Available online at ScienceDirect. Procedia Engineering 176 (2017 ) Dynamics and Vibroacoustics of Machines

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1 Available online at ScienceDirect Procedia Engineering 176 (2017 ) Dynamics and Vibroacoustics of Machines The fluid power elements and systems made of plastics Stryczek J.*, Banaś M., Krawczyk J., Marciniak L., Stryczek P. Department of Machine Design Fundamentals and Tribology, Fluid Power Research Group, Wroclaw University of Technology, Ignacego Łukasiewicza Street 7/9, Wrocław, Poland Abstract The Fluid Power Research Group from Wroclaw University of Technology have been concentrating their research and development activity on the application of plastics for the construction of hydraulic components. Their work focuses primarily on the gerotor pump, the pressure relief valve, the manifold and the hydraulic cylinder. The paper presents the theoretical basis of designing those elements, examples of the design solutions, as well as the experimental research results. A prototype of the hydraulic system featuring the plastic elements has been assembled and tested, and it proves to work properly The Authors.Published by by Elsevier Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility ofthe organizing committee of the International Conference on Dynamics and Vibroacoustics of Peer-review Machines. under responsibility of the organizing committee of the international conference on Dynamics and Vibroacoustics of Machines Keywords:hydraulic components; plastics; gerotor pump; production 1. Introduction Application of plastics instead of metals is a currently observed tendency in the making of fluid power elements and systems. The use of plastics is advantageous both for the construction and operational reasons, as it leads to: reduction of weight and size, reduction of vibration and noise, improvement of tribological properties, capability of self-sealing, increase in resistance to impurities, ability to work with a variety of working media including water and aggressive chemicals. It is also advantageous for technological reasons as it enables: manufacture of complicated profiles of the fluid power elements using a simple method of injection molding, simplifying and shortening of the production process. The application of plastics is profitable also for economic reasons as it: lowers the material costs,lowers the * Corresponding author. Tel.: ; address: jaroslaw.stryczek@pwr.edu.pl The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of the international conference on Dynamics and Vibroacoustics of Machines doi: /j.proeng

2 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) manufacture costs and reduces the production time. Nevertheless, along with the advantages, the use of plastics has some disadvantages, and those are: decrease in strength of the elements and the narrowing of the working pressure range, occurrence of the elements' size instability resulting from the change in the working fluid temperature and from the absorption of water solved in the working fluid. Hence, the researchers look for plastics which would be more useful in the making of fluid power elements and systems [1], as well as give examples of their application [2]. They also provide rules of designing plastic machine units such as, for instance, bearings [3]. In papers [4, 5], rules of designing and manufacturing plastic gear systems used in gerotor pumps were presented. The experimental research described in [6] proved the plastic gear system in the pump to work properly. In [7], a possible application of plastics as the material for the making of the slipper in the piston pump was shown. A plastic has also been successfully used for the making of hydraulic cylinders [8]. In [9], high applicability of plastics as well as special manufacture technologies of the plastics for the making of the high-pressure tanks and hydraulic cylinders are described. In that situation, the Fluid Power Research Group Wroclaw - FPRGWroclaw ( from Wroclaw University of Science and Technology, took up systematic research and development work aiming at: the development of the design and the manufacturing of the models of a set of the basic hydraulic elements made of plastics combining a gerotor pump, pressure relief valve, and a 2 position/2way directional control valve, and a hydraulic cylinder, the development, manufacturing, and testing of an exemplary fluid power system including those elements. Prior to the research and development work, it was assumed that at least half of the hydraulic machines' and elements' parts would be made of plastics. At the same time, they were supposed to be the essential parts, in terms of the design and operation of those units. 2. Plastics as the material for the construction of hydraulic components Analysis and selection of the plastic is the first and crucial stage of designing the fluid power components. The following selection criteria for the plastic were applied: possibly high strength manifested by high yield strength Re and a high Young's modulus, resistance to high temperature (T), high dimensional stability understood as a small linear elongation (W) and low material shrinkage (S) resulting from the injection process, small absorption (A) of water from the working fluid, low price and availability on the market, the processability by injection molding. For the construction of the hydraulic components, three types of material were selected, namely POM, PPS and PEEK. Their properties are presented in table 1. POM was treated as the base material featuring sufficient mechanical properties, the processability by the injection molding, as well as a low price and availability on the market. Another material used was PPS which showed higher strength but, at the same time, was harder for processing and more expensive. The material of the highest strength was PEEK, which was mechanically processed and, simultaneously, the most expensive. It is also possible to compose a special material combining the base material and additives. Tab. 1. Properties of the chosen plastic materials (polyoxymethylene - POM, polyphenylsulfone - PPS and polyetheretherketone PEEK) No Parameter Symbol Unit POM PPS PEEK 1. Yield strenght Re MPa Young's modulus E MPa Maximum work temperature T o C Shinkage S cm/cm Coefficient of linear expansion W cm/ o K Coefficient of water absorption A % Gerotor pump with plastic gears The design of the gerotor pump of q = 10 cm 3 /rev displacement has been presented in figure 1. The body of the pump consists of three parts, namely of the front part (1), the central part (2) and the back part (3). All the three parts

