24th International Colloquium Tribology - January 2024 121 Stick-Slip in Hydraulic Cylinders: New Test Methods & Simulation as a Tool for Selecting Coating Solutions for Piston Rods to Avoid Critical Operating Conditions Giuseppe Tidona 1* , Jürgen Molter 2 1 Competence Center for Tribology, Mannheim University of Applied Sciences 2 Mannheim University of Applied Sciences * Corresponding author: g.tidona@hs-mannheim.de 1. Introduction Hydraulic cylinders play a crucial role as linear actuators and are particularly in demand when high power density is required in applications such as crane booms or in underground mining. Under certain conditions, such as high working pressures and low travel speeds, an undesirable vibration phenomenon known as “stick-slip effect” is often observed [1]. Piston rods of most hydraulic cylinders are provided with a hard chrome coating. This coating not only provides excellent wear resistance, but also meets corrosion resistance requirements, making it an economically attractive option for a wide, technical range of applications. Since 2017, these hard chrome coatings have been subject to the provisions of the REACH regulation [1], which aims to reduce the use of hazardous chemicals to minimize environmental and health risks, creating a need to find coating alternatives that are as technically equivalent as possible but meet current health and environmental requirements. The paper addresses the stickslip behavior of hydraulic cylinder piston rod seals on different piston rod coatings. 2. Used test rig & specimen Table 1 shows the seal designs to be investigated [2]. The primary material of the rod seals used is the plastic polyurethane - a thermoplastic elastomer known for its high abrasion resistance, good resistance to oils and chemicals and good processability. Table 1: Seal designs to be investigated DS101 DS117 DS117R DS121 DS141 In addition to the classic hard chrome coating (HC), which will serve as a comparison reference in the investigations, a total of four other piston rod coatings (Tenifer QPQ-, S3PM-, WC-Co-HVOFand LIC-processed) were selected as mating surfaces for the rod seals. The different manufacturing processes for the surface modifications result in widely differing tribological surfaces, which have a significant influence on the friction and stick-slip behavior and will be considered in the evaluation of the results. A special long-stroke test rig was developed to study the stick-slip behavior of the different rod seal-piston rod pairings with high precision under variation of travel speeds, hydraulic pressure, and oil temperature. Figure 1 shows the pressure chamber used for the experimental investigation in a quarter section view as well as in a test run. Figure 1: Quarter section & full view of the pressure chamber with a rod seal (red), a guide ring (green) and a wiper (orange) 3. Load collective & test results The load collective is changed stepwise during an experiment. The results are presented as a function of pressure (T test -= 0 bar … 300 bar), thermostat temperature ( p test = 30-°C … 70-°C) and velocity (v test = 5 mm/ s … 25 mm/ s). Three characteristic values are introduced for the evaluation of the seal performance. The mean friction force value F RAVG provides information on the general friction level. The friction force is averaged over a stroke length. The maximum friction force F RMax describes the peak (red cross in fig. 2) after a change of direction and is a measure for the static coefficient of friction. The stick-slip range F RSS is introduced as a qualitative measure of the stick-slip effect and is calculated as the difference between the mean value of all maxima (golden line in fig. 2) and all minima (brown line in fig. 2). Figure 2: Friction force of a section of a stroke of the DS121 seal on a hard chrome plated piston rod at p = 100-bar, T = 30-°C and v = 5 mm/ s 122 24th International Colloquium Tribology - January 2024 New Test Methods & Simulation as a Tool for Selecting Coating Solutions for Piston Rods to Avoid Critical Operating Conditions Figure 2 shows an example of the friction force of the DS 121 seal design on a hard chromium-plated piston rod at p-= 100-bar, T = 30-°C and v = 5 mm/ s. After an initial static friction peak, a broad “band” is seen up to stop of the pressure chamber movement, where the friction force