Relating Laboratory Wear Testing to the In-Service Wear of Polymers

21

Relating

Laboratory

W e a r Testing to the

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 1, 2018 | https://pubs.acs.org Publication Date: September 12, 1985 | doi: 10.1021/bk-1985-0287.ch021

I n - S e r v i c e W e a r of P o l y m e r s

1

2

J.C.Anderson and P. K. Williamson 1

National Centre of Tribology, Risley, Warrington WA3 6AT, United Kingdom Lubrication and Wear Unit, Railway Technical Centre, Derby DE2 8UP, United Kingdom

2

A large quantity of wear data for polymers and polymer composites is now available from laboratory tests. Except in special circumstances, much of the in-service wear data is incomplete and cannot readily be compared with laboratory data. This paper draws together some laboratory and selected in-service wear data in an attempt to relate the two. The uncertainties in calculating specific wear rates from i n service data are discussed, as well as the data from laboratory tests which attempt to simulate particular modes of wear (eg abrasion, fretting). From the information presented, conclusions are drawn on the capability of predicting in-service wear from laboratory tests.

Over 70 d i f f e r e n t t y p e s o f polymer-based m a t e r i a l s are s o l d i n the UK a l o n e as dry b e a r i n g m a t e r i a l s . W h i l s t the number o f b a s i c polymers used i n these m a t e r i a l s i s l i m i t e d t o about 6 - v i z p o l y a c e t a l , n y l o n , p o l y e t h y l e n e , PTFE, p o l y i m i d e and p h e n o l i c - i t i s the i n c o r p o r a t i o n o f a g r e a t v a r i e t y o f f i l l e r s i n t o these polymers, and the d i f f e r e n t methods o f f a b r i c a t i o n employed, t h a t g i v e s r i s e t o the l a r g e number of commercially a v a i l a b l e m a t e r i a l s . The number o f polymer b e a r i n g m a t e r i a l s has grown s t e a d i l y over the l a s t twenty y e a r s , and t h i s growth i s bound t o c o n t i n u e as newer polymers, polymer b l e n d s and p o l y m e r / f i l l e r c o m b i n a t i o n s r e a c h the s t a g e o f commercial p r o d u c t i o n . Polymers and composites can be used as b e a r i n g m a t e r i a l s i n a wide range o f a p p l i c a t i o n s . They are p a r t i c u l a r l y a p p r o p r i a t e f o r b e a r i n g s i n which a f i l m o f l u b r i c a n t cannot be m a i n t a i n e d between the working surfaces at a l l times. The s e l e c t i o n o f an a p p r o p r i a t e m a t e r i a l , and the e s t i m a t i o n o f i t s l i k e l y performance i n a p a r t i c u l a r a p p l i c a t i o n , c a n become c o m p l i c a t e d because o f the combined e f f e c t s o f b e a r i n g p r e s s u r e , temperature, counterface roughness, sliding speed, and o t h e r parameters, on the wear r a t e o f the m a t e r i a l . Guidance on the s e l e c t i o n o f polymer m a t e r i a l s f o r b e a r i n g s i s a v a i l a b l e , f o r example This chapter not subject to U.S. copyright. Published 1985, American Chemical Society

Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

316

POLYMER WEAR AND ITS CONTROL

i n the E n g i n e e r i n g S c i e n c e s Data U n i t Item No 76029 C l \ the Wear C o n t r o l Handbook(21and i n the r e c e n t l y p u b l i s h e d Polymer M a t e r i a l s f o r Bearing Surfaces (3) A l l c o n t a i n u s e f u l i n f o r m a t i o n and i n r e f 3 comprehensive d a t a i s g i v e n f o r a l a r g e number o f c o m m e r c i a l l y a v a i l a b l e polymers and c o m p o s i t e s . T h i s d a t a was o b t a i n e d from t h r u s t b e a r i n g s and o t h e r t e s t s a t the N a t i o n a l Centre o f T r i b o l o g y . However c o n s i d e r a b l e d i f f e r e n c e s can e x i s t between the t e s t c o n d i t i o n s from such s o u r c e s and those p e r t a i n i n g t o the wide v a r i e t y o f p o s s i b l e service applications. The theme o f t h i s paper t h e r e f o r e i s the importance o f r e l a t i n g l a b o r a t o r y t e s t t o i n - s e r v i c e wear d a t a and the problems i n v o l v e d .


