1. Fachtagung TestRig - Juni 2022 61 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants Karl Jakob Winkler Gear Research Center (FZG), Technical University of Munich (TUM), Munich, Germany Benedikt Siewerin Gear Research Center (FZG), Technical University of Munich (TUM), Munich, Germany Dr. Thomas Tobie Gear Research Center (FZG), Technical University of Munich (TUM), Munich, Germany Prof. Karsten Stahl Gear Research Center (FZG), Technical University of Munich (TUM), Munich, Germany For a high efficiency regarding costs and environment, gearboxes need to be lubricated by high-performance lubricants. Such lubricants are continuously newly developed or existing lubricants are developed further in order to fulfill the steadily increasing demands of modern gearboxes. As the interaction between lubricant and gear material cannot be predicted reliably, each lubricant needs to be tested and characterized regarding the common gear failure modes scuffing, micropitting, wear and pitting. The FZG test methods cover the mentioned gear failures and can all be performed with the FZG back-to-back gear test rig. The results of the FZG test methods enable the comparison and classification of the lubricants regarding each gear failure. Furthermore, the characteristic values of the tests can be used for the rating and designing of gear sets according to the standard series ISO 6336. The following paper will introduce the FZG back-to-back gear test rig, will explain the fundamentals, procedures, main influences and benefits of the FZG test methods concerning scuffing and micropitting and will give a rough overview on the FZG test methods regarding wear and pitting. 1. Introduction For a high efficiency regarding costs and environment, gearboxes need to be lubricated by high-performance lubricants. Such lubricants are continuously newly developed or existing lubricants are developed further in order to fulfill the steadily increasing demands of modern gearboxes. Since the interaction between lubricant and gear material cannot be predicted reliably based on the knowledge of physical or chemical parameters alone, each lubricant needs to be tested and characterized regarding different gear failure modes. Gear failure modes influenced by the lubricant are scuffing, micropitting, continuous (or slow-speed) wear and pitting. In order to compare the performance of lubricants regarding these failure modes, standardized test methods are necessary. Such test methods were developed at the gear research center, whose German name is Forschungsstelle für Zahnräder und Getriebebau (FZG), of the Technical University of Munich. The FZG test methods can all be performed with the FZG back-to-back gear test rig. The results of the FZG test methods enable the comparison and classification of the lubricants regarding each gear failure. Furthermore, the characteristic values of the tests can be used for the rating and designing of gear sets according to the standard series ISO 6336 [13]. The following text will introduce the FZG back-to-back gear test rig, will explain the fundamentals, procedures, main influences and benefits of the FZG test methods concerning scuffing and micropitting and will give a rough overview on the FZG test methods regarding wear and pitting. 2. FZG Back-to-Back Gear Test Rig The standard FZG back-to-back gear test rig has a center distance of a = 91.5 mm [5] and is shown in Figure 2.1 as a schematic drawing. 62 1. Fachtagung TestRig - Juni 2022 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants Figure 2.1 FZG back-to-back gear test rig The test rig is driven by a three-phase asynchronous engine. The maximum speed of the motor is 3000 rpm. Test pinion and test gear are mounted on two parallel shafts which are connected to a drive gear stage with the same gear ratio. The shaft of the test pinion consists of two separate parts which are connected by a load clutch. A defined static torque is applied by twisting the load clutch using defined weights on the load lever or by twisting the load clutch with a bracing device. The torque can be controlled indirectly at the torque measuring clutch as a twist of the torsion shaft. The engine only has to provide the power loss of the two gear boxes in this closed power loop. For most of the standardized test procedures, the following test configuration regarding loading and test gears is chosen. The wheel is mounted on the torsion shaft and has the same speed as the engine. The load is applied in a way that the pinion is the driving gear and the gear is driven. The lubrication type of the test rig can be chosen between dip or spray lubrication. Lubrication with oil and grease is possible. The temperature of the lubricant can be controlled by a heating and cooling system. For spray lubrication, a lubrication device regulates the spray volume of usually 2 l/ min and the temperature. For dip lubrication, heating rods and cooling pipes in the gear box housing are used to control the temperature. The test rig is standardized in DIN ISO 146361 [5] and widely used all over the world for gear testing. 