surface fatigue stress on gears
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8/15/2019 Surface Fatigue Stress on Gears
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~ ) P e rg a m o n
Engineering FailureAna vsis,
V ol . 4 , N o . 2 , pp . 99 107 , 1997
( ~ 1 997 E l se v i e r S c i e nce L t d . A l l r i gh t s r e se rve d
P r i m e d i n G r e a t B r i t a in
1 3 5 0 6 3 0 7 , ' 9 7 $ 1 7 . 0 0 + 0 . 0 0
P I I : S 1 3 5 0 - 6 3 0 7 ( 9 7 ) 0 0 0 0 6 - X
S U R F A C E C O N T A C T F A T I G U E F A I L U R E S I N G E A R S
P . J . L . F E R N A N D E S a n d C . M c D U L I N G
Advanced Engineering and Test ing Services , CSIR, Private Bag X28, Auckland Park 2006, South Africa
R e c e i v e d 6 J a n u a r y
1 9 9 7 )
A b s t r a c t - - S u r f a c e
contact fatigue is the most co mm on cause of gear failure. It results in damage to contacting
surfaces which can s ignificantly reduce the load-carrying capacity of components , and may ult imately lead to
complete failure of a gear. Three types of contact fatigue damage are discussed, and a number of actual
examp les are presented to illustrate th is failure mod e in practice. © 1997 Elsevier Science Ltd.
1 . I N T R O D U C T I O N
T h e m o s t c o m m o n m o d e o f g e a r fa i lu r e e n c o u n t e r e d i n p r a c t ic e i s t h a t o f s u r fa c e c o n t a c t f a t ig u e .
T h i s m o d e o f f a i lu r e l e ad s t o c r a c k i n i t i a t i o n a t o r n e a r t h e c o n t a c t s u r f a c e , a n d m a y s u b s e q u e n t l y
l ea d t o d a m a g e v a r y i n g i n e x t en t f r o m m i c r o s c o p i c p i t ti n g t o s e v er e sp a l l in g . T h e m e t a l r e m o v e d
f r o m t h e s u r f a c e i n s u c h c a s e s e n t e r s t h e m a c h i n e s y s t e m , a n d c a n , i n t u r n , c a u s e a b r a s i v e w e a r a n d
f ai lu r e o f o t h e r c o m p o n e n t s . F u r t h e r m o r e , t h e p it s f o r m e d o n t h e d a m a g e d s u r fa c e l ea d t o th e
f o r m a t i o n o f st r es s c o n c e n t r a t i o n s , a n d s e rv e a s in i t i a ti o n s it es f o r o t h e r m o d e s o f g e a r fa i lu r e , e. g.
t o o t h b e n d i n g f a t i g u e [ 1]. I n t h is p a p e r , v a r i o u s t y p e s o f c o n t a c t f a t i g u e d a m a g e a r e r ev i e w e d a n d
i l lu s t r a te d u s i n g p r a c t i c a l e x a m p l e s . T h e c a u s e s a n d w a y s o f p r e v e n t i n g d a m a g e i n e a c h c a s e a r e
a l s o d i s c u s s e d b r i e f l y .
2 . M E C H A N I S M O F C O N T A C T F A T I G U E
W h e n e v e r t w o c u r v e d ( u s u al l y c o n v e x ) su r fa c e s a r e i n c o n t a c t u n d e r l o a d , t h e c o n t a c t o c c u r s
a l o n g a l in e o r p o i n t , o r , d e p e n d i n g o n t h e e la s t ic c o n s t a n t s o f t h e m a t e r i a l s c o n c e r n e d , a l o n g a v e r y
s m a l l c i r c u l a r o r el l i p t ic a l a r e a . A s a r e s u l t o f s u c h s m a l l c o n t a c t a r e a s , t h e s h e a r ( H e r t z i a n ) s t r e ss e s
w h i c h d e v e l o p a t a n d n e a r t h e s u rf a c e ar e c o n s e q u e n t l y v e r y h ig h . T h e m a x i m u m s h e a r s tr es s o c c u r s
a t s o m e d i s t a n c e b e l o w t h e s u r f a c e [ 2 ], a s i ll u s t r a t e d i n F i g . 1 .
