Lesson 8

Introduction to Tribology

Tribology is defined as the science and technology of interacting surfaces in relative motion, having its origin in the Greek word tribos meaning rubbing. It is a study of the friction, lubrication, and wear of engineering surfaces with a view to understanding surface interactions in detail and then prescribing improvements in given applications.

tribological adj.tribologist n.rub v. 擦 , 摩擦with a view toadv. 着眼于 , 以 ... 为目的 , 考虑到prescribe v.指示 , 规定 , 处 ( 方 ), 开( 药 )提出办法

The work of the tribologist is truly interdisciplinary, embodying physics, chemistry, mechanics, thermodynamics, and materials science, and encompassing a large, complex, and interwinded area of machine design, reliability, and performance where relative motion between surfaces is involved.

discipline n.纪律 , 学科v. 训练disciplinary adj.训练上的 纪律 的 , 惩戒性的学术的 , 科学的thermodynamics n.[ 物 ] 热力学embody vt.具体表达 , 使具体化 , 包含 , 收录encompass v.包围 , 环绕 , 包含或包括某事物Interwind v.互卷 , 互相盘绕交织

It is estimated that approximately one-third of the world's energy resources in present use appear as friction in one form or another. This represents a staggering loss of potential power for today's mechanized society. The purpose of research in tribology is understandably the minimization and elimination of unnecessary waste at all levels of technology where the rubbing of surfaces is involved.

Staggering adj.难以置信的；惊愕的

One of the important objectives in tribology is the regulation of the magnitude of frictional forces according to whether we require a minimum (as in machinery) or a maximum (as in the case of anti-skid surfaces) . It must be emphasized, however, that this objective can be realized only after a fundamental understanding of the frictional process is obtained for all conditions of temperature, sliding velocity, lubrication, surface finish, and material properties.

regulation n. 规则 , 规章 , 调节 , 校准

The most important criterion from a design viewpoint in a given application is whether dry or lubricated conditions are to prevail at the sliding interface. In many applications such as machinery, it is known that only one condition shall prevail (usually lubrication), although several regimes of lubrication may exist.

criterion n. ( 评判的 ) 标准 , 准据 , 规范prevail vi.流行 , 盛行 , 获胜 , 成功占上风 , 主导影响较大regime n.政体 , 政权状态，方式

There are a few cases, however, where it cannot be known in advance whether the interface is dry or wet, and it is obviously more difficult to proceed with any design. The commonest example of this phenomenon is the pneumatic tyre. Under dry conditions it is desirable to maximize the adhesion component of friction by ensuring a maximum contact area between tyre and road--and this is achieved by having a smooth tread and a smooth road surface.

proceed vi.进行 , 继续下去 , 发生pneumatic adj. 装满空气的 , 气力的 , 风力的 Adhesion n.粘着 , 附着粘附力 ; 附着力信奉 ; 追随 ; 支持 ; 同意 , 加入Tread n.走步；脚步踏板，梯面轮胎胎面

adhesion and cohesionadhesion and cohesion attractive forces between material bodies. attractive forces between material bodies. A distinction is usually made between an A distinction is usually made between an adhesiveadhesive

force, which acts to hold force, which acts to hold twotwo separate bodiesseparate bodies together (or to stick one body to another) and a together (or to stick one body to another) and a cohesivecohesive force, which acts to hold together force, which acts to hold together the like the like or unlike atoms, ions, or molecules of a single bodyor unlike atoms, ions, or molecules of a single body. . However, both forces result from the same basic However, both forces result from the same basic properties of matter. A number of phenomena can be properties of matter. A number of phenomena can be explained in terms of adhesion and cohesion. For explained in terms of adhesion and cohesion. For example, surface tension in liquids results from example, surface tension in liquids results from cohesion, and capillarity results from a combination cohesion, and capillarity results from a combination of adhesion and cohesion. The hardness of a of adhesion and cohesion. The hardness of a diamond is due to the strong cohesive forces diamond is due to the strong cohesive forces between the carbon atoms of which it is made. between the carbon atoms of which it is made. Friction between two solid bodies depends in part Friction between two solid bodies depends in part upon adhesion.upon adhesion.

