1 Introduction Hydraulic fluids play a decisive role on the wear and tear of hydraulic components and therefore the service life of the hydraulic system. The trend toward an everincreasing demand such as compact hydraulic systems, higher power density, lower tank volume, high oil circulation and lifetime filling requires a deeper understanding of the aging behavior and aging stability of hydraulic fluids. The current standard requirements such as DIN 51524 [1] for mineral oils include test standards such as the DIN EN ISO 4263-1 (Tost-Test) [2] for the aging of hydraulic fluids. This static test is based on thermal aging with the addition of air, water and copper. There are many other common oxidation tests for accelerated artificial oil aging [3 - 6]. Similar approaches can also be found in literature [7 - 9]. Parameters from real hydraulic systems such as high operating pressures or shear stress are not covered by these tests. A test method for the determination of oxidation stability in a highpressure piston pump is described in the JCMAS P045 Aus Wissenschaft und Forschung 25 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0028 Sensitivity analysis of operating parameters in hydraulic systems with respect to the aging of hydraulic fluids Tobias Schick, Jochen Hörer, Karl-Heinz Blum, Klaus Ellenrieder, Katharina Schmitz* In dieser Arbeit wird ein neuer Ölalterungsprüfstand vorgestellt, der auf einem realen hydraulischen System basiert. Dieser Prüfstand ermöglicht eine beschleunigte Fluidalterung unter einer hohen hydraulischen Belastung. Des Weiteren ist es möglich, die Betriebsparameter zu variieren und somit die verschiedenen Einflussfaktoren auf das Alterungsverhalten von Hydraulikflüssigkeiten zu bewerten. Unter verschiedenen Bedingungen gealterte Ölproben werden mittels FTIR analysiert. Anhand der Ergebnisse wird die jeweilige Alterungsrate des Fluids am Beispiel des Abbaus des Additivs Zink-Dialkyldithiophosphat (ZDDP) berechnet. Die Ergebnisse zeigen deutlich den Zusammenhang zwischen dem Alterungsverhalten und den verschiedenen Betriebsparametern des Prüfstands. Neben dem Abbau von ZDDP kann der Abbau des phenolischen Antioxidans (AO) und die Bildung von Varnish bedingt durch verschiedene Alterungsprodukte auf metallischen Oberflächen festgestellt werden. Schlüsselwörter Ölalterung, ZDDP, ZnDDP, FTIR, Fluidprüfstand, Hydraulik, Spektroskopie In this work, a novel oil aging test-rig based on a real hydraulic system is introduced. This device allows an accelerated fluid-aging rate under high hydraulic loads. Furthermore, it is possible to vary the operating parameters to evaluate the different factors that influence the aging behavior of hydraulic fluids. Oil samples, which are aged under different conditions, are analyzed using FTIR to estimate the rate of aging using the degradation process of Zinc dialkyl dithiophosphate (ZDDP). The results show a clear aging behavior dependent on different operating parameters in hydraulics. In addition to the degradation of ZDDP, the depletion of the phenolic antioxidant (AO) and the formation of a varnish film on metallic surfaces can be observed. Keywords Oil aging, ZDDP, ZnDDP, FTIR, fluid test rig, hydraulic, oil degradation, spectroscopy Kurzfassung Abstract * Tobias Schick Orcid-ID: https: / / orcid.org/ 0000-0002-2586-5401 Bosch Rexroth AG, Glockeraustraße 2 89275 Elchingen, Germany Jochen Hörer Karl-Heinz Blum Bosch Rexroth AG, 72160 Horb, Germany Klaus Ellenrieder Bosch Rexroth AG, 89275 Elchingen, Germany Univ.-Prof. Dr.-Ing. Katharina Schmitz Orcid-ID: https: / / orcid.org/ 0000-0002-1454-8267 Institute for Fluid Power Drives and Systems (ifas) RWTH Aachen University, 52074 Aachen, Germany relevant for oil aging can be identified and modelled in a practical manner. This data provide valuable insights into the design of new hydraulic systems in terms of high durability and low oil change intervals. 2 Experimental setup Figure 2 shows the schematic structure of the test-rig RFT-OA. The Rexroth A2FO10 bent-axis axial piston pump (1) is driven by a variable speed electric motor with up to 3000 rpm. Optionally, air (2) can be added to the test medium in the suction line with high precision. A Rexroth relief valve DBDS 10 K1X/ 400V (3) serves as a load and regulates the pressure up to 400 bar. After the valve, the hydraulic fluid flows back into the tank via a 10 µm filter (4) and a cooler (5). In addition, a polished copper sheet (7) which is a good catalyst for some aging reactions, can be placed in the tank. The system operates with a total volume between 8 to 16 l of hydraulic fluid. During the test, a special sampling-device, (9) automatically extracts oil samples at pre determined times. This enables uninterrupted operation of the test bench and ensures highly comparable and reproducible results. Depending on the choice of parameters, the RFT-OA shows the highest demands on the aging stability of a hydraulic fluid in a real hydraulic system. As much as possible, all hydraulic components of the test rig are constructed of stainless steel. The stainless steel is particularly effective for the oil tank, cooler and the piping. As we can