The results of the gear investigations were made available by Reintrieb GmbH (Vienna) for scientific evaluation and publication. 2 Test Methods, Test Rigs and Test Gears Test Methods The test methods presented in the following cover different gear failure modes, such as wear, scuffing, pitting and tooth root breakage [12]. Tooth root breakage and Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 1 Introduction According to the current state of the art power gearboxes must be lubricated, typically with a mineral or synthetic based oil. Since mineral or synthetic oils can lead to the pollution of the environment due to leakage, damages or improper disposal the application of such lubricants can be limited. If water would be suitable for lubrication, significantly environmental-friendlier gearboxes could be created. Figure 1 shows a possible application of gear boxes with water lubrication, e.g. a cruise ship entering endangered environments such as a fjord. Investigations on water and water-based gear lubrication show, that the lubricating film thickness is significantly lower compared to mineral or synthetic oil [2-6]. Due to the thin lubricating film thickness, impermissible scuffing and wear is expected. To cope with such a thin film thickness, extraordinary high surface hardnesses would have to be achieved. Tungsten carbide compositions are characterized by such extraordinary high surface hardness [7-10]. As tungsten carbide is not soluble in tungsten carbide, scuffing characterized by a welding of the surfaces is expected to be avoided. The usage of the tungsten carbide composition as gear material is equally aimed at achieving the necessary resistance against water corrosion which conventional steel does not provide. In this paper, the material-lubricant-systems of water lubrication combined with gears made from different tungsten carbide compositions are investigated regarding their behavior concerning wear as well as further gear failures such as scuffing, pitting and tooth root breakage. The innovative combination of tungsten carbide composite gears with water lubrication is a pioneering step towards sustainable gear sets and has been patented [11]. Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water Karl Jakob Raddatz, Thomas Tobie, Klaus Michaelis, Karsten Stahl* In this paper, the material-lubricant-systems of water lubrication combined with gears made from different tungsten carbide compositions are investigated regarding their behavior concerning wear as well as further gear failures such as scuffing, pitting and tooth root breakage. Keywords gears, boxes, synthetic based oil, mineral based oil, water-based, tungsten carbide, lubrication, scuffing, wear, pitting, tooth root breakage Abstract * Karl Jakob Raddatz (corresponding author) Thomas Tobie Klaus Michaelis Karsten Stahl Gear Research Center (FZG) Technical University of Munich, Garching, Germany Figure 1: Possible application of sustainable gear boxes with water lubrication, e.g. a cruise ship entering an endangered environment such as a fjord [1] pitting are fatigue failures, while scuffing and wear are mainly tribological failures. Figure 2 shows the investigated gear failures as well as their respective standardized test methods. Since wear is the critical gear failure mode for water lubricated gears, the wear amount was additionally measured during the pitting tests. This specific wear of the test gears was named ”high-speed wear” due to its higher circumferential speed compared to the conventional “slow-speed wear”. The experimental investigations were performed in the following order and are based on established and standardized test methods: ■ Tests based on the standardized test procedure DIN ISO 14635-1 [13] regarding the scuffing load carrying capacity. ■ Tests based on the standard test procedure FVA-Information 0/ 5 [14] regarding the tooth root bending strength. ■ Tests based on the standard test procedure C- PT/ 8,3/ 90/ 9: 10 according to FVA 2/ IV [15] regarding the pitting and high-speed wear (wheel rotational speed of n 2 = 1500 min -1 ) load carrying capacity. ■ Tests based on the standard test procedure C/ 0,05/ 90: 120/ 12 according to DGMK 377 [16] regarding the low-speed wear (wheel rotational speed of n 2 < 500 min -1 ) load carrying capacity. The standard test procedures C-PT/ 8,3/ 90/ 9: 10 and C/ 0,05/ 90: 120/ 12 are designations for the following test conditions: test gear type / circumferential velocity / temperature of the lubricant / load stage (KS). Table 1 shows the test