28 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 1. Introduction and Motivation An effective method to reduce CO 2 emission in mobile systems, especially in automotive applications, is the reduction of moving masses. Due to that, lightweight design is getting applied in an increasing number of products. A field which has still high potential for applying lightweight design are hydraulic systems. Actual hydraulics systems are often oversized, which leads to high masses and also to high costs. In order to identify the lightweight potential in a first step, each component of a hydraulic system has to be analysed concerning an optimization of masses. A high potential was identified for the main component of a hydraulic system, an axial piston pump. A possibility to realize lightweight design in the axial piston pump is the replacement of steelcomponents with components made of polymeric material or hybrid design. If these components are part of a tribological system, the replacement will lead to an unknown change of the behaviour of the tribological system. This is because in this application the behaviour of polymeric materials or hybrid design components is still unknown. Aus Wissenschaft und Forschung * Univ.-Prof. Dr.-Ing. Dr. h. c. Albert Albers M.Sc. Markus Blust Dipl.-Ing. Knut Wantzen Dr.-Ing. Benoit Lorentz IPEK - Institute of Product Engineering, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany A New Tribological Test Bench for Lightweight Hydraulic Axial Piston Pumps A. Albers, M. Blust, K. Wantzen, B. Lorentz* Eingereicht: 13. 3. 2016 Nach Begutachtung angenommen: 1. 7. 2016 Aus Gründen der Ressourcenschonung besteht bei hydraulischen Systemen in mobilen Anwendungen, wie beispielsweise den Axialkolbenpumpen, zunehmend die Forderung nach Einsparung von Gewicht und Kosten. Hierfür wird häufig die Leichtbaustrategie der Materialsubstitution genutzt. Angewendet auf Komponenten in tribologischen Systemen, wie das Kolben-Buchse System in einer Axialkolbenpumpe, führt dies oft zu einem unbekannten Verhalten in den tribologischen Kontakten. Daher wird auf die Anwendung der Materialsubstitution verzichtet und Leichtbaupotentiale bleiben ungenutzt. Um ungenutzte Potentiale im tribologischen System Kolben-Buchse einer Axialkolbenpumpe nutzen zu können, wurde am IPEK ein Prüfstand entwickelt, der die Charakterisierung der Eignung verschiedene Materialien ermöglicht. Der Prüfstand, das verwendete Kontaktmodell und erste Versuchsergebnisse werden vorgestellt. Schlüsselwörter Axialkolbenpumpe, Hydraulik, Leichtbau, Polymer- Werkstoffe, Prüfstand, Tribologie The aim to reduce the costs and the mass of hydraulic systems, for the example of an axial piston pump, is getting even more important. To reach this target, it is necessary to optimize the actual hydraulic systems by integrating lightweight designs. An often used design strategy is the so called “material substitution”. Typical material substitutions are replacements of steelcomponents with polymeric material or hybrid design components. If the substitution is affecting parts of a tribological system, the substitution changes the behaviour of the tribological system in an unknown way. Because of that, parts in a tribological system are mostly excluded from a lightweight design. To use the potential of the lightweight design also in parts of tribological systems, the IPEK developed a test bench, which makes it possible to investigate and characterize different materials for a substitution in the tribological contact between the pistons and the cylinder of an axial piston pump. The descriptions of the contact-modeling as well as first measurement results of the test bench are presented in this paper. Keywords axial piston pump, hydraulic, lightweight design, polymer materials, test bench, tribology Kurzfassung Abstract T+S_2_17 30.01.17 11: 59 Seite 28 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 The IPEK developed the test bench RPT to investigate lightweight materials and their behaviour in tribological contacts of an axial piston pump and to develop a scientific understanding of the tribological system. The analysis of the tribological contacts in an axial piston pump has shown that the contact between the pistons (figure 1, number 1) and the cylinder (figure 1, number 2) is highly stressed during operation and a replacement of the materials has a great effect on the tribological behaviour. Due to that, a test bench was designed based on a model of this tribological contact. A detailed description of the model is given in chapter 3. The test bench is presented in chapter 4 and first measurement results are shown in chapter 5. The IPEK has also developed a test bench for the tribological analysis of the contact between cylinder and control plate of an axial piston pump, the so called RPR test bench. The test bench RPR is presented in [2], [3] und [4]. 