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TECHNICAL ARTICLE Pressure Determination Approach in Specific Pressure Regions and FEM-Based Stress Analysis of the Housing of an External Gear Pump R. Cinar1 , M. Ucar2 , H.K. Celik3 , M.Z. Firat4 , and A.E.W. Rennie5 1 Department of Mechanical Education, Institute of Natural and Applied Sciences, Kocaeli University, Kocaeli, Turkey 2 Department of Automotive Engineering, Technology Faculty, Kocaeli University, Kocaeli, Turkey 3 Department of Agricultural Machinery, Faculty of Agriculture, Akdeniz University, Antalya, Turkey 4 Department of Animal Science, Faculty of Agriculture, Akdeniz University, Antalya, Turkey 5 Lancaster Product Development Unit, Engineering Department, Lancaster University, Lancaster, UK Keywords External Gear Pump, Hydraulic Pressure Measurements, Stress Analysis Correspondence H.K. Celik, Department of Agricultural Machinery, Faculty of Agriculture, Akdeniz University, Antalya Turkey Email: [email protected] Received: September 3, 2013; accepted: February 13, 2014 doi:10.1111/ext.12086 Abstract In this study, an application algorithm has been introduced to explore structural optimisation for gear pump housing. In the study, experimental and theoretical (analytical and numerical) methods are utilised. A commercial external spur gear pump which has 17.1 L min−1 volumetric flow rate with maximum pressure capacity of 250 bar has been considered for an application case study. In the experimental section of the study, four pressure sensors (emitters) were placed with angle intervals of 45◦ , 90◦ , 135◦ , and 180◦ on the pump’s housing to measure operating pressure values at specific pressure regions of the housing from inlet to outlet. According to experimental results of the pressure measurements, a response surface analysis (RSA) was carried out and then an estimation model (empirical equation), which could be used to calculate pressure values at any specific region of the housing, was obtained. According to the RSA results gained, it appears that the estimation model has 99.9% R2 value which can be used for adequately predicting accurate pressures at any region of the housing. Subsequently, this estimation model has been adapted for commercial finite element method (FEM)-based engineering software and from which stress distributions on the housing were simulated three dimensionally. FEM-based simulation outputs showed that there were no failure signatures on the pump housing. As the main conclusion of the study, it is seen that an estimation model gives an adequate approach to predict pressure values at any specific pressure region of the pump housing and, especially, stress distribution results has highlighted that a structural optimisation study may be suitable for the pump housing. Introduction One of the positive displacement types of hydraulic pump is the external gear pump. The gear pump is among the oldest and the most commonly used pumps in the wide range of application areas from agricultural machinery to heavy industrial machinery systems.1 It has become the main choice for applicants due to long life, minimum maintenance, high reliability, capability in high-pressure operations, etc. Basically, an external gear pump converts mechanical Experimental Techniques (2014) © 2014, Society for Experimental Mechanics power into hydraulic fluid power with high pressures. This converting operation is described as follows.2 The shaft of one of the gears is driven by a motor. This gear is known as the driving gear. It engages around the other, known as the driven gear. When the teeth of the two gears start disengaging (near the centre of the pump), the consequent low pressure sucks in the fluid. Upon further rotation, the fluid is trapped in between the void formed by this cavity and the housing. The fluid is then carried around towards 1 Stress Analysis of the Housing of an External Gear Pump the discharge side (outlet) of the pump with higher pressure magnitudes. As in principle, it cannot flow back through the very ‘‘small gap’’ between the teeth and the housing and neither through the engaged teeth of the gears, it is entirely ejected to the outlet. Therefore, unlike a fan (e.g.), the flow rate is expected not to depend on the pressure difference between the inlet and outlet. In some of the calculations, although it is assumed that there is no leakage from outlet to inlet, in practice, slip of the fluid through the gap and along the housing walls from the discharge side (high pressure) to the suction side (low pressure) is inevitable. Therefore, some of the drawbacks of using gear pumps include high noise, vibration, cavitation failure, and unavoidable outlet pressure