materials for tribology — 1 introduction

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MATERIALS FOR TRIBOLOGY- 1 Introduction D. Scott* Design to avoid wear and failure is impossibl~e if the designer does not know how his materials behave, especially how they wear and how they interact with lubricants Engineering design is the creation of instructions for making an article to satisfy a specific requirement. The materials of construction and the lubricant are important factors in such specifications. This journal has planned a series of articles which will deal with all types of material and those design aspects in which tribology plays a significant ~art. Each article will review material properties, applications and potential uses with the object of drawing the designer's attention to the wide variety of materials available to solve any given problem. The series will include: Materials for plain bearings Materials for rolling mechanisms New materials Substitute materials Material deterioration Materials will be classified by tribological properties such as resistance to scuffing and to abrasion as well as by more usual qualities such as strength and hardness. To take pro- per advantage of new and old materials the designer must adopt a multi-disciplinary, ie tribological, approach. The engineering properties of materials to be dealt with are those properties which need to be taken into account in de- signing a component to withstand the mechanical and thermal stresses to which it will be exposed and the environment in which it has to function. Technological progress continuously demands materials of improved properties and higher and higher strength-to-weight ratios. Mechanisms operating at high speeds, under heavy loads or in extreme environments need great strength. If they are in relative motion, they may also require great hardness and wear and corrosion resis- tance as well as structural and dimensional stability. Although new materials may meet stringent design require- ments beyond the capabilities of the more commonly used structural materials, their availability and cost will always make the commoner materials more attractive commercially and encourage research on ways of improving their proper- ties. The designer must not only search for improved ma- terials but also for cheaper materials and in some instances for suitable substitute indigenous materials. Today the de- signer has a vast range of materials from which to select .the one which best meets his requirements. A determining factor in the extensive use of many materials, apart from availability and cost, is the amenability of the material to manipulation and the degree to which the designer can con- trol and vary properties such as strength, hardness and ducti- lity. Often, his choice of materials is restricted by the manu- * Mr. Scott is Head of the Materials Group of the National Engineering Laboratory, East Kilbride, Scotland. facturing methods at his disposal or the type of malerial- forming plant available. STRENGTH OF MATERIALS Although structural features have an important effect on ma- terial properties, the fundamental properties of materials are primarily determined at atomic level. These articles will therefore consider the theory of material improvement. The maximum stress a material can withstand is that re- quired to shear the atomic bonds of its lattice. Theoretical maximum strengths have been achieved in whiskers and filaments but methods of using these directly in engineering components are not yet available. Whiskers neither contain nor give rise to dislocations but if it is essential that even the strongest materials should show some plasticity and work hardening then dislocations must be present and improve- ment in strength must be explicable in terms of stress re- quired to make dislocations move and interact with other dislocations or other microstructural features. The ability to control and regulate plastic flow may stimulate the develop- ment of materials better suited to manipulation than those now in use. MATERIALS AND TREATMENT Materials may be conveniently divided into three principal types, ferrous, non-ferrous and non-metallic. The abundance of iron and its alloys and their favourable economics and diverse properties make ferrous materials the desirable choice for tribological applications. Modern cast irons and steels find extensive use in tribe-engineering. The principal properties, methods of hardening and strengthening and uses of these materials will be discussed. Alloy steels can be tailored to specific applications: the effect of alloying ele- ments and the main uses of special steels will therefore be reported. In currently used steels the martensite transfor- mation produces the best combination of strength and ducti- lity but as hardness increases, ductility decreases and at strengths of about 300 0001b/in 2 (21 000kg/cm2), which are produced by conventional alloying and heat treatment, the ductility is diminished to levels at present considered un- acceptable for most engineering applications. If a thermo- mechanical treatment is used whereby austenite is strain- hardened before transformation to martensite, unusual ducti- lity, fatigue and impact properties are obtained. Ausforming may thus allow increased strength above the present usable limits without sacrifice in ductility. Ultra-high-strength alloy steels are available which may be formed in the air- cooled solution--treated state and which are precipitation hardened and strengthened by an ageing treatment. These maragtng steels have good toughness for their strength and are readily welded. Use of these steels eliminates quenching and associated distortion problems. From an economic point of view some newer manufacturing processes such as cold 14 TRIBOLOGY January 1968

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Page 1: Materials for tribology — 1 Introduction

