Theoretical and Experimental Research on the Yoke of the Axial Piston Pumps

Transcription

1 Theoretial and Experimental Researh on the Yoke of the Axial Piston Pumps Vasiliu Niolae, P.E., Ph.D., Professor University "Politehnia" of Buharest, Fluid Power Department Rosu Cristian Adrian, M.E., Ph.D., Researh Engineer University "Politehnia" of Buharest, Fluid Power Department Stefan Sorohan, M.E., Ph.D., Assoiated Professor University "Politehnia" of Buharest, Materials Strength Department Sandu Marin, M.E., Ph.D., Professor University "Politehnia" of Buharest, Materials Strength Department Abstrat The paper ontains theoretial and experimental researhes on the assembly made by the yoke and the inlined disk that equip a swashplate axial piston mahine. The theoretial studies onern the stress and defletion state of the assembly due to the arrying pressure of the hydrostati bearings, made by the slippers and the inlined disk. The simulations were made using the ANSYS program for the experimental state of loads, reated espeially to be implemented on an experimental test. To validate the finite element models of the assembly, a testing rig has been designed and built. The theoretial and experimental results are in good agreement and furthermore, simulations ould be done for the real state of loading. Introdution Beause of their major advantages, the hydrauli transmissions have imposed themselves over the other types of transmissions and they were adopted in all industrial branhes. The main omponents of a hydrauli transmission are the hydrauli mahines. For the transmissions that equip mobile platforms, the worldwide trend is to use the swashplate axial piston pumps (fig. 1). Even if this kind of displaement mahines are manufatured sine 197 [13], today researhers from all over the world are ontinuing to study the swashplate axial piston pumps to rise the operating pressure and to use eologial fluids, like pure water.

2 Figure 1 - Swashplate axial piston pump 1- front lid; slipper; 3 slipper pad; 4 ylinder blok; 5 - port plate housing; 6 bearing; 7 swashplate; 8 - swashplate housing; 9 pump asing; 10 port plate; 11 spring; 1 ontrol fork; 13 shaft. A representative domain for the high development of the axial piston pumps manufaturing is the aeronautis. In this industrial branh, light hydrauli transmissions that operate in very safe onditions are required. To make lighter hydrauli mahines, one must inrease the running pressure or redue the mass of the mehanial omponents. The mass redution must be done aording to an optimal design of the mahines geometry, taking into aount the stati and dynami stress and defletion states of the mehanial omponents. Reduing the mass has among limiting riteria the stiffness that would redue the maximum load. The pressure inrease implies other dangerous phenomena like material damage (failure), defletions (that affet the operating gaps between omponents in motion), and dry frition. To obtain a suessfully operating mahine, the designers must bear in mind the strutures strength and stiffness, the lubriation quality between surfaes in ontat, and the pressure equilibrium on the opposite walls of the same hamber. In this paper the authors present a method to obtain the proper material and the optimal geometry for a yoke. For this purpose, a parametrial model has been built and several simulations have been performed. The tested omponent To depit the method, the geometry of the yoke (fig ) has been taken from a SPV axial piston pump, made by ZTS Company. This mehanial omponent is a support for the inlined disk. The fores generated by the operating pressure is transmitted to the inlined disk by the piston and slipper. Furthermore the yoke takes all these loads and transmits them to the ase by two symmetrial bearings. The angle of the inlined disk ould be ontrolled by a servomehanism that ats on the yoke.

3 Figure - The tested yoke The yoke must be very stiff beause a small defletion on the inlined disk area ould have a bad influene on the hydrostati bearing operation. So when we design a yoke we must bear in mind to hoose a material that is not to elasti and an take all the loads without failing. The simulations have been made using the ANSYS 5.5 program. First, a stati simulation with ertain loading onditions was made, then we designed and built a test rig and validate the finite element method omputations by experimental measuring. To onlude the researh the real loading state of the yoke was then simulated. The finite element model To arry on the simulations of the yoke behavior we hoose the finite element method beause is more speifi to the struture researh. Beause the geometry is very omplex and we intend to study a large range of sizes for the hosen pump, we have developed a parametrial model of the yoke using the Autodesk Mehanial Desktop 5 software. The model was then imported into the ANSYS program and final arrangements of its geometry were made. To mesh the model we have hosen the SOLID 45 elements that are typial for this kind of simulation. The area we are interested to researh is loated near to the hole for the barrel driven shaft. In this area we developed a ontrolled finite element mesh and we used only briks. To avoid exesive solution time the rest of the yoke was meshed using briks, pyramids, and tetrahedrons (fig. 3).

