Proceedings of the 8th FPNI Ph.D Symposium on Fluid Power FPNI2014 June 11-13, 2014, Lappeenranta, Finland
FPNI2014-7827 CONSTRUCTION AND EXPERIMENTAL RESEARCH ON PLASTIC CYCLOIDAL GEARS USED IN GEROTOR PUMPS
Justyna Krawczyk*, Jarosław Stryczek Cathedral of Fundamental Machine Design and Tribology Wroclaw University of Technology, Wroclaw, Poland
ABSTRACT1
of collaborating components and to reduce noise. In terms of technology of injection moulding, being the main processing method, it is possible to obtain very complex shapes. The use of this method is also beneficial in economic terms as it allows significant reduction in the material and manufacturing costs. However, it should be remembered that plastics as construction materials have certain disadvantages. The manufactured elements show lower strength and a reduced load-carrying capacity range. During operation, dimensional instability caused by high temperature or water absorption may occur. In spite of the disadvantages, plastics are being increasingly used in the construction of hydraulic machines and appliances. Therefore, the Fluid Power Research Group (www.fprg.pwr.wroc.pl) from the Institute of Machine Design and Operation at Wroclaw University of Technology, began their work aimed at the development of a complete hydraulic system in which the basic components such as the pump, the cylinder and the valves, or at least their main parts, would be made of plastics. This article presents the design solution and the results of experimental research on a gerotor pump with gears made of POM and PPS, which are dedicated to that hydraulic system.
Paper presents the construction and results of experimental research of gerotor pump with gears made of POM and PPS. Research program included verification of design factors (axial and radial clearance), operational factors (impact of speed, pressure and temperature on the pump characteristics) and definition of durability characteristics. Results of research proves that the pump with gears made of plastics can operate at high speeds (up to 4000 rpm), at temperature of the working fluid in the range 25°C - 50°C, with the working pressure at the outlet of the pump 4 MPa. Presented results of long-term tests, shows that the pump with gears made of plastics worked for 40 hours maintaining a constant level of efficiency Q = 12.5 dm3, which represents 80% of the theoretical volumetric efficiency. INTRODUCTION In the construction of hydraulic machines and appliances, as well as in other areas, more and more often the aim is to reduce the weight and dimensions of manufactured parts and to cut down on their manufacturing costs. However, in order to achieve that goal, it is necessary to search for new construction materials and new manufacturing methods. Therefore, the trend of using plastics as construction materials is becoming increasingly apparent. They are used for making components such as pump bodies, pipes, cylinders or gears. [1, 2, 3] The application of plastics is advantageous in many respects. In terms of design, it is possible to significantly reduce the weight of produced items, to increase their ability to suppress vibration, to improve the tribological properties
MATERIAL SELECTION, PROPERTY TESTING, MATERIAL STRENGTH PARAMETERS OF PPS AND POM
*
Corresponding author. Tel.: +48 71 3203112; fax: +48 71 3227645. E-mail addresses:
[email protected] (Justyna Krawczyk),
[email protected] (Jarosław Stryczek),
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The work on the hydraulic system with plastic elements, started from the design and production of the gerotor pump. It was assumed that plastics in the pump would be used to make a set of gears. The body, the bearing and the shaft would be made of steel. It was also planned, that for the sake of the experimental research, two gear sets made of two types of plastics, with different mechanical properties, would be used. Copyright © 2014 by ASME
POM, it can be observed that in the range of strain ε = 0,15 %, the stress values σr grow linearly, for larger values of strain, stress values grow parabolically. Therefore, it is assumed that the value of deformation ε = 0,15 % separates the elasticity area from the plasticity area, and σr = 60 MPa stress corresponding to that deformation is at the same time yield point Re = 60 MPa. Similarly, the yield point was determined for the PPS. The stress-strain curve of PPS shows that in the range of
The construction of the pump, with efficiency of q = 10 cm3/rev, is shown in figure 1. The pump body consists of three parts: the front (1), middle (2) and back body (3), which are connected by a screw joint (4). Inside the middle body (2), gears (6,7) with internal meshing and tooth difference z2 – z1 = 7 – 6 = 1 are located. This assembly is driven by a shaft (5) and rotates in a bearing (8) located in the middle body (2). The shaft is fixed in plain bearings (9).