#

Laboratory

Wear Assessment

C o n s i d e r a b l e r e s e a r c h e f f o r t s have been made i n r e c e n t y e a r s i n t o the study and measurement o f the wear o f a range o f polymer-based m a t e r i a l s i n the l a b o r a t o r y . T h i s has r e s u l t e d i n the p u b l i c a t i o n o f numerous s t i m u l a t i n g papers which d e s c r i b e in detail the wear mechanisms p e r t a i n i n g t o r e s t r i c t e d t e s t c o n d i t i o n s and specific materials. U s e f u l and g e n e r a l r e f e r e n c e s on polymer t r i b o l o g y are given i n r e f s 4 , 5, 6. A d e s i g n e r c o u l d be f o r g i v e n f o r b e i n g somewhat c o n f u s e d by t h i s w e a l t h o f d a t a , w i t h perhaps l i t t l e o f i t a p p l i c a b l e t o h i s p a r t i c u l a r problem. In s e r v i c e a p p l i c a t i o n s , p a r t i c u l a r l y when exposed t o a changing environment, b e a r i n g s may be s u b j e c t e d t o a complex i n t e r a c t i o n o f wear p r o c e s s e s and c o n d i t i o n s , and no l a b o r a t o r y t e s t can p e r f e c t l y s i m u l a t e t h e s e . A l s o , by t h e i r very nature, mixed wear p r o c e s s e s i n v o l v e a s i g n i f i c a n t degree o f s t a t i s t i c a l v a r i a t i o n i n wear r a t e s and t h i s makes an accurate p r e d i c t i o n of in-service l i f e from l a b o r a t o r y t e s t s even more d i f f i c u l t (see r e f 6 ) . T h e r e f o r e i n l a b o r a t o r y t e s t s i t i s common t o choose s p e c i f i c c o m b i n a t i o n s o f l o a d , speed, c o n t a c t geometry ( p o i n t , l i n e or a r e a ) , motion ( c o n t i n u o u s r o t a t i o n , o s c i l l a t i o n , r e c i p r o c a t i o n or f r e t t i n g ) , c o u n t e r f a c e m a t e r i a l and s u r f a c e f i n i s h , ambient temperature and the presence or absence o f l u b r i c a n t or a b r a s i v e . The number o f p o s s i b l e t e s t combinations i s therefore vast, and y e t i t i s v i t a l l y i m p o r t a n t f o r the c o r r e c t c h o i c e t o be made i f the r e s u l t s are t o be o f v a l u e i n p r e d i c t i n g i n - s e r v i c e bearing l i f e . I n the B r i t i s h R a i l l a b o r a t o r i e s a t Derby a number o f s m a l l - s c a l e t e s t s are c a r r i e d out and the r e s u l t s g i v e n i n t h i s paper are from t e s t s which s i m u l a t e the f o l l o w i n g ; 3-body a b r a s i o n , low f r e q u e n c y fretting, c o n t i n u o u s r o t a t i o n and wear i n l o n g s t r o k e r e c i p r o c a t i n g motion. The 3-body a b r a s i o n t e s t i s c a r r i e d out on a r o t a t i n g l a p p i n g machine u s i n g f i n e l y d i v i d e d s i l i c a i n a l i g h t o i l as the a b r a s i v e slurry. The f r e t t i n g t e s t uses a d i s p l a c e m e n t o f 5 degrees and a f r e q u e n c y o f 5 Hz, which i s t y p i c a l o f the major v i b r a t i o n a l f r e q u e n c i e s o f bogie-mounted equipment i n d u c e d by t r a c k f o r c e s . The c o n t i n u o u s r o t a t i o n t e s t has a s t a n d a r d p i n - o n - r i n g c o n f i g u r a t i o n and the r e c i p r o c a t i n g t e s t c o m p r i s e s a b l o c k o f polymer l o a d e d onto a s t e e l p l a t e which r e c i p r o c a t e s over a 127mm s t r o k e a t 0.5 Hz. I n the NCT l a b o r a t o r i e s a v a r i e t y o f wear t e s t methods are used i n c l u d i n g t h r u s t washer, p i n - o n - d i s c and r e c i p r o c a t i n g t e s t s . From the l a r g e number o f polymers t h a t have been t e s t e d , two o b s e r v a t i o n s are worthy o f note h e r e . F i r s t l y , the p r e s e n c e o f a f i n e a b r a s i v e s l u r r y can i n c r e a s e the wear r a t e o f many polymers by about 2 #



21.