2 State of the Art Basic considerations of the kinematic processes and of the conditions in the tribological contact between the piston and the cylinder of an axial piston pump are shown by van der Kolk [5], Renius [6] and Regenbogen [7]. There are various materials available to decrease the overall mass of the system. The principle suitability of ceramics and polymers for their application in hydraulic systems is shown by Donders et al. [8]. The use of ceramics for components of hydraulic systems is discussed by Bartelt et al. [9] and Feldmann [10] and Schöpke [11]. Bartelt et al. [9] describe the development of test procedures for components of hydraulic systems made of ceramics. Feldmann [10] shows which developments in the field of ceramics could be of interest for hydraulic systems. A consequence of this work is that there is a potential of common ceramics for the use in components of hydraulic systems. He also shows that development is necessary to improve the components of hydraulic systems. Schöpke [11] works on simulation and testing of ceramics in the contact between cylinder and control plate of an axial piston pump. Investigations on PVD-coatings for optimization of friction and wear in the tribological contacts of a hydraulic axial piston pump are shown by Murrenhoff / Scharf [12], van Bebber [13] and Kleist et al. [14]. PVDcoated components of a hydraulic axial piston pump are tested with test benches by Murrenhoff / Scharf [12]. Their main finding is that there are PVD-coatings with high potential for the use in components of hydraulic axial piston pumps. The paper of van Bebber [13] contains investigations on PVD-coated pistons and control plate of a hydraulic axial piston pump and shows that with PVD-coatings a minimization of friction and wear is possible. The result of the investigations of Kleist et al. [14] is that the properties of PVD-coatings have to be adapted to the case of application in hydraulic systems. A multitude of polymers have been tested by Künkel [15] on a pin-on-disk test bench as well as on a journal bearing test bench. This way he gains knowledge for the estimation of the tribological behavior of the tested materials. The general tribology of polymers, polymer composites and ceramics is studied in [16]. In [17] especially tribological tests and results are presented. Pin-on-disk sliding friction and wear experiments were conducted on two different titanium alloys by Qu et al. [18]. Disks of titanium alloys were slid against fixed bearing balls composed of stainless steel, silicon nitride, alumina, and PTFE. One of the results was that a higher friction coefficient with larger fluctuation and higher wear rate was observed at lower sliding speed. Another result was that ceramic sliders experienced much higher wear and created more wear on the counterfaces than the stainless steel sliders did. 3 Test Bench Model The volumetric flow in an axial piston pump is realised by an oscillating backand forward movement of the pistons (figure 1, number 1) in the piston bushes of the cylinder (figure 1, number 2). During the forward movement, the pistons push the fluid out of the piston bushes and generate the oil pressure and volumetric flow. In the backward movement, the pistons suck new fluid into the pistons bushes. To realize the backand forward movement of the pistons and to translate the rotation of the driveshaft into a translation, the pistons are connected to a swashplate. The swashplate is directly connected to 29 Aus Wissenschaft und Forschung Figure 1: Axial piston variable pump, design Bosch- Rexroth, type A10VNO [1] T+S_2_17 30.01.17 11: 59 Seite 29 30 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 the driveshaft. This leads to a translational movement of the pistons. But due to the swashplate and the oil pressure in front of the pistons, the pistons get tilt in the piston bushes and generate a force on edge at the front of the piston bushes. According to that, the tribological contact of the RPT test bench is modeled as a line contact. As shown in figure 2, a piston rod with the diameter d k is oscillating up and down with a stroke of h k while