conditions for the different test methods. Test Gears For the experimental investigations regarding scuffing, pitting and wear, a test gear set with pinion and wheel of the FZG test gear geometry Type C is used. For the experimental investigations regarding tooth root breakage, the wheel of the Type C test gear set is used. Figure 3 Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 Figure 2: Investigated gear failures and their respective standardized test methods Figure 3: Exemplary photographic documentation of the test gear set Type C Test Speed Temperature Load Stages Torque Hertzian Pressure [17] Nominal Contact Stress [18] Symbol v t in m/ s T in °C KS T 1 in Nm p c in N/ mm 2 H0 in N/ mm 2 scuffing 8,3 30 - 75 (no cooling) KS 1 - KS 10 3,3 Nm - 266 Nm Material A KS 10: 2545 Material A KS 10: 2421 pitting and highspeed wear at n 2 = 1500 min -1 8,3 40 - 50 (active cooling) KS 4 KS 5 KS 6 KS 7 KS 8 KS 9 38 70 99 133 172 216 Mat. C / Mat. D 886 / 849 1202 / 1152 1430 / 1370 1657 / 1588 1884 / 1805 2112 / 2024 Mat. C / Mat. D 843 / 808 1144 / 1096 1360 / 1304 1579 / 1511 1793 / 1718 2010 / 1926 low-speed wear at n 2 < 500 min -1 0,05 0,57 2,76 25 - 30 (active cooling) Note: The Hertzian pressure p c according to Niemann [17] as well as the nominal contact stress H0 according to ISO 6336 [18] differ in their values due to the different modu lus of elasticity E documented in Table 3. Table 1: Test conditions for the different test methods Test Rigs The investigations for the evaluation of the gear failure tooth root breakage were conducted on a pulsator test rig and for evaluation of the gear failures wear, scuffing and pitting on a standard FZG back-to-back gear test rig. On the pulsator test rig a pulsating load is introduced on the gear teeth by clamping jaws. The pulsating load leads to a defined and calculable tooth root stress. Detailed information on the test rig and its function can be taken from literature [20, 21]. The test is performed until a tooth root breakage occurs or until the maximum load cycle number is reached. For most of the presented tests the maximum load cycle number usually was N max = 10 5 load cycles, only for selected tests the maximum load cycle number was increased up to N max = 6 · 10 6 load cycles. Figure 4 shows a schematic drawing of a pulsator test rig. On the FZG back to back test rig, a load is applied to the test gears by a closed power-loop implemented by a loading clutch and a load lever with weights. Detailed information on the test rig and its function can be taken Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 shows an exemplary photographic documentation of the test gear set Type C. Table 2 shows the corresponding material and lubricant used for the conducted tests. All gears are made out of tungsten carbide composite material with the different compositions A, B, C and D of tungsten, carbon and further alloying elements. The exact material compositions are proprietary knowledge of Reintrieb GmbH. Different kinds of water such as salt water, distilled water and tap water without salt were used for lubrication to investigate the behavior of the different materials with different water lubricants. The different kinds of water were not modified with additional chemicals or additives. Dedicated and comprehensive corrosion tests were conducted at an external laboratory on behalf of Reintrieb GmbH. Table 3 lists the main geometry, quality and material data of the applied test gear sets of Type C. The macrogeometry of the gear is created by sintering, the microgeometry of the gear flanks is realized by grinding. Profile modifications were specified in the form of tip reliefs for the pinion and wheel. Test Material Water scuffing A salt water tooth root breakage A, B, C, D no lubrication needed pitting and high-speed wear at n 2 = 1500 min -1 C, D distilled water low-speed wear at n 2 < 500 min -1 C, D tap water (without salt) Table 2: Materials and lubricants for the tests Description Symbol Value center distance a 91,5 mm normal module m n 4,5 mm number of teeth (pinion / wheel) z 1/ 2 16 / 24 face width b 14 mm pressure angle 20° helix angle 0° profile shift coefficient x 1/ 2 0,182 / 0,172 profile modifications — tip relief at pinion and wheel quality acc. DIN 3961 - 3967 [19] Q 5 - 10 roughness Ra Material A: Ra A = 0,6 - Material B: Ra B = 0,4 - Material C: Ra C = 0,2 - Material D: Ra D = 0,3 - modulus of elasticity E Material A: E A = 580 GPa Material B: E B = 580 GPa Material C: E C = 490 GPa Material D: E D = 450 GPa Table 3: