~ ) Pergamon
Engineering Failure Ana(vsis, Vol. 4, No. 2, pp. 99 107, 1997 (~ 1997 Elsevier Science Ltd. All rights reserved Primed in Great Britain 1350 6307,'97 $17.00 + 0.00
PII: S 1350-6307(97)00006-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 IN 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 Testing Services, CSIR, Private Bag X28, Auckland Park 2006, South Africa (Received 6 January 1 9 9 7 )
Abstract--Surface contact fatigue is the most common cause of gear failure. It results in damage to contacting surfaces which can significantly reduce the load-carrying capacity of components, and may ultimately lead to complete failure of a gear. Three types of contact fatigue damage are discussed, and a number of actual examples are presented to illustrate this failure mode in practice. © 1997 Elsevier Science Ltd.
1. I N T R O D U C T I O N
The m o s t c o m m o n m o d e o f gear failure e n c o u n t e r e d in practice is that o f surface c o n t a c t fatigue.
This m o d e o f failure leads to crack initiation at or near the c o n t a c t surface, a n d m a y subsequently lead to d a m a g e v a r y i n g in extent f r o m microscopic pitting to severe spalling. T h e metal r e m o v e d f r o m the surface in such cases enters the m a c h i n e system, and can, in turn, cause abrasive wear and failure o f other c o m p o n e n t s . F u r t h e r m o r e , the pits f o r m e d o n the d a m a g e d surface lead to the f o r m a t i o n o f stress c o n c e n t r a t i o n s , a n d serve as initiation sites for other m o d e s o f gear failure, e.g.
t o o t h bending fatigue [1]. In this paper, various types o f c o n t a c t fatigue d a m a g e are reviewed and illustrated using practical examples. T h e causes a n d ways o f preventing d a m a g e in each case are also discussed briefly.
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 two curved (usually convex) surfaces are in c o n t a c t u n d e r load, the c o n t a c t occurs along a line or point, or, d e p e n d i n g o n the elastic c o n s t a n t s o f the materials concerned, along a very small circular or elliptical area. As a result o f such small c o n t a c t areas, the shear (Hertzian) stresses which develop at a n d near the surface are c o n s e q u e n t l y very high. T h e m a x i m u m shear stress occurs at some distance below the surface [2], as illustrated in Fig. 1.
W h e n the c o n t a c t i n g stresses are repetitive, as is the case on the active flanks o f gear teeth, the cyclic compressive stresses induced cause differing elastic a n d plastic b e h a v i o u r in the near-surface material. D e p e n d i n g on the m i c r o s t r u c t u r e and grain orientation o f the material in this region, internal stress c o n c e n t r a t i o n s are f o r m e d which can ultimately lead to crack initiation. In practice, crack initiation usually occurs at inclusions in the stressed near-surface material, the m o s t deleterious being those inclusions which are hard, brittle and angular in shape.
D a m a g e due to c o n t a c t fatigue in gear teeth usually occurs in one o f three areas, viz along the pitch-line, in the a d d e n d u m (i.e. a b o v e the pitch-line), and in the d e d e n d u m (i.e. below the pitch line) [3]. A l o n g the pitch-line, only pure rolling stresses exist (Fig. 1), while a w a y f r o m the pitch- line, b o t h rolling a n d sliding stresses are experienced.* In the subsequent sections o f this paper, surface d a m a g e under b o t h pure rolling and rolling-sliding conditions, as well as a m o r e a d v a n c e d a n d severe m o d e o f surface d a m a g e called spalling, is discussed.
*Pure rolling stresses along the pitch-line occur only in spur, bevel and helical gears, and not in worm, spiral bevel or hypoid gears [4].
99
100 P. J. L. F E R N A N D E S and C. M c D U L I N G
///////////////////////////5-- ....
/ "
/- / J . J
- - Pure rolling
- - . J
Distance below surface
S h e a r stress
I
Fig. 1. Stress distribution at and near two contacting surfaces under pure rolling.
3. R O L L I N G C O N T A C T F A T I G U E
The stress distribution resulting from pure rolling conditions prevalent at the pitch-line is shown in Fig. 1. The maximum shear stress occurs at some distance, usually 0 . 1 8 - 0 . 3 m m [3], below the surface, and just ahead of the contact point. Cracks initiate at the point of maximum stress, and propagate essentially parallel to the surface. Continued rolling may cause the cracks to deviate up towards the contact surface, resulting in the removal of metal from the surface. The pits formed in this manner initially have sides perpendicular to the contact surface, as illustrated schematically in Fig. 2. Continued operation of the pitted surface, however, may cause a breakdown in the pit shape.
Pitting under pure rolling can occur even under proper lubrication conditions, since oil, as an incompressible fluid, will merely transmit the contact loads [5]. The pits formed are generally very small and seldom give more than a "frosted" appearance to the damaged surface. In some cases, the pits may not progress beyond their point of origin, and may even be self-healing [3, 5]. There are two unique characteristics of rolling contact fatigue pits which can be used to distinguish this type of damage from other forms of pitting.
