Structural Analysis and Optimized Design of Working Device. for Backhoe Hydraulic Excavator

Transcription

1 The 14th IFToMM World Congress, Taipei, Taiwan, October 5-30, 015 DOI Number: /IFToMM.14TH.WC.OS Structural Analysis and Optimized Design of Working Device for Backhoe Hydraulic Excavator Pang iaoping Gu ingtong Chen Jin Wang Yabin Chongqing University Chongqing University Chongqing University CHANG AN Abstract: since structural analysis has been rely too much on simulation softwares which cause great computer occupation and is hard to re-develop for optimization, the research on structural optimized design of working device based on classic mechanic theory was proposed. After taking research in structural analysis, the cross-section of working device was simplified to get the maximum stress in the standard working occasion, which was included in VB programmed software called self-compiled structural analysis software(ssas). Reliability of SSAS was verified through comparing result to ANSYS simulation. Then, structural optimization of working device had been realized with SSAS and genetic algorithm. Taking arm as an example, the result shows that: the weight is eliminated by 1.33% which is approximately 38.3 Kg. This research provides a new way for structural design and optimization of working device, which is significant to design original excavator with better quality. Keywords: backhoe excavator; working device; structural analysis; optimized design 1 Foreword Backhoe hydraulic excavator carries on earthwork by working device, structural strength of working device is an important sign of whole machine performance [1]; the improvement of whole life expectancy partly depends on structural strength of work device. In practical applications, the failure of whole machine often begins with the broken of working device, therefore, the structural analysis and optimized design is of great engineering significance. For now, the finite element method, a modern computing method based on structural mechanics, has been widely used in strength analysis []. To address the difficult analysis of modern complex model, finite element analysis method is widely applied with the assistance of computer technology and terms of numerical analysis. In engineering practice, the finite element analysis gives more accurate results in strength intensity, whereas, the calculating process is more complicated. Modern optimized design method is widely used in engineering practice, it makes the optimization into an automatic cycle instead of manually changing parameters and redesigning. however, the application in optimized design of hydraulic excavator still hasn t been achieved; designers still have to repeat the normal artificial process which is preliminary design - verification - redesign. Why not put structural analysis into optimization program to realize automatic optimization design? The reason is that the principles of current finite element analysis is dispersing model firstly, which produces huge amount of data; meanwhile, automatic boolean operation and meshing of new model tends to suspend when meets errors[3]; moreover, it s hard to apply load on different grid model. Since the structural design of the hydraulic

2 excavator working device, the boom and arm are thin-walled H-beam or I beam that are exposed to external loading like bending and twisting; which structures can be simplified to meet the basic assumptions of material mechanics[4]. In this paper, the structural analysis and optimized design of hydraulic excavator working device(excluding bucket) based on mechanics of materials is proposed to study. The working stress level plays a key role in evaluating performance of excavator working device. In order to reflect the stress state of certain sections objectively, 8 most dangerous stress observation points are selected on section according to the failure pattern of box welded structure and actual failure situations of excavator working device in practice. The most dangerous stress observation points are shown in Fig.. Structural Analysis.1 The simplification of cross section The statistical study of current excavator working device shows that general excavator working device (excluding bucket) can be simplified as tapered cross-section box beams with respect to length and cross section shape is constant. On the length direction, the thickness of upper plate, bottom plate and side plate are variable. The most common shapes of cross section parameters are shown in Fig. 1. Fig. Possible maximum stress points on cross-section. Stress analysis on section The hinge point forces of the boom and arm under standard working condition can be obtained by Excavator Performance Analysis Software[5]. By the way, the lateral force and the partial load are maximized on excavator working device[6] and the gravity is assumed on barycenter. Besides, local coordinate on the arm and boom are built shown as fig.3 and fig.4. Fig.3 Structural sketch of arm Fig.4 Structural sketch of boom Based on the boundary conditions, the maximum stress on section perpendicular to axis can be attained. The combination of stress on cross section of the working Fig.1 Structural parameter sketch of different cross-section equipment is complex; it can be decomposed into five parts to solve respectively; then, according to the fourth