3. Scuffing 3.1 Fundamentals regarding Scuffing Scuffing is an instantaneous failure, where the gear flanks are locally welded together due to the pressure and temperature conditions of the tooth contact. At the same time, there are no protective layers on the flank surfaces present to prevent a metal-to-metal contact. Figure 3.1 shows scuffing on a tooth flank. Figure 3.1 Scuffing on a tooth flank Owing to the kinematics, the local welds are torn apart, as soon as the gear mesh moves on. For a pinion and wheel made of the same material, the partner with positive sliding on the fast moving addendum gains material. The partner with negative sliding on the slow-moving dedendum loses material. The scuffing forms new untempered surfaces, so called friction martensite, with a hardness above 1000 HV. The surfaces of friction martensite remove material from the tooth flanks in consecutive contacts. Below the friction martensite, temperature-influenced zones can be localized, with a reduction of hardness due to short-term tempering. The tooth flank surface and geometry are successively destroyed, the load-carrying capacity near the surface is reduced, and unfavorable residual stresses and notch effects are introduced. This leads to increased dynamics and noise emission as well as to a decreased pitting capacity, with other failure to follow. [1, 21, 23] 3.2 Test-Method for Scuffing The standard test method for determining the scuffing load capacity is described in DIN ISO 146351 [5]. The test is conducted on the FZG back-to-back gear test rig. Standardized test gears of the type FZGA are applied for this test method. The test gears are designed as spur gears with z1 = 16 teeth on the pinion and z2 = 24 teeth on the gear, a normal module of mn = 4.5 mm, a pressure angle of α = 20° and no tooth flank corrections. The center distance is a = 91.5 mm and the effective tooth width is b = 20 mm for both partners. The test gears are made of the case hardening steel 16MnCr5, they are case-carburized to a surface hardness of 60 - 62 HRC and grinded to a surface roughness of Ra1/ 2 = 0,35/ 0,3 ± 0,1 μm. Figure 3.2 shows an exemplary test gear of the type FZG-A. [23] 1. Fachtagung TestRig - Juni 2022 63 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants Figure 3.2 Test gear type FZG-A The scuffing test consists of 12 load stages (LS) with a running time of 15 minutes each. After every stage, the torque of the pinion is increased. The first stage starts with a torque of T1,LS1 = 3.3 Nm and can be raised to a maximum of T1,LS12 = 534.5 Nm at load stage 12. The Hertzian contact pressure is increasing from pC,LS1 = 146 N/ mm2 to a maximum of pC,LS12 = 1841 N/ mm2. The speed of the wheel is n2 = 1.455 rpm, equivalent to a circumferential speed of vt = 8.3 m/ s. The gears are dip lubricated and beginning in load stage 5, the starting oil temperature is heated to ϑoil = 90 °C. Starting with load stage 4, the gears are to be examined after every stage including a close observation of possible damages on the tooth flanks. If the summed up width of the scuffing damages from all teeth is larger than the width of one flank, the test is finished and the last running step is identified as the failure load stage. The failure load stage allows a classification of the lubricant regarding scuffing. The test is ended when the failure criterion has been met and or when the load stage 12 has been completed without meeting the failure criterion. In order to ensure that the damage on the surface is significantly caused by scuffing, light microscopic inspections of the tooth flanks are conducted. For additional information and analysis, the wear of the test gear can be measured by weighing the gears after the last accomplished load stage. Figure 3.3 shows the scuffing test procedure according to DIN ISO 146351 [5] with the weighing of the gears at the end of the test. From the test result of the scuffing test, the scuffing temperature limit θS int according to the integral temperature method can be derived into the scuffing load capacity calculation for gears in practice. For cylindrical gears DIN 39904 [3] or ISO/ TS 633620/ 21 [14, 15] applies, for bevel gears DIN 39914 [4] or ISO/ TS 633620/ 21 [14, 15]. This temperature