Figure 1. Design and operation principle of the gerotor pump 1, 2, 3 – body parts, 4 – screw joint, 5 – shaft, 6, 7 – epicycloidal gears, 8 – bearing.
strain ε = 0,012 %, the stress values σr grow linearly, for larger values of the strain, the stress values grow parabolically. Same as in the case of POM, it is assumed that the value of deformation ε = 0,012 % separates the elasticity area from the plasticity area and σr = 180 MPa stress corresponding to that deformation, is simultaneously yield point Re = 180 MPa.
For the first phase of the research, it was decided to select a material generally available on the market, cheap and easy to process by means of the injection moulding method. It was also important that the chosen material had relatively good mechanical properties and a low water absorption coefficient. On the basis of the employed criteria for the preparation of the first gear set POM was selected as the construction material. For the second phase of the research it was decided that a material with higher strength properties than the previously used POM ought to be selected. The material was supposed to show greater dimensional stability, lower contraction, lower linear extension coefficient, higher resistance to high temperatures and lower water absorption coefficient than POM. That material, like POM, was supposed to be available on the market and easy to process by means of the injection moulding method. Taking all the criteria into account, PPS with additives was selected for the second phase of the research. Table 1 includes the basic properties of the two materials used. Before the selected materials were used to make the gears, they had undergone a static tensile test to determine their ultimate strength. It should be noted that this study does not take into account the fatigue tests. The results of the test are presented in figure 2. Analyzing the tensile curve for
Figure 2. Stress-strain diagram in a static tensile test for POM and PPS.[4]
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Table 1. Properties of polyoxymethylene (POM) and polyphenylensulfide (PPS).
No. 1. 2. 3. 4. 5. 6.
Property name Yield point Elasticity modulus The maximum operating temperature Contraction Coefficient of linear extension Coefficient of water absorption
Symbol Re E T S W A
Unit MPa MPa o C cm/cm cm/oK %
POM 60 3000 100 0,0285 0,0001 0,7
PPS 180 16000 160 0,005 0,00005 0,02
DESIGN SOLUTIONS: GEARS MADE OF POM AND PPS
pump (see figure 1) provides a desired axial clearance ha (see figure 3).
A view of the gears made of POM and PPS is shown in figure 3. These gears were made by means of the injection moulding method in a laboratory of the Institute of Machine Technology and Automation, using a specially designed form.
THE TEST STAND, RESEARCH PROGRAMME
a)
Research of the gerotor pump, with the gears made of POM and PPS was carried out in the laboratory of the Fluid Power Research Group at the Institute of Machine Design b)
Figure 3. Gears made of a) POM and b) PPS.
Analysis of figure 3 shows in both cases that for the making of the gears a range of solutions recommended in [5] was used. The gears feature: • a small height and streamline shape of the teeth, to ensure a high rigidity, they also have rounded corners at the base of the inner tooth, which is to reduce the notch effect - see (1) • a relatively large thickness of the rings under the toothings of the gears, which is intended to provide increased strength of gears - see (2) and (3) • 3 keys in the driving wheel with external teeth, which ensures uniform transmission of torque from the drive shaft, so as not to weaken the cross-section of the gears ring, the keys were placed under the heads of the teeth, the keyways have rounded corners which is to limit the impact of the notch - see (4) Surfaces of the gears’ teeth made by the injection moulding method are smooth. All teeth in the gears are in contact with each other and the intertooth radial clearances hr equal zero (see figure 3b). The side surfaces of the gears are also smooth, and proper fitting to the central body of the
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and Operation. The view of the test stand is shown in figure 4. The test pump (2) is driven by an electric motor (1). Supply pipes (3) provide the connection of the pump to the hydraulic system. Throttle valve (8) is used for loading the pump. The torque was measured by torque gauge (5), and the speed was measured by revolution-counter (4). The flow rate was measured by flowmeter (6) and the input and output pressure was measured by pressure sensors (7). The research programme for gears made of POM and PPS included: • research on the impact of the working fluid pressure and rotational speed on volumetric efficiency ηv, • research on the impact of oil temperature T on volumetric efficiency ηv. Moreover, the research programme for gears made from PPS included also: • research on ability to work at high rotational speeds n • research on the effect of axial clearance ha on volumetric ηv and total η efficiency, • long-term research. Copyright © 2014 by ASME
ranges and in the long-term research. Figure 6 shows that the pump successfully operated in a range of rotational speeds of n = 1500 – 4000 rpm. However, with the increasing rotational speed, the pressure at the outlet of the pump was reduced in the range po = 40 - 17 bar. It was done intentionally to approximately set constant motive power. Figure 6 also shows that pump volumetric efficiency ηv increases with the increase of rotational speed.