3 strength theory, synthetic stress of each point on cross section will be ready for numerical value comparison. (1)The stress produced by axial force F / A (1) 0 F axial force on the cross-section A section area ()Bending stress M 1 1 * x () Ix M bending moment produced by normal load 1 Ix inertia moment of the cross section to horizontal neutral axis neutral axis x the distance from the desired point to the (3)Shear stress of additional transverse bending moment caused by partial load and lateral force M * y (3) Iy M Additional transverse torque produced by 3 partial load and lateral force Wt Anti-torsion modulus of section (5)Shear stress produced by shear force. According to materials mechanics, the shear stress distribution follows the law of parabola on section. The value of shear stress on intermediate shaft is maximum while the value of torque on intermediate shaft is zero; therefore, the shear stress is not considered in stress synthesis generally [8]. However, in this case, the side plates may have large shear stress and cross-sectional area of side-bottom plates near welded joint changes rapidly. The shear stress is then calculated with following equation M * S 4 (5) * t * Ix M the shear force at section 4 S the static surface moment of bottom and side plates to neutral axis t the thickness of side plate M additional transverse bending moment caused by partial load and lateral force Iy Inertia moment of cross section to vertical neutral axis y the distance from the point we need to the vertical neutral axis.3 Software implementation The above calculation is compiled into excavator working device strength analysis software in Visual Basic language; it can quickly give the sectional stress state of excavator working device under given working condition. It s short for SSAS. The parameter input interface of the SSAS is shown as Fig. 5. (4)Stress of additional torque caused by partial load and lateral force. Due to the I-shaped hollow section consisting of thin-walled steel plate, it s assumed that shear stress on thin-walled steel section along the direction of steel plate is equal under torsional load[7]. M 3 1 (4) Wt

4 digging altitude of boom and arm[9], the selected condition need to meet the following conditions: a. Boom stays at the position where boom cylinder has maximum lever; b. Arm cylinder has maximum lever; c. Bucket stays at the position where it has the maximum digging force(inward 5 degree from the three-point line) and bucket tooth is under partial load and lateral force. Finally, input the mechanism parameters of dangerous condition, and select sectional shape 1 and geometry Fig.5 Input interface of section strength calculation The left side interface includes such parameter as the working conditions of excavator, the specific location of a section in local coordinate system, the rotating brake torque and width of bucket. In addition, working conditions can be determined after running the master program and inputting the working altitude of working device. Click on the Cross PRM button into interface shown in Fig. 6, in which section type is selected and structural parameter is set. After the setting, stress values parameter as shown in Fig. 5 and Fig. 6. In same working conditions, strength analysis is finished by ANSYS finite element analysis software and the SSAS. The FEA result is shown in Fig.7 and the compare result is stated in Fig.8. It s obvious that stress of these two patterns shares consistent trends and almost equivalent value. It firmly tells that self-compiled SSAS is as effective as finite element analysis software in structural strength analysis of backhoe excavator working device. of 8 observation points are available. Fig.6 Input interface of structural parameter of section.4 Case study To verify the validity of the SSAS, the SSAS analysis result of a backhoe hydraulic excavator arm is compared to that of finite element analysis. According to relevant standard about excavator attachment strength proofread and considering dangerous Fig.7 Stress distribution of arm FEA

5 side plates; these symmetrical side plates have same parameters; therefore, the thickness of side plates only depends on the location of cross-section, and the thickness is constant in each one of the three. In this optimization case, taking the parameters of sections at Lx1, Lx, Lx3 (shown in fig.9) as optimization variables. Values of Lx1, Fig.8 Contrast of stress from different analysis Lx, Lx3 are shown in Tab Structural Optimization and Design The backhoe hydraulic excavator working device can be optimized in terms of structure with the SSAS. Since Tab. 1 Coordinate value of cross-section Cross-section Lx1 Lx Lx3 Location/mm the optimization of the backhoe hydraulic excavator working device structure is a nonlinear programming research of constraints with multi-variables and multi-extreme points, it s hard to get the best solution with those most commonly used methods[10]. In this paper, on the basis of above strength calculation method, excavator arm structure optimization is taken on using genetic algorithms as optimization algorithm. The main optimization steps of the genetic algorithm are as follows: Firstly, establish an initial population of 80 members by changing the design variables, and iterations number is set to 95, the crossover probability is 0.9, mutation probability is 0.1. Each individual of initial population is generated by a random process; besides, fitness function is constructed refer to optimization objective; then, populations with higher fitness are produced through selection, crossover and mutation; finally, the best results are obtained using repeated genetic manipulation to guide population evolution toward optimal direction. 3.1 Design variables The arm optimization is mainly realized by adjusting the structural parameters of working device aiming at eliminating weight without decreasing overall strength security. The arm body mainly consists of upper plate, bottom plate, back side plates, middle side plates, front Fig.9 Sketch of the selected cross-section Without changing the mechanism parameters, the design variables are determined as followed: x { t1, t1, t, t3, t3, t4, Hx1, Hx3} { x, x, x, x, x, x, x, x } (6) t1 The thickness of arm bottom plate t1 The thickness of arm side plate on first section t The thickness of arm side plate on second section t3 The thickness of arm side plate on third section t3 The thickness of arm upper plate t4 The distance between the arm side plate and edge of bottom plate Hx1 The height of arm side plate at first section