3 602 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) of the body are connected by screw joints (4). Inside the central part of the body, there is an internal cycloidal gear system (6,7) of z 2 z 1 =7-6=1 tooth difference. The operation principle of the pump is the following: the gears are driven by the shaft (5), and they rotate in the slidebearing (8). The working fluid supplied to the pump through the inlet (I), gets into the inlet chamber, and from the inlet chamber (CR I ), into the spinning intertooth chambers (T) made between the teeth of the epicycloidal gears. From the intertooth chambers, it is pumped into the outlet chamber (CR o ), and next, into the outlet (O). Fig. 1. Design of the gerotor pump 1, 2, 3 body parts, 4 screw joints, 5 shaft, 6, 7 epicycloidal gears, 8- bearing ring, I inlet, O outlet, CR I inlet chamber, CR O outlet chamber, T intertooth displacement chamber Figure 2 illustrates the gear system used in the gerotor pump, and figure 2a depicts the POM gears, figure 2b shows the PPS gears, whereas figure 2c, the PEEK gears. The gears have been designed according to the rules given in [10, 11]. The gears feature small height, a streamlined tooth profile (1), relatively thick rings under the gearing (2, 3), and three keys in the driving external gear (4). The POM and PPS was made by means of injection molding, while the PEEK gears, by means of machining. The gears' teeth are smooth. All the teeth in the gear system get into mesh with zero intertooth radial clearances h r. The side surface of the gears is also smooth, and the accurate fitting of the pump's central body provides the required axial clearance h a (figure 2). Fig. 2 The gears made of a) POM, b) PPS and c) PEEK Along with the manufacturing process, the FEM analysis of the gear system was being carried out [11]. Exemplary stress and strain of the POM gear system loaded with torque M = 7 MPa and pressure p = 4 MPa are presented in figure 3. The figure shows that the stress in the gears did not exceed the maximum allowed values. The

4 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) figure depicts also the gears' deformation generating intertooth clearances h rp which result in the leakage inside the pump and in a decrease of the pump's volumetric efficiency. The systematic FEM analysis [12] showed that the POM gears can work within pressure range of p = 6 MPa. The research on the gerotor pump with the plastic gears was conducted on a test stand in the laboratory of the Fluid Power Research Group from Wroclaw University of Technology. The research results are presented in figure 4. Figure 4a shows that highest values of volumetric efficiency η v are achieved in the case of using the PEEK. The material has the highest strength and deformations of gears are smallest. The efficiency of the pump with POM gears is higher than efficiency of the pump with PPS gears. Hence, it may be concluded that the POM gears, while working, absorb water contained in the working fluid (see table 1), boost their size and, consequently, self-seal the pump. The PPS gears practically do not absorb water, so their size remains unchanged. In that case, a pressure increase in the pump leads to an increase in the size of the internal clearances and simultaneously, a decrease in volumetric efficiency η v. An increase in rotational speed n of the pump boosts efficiency η v. a) b) Fig.3. Visualisation of the FEM analysys results of the POM gears: a) stress, b) strain Figure 4b shows that a rise of the working fluid temperature results in a fall of volumetric efficiency η v. It is a typical phenomenon, as along with the temperature rise, the working fluid viscosity lowers, which results in an increase of the leakage through the internal clearances of the pump, and consequently, in the efficiency reduction. A long-lasting (50 hours) test of the pump with the PPS gears was done. The result of the test was positive, namely the pump was working failure-free. After the test, on the surface of the teeth and on the end faces of the gears, only some wear traces caused by the parts' collaboration were observed. Fig.4 Results of the research on the gerotor pump featuring plastic gears a) Volumetric efficiency of the pump depending on the plastic type b) Volumetric efficiency of the pump depending on the working fluid temperature (viscosity)