ripple. The ripples are the main source of the vibration, noise wearing, and efficiency loss of gear machineries.3 During the pumps operation, cavitation and especially alternating high-pressure spikes are very undesirable and essentially impact on the structural stress and life time of the housing, side plates, bearings and gears, therefore, they may be responsible for structural failure of these components. In the literature, there exists a significant amount of analytical, numerical, and experimental studies to address the gears tooth form, cavitation, flow characteristics, leakage from gaps, and pressure/flow ripples of external gear pumps. In addition, the literatures offer limited information regarding the other structural components’ analysis details for this type of hydraulic pump and motors with external and internal gears.4 – 17 In this study, an application algorithm has been developed for structural optimisation exploration of external gear pump housing. A sample high-pressure external spur gear pump has been considered, with the focus on the structural analysis and structural optimisation exploration of its housing. The study centred on the algorithm that can be summarised as follows: firstly, the alternating pressure distribution issue on the gear pump from inlet to outlet and an experimental setup are introduced with the aim to obtain experimental data for alternating pressure distribution at the inside pressure surfaces in the pump housing, within defined operating conditions. Then the experimental data is analysed utilising response surface modelling (RSM). According to the statistical analysis, an estimation model (empirical equation) is derived that extracts the exposed pressure values at any specific region of the pump housing from inlet to outlet, numerically. Subsequently, the pressure equation is employed in the finite element analysis (FEA) to investigate 2 R. Cinar et al. the structural deformation behaviour of the pump housing. According to the simulation outputs, the structural optimisation approach for the pump housing is discussed. Application Algorithm Generally, maximum operating pressures are considered in design calculations for gear pump systems; however, pressure distribution is not uniform in the pressurised surface of the housing from inlet to outlet. This phenomenon creates some difficulties in order to reach the optimum structural design parameters of the pump housing. At this point, it is true to say that it is important to predict pressure values at any specific region (between the inlet and the outlet) of the housing in order that it is possible to calculate stress distribution, which is one of the evaluation parameters required for the optimum structural design process. Considering this issue, an application algorithm that includes experimental and theoretical (analytical and numerical) methods has been developed which can be used in pre-design decision processes and to determine the likelihood for structural optimisation potential of the external gear pump housing. The application steps of the algorithm are detailed in Fig. 1. Pressure Distribution Issue in External Gear Pumps and Experimental Setup In the literature, the most commonly used pressure distribution approach for the external gear pump is a linear approach that is defined by the gear rotation angles and maximum operating pressure magnitudes (Fig. 2). In this approach, no leakage is assumed during the pumping operation and the pressure at a specific angle’s region (Pi ) is expressed as per Eq. 1 given below.4,5 β Pi = Pmax (1) π As detailed in Fig. 2, the pressure distribution is seen as non-uniform from inlet to outlet and the maximum pressure is defined as Region IV in the gear pump housing. However, this knowledge has been considered in a linear approach assuming no leakage, and an experimental study has also been set up in order to determine experimental pressure distribution values with inevitable internal leakage at specific regions in the external gear pumps. It was described as an estimation model which might be used to load the housing correctly for its structural deformation potential. In the experimental setup, a commercial external spur Experimental Techniques (2014) © 2014, Society for Experimental Mechanics Stress Analysis of the Housing of an External Gear Pump R. Cinar et al. Figure 1 Application algorithm for exploration of structural optimisation potential. gear pump (HEMA Industry Inc., Tekirdağ, Turkey), which has 17.1 L min−1 volumetric flow rate and a maximum pressure capacity of 250 bar with gears of 2.75 module and 12 teeth, was utilised. The pump was driven at 1500 min−1 revolutions by a four pole electric motor (GAMAK™; Makina Sanayi A.