MATERIALS FOR TRIBOLOGY- 1 Introduction

D. Scott*

Design to avoid wear and fai lure is impossibl~e if the des igner does not know how his ma te r i a l s behave, especia l ly how they wear and how they in terac t with lubr icants

Engineering design is the creation of instructions for making an article to satisfy a specific requirement. The materials of construction and the lubricant are important factors in such specifications. This journal has planned a series of articles which will deal with all types of material and those design aspects in which tribology plays a significant ~art. Each article will review material properties, applications and potential uses with the object of drawing the designer's attention to the wide variety of materials available to solve any given problem. The series will include:

Mater ia l s for plain bear ings

Mater ia l s for rol l ing mechan i sms

New mate r i a l s

Substitute ma te r i a l s

Mater ia l de ter iora t ion

Mater ia ls will be c lass i f ied by t r ibological p rope r t i e s such as r e s i s t ance to scuffing and to abrasion as well as by more usual quali t ies such as s t rength and hardness . To take pro- per advantage of new and old ma te r i a l s the des igner must adopt a mul t i -d isc ip l inary , ie t r ibological , approach.

The engineer ing p rope r t i e s of ma te r i a l s to be dealt with are those p rope r t i e s which need to be taken into account in de- signing a component to withstand the mechanical and the rma l s t r e s s e s to which it will be exposed and the environment in which it has to function. Technological p r o g r e s s continuously demands ma te r i a l s of improved p rope r t i e s and higher and higher s t reng th- to -weigh t ra t ios . Mechanisms operat ing at high speeds , under heavy loads or in ex t r eme envi ronments need great s t rength. If they are in re la t ive motion, they may also requ i re great hardness and wear and co r ros ion r e s i s - tance as well as s t ruc tura l and dimensional stabili ty.

Although new mate r i a l s may meet s t r ingent design requ i re - ments beyond the capabi l i t ies of the more commonly used s t ruc tu ra l mate r ia l s , their availabili ty and cost will always make the commoner ma te r i a l s more a t t rac t ive commerc ia l ly and encourage r e s e a r c h on ways of improving the i r p rope r - t ies . The des igner must not only sea rch for improved ma- t e r i a l s but also for cheaper ma te r i a l s and in some ins tances for sui table subst i tute indigenous mate r i a l s . Today the de- s igner has a vast range of ma te r i a l s f rom which to se lec t

.the one which best meets his r equ i remen t s . A de te rmin ing factor in the extensive use of many mate r i a l s , apar t f rom availability and cost , is the amenabil i ty of the mate r ia l to manipulation and the degree to which the des igner can con- t ro l and vary p rope r t i e s such as s t rength, ha rdness and ducti- lity. Often, his choice of ma te r i a l s is r e s t r i c t e d by the manu-

* Mr. Scott is Head of the Mate r ia l s Group of the National Engineer ing Laboratory , East Kilbride, Scotland.

factur ing methods at his d isposa l or the type of ma le r i a l - forming plant available.

STRENGTH OF MATERIALS

Although s t ruc tu ra l fea tures have an impor tant effect on ma- t e r i a l p rope r t i e s , the fundamental p rope r t i e s of ma te r i a l s a re p r imar i ly de t e rmined at a tomic level. These a r t i c l e s will t he re fo re cons ide r the theory of mate r ia l improvement . The maximum s t r e s s a ma te r i a l can withstand is that r e - quired to shea r the atomic bonds of i ts lat t ice. Theore t i ca l maximum s t r eng ths have been achieved in wh i ske r s and f i laments but methods of using these d i rec t ly in engineer ing components a re not yet avai lable. Whiskers ne i ther contain nor give r i s e to d is loca t ions but if it is e s sen t i a l that even the s t ronges t ma te r i a l s should show some plas t ic i ty and work hardening then d is loca t ions must be p re sen t and improve - ment in s t rength must be expl icable in t e r m s of s t r e s s r e - quired to make dis locat ions move and in te rac t with o ther d is locat ions or o ther m i c r o s t r u c t u r a l f ea tu res . The ability to control and regulate p las t ic flow may s t imula te the develop- ment of ma te r i a l s be t t e r sui ted to manipulation than those now in use.