5 604 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) In summary, it is concluded that the correct operation and achieved technical parameters of gerotor pump depend on the design of the gerotor gears system.during operation these gears are deformed, which is causedby mechanical loads (torque and intertooth force) and hydraulic loads (pressure of the working fluid).deformations cause increased friction of gears and formation of increased internal gaps in the pump. As a result, it reduce the working pressure and causes a decrease in volumetric and totalefficiency of the pump.therefore, the design process should consider the following challenges. You should develop a design solution and choose the material to achieve the highest possible stiffness and strength of the gears and the stability of their shape and dimensions.at the same time gears material should be resistant to high temperatures and the impact of various working fluids (oil, water, aggressive chemicals).the strength of gears, the stability of dimension and the accuracy of surface is achieved during the process for production.therefore, the effectiveness of using both the machining, as well as injection molding technology was confirmed,wherein for the preparation of the models and prototypes more useful and less expensive is machining technology. 4. Pressure relief valve made of plastics The design of a pressure relief valve of Q = 15 dm 3 /min flow rate is presented in figure 5a. The basic elements of the valve are the poppet (1) and the valve seat (2). They are closed in the body of the valve (3). The poppet (1) is pressed to the seat (2) by the spring (4), and the pushing force is set by means of the knob (5). The valve is closed by the upper cover (6) and by the lower cover (7). All the elements, except for the spring, are made of plastics. The operation principle of the valve is the following: the working fluid gets into the valve through the inlet (I); if the force working on the poppet (1) is bigger than the spring tension force, the poppet will go up and the working fluid is let through the outlet (O) into the tank. A view of the valve's model made of POM has been presented in figure 5b. In the figure, the inlet (I) and the outlet (O) of the valve have been marked. a) b) Fig. 5 Design of the pressure relief valve a)the pressure relief valve: 1- poppet, 2 valve seat, 3 body, 4 spring, 5 knob, 6,7 - cover, I- inlet, O outlet b)view of the valve Q = 15 dm 3 /min The valve was experimentally verified, and then subjected to the design optimization consisting in mutual matching of the poppet (1) and the valve seat (2), as well as in the spring selection (3). The flow characteristics defined during the process are presented in figure 6a. The figure shows that the optimization caused equalization of the characteristics and resulted in the determination of the flow rate through the valve, to the assumed value of Q = 15 dm 3 /min. On the other hand, figure 6b shows that the flow characteristics of the plastic valve are similar to the flow characteristics of the metal valve. A subsequent experimental study showed that a plastic valve as the pump can operate correctly in the pressure range of p = 6 MPa

6 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) Fig. 6 Characteristics of the plastic pressure relief valve Q = 15 dm3/min a) Characteristics of the valve before and after the poppet profile optimization b) Comparison of the plastic and the metal valve's characteristics (industrial valve) 5. Directional control valve made of plastics Based on the proven design solution of the pressure relief valve, a pneumatically controlled 2/2-way directional control valve of Q = 15 dm 3 /min flow rate was built. The design of the valve is presented in figure 7a. a) b) Fig. 7 Design of the 2/2-way directional control valve, Q = 15 dm 3 /min a) The directional control valve: 1 - poppet, 2 valve seat, 3 body, 4 spring, 5 knob, 6,7 covers, 8 piston, 9 stem, 10 spring, J inlet, O outlet, C control port b) View of the 2/2-way directional control valve, Q = 15 dm 3 /min Components of the directional control valve such as the poppet (1), the seat (2), the body (3), the spring (4), the knob (5) and the covers (6, 7) remain the same as the ones in the pressure relief valve. However, in the lower part of the body (3), the piston (8) collaborating with the poppet's stem (9) and the spring (10) is located. The operation principle of the valve is as follows: through the opening (C) in the bottom cover (7) compressed air is fed, which lifts the piston (8), the stem (9) and the poppet (1) against the seat (2). Through the gap between the poppet and the