W h e n t h e c o n t a c t i n g s t re s se s a r e r e p e ti t iv e , a s is t h e c a s e o n t h e a c t i v e f la n k s o f g e a r t e e th , t h e
c y c l ic c o m p r e s s i v e s t r es s e s i n d u c e d c a u s e d i f f er i n g el a st i c a n d p l a s t ic b e h a v i o u r i n t h e n e a r - s u r f a c e
m a t e r i a l . D e p e n d i n g o n t h e m i c r o s t r u c t u r e a n d g r a i n o r i e n t a t i o n o f t h e m a t e r i a l in t h is re g i o n ,
i n t e r n a l s t r es s c o n c e n t r a t i o n s a r e f o r m e d w h i c h c a n u l t i m a t e l y l e a d t o c r a c k i n i t ia t i o n . I n p r a c t i c e ,
c r a c k i n i t i a t i o n u s u a l l y o c c u r s a t i n c l u s i o n s i n th e s t r es s e d n e a r - s u r f a c e m a t e r i a l , t h e m o s t d e l e t e r i o u s
b e i n g t h o s e i n c l u s i o n s w h i c h a r e h a r d , b r i t t le a n d a n g u l a r i n sh a p e .
D a m a g e d u e t o c o n t a c t f a t i g u e i n g e a r t e e th u s u a l l y o c c u r s i n o n e o f t h r e e a r e as , v i z a l o n g t h e
p i t c h - li n e , i n t h e a d d e n d u m ( i.e . a b o v e t h e p i t c h -l i n e) , a n d i n t h e d e d e n d u m ( i. e. b e l o w t h e p i tc h
l in e ) [3 ]. A l o n g t h e p i t c h - l i n e , o n l y p u r e r o l l i n g s t r e s s es e x i s t ( F i g . 1 ), w h i l e a w a y f r o m t h e p i t c h -
l in e , b o t h r o l l i n g a n d s l id i n g s t re s s es a r e e x p e r i e n c e d . * I n t h e s u b s e q u e n t s e c t i o n s o f t h is p a p e r ,
s u r fa c e d a m a g e u n d e r b o t h p u r e r o l l in g a n d r o l l in g - s l id i n g c o n d i t i o n s , a s w e ll as a m o r e a d v a n c e d
a n d s e v e re m o d e o f s u r f ac e d a m a g e c a l le d s p a l l in g , i s d i s c u s s ed .
*Pure rolling stresses along the pitch-line occur only in spur, bevel and helical gears, and not in worm, spiral bevel or
hypo id gears [4].
99
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P . J . L . F E R N A N D E S a nd C. M c D U L I N G
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F i g . 1 . S t r e s s d i s t r i b u t i o n a t a n d n e a r t w o c o n t a c t i n g s u r f a c e s u n d e r p u re r o l l in g .
3 . R O L L I N G C O N T A C T F A T I G U E
T h e s t re s s d i s t r ib u t i o n r e s u l t in g f r o m p u r e r o l l in g c o n d i t i o n s p r e v a l e n t a t t h e p i t c h - l in e i s s h o w n
i n F i g . 1 . T h e m a x i m u m s h e a r s tr es s o c c u r s at s o m e d i s t a n c e , u s u a l l y 0 . 1 8 - 0 . 3 m m [ 3], b e l o w t h e
s u r fa c e , a n d j u s t a h e a d o f t h e co n t a c t p o i n t . C r a c k s i n i t ia t e a t t h e p o i n t o f m a x i m u m s tr es s, a n d
p r o p a g a t e e s s e n t i a l l y p a r a ll e l t o th e s u r fa c e . C o n t i n u e d r o l l i n g m a y c a u s e t h e c r a ck s t o d e v i a t e u p
t o w a r d s t h e c o n t a c t s u r fa c e , r e s ul t in g i n t h e r e m o v a l o f m e t a l f r o m t h e s u rf a ce . T h e p i t s f o r m e d i n
t h i s m a n n e r i n i t i a l ly h a v e s i d es p e r p e n d i c u l a r t o t h e c o n t a c t s u r f ac e , a s i ll u s tr a t ed s c h e m a t i c a ll y i n
F i g . 2 . C o n t i n u e d o p e r a t i o n o f t h e p it te d s u r fa c e, h o w e v e r , m a y c a u s e a b r e a k d o w n i n th e p i t s h a p e .