Firstly, the formation of rolling contact fatigue pits occurs with no surface plastic deformation.
This is contrary to pitting under sliding-rolling conditions, as shown in Section 4. Secondly, in components with a case-hardened layer consisting of martensite with little or no retained austenite,
\ \ \ \
\ . .
/
/ /
/,"
j . J " ./- j j - - Y
- \ \ \ \ . , .\
\
\ Fig. 2. Location and orientation of subsurface cracks under pure rolling conditions.
Surface contact fatigue failures in gears 101 rolling contact fatigue leads to the formation of a microstructural feature referred to as "butterfly wings" [3]. These are formed when plastic deformation is constrained by the surrounding material, and is more c o m m o n when shear stresses are extremely high.
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
Pure rolling conditions prevail when the surface velocities o f two contacting curved bodies are the same. However, if these velocities are different, an element of sliding is introduced which significantly alters the stress distribution in the surface and near-surface material. Depending on the relative velocities o f the contacting bodies, rolling and sliding may occur in the same d i r e c t i o n * -
positive sliding--or
in opposite directionsnegative sliding.
The effect o f the latter is that the surface material is rolled in one direction, and pushed (sliding) in another, therefore resulting in higher stresses than those encountered in positive sliding [4]. The modified stress distribution in the surface and near-surface material resulting from combined rolling and sliding is shown in Fig. 3. The position of maximum shear stress is moved closer to the contacting interface, and crack initiation therefore occurs at the surface.Gear teeth have complex combinations of sliding and rolling, which vary along the profile o f each tooth, as illustrated in Fig. 4. In the addendum, the direction of rolling and sliding is the same, and positive sliding conditions therefore prevail. In the dedendum, however, the direction o f rolling is opposite to that of sliding, and negative sliding conditions exist. Contact fatigue is therefore more likely to initiate in the dedendum, and pitting in this region is usually very severe, and often acts as a precursor to tooth bending fatigue [1].
In practice, it is c o m m o n that contact fatigue damage will first occur in the dedendum of the smaller gear (which is usually the driving gear) of a gear set [4, 5]. This is explained by the fact that the smaller gear will undergo more revolutions, and therefore each tooth will experience a larger number of stress cycles. In order to prevent premature failure in such cases, it is c o m m o n to make the smaller, driving gear harder than the other gears in the gear set.
tress
~.J . f
/ . ~ - ~ i ...
beloy
~ D i s t a n c e / ~ Pure s ~ ~
surface l/ Comb.m..ed rolling
Fig. 3. Stress distribution at and near two contacting surfaces under slidin~rolling conditions.
*The direction of rolling is the direction in which the point of contact between two bodies moves, and is always opposite to the direction of rotation.
102 P.J.L. FERNANDES and C. McDUL1NG
\.
j J / / / /
/ . ~ D r i v i n g g e a r
/ / \,
\
B e g i n n i n g o f c o n t a c t
/
R - r o l l i n g S - s l i d i n g
D r i v i n g g e ~ m .
f
_ - r - - !
/ \ \
~\. \ ' \ \
. \
\
\
E n d o f c o n t a c t
Fig. 4. Combination of sliding and rolling in gear teeth.
A n o t h e r region in which contact fatigue damage is frequently encountered is at the lowest point o f single tooth contact, i.e. the point at which contact is made with the tip o f the matching tooth [3]. Since the contact area in this case is very small, high stresses are generated, even under normal loads. Moreover, the lowest point o f single tooth contact is always in the dedendum o f the matching tooth, and sliding speeds, both in approach and recess, are at a maximum. The negative sliding conditions in these regions, together with the high stresses and high sliding speeds, lead to rapid initiation o f damage. Figure 5 shows the driving gear o f a 1.5 ton lever hoist in which severe pitting in the dedendum has occurred at the point o f single tooth contact [6]. The extent o f damage observed occurred after only 500 cycles under test conditions.
Unlike contact fatigue damage under pure rolling conditions, the sliding rolling action causes plastic deformation o f the surface material, and this can usually be detected using metallographic analysis. The extent o f plastic deformation, and hence o f contact fatigue damage, can be reduced
Surface contact fatigue failures in gears 103
it ~4 t } t )
Fig. 5. Severe pitting in the d e d e n d u m o f the driving gear o f a lever hoist. Pitting initiated at the lowest point of single tooth contact.
effectively by ensuring that correct lubrication conditions are maintained. Furthermore, it has been shown that sliding-rolling damage can be minimized by achieving a surface hardness on the matching bodies greater than H R C 60 [4]. Retained austenite, at levels o f 10-20%, in the case-hardened layer also minimizes the extent o f contact fatigue since this microstructural constituent deforms plastically to form larger contact areas, and hence lower contact stresses. However, austenite reduces the fatigue strength of the material, and can therefore introduce other deleterious effects.
