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Pitting fatigue of gears—some ideas on appearance, mechanism and lubricant influence
RESEARCH REPORTS Pitting fatigue of gears - some ideas on a p p e a r a n c e , m e c h a n i s m and lubricant influence W. J. Bartz and V. KriJger*
The reliability of gears depends strongly on the prevention of failures during operation. Evaluation of gear tooth failures is necessary in order to avoid or to retard the same failure recurring in comparable situations. Therefore, an objective and unambiguous analysis of the reasons for the failures is necessary. The evaluation of the influence of lubricants may be very difficult ~- 3 because many of the fundamental relationships between lubricant properties and types of gear failure have not yet been exactly determined. At the moment it is only possible to state that fractures of gear teeth cannot be influenced by lubricants, while scuffing and scoring may be avoided by using suitable lubricants and no generally accepted results are available for the relationship between lubricant and pitting fatigue. A deeper understanding of lubricants must be achieved before they will be recognized as functional or design elements rather than purely operational materials 4,s. It would then be necessary to consider the lubricants at the design and calculation stage of gear development. Type of appearance Pitting is to be regarded as a type of fatigue failure, characterized by the fact that particles of material will break off the surfaces of the gear teeth after a certain number of meshing cycles has been reached. These material losses will be preceded by cracks which have been caused by severe damage of the substrate structure beneath the surface (Fig 1). These cracks are often orientated to the tooth surfaces and form an angle of about 45 ° with the surface (Fig 2). The pits often show a shell-shaped appearance, as can be seen from Fig 3. In Fig 4, the contours of the material shortly to be broken off may clearly be recognized. The substrate material has not been completely broken off, but this cannot be avoided because the crack is orientated in a direction parallel to the surface (Fig 5). The final breaking of the particle will then occur as a forced rupture. The plastic deformation caused by shearing of the material during the fracture process can clearly be seen in Figs 6 and 7. It should be obvious that after destroying the structure by fatigue to the degree shown in Fig 1, pitting cannot be avoided, although the photograph shows no indication of the surface breaking. All possible measures have to be taken to eliminate the destruction of the structure by cracks. Because of their influence on the reliability of gears and the necessity of taking preventive measures, running-in pitting and progressive pitting have to be distinguished 6- 9. Running-in pitting may occur during the first operational phase of new gears, especially in those made from unhardened or soft materials. Usually this pitting only increases until the local and limited surface 'peaks' have been removed and hence local overloads have been reduced to such a degree that the contact area now available will be large enough to transmit the load without any surface * Institute of Petroleum Research, 3 Hanover, Germany
Fig 1
Structure of destroyed material caused by cracks
which precede the breaking away of particles
Fig 2 Cracks and pitting on a gear moth. The cracks are orientated to the surface to form an angle of about 45 ° with the surface
Fig 3
Shell-shaped pits
TRIBOLOGY October 1973
191
Fig 6 Final breaking away of a particle by forced rupture; deformations are caused by shearing of the material
Fig 4
Contours of a particle about to break off
Fig 5 A particle almost disconnected from the substrate material by cracks orientated in a direction parallel to the surface. failures. The development of this kind of pitting not only ceases, but these pits in general may diminish by correction of the meshing gear wheels. In contrast progressive pitting fatigue will still increase rapidly after the local overloads have been reduced by the running-in process. Developing of pitting will now be more and more pronounced with running time, until the unimpaired remaining surface area will not be sufficient to transmit the load. Progressive increasing damage of the surface results. To evaluate pitting fatigue it is necessary to consider the type of failures which could possibly occur. Unfortunately it is difficult to decide whether pitting fatigue will cease or not. Pitting is insignificant and can be considered as negligible if the pits are small and few in number: then no impairment of the operational characteristics can be recognized (Fig 8). On the other hand large pitting fatigue (Fig 9) may rapidly result in complete destruction of the gear tooth surface (Fig 10). As a result of the high specific overloads, fractures of the whole tooth may shortly end the life of the gear wheel (Fig 11).
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TRIBOLOGY October 1973
Fig 7 Higher magnification of the fracture surface shown in Fig 6
Fig 8
Insignificant pitting fatigue
~~ 0,78a Fig 9
Progressive p i t t i n g f a t i g u e
-
- -~
- - O,ha
IZ I
Fig 1 2
Fig 1 0 fatigue i!i
¸
Tooth surface destroyed by extensive pitting
?