P i t t i n g u n d e r p u r e r o l l i n g c a n o c c u r e v e n u n d e r p r o p e r l u b r i c a t i o n c o n d i t i o n s , s i n c e o i l , a s a n
i n c o m p r e s s i b l e f l u i d , w i l l m e r e l y t r a n s m i t t h e c o n t a c t l o a d s [ 5 ] . T h e p i t s f o r m e d a r e g e n e r a l l y v e r y
s m a l l a n d s e l d o m g i v e m o r e t h a n a " f r o s te d " a p p e a r a n c e t o t h e d a m a g e d s u r fa c e . I n s o m e c a se s ,
t h e p i ts m a y n o t p r o g r e s s b e y o n d t h e ir p o i n t o f o r ig i n , a n d m a y e v e n b e s el f -h e a l i n g [ 3 , 5] . T h e r e
a r e t w o u n i q u e c h a r a c t e r i s t i c s o f r o l l i n g c o n t a c t f a t i g u e p i t s w h i c h c a n b e u s e d t o d i s t i n g u i s h t h i s
t y p e o f d a m a g e f r o m o t h e r f o r m s o f p i tt in g .
F i r s t l y , t h e f o r m a t i o n o f r o l l i n g c o n t a c t f a t i g u e p i t s o c c u r s w i t h n o s u r f a c e p l a s t i c d e f o r m a t i o n .
T h i s i s c o n t r a r y t o p i tt i n g u n d e r s l id i n g - r o l l i n g c o n d i t i o n s , a s s h o w n i n S e c ti o n 4 . S e c o n d l y , i n
c o m p o n e n t s w i t h a c a s e - h a r d e n e d l a y e r c o n s i s t i n g o f m a r t e n s i t e w i t h l i tt le o r n o r e t a i n e d a u s t e n i t e ,
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F i g . 2 . L o c a t i o n a n d o r i e n t a t i o n o f s u b s u r f a c e c r a c k s u n d e r p u r e r o l l i n g c o n d i t i o n s .
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Surface contact fatigue failures n gears 101
r o l li n g c o n t a c t f a ti g u e le a d s t o th e f o r m a t i o n o f a m i c r o s t r u c t u r a l f e a t u r e r e f e rr e d to a s b u t t e r f l y
w i n g s [ 3 ]. T h e s e a r e f o r m e d w h e n p l a s t ic d e f o r m a t i o n is c o n s t r a i n e d b y t h e s u r r o u n d i n g m a t e ri a l ,
a n d is m o r e c o m m o n w h e n s h e a r s tr es s es a r e e x t re m e l y h ig h .
4 . S L I D I N G R O L L I N G C O N T A C T F A T I G U E
P u r e r o l l i n g c o n d i t i o n s p r e v a i l w h e n t h e s u r f a c e v e lo c i t ie s o f t w o c o n t a c t i n g c u r v e d b o d i e s a r e
t h e s a m e . H o w e v e r , i f t h e s e v e l o c it i e s a r e d i f f e re n t , a n e l e m e n t o f sl i d i ng i s i n t r o d u c e d w h i c h
s i g n i fi c a n t ly a l t e r s t h e s t r es s d i s t r i b u t i o n i n th e s u r f a c e a n d n e a r - s u r f a c e m a t e r i a l . D e p e n d i n g o n t h e
r e la t iv e v e lo c i ti e s o f t h e c o n t a c t i n g b o d i es , r o l l in g a n d s li d in g m a y o c c u r i n t h e s a m e d i r e c t i o n * -
posi t ive sl iding--or
i n o p p o s i t e d i r e c t i o n s
negat ive sliding.