6 x3 The height of arm side plate at third section 3. The objective function Structural optimization of excavator working device means that the weight of the excavator working device is reduced without increasing the maximum stress on section. Since the mass is proportional to volume and the length of arm is constant, the optimization target is to keep the cross-sectional area as small as possible. The effect principle of excavator working device mass in vertical and horizontal mining is consistent, that is to say, performance improvement in vertical mining can make sure overall comprehensive performance advancement[11]. Hence, selective digging pattern is acceptable. To avoid the misleading of different cross-section area in orders of magnitude, a normalized objective function is defined as follow: m min( A, A,... A n ) 1 An 1 m 1,,3(7) 1 The ratio of the minimum area to optimized area on arm first cross-section; The ratio of the minimum area to optimized area on arm second cross-section; 3 The ratio of the minimum area to optimized area on arm third cross-section. In summary, the objective function of arm optimization is defined as 3.3 Constraints F( ) ( ) / (8) To ensure the optimized arm can work regularly during the whole lifespan of the excavator,structural optimization of backhoe hydraulic excavator working device has to obey the law of excavator working device strength requirements. Strength check of each section obeys the following constraints: The first section of arm: 1 1 ( 1 the optimized stress of first arm section; 1 the stress of first arm section before optimization, the following similar) The second section of arm: The third section of arm: 3.4 Optimization case study 3 3 The arm structural parameters of previous verified excavator model is optimized with the above optimization way. Structural parameters of arm before and after optimizing are shown in Tab.. Tab. Contrast of arm structural parameter before and after optimization Parameter at1 at1 at3 at4 Before After Parameter at at3 Hx1 Hx3 Before After The most desirable fitness function value F (x) = From the parameters comparison of before and after optimization in Tab., It s clear that the thickness of the arm side plates is decreased with the upper plate almost unchanged, and bottom plate has some increment. The thickness of side plates are never thicker than that of upper and bottom plates before and after optimization, which goes consistently with the law that the thickness of ribbed plate is never thicker than that of base plates. Combining the I structure of arm and variation tendency of plates in optimization process, it s concluded that the upper and bottom plate play the most important role in arm structure, whereas, side plates acts like stiffener. Taking advantage of those parameters in Tab., the areas of cross-section1,,3 can be easily calculated as is shown in Tab. 3.

7 Tab. 3 Contrast of arm section area before and after optimization The work reported in this paper is funded by the National Natural Science Foundation of China (NO ). Cross-section AA1 AA AA3 Before/mm After/mm Variation -.7% -.49% 0.00% The data in Tab.3 shows that the optimized area of the first and second section has been reduced, while the area of third cross-section is substantially constant. Analyzing the established three-dimensional model of arm before and after optimization, it s found that the volume of arm is decreased by 1.33% that is 38.3kg decrement in weight. 4 Conclusion (1) After analyzing the structural characteristics of backhoe hydraulic excavator working device, the boom and arm section is simplified; then, based on classical mechanics, self-compiled structure analysis software is programmed in VB. Finally, the case study demonstrates the feasibility and reliability of the method. () On the basis of self-compiled structural analysis software, structural optimization of backhoe hydraulic excavator working device is proposed. The optimization of arm structure parameter with the software shows that upper and bottom plates are the main working components, which is consistent with the actual design principle of arm. (3) The quick and easy optimized design of backhoe hydraulic excavator is realized in this research which provides the basis for collaborative design considering structure and mining performance. Meanwhile, the research is a good theoretical and technical support for References [1] Single Bucket Hydraulic Excavator [M]. Self-edited textbook of Chongqing University, []Chen Jian,Zhou in,liu in. Optimization Design of the Hydraulic Excavator Arm[J]. Construction Machinery and Equipment,008,39(7):0 1. [3] Song Zhian,Yu Tao,LI Hongyan. The finite element analysis of mechanical structure[m]. National Defence Industry Press, 01.. [4] Liu Benxue,The Finite Element Analysis of Backhoe Hydraulic Excavator Working Device[D].i an:chang an university.003. [5]Chongqing University. Excavator Performance Analysis Software.014. [6] Ren Zhigui,Chen Jin,Wang Shuchun, Pang iaoping. Dynamic Stress Test and Transient Analysis of Hydraulic Excavator[J]. Journal of South China University of Technology: Natural Science Edition,014,40(1):-7. [7] G.M.L.GLADWELL, Structural Analysis[M]. Springer,009: [8] Chen Tianfu,Feng iangui. Mechanics of Materials, Chongqing University Press [M], [9] Chen Guojun. Hydraulic excavator [M]. Huazhong University of Science Press, [10] Chen Yihua, Wang Kairong, He Renbin. Optimization Methods. Chongqing: Mathematics school of Chongqing University.004. [11] Huang Bin. Optimization of the Hydraulic Excavator Working Device[D]. Changsha: Central south university original design excavator which is of great theoretical significance and industrial applications. Acknowledgements