!
State of stress for the cylinder/plate system
Nevertheless certain other theories are proposed and discussed as the reason for pitting fatigue, although it is unlikely that they could be very important (Table 1). It is generally accepted that pulsating and alternating stresses will lead to pitting of the material, followed by fatigue. Shearing stresses beneath the surface
During contact, under load, of two rolling parts, a state of stress will be built up. This is shown in Fig 12 in a simplified form for the cylinder/plate system. A pressure field is developed, due to the load on the flattened contact area, and this is characterized by a semicircular pressure distribution. Beneath the contact surface of the substrate a state of stress will be induced. The shearing stresses r = 1 / 2 ( O x - Oy),
Fig 11 Whole gear tooth destroyed by extensive pitting fatigue
Reason for pitting fatigue Pitting, preceded by material fatigue, is mainly responsible for gear tooth surface failures. The main reason for pitting fatigue is thought to be due to the critical contact pressure or Hertzian pressure for a specific material being exceeded.
control the cracks which precede pitting fatigue. These stresses reach their maximum rmax = 0.30po at z = 0, 78a beneath the surface (a is the smaller radius of the flattened area). These principal shear stresses form an angle of 45 ° with the surface. During rolling of the two surfaces the principal shear stresses will increase from zero to a maximum and then decrease again to zero; therefore these stresses have to be defined as pulsating stresses. However, the shear stresses orientated in a direction parallel to the surface, ie in an orthogonal direction to the normal component of force, have to be distinguished from the principal stresses. These shear stresses have to be defined as alternating stresses, the maxima of which vary between forth = +0.25po and ror~ = -0.25po at z = 0, ha. It is accepted that these shear stresses will initiate the first cracks beneath the surface once a certain critical value has been exceeded.
TRIBOLOGY October 1973
193
Tangential stresses at the surface The state of stress discussed above will now be established between rolling parts. For meshing gear wheels this is only valid at the pitch line. At the root and tip of the teeth an increasing slidin~ action occurs and results in sliding friction forces, that reverse their direction at the pitch point. The orthogonal friction forces will lead to normal and shear stresses, orientated in a direction parallel to the surface. In this way the surfaces are subjected to tensile and compressive alternating stresses. Surface cracks result if certain critical stresses are exceeded. Because of these additional stresses the depth of the maxima of the shear stresses is reduced and the danger o ~ crack formation is increased. Formation of cracks is due to two effects 1°. Firstly, cracks will start directly below or at the surface, if the pulsating and alternating stresses exceed certain critical values which depend on the substrate material. It is possible that these cracks will start at specific points especially favoured by cracks, for instance at processing scars. They can also be caused by strain-hardening of the surface due to the limit of elasticity being exceeded. In addition, anisotropy of the compressibility of embedded crystallites may lead to local stresses initiating cracks along the crystal boundaries. However, the pulsating and alternating stresses will give rise to plastic deformations, in the course of which energy will be stored within the deformation layer. Whether most of the cracks which lead to pitting fatigue will start at the surface or occur beneath the surface, is still under debate.
Controlling and influencing pitting fatigue Effects With reference to the mechanism of pitting fatigue formation, discussed above, the following effects should influence the development of pits on gear tooth surfaces: material surface treatment surface finishing toothing and tooth profile operating conditions lubricants.
Table 1
Possible reasons for pitting fatigue
Surface fatigue caused by pulsating and alternating stresses Shear stresses caused by rolling contact pressure Tangential stresses caused by frictional effects Overload at the surface by cavitation effects Local welding of the surfaces Overload at the surface caused by tensions Internal oxidation effects caused by plastically deformed zones Fretting corrosion effects
Table 2 failures
Possible influence of lubricants on pitting fatigue
(a) Reducing the development of internal cracks Reduction of Hertzian pressure by increasing the elastohydrodynamically transmitted load by: Higher rated viscosity Higher pressure coefficient of viscosity Lower temperature dependence of viscosity (b) Reducing the development of cracks at the surface Reducing the solid body friction coefficient by: Higher operated viscosity Suitable type of lubricant Suitable additives Reducing splitting effects of cracks by: Higher operational viscosity Suitable additives
Only the influence of lubricants will be considered.