ANDERSON AND WILLIAMSON

Laboratory Vs. In-Service Polymer Wear 317

o r d e r s o f magnitude. F i g u r e 1 i n d i c a t e s t h e range o f wear r a t e s o f seven polymers o f d i f f e r e n t t y p e s under d i f f e r e n t wear t e s t c o n d i t i o n s and t h e marked i n c r e a s e i n wear under a b r a s i v e c o n d i t i o n s i s shown f o r f o u r out o f the seven polymers. The i n c r e a s e was l e s s pronounced f o r the f i l l e d n y l o n . However b o t h t h e p o l y u r e t h a n e e l a s t o m e r s and u l t r a - h i g h m o l e c u l a r weight p o l y e t h y l e n e d i d n o t g i v e h i g h e r wear r a t e s under 3-body a b r a s i o n compared w i t h o t h e r wear p r o c e s s e s , and t h i s i s a s s o c i a t e d w i t h t h e i r h i g h f r a c t u r e toughness. F i g u r e 1 a l s o i l l u s t r a t e s t h e second o b s e r v a t i o n , namely t h a t t h e wear r a t e s o f t h e m a t e r i a l s a r e ranked d i f f e r e n t l y i n each type o f wear t e s t . A filled polyimide, f o r example, gave a v e r y low wear r a t e under c l e a n r e c i p r o c a t i n g c o n d i t i o n s where a t r a n s f e r f i l m can form, b u t under a b r a s i v e c o n d i t i o n s wore v e r y r a p i d l y . In o r d e r t o produce d a t a i n a form which can be a n a l y s e d d i r e c t l y when g e n e r a t e d under d i f f e r e n t t e s t c o n d i t i o n s , a " s p e c i f i c wear r a t e " (swr) i s commonly used. S p e c i f i c wear r a t e i s d e f i n e d as t h e volume o f wear p e r u n i t a p p l i e d l o a d p e r u n i t s l i d i n g d i s t a n c e and i s p r o p o r t i o n a l t o t h e s l o p e o f t h e wear d e p t h - s l i d i n g d i s t a n c e c u r v e . I t i s sometimes a l s o r e f e r r e d t o as a "wear f a c t o r " . Following a r u n n i n g - i n p e r i o d , t h e volume o f wear tends t o be p r o p o r t i o n a l t o t h e a p p l i e d l o a d and s l i d i n g d i s t a n c e u n l e s s t h e mechanism o f wear changes. As an example o f t h e l a t t e r , work on u l t r a - h i g h m o l e c u l a r weight p o l y e t h y l e n e has i n d i c t e d t h a t a t r a n s i t i o n i n wear r a t e can o c c u r and t h i s i s thought t o be a s s o c i a t e d w i t h t h e o n s e t o f s u r f a c e f a t i g u e on t h e polymer, a f t e r a c e r t a i n s l i d i n g d i s t a n c e ( 7 ) and t h e r e f o r e care would be needed when u s i n g t h e swr f o r t h i s m a t e r i a l . When a t t e m p t i n g t o e x t r a p o l a t e l a b o r a t o r y wear d a t a t o p r e d i c t the l i f e o f a s e r v i c e b e a r i n g , one f u r t h e r l i m i t a t i o n i s t h a t o f scaling. T h i s i s n o r m a l l y c a r r i e d f o r two r e a s o n s ; f i r s t l y to reproduce s e r v i c e b e a r i n g p r e s s u r e s on a s m a l l s c a l e l a b o r a t o r y r i g and s e c o n d l y t o a c c e l e r a t e t h e wear p r o c e s s . In l a b o r a t o r y t e s t s the f r i c t i o n a l heat g e n e r a t i o n and d i s s i p a t i o n w i l l be d i f f e r e n t from s e r v i c e and the wear r a t e s o f many polymers a r e s e n s i t i v e t o temperature. Also, t h e wear d e b r i s g e n e r a t e d i n s m a l l s c a l e l a b o r a t o r y t e s t s can escape more r e a d i l y than «in l a r g e r c o n f o r m a l b e a r i n g s and t h e r e t e n t i o n o f wear d e b r i s i s known t o a f f e c t wear r a t e a d v e r s e l y i n some cases C8), Other f a c t o r s t o be c o n s i d e r e d a r e t h e r a t i o o f c o n t a c t a r e a t o parameters such as s u r f a c e f i n i s h , polymer and f i l l e r p a r t i c l e s i z e and m e t a l g r a i n s i z e . Finally, with regard t o laboratory t e s t i n g a l l the important s e r v i c e c o n d i t i o n s must be i n c o r p o r a t e d i f t h e r e s u l t s a r e t o be o f value t o the designer. F o r example t h e e f f e c t o f a b r a s i v e s has a l r e a d y been mentioned, b u t i n a i r c r a f t a p p l i c a t i o n s t h e leakage o f f u e l o r o t h e r f l u i d s onto polymer b e a r i n g s can be a major f a c t o r i n t h e i r performance. I n the nuclear industry candidate bearing materials f o r exposed a p p l i c a t i o n s must have been t e s t e d under suitably irradiated conditions. However, because o f t h e s e d i f f i c u l t i e s , candidate materials f o r an a p p l i c a t i o n s h o u l d n o t be s e l e c t e d by an a r b i t r a r y method i f a l a b o r a t o r y t e s t i s not f e a s i b l e . The p u b l i s h e d d a t a s h o u l d be c o n s u l t e d t a k i n g i n t o account t h e parameters o f t h e a p p l i c a t i o n w i t h a c r i t i c a l a p p r a i s a l o f the environmental c o n d i t i o n s . An i n - s e r v i c e t r i a l can be undertaken and t h e problems a s s o c i a t e d w i t h t h i s approach are c o n s i d e r e d n e x t .



Legend Filled

PTFE


Polyacetal Filled

nylon

Polyurerhane elastomer Filled polyimide Fabric reinforced phenolic resin U l t r a high molecular weight polyethylene

Abrasion

Continuous rotation

Reciprocation

Fretting F i g u r e 1. Comparison o f l a b o r a t o r y wear o f 7 polymer c o m p o s i t e s under d i f f e r e n t l a b o r a t o r y wear t e s t s .


21.