Figure 4. Test stand in the laboratory of FPRG 1 – electric motor, 2 – test pump, 3 – inlet and outlet supply pipes, 4 – revolution-counter, 5 – torque gauge, 6 – flowmeter, 7 – pressure sensors, 8 – throttle valve.
RESEARCH RESULTS AND DISCUSSION OF THE RESULTS Figure 5 shows the results of the impact of speed and temperature of the working fluid on the efficiency characteristics of the pump with POM gears. The figure shows that the pump successfully worked in a range of speeds n = 750 – 2000 rpm. The maximum working pressure po = 6 MPa was obtained at the highest speeds n = 1500 rpm and n = 2000 rpm, with a 60 - 80 % volumetric efficiency ηv. The figure also shows that pump volumetric efficiency ηv increases with the increase of the temperature of the working fluid from 25oC to 50oC. As a result of the high temperature impact, dimensional stability decreases in the POM gears, the size of the gears increases, additionally intensified by absorption of water from oil by the material. These phenomena lead to reduction of radial clearance hr and axial clearances ha, the pump self-sealing and internal leakages reduction, and consequently, to an increase in the pump’s volumetric efficiency.
Figure 6. Volumetric efficiency characteristics, depending on the output pressure ηv = f(po), for different rotational speeds of the pump shaft n = 1500 ÷ 4000 rpm.
Figure 7 shows the influence of the working fluid temperature T on volumetric ηv and the total η efficiency of pump containing gears made of PPS. The figure shows that an increase in temperature of the working fluid from T = 25oC to T = 50oC reduced the range of operating pressures of the pump from po = 40 bar to po = 30 bar and a decrease in volumetric ηv and total η efficiency. This is explained by the fact that when temperature increases, the viscosity of the oil decreases, leads to reduction of the flow resistance through the internal clearances in the pump and to an increase in the internal leakage, which, consequently reduce the pump’s efficiency.
Figure 5. Volumetric efficiency characteristics for gears made of POM, depending on the output pressure ηv=f(po), for different values of rotational speed and temperature of working fluid.
Figure 7. Efficiency characteristics, depending on the output pressure ηv = f(po), η = f(po) for different values of the hydraulic oil temperature t = 25, 50oc.
Basic study on the impact of pressure, temperature of the working fluid and rotational speed on the pump characteristics were repeated for the material used at the second stage of the research, namely for PPS. This material features higher strength properties than POM, which is why it was decided to expand the research scope, mainly in order to check how the material would behave in the higher speed
PPS is a more stable material than POM. It does not change its dimensions under the influence of high temperature and has a lower coefficient of water absorption. Therefore, for gears made of PPS, it was decided that the impact of the very design and specifically the impact of axial clearances ha on the efficiency characteristics of the pump should be checked. Figure 8 shows what effect axial
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efficiency ηv = 80% remained unchanged during the entire trial. After the research, the pump was disassembled. Inspection of the gears did not show any visible signs of wear. Maintaining constant efficiency, and no signs of wear indicate proper operation of the pump during the long-term test.
clearance ha has on volumetric ηv and total η efficiency of the pump. The figure shows that the reduction of the axial clearance from ha = 0.14 mm to ha = 0.04 mm resulted in the widening of the scope of the working pressure from po = 15 bar to po = 40 bar, while maintaining of volumetric efficiency ηv = 40% and increasing of the total efficiency from η = 20% to η = 30%.
CONCLUSIONS Table 2 illustrates technical specifications obtained during testing of the gerotor pump with gears made of plastics. The experimental research carried out on the gerotor pump with gears made of plastics confirm that plastics as construction materials may be used in fluid power. They also proved the procedure employed for the designing of the plastic gears to be adequate, and therefore it should be used in the next phases of the research. A key role in the operation of the pump play epicycloidal gears with internal meshing made of POM and PPS. Material POM proved to be less stable. Under the influence of temperature and water absorption from the working fluid, the gears made of POM increased their overall size, which lead to self-sealing of the pump and improvement of its characteristics. PPS is a more stable material, the gears’ dimensions are not increased under the influence of high temperature or as a result of water absorption from the working fluid. Significant impact on the technical parameters of the pump with PPS gears has the design precision. Reducing the axial clearance from ha = 0,04 mm to ha = 0,14 mm resulted in the widening of the scope of the pump pressure and an increase in the total efficiency of the pump. Therefore, it is estimated that POM and PPS are recommended for the making of pump gears. However, it appears that in the future, it would be better to use a more stable material like PPS and try to improve the manufacturing precision of gears in order to obtain smaller internal clearances (for example, axial clearance ha = 0.02 mm). The test results do not exclude the reason of the search for other plastics featuring higher strength and higher dimensional stability, and which could be used for the making new gears . Material, however, is not the only factor determining the manufacture of high-strength gears, the others are proper design, precise injection moulding, proper selection of the injection parameters, and finally the correct assembly of the pump. It is expected that further improvements introduced in the whole process of designing the pump will enable obtaining better technical characteristics, such as operation at higher pressures po = 6MPa, volumetric efficiency ηv = 70%, and total efficiency η = 60%. In addition development of the plastic body of the pump is planned.