MATERIALS AND TREATMENT

Mate r ia l s may be conveniently divided into th ree pr incipal types, f e r rous , non - f e r rous and non-meta l l i c . The abundance of iron and i ts al loys and the i r favourable economics and d ive r se p rope r t i e s make f e r ro u s ma te r i a l s the des i r ab l e choice for t r ibological appl icat ions. Modern cas t i rons and s t ee l s find extensive use in t r i be -eng inee r ing . The pr incipal p rope r t i e s , methods of hardening and s t rengthen ing and uses of these ma te r i a l s will be d i scussed . Alloy s t ee l s can be ta i lored to speci f ic applicat ions: the effect of al loying e le- ments and the main uses of specia l s t ee l s will t he r e fo re be repor ted . In cur ren t ly used s t ee l s the mar t ens i t e t r a n s f o r - mation produces the bes t combination of s t rength and ducti- lity but as h a rd n es s i n c r e a s e s , ductility d e c r e a s e s and at s t r eng ths of about 300 0001b/in 2 (21 000kg/cm2), which a re produced by conventional alloying and heat t r e a tmen t , the ductility is d iminished to levels at p resen t cons ide red un- acceptable for most engineer ing applicat ions. If a t h e r m o - mechanical t r ea tmen t is used whereby aus teni te is s t r a i n - hardened before t r ans fo rma t ion to mar t ens i t e , unusual ducti- lity, fatigue and impact p rope r t i e s a re obtained. Ausforming may thus allow inc rea sed s t rength above the p re sen t usable l imi ts without s ac r i f i ce in ducti l i ty. U l t r a - h i g h - s t r e n g t h alloy s t ee l s a re available which may be formed in the a i r - cooled solut ion-- t rea ted s ta te and which a re prec ip i ta t ion hardened and s t rengthened by an ageing t r ea tment . These maragtng s t ee l s have good toughness for the i r s t r eng th and a re readi ly welded. Use of these s t ee l s e l imina tes quenching and as soc ia t ed d is tor t ion p rob lems . F r o m an economic point of view some newer manufactur ing p r o c e s s e s such as cold

14 TRIBOLOGY January 1968

Page 2: Materials for tribology — 1 Introduction

extrusion, which improve material properties, should be con- sidered: so also should economical duplex heat treatments such as martempering.

HIGH TEMPERATURES

In high t e m p e r a t u r e appl icat ions , h igh- speed tool s t e e l s and s i m i l a r spec ia l s t ee l s and nickel- and coba l t -based al loys will be de sc r ibed . Cor ros ion r e s i s t a n c e r e q u i r e s carefu l ma te r i a l se lec t ion and the use of s t a in l e s s s t ee l s , or non- f e r rous me ta l s such as a luminium, nickel, ch romium, t i tanium and the i r a l loys and of non-meta l l i c m a t e r i a l s will t h e r e f o r e be reviewed. To r e s i s t ab ra s ive wear the des igne r must choose hard m a t e r i a l s such as cemen ted ca rb ides , c e r m e t s and even diamond.

COMPOSITES

The t he rm a l and s t r e s s p rob l ems a s soc i a t ed with advanced t r i b o - e n g i n e e r i n g requ i re grea t s t reng th , l a rge e las t ic modu- lus and light weight: s a t i s f ac to ry m a t e r i a l s a re not cu r r en t ly avai lable. Conventional m a t e r i a l s have been improved by orthodox methods a lmos t to the l imit of the i r potential mecha- nical p r o p e r t i e s so that new types of ma t e r i a l a r e having to be developed. Compos i t e s , which combine m a t e r i a l s of d i s - s i m i l a r mechanica l and physical p r o p e r t i e s , can have pro- pe r t i e s supe r io r to one or both of the i r cons t i tuents . The re a re two pr incipal types. In one, a ma t r ix may be r e in fo rced with f i b re s or pa r t i c l e s to improve i ts p r o p e r t i e s . In the o ther the ma t r ix is essen t ia l ly a glue to hold toge ther f i b r e s or pa r t i c l e s which have valuable p r o p e r t i e s but which by t hemse lves cannot be used as eng ineer ing m a t e r i a l s . The bonding of the f i b re s to the ma t r ix a f fec ts the s t r eng th s ince it is through this bond that s t r e s s is t r a n s m i t t e d f rom one f ibre to the next, the ma t r ix being only lightly s t r e s s e d . Cer ta in diff icul t ies need to be su rmounted before compos i t e s achieve the i r full potential . Whiske r s and f ib re s a r e expen- s ive and have p r o b l e m s with s t r e s s concent ra t ion at the i r ends which can influence c rack init iat ion and propagation. Conventional methods a r e not sui table for the manufacture of components f rom compos i t e s nor for the fabr ica t ion and joining of f i b r e - r e i n f o r c e d ma te r i a l s . By employing re in - f o r c e m e n t s of oxide on non-meta l l i c wh i ske r s which approach the theore t i ca l s t reng th , very high u l t imate s t r eng ths in com- pos i tes a r e poss ib le . Glass , carbon, s i l icon ni t r ide and alumina a re a t t r ac t ive n o n - m e t a l s . Bes ides rep lac ing meta ls , c e r a m i c s may be used as coat ings to complemen t de s i r ab l e meta l c h a r a c t e r i s t i c s by adding r e f r a c t o r y p r o p e r t i e s , insu- lation and e ros ion , wear , oxidation and co r ro s ion r e s i s t a n c e .