Laboratory Vs. In-Service Polymer Wear


In S e r v i c e Wear Assessment Once a p r o t o t y p e b e a r i n g i s i n s t a l l e d i n a s e r v i c e a p p l i c a t i o n i t i s o f t e n d i f f i c u l t , by v i r t u e o f the s e r v i c e duty r e q u i r e m e n t s , f o r the t r i b o l o g i s t t o g a i n access t o i t or t o have s u f f i c i e n t time t o make adequate measurements. D i s a s s e m b l y can be d i f f i c u l t o r i m p o s s i b l e , and the accuracy o f the wear measurement may w e l l be i n f e r i o r t o t h a t of a laboratory t e s t . To t a k e an extreme example, a r t i f i c i a l hip j o i n t s cannot be removed from the body f o r r o u t i n e wear measurement, and wear i s a s s e s s e d from p e r i o d i c X-ray photographs o f the j o i n t from which the t h i c k n e s s o f the polymer l i n i n g can be e s t i m a t e d . A similar problem e x i s t s i n many e n g i n e e r i n g examples where b e a r i n g s are not a c c e s s i b l e (the n u c l e a r , o f f s h o r e o i l and aerospace i n d u s t r i e s ) o r where they cannot be d i s m a n t l e d d u r i n g normal s e r v i c e . S i n c e wear and f r i c t i o n are system-based p r o p e r t i e s , i t i s a l s o important t o measure o t h e r f a c t o r s such as b e a r i n g t e m p e r a t u r e , a l t h o u g h t h i s g e n e r a l l y o n l y c a r r i e d out i n c r i t i c a l a p p l i c a t i o n s . S e r v i c e wear can comprise a complex m i x t u r e o f wear p r o c e s s e s and the e f f e c t o f machine v a r i a b l e s such as v i b r a t i o n , i n g r e s s o f d i r t and corrosion products needs to be taken into account f o r each a p p l i c a t i o n , a l t h o u g h a q u a n t i t a t i v e assessment o f the e f f e c t o f each i s v i r t u a l l y impossible. A l l the above makes the comparison o f the r e l a t i v e i n - s e r v i c e performance o f d i f f e r e n t m a t e r i a l s f a r from a c c u r a t e . The c a l c u l a t i o n o f s p e c i f i c wear r a t e f o r a g i v e n a p p l i c a t i o n may r e q u i r e e s t i m a t e s t o be made o f the average l o a d i f i t i s dynamic, but t h i s would not take account o f the p o s s i b l e e f f e c t o f the few h i g h l o a d c o n d i t i o n s . Moreover e s t i m a t i n g the r e l a t i v e d i s t a n c e o f s l i d i n g between the s u r f a c e s may be v e r y i m p r e c i s e . For a c o n t i n u o u s l y r o t a t i n g s h a f t the e s t i m a t e of sliding d i s t a n c e may be more a c c e p t a b l e . However, f o r an i n t e r m i t t e n t l y o p e r a t i n g support s h a f t b e a r i n g f o r , say, a railway vehicle braking system, the estimate i n c l u d e s not only the number o f brake a p p l i c a t i o n s w i t h i n the t r i a l p e r i o d , but a l s o the amount o f r e l a t i v e p i n / b u s h movement t h a t o c c u r s as a r e s u l t o f the " r a t t l i n g " o f the b r a k e g e a r d u r i n g the p e r i o d when i t i s h a n g i n g f r e e l y . O b v i o u s l y con s i d e r a b l e e r r o r s c o u l d a r i s e i n such an e s t i m a t e s i n c e i t i s based on "average v a l u e s " f o r t r a i n speed, b o g i e v i b r a t i o n a l frequency and angular displacement, and frequency o f a p p l i c a t i o n , dynamic f o r c e s from bogie o s c i l l a t i o n and c e r t a i n component w e i g h t s . Moreover, a c a l c u l a t i o n must be made o f the worn volume based on the measured i n c r e a s e i n b u s h i n g diameter, which n o r m a l l y o c c u r s e s s e n t i a l l y a t one p o s i t i o n around the c i r c u m f e r e n c e as a r e s u l t o f a u n i d i r e c t i o n a l load. I n o t h e r i n s t a n c e s , the bushes are " b e l l e d o u t " a t t h e i r ends. C a l c u l a t i o n s and e s t i m a t i o n s have t o be made f o r each o f these wear volumes where n e c e s s a r y . I n the case o f a s e r v i c e t r i a l o f polymer b u s h i n g f o r a r a i l c a r brake l i n k a g e the e s t i m a t e d t o t a l r e l a t i v e p i n / b u s h s l i d i n g d i s t a n c e as a r e s u l t o f o c c a s i o n a l b r a k e a p p l i c a t i o n s was o n l y 0.3% o f t h a t a r i s i n g from the f r e t t i n g o f the s u r f a c e s d u r i n g f r e e movement o f the b r a k e g e a r over the t r i a l p e r i o d . U s i n g the e s t i m a t e d volume wear o f the b u s h i n g f o r each m a t e r i a l t o g e t h e r w i t h the d i s t a n c e o f s l i d i n g and the b u s h i n g l o a d , the s p e c i f i c wear r a t e i n s e r v i c e f o r each o f the f o u r m a t e r i a l s c o u l d be c a l c u l a t e d . The average v a l u e s are p l o t t e d i n F i g u r e 2 t o g e t h e r w i t h the v a l u e s o b t a i n e d from each o f the


319


320


10

10 E

10

< or

10

< 10

10

CL

to Abrasion

Reciprocation

Continuous rotation LABORATORY DATA

x

Polyacetal

f

Fabric

reinforced

*

Fabric

reinforced

Fretting

A Filled phenolic phenolic

CALCULATED FROM SERVICE DATA

nylon resin

resin + friction modifier

F i g u r e 2. R a i l c a r and b r a k e g e a r bushes c o m p a r i s o n o f s e r v i c e and laboratory data.


21.



321

laboratory tests. I t i s e v i d e n t t h a t t h e band o f s e r v i c e v a l u e s l i e s close t o that f o r the abrasion t e s t . In p r a c t i c e , the large i n i t i a l c l e a r a n c e s i n t h i s equipment a l l o w e d t h e e n t r y o f l a r g e volumes o f a i r b o r n e dust ( o r i g i n a t i n g from t h e b r a k e s and t h e t r a c k b a l l a s t ) and i t can be assumed t h a t t h e main cause o f t h e wear i s t h e f r e e movement o f t h e l i n k a g e e s p e c i a l l y when i n t h e presence o f a b r a s i v e dust.