T h e e f f e c t o f t h e l a t t e r i s t h a t t h e s u r f a c e
m a t e r i a l i s r o l le d i n o n e d i r e c t i o n , a n d p u s h e d ( s li d i n g ) i n a n o t h e r , t h e r e f o r e r e s u l t i n g in h i g h e r
s t re s s e s t h a n t h o s e e n c o u n t e r e d i n p o s i t i v e s li d i n g [4 ]. T h e m o d i f i e d s t re s s d i s t r i b u t i o n i n t h e s u r f a c e
a n d n e a r - s u r f a c e m a t e r i a l r e s u l t in g f r o m c o m b i n e d r o l l i n g a n d s l id i n g i s s h o w n i n F ig . 3 . T h e
p o s i t i o n o f m a x i m u m s h e a r s t re s s is m o v e d c l o s e r t o t h e c o n t a c t i n g i n t e r f ac e , a n d c r a c k i n i t i a ti o n
t h e r e f o r e o c c u r s a t t h e s u r f a c e .
G e a r t e e th h a v e c o m p l e x c o m b i n a t i o n s o f s li d in g a n d r o ll in g , w h i c h v a r y a l o n g t h e p r o fi l e o f e a c h
t o o t h , a s i l l u s t r a te d i n F i g . 4 . In t h e a d d e n d u m , t h e d i r e c t i o n o f r o l l i n g a n d s l id i n g is th e s a m e , a n d
p o s i t i v e s l id i n g c o n d i t i o n s t h e r e f o r e p r e v a il . I n t h e d e d e n d u m , h o w e v e r , t h e d i r e c t i o n o f r o l l in g is
o p p o s i t e t o t h a t o f s l id i n g , a n d n e g a t i v e s l id i n g c o n d i t i o n s e x is t . C o n t a c t f a t i g u e is t h e r e f o r e m o r e
l i k el y to i n i t i a t e in t h e d e d e n d u m , a n d p i t t i n g i n t h i s r e g i o n i s u s u a l l y v e r y s e v e re , a n d o f t e n a c t s a s
a p r e c u r s o r t o t o o t h b e n d i n g f a t i g u e [ 1 ] .
I n p r a c t ic e , it is c o m m o n t h a t c o n t a c t f a t i g u e d a m a g e w il l f ir s t o c c u r i n t h e d e d e n d u m o f t h e
s m a l l e r g e a r ( w h i c h i s u s u a l l y t h e d r i v i n g g e a r ) o f a g e a r s e t [ 4, 5 ]. T h i s i s e x p l a i n e d b y t h e f a c t t h a t
t h e s m a l l e r g e a r w il l u n d e r g o m o r e r e v o l u t i o n s , a n d t h e r e f o r e e a c h t o o t h w i ll e x p e r i e n c e a la r g e r
n u m b e r o f s tr es s c y cl es . I n o r d e r t o p r e v e n t p r e m a t u r e f a il u r e i n s u c h c a se s , it is c o m m o n t o m a k e
t h e s m a l l e r, d r i v i n g g e a r h a r d e r t h a n t h e o t h e r g e a r s in t h e g e a r s et .
t r e s s
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b e l o yD i s t a n c e / ~ P ure s ~ ~
surfac e l / Comb.m. .ed roll ing
Fig. 3. Stress distribution at and near two contacting surfaces under slidin~ro lling cond itions.
*The direction of rolling is the direction in w hich the po int of contact between wo b odies mo ves, and is always oppo site
to the direction of rotation.
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Fig. 4. Com bination of sliding and rolling in gear teeth.