Metals for use at high t e m p e r a t u r e s can be s t r eng thened by d i s p e r s i n g non-me ta l l i c pa r t i c l e s in them. Such m a t e r i a l s can maintain useful p r o p e r t i e s at t e m p e r a t u r e s within 50--100°C of the mel t ing point of the ma t r ix metal . Only sma l l amounts of the d i spe r so id a r e requ i red . Nickel a l loys with thor ium (TD nickel) a r e now c o m m e r c i a l l y available: so a r e synthet ic diamond and sapphi re , whils t new graph i tes and other new m a t e r i a l s such as the ca rb ides , bo r ides and ni- t r i de s of ce r ta in me ta l s approach the ha rdnes s of diamond. Informat ion r ega rd ing such new m a t e r i a l s will be r e p o r t e d in the a r t i c l e on new ma te r i a l s .

WHAT DOES THE DESIGNER SPECIFY

Adequate ma te r i a l p r o p e r t i e s for design a re usually ensu red by ind i rec t means , mainly by the des igner speci fying chemi - cal ana lys i s , heat t r ea tmen t and mechanica l p r o p e r t i e s . Such spec i f ied p r o p e r t i e s may not be d i rec t ly r e p r e s e n t a t i v e in s e rv i ce . The mos t impor tant ma te r i a l p roper ty may be r e - s i s t ance to abras ion or r e s i s t a n c e to scuffing, or to ro l l ing contact fatigue, o r to lubricant at tack, or to co r ros ion : the envi ronment may a lso be impor tan t . A proper ty such as d imens iona l s tabi l i ty may comple te ly de t e rmine the s e r v i c e life.

WEAR AND CORROSION

Wear may take many f o r m s depending upon su r face topo- graphy, contact condi t ions and env i ronment but genera l ly t he re a r e two main types; mechanica l and chemica l . Mecha-

nical wear is r e l a t ed to f r ic t ion , ab ras ion and fatigue. Chemi- cal wear a r i s e s f r o m su r f ace a t tack by reac t ive compounds and the subsequent b reak ing or rubbing away of the reac t ion products . The d i f ferent types of wear may occur s imu l - taneously .