see in Figure 3 the aging process is not only measurable by serveral chemical analysing techniques but also visible for the naked eye. The degradation products of additives and base oil generate molecules with a high specific mass, which are known as varnish. Varnish is not soluable in unpolar oils and forms an orangecolored film on all metal surfaces. This effect can lead to problems such as sticking valves, especially when the system is switched off and cooled down. Prior to each new experiment we clean our RFT-OA test-rig by adding a special varnish-removing additive and run the system for several hours. The system is then flushed three times with fresh oil. 3 Evaluation method We use a fully formulated API group I, ISO VG 32 mineral oil with a zinc-based and phenolic antioxidant containing additive system for our experiments. Mineral oils cover about 75 % of the market share of hydraulic fluids [11]. One of the most important additives in this formulation is the anti-wear (AW) additive zinc dialkyl dithiophosphate (ZDDP). ZDDP is not only reducing friction and wear by forming a protective tribo-film on metal surfaces but also ZDDP acts as extreme pressure (EP) additive and additionally offers antioxidant properties [12]. For these reasons, we focus our attention on the de- Aus Wissenschaft und Forschung 26 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0028 [10]. In this test method, the hydraulic fluids are aged and assessed in a practical hydraulic circuit comprising a Rexroth A2FO10 bent-axis pump, a load valve, a filter unit and an oil cooler as well as a copper catalyst in the tank and the addition of air. In addition to the requirements of the JCMAS P045, the aging test rig RFT-OA “Rexroth-Fluid-Test-Oil-Aging” (Figure 1) developed by Bosch Rexroth allows the variation of all operating parameters, such as operating pressure, pump speed, oil temperature as well as accurately defined contamination with air and water. As a result, all influence parameters Figure 1: Test-rig RFT-OA Figure 2: Schematic structure of the RFT-OA test-rig Figure 3: Formation of a varnish film on the metal surfaces inside the tank during the aging test pletion of ZDDP to calculate an oil-aging rate with respect to the different hydraulic parameters. The degeneration of ZDDP occurs in several steps (Figure 4). During the first reaction, an oxygen atom replaces one sulfur of the dithiophosphate, which leads to a monothio molecule. This monothiophosphate still has some helpful AW-properties. During the second oxidation step the monothio molecules were oxidized to phosphates. In our work we use Fourier-transform infrared spectroscopy (FTIR) to measure the content of AO, ZDDP and its degradation products. A dataset with several FTIR measurements of our reference experiment (350 bar, 1500 rpm, 80 °C, 16 l, air 1 l/ h) is shown in Figure 5. We can identify the degeneration of ZDDP using the IRabsorption spectra evaluated at 654 cm -1 . For the monothiophosphate species, the same method is used at 1018 cm -1 . The concentration of phosphates correlates to the absorption band at 1800 cm -1 . Initially the ZDDP is consumed, which leads to the formation of the monthiophosphate species during the first 300 hours of the test. Once this process is completed, the monothio species reach a maximum content and begin to deplete to phosphates. After 500 h of test time, there is no ZDDP present. Large quantities of phosphates have been produced, and a residual amount of monothiophosphates can also be detected. There are characteristic points which are important: the maximum concentration of monothio species at 250 hours and the (nearly) complete ZDDP decomposition at 300 hours. All these points in time correlate with the rate of oil aging. The second additive to consider is the phenolic antioxidant which appears at 3650 cm -1 in the IR-absorption spectra. In general, the antioxidant is designed to protect the other oil components from oxidative aging. At the beginning of the experiment, only a small amount of AO is consumed. At 250 h, this changes abruptly. In the second half of the experiment, the concentration of antioxidant drops significantly faster. After 500 h, approx. 70 % of the original amount is still available. Aus Wissenschaft und Forschung 27 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0028 Figure 4: Degeneration process of ZDDP Oil volume [l] Pump speed [rpm] Load pressure [bar] Temperature (Tank) [°C] Coppercatalyst Air supply [l/ h] Time max. Monothiophosphates [h] Time max. Monothiophosphates volume compensated [h/ l] 16 1500 350 80 Yes 0 400 25 8 1500 350 80 Yes 0 200 25 13 3000 350 80 Yes 0 200 15,4 13 1500 175 80 Yes 0 >500 >38,5 16 1500 350 100 No 0 300 18,8 13 1500 350 80 No 0 400 30,8 16 1500 350 80 Yes 1 250 15,7 Table 1: Overview of the performed experiments Figure 5: Results of FTIR-analysis during the oil aging process at the RFT-OA test-rig Load pressure In this aging test, the load pressure has an unusually large influence on the aging behavior. At half the pressure, the maximum of the monothiophosphates is no longer reached within 500 h. At constant tank temperature, the point of the