can e.g. also be used as a basis of a relative comparison of different oils. It is also possible to relatively compare results of different scuffing test methods. Figure 3.3 Scuffing test procedure according to DIN ISO 146351 [5] including weighing of gears at the end of the test The test method according to DIN ISO 146351 [5] is referred to as A/ 8,3/ 90 and used for industrial gear oils. “A” refers to the test gear type FZGA, “8,3” refers to the circumferential speed of vt = 8.3 m/ s and “90” refers to the oil temperature of ϑoil = 90 °C. Based on the test method A/ 8,3/ 90, there are further test methods with intensified, more severe operating conditions for lubricants with higher scuffing load capacities. The intensified operating conditions ensure, that lubricants with high scuffing load capacities are comparable instead of all achieving the load stage 12. An exemplary test method with intensified operating conditions is the test method A10/ 16,6R/ 120 according to DIN ISO 146352 [6], which is used for automotive gear oils such as API GL 4. “A10” refers to the test gear type FZGA with a reduced face width of b = 10 mm, “16,6R” refers to a rotational reversed, increased circumferential speed of vt = 16,6 m/ s and “120” refers to an increased oil temperature of ϑoil = 120 °C. Further scuffing test methods using the FZG back-toback gear test rig are shown in Figure 3.4. Figure 3.4 Test methods for scuffing load carrying capacity using the FZG back-to-back gear test rig 64 1. Fachtagung TestRig - Juni 2022 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants 3.3 Influences on Scuffing 3.3.1 Protective Layers Scuffing is not observed when protective layers prevent metal-to-metal contact of the surfaces [20]. The protective layers can be maintained by the lubricant in different ways as shown in Figure 3.5. Figure 3.5 Protective layers maintained by the lubricant Hydrodynamic lubrication prevents the welding when the hydrodynamic film thickness is greater than the surface roughness. The operating viscosity determined at the bulk temperature of the gear is one of the main influence factors on the film thickness. Lubrication by physically adsorbed layers can occur when polar components in the base oils and additives are physically adsorbed on the surface and thus prevent a metal-to-metal contact. The adhesive forces between the molecules and the surface are reduced at higher temperatures and the protection is reduced. Lubrication by chemical reaction layers is possible at higher temperatures when chemical reactions protect the tooth flank surfaces from scuffing. The high temperatures which are necessary for the chemical film formation are locally and temporarily present in the contact. In the locations of lower temperature, as for example in valleys, hardly any reaction layers can be detected. Furthermore, no reaction layers can be found in scuffed areas, probably due to excessive temperatures and decomposition phenomena. [2] 3.3.2 Lubricant Viscosity The scuffing load capacity of unadditivated mineral oils increases with increasing viscosity. Using an ISO VG 460 oil instead of an ISO VG 46 oil results in a scuffing load capacity three times higher as shown in Figure 3.6 [21]. The increase of the scuffing load capacity for unadditivated oils is almost independent of the pitch-line velocity [1]. Where additive reactions are predominantly responsible for the surface protection, the influence of viscosity is comparatively low [1]. [2] Figure 3.6 Influence of oil viscosity on the scuffing load capacity of unadditivated lubricants [21] 3.3.3 Temperature The temperature is found to have a contradictory effect on the scuffing capacity. With increasing temperature, the lubricant viscosity, the film thickness and thus the scuffing load capacity of unadditivated mineral oils decrease. For oils with additives, the increasing temperature enhances the additive reactivity and compensating or overcompensating the decrease of the viscosity and eventually increasing the scuffing load capacity as shown in Figure 3.7. Figure 3.7 Influence of oil temperature on the scuffing load capacity of an additivated and unadditivated oil [30] The temperature affects the protective layers in various forms such as oil, bulk, flash or total contact temperature. To ensure sufficient protection over a wide temperature range, the different components of the base oil and the 1. Fachtagung TestRig - Juni 2022 65 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants additive package must provide overlapping activity. Figure 3.8 shows the temperature gap between the protective layers due to viscosity and additives. [30] Figure 3.8 Temperature