Highly loaded hypoid gears, which are preferably used in vehicle axle drives, exhibit an unfavorable combination of high sliding velocities and high contact stresses in tooth contact with regard to the failure mode scuffing. For this reason, the scuffing load carrying capacity is a decisive design criterion in the development of hypoid gears. In addition to the gear geometry, the entire tribological system of the gearbox must also be considered. Beyond the loads and speeds that occur during gear operation, the lubricant used and its additives have a significant influence on scuffing load capacity. Since it is generally impossible to determine the influence of the lubricant on the load carrying capacity of gears on the basis of physical or chemical oil data alone, experimental test methods are necessary.
In order to be able to investigate and classify the scuffing resistance of hypoid oils, an experimental test method was developed based on theoretical investigations. To determine the test procedure and the corresponding load stages in the newly developed test, the flash temperature according to ISO/TS 10300- 20:2021 was used as the relevant variable. In order to achieve a reproducible test procedure, a running-in procedure was specified based on the results of experimental investigations on the FZG hypoid back-to- back test rig.
This article presents the development and detailed description of a new test method for classifying and investigating the scuffing resistance of highly loaded hypoid oils, as well as initial experimental results.
Hypoid gears are classified as angular gears. Due to their geometry, they are, in comparison to spur or bevel gears, characterized by a sliding velocity component both in the direction of the tooth profile and in the longitudinal direction of the tooth. As a result, hypoid gears exhibit an increased overall sliding velocity during tooth contact. Since hypoid gears have a wide range of application in axle drives of vehicles, extreme operating conditions are to be expected, including high speed and shock loading. The combination of high speeds resulting in high sliding velocities and simultaneous shock loadings or high loads subjects hypoid gears to operating conditions with a high risk of scuffing.
High load carrying gear oils are used to increase the scuffing load carrying capacity of hypoid gears. These oils are usually enriched with a large quantity of additives. The performance of a lubricant is influenced by the base oil type, the oil viscosity, the additive types, and the quantity of additives . Since it is generally impossible to determine the influence of the lubricant on the load carrying capacity of gears on the basis of physical or chemical oil data alone, experimental test methods are necessary.
This article presents the development and detailed description of a new test method used for classifying and investigating the scuffing resistance of highly loaded hypoid oils, as well as initial experimental results.
2 Failure mode scuffing
Scuffing is a type of spontaneous damage in which the tooth flank surfaces are welded due to an excessively high local temperature occurring during tooth contact. The relative movement of the tooth flanks of the pinion and wheel causes immediate separation of the welded surfaces. The resulting scuffing marks always develop in the direction of the sliding velocity and occur in the corresponding flank areas of the pinion and wheel. Given a pinion and wheel made of the same material, the partner undergoing positive sliding gains material, and the partner undergoing negative sliding loses material. [2-3]
Figure 1 shows an example of scuffing failure at a hypoid gear flank. The scuffing marks are oriented in the direction of the sliding velocity.
Metallographic examinations of flank areas with scuffing show friction martensite with an underlying tempering structure is present at the surface. This suggests high-temperature exposure above the austenitizing temperature of about 800°C, with subsequent quenching and rehardening. 
In addition to geometrical gear dimensions and gear set operating conditions, lubrication conditions also have a relevant impact on the failure mode scuffing. To avoid scuffing, metallic contact between sliding elements must be averted. Increasing oil viscosity facilitates separation of the tooth flanks during tooth contact. However, an increase in oil temperature causes a drop in oil viscosity, and thus an increase in the risk of scuffing. To increase the scuffing load carrying capacity of a gear oil, extreme pressure additives (EP) are therefore added to the oil. When the additives are activated by appropriate contact temperatures, they form a tribological protective layer that prevents metallic contact between the tooth flanks. This increases the scuffing load carrying capacity of the gear. 
Experience shows the scuffing load capacity of a gear oil increases with rising anti-wear (AW) and EP additive concentration . Gear oils of classification API GL-5 should be used for hypoid gears subjected to highly loaded/shock conditions .
The current, internationally accepted calculation method for determining the scuffing load carrying capacity of bevel and hypoid gears is ISO/TS 10300-20 . This method is based on research project FVA 519 . An elementary input variable is the scuffing load carrying capacity of the gear oil represented by the permissible contact temperature, which can only be determined experimentally.
