at the contact. However, they are useful when the surface texture is produced by the same machining method. Size, shape, and slope of the asperities affect the friction characteristics and the resulting wear rate and heat dissipation in the contact. Fig.7 below shows different surface textures with equivalent surface roughness values.

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Fig. 7 Various surfaces with the same RA for different machining processes Surface textures are in the following order from top to bottom: lapping, grinding, super-finishing, honing, and machining processes.

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Examples:

1. The profile shown represents a sample contour of a machined surface. The actual deviations from the reference plane x-x, which is located such that the sectional areas above and below it are equal and shown on the diagram below. Determine: (a) The C.L.A (b) R.M.S average (c) Max. Peak to valley height A=7, B=19, C=27, D=19, E=17, F=15, G=9, H=3, I=13, J=15, k=19, L=11, M=22, N=13, O=8, P=3, Q=1, R=11 um.

2. Calculate the standard deviation of the peaks and the roughness average value for a surface texture where the peak ordinates distribution is governed by the following relationship; Z = (2x10-6 + L2/16), where L is the sampling length interval, L= 1 mm. 3. A sinusoidal and triangular profiles with wave length  as shown in the figure, calculate the relationships between the maximum amplitudes of the two profiles which give the same values of roughness, Ra and standard deviation, . Expression for the sinusoidal and triangular profiles of wavelength  respectively are: Z(x) = Ao sin(2 x/) and Z(x) = 4 A1 x/, where Ao and A1 are the maximum amplitudes of the sinusoidal and rectangular profiles. (Note x =  /4) {Note: Cos2x = ½ (1 + cos2x) and Sin2x = 1/2 (1 – cos2x)}

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 Friction Theories  Whenever a body slides or rolls on top of another a resistance to motion is experienced. The resistive force which is parallel to the direction of motion is called the ‘friction force’. The tangential force required to initiate friction is called the ‘static friction force’ and the force required to maintain sliding is the ‘kinetic (dynamic) friction force’. Kinetic friction force is lower than static friction force.  Amanton’s laws of friction: there are two laws of friction, the first states that the friction is independent of the apparent area of contact between the contacting bodies (friction force is governed by the actual area of contact) and the second states that the friction force is directly proportional to the normal applied load on the contact. In addition to Amanton’s laws a third law was introduced by Coulomb (1785) which states that the kinetic friction force is independent of the speed of sliding (friction force is governed by the actual area of contact).  Types of surface interaction I.

Asperity interlocking where motion can’t take place without deformation of the asperities usually of the softer asperities in the contact as shown in Fig.8 (a).

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Fig.8 (a) II.

Macro-displacement where a harder asperity for example A loaded against a softer solid surface B causes displacement of material B during motion producing of what is called plough effect as shown in Fig.8(b). A

B Fig.8 (b)  Energy loss due to friction. This loss between contacting surfaces takes place due to deformation of the surfaces. The deformation can be elastic, plastic or fracture. i.

Plastic deformation is the major source of energy loss in most practical situations.

ii.

Energy loss due to fracture in most cases of sliding motion is smaller than the plastic deformation. One reason for this is that a wear particle is not formed at each asperity contact, but rather for most metal contacts operating in normal atmospheric conditions, an asperity is required to make a large number of contacts (1000) before it breaks thus forming a wear particle.

 The Simple Adhesion Theory of Friction 1.

Contacting asperities deform plastically due to the normal applied load (W). This plastic flow over the contacts increases until the real area is large enough to support the applied load. For an ideal elastic-plastic material; W = A P o, where A is the actual area of contact, P o = yield pressure of the softer material.

2.

The friction force given by Bowden and Tabor is governed by the following equation; F = friction force due to interlocking of asperities (F adh.) + Friction force due to macrodisplacement (Fploughing) = AS + FPl; where S= shear stress required to cause plastic flow and final fracture, and F Pl = the force required to plough into the soft metal. F Pl is small compared to the Fadh and thus can be neglected in the above equation, hence F = AS = WS/P o and the

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coefficient of friction μ = F/W=S/Po. The value of μ can be reduced by adding a substrate to softer material and thus reducing the value of S such as in bearings (Be-bitted Bearings). For most metals S < 1/5 Po and hence μ  0.2. However many metal combinations give μ  0.5 and metals in vacuum give much higher values of μ. This led Bowden and Tabor to re-examine their simple theory and formulate new set of equations.

Tutorials

[Assume for the below problems; μ = μadhesive + μploughing] 1. A hard steel ball of diameter D is rubbed a cross a soft metal surface, it plows out a groove of width d. If d is very much less than D, derive an expression for the plowing contribution to the friction coefficient. {μ = d/D}

2. A hard steel ball is slid across a soft metal surface under two different loads. In one case the friction coefficient is 0.22 and the groove width is 0.3mm; in the other case the friction coefficient is 0.25 and the width is 0.8 mm. Calculate the diameter of the slider and the adhesive contribution to the coefficient of friction. {d=5.3 mm; μ = 0.202}.