gap between protective layers due to viscosity and additives [30] 3.3.4 Additives The influence of additives on the scuffing load capacity is determined by the type and concentration of the additives. Different additive types show different influences regarding the scuffing performance of the lubricant [1]. While a sulphurised natural oil shows the same or similar scuffing increase over the whole speed range, a 0.2 % concentration of ZnDTP gives a large scuffing improvement in the high-speed range, and a 0.2 % concentration of sulphur-phosphorus gives an even larger scuffing improvement. For an increase of the additive concentration, an increase of the scuffing load capacity is expected. Especially when comparing unadditivated mineral oils with additivated mineral oils, a significant increase of the scuffing load capacity is observed [1]. When comparing additivated oils with increasing additive concentration, an almost linear increase of the scuffing load capacity is followed by a saturation effect at higher additive concentrations. Figure 3.9 shows the influence of a tricresyl phosphate (TCP) additive concentration on the scuffing load capacity. [2] Figure 3.9 Influence of a tricresyl phosphate (TCP) additive concentration on the scuffing load capacity [21] 3.4 Benefits of the Standardized Scuffing Test Benefits of the standardized test method regarding the scuffing load capacity include the following: • The scuffing test can be conducted with the FZG backto-back gear test rig, such as numerous other tests regarding the gear load carrying capacity. • The total running time of the scuffing test takes only 3 hours, examination time excluded. • Lubricants can be classified in relation to one another and the results can be used for comparative assessment and further evaluation. • The failure load stage as a characteristic value can be used for gear ratings and designs according to ISO/ TS 6336-20/ 21 [14, 15]. Part 20 and part 21 of ISO/ TS 6336 cover the calculation of the scuffing load capacity according to the flash and integral temperature method. • Alternative operating conditions can be chosen to achieve reasonable test results for lubricants with different scuffing behavior. • The test procedure as well as the test rig and the test gears are standardized, allowing a common use and a good comparability of the results. 4. Micropitting 4.1 Fundamentals regarding Micropitting Micropitting is a phenomenon which can generally be observed in lubricated, highly loaded rolling and sliding contacts. Micropitting can limit the life time of gears and can thus be included when referring to gear failures. Micropitting can particularly be found on gears with a 66 1. Fachtagung TestRig - Juni 2022 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants high surface hardness together with unfavorable lubricating conditions. Oil viscosity, additives, peripheral speed, tooth flank surface (processing, roughness), heat treatment as well as the operating temperature influence the damage occurrence, development and intensity. Micropitting can be identified with the naked eye as a beamless grey blotch on the tooth flank. With a higher magnification, small-sized cracks and little disruptions are visible. Micropitting normally occurs on the dedendum flank of the driving gear. It propagates gradually over the flank and in extreme cases covers the whole active flank surface. Micropitting can cause profile deviations on the active flanks. In this case dynamical additional forces and gear noise can increase. Figure 4.1 Micropitting on a tooth flank The formation of micropitting can be explained with the following model concept. At first, plastic deformations of the surface roughness peaks take place. On the deformed areas detrimental chemical reactions caused by the lubricant additive package can lead to a reduction of the resistance of the surface layer. Under the influence of mechanical stresses, crack initiation can take place. The crack propagation, which is influenced by the main shear stresses, shows partly endurance failure characteristics. This results in the development of microscopic pits, caused by merging of single cracks, by tearing of undercut material parts or by remigration of cracks to the surface. Profile deviations occur by a continuing repetition of these processes. Consequently, micropitting is a fatigue damage that is initiated directly at the surface of the gear flanks due to stress concentrations at asperity contacts when the gears are operated in the regime of mixed or boundary lubrication. [31] 4.2 Test Method for Micropitting The standard test method for determining the micropitting load capacity of gears