In plain bea r ings the load is t r a n s m i t t e d between moving points by s l id ing contact and the c r i t e r i on of s a t i s f a c t o r y p e r f o r m a n c e of the bea r ing is min imum wear of the compo- nents toge ther with f r e e d o m f r o m s e i z u r e and f r e e d o m f r o m mechanica l fa i lure by deformat ion or fatigue. To c a r r y a hard s t ee l shaft , which is usual ly spec i f ied for i ts mechan ica l p r o p e r t i e s , a bear ing ma t e r i a l must be soft to avoid wear of the h a r d e r ma t e r i a l yet s t rong enough to wi ths tand heavy loads without d i s to r t ion and without fatigue. The s u c c e s s of white meta l bea r ings is genera l ly r e g a r d e d as being due to c o r r e c t c o m p r o m i s e between so f tnes s to avoid wear and s t r eng th to r e s i s t fatigue. Substi tute bea r ings offer a com- p r o m i s e between al loys soft enough to avoid wear , those hard enough to r e s i s t fatigue, and those able to r e s i s t c o r r o - sion by the use of a soft over lay on a s t rong bea r ing metal . F u r t h e r economy can be ef fec ted by using, say, c o p p e r / l e a d as an in te r lay between a s tee l base and a soft over lay . The role of the over lay as a s ac r i f i c i a l f i lm to avoid bea r ing fa i lure by fatigue of such base m a t e r i a l s as a luminium is being expanded. P l a s t i c s - l i n e d bea r ings a r e potential ly sui table for most appl ica t ions and when ma t e r i a l capable of being fo rmed to, and maintaining, c lose t o l e r a n c e s becomes avai lable , someth ing of a revolut ion in bear ing design may occur . Under more arduous s l id ing contact condi t ions where mating s u r f a c e s come into contact , r e s i s t a n c e to scuff ing is an impor tant p roper ty . This does not s e e m to be r e l a t ed to ma te r i a l ha rdnes s : compatabi l i ty of ma te r i a l pa i r s appea r s to be the dominant fac tor . Surface t r e a t m e n t s which lubr i - cate , such as sulphoning and soft ni t r iding, can aid des ign in many s l id ing-con tac t appl ica t ions . In ro l l ing e l emen t s with coun te r fo rma l a s s e m b l i e s , the concent ra t ion of s t r e s s n e c e s - s i t a t e s m a t e r i a l s of g rea t s t r eng th which usually fail by ro l l ing contact fatigue. The eng ineer ing r e q u i r e m e n t s of b a l l - b e a r i n g s t e e l s a r e d imens iona l s tabi l i ty , g rea t h a r d n e s s to r e s i s t wear , a high e las t ic l imi t to avoid p las t ic d e f o r m a - tion under load and good fatigue r e s i s t a n c e to contend with a l t e rna t ing s t r e s s e s . In h i g h - t e m p e r a t u r e use m a t e r i a l s must a lso have adequate hot h a r d n e s s and must r e s i s t oxida- tion. Under condi t ions of un lubr ica ted rol l ing contact the useful life of ro l l ing e l emen t s is not l imi ted by fatigue but by wear so that hard m a t e r i a l s such as s i n t e r e d ca rb ide s a r e r equ i red .

Abras ion caused by ploughing or gouging of hard pa r t i c l e s , deb r i s etc agains t a sof te r working su r f ace is a s e r i ous cause of wear . T h e r e a r e indicat ions that abras ion , p r i m a r i l y a crude machining p r o c e s s , is r e l a t ed to indentat ion h a r d n e s s . Hard w e a r - r e s i s t a n t m a t e r i a l s will be d i s c u s s e d in this con- text. Economic r e a s o n s may dic ta te that expensive al loys should be abandoned in favour of su r face t r e a t m e n t s appl ied to o rd inary s t e e l s to i n c r e a s e s t r eng th and improve wear r e s i s t a n c e . The role of the common t r e a t m e n t s such as c a r - bur iz ing and n i t r id ing which depend upon the diffusion of hardening e l emen t s into the su r f ace of the base ma te r i a l will be covered . E l ec t ro -depos i t i on of ha rd meta l such as c h r o - mium and hard facing or su r fac ing techniques to provide spec i f ic alloy or c e r a m i c coat ings on components manufac- tu red in mild s t ee l or o the r cheap, eas i ly produced and shaped ma t e r i a l will be outlined.

Cor ros ion p r o b l e m s r eq u i r e spec ia l cons idera t ion . Genera l ly to r e s i s t co r ro s ion , me ta l s must f o r m a dense , tough, i m p e r - vious oxide l ayer which r e s i s t s c rack ing under load and p r e - vents a t tack of the meta l by the c o r r o s i v e envi ronment . T h e r e is a range of potential m a t e r i a l s sui table for wi thstanding hos t i le env i ronment s and e l iminat ing phenomena such as hydrogen e m b r i t t l e m e n t and s t r e s s co r ro s ion .

FRICTION MATERIALS

The aeronaut ica l and automobile i ndus t r i e s make daily m o r e s e v e r e demands on f r ic t ion m a t e r i a l s . High quality brake m a t e r i a l s must have high coef f ic ien ts of f r ic t ion , s tabi l i ty at al l opera t ing t e m p e r a t u r e s , good wear r e s i s t a n c e and s t rength , high conductivity and c o r r o s i o n s tabi l i ty . At high opera t ing t e m p e r a t u r e s wear of organic m a t e r i a l b e c o m e s

TRIBOLOGY January 1968 15

Page 3: Materials for tribology — 1 Introduction

e x t r e m e l y s e v e r e . P r o g r e s s i s l ikely to be in the develop- ment of s i n t e r e d and c e r m e t f r ic t ion m a t e r i a l s .