3 Current test methods for determining the scuffing load carrying capacity of gear oils
The current, globally recognized test for examining the scuffing load carrying capacity of highly additivated gear oils with respect to their applicability in highly loaded hypoid gear transmissions is the L42 test. In this test, gear oil is evaluated in an axle test rig at high circumferential speeds and shock loads. The test method is defined in ASTM standard D7452 . To pass the L42 test, the tooth flanks of the test hypoid gear may have a maximum scuffing area, as compared to the average scuffing area of the passed reference oils used to calibrate the test rig. Among other things, passing the L42 test is necessary for the test oil to receive the API GL-5 classification . The L42 test classifies gear oils with regard to their applicability, but it does not provide information on the permissible contact temperature.
According to the current state of the art, there are a large number of different test methods which — with as little effort as possible and small quantities of oil required — enable a conclusion to be made regarding the scuffing load carrying capacity of lubricants. Figure 2 shows a selection of possible oil test methods.
However, Wirtz  showed by way of experimental investigations that no correlation can be established between the results on simple testing machines and the behavior of the lubricants during gear tooth contact. This shows that only test methods using gears enable reliable applicability of the determined lubricant strength values to practical gears. The test method selected can be of great importance when testing a lubricant, since the given method can have an elementary effect on the results and their load carrying capacity for applicability to practical gear units. 
Spur gears have been used to develop various methods for testing the scuffing load carrying capacity of gear oils, e.g. the standardized test methods A/8.3/90  and A10/16.6R/90 . The preceding are step tests using an FZG back-to-back test rig. The load stages are defined in DIN ISO 14635-1:2006 , from load stages 1 to 12. However, since both test methods allow neither differentiation of API GL-4 and GL-5 oils nor determination of the scuffing load carrying capacity of an API GL-5 oil, the S-A10/16.6R/90 shock test  was developed. The test gear, A10, as well as the load stages are defined in the DIN ISO 14635- 2:2010  standard. In contrast to the step tests, the expected failure load stage is applied directly. So, the test gear undergoes no running-in procedure and is, therefore, significantly more vulnerable to scuffing . If the test gear passes through the load stage without damage, the oil then achieves a PASS. If scuffing occurs, then it achieves a FAIL. The test conditions are:
- Wheel drives pinion.
- Pitch line velocity 16.6 m/s.
- Oil starting temperature of 90°C.
No oil cooling takes place during the test run. The oil start temperature can be varied. Figure 3 shows test results using various API GL-4 and GL-5 oils. Gear oils classified as API GL-4 show a PASS in load stage 8 and a FAIL in load stage 9. GL-5 lubricants show a PASS in load stage 9 and a FAIL in load stage 10 .
Another test for examining hypoid gear oils is the A44/Cr hypoid scuffing test. The latter was developed by Langenbeck  and modified by Richter . This procedure tests the scuffing load capacity of hypoid gear oils using a hypoid test gear with a hypoid offset of 44 mm. The A44/Cr test is a step test on a hypoid back-to-back test rig. The operating conditions are summarized in Table 1. 
Experimental investigations by Reimann, et al.  using the A44/Cr test show it is possible to differentiate between API GL-4 and GL-5 oils. However, since gear oils of the API GL-5 class can pass the test without damage, it is not possible to differentiate between these oils on the basis of their scuffing load carrying capacity or to determine their exact permissible contact temperature. Currently, it isn’t possible to carry out the test because the A44 test hypoid gear is no longer available. Also, there is no information on the microgeometry design of the A44 test hypoid gear for an exact redesign of the hypoid gear. The basic geometry data are presented in Table 2.
4 Development of the new test method
The aim of the new test method is to enable the testing and differentiation of hypoid gear oils with regard to their scuffing resistance. In addition, the test method should allow a differentiation to be made between API GL-4 and GL-5 oils. According to the hypoid oil field of application, a hypoid gear is used as the test gear. Furthermore, it should be possible to determine the permissible scuffing temperature as a strength value for the scuffing resistance of a gear oil. The permissible scuffing temperature can be calculated according to ISO/TS 10300-20  as a function of the failure load stage achieved in a suitable scuffing test.
4.1 Test equipment
The testing machine used is a hypoid-back-to-back test rig. “It works according to the principle of a closed power circuit (see Figure 4). Two hypoid gear sets, the test gear set and the slave gear set, are connected to a spur gear set over to parallel shafts by use of a mechanical load clutch, which allows the application of a defined torque. The torque applied to the test pinion is measured using strain gauges that are mounted on the torsional shaft. The electric motor only injects the power losses induced by the components of the hypoid-back-to-back test rig. The test rig offers the possibility of injection or dip lubrication as well as a combination of both types of lubrication. The oil temperature in the gearboxes can be regulated by use of oil supply units.” 