Main geometry, quality and material data of the test gear sets of Type C 3 Test Results 3.1 Scuffing Scuffing was examined in an initial screening test with a test gear set made from the material A and lubricated with salt water. The temperature during the test reached up to 75 °C without active cooling or heating. During the test no scuffing occurred up to the load stage KS 10, but the test was terminated by tooth root breakage during the test cycle at load stage KS 10. Figure 6 shows the photographic documentation of the scuffing test. The examination of the broken surfaces revealed that there was one smooth fatigue fracture and several rough forced ruptures. Obviously, the fatigue tooth root fracture occurred first, followed by consecutive forced ruptures. Due to the fact, that not the scuffing load carrying capacity, but the tooth root bending strength was the limiting gear failure, subsequent investigations regarding the tooth root breakage were performed with the help of a pulsator test rig. Further tests specifically aimed at the scuffing load carrying capacity were not conducted. However, for all following tests with cri- Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 from literature [13, 22, 23]. The tests are performed until a load stage defined by the test method is reached or until scuffing respectively pitting occurs. The test method for wear does not define a maximum wear amount but compares the wear amounts after the different test cycles. The wear amounts correspond to the weight change measured with a precision scale. Figure 5 shows a schematic drawing of a FZG back-to-back test rig used for the scuffing, pitting and high-speed as well as lowspeed wear tests. Figure 4: Schematic drawing of a pulsator test rig Figure 5: Schematic drawing of a FZG back-to-back test rig Figure 6: Photographic documentation of the scuffing test, no scuffing occurred until test termination by tooth root breakage during load stage KS 10 ing capacity against tooth root breakage ranging from low to high values compared to conventional steel. It must be noted that the values are based on a limited number of tests and are not statistically validated. However the difference between the materials were very distinct and thus fulfil the screening function sufficiently. For the carbide composite materials A and B, the measured endurable nominal tooth root stress is 30 % lower compared with the reference case carburized steel. For material A, the nominal tooth root stress at load stage 10 of the scuffing test correlates with the endurable nominal tooth root stress stated in Figure 7, thus confirming the plausibility of the test results as well as the reproducibility between pulsator and FZG back to back test rig. The carbide composite materials C and D show endurable nominal tooth root stresses above the reference case carburized steel. It was thus decided to use materials C and D for further investigations regarding pitting and highspeed wear as well as low-speed wear. 3.3 Pitting and High-Speed Wear The tests regarding pitting and high-speed wear were conducted with the materials C and D. Distilled water was used as a lubricant for the test gear set Type C. All tests were performed at a circumferential speed at the pitch point of v = 8,3 m/ s, while the load stages (KS) were varied between the test cycles. After each test cycle, the gear flanks were photographically documented, the tooth flank profile 3D-measured, and the entire gear was weighed on a precision scale. The weight-measured amount of wear was used to derive the linear wear coefficient c lt [24]. The important and often used linear wear coefficient c lt is directly proportional to the amount of wear and is shown on the ordinate axis of the following graphs. Figure 8 shows the results of the pitting and high-speed wear test for the material C. Figure 9 shows the results of the pitting and high-speed wear test for the material D. Figure 10 shows the photographic documentation and 3D- Aus Wissenschaft und Forschung 30 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 tical scuffing conditions, especially during pitting and high-speed wear tests, scuffing was not observed. 