Ş., İstanbul, Turkey) which has a power capacity of 11 kW. All of the hydraulic equipment and the electric motor have been situated on a chassis frame. The pressure measurements were conducted by utilising a computeraided measurement system with five GEMS™ (Gems Sensors & Controls, Plainville, MA)-3200-heavy duty series pressure transmitters, which have a measurement range of 0–400 bar. Four of them were placed on the pump housing with 45◦ intervals from the inlet to the outlet lines and the fifth one was placed after the outlet line. In order to verify this, a check valve and an analogue pressure gage are also placed on the output line just before the oil tank. In the experiment, AKSOIL HYDROTRANS ISO 46 hydraulic oil was used. The pressure measurements in the experiment were conducted by stepping up the outlet pressure in 10-bar interval from zero pressure to a maximum pressure value of 250 bar (i.e. 25 steps representing Experimental Techniques (2014) © 2014, Society for Experimental Mechanics each of the recurrences). Pressure data from all of the pressure transmitters were recorded for 30 s after stable regime time for each of the pressure steps. These measurement procedures had three recurrences. Placement of the pressure transmitters and the experimental setup details can be seen in Fig. 3. Results of the Experimental Practice According to the data collected from the pressure transmitters at the different regions on the pump housing, the pressure distribution behaviour of this gear pump housing was obtained experimentally between the inlet and the outlet. The data was also analysed utilising the RSM methodology which is a useful tool in modelling curvature effects in many scientific disciplines. Statistical analysis has also extracted an estimation model that gives pressure (Pi ) values at any specific region depending on outlet pressure (Pmax ) and gear rotation angle (β). The quadratic model obtained from the response surface analysis (RAS) is given as: 2 2 + β4 x2i + β5 x1i x2i + i yi = β0 + β1 x1i + β2 x2i + β3 x1i (2) 3 Stress Analysis of the Housing of an External Gear Pump R. Cinar et al. Figure 2 Pressure distribution and pressure regions at the gear pump housing. where, yi is the pressure at specific region (Pi ) (bar), x 1i is the outlet pressure (maximum pressure) (Pmax ) (bar) and x 2i is the gear rotation angle (β) (◦ ); β 0 is a constant coefficient, β 1 , β 2 , β 3 , β 4 , and β 5 are the interaction coefficients of linear, quadratic, and second-order terms, respectively; and i is the error term. The quality of the fit of the model in Eq. 2 was evaluated using the coefficient of determination (R2 ). The statistical significance of the polynomial model used in this study was determined by the F-test. The significance of the regression coefficients was tested by Student’s t-test. The results of the RAS are presented in Table 1 and the predicted response is described as follows: yi = −9.550433 + 0.511168x1i + 0.195402x2i 2 2 + 0.000065669x1i − 0.0008x2i + 0.002824x1i x2i (3) As can be seen from Table 1, the overall regression F value of 15448.6 and the low probability value (P < 0.0001) indicate that the model was significant for the response variable pressure at a specific region. The estimated parameters for the statistics indicated a well-established pressure pattern which can be described by a response surface model with linear, quadratic, and cross-product terms [see partial Ftest and F-test for overall (total) regression results in Table 1]. For this model, both variables x 1i and 4 x 2i and their interaction seem to be very important parameters (t-test; in Table 1). The canonical analysis indicates that the predicted response surface is shaped like a saddle (Fig. 4). The eigenvalue of 13.37 shows that the valley orientation of the saddle is less curved than the hill orientation, with eigenvalue of −19.39. As the canonical analysis resulted in a saddle point, the estimated surface does not have a unique optimum. However, the ridge analysis indicated that maximum yields will result from higher values of outlet pressures and gear rotation angles. A contour plot of the predicted response surface, shown in Fig. 4, confirms this conclusion. The coefficient of determination (R2 = 99.9%) obtained in this study was rather high, indicating that 99.9% of the variability in the response could be explained by the model (Eq. 3). On the basis of these results, the response surface model obtained in this study for predicting the pressure at specific region was considered reasonable. In addition to the contour plot in Fig. 4, correlation between raw experimental results, estimated results, and theoretical (From Eq. 1) results