1、 Chapter 3 Introduction to TribologyTribology is the science and technology of interacting surfaces in relative motion which involves friction,wear and lubrication.Understanding tribological principles is essential for the successful design of machine elements.Importance of Tribology Friction and we
2、ar can cause a staggering loss of potential power for today mechanized society.The purpose of research in tribology is to minimize and eliminate unnecessary waste at all levels of technology where the rubbing of surfaces is involved.Complexity of TribologyInterdisciplinary nature:It requires a varie
3、ty of knowledge such as physics,chemistry,mechanics,thermodynamics,and material science.Tribology problems are often coupled with a large,complex,and interwinded area of machine design,reliability,and performance where relative motion between surfaces is involved.One of the important objectives in t
4、ribology is the regulation of the magnitude of frictional forces according to whether we require a minimum friction(as in machinery)or a maximum(as in the case of anti-skid surfaces).Friction Laws of FrictionThe friction force is proportional to the normal load.The friction force is not dependent on
5、 the apparent area of the contacting solids.The friction force is independent of the sliding velocity.Bowden-Tabor Theory of Friction Friction is the force required to break the welds formed at the points of contact.The area of real contact between couples is proportional to the load;thus the fricti
6、onal force should be proportional to the load.Since the area of real contact is independent of the apparent surface area of the body,the friction should be independent of the apparent surface area.The coefficient of friction is the shear strength of the junctions divided by the yield pressure of the
7、 softer component.Tribological SolutionsBy using solid surface coatings with low resistance to transverse shear By lubricating techniquesBy improving the bulk materials or metal surfaces through surface modification techniquesBy interposing rolling elements such as balls,cylinders and the like betwe
8、en the two surfacesBy using magnetic and similar force fields to carry load without mechanical contact WearWear may be viewed as the progressive loss of substance from the operating surface of a body occurring as a result of loading and relative motion at the surface.The mechanism of wear is less un
9、derstood than that of friction and lubrication.Wear can be classified by the physical nature of the underlying process,such as abrasion,adhesion,and fatigue.In the same way as friction,wear of machinery is sometimes advantageous.The initial wear resulting in better mating of components(running-in),i
10、s evidently desirable.Polishing and grinding of many materials,involving positive application of wear,is a technology process where material removal can be considered as abrasive wear.WearAdhesive WearSolid-phase welding forms between two contact asperities after normal load applies.As the moving as
11、perities pass,the microscopic weld often breaks and material is removed from the surface with the lower yield strength to another.Abrasive WearAbrasive wear occurs when two interacting surfaces are in direct physical contact and one is significantly harder than the other.The general laws of abrasive
12、 wear:1)Wear increases with sliding distance.2)Wear increases with normal applied load.3)Wear decreases as the hardness of the sliding surface increases.Fatigue wear is caused by the propagation of subsurface damage to the surface due to cyclic loadings,i.e.by fatigue mechanism.Fatigue wear occurs i
13、n non-conformal machine elements due to high stresses to the solid material,even in well-lubricated situations or“non-contact”state.Life of fatigue wear should be treated statistically due to its so many influencing uncertainties.Fatigue WearLubricationThe object of lubrication is to reduce friction
14、,wear,and heating of machine parts which move relative to each other.Five distinct forms of lubrication:l 1.Hydrodynamic;l 2.Hydrostatic;l 3.Elastohydrodynamic;l 4.Boundary;l 5.Solid-film Hydrodynamic LubricationThe load-carrying surfaces of the bearing are separated by a relatively thick film of lu
15、bricant,so as to prevent metal-to-metal contact.The film pressure is created by the moving surface itself pulling the lubricant into a wedge-shaped zone at a velocity sufficiently high to create the pressure necessary to separate the surfaces against the load on the bearing.The stability can be expl
16、ained by the laws of fluid mechanics.Hydrostatic LubricationIt is obtained by introducing the lubricant,into the load-bearing area at a pressure high enough to sepa-rate the surfaces with a relatively thick film of lubricant.Unlike hydrodynamic lubrication,motion of one surface relative to another i
17、s not required.Hydrostatic lubrication should be considered when designing bearings where the velocities are small or zero and where the frictional resistance needs to be an absolute minimum.Elastohydrodynamic LubricationIs a form of hydrodynamic lubrication where elastic deformation of the lubricat
18、ed surfaces becomes significant.Is usually associated with non-conformal surfaces.The mathematical explanation requires the Hertzian contact theory of contact stress and fluid mechanics.Boundary LubricationThe highest asperities may be separated by lubricant films only several molecular dimensions i
19、n thickness.The viscosity of the lubricant is not of as much importance with boundary lubrication as is the chemical composition.Solid-film LubricationWhen bearings must be operated at extreme temperatures,the ordinary mineral oils are not satisfactory.A solid-film lubricant such as graphite or molybdenum disulfide must be used Mixed LubricationIf the pressures in machine elements are too high,or running speeds are too low,asperity contact occurs,the regime is a combination of boundary film,metal contact,and fluid film.The end of Chapter 3