The test hypoid gear set is the G44 hypoid test gear and is made of 18CrNiMo7-6 steel. The basic geometry data are presented in Table 2. The load carrying capacity of the G44 hypoid test gear was previously investigated in the research project FVA 411 . The test gear geometry is designed with the aim of preferably causing failures on the gear flank. Failures such as tooth root breakage should be reliably avoided by virtue of the design.
Four different gear oils based on mineral oil were used as test oils.
Table 3 provides an overview of the oils examined. Oils 1 and 2 were based on the standard oil FVA 3 with different concentrations of the EP additive “Anglamol 99.” Oil 1 had a concentration of 4% Anglamol 99, and oil 2 had 6.5%. Regarding oils 3 and 4, neither the composition nor the type of the additive was known. Oil 2 and oil 4 were classified according to API GL-5 and had passed the L42 test. Oil 1 was also tested in the L42 test and failed. Oil 1 exhibited performance comparable to that of an API GL-4 oil. According to the manufacturer, oil 3 should be assigned to the API GL-3/4 class. Consequently, it was assumed that oil 3 would fail the L42 test.
4.2 Definition of the gear running-in procedure
Since the flank roughness has a significant influence on the scuffing load carrying capacity of gears, a gear running-in procedure was defined . The aim of the gear running-in procedure was to minimize the influence of manufacturing-related fluctuations in the flank roughness of the test gears on the test result. The gear running-in procedure smooths the tooth flanks of the test gears to a comparable roughness level, depending on the test oil. To ensure a complete gear running-in procedure, the pinion torque, pinion speed, duration of the running-in, and choice of oil were defined on the basis of calculation results of a loaded tooth contact analysis (LTCA), findings from the state of the art, and experimental investigations.
The drive torque selected should be as high as possible compared to the test torque in order to smooth as large a flank area as possible, i.e., ideally the complete contact pattern under maximum test load. In combination with the drive torque, the selected speed should be as low as possible in order to allow a thin lubricant film thickness ensuring removal of the roughness peaks. A high load also favors the generation of a thin lubricant film. The rotational speed at pinion is set to 120 min-1 since operation of the test rig is possible without drive motor control problems. A torque of 140 Nm was defined at the pinion by way of a local load carrying capacity recalculation based on tooth contact analysis. Particular care was taken to ensure no scuffing occurred during running-in when using an API GL-4 oil. Figure 5 shows the local safety factors against scuffing on the coast flank of the G44 test hypoid gear. The safety factors against scuffing were calculated according to the calculation method developed in the research project FVA 519 . It can be clearly seen that the local safety factors against scuffing in the entire tooth contact were above 1.5, so no scuffing should occur. However, slight scuffing did occur during experimental investigations using test oil 1 (API GL-4). The pinion torque was therefore reduced to 100 Nm.
A simulative analysis of the contact pattern development at the load stages in the main test phase using TCA showed that the most scuffing-critical flank area was largely run in at a torque at pinion of 100 Nm.
In principle, two options were available for the choice of running-in oil, i.e., either a high load carrying capacity gear oil to avoid scuffing during running-in or using the test oil. Klein  performed experiments on a bevel gear and a hypoid gear. The high load carrying capacity gear oil consisted of the identical base oil and additive package as the test oil, but it had a higher amount of the additive package, hence also a higher scuffing load carrying capacity. After running-in, both gear sets (running-in with the test oil and with the high load carrying capacity gear oil) exhibited a comparable flank roughness. It can be concluded that sufficient smoothing of the tooth flank is possible using both oils. However, experiments on discs and spur gears have shown that, in addition to the surface roughness, the damage and friction behavior is largely dependent on tribo-induced layers that can already appear on the flanks during the running-in process . The additivation in the different gear oils may lead to different tribo-induced layers. Thus, the use of a separate high load carrying running-in oil did not prove to be expedient.
The necessary duration of the running-in procedure was determined experimentally. For this purpose, the running-in procedure was interrupted at defined intervals, the wheel removed, and the flank roughness measured on a Klingelnberg P40 gear measuring center. The roughness measurements were performed at a measuring distance of 4.8 mm and a cut off of 0.8 mm in the profile direction at the center of contact pattern on three teeth distributed over the circumference. Figure 6 shows the results of the flank roughness measurements during running-in using the test oils. In the tests using oils 2 and 3, roughness measurements were taken after 100,000, 150,000, and 200,000 load cycles at pinion. A smoothing of the tooth flank took place and an almost constant roughness level was already established in the first 100,000 load cycles at pinion. A running-in duration of 150,000 load cycles at pinion was specified, in order to ensure complete running-in of the gear teeth. The tests using oils 1 and 4 confirmed sufficient smoothing of the tooth flank had indeed taken place after 150,000 load cycles at pinion.