3.2 Tooth root breakage The tests regarding tooth root breakage were conducted as a screening of the tooth root bending strength for four different tungsten carbide composition materials. The wheel of the Type C test gear was chosen for testing on the pulsator test rig, where no lubrication is necessary. Figure 7 shows the results of the tests regarding the tooth root bending strength of the different tungsten carbide composite materials (blue) and a commonly used case carburized steel 18CrNiMo7-6 as comparative reference (green). The moduli of elasticity of the tungsten carbide composite materials are significantly higher compared to the conventional steel. The higher moduli of elasticity lead to an increased notch effect and sensitivity for the tungsten carbide composite materials. In consideration of the increased notch sensitivity, the tungsten carbide composite materials show a load carry- 0 200 400 600 800 1000 1200 1400 1600 1800 Material A Material B Material C Material D 18CrNiMo7-6 Values 713 734 1207 1666 1109 long life endurable nominal tooth root stress 2 Figure 8: Results of a pitting and high-speed wear test for material C Figure 7: Results of the tests regarding the tooth root bending strength of different tungsten carbide composite materials (long life) measurements of the tooth flank profile after the respective test cycles for material D. The flanks optically show typical running marks as well as an increasing profile form deviation in the area below the pitch point. This profile form deviation is likely caused by wear due to unfavorable tribological conditions in the area below the pitch point. Wear equally affects the area above the pitch point, even though the impacts observed are not as noticeable as below the pitch point. The reduced wear above the pitch point is likely supported by the applied tip reliefs. During all tests, no pittings were observed. The results show that the linear wear coefficient does not correlate well with the load. There is a tendency that for gears made from the material C a lower wear is measured compared to the gears made out of the material D. This result correlates with and is supported by the findings of the external laboratory that material C shows a reduced corrosion compared to material D. The other tendency showing is, that a longer test cycle duration leads to lower wear. This is observed for the last test cycle run of material D, where the wear is reduced significantly even though the load stage is increased from KS 7 to KS 8. This observation might correlate with an increased running-in of the tooth flanks with an increased time of the test cycle. In general, the linear wear coefficients for the water lubricated gears made from tungsten carbide composite are increased compared to experience-based values for conventional steel gears with oil lubrication. 3.4 Slow-Speed Wear The slow-speed wear tests were conducted with the material C and D. Tap water without additional salt was used as a lubricant for the test gear set Type C. All tests were performed at load stage KS 7, while circumferential speeds at the pitch point varied between the test cycles. After each test cycle, the gear flanks were photographically documented, 3D-measured regarding the tooth flank profile and the entire gears weighed on a precision scale. The weightmeasured amount of wear was used to derive the linear wear coefficient c lt . Figure 11 shows the results of the slow-speed wear test for the material C. Figure 12 shows the photographic documentation of the gear Aus Wissenschaft und Forschung 31 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 Figure 10: Photographic documentation and 3D-measurements of the tooth flank profile (material on the left side of the line) after the respective test cycles for material D Figure 11: Results of the slow-speed wear test for material C Figure 9: Results of a pitting and high-speed wear test for material D ■ Wear is the main limiting factor for the gear endurance. The amount of wear can vary with the operating conditions such as load and rotational speed as well as the composition of the tungsten carbide material. ■ Further investigations are required for the optimization of the material-lubricant-system. The scientific conclusions show the great potential regarding the water lubrication of tungsten carbide composite gears: ■ The prove of concept for the innovative combination of tungsten carbide composite gears and water lubrication has been brought forward with the presented investigations. ■ Tooth root breakage, scuffing and pitting do not limit the industrial application of the presented material-lubricant-system in gear boxes, when appropriate technical boundary conditions regarding wear are defined. ■ Gear boxes for industrial applications with appropriate wear allowances and frequent maintenance services (e.g. harbor tugboats) can be designed and further