A n o t h e r r e g io n i n w h i c h c o n t a c t f a ti g u e d a m a g e i s f r eq u e n t ly e n c o u n t e r e d i s at th e l o w e s t p o i n t
o f s i n g le t o o t h c o n t a c t , i . e. t h e p o i n t a t w h i c h c o n t a c t i s m a d e w i t h t h e ti p o f t h e m a t c h i n g t o o t h
[ 3 ] . S i n c e t h e c o n t a c t a r e a i n t h i s c a s e i s v e r y s m a l l , h i g h s t r e s s e s a r e g e n e r a t e d , e v e n u n d e r n o r m a l
l o a d s . M o r e o v e r , t h e lo w e s t p o i n t o f si n g le t o o t h c o n t a c t is a l w a y s i n th e d e d e n d u m o f th e m a t c h i n g
t o o t h , a n d s l id i n g s p e e d s , b o t h i n a p p r o a c h a n d r e c es s , a r e a t a m a x i m u m . T h e n e g a t i v e s l id i n g
c o n d i t i o n s i n t h e s e r eg i o n s , t o g e t h e r w i t h t h e h i g h s t r es s e s a n d h i g h s l i d i n g s p e e d s , l e a d t o r a p i d
i n i t ia t i o n o f d a m a g e . F i g u r e 5 s h o w s t h e d r i v i n g g e a r o f a 1 .5 t o n l e v er h o i s t i n w h i c h s e v e re p i t ti n g
i n th e d e d e n d u m h a s o c c u r r e d at t h e p o i n t o f s in g l e t o o t h c o n t a c t [ 6]. T h e e x t e n t o f d a m a g e o b s e r v e d
o c c u r r e d a f t e r o n l y 5 0 0 c y c l e s u n d e r t e s t c o n d i t i o n s .
U n l i k e c o n t a c t f a ti g u e d a m a g e u n d e r p u r e r o l li n g c o n d i t i o n s , t h e s l id i n g r o l l in g a c t i o n c a u s e s
p l a s ti c d e f o r m a t i o n o f t h e s u rf a c e m a t e r i a l , a n d t h i s c a n u s u a l l y b e d e t e c t e d u s i n g m e t a l l o g r a p h i c
a n a l y s i s . T h e e x t e n t o f p l a s t ic d e f o r m a t i o n , a n d h e n c e o f c o n t a c t f at i g u e d a m a g e , c a n b e r ed u c e d
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S u r f a c e c o n t a c t f a t i g u e f a i l u r es i n g e a r s 1 03
i t ~ 4 t t
F i g . 5 . S e v e r e p i tt i n g i n t h e d e d e n d u m o f t h e d r i v i n g g e a r o f a l e v e r h o i s t . P i t t i n g in i t i a t e d a t t h e l o w e s t p o i n t
o f s i n gl e t o o t h c o n t a c t .
e f fe c ti v e ly b y e n s u r i n g t h a t c o r r e c t l u b r i c a ti o n c o n d i t i o n s a r e m a i n t a i n e d . F u r t h e r m o r e , i t h as b e e n
s h o w n t h a t s l i d i n g - r o ll i n g d a m a g e c a n b e m i n i m i z e d b y a c h i ev i n g a s u r fa c e h a r d n e s s o n t h e m a t c h i n g
b o d i e s g r e a te r t h a n H R C 6 0 [ 4] . R e t a i n e d a u s t en i t e, a t l e v el s o f 1 0 - 2 0 % , i n t h e c a s e - h a r d e n e d l a y e r
a l s o m i n i m i z e s t h e e x t e n t o f c o n t a c t f a t i g u e s i n ce th i s m i c r o s t r u c t u r a l c o n s t i t u e n t d e f o r m s p l a s t i c a ll y
t o f o r m l a r g e r c o n t a c t a r e a s , a n d h e n c e l o w e r c o n t a c t s t r e s s e s . H o w e v e r , a u s t e n i t e r e d u c e s t h e
f a t i g u e s t r e n g t h o f t h e m a t e r i a l , a n d c a n t h e r e f o r e i n t r o d u c e o t h e r d e l e t e r i o u s e f fe c ts .