were explored and the plots were presented for the measurement points at 45◦ , 90◦ , 135◦ , and 180◦ on the pump’s housing, given in Fig. 5. From initial observation of these charts, it can be seen that the results show a higher difference than the others at the first measurement points and then it has an overlap at the Experimental Techniques (2014) © 2014, Society for Experimental Mechanics Stress Analysis of the Housing of an External Gear Pump R. Cinar et al. Figure 3 Experimental setup. Table 1 Response surface analysis result Equation parameters β0 β1 β2 β3 β4 β5 R2 = 0.999 t value Pr > |t| Partial F-ratio test 1.429313 −6.68 0.013763 37.14 0.024904 7.85 0.00004785 1.37 0.00011 −7.9 0.000061541 45.88 F value = 15448.6 (total regression) <.0001 <.0001 <.0001 0.173 <.0001 <.0001 37561.1 (0.0001) Estimate −9.550433 0.511168 0.195402 0.000065669 −0.00087 0.002824 SE last measurement point, that is, the outlet line of the pump housing. The reason for these differences can be explained as follows: in the theoretical approach, no leakage at the pumps gear mesh is assumed; however, leakage is inevitable in physical practice. Therefore, a pressure rise is seen in the charts for the experimental results, especially at the first measurement points. Conversely, there is good correlation between the raw experimental and estimated results plotted in the charts. As the charts show, linear pressure is observed from the inlet to the outlet in the pump, producing an uncomplicated degree of estimation. This correlation has proved that the estimation model, which is extracted from the statistical analysis, adequately reflects the physical practice scenarios with this pump. Experimental Techniques (2014) © 2014, Society for Experimental Mechanics 7.76 (0.0007) 2105.24 (0.0001) Probability = < 0.001 Model Adequacy Checking Usually, it is necessary to check the model in Eq. 3 to ensure that it provides an adequate approximation to the real system. Unless the model shows an adequate fit, proceeding with the investigation and optimisation of the likely fitted response surface will give poor or misleading results. The residuals from the least squares fit play an important role in judging model adequacy. The model adequacy or the appropriateness of the fit of the proposed model to the experimental data can be assessed by applying the diagnostic plots, such as a histogram of the residuals and the predicted versus actual value plots. Figure 6 shows these two diagnostic plots for the pressure at a specific region. A histogram plot indicates whether the residuals follow 5 Stress Analysis of the Housing of an External Gear Pump Figure 4 Contour plot of predicted response surface. a normal distribution, in which case the data is normally distributed. As can be seen in Fig. 6, the predicted values of pressure obtained from the model (Eq. 3) and the actual experimental data were in good agreement. FEM-Based Stress Analysis of the Pump Housing In this step of the study, it focuses on the structural deformation exploration of the gear pump housing by utilising finite element method (FEM)-based stress analysis. The results of numerical calculations are strongly dependent on boundary conditions, e.g. mounting, connection of the single housing components, etc. For this reason, the boundary conditions have to be set exactly to achieve results comparable with the measurements.18 The experimental study in the previous section had extracted an estimation model which described pressure values at any specific region depending on outlet pressure values and gear rotation angle; thus, in the finite element analysis (FEA), applied pressure magnitudes in the housing separated pressure surfaces (7 pressure sections in total) from inlet to outlet having been calculated using this model (Eq. 3). In the stress analysis, maximum outlet pressure is defined as 250 bar and pressure distribution from inlet to outlet is applied on the pressure surface of the housing. Experimental results also determined that there was a negative pressure effect at the inlet of 9.55 bar (maximum). This condition was also considered in FEA. In addition to this, fluid pressure and bearing loads from gear axles on the pump-bearing components were considered. In the gear pump, pressure distribution is non-uniform and, as is mentioned previously, the 6 R. Cinar et al. maximum pressure region is laid out between the region of π and 3π/2 (Region IV). This phenomenon pushes the gears from the outlet to the inlet side which causes bearing loads. In fact, the direction of these loads affects the bearings with a convenient force angle from the outlet to the inlet, however, to load the housing in a worst scenario, it is assumed