7 606 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) seat, the working fluid flows between the inlet to the directional control valve (I) and the outlet (O). A view of the model of the directional control valve is shown in figure 7b. In conclusion it is noted that the correct operation and achieved technical parameters of the pressure relief valve and control valve made of plastic, depends mainly on the construction solution of popet seat unit and associated with this unitinlet and outlet channels, and they also depends on the selection of material. The popet seat unit is loaded with pressure and with the spring tension force. In the closed state, the pressure causes deformation of that unit and the formation of internal leakage. In the open state, the pressure causes a change in shape of the system of inlet channel, popet seat gap, drainage channel. The result is a variable flow conditions and variable pressure, which is not conducive to stable operation of the valves. Therefore, in the design process of valves from plastic following challengesshould be considered. The shape of popet seat unit and the material for this unit should be designed so as to ensure tightness and stiffness in the closed state in a situation of pressure load and spring tension force. At the same time this unit and cooperating with itchannels should have an optimum shape and size for the stability of flow processes occurring in the open state. As in the caseof pumps, as well as for valves, it is important to ensure high accuracy of shape and dimensions that can be achieved from both the machining, as well as injection molding technology. 6. Hydraulic cylinder made of plastic The design of a double action hydraulic cylinder made of plastics is presented in figure 8a. The cylinder has a diameter of 50 mm and a stroke of 400 mm. a) b) I (O) Fig. 8. Design of the double action hydraulic cylinder made of plastics ϕ = 50 mm, L = 400 mm a) The cylinder: 1 barrel (tube), 2, 3 caps, 4 piston rod, 5-piston, 6 -screws, 7, 8, 9 seals b) View of the cylinder model The cylinder consists of a barrel (1) closed on both sides with caps (2, 3). Inside the barrel, there is a rod (4) connected with a piston (5). The whole cylinder is jointed by screws (6). The covers, the rod and the piston are sealed by rings (7, 8, 9) of different types. The piston and he screws are made of steel, while the other components of the cylinder are made of plastics. The connection ports of the cylinder are labeled as (I) and (O). A view of the cylinder's model is shown in figure 8b. The hydraulic cylinder was subject to FEM analysis.

8 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) a) b) Fig. 9. Results of tests on the hydraulic cylinder made of plastics a)view of the stress and deformation in the actuator made of POM b)volumetric efficiency of the cylinder depending on pressure ηv= f (p) Sample results of the analysis are shown in figure 9a. It was found that loading the cylinder's barrel with pressure P = 5 MPa did not result in the exceeding the allowable stress. At the same time, the deformation of the cylinder's barrelperpendicular to the axis of the cylinder was small enough to be compensated by the elastic deformation of the piston seals. Next, the cylinder was subjected to verification tests on the test stand, and the test results are illustrated in figure 9b. The figure shows that the cylinder worked steadily within the working pressure range up to p = 5 MPa, which was assumed for the FEM analysis, and at relatively high volumetric efficiency of η v = %. In the case of the actuator, correct operationand achieved technical parameters depend mainly on the design of tube - piston rod sealsunit.under the influence of the pressure tube is deformedwhich is increasing the gap between the piston rod and tube,and sealing does not ensure inner tightness of the cylinder.the gap varies depending on the position of the piston rod in tube, and therefore the tightness of the cylinder is also affected, and the movement of the piston may be uneven.therefore, in the design of actuators there are the following challenges.tube should be made of a material with the greatest possible strength.it should have a thickness which will provide as low as possible transverse strains.appropriate material and thickness of the tube wall, are to ensure minimal changes in the gap between the tube and the piston rod and the ability to seal the cylinder.it is extremely important to maintain the circularity and accuracy of the inner surface of the tube, which cooperates with the seals.this is achieved by a special technology for producing a tube of plastics.accuracy of production of the tube determines the durability and reliability of the cylinder. 7. A hydraulic system featuring plastic components as an example of plastic components' practical application The fluid power components made of plastics presented above were used for the making of a prototype hydraulic system [15]. A view of the system is shown in figure 10. The system was launched in the laboratory of the FPRG at Wroclaw University of Technology. It worked properly performing the basic functions, i.e. extension and retraction of the piston rod at different loads. The system reached the operating pressures of p=3 4MPa. That confirmed the possibility of using the prototype system as a low-pressure system which can be used in different fields of machine design.

9 608 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) Fig.10. View of the hydraulic system: 1 pump, 2 pressure relief valve, 3 directional control valve, 4 hydraulic cylinder,5 electric box, 6 motor, 7 tank Plastics have also been used for the designing and making of the body of an external involute gear micropump, featuring displacement of Q = 0.3 dm 3 / min and n = 1500 rev/min (see figure 11). As given in [16], the development of a plastic body of the gear pump is aimed at the reduction of its weight, dampening of vibration and noise reduction. From the pump, bearings have been eliminated, as the holes made on the sides of the pump, simultaneously become the bearing seats, where the gear pins rotate. Fig. 11 The external involute gear micropump with the plastic body Q = 0.3 l/min, n = 1500 rev/min[16]