two half shells are pressed with their edges on the rod with a defined force F K and realizes the tribological line contact. The diameter of the rod is variable in a range of 12 to 20 mm. To realize a realistic load in the contact area, it is possible to apply a force FK with a maximum of 6 kN on every half shell. Unlike the real system, the forces are kept constant, which models a worst-case condition. In the real system, during the different phases of pushing and sucking the oil, the force decreases periodically and unloads the tribological contact. Because of the simple geometry of the piston rod and the half shells, it is easily possible to manufacture them out of a wide range of lightweight materials and with various topographies. Furthermore the two half shells makes it possible to test two test specimen at the same test. In the opposite to the real system, the piston rod and the two half shells are working under an oil bath lubrication without any oil pressure. Compared with this, in the real system occurs unsteady oil pressure and thereby an unsteady form of pressure lubrication. The oil pressure has a load supporting effect which leads to less friction and wear. Due to that, lubrication without pressure is a worst case scenario for the tribological behaviour of the contact. Because of that, at the RPT the oil bath lubrication is realized. It is possible to use various lubricants and to temper them with a heating and cooling system within a range T Fluid , with the constraint that it is not possible to realize oil temperatures below 20 degree. An overview of the properties of the RPT test bench and their ranges is given in table 1. The RPT is equipped with a measurement system which allows measuring the occurring friction force between the piston rod and the two half shells. Wear measurement is realized in an indirect way: the half shells are weighed before and after testing. To get a sufficient statistical probability of the test results, the RPT test bench also features the possibility for testing 8 tribological contacts with the same testing-parameters at the same time. 4 Design of the Test Bench Figure 3 shows the realized design of the RPT test bench. The main component is the test chamber (figure 3, num- Aus Wissenschaft und Forschung Figure 2: Modeling of the test bench RPT Figure 3: Design of the test bench RPT Table 1: Characteristics of the test bench RPT Diameter of the pistons d K 12 - 20mm Pressure load F K < 6 kN Number of strokes n K < 500 1/ min Height of stroke < 30 mm Oil lubrication process oil-bath lubrication Lubricant temperature T Fluid 20 - 80 °C Number of tribological during simultaneous testing 8 T+S_2_17 30.01.17 11: 59 Seite 30 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 ber 1), placed on a test bench table (figure 3, number 6). The test chamber integrates four test units (figure 3, number 2): Two test units in front and two test units on the back of the gearbox (figure 3, number 4). Every test unit allows a testing of two tribological contacts. The test bench is driven by an electric engine (figure 3, number 5). The gearbox transmits the drive power to two parallel shafts. At the end of the both shafts are two eccentrics (figure 3, number 3). With the eccentrics the rotation is translated in an oscillating movement which drives the piston rods in the test units. Figure 4c shows a cross-sectional view of a test unit. The conrod (figure 4c, number 1) in combination with the eccentric realizes the oscillating vertical movement of a carrier (figure 4c, number 2). This carrier is radial guided by a sleeve bearing (figure 4c, number 3). On this carrier, the piston rod specimen (figure 4c, number 4) is clamped by a clamping fixture (figure 4, number 5). At the lower end of the specimen a second carrier (figure 4, number 6) is clamped (figure 4, number 7) to the piston rod. This carrier is also guided by a sleeve bearing (figure 4, number 8) and guarantees a radial guiding of the piston rod specimen. The both half shells (figure 4c, number 9) are fixed in two holders, which are at one side simply supported (figure 4a, number 12 and figure 4b) and on the other side the hydraulic cylinders (figure 4c, number 10) are pressing them down. A constant pressure load of the hydraulic cylinders is guaranteed by a particular closed loop control with a pressure proportioning valve. Due to the pressure load of the hydraulic cylinders, the edges of the half shells are pressed against the piston rod specimen. Every test unit realizes