investigated regarding the behavior of the material-lubricant-system in practical usage. Aus Wissenschaft und Forschung 32 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 flanks and 3D-measurements of the tooth flank profile after the respective test cycles for material C. The flanks show typical running marks as well as an increasing profile form deviation in the area above and below the pitch point. This profile form deviation is likely caused by an increased wear due to the slow circumferential speed of the gears. The slow circumferential speed is leading to increased unfavorable tribological conditions and increased wear of the slow-speed wear tests compared to the high-speed wear tests. Figure 13 shows the results of the slow-speed wear test for the material D. The results show that the linear wear coefficient is significantly higher compared to the values of the pitting and highspeed wear test from Figure 8 and Figure 9. The linear wear coefficient decreases with higher rotational speeds, indicating certain film formation properties for water as a lubricant. Material C shows a lower wear compared to material D, which is consistent with the results from the high-speed wear tests as well as the increased corrosive behavior of material D. As observed from the high-speed wear tests, a tendency of lower wear caused by a longer tests cycle is observed. In general, the linear wear coefficients for the water lubricated gears made from tungsten carbide composite are increased compared to experience-based values for conventional steel gears with oil lubrication 4 Conclusions The presented investigations on water lubricated gears made from tungsten carbide composite lead to the following scientific conclusions: ■ Gears made from tungsten carbide composite material can be operated in practical transmissions with pure water lubrication if a certain amount of wear can be tolerated. ■ The test results regarding tooth root bending, scuffing, and pitting are comparable to conventionally manufactured and oil lubricated steel gears. Figure 13: Results of the slow-speed wear test for material D Figure 12: Photographic documentation and 3D-measurements of the tooth flank profile (material on the left side of the line) after the respective test cycles for material C ■ When more experience is gathered and further optimizations are performed, a much broader scope of industrial applications (e.g. cruise ships) can be realized. The benefits of these research results are: ■ Water as lubricant is abundantly available and does not have to be refined in energy consumptive processes. ■ Water as a lubricant is completely biodegradable and sustainable. ■ The challenges of corrosion and wear were effectively counteracted by the usage of tungsten carbide composite material. ■ The findings motivate further optimizations of the material (e.g. composition and alloying elements) as well as the lubricant (e.g. biodegradable additives or thickener). Summarizing the aforementioned results and conclusions, it can be stated, that the sustainable lubrication of gears with water has been technically proven and that further developments regarding optimized lubricant-material-systems are promising. Acknowledgement This work was funded by the “Austrian COMET-Program” (project InTribology1, no. 872176) via the Austrian Research Promotion Agency (FFG) and the federal states of Niederösterreich and Vorarlberg and was carried out within the “Excellence Centre of Tribology” (AC2T research GmbH). References [1] Armygov, A., 2022, Cruise Liners On Geiranger Fjord. Article-ID: 552192805. www.shutterstock.com [2] Jeng, Y.-R., Huang, Y.-H., Tsai, P.-C., and Hwang, G.-L., Tribological Properties of Carbon Nanocapsule Particles as Lubricant Additive, Journal of tribology, 136 4, pp. 418011 - 418019, 2014. 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Jahrgang · eOnly Sonderausgabe 2/ 2022 DOI 10.24053/ TuS-2022-0035 [19] Deutsches Institut für Normung e.V. (DIN), Geometrie von einzelnen außen- oder innenverzahnten Stirnrädern, DIN 3961 - 3967. Deutsche Norm, 1978 - 1986. [20] Fuchs, D., Schurer, S., Tobie, T., and Stahl, K., A model approach for considering nonmetallic inclusions in the calculation of the local tooth root load-carrying capacity of high-strength gears made of high-quality steels, Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 233 21-22, pp. 7309 - 7317, 2019. DOI: 10.1177/ 0954406219840676 [21] Weber, C., Tobie, T., and Stahl, K., 2019, Rapid and Precise Manufacturing of Special Involute Gears for Pro-