5 . S P A L L I N G
S p a l l i n g i s d e f i n e d b y t h e SM Metals Reference Book [7 ] a s t h e c r a c k i n g a n d f l a k i n g o f p a r t i c l e s
o u t o f a s u r f a c e . I t r e f e r s t o t h e f o r m a t i o n o f l ar g e a n d d e e p p i t s o n c o n t a c t i n g s u r f a c e s , w h i c h
s i g n i fi c a n t ly r e d u c e t h e l o a d - c a r r y i n g c a p a c i t y o f c o m p o n e n t s . T h e p i t s t h e m s e l v e s a c t as s t re s s
c o n c e n t r a t i o n s w h i c h m a y l e a d t o o t h e r m o d e s o f fa i lu r e , w h il e th e m a t e ri a l l o s t f r o m t h e s u r f a ce
c a n c a u s e d a m a g e e l s e w h e r e i n t h e c o m p o n e n t . T w o m e c h a n i s m s o f s p al li n g h a v e b e e n i d e n ti f ie d
[ 5 ]
F i r s t l y , s p a l l i n g m a y o c c u r a s a c o n t i n u a t i o n o f p i t ti n g r e s u l t in g f r o m r o l l i n g o r s l i d i n g - r o l l in g
c o n t a c t f a t i g u e . I n s o m e i n s t a n c e s , t h e c r a c k s i n i t i a t e d b y t h e s e m e c h a n i s m s w i l l p r o p a g a t e i n t o t h e
m a t e r i a l , a n d u l t i m a t e l y r e s u l t in t h e l o s s o f l ar g e p i ec e s o f m e t a l f r o m t h e c o n t a c t s u r f a c e . A n
e x a m p l e o f t h i s is g iv e n i n F i g . 6 , w h i c h s h o w s s e v e r e s p a l li n g w h i c h o c c u r r e d a s a r e s u l t o f r o ll i n g
c o n t a c t f a t i g u e p i t t i n g a l o n g t h e p i t c h - l i n e o f a h el i c a l g e a r . I n s o m e c a s es , p i t t i n g w i ll s t a r t a t o n e
p o i n t , a n d p r o g r e s s o u t w a r d s a n d u p w a r d s a l o n g t h e t o o t h p r o f i le t o f o r m w h a t is k n o w n a s th e
c y c l o n e e f f ec t [ 3 ] . T h i s i s c l e a r l y s h o w n i n F i g. 7 f o r t h e c a s e o f a s m a l l h e li c a l g e a r f r o m a n
a u t o m a t i c g e a r b o x [ 8 ] .
I n t h e s e c o n d m e c h a n i s m o f s p a l li n g , a l s o k n o w n a s c a s e c r u s h i n g o r s u b - c a s e f a t i g u e [4 ],
f a t i g u e c r a c k s i n i t i a t e a t t h e m e t a l l u r g i c a l n o t c h f o r m e d b e t w e e n t h e h a r d e n e d c a s e a n d c o r e . S i n c e
t h i s i n t e r f a c e i s u s u a l l y v e r y d e e p , t h e d e p t h a n d s i ze o f t h e p i t s f o r m e d a r e s i g n i fi c a n t ly l a r g e r t h a n
t h o s e e n c o u n t e r e d i n r o l l i n g o r s l i d i n g - r o l l i n g c o n t a c t f a t i g u e . F i g u r e 8 s h o w s a s p u r g e a r i n w h i c h
l a r g e, d e e p p i t s h a v e f o r m e d a l o n g t h e e n t i r e a c t i v e f l an k a s a r e s u l t o f s u b - c a s e f a t i g u e [ 9 ] . T h e
m e c h a n i s m o f c r a c k i n i t i a t i o n i s t h e s a m e a s i n t h e o t h e r m o d e s o f c o n t a c t f a t ig u e . S i n c e t h i s t y p e
o f s p a l li n g i s e s s e n t i al l y d u e t o t h e d i s p a r i t y i n t h e m e c h a n i c a l p r o p e r t i e s o f th e c a s e a n d c o r e ,
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S u r f a c e c o n t a c t f a t i g u e f a i l u r es i n g e a r s 1 0 5
F i g . 7. ( a ) A c t i v e f la n k o f a h el i c al g e a r s h o w i n g t h e c y c l o n e e f fe c t, a n d ( b ) h i g h - m a g n i f i c a t i o n p h o t o g r a p h
o f t h e s a m e a r e a s h o w i n g h o w p i t t i n g h a s e x t e n d e d u p w a r d s a n d o u t w a r d s .