highest temperature in the system at the load valve is mainly influenced by the set pressure, which is the reason for the strong influence. Temperature The oil temperature also has a decisive influence on aging behavior. However, the expectation of an arrhenius equation is not met even if the missing copper catalyst in the fifth experiment is considered. One possible reason for this effect could be that an oil formulation is a complex mixture of substances in which many different reactions take place simultaneously which influences each other. Copper Catalyst The copper catalyst in the tank has a comparatively lower but still measurable influence on the aging rate. In the sixth test without the catalytic converter, the time until the maximum monothiophosphate content is reached increases to 30,8 l/ h. Air supply The oxygen content in the oil is increased by the addition of air, which leads to a significantly higher degradation rate of ZDDP. 5 Conclusion Oil samples were aged in a hydraulic test-rig under different operating parameters. The experiments show that the aging of hydraulic fluids is dependent on many different factors. Using the example of the degradation of ZDDP, the individual influences can be evaluated more accurately. This process cannot be fully covered by artificial aging methods. References [1] DIN 51524-3: 2017-06, Pressure fluids - Hydraulic oils -Part 3: HVLP hydraulic oils, Minimum requirements [2] DIN EN ISO 4263-1: 2004, Petroleum and related products - Determination of the ageing behavior of inhibited oils and fluids - TOST test -Part 1: Procedure for mineral oils (ISO 4263-1: 2003) Aus Wissenschaft und Forschung 28 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0028 4 Results and discussion In this work, several experiments with different operating parameters were carried out at the test-rig RFT-OA. During the experiments, oil samples were taken at 50 hours intervals and evaluated using the previously described methods. Table 1 shows an overview of the tests performed with the various parameters. The point in time of maximum concentration of the monothiophosphates is selected as evaluation criterion for the measure of ZDDP depletion. A high value until the maximum concentration is reached means a slower aging rate and vice versa. Due to the different oil quantities in the experiments, it is necessary to compensate the results accordingly. Figure 6 shows the course of the monothiophosphate concentration with different oil volumes and otherwise identical parameters on the test rig. If the oil quantity is halved from 16 l to 8 l with otherwise constant parameters, the time until the maximum of the monothiophosphates is reached, is also cut in half from 400 h to 200 h. This result shows that in both cases an identical aging rate is present during the degradation process of the ZDDP. By standardizing the results for the oil quantity, all other experiments can be made comparable. The value of 25 l/ h for the first two tests is used as reference. Pump speed If the speed and thus the flow rate are increased, each molecule in the oil passes a critical tribological juncture, with high temperature or a narrow shear gap more often. This significantly increases the depletion rate of the ZDDP, which leads to a low value of 15,5 h/ l until the maximum monothiophosphate concentration is reached. Figure 6: Results of FTIR-analysis of two different experiments with different oil quantities [3] ASTM International. D6186-19 Standard Test Method for Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry (PDSC). West Conshohocken, PA; ASTM International (2019) [4] ASTM International. D2272-14a Standard Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel. West Conshohocken, PA; ASTM International (2014) [5] ASTM International. D6514-03(2019)e1 Standard Test Method for High Temperature Universal Oxidation Test for Turbine Oils. West Conshohocken, PA; ASTM International (2019) [6] ASTM International. D5846-07(2017) Standard Test Method for Universal Oxidation Test for Hydraulic and Turbine Oils Using the Universal Oxidation Test Apparatus. West Conshohocken, PA; ASTM International (2017) [7] Dörr, N., Brenner, J., Ristić, A. et al. Correlation Between Engine Oil Degradation, Tribochemistry, and Tribological Behavior with Focus on ZDDP Deterioration. Tribol Lett 67, 62 (2019). [8] Besser, C., Agocs, A., Ronai, B. et al. Generation of engine oils with defined degree of degradation by means of a large scale artificial alteration method. Tribology International, Volume 132 (2019), Pages 39-49 [9] Vito Tič, Tadej Tašner, Darko Lovrec, Enhanced lubricant management to reduce costs and minimise environmental impact. Energy, Volume 77 (2014), Pages 108-116 [10] JCMAS P 045: 2004, Hydraulic Fluids for Construction Machinery - Test Method for Indicating Oxidation Stability in High Pressure Piston Pump [11] Bauer G. (2011) Druckflüssigkeiten. In: Ölhydraulik. Vieweg+Teubner [12] Spikes, H. The History and Mechanisms of ZDDP. Tribology Letters 17, 469-489 (2004) Aus Wissenschaft und Forschung 29 Tribologie + Schmierungstechnik · 69. Jahrgang · eOnly Sonderausgabe 1/ 2022 DOI 10.24053/ TuS-2022-0028