Figure 8. Efficiency characteristics, depending on the output pressure ηv = f(po),, η = f(po) for different values of the axial clearance in the pump ha = 0,14; 0,04 mm
As indicated in [6], the reducing of axial clearance ha causes a reduction of internal leakage, and thus an increase in volumetric efficiency ηv. At the same time, assumed axial clearance ha = 0,04 mm is sufficient for the free rotation of the gears relative to the stationary surfaces of the pump body (see figure 1), which do not generate mechanical losses resulting from friction. This is confirmed by the view of the side surfaces of the gears, which after experimental research remain clean, non-cracked and showing no signs of mechanical collaboration with pump body.
Figure 9. Volumetric efficiency during long-term trials ηv = f(t)
Due to the positive results of the basic research on the pump with PPS gears it was decided that they would be subject to long-term test. Figure 9 shows a diagram of volumetric efficiency ηv during a forty hour long test at a pressure of po = 8 bar. The figure shows that volumetric
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Table 2. Technical specifications of gerotor pump with epicycloidal gears made of POM and PPS.
No. 1. 2. 3. 4.
Specification Efficiency The maximum pressure Volumetric efficiency Total efficiency
5.
Speed range
6.
Oil temperature
7.
The total operating time of the pump
Symbol Q pmax ηv η nmin nmax Tmin Tmax t
Unit cm3/rev bar % % rpm o
C
h
POM 10 60 80 750÷ 2000 25 ÷ 50 -
PPS 10 40 40 30 1500 ÷ 4000 25 ÷ 50 40
REFERENCES [1] 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 465-479. [2] Bonanno A., Matarialli plastici, caratte ristiche a limitti., Oleodynamica Pneumatica, apricle 2008, p-p 68-73. [3] Bełzowski, A. ; Stróżyk, P., Assessment of repair reinforcement of polyester-glass fibre pipe. KOMPOZYTY Nr 2/2008, pages. 179-184. [4] Biernacki K., Internal report: strength research of plastics, [5] Stryczek, J., Bednarczyk S., Biernacki K., Application of plastics in manufacture of the gerotor pump .The Twelfth Scandinavian International Conference on Fluid Power SICFP’11. May 18- 20, 2011, Tampere Finland. Vol. 3(4), pages 369-383. [6] Stryczek J., Koła zębate maszyn hydraulicznych, Oficyna Wydawnicza Politechniki Wrocławskiej. Wrocław 2007. http://www.dbc.wroc.pl/dlibra/docmetadata?id=20045&from=&dirids=1 [7] Krawczyk J., Stryczek J., Układ hydrauliczny z elementami wykonanymi z tworzyw sztucznych, Górnictwo Odkrywkowe, 2013, R. 54, nr 3/4, s. 52-57. [8] Krawczyk J., Stryczek J., Badania pompy gerotorowej z kołami zębatymi wykonanymi z PPS, HiP 1/2014 [9] Stryczek J., Antoniak P., Banas M., An Idea of the Electronic Control System for the Multifunctional Hydraulic Machine, Journal of Vibroengineering, September 2010, vol. 12, Issue 3, pages 329 - 332 [10] Stryczek, J., Bednarczyk S., Biernacki K., Gerotor pump with POM gears: Design, production technology, Archives of Civil and Mechanical Engineering (2014), research, http://dx.doi.org/10.1016/j.acme.2013.12.008 [11] Stryczek, J., Bednarczyk S., Biernacki K., Strength analysis of the polyoxymethylene cycloidal gears of the gerotor pump, Archives of Civil and Mechanical Engineering (2014), http://dx.doi.org/10.1016/j.acme.2013.12.005 [12] Wieleba W., Bezobsługowe łożyska ślizgowe z polimerów termoplastycznych, Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław 2013
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