Comparison Of L a b o r a t o r y

And I n - S e r v i c e

Assessments

I n c e r t a i n f a i r l y r a r e c i r c u m s t a n c e s a s u f f i c i e n t number o f m a t e r i a l s are t e s t e d i n a s i n g l e s e r v i c e a p p l i c a t i o n t o a l l o w a d i r e c t i n - s e r v i c e comparison t o be c a r r i e d o u t a l t h o u g h a comparison w i t h t h e laboratory data requires considerable estimation. Nevertheless this can be a u s e f u l e x e r c i s e . I t cannot be d e n i e d t h a t d a t a t a k e n from a s e r v i c e t r i a l compares b e a r i n g m a t e r i a l s i n t h e i r t r u e w o r k i n g environment, a l t h o u g h a good performance i n one a p p l i c a t i o n w i l l n o t n e c e s s a r i l y ensure t h e same i n a n o t h e r , u n l e s s t h e m i x t u r e o f wear mechanisms i s i d e n t i c a l . However, duty r e q u i r e m e n t s and economic c o n s i d e r a t i o n s do n o t always a l l o w f u l l i n - s e r v i c e t r i a l s t o be u n d e r t a k e n . An a l t e r n a t i v e approach i s t o a s s e s s a s e r v i c e a p p l i c a t i o n i n terms o f t h e r e l a t i v e importance o f b a s i c wear mechanisms ( a b r a s i o n , f a t i g u e , adhesion, i m p a c t / s l i d e , e r o s i o n e t c ) and t h e geometry and motion. S m a l l s c a l e t e s t s a r e then c a r r i e d o u t i n t h e l a b o r a t o r y t o s i m u l a t e t h e s e mechanisms and parameters i n o r d e r t o produce r e l e v a n t wear d a t a . The performance d a t a f o r each m a t e r i a l w i t h r e f e r e n c e t o each o f these t e s t s w i l l t h e r e f o r e grade t h e s u i t a b i l i t y o f t h a t m a t e r i a l t o t h e proposed a p p l i c a t i o n . I n many l a b o r a t o r i e s (BR and NCT) f u r t h e r t e s t s a t e i t h e r f u l l o r reduced s c a l e a r e c a r r i e d o u t on r i g s d e s i g n e d t o s i m u l a t e s p e c i f i c applications. Candidate m a t e r i a l s w i t h s u i t a b l e p r o p e r t i e s chosen from t h e s m a l l s c a l e mechanism t e s t s a r e then r u n on t h e s e r i g s under r e a l i s t i c or scaled loads, speeds, g e o m e t r i e s , t y p e s o f motion and surface f i n i s h e s . Sometimes t h e a c t u a l s e r v i c e component can be o p e r a t e d i n t h e l a b o r a t o r y t o a v o i d t h e n e c e s s i t y t o manufacture a special r i g . In e i t h e r instance i t i s s t i l l not p o s s i b l e t o simulate a l l f a c t o r s which a f f e c t wear such as v i b r a t i o n , shock l o a d s , a b r a s i o n from a i r b o r n e dust and c o r r o s i o n . In t h e f o l l o w i n g paragraphs d a t a from s e r v i c e and l a b o r a t o r y a r e compared f o r seven case h i s t o r i e s , f o u r from t h e r a i l w a y i n d u s t r y , and t h r e e from o t h e r a r e a s . The r e l a t i o n s h i p between performance i n t h e component and on a l a b o r a t o r y r i g i s demonstrated on a c o r r e l a t i o n graph such as i n F i g u r e 3 comparing t h e s p e c i f i c wear r a t e s from t h e two s o u r c e s on l o g a r i t h m i c c o o r d i n a t e s . A p e r f e c t c o r r e l a t i o n would be shown by p o i n t s l y i n g a l o n g t h e l i n e a t 45° t o t h e x - a x i s and a l l p o i n t s below t h i s l i n e i n d i c a t e a h i g h e r wear r a t e i n t h e l a b o r a t o r y t e s t than i n s e r v i c e , t h a t i s a c o n s e r v a t i v e o r s a f e l i f e p r e d i c t i o n . Above t h i s l i n e t h e v a l u e o b t a i n e d i n s e r v i c e i s h i g h e r than would be e x p e c t e d from l a b o r a t o r y t e s t s and, on t h e f i g u r e , t h e d o t t e d l i n e s i n d i c a t e t h e d e v i a t i o n from e x a c t c o r r e l a t i o n . I n t h e case o f human h i p and knee p r o s t h e s e s , some d a t a i s a v a i l a b l e (9) which compares wear t e s t r e s u l t s from j o i n t s i m u l a t o r s , and o t h e r l a b o r a t o r y t e s t s w i t h " i n - v i v o " wear r a t e s . In j o i n t s i m u l a t o r s t h e a m p l i t u d e and frequency o f motion and t h e c y c l i c l o a d s r e l a t e d t o walking are simulated as c l o s e l y as p o s s i b l e . The




acetabular cup

SPECIFIC

Figure

WEAR

RATE , m / N m. 3

LABORATORY

3. C o r r e l a t i o n o f l a b o r a t o r y

TESTS

and s e r v i c e wear d a t a .



21.