4 Figure 3 - The finite element model The steady state analysis of the yoke in ertain loading ondition Before simulating the real loading state of the yoke we must validate the finite element model. So a simulation set was performed taking into aount a partiular loading type. For this simulation we onsidered that the axial fore is ast evenly on all the slippers surfaes. This loading ase orresponds to the motor operating ase of the displaement mahine when the flow valve is partialy opened and all the pistons are under pressure. To simulate this kind of load on a test rig, we intend to use a hydrauli ylinder. The load fore values orrespond to the pressure fore developed on the hydrauli ylinder for 0, 40, 60 and 80 bars. Beause of the linearity of the omputation we present the results only for the 80 bar pressure (figs 4, 5 and 6). Figure 4 - The stress diagram [N/mm ] on the 1 st diretion for 80 bar

5 Figure 5 - The stress diagram [N/mm ] on the nd diretion for 80 bar Figure 6 - The equivalent (Von Mises) stress diagram for 80 bar [N/mm ] The experimental measurements The experimental rig The designed rig is shown in figures 7 and 8. The rig struture is omposed by two flanges ( and 11), 4 strength bars (3), a flange (5) for axial onstrain of the hydrauli ylinder (4), a mehanial adapter needed to transmit the pressure fore from the motor to the ylinder blok, and some omponents taken from the SPV pump: the ylinder blok (7), the yoke (10), the slippers and pistons (1). To press the pistons on the swashplate a bored plate (9) has been attahed.

6 Figure 7 - The experimental rig Figure 8 - The layout of the rig 1 nut; lower flange; 3 strength bar; 4 hydrauli ylinder; 5 axial onstraining flange; 6 mehanial adaptor; 7 ylinders blok; 8 pump ase; 9 bored flange; 10 yoke; 11 upper flange

7 The pressure from the hydrauli ylinder has been supplied from a hydrauli pressure soure. The onnetion between pressure soure and hydrauli ylinder has been made by an eletrohydrali flow valve. To measure the yoke strain state we have used resistive transduers plaed on the bak of the yoke (fig. 9). The eletrial information has been amplified and proessed using Hottinger equipments. To avoid the errors introdued by temperature differene an idential resistive transduer has been plaed on the rig. Figure 9 - The resistive transduers plaement The experimental measurements The experimental measurements have been done within the Fluid Power Control Laboratory from the University "Politehnia" of Buharest. To measure the strain state of the yoke, we took from the SPV pump the assembly onsisting of ase, bearings, yoke, inlined disk, slippers, pistons, and ylinders blok. During the testing measurements an axial fore has been applied on the ylinder blok. The load has been transmitted by the pistons, the slippers, and the mehanial adaptors to the inlined disk that relays on the yoke. The yoke is onneted to the ase by two radial-axial bearings. During testing period six measurement sets have been performed. The pressure in the hydrauli motor has been set to the 0, 0, 40, 60, 80 bar. To measure the defletions of the yoke four-tensometri rosettes have been used. A rosette ontains three transduers desribed by the letters a,b, (figure 10). Eah rosette is marked with a number between 1 and 4. Their loation is indiated in fig. 11. Figure 10 - Tensometri rosette and the main diretions

8 Figure 11 - The tensometri rosette positions Starting from the measured strain, the prinipal diretions and the prinipal stresses an be omputed using the following relations [8]. For a measurement point: ε 1, ε = a + ε b 3 + ε ± 3 ( ε ε ) + ( ε ε ) + ( ε ε ) a b b a σ 1, = E ε 3 a + ε b 1 ν + ε 1 3 b φ = artan εa ε ( ε ε ) b ± 1 + ν ε, ( ε ε ) + ( ε ε ) + ( ε ε ) a b b a ε med ε = a + ε b 3 + ε where: ε - measurement strain on eah transduer of one rosette; a, b, σ 1, - the prinipal stresses; φ 1, - the prinipal diretions; ν - the Poisson ratio; E - the Young modulus. If ε a ε med, then φ = φ. If ε a εmed, then φ = φ. If ε a = εmed, then φ = ±45. > 1 < The following relation gives the equivalent stress, omputed with the Von Mises riteria: o σ eq = σ 1 + σ σ σ 1 The next table ontains the omputed values of the main diretions and the stresses on these diretions for the four measurements points indiated in fig. 11. Pressure Measurement ϕ σ 1 σ σ eh