is described in the document FVA Informationsblatt 54/ 7 [10] respectively recently published DIN 399016 [7]. The test is conducted on the FZG back-to-back gear test rig. Standardized test gears of the type FZGC are applied for this test method. These test gears are designed as spur gears with z1 = 16 teeth on the pinion and z2 = 24 teeth on the gear, a normal module of mn = 4.5 mm, a pressure angle of α = 20° and no tooth flank corrections. The center distance is a = 91.5 mm and the effective tooth width is b = 14 mm for both partners. The test gears are made of the case hardening steel 16MnCr5, they are case-carburized to a surface hardness of 750 HV and grinded to a surface roughness of Ra1/ 2 = 0,5 ± 0,1 μm. Figure 4.2 shows an exemplary test gear of the type FZG-C. [10] Unlike the test gear type FZG-A, the FZG-C type gears used here are balanced with regard to sliding velocity and therefore are not prone to scuffing failures. Figure 4.2 Test gear type FZG-C The micropitting test consists of a load stage test with 6 load stages and an endurance test with 6 intervals. The running time per load stage is 16 h whereas the running time per interval is 80 h. The load stage test starts with a torque of T1,LS5 = 70 Nm at load stage 5 and can be raised to a maximum of T1,LS10 = 265,1 Nm at load stage 10. The Hertzian contact pressure is increasing from pC,LS5 = 795 N/ mm2 to a maximum of pC,LS10 = 1547 N/ mm2. The speed of the pinion is n1 = 2250 rpm, equivalent to a circumferential speed of vt = 8.3 m/ s. The gears are spray lubricated with an injection volume of 2 l/ min and the oil temperature should be chosen according the operating temperature in the subsequent application of the lubricant. Usual temperatures range between is ϑoil = 40 - 120 °C. Figure 4.3 shows the test procedure of the micropitting test according to FVA 54/ 7 [10] respectively DIN 3990-16 [7]. 1. Fachtagung TestRig - Juni 2022 67 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants Figure 4.3 Test procedure of the micropitting test according to FVA 54/ 7 [10] respectively DIN 3990-16 [7] The test gears are removed from the test rig after every load stage and interval. The profile form deviation of the test gears is measured, flank photos of the micropitted areas are taken and the material loss is weighed. The failure criterion for the load stage test is an average profile form deviation of ffm > 7,5 μm whereas the failure criterion for the endurance test is an average profile form deviation of ffm > 20 μm. The test is additionally stopped, if pitting, scuffing or wear occurs. The test is ended when the failure criterion has been met or when the last load stage has been completed without meeting the failure criterion. Generally, the load stage test and the endurance test are performed. If the failure criterion of the load stage test is met in a load stage ≤ 7, then the endurance test is not conducted. If required, a modified endurance test can be performed on a lower load stage. The result of the load stage test is the failure load stage, the result of the endurance test is information on the progression of the micropitting at a higher number of load cycles. According to the failure load stage, the lubricant can be classified into the classes GFT-low, GFT-medium or GFT-high regarding the micropitting load capacity. GFT is the abbreviation for the German word Graufleckentragfähigkeit meaning micropitting load capacity. Figure 4.4 shows exemplary results of a micropitting test according to FVA 54/ 7 [10] with three exemplary lubricants of the classes GFT-low, GFT-medium and GFT-high. Figure 4.4 Exemplary results of a micropitting test according to FVA 54/ 7 [10] The test method according to FVA 54/ 7 [10] respectively DIN 399016 [7] is referred to as C/ 8,3/ 90. “C” refers to the test gear type FZGC, “8,3” refers to the circumferential speed of vt = 8.3 m/ s and “90” refers to a chosen oil temperature for the lubricant of ϑoil = 90 °C. The complete test C/ 8,3/ 90 needs 6 - 8 weeks of time and approximately 30 l of oil for the spray lubrication system. A shorter alternative to C/ 8,3/ 90 is the so called micropitting short test GFKT according to DGMK 575 [27]. GFKT is the German abbreviation for Graufleckenkurztest, which means micropitting short test. The differences between the micropitting short test and the micropitting test are the dip lubrication and a load stage test with only two load stages. The dip lubrication of the micropitting short test needs 1.5 l of lubricant for the gear box, which is significantly less compared to the 30 l of the micropitting test. For the load stage test, the two load stages 7 and 9 are tested, which