Such a combination, however, would produce a disastrously low coefficient of friction under wet conditions. In the latter case, an adequate tread pattern and a suitably textured road surface offer the best conditions, although this combination gives a lower coefficient of friction in dry weather.

Disastrously adv.悲惨地Disastrous adj.灾难性的 , 悲惨的 ; 招致不幸的 ; 造成重大伤亡的disaster n.灾难 , 天灾 , 灾祸Texture vi.具有…质地 / 纹理具有…结构 / 构造

The several lubrication regimes which exist may be classified as hydrodynamic, boundary, and elastohydrodynamic. The different types of bearing used today are the best examples of fully hydrodynamic behaviour, where the sliding surfaces are completely separated by an interfacial lubricant film.

Interfacial adj.界面的 , 分界面的 , 面间的

Boundary or mixed lubrication is a combination of hydrodynanfic and solid contact between moving surfaces, and this regime is normally assumed to prevail when hydrodynamic lubrication fails in a given product design. For example, a journal bearing is designed to operate at a given load and speed in the fully hydrodynamic region, but a fall in speed or an increase in load may cause part solid and part hydrodynamic lubrication conditions to occur between the journal and bearing surfaces.

This boundary lubrication condition is unstable, and normally recovers to the fully hydrodynamic behaviour or degenerates into complete seizure of the surfaces. The pressures developed in thin lubricant films may reach proportions capable of elastically deforming the boundary surfaces of the lubricant, and conditions at the sliding interface are then classified as elastohydrodynamic.

recover vt.重新获得 , 恢复Degenerate vt.使衰退 , 使退化a. n. 腐化的

[ 者 ]seizure n.捉 , 没收 , 查封 ,

咬死bearing seizure轴承咬死Proportion n.比例 , 均衡 ,面积 , 部分vt. 使成比例 / 均

衡

It is now generally accepted that elastohydrodynamic contact conditions exist in a variety of applications hitherto considered loosely as belonging to the hydrodynamic or boundary lubrication regimes; for example; the contact of mating gear teeth.

hitherto adv. until this time 迄今 , 至今loosely adv.宽松地 , 模糊地 ,不确切地

Solid lubricants exhibit a compromise between dry and lubricated conditions in the sense that although the contact interface is normally dry, the solid lubricant material behaves as though initially wetted.

compromise n. 妥协 , 折衷v. 妥协 , 折衷 , 危及 ... 的安全

This is a consequence of a physico-chemical interaction occuring at the surface of a solid lubricant lining under particular loading and sliding conditions, and these produce the equivalent of a lubricating effect.

Lining n.加衬里 , 内层 , 衬套

What is Tribology?

• Tribology is defined as the science of interacting surfaces in relative motion. The word tribology comes from the Greek tribos, meaning rubbing. In any machine there are lots of component parts that operate by rubbing together. Some examples are bearings, gears, cams and tappets, tyres, brakes, and piston rings. All of these components have two surfaces which come into contact, support a load, and move with respect to each other. Sometimes it is desireable to have low friction, to save energy, or high friction, as in the case of brakes. Usually we don't want the components to wear so they are lubricated.

• The study of friction, wear, lubrication and contact mechanics are all important parts of tribology. Related aspects are surface engineering (the modification of a component's surface to improve its function, for example by applying a surface coating), surface roughness, and rolling contact fatigue (where repeated contacts causes fatigue to occur).

When Two Surfaces Are Pressed Together

Surfaces may look smooth, but on a microscopic scale they are rough. When two surfaces are pressed together, contact is made at the peaks of the roughness or asperities. The real area of contact can be much less than the apparent or nominal area. At the points of intimate contact, adhesion, or even local welding, can take place. If we want to slide one surface over the other then we have to apply a force to break those junctions.