Influence of lubricants Table 2 indicates the possible influences of lubricants on pitting fatigue. These are divided into the effects caused by internal cracks and surface cracks. Control of pitting fatigue due to cracks starting beneath the surface seems to be possible only by reducing the contact pressure (Hertzian pressure), i.e. by reducing the pulsating and alternating stresses. For given stress conditions this can only be achieved by increasing the elastohydrodynamic component of transmitted force. Considering the fundamental elastohydrodynamic relationships, it follows that this component is controlled by the viscosity at the point of contact. As a measure against pitting fatigue, the local or operational viscosities have to be increased. Viscosity-temperature and viscosity-pressure behaviour must be taken into account. Higher operational viscosities can be achieved by higher rated viscosity, higher pressure coefficients of viscosity and lower viscosity temperature dependencies. One possibility of retarding the formation of cracks which start at the surface is to reduce the sliding friction
194
TRIBOLOGY October 1973
coefficient at the point of contact. This leads to smaller tangential stresses at the surface. Again this could be caused by higher operational viscosities at the point of contact. It is well known that different base oils or lubricants, eg synthetic oils, may cause different sliding friction effects. It is also to be expected that gear lubricants containing additives which cause adsorption and reaction layers on the gear tooth surfaces at the points of contact, will also reduce or retard the development of pits. It is well known that the frictional behaviour of such surface layers is better than that of the substrate itself. Once cracks have started, breaking away of particles and thus pitting fatigue may be accelerated by the splitting action of the oil. During the rolling and sliding action of two meshing gear tooth surfaces, the lubricant will be forced into existing cracks and pressurized. When the pressure is suddenly released at the point of contact, some particles may be broken off by the oil trapped within
the cracks. At some stage which is dependent on the driving pinion and driven gear wheel the cracks will be 'opened' by tangential forces near the root of the teeth, during which the lubricant can be forced into the cracks. Near the tip of the teeth these cracks will be compressed and 'closed', which leads to much less pitting fatigue in this area of the gear tooth surface. A higher viscosity is obtained by using lubricants or additives with a higher interfacial tension and this results in a less pronounced 'creeping behaviour' in the lubricant and the possibility of reducing the 'splitting' effect.
Further investigations All effects which decrease the maximum shear stresses beneath the surfaces, or tangential stresses at the surfaces of gear teeth should produce a reduction in pitting fatigue failures. The remarkable influence of the operational viscosity at the point of contact in decreasing pitting fatigue effects has been stated and confirmed. Further investigations are necessary before the influence of base oils and additives is accepted. Recently an extensive and systematic research programme (involving the use of an FZG-Gear-Tester with heat-treated and case-hardened gear wheels made from 42CrMo4 and 16MnCr5 respectively, ground to a maximum peak-to-valley height of about 2 to 3/am) has begun. The following additives are under investigation: Zinc-di-iso-propyldithiopho sphate (i-C3ZnDDP) Zinc-di-n-octyldithiophosphate (n-C8ZnDDP) Lead-di-n-octyldithiopho sphate (n-CsZnDDP) Tricresylphosphate Hexachlorethane
Dibenzyldisulfide Molybdenum disulfide Several anti-wear and ep-additives, commercially available. The influence of several synthetic base oils will then be investigated and the results presented in the near future.