The following list summarizes the defined operating conditions for the gear running-in procedure:
- Torque at pinion: 100 Nm.
- Rotational speed at pinion: 120 min-1.
- Duration: 150,000 load cycles at pinion.
- Regulated oil temperature at 90°C.
- Use of the test oil for running-in.
After the running-in procedure, the oil was changed in order to remove wear particles.
4.3 Definition of the operating conditions of the main test phase
The operating conditions were oriented by the existing modified A44/Cr hypoid scuffing test. Since the A44 and G44 hypoid test gears were of comparable size, the rotational speed at pinion was set to the maximum possible test rig rotational speed of 4,550 min-1. In order to provoke scuffing, the scuffing test was performed on the coast flank and wheel drives pinion . Applied to a vehicle, this means forward motion with engine braking or recuperation in an electric powertrain. To transfer the load stages of the A44/Cr hypoid scuffing test to the more scuffing-critical G44 test hypoid gear, the contact temperature was used as a comparative parameter, because the latter is the decisive influencing parameter with regard to scuffing. The approach chosen for determining the main test phase load stages in the new test method using the G44 test hypoid gear can be divided into three steps:
1. Best possible redesign of the A44 hypoid gear.
2. Calculation of the maximum contact temperature per each load stage of the A44/Cr scuffing test.
3. Calculation of the load stages for the G44 hypoid test gear.
Figure 7 illustrates the approach chosen.
The geometry of the A44 hypoid gear must be known in order to calculate the contact temperatures occurring during each load stage in the A44/Cr scuffing test. Given that A44 hypoid gear was not available and no microgeometry information was available, an exact redesign of the A44 hypoid gear was not possible. However, an attempt was made to achieve the best possible approximation. An initial rough design was performed on the basis of the known macrogeometry data and manufacturing processes. A best possible reproduction of the ease-off was made on the basis of contact pattern photos.
The next step was to calculate the maximum contact temperature per load stage of the A44/Cr scuffing test. Since the microgeometry of the A44 hypoid gear was not precisely reproduceable, the maximum contact temperatures were calculated using the simplified calculation method according to FVA 519 . This calculation method basically agrees with the standardized calculation method according to ISO/TS 10300-20 . The advantage is the simplified calculation method enabled a very good determination of the contact temperature in the absence of exact microgeometry knowledge. Since the tooth flank stress has an influence on the contact temperature and is strongly affected by the effective face width, the effective face width was determined using a replica of the A44 hypoid gear by means of an LTCA. The maximum contact temperature was determined using the effective face width as basis for calculation.
Possible deviations resulting from the redesign of the microgeometry and possibly having a significant influence on the calculation result are therefore not considered.
Finally, the load stages for the new test method were determined. The iterative procedure shown in Figure 7 was followed for this purpose. The iteration variable was the pinion torque. The difference between the maximum contact temperature of the A44 hypoid gear and the G44 hypoid gear was used as the abort criterion for the iteration. The abort criterion was reached when the difference between the maximum contact temperatures of the A44 and G44 hypoid gear was less than 1%. The contact temperatures of the G44 test gear were determined in the same way as the contact temperatures of the A44 test gear. The simplified calculation method according to FVA 519  was also used to ensure a consistent calculation process between the recalculation of the A44 and G44 hypoid gears. Reaching the abort criterion defined the pinion torque of the G44 hypoid gear for the corresponding load stage.
Table 4 summarizes the 14 defined load stages for the new test method using the G44 hypoid gear. Since the pinion torque over the load stages can be described by a polynomial function, and the G44 hypoid gear has sufficient safety with regard to other types of damage considering the number of load cycles per load stage, it was possible to extend the load stages. Each load stage was run for 10 minutes at an oil start temperature of 90°C. The oil temperature was not controlled during the test run and was able to escalate freely. The lubrication type used was dip lubrication.
4.4 Evaluation of a test run
Photographic documentation of the tooth flanks was made after each load stage and used to evaluate the test. The evaluation was performed on the wheel because it was more accessible to the camera in the gearbox. If the scuffing area occupied at least 5% of the contact pattern under load, the load stage was defined as the failure load stage. Figure 8 shows an exemplary measurement.