The Force of Friction

• The friction force is the resistance encountered when one body moves relative to another body with which it is in contact. The static friction force is how hard you have to push something to make it, whilst the dynamic friction force is how hard you push to keep it moving. The ratio of the frictional force F to the normal force W is called the coeffiecient of friction and given the Greek symbol m (pronounced mew).

• Usually we want low friction (in a car engine for example) so we do not waste excessive energy getting it moving. But in same case we need high friction, in brakes for example. Friction is also important for car tyres to grip the road and between shoes and the ground for walking.

Keeping the Surfaces Apart-Lubrication

If we put a layer of oil between the surfaces then we can separate them and easily slide one over the other with reduced friction and wear. Mineral oils are the most common lubircants, but other low shear strength materials are also used; for graphite, PTFE, and soft metals like lead or gold.

The selection of the best lubricant and understanding the mechanism by which it acts

to separate surfaces in a bearing or other machine componment is a major area for study in tribology.

When Things Wear Out

• If one surface is slid over another then the asperities come into contact and there is a possibility that wear can occur. The breaking of all the little junction can cause material removal (called adhesive wear). Or the asperities of a hard surface can plough grooves in a soft surface (called abrasive wear).

• Wear is usually unwelcome; it leads to increased clearances beween moving components, increased mechanical loading and maybe even fatigue. But in grinding and polishing process the generation of high wear rates is desirable.

• As well as adhesive and abrasive wear, there are other mechanisms whereby material can be removed from a surface. Erosive wear occurs when particles (or even water droplet) strike a surface and break off a bit of the material. Hard particles can become trapped in contacts and cause material to be removed from one or both of the surfaces. One of the main reasons for frequent change of car engineoil is that it becomes contaminated

with hard debris particles that can wear out the engine components.

• Stress and Strain at the Contact• The design of rolling bearings and gears is such

that the load is supported on a small area. This leads to high stresses (about the highest stresses we find in any branch of engineering) over small areas of the components. This can cause high frcition, wear, and contact fatigue. Contact mechanics is therefore an important part of tribology.

• The analysis of contact stress is frequently difficult. Simple component geometries can be analysed using hand calculations. More complex component shapes frequently require analysis by numerical methods

• Tribology Down the Centuries

• Early civilisations developed quite sophisticated tribological devices such as potter’s wheels, door hinges and wheeled carriages. The carvings on the tomb at Saqqara shows an Eygptian tribologist bending down to lubricate the sled that carries a statue of Ti (c. 2400 BC).

• Military engineers rose to prominence in the days of the Roman empire by devising both war machinery and methods of fortification, using tribological principles. War ships (c. 50 AD) recovered from Lake Nemi near Rome, contain broze balls and rollers used to support rotating platforms

• It was the renaissance engineer-artist, Leonardo da Vinci (1452-1519), celebrated engineer, painter, and sculpter, who discovered that the tangential force of friction between moving solid bodies is proportional to the normal force. His notebooks show many designs for

moving parts and

machines that

show a remarkable

similarity to those

in use today

• Several of the innovative designs behind John Harrison's (1693-1776) marine chronometers where based on a tribological understanding of the moving parts. He designed and build clocks that required no lubrication. The properties of oil could not be kept constant over long periods of time or with climate changes. So, the removal of the requirement for lubrication allowed the clocks to remain accurrate over sea jouneys lasting many months.

• The coming of the computer age has provided new challenges for tribologists. The interface between the reading head and the magnetic disk in a computer hard disk requires careful design and lubrication to minimise

friction and reduce the

likelihood of disk

crashes and damage.

Want to Know More?

• There are several excellent books on the subject of tribology (take a look at our book list - especially the undergraduate texts section).

• We have put together a page of links to some of the best web sites with a tribological flavour. This page also included links to several professional bodies and learned socities that represent the interests of tribologists around the world.

• The WWWTribology@Sheffield web pages contain details of our research into a several aspects of tribology especially wear and lubrication.

• Or you may like to look at the Tribology of Machine Elements Course that we teach to undergraduate students at Sheffield.