323

e s t i m a t i o n o f an " i n - v i v o " s p e c i f i c wear r a t e i s a g a i n d i f f i c u l t s i n c e assumptions have t o be made c o n c e r n i n g p a t i e n t a c t i v i t y . In Figure 3 " i n - v i v o " and l a b o r a t o r y t e s t wear r a t e s a r e compared f o r u n f i l l e d PTFE and f o r u l t r a - h i g h m o l e c u l a r weight p o l y e t h y l e n e . For both materials the laboratory wear test data were obtained from pin-on-plate reciprocating t e s t s i n which t h e environment and c o u n t e r f a c e topography i n a h i p j o i n t were s i m u l a t e d as c l o s e l y as possible. A l s o i n t h e same f i g u r e a r e d a t a f o r knee j o i n t s t e s t e d i n a simulator compared w i t h estimated wear r a t e s from " i n - v i v o " measurements. I n a l l cases t h e agreement o f l a b o r a t o r y and " i n - v i v o " wear r a t e s i s w i t h i n a f a c t o r o f t h r e e which demonstrates t h e c a r e t a k e n i n t h e i r c a l c u l a t i o n and i n c l o s e l a b o r a t o r y s i m u l a t i o n o f t h e i m p o r t a n t wear parameters. A comparison between t h e l a b o r a t o r y and i n - s e r v i c e wear assessment o f two l a r g e marine b e a r i n g s i s shown i n F i g u r e 4. In s e r v i c e , t h e l o a d on, and motion o f , these b e a r i n g s i s d i c t a t e d by wind and s e a c o n d i t i o n s . I n o r d e r t o c a l c u l a t e a s p e c i f i c wear r a t e f o r the i n - s e r v i c e bearings, an average l o a d and a m p l i t u d e o f motion was d e r i v e d . From measurements o f t h e d i a m e t e r s o f b e a r i n g s a f t e r 2.5 y e a r s o f u s e , average wear depths were c a l c u l a t e d . The l a b o r a t o r y wear d a t a was o b t a i n e d from t h r u s t washer t e s t s on i d e n t i c a l m a t e r i a l c o m b i n a t i o n s a t t h e mean i n - s e r v i c e l o a d and a m p l i t u d e o f o s c i l l a t i o n . The agreement f o r b e a r i n g 1 i s remarkably good c o n s i d e r i n g t h e assump t i o n s made, and t h e f a c t t h a t t h e a c t u a l ( j o u r n a l ) b e a r i n g s were an o r d e r o f magnitude l a r g e r t h a n t h e t e s t ( t h r u s t ) b e a r i n g s . In the case o f b e a r i n g 2 t h e i n - s e r v i c e wear r a t e was lower than t h e l a b o r a t o r y r a t e by a f a c t o r o f 6-7. The r e a s o n f o r t h i s i s t h a t t h e a x i s o f motion o f t h i s b e a r i n g was b e l i e v e d t o be f u r t h e r removed from the p r e v a i l i n g wind and s e a c o n d i t i o n s compared w i t h b e a r i n g 1. A l s o i n f i g 4 i s t h e comparison o f i n - s e r v i c e and l a b o r a t o r y wear d a t a f o r s m a l l b e a r i n g s f o r an e l e c t r i c a l charge conveyor. The r e a s o n f o r t h e h i g h e r than p r e d i c t e d i n - s e r v i c e wear o f t h e s e b e a r i n g s was that, i n t h e presence o f , h i g h v o l t a g e s , an e l e c t r i c a l discharge o c c u r r e d through t h e t h i n polymer l a y e r between t h e s t e e l b a c k i n g and the b e a r i n g p i n . Consequently, some o f t h e bronze f i l l e r i n t h e polymer was d e p o s i t e d on t h e c o u n t e r f a c e , causing a s i g n i f i c a n t i n c r e a s e i n t h e roughness o f t h e p i n and hence i n t h e wear r a t e o f t h e polymer. F o l l o w i n g t h i s f i n d i n g , s t e p s were t a k e n t o e l i m i n a t e t h i s discharge, and t h e wear r a t e s o f replacement b e a r i n g s appear t o be s i m i l a r t o those o b t a i n e d i n t h e l a b o r a t o r y . Within t h e BR l a b o r a t o r i e s b o t h s m a l l - s c a l e wear p r o c e s s s i m u l a t i o n and l a r g e - s c a l e s e r v i c e s i m u l a t i o n r i g s a r e used b u t t h e d a t a r e g a r d i n g t h e f o l l o w i n g r a i l w a y a p p l i c a t i o n s o r i g i n a t e o n l y from the s m a l l - s c a l e t e s t s . T h e r e f o r e f o r each m a t e r i a l a s e t o f f o u r l a b o r a t o r y wear v a l u e s a r e p l o t t e d on t h e c o r r e l a t i o n graphs r e l a t i n g t o ; three-body a b r a s i o n , low frequency f r e t t i n g , c o n t i n u o u s r o t a t i o n and r e c i p r o c a t i n g s l i d i n g . F i g u r e 5, f o r i n s t a n c e , shows t h e f o u r l a b o r a t o r y v a l u e s p l o t t e d a g a i n s t t h e c a l c u l a t e d wear r a t e from s e r v i c e f o r each o f t h e t h r e e m a t e r i a l s used i n a t r i a l on a r a i l c a r a n t i - r o l l s u s p e n s i o n system. The wear i n s e r v i c e was s i m i l a r f o r t h e t h r e e m a t e r i a l s and t h e 1:1 c o r r e l a t i o n l i n e i n d i c a t e s t h a t t h e abrasion t e s t s i n the laboratory g r e a t l y over-estimate the observed s e r v i c e wear whereas t h e c o r r e l a t i o n w i t h t h e o t h e r results i s



POLYMER WEAR AND

SPECIFIC

F i g u r e 4.