that the maximum pressure is affecting the gears at the regions between π/2 and 3π/2 which provides a direct force from the outlet to the inlet in the housing for bearing loads (Fig. 7). According to the calculations that consider the gears profile dimensions and pressure at each gear tooth gap, the radial acting total force on the gear was calculated as 12780 N. Hence, each of the gears bearing forces 6390 N since the gear bearing is symmetric. In addition to the loading boundary condition definitions present in the FEA setup, the side covers of the pump are assumed to be rigid parts and the cover surfaces of the pump have been restricted by using frictionless support features. Bolt connection holes are restricted using cylindrical support features in the FEM code as well. In the analysis, three-dimensional (3D) models of the housing and bearings were created in SolidWorks 3D parametric solid modelling design software while Ansys Workbench commercial FEM code was utilised for stress analysis. Material properties of high strength 7000 series aluminium alloy (7020-T6), stainless steel, and highleaded tin bronze (SAE 660) have been assumed for the gear pump housing, bearing bushings, and sliding bearings, respectively (Table 2).19 – 21 The analysis has been set up with 3D, linear, static, and isotropic material model assumptions. The FE model of the gear pump components (housing and bearings) used in the FEA have been created using Ansys Workbench meshing functions with a curvature-based meshing approach.22 After completion of the pre-processes in FEA, simulations were run. Von Mises equivalent stress distribution and deformation behaviour of the housing under defined boundary conditions were obtained. According to the simulation outputs, it is shown that the maximum equivalent stress appeared on the pump housing as 56.916 MPa and maximum deflection was 0.00844 mm. Mesh structure details and related simulation outputs are presented in Fig. 8. Discussion for the FEA Results As it can be seen in Fig. 8, the simulation outputs present a good visualisation of the stress distribution and deformation behaviour of the Experimental Techniques (2014) © 2014, Society for Experimental Mechanics R. Cinar et al. Stress Analysis of the Housing of an External Gear Pump Figure 5 Correlation between raw experimental, theoretical (Eq. 1) and estimated results (Eq. 3). Figure 6 Diagnostic plots. housing under defined boundary conditions. The stress distribution is not uniform as non-uniform pressure effects are evident at the inside surfaces of the housing. Although, applied pressure values increase progressively from inlet to outlet, the stress distribution is not seen to correlate with these progressive pressure values. Maximum stress is seen at the outlet line of the housing as is expected; Experimental Techniques (2014) © 2014, Society for Experimental Mechanics however, the lower stress magnitude is seen at a pressure region of P3 than P0 , which is the lowest pressure magnitude. This can be explained as a result of the geometric structure of the housing. Addition to this, degree of freedom of bolt holes in the housing was restricted in the FEA. Hence, lower stress magnitude as a result of limited deformation behaviour at these holes is seen at pressure region 7 Stress Analysis of the Housing of an External Gear Pump R. Cinar et al. Figure 7 Pressure and bearing loads effect in the gear pump housing. Table 2 Material properties Material properties used in FEA Modulus of elasticity (GPa) Poisson’s ratio (−) Yield strength (MPa) Tensile strength (MPa) Density (kg m−3 ) Housing Bearing bushings Sliding bearings 72 0.33 280 350 2780 210 0.28 230 400 7700 100 0.32 140 240 8930 of P3 . According to the results obtained in the simulation, safety factors (SF) have been calculated considering the yield strength point of the housing material (280 MPa) for seven pressure surfaces inside the housing from inlet to outlet. In order to visualise the change of SF at the housing pressure surfaces from inlet to outlet, the maximum stress values for each of the pressure regions and SF distribution chart have been given in Fig. 9. In Fig. 9, the non-uniform distribution of SF is seen more clearly. The maximum SF was at the pressure region of P3 (13.746) and the minimum SF was at the pressure region of P6 (Outlet) (4.920). These SF values obtained can be interpreted as quite high due to the thickness of the housing which has been designed with a high SF. Conversely, in the literature, it is suggested that the gap between gear teeth and 8 inside surfaces of the housing should be between the values of 0.005 and 0.020 mm for low operation pressure conditions and 0.0025 and 0.010 mm for