References 1 Bartz,W. J. 'Concerning the role of lubricants in the damage of machine components,' (in German)Der Maschinensehaden, Vo142, No 3 (1969) pp 65-76 2 Bartz,W. J. 'The influence of lubricating oils on gear wheel damage' (in German)Antriebstechnik, Vol 9, No 10 (1970 pp 361-367 3 Bartz,W. J. 'The influence of the theological properties of lubricants on the frictional wear of gear wheels' (in German) VDI-Z, Vol 113, No 2 (1971)pp 142-148 4 Bartz,W. J. 'On the importance of elastohydrodynamic lubrication in gear wheel mating,' (in German), Konstruktion, Vol 23, No 7 (1971) pp 257-262 5 Bartz,W. J. 'The lubricating oil as a funetaonal component,' (in German) Mineral61teehnik, Vol 16, Nos 7/8 (1971) pp 1-34 6 Dudley, D. W. and Winter, H. 'Gearing,' (in German) Springer-Verlag (1961 ) 7 Haniseh,F. 'The influence of lubricants on gear wheel damage,' (in German) Allianz-Erfahrungsberichte, Vol 2, 1965 8 'Gear wheel damage,' (in German) ZF-Norm 201, Zahnradfabrik Friedriehshafen AG 9 'Gear tooth wear and failure', AGMA-Norm 110.02 (1951) American Gear Manufacturing Association 10 Bartz,W. J. 'An investigation into the influences of modern lubricants on the pitting fatigue at rolling and sliding contacts' (in German) Forschungsheft Forschungsvereinigung Antriebstechnik, Vol 2 (1971)
On some aspects of metallurgical changes during gear pitting V. C. Venkatesh and R. Krishnamurthy* Pitting phenomenon in gears was studied in a recirculating power type test rig. Micrographs revealed relative grain coarsening around pitted areas, together with transcrystalline and intergranular cracks while macrophotographs defined pit configuration with the surface cracks. Pitting is a surface fatigue failure on the material as a result of repeated surface or subsurface stresses that exceed the endurance limit of the material 1. It is characterized by the removal of material and the formation of pits.
Mechanism of failure Different theories exist explaining the mechanism of failure. Due to rolling and sliding action between two surfaces, stresses are induced and the stress distribution is as shown in Fig 1. When the sub-surface shear stress exceeds the endurance limit of the material a fatigue crack initiates and propagates parallel to the surface, isolating and removing a piece of surface material to form a pit 2. Sometimes oil enters a fatigue crack nucleated at the surface and the crack propagates under the hydraulic wedge action of the trapped oil 3. An attempt is now made to explain the mode of failure.
Material:
0.22%C 0.44%C Treatment: untreated Type of manufacture grinding
mild steel - 130 BHN mild steel - 130 BHN (soft) and finish: hobbing without
r stress
////// Distance 1 below surface
//~p uPure = slidring~ ~ f.~-Combined roll ing J
and sliding
Experimental Material specification The material specification and the test conditions were: * Department of Mechanical Engineering, Indian Institute of Technology, Madras 600036, India
Fig 1 Stressdistribution in contacting surfaces2due to rolling, sliding and combinedeffect
TRIBOLOGY October 1973
195
The reliability of gears depends strongly on the prevention of failures during operation. Evaluation of gear tooth failures is necessary in order to avoid or to retard the same failure recurring in comparable situations. Therefore, an objective and unambiguous analysis of the reasons for the failures is necessary. The evaluation of the influence of lubricants may be very difficult ~- 3 because many of the fundamental relationships between lubricant properties and types of gear failure have not yet been exactly determined. At the moment it is only possible to state that fractures of gear teeth cannot be influenced by lubricants, while scuffing and scoring may be avoided by using suitable lubricants and no generally accepted results are available for the relationship between lubricant and pitting fatigue. A deeper understanding of lubricants must be achieved before they will be recognized as functional or design elements rather than purely operational materials 4,s. It would then be necessary to consider the lubricants at the design and calculation stage of gear development. Type of appearance Pitting is to be regarded as a type of fatigue failure, characterized by the fact that particles of material will break off the surfaces of the gear teeth after a certain number of meshing cycles has been reached. These material losses will be preceded by cracks which have been caused by severe damage of the substrate structure beneath the surface (Fig 1). These cracks are often orientated to the tooth surfaces and form an angle of about 45 ° with the surface (Fig 2). The pits often show a shell-shaped appearance, as can be seen from Fig 3. In Fig 4, the contours of the material shortly to be broken off may clearly be recognized. The substrate material has not been completely broken off, but this cannot be avoided because the crack is orientated in a direction parallel to the surface (Fig 5). The final breaking of the particle will then occur as a forced rupture. The plastic