23rd International Colloquium Tribology - January 2022 57 Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water Karl Jakob Winkler Gear Research Center (FZG), Technical University of Munich, Garching, Germany Corresponding author: karljakob.winkler@tum.de Thomas Tobie Gear Research Center (FZG), Technical University of Munich, Garching, Germany Klaus Michaelis Gear Research Center (FZG), Technical University of Munich, Garching, Germany Karsten Stahl Gear Research Center (FZG), Technical University of Munich, Garching, Germany 1. Introduction The preservation of the planet earth as our habitat is of great importance. One contribution to the preservation of our planet is the use of environment-friendly lubricants. The idea is to lubricate gears with water instead of conventional mineral or synthetic oil. The cylindrical test gears are made of carbide composite to achieve the necessary resistance against water corrosion and gear failures. The innovative combination of carbide composite gears and water lubrication is a step to sustainable gear sets and has been patented [4]. The investigations were conducted on a Pulsator rig (bending strength) and a FZG back-to-back gear test rig using established and standardized test methods. The results of the gear investigations were made available to the gear research center FZG by the Reintrieb GmbH (Vienna) for further scientific evaluation. 2. Experimental Investigations The following experimental investigations are conducted according to the corresponding test procedures: • Tests based on a unified test procedure [2] regarding the tooth root bending strength. • Tests based on the standardized test procedure DIN ISO 146351 [3] regarding the scuffing load carrying capacity. • Tests based on a unified test procedure [5] regarding the pitting and high-speed wear (wheel rotational speed of n 2 = 1500 min -1 ) load carrying capacity. • Tests based on a unified test procedure [1] regarding the low-speed wear (wheel rotational speed of n 2 < 500 min -1 ) load carrying capacity. Table 1 shows the corresponding material and lubrication used for the conducted tests. The material underly a non-disclosure agreement by the Reintrieb GmbH. All gears are made out of carbide composite material with different compositions of tungsten carbide and further alloying elements. Table 1: Material and lubrication for the tests Test Material Water tooth root bending trength A, B, C, D no lubrication needed scuffing A salt water 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 lists the main geometry data of the applied test gear set Type C. The pulsator tests were performed on the pinion of this gear type. Table 2: Main geometry data of the test gear Type C Description Symbol Value center distance a 91,5 mm normal module m n 4,5 mm number of teeth z 16 / 24 width b 14 mm pressure angle α 20° helix angle β 0° 58 23rd International Colloquium Tribology - January 2022 Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water Figure 1 shows an exemplary photographic documentation of the test gear set Type C. Figure 1: Exemplary photographic documentation of the test gear set Type C Figure 2 shows the FZG back-to-back test rig used for the scuffing, pitting and high-speed as well as low speed wear tests. Figure 2: FZG back-to-back tests rig 3. Results The tests regarding the tooth root bending strength were conducted as an initial screening for the different composite materials. Figure 3 shows the results of the tests regarding the tooth root bending strength of the different carbide composite materials (blue) and a commonly used case carburized steel 18CrNiMo7-6 as comparative reference (green). It must be noted that the values are based on a limited number of tests and are not statistically validated. Figure 3: Results of screening tests regarding the tooth root bending strength (long life) For the carbide composite materials A and B the measured endurable nominal tooth root stress is 30 % lower compared to the reference case carburized steel. The carbide composite materials C and D show endurable nominal tooth root stresses above the reference case carburized steel. It is thus decided to use the materials C and D for further investigations regarding the pitting and high-speed wear as well as the low-speed wear. Parallel to the tests regarding the tooth root bending strength, a screening test regarding the scuffing load carrying capacity was conducted for the material A. The load stage 9 was completed without scuffing, while in load stage 10 the gear set failed due to tooth root breakages at the pinion and the wheel. The nominal tooth root stress at load stage 10 correlates with the endurable nominal tooth root stress from Figure 3. Further tests specifically aimed at the scuffing load carrying capacity were not conducted. However, for all following tests with critical scuffing conditions, especially the pitting and highspeed wear tests, scuffing was not observed. Figure 4 shows an exemplary result of a pitting and highspeed wear test. For all tests, pittings were not observed. The linear wear coefficient clT is an important parameter to measure wear. The results show, that the linear wear coefficient does not clearly correlate with the load. Figure 4: Exemplary results of a pitting and high-speed wear test (material D) 23rd International Colloquium Tribology - January 2022 59 Scientific Evaluation of Investigations on the Load Carrying Capacity of Carbide Cylindrical Gears Lubricated with Water Figure 5 shows exemplary results of a low-speed wear test. The results show, that the linear wear coefficient is significantly higher compared to the values of the pitting and high-speed wear test from Figure 4. The results furthermore show, that the linear wear coefficient decreases with higher rotational speeds, indicating certain film formation properties of water. Figure 5: Results of a low-speed wear test (material D) For high-speed and low-speed wear tests, the linear wear coefficients were increased compared to experience-based values for conventional steel gears with oil lubrication. The material D showed a corrosive behaviour and thus higher wear compared to the material C. 4. Conclusions The investigations showed the following results: • Lubrication of carbide gears with water is technically possible. • The test results regarding tooth root bending, scuffing and pitting are comparable to conventionally manufactured and oil lubricated steel gears. • The main factor limiting the gear life time is wear. The amount of wear can vary with the operating conditions such as load and rotational speed. • Further investigations are required for the optimization of the material-lubricant system. The benefits of these research results are: • High potential of sustainable lubrication with water instead of mineral or synthetic oil was proven. • Environmentally friendly lubrication of gears with water is possible in nature reserves both on land and at sea. Acknowledgement: This work was funded by Reintrieb GmbH as well as the project COMET InTribology1, FFG-No. 872176 (project coordinator: AC2T research GmbH, Austria). References [1] Bayerdörfer, I., et al., Method to Assess the Wear Characteristics of Lubricants FZG Test Method C/ 0,05/ 90: 120/ 12, Hamburg, Anmeldung: 1997. [2] Bergmann, C., et al., FVA-Merkblatt Nr. 0/ 5 - Empfehlung zur Vereinheitlichung von Pulsatorversuchen zur Zahnfußtragfähigkeit von vergüteten und gehärteten Zylinderrädern, Frankfurt, Anmeldung: 1999. [3] Deutsches Institut für Normung e.V. (DIN), Zahnräder - FZG-Prüfverfahren - Teil 1: FZG-Prüfverfahren A/ 8,3/ 90 zur Bestimmung der relativen Fresstragfähigkeit von Schmierölen (ISO 14635- 1: 2000), DIN ISO 14635-1: 2006, Berlin, Anmeldung: 2006. [4] Lais, S., Getriebe. Inhaber: Reintrieb GmbH, Ed., Patentschrift EP2614000, Anmeldung: 2011, Publikation: 2018 [5] Schedl, U., et al., Einfluss des Schmierstoffes auf die Grübchenlebensdauer einsatzgehärteter Zahnräder im Einstufen- und im Lastkollektivversuch, Frankfurt/ Main, Anmeldung: 1997.