STANDARDS

Modern technology is c r ea t i ng such demands for spec ia l ma- t e r i a l s that no m a t e r i a l r e m a i n s of only academic i n t e r e s t for long. As soon as a new m a t e r i a l is ava i lab le and i ts p ro - p e r t i e s have been de te rmined , some use can be found for it. The absence of any recogn ized code of p r ac t i c e or even well e s t ab l i shed or s t anda rd l i s t s for evaluat ing new m a t e r i a l s for t r i b o - d e s i g n r e q u i r e m e n t s means that su i tab i l i ty can be r e l i ab ly e s t ab l i shed only by p e r f o r m a n c e in p rac t i ce . Acce l - e r a t ed s imula t ed s e r v i c e t e s t ing can avoid f u l l - s c a l e - t e s t de lays but such t e s t ing mus t be c a r r i e d out caut iously and an unde r s t and ing of the modes of fa i lu re is e s s e n t i a l in s e l ec - t ing a r ea sonab le a c c e l e r a t e d p rocedure . Fa i lu re inves t iga - t ion i s an impor t an t aspec t of m a t e r i a l s sc ience . Improve - men t s in p e r f o r m a n c e can be aided by a b e t t e r unde r s t and ing of the m e c h a n i s m s of fa i lu re and i ts con t ro l l ing f ac to r s . The study of m a t e r i a l f a i lu re is thus impor t an t as it can lead to b e t t e r des igns embodying g r e a t e r r e l i ab i l i ty as well as eff iciency of s e rv i ce . A study of the de t e r i o r a t i on of m a t e r i -

a l s in s e r v i c e can help develop m e a n s of p ro long ing t h e i r l ife.

E n g i n e e r s usua l ly des ign componen t s f r o m well e s t a b l i s h e d m a t e r i a l s . The t ime between the deve lopment of a new or improved m a t e r i a l and i ts use is p r o g r e s s i v e l y shor t en ing . In th is s e r i e s of a r t i c l e s we sha l l d raw a t ten t ion to new ma- t e r i a l s and t h e i r spec i f ic p r o p e r t i e s and potent ia l u s e s in the hope of speed ing up the t r a n s i t i o n f r o m deve lopment to use . We sha l l t ry to provide in fo rma t ion in a fo rm the d e s i g n e r can read i ly i n t e r p r e t . New m a t e r i a l s often sa t i s fy some ad- vanced des ign r e q u i r e m e n t s bt/t t h e i r p r o p e r t i e s may not a lways sui t convent ional p r a c t i c e . P r o p e r l y approached th i s can s t imu la t e new des ign concepts to make use of unusua l combina t ions of p r o p e r t i e s . The modern p r o f e s s i o n a l t r i - bologis t should have the in te l l ec tua l f lexibi l i ty to e n c o m p a s s the va r ious d i sc ip l ines in his f ie ld and should s t r i v e for a m a t e r i a l s sc i ence approach to h is p r o b l e m s . M a t e r i a l s s c i ence is moving so fas t that it may not be long be fo re com- pu te r s will be able to p r e s c r i b e m a t e r i a l s to p a r t i c u l a r spec i f i ca t ions . Until th i s ideal s t a t e for a f f a i r s is r e a c h e d t r i bo log i s t s mus t re ly on des ign data s h e e t s and full d e s c r i p - t ions of m a t e r i a l s and t h e i r p r o p e r t i e s . We hope that the fo r thcoming s e r i e s of a r t i c l e s will make a useful c o n t r i - bution.

Materials for plain bearings

Number 2 in the

P. T. Holligan*

series on materials for tribology

This a r t i c l e s u m m a r i z e s the a t t r i bu t e s and s h o r t c o m i n g s of the common bea r ing ma te r i a l s - -whi t e me ta l s , a l u m i n i u m - base a l loys , c o p p e r - b a s e a l loys-- that d e s i g n e r s a r e a l r eady using. It d i s c u s s e s load c a r r y i n g capaci ty , oil f i lm t h i c k n e s s and wear and c o r r o s i o n r e s i s t a n c e . Although these m a -

t e r i a l s a r e facing i n c r e a s i n g compet i t ion f rom p l a s t i c s and o ther spec ia l m a t e r i a l s , they p o s s e s s c e r t a i n c h a r a c t e r i s t i c s . t echn ica l and economic , which a r e l ikely to offer the d e s i g n e r the bes t solut ion for many y e a r s yet .