deformation caused by shearing of the material during the fracture process can clearly be seen in Figs 6 and 7. It should be obvious that after destroying the structure by fatigue to the degree shown in Fig 1, pitting cannot be avoided, although the photograph shows no indication of the surface breaking. All possible measures have to be taken to eliminate the destruction of the structure by cracks. Because of their influence on the reliability of gears and the necessity of taking preventive measures, running-in pitting and progressive pitting have to be distinguished 6- 9. Running-in pitting may occur during the first operational phase of new gears, especially in those made from unhardened or soft materials. Usually this pitting only increases until the local and limited surface 'peaks' have been removed and hence local overloads have been reduced to such a degree that the contact area now available will be large enough to transmit the load without any surface * Institute of Petroleum Research, 3 Hanover, Germany
Fig 1
Structure of destroyed material caused by cracks
which precede the breaking away of particles
Fig 2 Cracks and pitting on a gear moth. The cracks are orientated to the surface to form an angle of about 45 ° with the surface
Fig 3
Shell-shaped pits
TRIBOLOGY October 1973
191
Fig 6 Final breaking away of a particle by forced rupture; deformations are caused by shearing of the material
Fig 4
Contours of a particle about to break off
Fig 5 A particle almost disconnected from the substrate material by cracks orientated in a direction parallel to the surface. failures. The development of this kind of pitting not only ceases, but these pits in general may diminish by correction of the meshing gear wheels. In contrast progressive pitting fatigue will still increase rapidly after the local overloads have been reduced by the running-in process. Developing of pitting will now be more and more pronounced with running time, until the unimpaired remaining surface area will not be sufficient to transmit the load. Progressive increasing damage of the surface results. To evaluate pitting fatigue it is necessary to consider the type of failures which could possibly occur. Unfortunately it is difficult to decide whether pitting fatigue will cease or not. Pitting is insignificant and can be considered as negligible if the pits are small and few in number: then no impairment of the operational characteristics can be recognized (Fig 8). On the other hand large pitting fatigue (Fig 9) may rapidly result in complete destruction of the gear tooth surface (Fig 10). As a result of the high specific overloads, fractures of the whole tooth may shortly end the life of the gear wheel (Fig 11).
192
TRIBOLOGY October 1973
Fig 7 Higher magnification of the fracture surface shown in Fig 6
Fig 8
Insignificant pitting fatigue
~~ 0,78a Fig 9
Progressive p i t t i n g f a t i g u e
-
- -~
- - O,ha
IZ I
Fig 1 2
Fig 1 0 fatigue i!i
¸
Tooth surface destroyed by extensive pitting
?
!
State of stress for the cylinder/plate system
Nevertheless certain other theories are proposed and discussed as the reason for pitting fatigue, although it is unlikely that they could be very important (Table 1). It is generally accepted that pulsating and alternating stresses will lead to pitting of the material, followed by fatigue. Shearing stresses beneath the surface
During contact, under load, of two rolling parts, a state of stress will be built up. This is shown in Fig 12 in a simplified form for the cylinder/plate system. A pressure field is developed, due to the load on the flattened contact area, and this is characterized by a semicircular pressure distribution. Beneath the contact surface of the substrate a state of stress will be induced. The shearing stresses r = 1 / 2 ( O x - Oy),
Fig 11 Whole gear tooth destroyed by extensive pitting fatigue
Reason for pitting fatigue Pitting, preceded by material fatigue, is mainly responsible for gear tooth surface failures. The main reason for pitting fatigue is thought to be due to the critical contact pressure or Hertzian pressure for a specific material being exceeded.
control the cracks which precede pitting fatigue. These stresses reach their maximum rmax = 0.30po at z = 0, 78a beneath the surface (a is the smaller radius of the flattened area). These principal shear stresses form an angle of 45 ° with the surface. During rolling of the two surfaces the principal shear stresses will increase from zero to a maximum and then decrease again to zero; therefore these stresses have to be defined as pulsating stresses. However, the shear stresses orientated in a direction parallel to the surface, ie in an orthogonal direction to the normal component of force, have to be distinguished from the principal stresses. These shear stresses have to be defined as alternating stresses, the maxima of which vary between forth = +0.25po and ror~ = -0.25po at z = 0, ha. It is accepted that these shear stresses will initiate the first cracks beneath the surface once a certain critical value has been exceeded.