WEAR

R A T E , m /Nm 3

LABORATORY

C o r r e l a t i o n of laboratory

ITS CONTROL

TESTS

and s e r v i c e wear d a t a .


WILLIAMSON



ANDERSON AND

SPECIFIC

Figure

5.

WEAR

RATE,

m / Nm 3

LABORATORY


and

TESTS

service

data.



326


reasonable. A s h o r t d i s c u s s i o n o f the r e l a t i o n s h i p o f t h i s d a t a t o the known s e r v i c e environment f o r the f o u r r a i l w a y a p p l i c a t i o n s i s given l a t e r . F i g u r e 6 shows the d a t a f o r the r a i l c a r b r a k e g e a r a p p l i c a t i o n d i s c u s s e d e a r l i e r but here the s e r v i c e wear r a t e s are c a l c u l a t e d t o be h i g h e r than f o r the a n t i - r o l l bar b e a r i n g s a l t h o u g h the v e h i c l e i s similar. The 1:1 l i n e c o r r e l a t i o n here passes t h r o u g h the l a b o r a t o r y a b r a s i v e wear v a l u e s and c o r r e s p o n d s t o the s e r v i c e o b s e r v a t i o n o f l a r g e d e p o s i t s o f a b r a s i v e dust i n these b u s h i n g s . Two f u r t h e r t r i a l s have been c a r r i e d out on the t r e a d b r a k e system o f the Advanced Passenger T r a i n (APT) and the High Speed T r a i n (HST) u s i n g polymer bushes i n the l i n k a g e , and the c o r r e l a t i o n graph on F i g u r e 7 a g a i n shows a s p r e a d o f l a b o r a t o r y v a l u e s f o r each s e r v i c e wear c a l c u l a t i o n . I t s h o u l d be p o i n t e d out t h a t the p o l y a c e t a l bush on the HST t r i a l was run a g a i n s t a l o w - t e m p e r a t u r e - n i t r i d e d s t e e l p i n as opposed t o a case hardened s t e e l f o r the o t h e r a p p l i c a t i o n s . To date o n l y the c o n t i n u o u s r o t a t i o n a l wear t e s t has been c a r r i e d out on t h i s combination o f m a t e r i a l s and the l a b o r a t o r y d a t a on F i g u r e 7 i s therefore r e s t r i c t e d . The 1:1 c o r r e l a t i o n l i n e passes between the a b r a s i o n and the o t h e r l a b o r a t o r y t e s t r e s u l t s , i n d i c a t i n g a mixed wear p r o c e s s , w i t h o n l y a s m a l l a b r a s i v e wear component. The d a t a from the above f o u r r a i l w a y a p p l i c a t i o n s can be f u r t h e r a n a l y s e d s i n c e one m a t e r i a l , p o l y a c e t a l , was used i n a l l o f the t r i a l s and F i g u r e 8 shows the v a r i a t i o n o f the c a l c u l a t e d s e r v i c e wear r a t e s . The h i g h e s t wear was i n the r a i l c a r a p p l i c a t i o n and i s a s s o c i a t e d w i t h h i g h i n i t i a l c l e a r a n c e s and a f a i r l y l o n g s e r v i c e l i f e r e s u l t i n g i n c o n s i d e r a b l e r a t t l i n g o f the b r a k e g e a r and the e n t r y o f a h i g h p r o p o r t i o n of airborne dust. The APT t r e a d b r a k e t r i a l , i n contrast, was v e r y much s h o r t e r and the i n i t i a l c l e a r a n c e s were v e r y s m a l l r e s u l t i n g i n a l a r g e r e d u c t i o n i n a b r a s i v e dust contamination. However, the b r a k e g e a r s u p p o r t p i n s on the APT were a x l e mounted which r e s u l t e d i n h i g h t r a c k impact f o r c e s b e i n g t r a n s m i t t e d t h r o u g h the p i n / b u s h assembly which would produce an i n c r e a s e i n wear r a t e . The HST t r e a d b r a k e s u p p o r t bushes are mounted on the b o g i e a t a g r e a t e r d i s t a n c e from the t r a c k than the r a i l c a r and APT bushes, and, t h e r e f o r e , the volume o f a b r a s i v e dust e n t e r i n g the b e a r i n g s was much reduced. The wear r a t e s c a l c u l a t e d from these three s e r v i c e a p p l i c a t i o n s are t h e r e f o r e i n l i n e w i t h the s e v e r i t y o f the b e a r i n g environment and, i n p a r t i c u l a r , the amount o f a b r a s i v e a i r b o r n e dust e n t e r i n g the b e a r i n g . The f i n a l s e r v i c e r e s u l t shown i n f i g u r e 8 r e f e r s t o the bushes i n the a n t i - r o l l system o f r a i l c a r s designed by B r i t i s h R a i l f o r o p e r a t i o n i n Taiwan, where the i n i t i a l c l e a r a n c e s were s m a l l and the o p e r a t i o n a l l e g e d l y smooth. However some e v i d e n c e o f a b r a s i v e wear was seen on the s u r f a c e s o f these bushes r e s u l t i n g i n a f a i r l y h i g h wear r a t e w i t h r e s p e c t t o t i m e i n s e r v i c e . The c a l c u l a t e d swr i s therefore rather l e s s t h a n might have been e x p e c t e d from the o b s e r v a t i o n s o f the worn b e a r i n g s and the o n l y p o s s i b l e r e a s o n s are t h a t the v a l u e s f o r the a n g u l a r d i s p l a c e m e n t , frequency o r l o a d are overestimated, o r o t h e r u n u s u a l e n v i r o n m e n t a l c o n s i d e r a t i o n s have not been t a k e n i n t o account.