3. Two hard conical sliders of semi-angles 75 o and 80o are slid across an un-lubricated metal surface and the measured friction coefficients are in the ratio 11:10. The experiment is then repeated with the surfaces lubricated and the measured friction coefficients are then in the ratio 13:10. Find the coefficients of adhesive friction in the two cases. Also determine the contamination coefficient C when the junction angles for both cases is zero (use graph Fig.2.12 from J.Halling text book). {μdry = 0.5, μlub = 0.08, C1 = 0.9 and C2 = 0.25}. Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

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Fig.2.12

 Wear Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

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 Definition: It is defined as the progressive loss of substance from the surface of a body brought about by mechanical action.  Types of wear: Burwell has classified five mechanisms of wear namely; 1. Adhesive wear 1.1 Wear particles are displaced due to adhesion between opposing surface asperities. Usually the softer metal will be removed. If the asperities are the same, wear will take place from both asperities. The size of the wear particles depends on where the shearing takes place. If work hardening extends well into the asperity, the tendency will be to produce large wear particles. In order to minimize the rate of wear, we need to minimize the area of contact A. Area of contact (A) = Load (W)/Compressive yield stress (y) or Po. Reducing the load on the surfaces will reduce the wear. The second way to reduce the area A is to increase y i.e. the hardness of the metal. This is why hard pencils (H2 and above) write less clearly than soft pencils such as HB pencil. The wear rate for adhesive wear mechanism per unit slid distance is mathematically given by; V = K W/3Po where W = normal applied load, Po = yield pressure of the softer material and K is the probability of an asperity contact producing a wear particle. Note that as P o increases V decreases.

1.2 Rowe has modified the simple adhesion theory to include the effect of surface films and derived the following expression for the total wear volume; V = Km (1 +  μ2 )1/2  W/Po, where Km = a constant for sliding metals independent of lubricant properties or of surface films,  = the fractional surface film defect and is characteristic of the lubricant,  = Am/A where Am = real contact area and A = W/ P o the apparent area of contact,  = a constant, and μ is the coefficient of friction

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2. Abrasive wear: It is defined as the ploughing action of a hard asperity into a soft metal. For example, the action of sand paper over a metal surface. The total volume of wear particles displaced due to abrasive wear per unit distance of travel is given by; V = 2 W Cot  /лPo , where W = normal applied load,  = ½ angle of the conical hard asperity (assumed shape of asperity) as shown in Fig.9, P o = yield pressure of the softer material. Note that as  increases V decreases. The effect of asperity shape on the form of abrasive wear is shown in Fig.10. W

Fig.9

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Fig.10 Effect of asperity shape on the form of abrasive wear 3. Fatigue Wear 3.1 Rolling contact: During rolling the opposing surfaces are separated by a lubricant film as in antifriction bearings. The surfaces experience large stresses transmitted through the lubricant film. The nature and magnitude of these stresses can be found using the Hertezian equations. These show that the maximum compressive stresses occur at the surface and the maximum shear stresses occur some distance below the surface as shown in Fig.11. In rolling contact, fatigue takes place after a critical number of revolutions. Before this critical condition very little wear takes place and the bearing operates normally until the wear particles are detached and this occurs when the useful life of the bearing is terminated. The life of the bearing is shown to be inversely proportional to the cube of the applied load; N = Constant/W 3. N=bearing life in rpm, W=applied load. It has been shown that the position of maximum shear stress (where undersurface defect takes place) in pure rolling is proportional to Y  (WR)1/3 for a ball and Y  (WR)1/2 for a cylinder, where R = radius and W=load. To reduce the effect of stresses transmitted to the opposing surfaces by the oil film, extreme pressure additives are added to the lubricant.

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(a)

(b) Fig. 11 (a) Actual isochromatics obtained for the contact of a cylinder and a plane due to normal load alone (b) Isochromatics due to combined normal and tangential loads (c) Effect of contact pressure on elastic/plastic behavior of a material

(c)

3.2 Sliding contact: Wear particles are formed after a large number of contacts in a sliding process due to adhesion or abrasion of opposing asperities. These asperities experience a sufficient number of contacts and deformations to produce a fatigue fracture of the asperities. 4. Corrosive wear: When rubbing takes place in a corrosive environment, gaseous or liquid, then surface reactions take place and reaction products (oxides) are found on one or both surfaces. These reaction products (oxides) are poorly adhere to the surfaces, and further rubbing causes their removal. Thus Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