TRIBOLOGY October 1973
193
Tangential stresses at the surface The state of stress discussed above will now be established between rolling parts. For meshing gear wheels this is only valid at the pitch line. At the root and tip of the teeth an increasing slidin~ action occurs and results in sliding friction forces, that reverse their direction at the pitch point. The orthogonal friction forces will lead to normal and shear stresses, orientated in a direction parallel to the surface. In this way the surfaces are subjected to tensile and compressive alternating stresses. Surface cracks result if certain critical stresses are exceeded. Because of these additional stresses the depth of the maxima of the shear stresses is reduced and the danger o ~ crack formation is increased. Formation of cracks is due to two effects 1°. Firstly, cracks will start directly below or at the surface, if the pulsating and alternating stresses exceed certain critical values which depend on the substrate material. It is possible that these cracks will start at specific points especially favoured by cracks, for instance at processing scars. They can also be caused by strain-hardening of the surface due to the limit of elasticity being exceeded. In addition, anisotropy of the compressibility of embedded crystallites may lead to local stresses initiating cracks along the crystal boundaries. However, the pulsating and alternating stresses will give rise to plastic deformations, in the course of which energy will be stored within the deformation layer. Whether most of the cracks which lead to pitting fatigue will start at the surface or occur beneath the surface, is still under debate.
Controlling and influencing pitting fatigue Effects With reference to the mechanism of pitting fatigue formation, discussed above, the following effects should influence the development of pits on gear tooth surfaces: material surface treatment surface finishing toothing and tooth profile operating conditions lubricants.
Table 1
Possible reasons for pitting fatigue
Surface fatigue caused by pulsating and alternating stresses Shear stresses caused by rolling contact pressure Tangential stresses caused by frictional effects Overload at the surface by cavitation effects Local welding of the surfaces Overload at the surface caused by tensions Internal oxidation effects caused by plastically deformed zones Fretting corrosion effects
Table 2 failures
Possible influence of lubricants on pitting fatigue
(a) Reducing the development of internal cracks Reduction of Hertzian pressure by increasing the elastohydrodynamically transmitted load by: Higher rated viscosity Higher pressure coefficient of viscosity Lower temperature dependence of viscosity (b) Reducing the development of cracks at the surface Reducing the solid body friction coefficient by: Higher operated viscosity Suitable type of lubricant Suitable additives Reducing splitting effects of cracks by: Higher operational viscosity Suitable additives
Only the influence of lubricants will be considered.
Influence of lubricants Table 2 indicates the possible influences of lubricants on pitting fatigue. These are divided into the effects caused by internal cracks and surface cracks. Control of pitting fatigue due to cracks starting beneath the surface seems to be possible only by reducing the contact pressure (Hertzian pressure), i.e. by reducing the pulsating and alternating stresses. For given stress conditions this can only be achieved by increasing the elastohydrodynamic component of transmitted force. Considering the fundamental elastohydrodynamic relationships, it follows that this component is controlled by the viscosity at the point of contact. As a measure against pitting fatigue, the local or operational viscosities have to be increased. Viscosity-temperature and viscosity-pressure behaviour must be taken into account. Higher operational viscosities can be achieved by higher rated viscosity, higher pressure coefficients of viscosity and lower viscosity temperature dependencies. One possibility of retarding the formation of cracks which start at the surface is to reduce the sliding friction
194
TRIBOLOGY October 1973
coefficient at the point of contact. This leads to smaller tangential stresses at the surface. Again this could be caused by higher operational viscosities at the point of contact. It is well known that different base oils or lubricants, eg synthetic oils, may cause different sliding friction effects. It is also to be expected that gear lubricants containing additives which cause adsorption and reaction layers on the gear tooth surfaces at the points of contact, will also reduce or retard the development of pits. It is well known that the frictional behaviour of such surface layers is better than that of the substrate itself. Once cracks have started, breaking away of particles and thus pitting fatigue may be accelerated by the splitting action of the oil. During the rolling and sliding action of two meshing gear tooth surfaces, the lubricant will be forced into existing cracks and pressurized. When the pressure is suddenly released at the point of contact, some particles may be broken off by the oil trapped within
the cracks. At some stage which is dependent on the driving pinion and driven gear wheel the cracks will be 'opened' by tangential forces near the root of the teeth, during which the lubricant can be forced into the cracks. Near the tip of the teeth these cracks will be compressed and 'closed', which leads to much less pitting fatigue in this area of the gear tooth surface. A higher viscosity is obtained by using lubricants or additives with a higher interfacial tension and this results in a less pronounced 'creeping behaviour' in the lubricant and the possibility of reducing the 'splitting' effect.