exhibits a relaxing behavior. A closer look reveals a sawtooth pattern in the friction signal, which is typical for a stick-slip affected motion [3]. Table 2 shows a section of the comparison of the test results for the DS121 seal design on a selection of piston rods for the pressures p = 100 … 200 bar. Green stands for a low and red for a high characteristic value. Although stickslip can be observed for some seal designs under certain load parameter combinations, the best performance is achieved with the LIC rod as it has similar friction properties and a lower stick-slip tendency than the hard chromium plated piston rod. The worst performance is seen with the QPQ-processed rod. The rough surface results in high characteristic friction values, increased wear of the rod seal and leakage. Table 2: Section of the test results of DS121 on selected piston rods 4. MATLAB & ABAQUS-simulations Parallel to the experimental investigation, simulations with the FEM-program ABAQUS were carried out. These simulations make it possible to determine the deformation of the seals under pressurization and to analyze the resulting pressure profiles of the seal body on the piston rod [4]. With the help of the inverse Reynolds equation, it is possible to calculate the volumetric flow rate of the lubricant transported through the contact as a thin lubricating film which is used to draw conclusions about the stick-slip-tendency of a seal design. The investigations are further supplemented by a MAT- LAB Simulink model of a hydraulic cylinder. It allows the precise analysis of the operating behaviour as a function of speeds, temperatures and pressures. To adequately account for the viscoelastic behavior of the seals, the extended Maxwell model is introduced as a rheological model and stiffness and damping coefficients are defined [5]. To fully analyze the influence of operating conditions, the hydraulic cylinder must be embedded in a hydraulic system and equipped with position and speed control [6]. 5. Conclusion As part of the NoChromeNoStickSlip project, a test rig was developed for the analysis of the stick-slip behavior of rod seals on different piston rods. During the test series, the LIC piston rod emerged as the favorite for the mentioned parameter range. Using the results of the FEM simulations, the friction effects in the tests could be attributed to specific geometric features of the seals. As more knowledge is gained in the project, new designs will be derived that exhibit improved frictional behavior and reduced stick-slip tendencies. The Simulink model provides basic information, but material and friction data sets are needed to make qualitative predictions about the stick-slip behavior of a seal in service. 6. Acknowledgements The authors would like to thank the funding program “Zentrales Innovationsprogramm Mittelstand” of the BMWK for the funds provided to carry out the project. References [1] Skowrońska, J., Kosucki, A., & Stawiński, Ł. (2021). Overview of Materials Used for the Basic Elements of Hydraulic Actuators and Sealing Systems and Their Surfaces Modification Methods. Materials (Basel, Switzerland), 14(6), 1422. https: / / doi.org/ 10.3390/ ma14061422 [2] Elbe-Dichtungen.de. Accessed on 15.05.2023; Available at: https: / / elbe-dichtungen.de/ [3] Popov, V. (2010). Kontaktmechanik und Reibung, Von der Nanotribologie bis zur Erdbebendynamik (3 rd ed.) Berlin, Germany: Springer-Verlag, ISBN: 978-3-662- 45974-4. [4] Nissler, U. (2015). Dichtheit von Hydraulikstangendichtringen aus Polyurethan: Einfluss von Geometrieveränderungen an PU-Nutringen auf deren Dichtverhalten und Vergleich verschiedener Dichtheitsbewertungen. Dissertation. Universität Stuttgart, Institut für Maschinenelemente. Available at: https: / / elib.uni-stuttgart.de/ bitstream/ 11682/ 4633/ 1/ Dissertation_Nissler.pdf. ISBN: 978-3-936100-62-4. [5] Moldenhauer, P. (2014). Modellierung und Simulation der Dynamik und des Kontakts von Reifenprofilblöcken. Dissertation. Technische Universität Bergakademie Freiberg, Fakultät für Maschinenbau, Verfahrens- und Energietechnik, Freiberg. Accessed on 17.07.2022; Available at: https: / / tubaf.qucosa.de/ api/ qucosa%3A22727/ attachment/ ATT-0/ [6] Will, D. and Gebhardt, N (2011). Hydraulik: Grundlagen, Komponenten, Schaltungen (5 th ed.), Dresden, Deutschland: Springer Verlag, ISBN: 978-3-642- 17242-7.