two tribological contacts. During operation, the pressure of the half shells on the piston rod creates a friction force. This force leads to a strain in the four hanging rods of the half shell holders (figure 4c, number 11). Compressiontension resistance gauges are located on the hanging rods. With them it is possible to measure the strain and therewith the friction force between the piston rod and the two half shells. The compressiontension resistance gauges are measuring a constant load of the hydraulic cylinders superposed with an oscillating load of the friction force. The constant part of the measured signal could be taken for the calculation of the force F K on the half shells (based on geometrical properties of the test units). The oscillating part of the signal is nearly the friction force of the tribological contacts of the two half shells. Assuming that the friction forces are the same in both contacts, the half of the measured friction force can be correlated with the friction force of one contact. Furthermore the arrangement of the resistance strain gauges is chosen in a way that the measurement is not influenced by the temperature of the fluid. At the moment only two of the four test units of the test bench RPT are equipped with compressiontension resistance gauges. Additional test units could be also equipped with compressiontension resistance gauges. The test chamber is filled by the test fluid and tempered by a continuous-flow heater and a cooler within a range T Fluid . A special control circuit with thermocouples in the test chamber guarantees a constant temperature of the test fluid. 31 Aus Wissenschaft und Forschung Figure 4: Test unit test bench RPT, a.) overall view, b.) principle sketch, c.) sectional view T+S_2_17 30.01.17 11: 59 Seite 31 32 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 5 Validation For a first validation of the test bench RPT and as a reference for further investigations, test components made of common materials for the pistons and piston bushes in axial piston variable pumps were taken under investigation. The characteristics of the test specimen and the lubricant are shown in table 2. The geometries of the test specimen are shown in figure 5. Figure 6 shows the defined test procedure for the validation of the test bench RPT. The pressure in the hydraulic cylinders is kept constant at 21,4 bar. There is also a constant number of strokes of 250 per min (broken line in figure 6) and a constant oil bath temperature of 50 °C. The duration of the validation test is 60 min. The test procedure was repeated two times. Each time with completely new test specimen and the friction force was measured with both test units equipped with compressiontension resistance gauges. In figure 7 and figure 8 the results of both validation runs are shown. Based on the signals of the compressiontension resistance gauges, the friction force and the pressure load F K on the edges of the half shells were calculated. A comparison of figure 7 with figure 8 shows reproducible results for the friction forces and pressure loads F K in each validation run and also reproducible results in each test unit. Therewith the test bench RPT is verified based on the test procedure shown in figure 6. In figure 7 and figure 8 the friction forces are increasing over the duration of the validation test. A reason for this could be the increase of the contact area between pistons and half shells because of the wear. Aus Wissenschaft und Forschung Table 2: Validation parameters test bench RPT Material of the pistons Heattreated steel Material of the half shells Nodular graphit cast iron Lubricant H-LP 32 (DIN 51524) Diameter of the pistons d k 14,5 mm Figure 5: Geometry of the test specimen for the validation of the test bench RPT Figure 6: Validation test procedure for the test bench RPT T+S_2_17 30.01.17 11: 59 Seite 32 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 In figure 9 and figure 10 extracts of the friction forces of both validation runs are shown. A comparison of figure 9 with figure 10 shows also reproducible results regarding the profile of the measured friction forces. There is a phase shift between the signals of the test units 1 and 2 in both figures because of the different positions of the eccentrics of the test units. Figure 9 and figure 10 show increasing friction forces depending on the moving direction of the piston. 