ITS CONTROL


POLYMER WEAR AND

SPECIFIC

Figure

7.

WEAR

RATE,

m'/Nm

-

LABORATORY


and

TESTS

service

data.



21.


Anti - roll bar bearings


H ST treadbrake bushes

A PT treadbreak bushes

Railcar brakegear bushes

F i g u r e 8. R e l a t i o n s h i p between s e r v i c e and s p e c i f i c wear r a t e and application.


329

330



Recommendations The paper has o u t l i n e d the problems which can a r i s e when u s i n g wear d a t a from l a b o r a t o r y t e s t s or from s e r v i c e t r i a l s . The examples illustrate the p o t e n t i a l e r r o r o f wear r a t e s and hence life p r e d i c t i o n s . I t would be i n a p p r o p r i a t e t o c o n c l u d e t h i s paper w i t h o u t some p o s i t i v e recommendations which may h e l p t o reduce t h i s e r r o r . Firstly, i t i s c l e a r t h a t i f l a b o r a t o r y t e s t s are t o be c a r r i e d out then they must s i m u l a t e the s e r v i c e environment i f they are t o give a reasonable p r e d i c t i o n of expected l i f e . A good s i m u l a t i o n i s n e c e s s a r y f o r a good c o r r e l a t i o n . I f the wear d a t a s u p p l i e d by a polymer b e a r i n g s u p p l i e r has t o be r e l i e d upon, f u l l d e t a i l s o f the t e s t c o n d i t i o n s s h o u l d be o b t a i n e d . I n a d d i t i o n , the a p p l i c a t i o n must be a s s e s s e d i n terms o f the l i k e l y wear p r o c e s s e s . Where p o s s i b l e , an independent assessment o f a m a t e r i a l ' s performance s h o u l d be sought. Where i n - s e r v i c e t e s t s are used t o s e l e c t m a t e r i a l s the d e s i g n e r s h o u l d be c a u t i o u s about u s i n g the d a t a from one t r i a l t o p r e d i c t performance i n a n o t h e r . Even i f the l o a d , motion and speed are s i m i l a r , u n f o r e s e e n e n v i r o n m e n t a l changes can markedly a l t e r the wear performance o f polymer b e a r i n g s . The wear d a t a on m a t e r i a l s o b t a i n e d from p u b l i s h e d l i t e r a t u r e may not be r e l e v a n t t o the s e r v i c e c o n d i t i o n s proposed and a d v i c e from an independent source s h o u l d be sought i n such c a s e s . Conclusions (a)

(b)

(c)

(d)

O b t a i n i n g an a c c u r a t e p r e d i c t i o n o f the l i f e o f a b e a r i n g from s p e c i f i c wear r a t e s g e n e r a t e d i n a l a b o r a t o r y t e s t i s not an easy t a s k . Not o n l y do the i m p o r t a n t wear parameters o f the a p p l i c a t i o n have t o be i d e n t i f i e d and s i m u l a t e d but environ mental f a c t o r s can have a s i g n i f i c a n t e f f e c t on i n - s e r v i c e wear rates. Not o n l y do s p e c i f i c wear r a t e s o f m a t e r i a l s v a r y w i t h the type of laboratory t e s t ; the r a n k i n g o f m a t e r i a l s a l s o changes w i t h t e s t type. Where s e r v i c e c o n d i t i o n s can be d e f i n e d and s i m u l a t e d i n the laboratory, then the c o r r e l a t i o n between i n - s e r v i c e and l a b o r a t o r y t e s t wear r a t e s can be c l o s e ; t h a t i s i n agreement t o w i t h i n a f a c t o r o f 2 or l e s s . C o n v e r s e l y where s e r v i c e c o n d i t i o n s are v a r i a b l e and cannot be c l o s e l y simulated, or i f wear r a t e d a t a i s used t o p r e d i c t l i f e w i t h o u t c o n s i d e r a t i o n o f the a p p r o p r i a t e factors, then the p r e d i c t e d l i f e c o u l d be i n e r r o r ( i e o p t i m i s t i c ) by an o r d e r o f magnitude or more.

Acknowledgments The a u t h o r s w i s h t o thank Dr T N Marsham, Managing D i r e c t o r , N o r t h e r n Division, UKAEA and Dr A H Wickens, D i r e c t o r of Engineering Development and Research, British Rail, f o r t h e i r permission to p u b l i s h t h i s paper.


21.

ANDERSON A N D WILLIAMSON

Laboratory Vs. In-Service Polymer Wear 331

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3.

4. 5.

6. 7. 8.

9.

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R E C E I V E D March 26, 1985


Relating Laboratory Wear Testing to the In-Service Wear of Polymers

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