Further investigations All effects which decrease the maximum shear stresses beneath the surfaces, or tangential stresses at the surfaces of gear teeth should produce a reduction in pitting fatigue failures. The remarkable influence of the operational viscosity at the point of contact in decreasing pitting fatigue effects has been stated and confirmed. Further investigations are necessary before the influence of base oils and additives is accepted. Recently an extensive and systematic research programme (involving the use of an FZG-Gear-Tester with heat-treated and case-hardened gear wheels made from 42CrMo4 and 16MnCr5 respectively, ground to a maximum peak-to-valley height of about 2 to 3/am) has begun. The following additives are under investigation: Zinc-di-iso-propyldithiopho sphate (i-C3ZnDDP) Zinc-di-n-octyldithiophosphate (n-C8ZnDDP) Lead-di-n-octyldithiopho sphate (n-CsZnDDP) Tricresylphosphate Hexachlorethane
Dibenzyldisulfide Molybdenum disulfide Several anti-wear and ep-additives, commercially available. The influence of several synthetic base oils will then be investigated and the results presented in the near future.
References 1 Bartz,W. J. 'Concerning the role of lubricants in the damage of machine components,' (in German)Der Maschinensehaden, Vo142, No 3 (1969) pp 65-76 2 Bartz,W. J. 'The influence of lubricating oils on gear wheel damage' (in German)Antriebstechnik, Vol 9, No 10 (1970 pp 361-367 3 Bartz,W. J. 'The influence of the theological properties of lubricants on the frictional wear of gear wheels' (in German) VDI-Z, Vol 113, No 2 (1971)pp 142-148 4 Bartz,W. J. 'On the importance of elastohydrodynamic lubrication in gear wheel mating,' (in German), Konstruktion, Vol 23, No 7 (1971) pp 257-262 5 Bartz,W. J. 'The lubricating oil as a funetaonal component,' (in German) Mineral61teehnik, Vol 16, Nos 7/8 (1971) pp 1-34 6 Dudley, D. W. and Winter, H. 'Gearing,' (in German) Springer-Verlag (1961 ) 7 Haniseh,F. 'The influence of lubricants on gear wheel damage,' (in German) Allianz-Erfahrungsberichte, Vol 2, 1965 8 'Gear wheel damage,' (in German) ZF-Norm 201, Zahnradfabrik Friedriehshafen AG 9 'Gear tooth wear and failure', AGMA-Norm 110.02 (1951) American Gear Manufacturing Association 10 Bartz,W. J. 'An investigation into the influences of modern lubricants on the pitting fatigue at rolling and sliding contacts' (in German) Forschungsheft Forschungsvereinigung Antriebstechnik, Vol 2 (1971)
On some aspects of metallurgical changes during gear pitting V. C. Venkatesh and R. Krishnamurthy* Pitting phenomenon in gears was studied in a recirculating power type test rig. Micrographs revealed relative grain coarsening around pitted areas, together with transcrystalline and intergranular cracks while macrophotographs defined pit configuration with the surface cracks. Pitting is a surface fatigue failure on the material as a result of repeated surface or subsurface stresses that exceed the endurance limit of the material 1. It is characterized by the removal of material and the formation of pits.
Mechanism of failure Different theories exist explaining the mechanism of failure. Due to rolling and sliding action between two surfaces, stresses are induced and the stress distribution is as shown in Fig 1. When the sub-surface shear stress exceeds the endurance limit of the material a fatigue crack initiates and propagates parallel to the surface, isolating and removing a piece of surface material to form a pit 2. Sometimes oil enters a fatigue crack nucleated at the surface and the crack propagates under the hydraulic wedge action of the trapped oil 3. An attempt is now made to explain the mode of failure.
Material:
0.22%C 0.44%C Treatment: untreated Type of manufacture grinding
mild steel - 130 BHN mild steel - 130 BHN (soft) and finish: hobbing without
r stress
////// Distance 1 below surface
//~p uPure = slidring~ ~ f.~-Combined roll ing J
and sliding
Experimental Material specification The material specification and the test conditions were: * Department of Mechanical Engineering, Indian Institute of Technology, Madras 600036, India
Fig 1 Stressdistribution in contacting surfaces2due to rolling, sliding and combinedeffect
TRIBOLOGY October 1973
195