results in a maximum duration of 6 days including a repeat test run for the micropitting short test. The micropitting short test cannot replace the micropitting test in terms of information and significance, but can serve as a complementary screening test for the authoritative micropitting test according to FVA 54/ 7 respectively DIN 399016 [7]. Figure 4.5 shows exemplary results of a micropitting short test according to DGMK 575 [27]. Based on the determined failure load stage in the micro-pitting test, a critical specific lubricant film thickness for the investigated lubricant at the present test conditions can be determined. This value can be introduced as a kind of strength value for the lubricant-material paring into the rating method acc. to ISO/ TS 6336-22 [16] . This allows the gear designer to consider the micropitting performance of the used lubricant within the load capacity calculation of gears in practice. Figure 4.5 Exemplary results of a micropitting short test according to DGMK 575 [27] 4.3 Influences on Micropitting 4.3.1 Lubricant With a higher lubricant viscosity an increase in the micropitting resistance can be expected [9]. This is explained by the increasing lubricant film thickness. Further investigations rank the influence of the viscosity in comparison to other parameters as rather low [25]. The 68 1. Fachtagung TestRig - Juni 2022 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants composition of the lubricant is attributed with a high influence on the micropitting resistance [11]. In experimental studies, synthetic base oils showed a reduced formation of micropitting compared to mineral oils. Figure 4.6 shows the influence on the micropitting load capacity of the viscosity and additive package of a lubricant. Regarding lubricant 1b, the additive package of lubricant 3b with a comparable viscosity shows a greater influence on the micropitting load capacity than the influence of the viscosity of lubricant 1a with the same additive package. Figure 4.6 Influence of the viscosity and additive package of a lubricant on the micropitting load capacity Generally speaking, it can be stated that the chemical-physical interactions between additive components and material are of decisive importance for the micropitting load capacity of a gear set. A reliable determination of the influence of the additive on the micropitting load capacity must be carried out experimentally by application of the standardized micropitting tests. Investigations with dipand spray-lubricated gears were carried out [25]. The dip lubricated tests showed less pronounced micropitting under otherwise identical operating conditions. The less pronounced micropitting for dip-lubricated gears is attributed to the better cooling. [18, 31] Operating Conditions The tooth flank pressure has a fundamental influence on the occurrence of micropitting. Although micropitting can already occur at relatively low loads and after a short running time, higher torques or pressures on the tooth flanks generally lead to an increased formation of micropitting [8, 11]. An increasing test temperature leads to a decreasing film thickness and is generally considered to have a negative influence on the micro-pitting load capacity. It should be noted, however, that if the temperature is lowered too much, additives cannot develop their full protective effect. Figure 4.7 shows the influence of the test temperature on the micropitting load capacity of different lubricants. For lubricant M2, the additives obviously compensate the decreasing film thickness and thus lead to higher micropitting load capacity at higher temperatures. The majority of the lubricants show a decreasing micropitting load capacity with an increasing temperature. As the operating temperature is an independent influence parameter, it is strongly recommended to run the micropitting test in the temperature range of the subsequent lubricant application. Figure 4.7 Influence of the test temperature on the micropitting load capacity With the increase of the circumferential speed and consequentially a greater lubricant film thickness, an increase of the micropitting load capacity can be expected. However, very high speeds for dynamic reasons and increase heat input can have negative effects. [11, 31] 4.3.2 Gear Geometry and Manufacturing The reduction of the area of negative sliding and the reduction of the absolute sliding velocities in this area, reduce the formation of micropitting. Accordingly, a positive profile shift can reduce the formation of micropitting. Profile corrections such as tip reliefs have a favorable effect on the micropitting load capacity, if the maximum pressure