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corrosive wear requires both corrosion and rubbing. To protect surfaces from corrosive wear a lubricant is introduced. However sometimes water is dissolved in oil, which invokes corrosive wear. Corrosion is not always a deleterious phenomenon. Oxide films and other corrosion products prevent adhesion of metal asperities and metallic wear in vacuum where oxide films can’t form is generally very high. 5. Fretting wear: Fretting occurs where low amplitude vibratory motion takes place between two metal surfaces loaded together, e.g. shrink fits, bolted parts and splines. Basically fretting is a form of adhesive wear, the normal load causing adhesion between asperities and the vibrations causing rupture. Commonly fretting is combined with corrosion and is termed fretting corrosion.  Factors affecting wear 

Wear under vacuum condition increases, since asperities junctions grow larger unimpeded by the presence of surface films in the junction. Thus ‘cold welds’ are made over large areas of asperity contact resulting in high friction coefficients. In order for tangential rubbing motion to proceed, these welded bonds must be broken.

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Wear is lower in atmospheric condition since oxide films formed on the surfaces minimize junction growth which occurs in vacuum and thus reduces friction and wear rate.

 Boundary lubrication 

Boundary lubrication refers to the situation where an oil film is present between the two rubbing surfaces but its thickness is insufficient to prevent asperity contact through the film, as in starting up and running down of machinery. Boundary lubrication can be considered in the same way as an oxide film, i.e. it limits, but doesn’t prevent metallic contact at asperities.

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Boundary lubrication can reduce wear by limiting the access of a corrosive liquid or gas when a bearing is operated in a hostile environment. It has been found that one or two molecular layers of lubricant are effective in reducing wear by a factor greater than 1000.



Under high pressures and the resulting high temperatures, the contacting surfaces are prone to wear as in hypoid gears, where the organic lubricants are ineffective and they break down under these conditions. Additives are used to counteract these high pressures e.g. organic chlorine or sulphur compounds. These additives are stable at room temperature but react with the metal at high temperatures forming metal chloride or sulphide films which inhibit welding of the asperities and thus reduce wear to an acceptable level.

 Prevention of wear 

Contacting surfaces can be kept apart by hydrodynamic lubrication as in the journal bearing operation, thus reducing wear to a minimum.



Boundary lubricants are used during start-up and run down of machinery to reduce asperity contacts and thus reduce wear.

 Hard materials wear less rapidly than softer ones. Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

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

Avoid use of compatible materials (similar)



Surfaces should be smooth and dirt particles must be excluded between contacting surfaces by filtration and corrosive atmospheres must be excluded where possible.

Problems: 1. In a wear test, a bronze annulus having an outside diameter of 25 mm and an inside diameter of 15 mm is placed with its flat face resting on a carbon steel plate under a normal load of 100 N and rotated about its Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

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axis at 5 Hz for 20 hours. At the end of the test, the specimens are separated and weighed, and it is found that the mass losses of bronze and steel due to adhesive wear are 25 mg and 8 mg respectively. Using the materials data given below, calculate the wear coefficients for bronze and steel. Child’s expresses the severity of adhesive wear by a wear coefficient K given by the expression; V = K W S/H, where V is the wear volume, W is the normal load, S is the slid distance and H is the hardness of the softer material. [Hsteel = 2400 MPa; Hbronze = 800 MPa; ρsteel = 7.8 Mg/m3; ρbronze = 8.4 Mg/m3]. {Ans. Kbronze = 10.53 E-07; Ksteel = 3.63 E -07}

2. A rigid cutter is used to cut a medium carbon steel bar of 5 mm diameter. The hardness of the carbon steel is 2 GPa. The width of cut is 0.5 mm. It took 5 min. to cut the bar, and the energy expended was 50 watts. The coefficient of friction between the cutter and the steel bar is 0.3. Calculate the wear coefficient of the steel bar during the cutting process.

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3. A hard steel surface of conical asperities of an average semi-angle of 60 o slides on a soft lead surface of hardness H=75 MPa under a load of 10 N. Determine the volume of lead displaced in unit slid distance. Given that the volume of lead material removed is 10 -6 m3 for a sliding distance of 1 km, calculate the wear coefficient of lead.

Contact of Surfaces – Chapter 3

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Chapter 4 - Tribological Properties of Solid Polymers

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Chapter 5 – Lubricant Properties

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Chapter 6 –Lubricating Systems Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

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Hydrophobic element

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Chapter 7 –Lubricated Thrust Bearings

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Chapter 8 –Lubricated Journal Bearings

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Chapter 9 – Bearing Selection

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Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

Date: 16 Sept. 2018

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Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

Date: 16 Sept. 2018

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Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

Date: 16 Sept. 2018

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Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

Date: 16 Sept. 2018

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Dr. Mohamed Nabhan Mechanical Engineering Department - University of Bahrain Tribology, MENG473

Date: 16 Sept. 2018

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