6 Conclusion A test bench for the analysis of the tribological behavior of the contact between piston and piston bush in the axial piston variable pump was developed. The main objective of the developed test bench is to test the tribological behavior of lightweight materials in this specific contact. The aim of the test bench is the modelling of tribological conditions like they are observed in the real system. Measurements done for a first validation of test bench indicated reproducible test results. Therewith the developed test bench is verified. There will be tests with lightweight materials in the future. Acknowledgments The authors would like to thank the German Federal Ministry of Education and Research for financial support within the framework of the research project “LHYDIA - Leichtbauhydraulik im Automobil”. References [1] Matthies, H. J., Renius, K. T.: Einführung in die Ölhydraulik. 7. Aufl., Vieweg+Teubner Verlag, Wiesbaden, 2012 33 Aus Wissenschaft und Forschung Figure 7: Results of the first validation run Figure 8: Results of the second validation run Figure 9: Extract of the friction forces of the first validation run Figure 10: Extract of the friction forces of the second validation run T+S_2_17 30.01.17 11: 59 Seite 33 34 Tribologie + Schmierungstechnik 64. Jahrgang 2/ 2017 [2] Albers, A., Blust, M., Lorentz, B., and Burger, W., 2014, “A New Tribological Test Bench for Light-weight Hydraulic Components”, In Proceedings TAE - 19 th International Colloquium Tribology - Industrial and Automotive Lubrication 2014, TAE - Technische Akademie Esslingen e.V. [3] Blust, M., Lorentz, B., and Burger, W, 2014, “Validation of a New Tribological Test Bench for Lightweight Hydraulic Components”, In Proceedings NORDTRIB 2014 - 16 th Nordic Symposium on Tribology, Danish Technological Institute [4] Blust, M., Lorentz, B., and Burger, W, 2014, “Validation of a New Tribological Test Bench for Lightweight Hydraulic Components”, TRIBOLO-GIA - Finish Journal of Tribology 2 vol 32/ 2014, 3-11 [5] van der Kolk, H.-J.: Beitrag zur Bestimmung der Tragfähigkeit des stark verkanteten Gleitlagers Kolben/ Zylinder an Axialkolbenpumpen der Schrägscheibenbauart. Dissertation, Universität (TH), Karlsruhe. Fakultät für Maschinenbau, 1972. [6] Renius, K. T.: Untersuchungen zur Reibung zwischen Kolben und Zylinder bei Schrägscheiben-Axialkolbenmaschinen. Düsseldorf: VDI-Verlag (VDI-Forschungsheft / Verein Deutscher Ingenieure, 561), 1974. [7] Regenbogen, H.: Das Reibungsverhalten von Kolben und Zylinder in hydrostatischen Axialkolbenmaschinen. Zugl.: Braunschweig, Univ., Diss., 1978. Düsseldorf: VDI-Verlag (VDI-Forschungsheft / Verein Deutscher Ingenieure, 590), 1978. [8] Donders, S., Kane, B., Seifert, V.: Einsatz von Kunststoffen und Keramik in Hydraulikkomponenten, 4. Internationales Fluidtechnisches Kolloquium, Dresden, 2004. [9] Bartelt, H. C., Scheunemann, P., Feldmann, D. G.: Methoden der Qualifizierung von Keramik als Werkstoff für Bauteile der Fluidtechnik, 4. Internationales Fluidtechnisches Kolloquium, Dresden, 2004 [10] Feldmann, D. G.: The use of ceramic materials for fluid power components. 80th Anniversary of Lithuanian University of Agriculture, 2004, S. 60-64 [11] Schöpke, M. Gestaltung, Berechnung und Erprobung hochbeanspruchter Keramikbauteile - Lagerplatte und Umsteuerplatte für eine Axialkolbenmaschine, Dissertation TU Hamburg - Harburg 1998, Fortschritt - Berichte VDI, Reihe 1, Nr.308 [12] Murrenhoff, H.; Scharf, S.: Verschleißreduzierung in hydraulischen Verdrängereinheiten durch Verwendung PVDbeschichteter Komponenten. Materialwissenschaft und Werkstofftechnik, 2004, 35. Jg., Nr. 10-11, S. 881-888 [13] Van Bebber, D. G.: PVD-Schichten in Verdrängereinheiten zur Verschleiß- und Reibungsminimierung bei Betrieb mit synthetischen Estern. Aachen, RWTH, Diss., 2003 [14] Kleist, A., Nevoigt, A., Lehner, S., Donders, S.: Oberflächenbeschichtungen - neue Möglichkeiten für die Fluidtechnik, Ölhydraulik und Pneumatik, Band 40, Heft 4, Seite 263-268, 1996 [15] Künkel, R.: Auswahl und Optimierung von Kunststoffen für tribologische Systeme, Dissertation Universität Erlangen-Nürnberg 2005 [16] Stachowiak, G. W.; Batchelor, A. W.: Engineering Tribology. 3. Aufl., Butterworth-Heinemann, Burlington, 2005 [17] Czichos, H.; Habig, K.-H.: Tribologie-Handbuch: Tribometrie, Tribomaterialien, Tribotechnik. 3. Aufl., Vieweg +Teubner Verlag, Wiesbaden, 2010 [18] Qu, J., Blau, P. J., Watkins, T. R., Cavin, O. B., & Kulkarni, N. S. (2005). Friction and wear of titanium alloys sliding against metal, polymer, and ceramic counterfaces. 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