is shifted to a tooth flank area that has a significantly lower tendency to micropitting. With regard to a size influence, it was shown that an increase in the number of teeth and a reduction in the module can have a positive effect on the micropitting load capacity. The positive effect is associated with the reduction of the meshing section and the reduction of the absolute sliding velocities. [25, 31] The surface roughness of the tooth flanks was found to be a significant factor regarding the development of micropitting. A high surface roughness has a negative effect on the micropitting load capacity, as do pairs of smooth and rough gears. The effect of the surface roughness can be explained by excessive micro stresses which occur at the roughness peaks. [9, 11, 18] 4.4 Benefits of the Standardized Micropitting Test Benefits of the standardized test method regarding the micropitting load capacity include the following: • The micropitting test can be conducted with the standardized FZG back-to-back gear test rig, such as numerous other tests regarding the gear load carrying capacity. 1. Fachtagung TestRig - Juni 2022 69 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants • Besides the result of the failure load stage as characteristic value, the test gives information on the development of the micropitting formation. • Lubricants can be classified in relation to one another and the results can be used for comparative assessment and further evaluation. • The failure load stage as a result can be used for gear ratings and designs according to ISO/ TS 6336- 22 [16]. Part 22 of the ISO 6336 series covers the calculation of the micropitting load capacity. • The micropitting short test can be used as a complementary screening test for the authoritative micropitting test. • The test procedure as well as the test rig and the test gears are standardized, allowing a common use and a good comparability of the results. 5. Further Test Procedures 5.1 Wear Wear is the continuous removal of material from the surface of the gear flanks with each revolution. Wear occurs on the addendum and dedendum of the tooth flank. In more advanced cases of wear, a horizontal line is revealed due to the rolling rather than sliding at the pitch diameter. Figure 5.1 shows wear on a tooth flank including a pronounced horizontal line at the pitch diameter. Figure 5.1 Wear on a tooth flank including a pronounced horizontal line at the pitch diameter (left arrow) Undesirable consequences of wear are the change in the profile form and the wearing off of the hardened case. Desirable consequences of wear are the smoothing of the tooth flanks and the improving of the contact pattern due to preconditioning, so called “running in”. Consequences of wear can be the increase of additional dynamic forces and in well-advanced stages, wear can lead to tooth root breakage. Figure 5.2 shows the schematic development of a gear tooth affected by wear Figure 5.2 Development of a gear tooth affected by wear The lubricant has a significant influence on the wear behavior, especially the viscosity of the base oil and the additives of the lubricant. The lubricant influence on the wear behavior can be determined using the standardized test method according to DGMK 377 [29]. This standardized wear test can be conducted with the FZG back-toback gear test rig as well. The results of the wear test enables a comparison of the wear behavior of different lubricants by classifying the lubricants in defined wear categories. The linear wear coefficients from the wear test can be used to calculate the expected wear for a field application according to the method of Plewe [22]. [19, 26] 5.2 Pitting Pitting is a fatigue failure on the tooth flank due to the rolling-sliding contact of the tooth flanks. The crack initiation is at or near the surface and the crack propagation leads into the material at a characteristic angle. Due to the crack propagation and persisting load on the damaged tooth flank, shell shaped material losses can occur. Consequences can be an increase of noise and vibrations and in case of a progressive pitting failure development, tooth breakage. Pittings predominantly occur in areas with negative specific sliding. The size and shape of the pittings vary dependent on the gear material. Figure 5.3 shows an exemplary pitting on a tooth flank of a case carburized test gear. Figure 5.3 Pitting on a tooth flank of a case carburized test gear (additional micropitting on tooth flank) 70 1. Fachtagung TestRig - Juni 2022 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants Main influence parameters on pittings include the Hertzian contact load, the tangential