5. S P A L L I N G
Spalling is defined by the A S M Metals Reference Book [7] as "the cracking and flaking of particles out of a surface". It refers to the formation of large and deep pits on contacting surfaces, which significantly reduce the load-carrying capacity o f components. The pits themselves act as stress concentrations which may lead to other modes of failure, while the material lost from the surface can cause damage elsewhere in the component. Two mechanisms of spalling have been identified
[5].
Firstly, spalling may occur as a continuation of pitting resulting from rolling or sliding-rolling contact fatigue. In some instances, the cracks initiated by these mechanisms will propagate into the material, and ultimately result in the loss o f large pieces of metal from the contact surface. An example of this is given in Fig. 6, which shows severe spalling which occurred as a result o f rolling contact fatigue pitting along the pitch-line of a helical gear. In some cases, pitting will start at one point, and progress outwards and upwards along the tooth profile to form what is known as the
"cyclone" effect [3]. This is clearly shown in Fig. 7 for the case of a small helical gear from an automatic gearbox [8].
In the second mechanism of spalling, also known as "case crushing" or "sub-case fatigue" [4], fatigue cracks initiate at the metallurgical notch formed between the hardened case and core. Since this interface is usually very deep, the depth and size of the pits formed are significantly larger than those encountered in rolling or sliding-rolling contact fatigue. Figure 8 shows a spur gear in which large, deep pits have formed along the entire active flank as a result of sub-case fatigue [9]. The mechanism of crack initiation is the same as in the other modes of contact fatigue. Since this type of spalling is essentially due to the disparity in the mechanical properties of the case and core,
104 P . J . L . F E R N A N D E S and C. M c D U L I N G
Fig. 6. Severe spalling along the pitch-line of a helical gear resulting from rolling contact fatigue.
Surface contact fatigue failures in gears 105
Fig. 7. (a) Active flank of a helical gear showing the "cyclone" effect, and (b) high-magnification photograph o f the same area showing how pitting has extended upwards and outwards.
106 P.J.L. FERNANDES and C, McDULING
Fig. 8. Sub-case fatigue or spalling on the active flank of a spur gear.
increasing the hardness (or strength) o f the core t h r o u g h material selection or heat t r e a t m e n t can effectively reduce the incidence o f spalling. Alternatively, by increasing the depth o f the case, the core~case interface can be m o v e d to a position o f lower stress.
6. S U M M A R Y
Surface c o n t a c t fatigue is the m o s t c o m m o n cause o f gear failures. Three forms o f surface c o n t a c t fatigue d a m a g e have been identified, d e p e n d i n g on the stress distribution at a n d near the surface.
F r o m the preceding sections, the characteristics o f each type o f failure can be s u m m a r i z e d as follows:
(a) Rolling c o n t a c t fatigue occurs along the pitch-line o f gear teeth a n d leads to the f o r m a t i o n o f m i c r o s c o p i c pits.
(b) Sliding-rolling c o n t a c t fatigue occurs a w a y f r o m the pitch-line, but p r e d o m i n a n t l y in the d e d e n d a o f gear teeth, where negative sliding conditions prevail. This f o r m o f d a m a g e leads to the f o r m a t i o n o f surface pits which m a y act as initiation sites for other m o d e s o f failure.
(c) Spalling refers to the f o r m a t i o n o f large, deep pits on c o n t a c t i n g surfaces, and is either a c o n t i n u a t i o n o f rolling or rolling sliding c o n t a c t fatigue damage, or m a y be due to cracking at the c a s e - c o r e interface.
A n u m b e r o f real, practical examples o f gear failures have been used to illustrate the various types o f c o n t a c t fatigue damage.
Acknowledgement The assistance of the staffofthe Advanced Engineering and Testing Services Programme, Mattek. CSIR, in the preparation of this paper is gratefully acknowledged.
Surface contact fatigue failures in gears R E F E R E N C E S
107
I. Fernandes, P. J. L., Engineering Failure Analysis, 1996, 3(3), 219 225.
2. Shigley, J. E., Mechanical Engineering Design, 1st metric edn. McGraw-Hill, Singapore, 1986, pp. 78 80.
3. Alban, L. E., Systematic Analysis of Gear Failures. American Society for Metals, Metals Park, OH, 1985, pp. 94-106.
4. Wulpi, D. J., Understandin9 How Components Fail. American Society for Metals, Metals Park, OH, 1985, pp. 183 204.
5. ASM Metals Handbook, Failure of Gears, Vol. 10, Failure Analysis and Prevention, 8th edn. American Society for Metals, Metals Park, OH, 1975, pp. 507 524.
6. McDuling, C., Test Report. Advanced Engineering and Testing Services, CSIR, 1996.
7. A S M Metals Reference Book, 2nd edn. American Society for Metals, Metals Park, OH, 1983, p. 67.
8. Heyes, A., Failure Analysis Report. Advanced Engineering and Testing Services, CSIR, 1996.
9. James, A., Failure Analysis Report. Advanced Engineering and Testing Services, CSIR, 1994.