10 J. Stryczek et al. / Procedia Engineering 176 ( 2017 ) Summary The results of the construction, technological and experimental work taken up by the FPRG confirm the possibility of using plastics for the making of typical fluid power elements. Those are elements such as a gerotor pump, a pressure relief valve, 2/2-way directional control valve and a hydraulic cylinder. Models of those elements made by the FPRG proved to work properly at pressure p= 4-5 MPa and flow rate of Q = 15 dm 3 /min. The design and research work carried out shows that the correct operation and achieved technical parameters of hydraulic components made of plastic depends on: - design solution of basic units and parts of these devices (gerotor pump - a set of gerotor gears, valves - popet / seat unit, cylinder - a set of tube / piston rod / seals) - the selection of material, which should have a high strength, dimensional stability, heat resistance and resistance to impact of the different working media, - selection of components productiontechnology (machining, injection molding); these technologies should ensure the accuracy of the shape and dimensions, as well as accuracy and texture of surface. Improving those factors should lead to the making of the basic fluid power elements' prototypes such as pumps, valves or cylinders, which would work reliably at pressure p = 6 MPa and flow rate Q = 1-15 (20) dm 3 /min. The FPRG Wroclaw from Wroclaw University of Science and Technology are systematically working to achieve those objectives. References [1] Bonanno A., Materialli plastici, caratte ristiche a limitti., Oleodynamica Pneumatica, apricle 2008, p-p [2] Bonanno A., Materialli polimerici applicaziani, Olevdynamica Pneumatica, Aprile, 2008, p [3] Wieleba W., Bezobsługowe łożyska ślizgowe z polimerów termiplastycznych (Maintenance-free slide bearings made of theromoplastic polymers) Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław 2013 [4] Stryczek J., Bednarczyk S., Zienowicz Z., Gerotor pumps with plastic gears, Hydraulika i Pneumatyka, 2006, R. 26, nr 5, s [5] Stryczek J., Bednarczyk S., Biernacki K., Application of plastics in manufacture of the gerotor pump, The Twelfth Scandinavian International Conference on Fluid Power, May , Tampere, Finland [6] Krawczyk J., Stryczek J., Construction and experimental research on plastic cycloidal gears used in gerotor pumps, 8th FPNI Ph.D Symposium on Fluid Power [Dokument elektroniczny], Lappeenranta, Finland, June 11-13, [New York] : ASME, cop s [7] Rokala M., Kaskinen K.T., The effect of PV-rate on PEEK-made slipper behaviour in water hydraulic axial piston unit, The Twelfth Scandinavian International Conference on Fluid Power, SCIFP 11, vol 1(4) Tampere, Finland, May 18 20, 2011, pp [8] Harnisch M., Kunststoffe n fluidtechnischen Antrieben Oelhydraulik und Pneumatik, 11 12/2013, pp [9] Błażejewski W., Kompozytowe zbiorniki wysokociśnieniowe wzmocnione włóknami wg wzorów mozaikowych (High-pressure composite mosaic pattern fibre-reinforced tanks), Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław 2013 [10] Biernacki K., Selection of the optimum tooth profile for plastic cycloidal gears, Journal of Mechanical Engineering Science 2014, vol. 228 (18), pages , SAGE [11] Biernacki K., Stryczek J.: Analysis of stress and deformation in plastic gears used in gerotor pumps, The Journal of Strain Analysis for Engineering Design, October 2010, vol. 45, Issue 7, pages [12] Stryczek, J., Bednarczyk S., Biernacki K., Strength analysis of the polyoxymethylene cycloidal gears of the gerotor pump, Archives of Civil and Mechanical Engineering, vol. 14, no. 4, 2014, pages [13] Marciniak L., Banaś M., Badania eksperymantalne prototypowego zaworu przelewowego z tworzyw sztucznych (Experimental research on the prototype pressure relief valve made of plastics), Napędy i sterowania hydrauliczne i pneumatyczne 2015 : międzynarodowa konferencja naukowo-techniczna (Hydraulics and pneumatics 2015: international scientific-technical conference), Szklarska Poręba-Jakuszyce, marca Wrocław : Ośrodek Doskonalenia Kadr SIMP, s [14] Stryczek P., Banaś M., Badania prototypu eksperymentalnego siłownika hydraulicznego z tworzyw sztucznych (Research on the prototype of an experimental hydraulic actuator made of plastics), Napędy i sterowania hydrauliczne i pneumatyczne 2015 : międzynarodowa konferencja naukowo-techniczna (Hydraulics and pneumatics 2015 : international scientific-technical conference), Szklarska Poręba- Jakuszyce, marca Wrocław : Ośrodek Doskonalenia Kadr SIMP, s [15] Stryczek J., Bednarczyk S., Biernacki K., Wołodźko J., Koncepcja układu hydraulicznego z elementami wykonanymi z tworzyw sztucznych (A concept of a fluid power system featuring plastic elements), Międzynarodowa konferencja naukowo-techniczna Napędy i Sterowania Hydrauliczne i Pneumatyczne 2012, SIMP, Maj , Wrocław [16] Rodionov L., Rekadze P., Stryczek J., A gear micropump without bearings production, Applied Mechanics and Materials, vol. 775 (2015), pages