load, the sliding due to the gear geometry, micropitting formation, the temperature, the material, the heat treatment and the lubricant. A high lubricant viscosity reduces the risk of pitting formation. Additives such as EP-additives can effectively increase the pitting load capacity. Standardized rating methods for the pitting load capacity such as ISO 63362 [17] only consider the nominal viscosity of the lubricant. Chemical related influences of additive components are neither taken into account nor predictable. Thus, the experimental evaluation of the lubricant pitting performance is necessary. Standardized test methods exist in the form of the pitting test according to FVA 2/ IV [24] respectively CEC L-108-19 [28] which can be performed with the FZG back-to-back gear test rig. The results of the pitting test can be used for a comparison of the pitting behavior of different lubricants by classifying the lubricants according to the pitting lifetime. The surface roughness and micropitting formation have a critical influence on the test result. To reduce these interfering influences, a new CEC test method CEC L-108-19 [28] based on the pitting test according to FVA 2/ IV [24] was designed. Main changes include a reduced surface roughness of the test gears and defined reference lubricants including precision data regarding repeatability and reproducibility of the test method. [12] 6. Conclusion The FZG back-to-back gear test rig as a multi-purpose test rig allows to investigate different gear failures. Gear failures affected by lubrication include scuffing, micropitting, wear and pitting. In order to systematically evaluate the lubricant performance regarding those gear failures, standardized test methods were developed at the FZG. Scuffing can be described as an instantaneous welding of the mating surfaces of two gears. Micropitting can be described as numerous small cracks due to surface fatigue, visible as a dull matte surface on the gear flank. Wear is the continuous removal of material from the surface of the gear flanks with each revolution. Pitting is a fatigue failure on the tooth flank which leads to shell shaped material losses. The lubricant with its base oil, viscosity and especially additives affects the above mentioned gear failures. The tribological interaction between lubricant and gear cannot be predicted reliably. Thus, an experimental evaluation of the lubricant performance is necessary. The standardized FZG test methods allow for a relative comparison of gear lubricants as well as a systematic classification. The results of the test methods include characteristic values which can be used for the rating and designing of gear sets according to the standard series ISO 6336 [13]. Besides scuffing, micropitting, wear and pitting, the FZG back-to-back gear test rig can be used to determine gear characteristics such as gear efficiency, vibration as well as further fatigue failures such as tooth root breakage and tooth flank fracture. Furthermore the FZG test rig can be used to investigate and quantify influences from different gear materials, heat treatments, manufacturing processes, gear geometries or operating conditions on the gear performance. Consequential, strength numbers and characteristic values can be derived, which are typically required for a reliable gear rating and design. The test methods are constantly scrutinized and if needed enhanced. The modification and variation of operating conditions realize a broad bandwidth of lubricants that can be tested. The steady development enables a long-lasting comparison of the lubricants and a sustainable evaluation of their influence on the gear load carrying capacity. The standardized test rig, well defined test procedures and specially designed test gears that are suitable to investigate the desired failure limits allow a widespread use of the FZG back-to-back gear test rig and the FZG gear lubricant test methods. They are basic requirements for reliable and repeatable test results to characterize the performance of modern gear lubricants. 1. Fachtagung TestRig - Juni 2022 71 FZG Test Methods - Testing and Characterization of High-Performance Gear Lubricants Bibliography [1] Collenberg, H. F., Untersuchung zur Fesstragfähigkeit schnelllaufender Stirnradgetriebe,” Dissertation, Forschungsstelle für Zahnräder und Getriebebau, Technische Universität München, 1990. [2] Collenberg, H. F., Schlenk, L., Michaelis, K., and Höhn, B.-R., “Effect of Temperature on the Scuffing Load Capacity of EP Gear Lubricants,” Tribotest Journal, 7(4), pp.317 - 332, 2001. [3] Deutsches Institut für Normung e.V. 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