9 [bar] point [grd] [N/mm ] [N/mm ] [N/mm ] 0 1 3,4 18,76 3,04 17,44,4 3,7 0,9 31,8 3 3,74 18,86-4,76 1,64 4-1,74 3,37-3,97 34, ,18 37,1 6,3 34,39 4,03 60,75-0, ,31-9, -41,69 4-1,49 60,67-7,47 64, ,09 5,65 8,15 49,09 4,86 8,85 -,45 83,5 3 1,7 51,17-13,07 58,81 4-1,9 104, 3,84 10, ,6 71,64 10,95 66,8 5,16 106,14-3, ,79 65,63-18,13 76,3 4 -,37 113,15-13,16 10,3 Analysis Results & Disussion The results obtained by experimental measurements are in good agreement with the theoretial ones obtained by numerial simulation. So the onlusion is that the finite element model is equivalent with the real one. The measured data obtained for the no 4 rosette (fig. 11) and the theoretial data obtained by finite element simulation (figs 4, 5, and 6) are represented in the table below. Pressure [bar] σ 1 [N/mm ] experimental σ 1 [N/mm ] theoreti σ [N/mm ] experimental σ [N/mm ] theoreti σ eq [N/mm ] experimental σ eq [N/mm ] theoreti 0 3, , , , , , , , , , , , We mention that the material properties taken into aount in both the theoretial and experimental data proessing orrespond to a regular steel, having Young Modulus = 1*10 4 N/mm and Poisson Ratio = 0,3.

10 The steady state analysis of the yoke in real loading onditions Next we studied the steady state stresses and displaements of the yoke due to the hydrostati pressure applied on the inlined disk. The pressure fores are transmitted to the inlined disk by pistons and slippers. The inlined disk relies on the yoke, so the fore is transmitted to the mahine asing by the bearings that allow the rotary motion of the yoke against asing. For simulations we have used the same finite element model presented above. Beause the area we are interested in is loated in the middle of the yoke, the bearing surfaes an be onsidered as being ompletely embeded, and the swashplate and the yoke are the same body. On the slippery surfae of the swashplate, a pressure was applied foused on small area that represent the slipper prints. To load the yoke, we have onsidered that only five pistons from nine are ative and they are symmetrially disposed against the rotary axis of the yoke (fig. 1). Figure 1 - The yoke analysis in operating onditions In figure 13 is shown the total displaements diagram of the yoke relative to the fixed boundary. The maximum value of the displaement is found on the area around the hole for the barrel driving shaft, and has the same size with the arrying film thikness of the hydrostati bearing.

11 Figure 13 - The total displaement diagram In figure 14 and 15 are shown the von Mises stress diagrams. The figure 14 shows the maximum value of the stress ours on the embeded area. This value is not real beause the embed does not exist. Instead of an embed in this area exists a bearing that has a ertain stiffness. But we are not interested to researh the onnetion between the yoke and the ase. Beause the area we are interested to researh is far from the boundary, we aproximated this onetion as embed. If the interested area is visualized the stress values will onsiderable derease (figure 15). Figure 14 - The equivalent stresses diagram (von Mises) [N/mm ]

12 Figure 15 - The equivalent stresses diagram around the applied loads area [N/mm ] Conlusion In this paper the authors have proposed a method for designing a yoke. To exemplify the method, a real model has been studied in both theoretial and experimental way. The studied mehanial part has been taken from a ZTS swashplate pump. This type of pump is urrently studied all over the world beause of its tehnial benefits in most of the industrial appliations. Beause of its omplex geometry the pump yoke annot be studied with lassial material strength method. Thus a numerial method should be hosen and we have used the finite elements method beause this method takes suessfully into aount the stress onentrators. To model suh omplex geometry we have used the Autodesk Mehanial Desktop software and then we have exported the model in the ANSYS CAD module. Next a simplifying work has been performed in order to obtain an equivalent model. This model has been meshed very arefully in the area we are interested to keep under surveillane (the area where the slippers slide on the inlined disk) and then strutural analysis was performed. The finite element analysis was made for two loading situations: the one that model the real operating ase and the one that expresses the experimental loading ase. The experimental loading ase models a peuliar motor operation mode of this type of displaement mahine. To measure the stress state of the yoke a testing rig has been designed and built that simulates the motor operating situation of the displaement mahine. The data aquisition has been performed using Hottinger equipment. Finally we have ompared the both theoretial and experimental results and we have found them in good agreement. This inreased the trust level in the finite element model and further simulations ould be developed on it with ANSYS. Referenes 1. Dally J.W., Riley W.F. Experimental Stress Analysis. MGraw Hill Book Co., New York, 1978.

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