The p r i m a r y funct ions of a bear ing, of wha teve r type, a r e to locate re la t ive ly moving m e m b e r s , and to t r a n s m i t force be - tween these m e m b e r s with the max imum eff ic iency and the min imum power loss .

F ive known methods of des ign ing b e a r i n g s u r f a c e s to mee t these r e q u i r e m e n t s have a l r eady been descr ibed1: the p r e s e n t a r t i c l e d i s c u s s e s me ta l l i c p la in b e a r i n g s in which the object of the des ign i s to keep the moving s u r f a c e s s epa ra t ed by a f i lm of fluid. Th i s involves cons ide ra t ion of the m a t e r i a l s of the moving sur faces , of the fluid s e p a r a t i n g them and of the method of locat ion of these su r faces .

THE BEARING ASSEMBLY

A bea r ing a s sembly cons i s t s of the fol lowing m e m b e r s :

1 The housing or s t r u c t u r e in which the bea r i ng is located.

2 The bea r ing or bea r i ng m a t e r i a l which may or may not be in teg ra l with the housing.

3 The o ther moving m e m b e r .

4 The lubr i can t or fluid s epa ra t i ng the two moving m e m - be r s , which i s in t roduced by su i tab le m e a n s depending upon the ove ra l l des ign of the m e c h a n i s m .

CONDITIONS OF OPERATION

The condi t ions under which the bea r ing a s s e m b l y mus t ope r - ate va ry widely with d i f ferent types of m e c h a n i s m . They range f rom s teady un id i rec t iona l loading of va ry ing magn i -

* Mr. Holligan was unti l r ecen t ly Senior Techn ica l Adv i se r to the s a l e s o rgan i sa t ion of G lac i e r Metal Company, Alper ton, Wembley, Middlesex and is now a consul tan t .

tudes , in such appl ica t ions as tu rb ine , g e n e r a t o r , g e a r box and c e r t a i n m a c h i n e - t o o l b e a r i n g s , to dynamic loadings , v a r y - ing in both magni tude and d i rec t ion , found for example in i n t e rna l combus t ion engines , r e c i p r o c a t i n g pumps and c o m - p r e s s o r s . In addit ion the r e l a t i ve veloci ty of the moving p a r t s c o v e r s a wide range , while the t e m p e r a t u r e at the ac tua l b e a r i n g s u r f a c e s d i f f e r s g rea t ly in d i f f e ren t appl ica t ions .

The de s igne r of a m e c h a n i s m mus t take a l l t hese v a r i a b l e s into account in dec id ing upon the type and des ign of b e a r i n g to adopt, and the a p p r o p r i a t e method of lubr ica t ion . The de- ta i led des ign of the b e a r i n g a s s e m b l y and of i t s l ub r i ca t i on s y s t e m a r e outs ide the scope of th i s a r t i c l e , but the d e m a n d s imposed upon the b e a r i n g m a t e r i a l will now be cons ide red .

REQUIREMENTS OF BEARING SURFACES

Two componen t s of the b e a r i n g a s s e m b l y r e q u i r e c o n s i d e r a - tion:

1 The b e a r i n g su r f ace

2 The shaf t or o the r moving m e m b e r

The shaf t

It will be s i m p l e r to dea l f i r s t with the shaf t or o the r moving shr face , as i t s des ign and m a t e r i a l a r e d ic ta ted p r i m a r i l y by s t r e s s and by economic cons ide ra t i ons . F e r r o u s m a t e r i a l s ( s tee l or cas t i ron) a r e mos t commonly employed and fo r economic r e a s o n s unha rdened ca rbon s t ee l or one of the ca s t i rons , such as sphe ro ida l g raph i t i c i ron for shaf ts , or o r d i n - a ry s t r u c t u r a l i ron for bedpla tes , a r e m o s t widely used.

Where the m e m b e r i s sub jec ted to s t r e s s e s h ighe r than those m a t e r i a l s can sus ta in without de fo rma t ion or fa i lure , h e a t -

16 TRIBOLOGY J anua r y 1968