The 5% criterion was determined experimentally for the G44 hypoid gear using all of the test oils. Figure 9 shows the development of scuffing area in relation to the contact pattern under load over the load stages of the new test method using the test oils. Oils 1 and 3 were tested twice. In some cases, minimal scuffing was identified during the lower load stages. However, when the scuffing area exceeded 5% of the contact patter under load, it increased sharply during the next load stages. Thus, the 5% threshold was defined as a failure criterion. The interpretation of the experimental results on the scuffing load carrying capacity of the test oils is presented in section 5.1.
5 Initial experimental results
5.1 Summary and interpretation of initial experimental tests
Based on the evaluation shown in Figure 9, the determined failure load stages of the test oils are summarized in Figure 10. According to the API GL classification, oil 3 (API GL-3/4) showed the lowest failure load stage, followed by oil 1 (API GL-4) and oils 2 and 4 (both API GL-5), which showed the highest failure load stage. In one test run using oil 3, no flank documentation of sufficient quality was available for evaluating the third load stage, so the failure load stage could have been either three or four. In Figure 10, this fact is indicated by an error bar, and in Figure 9 by a dashed line.
A repeat test was performed in each case using oils 1 and 3. In Figure 9, the scuffing area varied between the two tests with the same oil, but an equal failure load stage was nevertheless determined. This indicates a presumably good level of repeatability when using the new test method and confirms the suitability of the defined failure criterion for the G44 test hypoid gear. Furthermore, it was possible to generate different failure load stages within oils of the API GL-5 class, so the test method permitted the comparability of API GL-5 oils based on their scuffing load carrying capacity. Based on the test results shown, oils that can pass at least load stage five without damage were presumed to have the scuffing load carrying capacity of an API GL-5 oil. However, further tests will be necessary in order to confirm this.
In experiments using the S-A10/16.6R/90 spur gear scuffing test, oils 2 and 4 demonstrated a lower scuffing load carrying capacity than other API GL-5 oils. However, the new test method has up to 14 load stages to ensure the differentiation of hypoid oils.
5.2 Findings from experimental investigations
The G44 test hypoid gear exhibited wheel tooth tip contact at the higher load stages. This result was observed in single tests starting at load stage 6. Due to the increase in flank contact stresses at the tooth tip of the wheel combined with high sliding velocities, scuffing also occurred at that location. Figure 11 shows an example of the situation described on a G44 test gear that had been driven.
The scuffing marks at the wheel tooth tip caused by the excessive contact stress was not considered in the evaluation of the scuffing area in order to avoid falsification of the test results. Since the tooth tip of the wheel is in mesh in the outgoing tooth contact and therefore has no influence on the lubricant film formation in the remaining tooth contact, this was considered permissible. However, in regard to standardizing the new test method, it will be necessary to adjust the microgeometry of the coast flank of the G44 hypoid test gear.
6 Summary and outlook
This article presents the development of a new scuffing test method used for investigating the scuffing load carrying capacity of gear oils with high additivation. The new test method consists of a running-in procedure and a step test of up to 14 load stages as the main test phase, using a hypoid test gear. The test is performed on a hypoid back-to-back test rig. The operating conditions were determined based on the current knowledge available, local and simplified load carrying capacity calculations, and experiments. In addition, a failure criterion was defined on the basis of experimental findings. If the scuffing area exceeded 5% of the load carrying flank area, the corresponding load stage was defined as a failure load stage.
Initial experiments have shown it is indeed possible to differentiate amongst gear oils with high additivation relative to their scuffing load carrying capacity. Oils that can pass at least load stage five without damage are presumed to have the scuffing load carrying capacity of an API GL-5 oil. However, further tests will be necessary in order to confirm this conclusion. Based on the initial test results, the new test method achieved consistent results. Furthermore, it was observed that, in regard to standardization of the new test method, it will be necessary to adjust the microgeometry of the coast side of the test hypoid gear.
The authors would like to thank for the sponsorship and support received from the Bundeswehr Research Institute for Materials, Fuels, and Lubricants (WIWeB).
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Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Suite 500, Alexandria, Virginia 22314. Statements presented in this paper are those of the authors and may not represent the position or opinion of the American Gear Manufacturers Association. (AGMA) This paper was presented October 2022 at the AGMA Fall Technical Meeting. 22FTM01