high operation pressure conditions to avoid excessive and undesirable fluid leakage from the gear-housing gaps through the inlet side.5 The simulation results determined that the maximum deflection of the housing under defined boundary conditions was 0.00844 mm. This value stands within the suggested values for the gear-housing operating gap. This knowledge may allow design decisions with high SF to be made to keep this gap between suggested values, which might be a design constraint. However, all the design constraints must be met in the gear pumps total design process. Calculated high values of SF may lead us to think about a structural optimisation to avoid unnecessary material usage for the housing. Concluding Remarks As a main concluding remark, considering to the simulation results and SF calculations, it can be highlighted that a structural optimisation study could be undertaken with the aim of reducing the housing weight that would also provide a cost saving by selecting optimum values for the housing thickness. As such, firstly, size optimisation techniques could be considered which may provide uniform thickness Experimental Techniques (2014) © 2014, Society for Experimental Mechanics R. Cinar et al. Stress Analysis of the Housing of an External Gear Pump Figure 8 Mesh structure details and simulation outputs: deformation behaviour and Von Mises stress distributions. for the entire housing, although non-uniform stress distribution may be an obstacle. As such, the outlet side will need to be a thicker material than the inlet side of the pump housing. Therefore, it may be considered that a total structural optimisation approach should be undertaken that considers topology, shape, and size optimisation techniques in order. Experimental Techniques (2014) © 2014, Society for Experimental Mechanics Additionally, a number of points can be summarised as follows, as final conclusion remarks of the whole study presented in this paper: 1. An application algorithm has been introduced for structural optimisation exploration of external gear pump housing and it was implemented with success within a case study for a sample external spur gear pump. 9 Stress Analysis of the Housing of an External Gear Pump R. Cinar et al. Figure 9 Safety factor distribution chart for each of the pressure regions in the housing. 2. In the case study, an experimental study has been set up and an estimation model has been extracted which is adequately describes the exposed pressure values at any specific region of the pump housing from inlet to outlet, numerically. 3. FEM-based stress analysis has been conducted to investigate the structural deformation behaviour of the pump housing, which states that the maximum stress on the housing was 56.916 MPa and maximum deflection was 0.00844 mm. This deflection value is within the limit gear-housing gaps tolerance as suggested in the literature. 4. According to SF calculations considering material yield strength point, non-uniform stress distribution is seen which is not in accordance with the pressures applied to the pressure surfaces of the housing. 5. SF calculation showed that the maximum SF was 13.746 and the minimum SF was 4.920 for the pressure regions of P3 and P6 (Outlet) , respectively. 6. Final evaluation of the case study has determined that an optimisation study could be suggested that aims to make a reduction on the material usage of the housing considering all design constraints defined for the total design process of the gear pump design. To do this, topology, shape, and size optimisation techniques may be considered in order to obtain optimum geometrical shape and the size of the housing. Acknowledgments This study was supported financially by the Scientific Research Fund of Kocaeli University (Project No: 2012/68) and partially supported by the Scientific Research Projects Coordination Unit of Akdeniz University. Nomenclature Pi (yi ) Pressure at specific region (bar) 10 Pmax (x 1i ) β(x 2i ) β 0–5 i Maximum pressure (bar) Rotation angle of the pump gears (o ) Empirical equation coefficients (−) Equation error term (−) Subscripts i Index for specific rotation angle of the pump gears Inlet Inlet port of the gear pump housing Outlet Outlet port of the gear pump housing max Maximum min Minimum Abbreviations FEA FEM RSA RSM SF Finite element analysis Finite element method Response surface analysis Response surface modelling Safety factor References 1. Ragunathan C., Manoharan C. (2012) ‘‘Dynamic Analysis of Hydrodynamic Gear Pump Performance Using Design of Experiments and Operational Parameters’’. IOSR Journal of Mechanical and Civil Engineering6: 17–23. 2278–1684. 2. 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