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1 06 P . J . L . F E R N A N D E S a nd C , M c D U L I N G
Fig. 8. Sub-case fatigue or spalling on the active flank of a spur gear.
i n c r e a s i n g t h e h a r d n e s s ( o r s tr e n g t h ) o f t h e c o r e t h r o u g h m a t e r i a l s e l e c t io n o r h e a t t r e a t m e n t c a n
e f fe c t iv e l y r e d u c e t h e i n c i d e n c e o f s p al l in g . A l t e r n a t i v e l y , b y i n c r e a s i n g t h e d e p t h o f th e c a s e , t h e
c o r e ~ c a s e i n t e r f ac e c a n b e m o v e d t o a p o s i t i o n o f l o w e r s tr es s .
6 . S U M M A R Y
S u r f a c e c o n t a c t f a ti g u e is th e m o s t c o m m o n c a u s e o f g ea r fa i l ur es . T h r e e f o r m s o f s u r fa c e c o n t a c t
f a t i g u e d a m a g e h a v e b e e n i d e n t i f i e d , d e p e n d i n g o n t h e s t r e s s d i s t r i b u t i o n a t a n d n e a r t h e s u r f a c e .
F r o m t h e p r e c e d i n g s e c t i o n s, t h e c h a r a c t e ri s t i c s o f e a c h t y p e o f f a i lu r e c a n b e s u m m a r i z e d a s f o l l o w s :
( a) R o l l i n g c o n t a c t f a t i g u e o c c u r s a l o n g t h e p i t c h - li n e o f g e a r t ee t h a n d l e ad s t o th e f o r m a t i o n o f
m i c r o s c o p i c p i t s .
( b ) S l i d i n g - r o l l i n g c o n t a c t f a t ig u e o c c u r s a w a y f r o m t h e p i t c h -l i n e , b u t p r e d o m i n a n t l y i n t h e
d e d e n d a o f g e a r te e t h , w h e r e n e g a t i v e s l id i n g c o n d i t i o n s p r e v a il . T h i s f o r m o f d a m a g e l e a d s t o
t h e f o r m a t i o n o f s u r fa c e p i t s w h i c h m a y a c t a s i n i t i a ti o n s it es f o r o t h e r m o d e s o f fa i lu r e .
( c) S p a l l i n g r e f e r s t o t h e f o r m a t i o n o f l a r g e , d e e p p i t s o n c o n t a c t i n g s u r f a c e s , a n d i s e i t h e r a
c o n t i n u a t i o n o f r o l li n g o r ro l l i n g s l id i n g c o n t a c t f a t i g u e d a m a g e , o r m a y b e d u e t o c r a c k i n g at
t h e c a s e - c o r e i n t e rf a c e .
A n u m b e r o f r e al , p r a c t i c a l e x a m p l e s o f g e a r f a il u r e s h a v e b e e n u s e d t o i l l u s tr a t e t h e v a r i o u s t y p e s
o f c o n t a c t f a t i g u e d a m a g e .
cknowledgement The ass istance of the s taffof the Advanced Engineering and Test ing Services Programme, Mattek. CSIR,
in the preparation of this paper is gratefully acknow ledged.
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S ur fac e c on t ac t f a t i gue f a i l u r e s i n gea r s
R E F E R E N C E S
107
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4 . W ulp i , D . J . , Unders tandin9 How Componen ts Fai l . A m er i c an S oc i e ty fo r M e t a l s , M e t a l s P a rk , O H , 1 9 85 , p p . 1 83 2 0 4.
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