S. C. Bende, N. P. Awate / Mechanica Confab

COMPUTER AIDED DESIGN OF EXCAVATOR ARM: FEM APPROACH Sachin B. Bende1, N. P. Awate2 1

2

M. Tech. Scholar, GHRCE, Nagpur, India Assistant Professor, GHRCE, Nagpur, India

Abstract The present review provides brief information about the excavator arm mechanism. The use of aluminum alloy material in excavator arm can increase the volumetric capacity of the bucket. In this paper, it is proposed to study the concept of finite element method and its application to the excavator arm design. As the present mechanism used in excavator arm is subjected to torsional and bending stresses, so it is necessary to design a new mechanism. The finite element method was used to find out the optimized design solution. Pro-E software was used for CAD design. Finite Element Methodology was used to find out dynamic fatigue failure of excavator arm mechanism design using ANSYS FEM software for different material properties. Finite Element Methodology increased the productivity of the excavator arm mechanism design, helped to conceptualize the product, reduced time required to design and analysis. Keyword: Excavator Arm, CAD, FEM, Digging, Lifting 1. Introduction Excavators are intended for excavating rocks and soil. Excavators may have a mechanical or hydraulic drive. Hydraulic excavators are the most important group of excavators. Typical hydraulic backhoe excavator linkages are shown in Figure 1. It consists of four link members: the bucket, the stick, the boom and the revolving superstructure (upper carriage). There is almost limitless range of sizes of backhoe, from hoes mounted on small agricultural tractors used in residential construction all the way up to huge crawler-mounted hoes capable of handling some of the heaviest work in industrial jobs. These excavators are also operated with other attachments such as clamshell, dragline, drilling equipment scarifies for braking pavements and frozen soils. On the other hand, the work functions of the backhoe often overlap those of other machines such as front-end loaders, tractor shovel, scrapers, clamshells and draglines. In addition, it is particularly useful for trenching foundation footing excavation, basement excavation and similar works [1, 2]. The useful task of backhoe hydraulic excavator is to free and/or remove surface materials such as soil from its original location and transfer it to another location by lowering the bucket, digging, pushing and/or pulling soil then lifting, swinging and emptying the bucket. The excavation of this task is usually performed by a human operator who controls the motion of the machine manually by using the visual feedback provided through his or her own eyes. In many current applications of excavations, the semiautonomous applications or even automatic operations of the machine is desirable and sometimes even necessary. Automation of excavation control system for effective use in the dark severe weather, hazardous and/or unhealthy environments, terrestrial, lunar and planetary excavation calls for a robotic system able to perform the planned digging work. Based on the aforementioned Vol. 2, No. 1, January 2013

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argument, the development of an accurate computer model is essential to describe the kinematics and dynamics of excavator for transferring surface materials [3, 4].

Figure 1. Typical Hydraulic Backhoe Excavator [3] 2. Methodology 2.1. Design Requirements Uni-Graphics NX software was used to develop a three dimensional model of a proposed hydraulic excavator arm. An exhaustive design process was carried out before the development of the three-dimensional model. This is a classic strength to weight design optimization problem. The reduction of mass not only allows for a greater load carrying capacity of the excavator, but a significant cost reduction. Aspects such as materials and specific designs are crucial in the development process. Therefore, measurements and criteria were established, so that the model met the following criteria: • • • •

The model must withstand a minimum load of 10,000 Newton either in compression or in tension without failure. The material of the model must be such that the manufacturing process is cost efficient. The model must be such that the requirements for fabrication are optimized. The final product must be durable and reliable [15].

2.2. Model Building Uni-Graphics (UG) software was used to build 3D geometric models of the frame, working arms, bucket rods and buckets, bases on drawings of the excavator. During modeling, threaded holes were omitted; chamfers, transportation lifting lug and other insignificant factors were also neglected [8]. Figure 2 shows the geometric model of a mechanism assembled from the above components.

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Figure 2. Three-Dimensional Model [8] ANSYS-oriented software prepared in GFEM module of UG, specifically, its FE preprocessing feature, was used to build a FE model. Figure 3 shows the FE model at a certain operating position.

Figure 3. Three-Dimensional FE Model [8] 2.3. Defining Typical Operating Conditions Operating conditions of a back-digging, single bucket hydraulic excavator mechanism was categorized into four types. To address failure problems in engineering, the paper added a fifth operating condition viz. rotary braking operating condition. Position characterizations and load computations of these operating conditions are shown below. Operating Condition 1: The cylinder of the working arm retracts completely, the cylinder of the bucket rod has the greatest force arm; the tips of teeth are on the extended line of the line segment passing through the joint of bucket and bucket rods, and the joint of bucket rod and working arm. Under this condition, the load is composed of gravity, tangential force and side force. Vol. 2, No. 1, January 2013

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Operating Condition 2: Cylinders of the working arm and the bucket rod have the greatest force am; the tips of teeth are on the extended line of the line segment passing through the joint of bucket and bucket rod, and the joint of bucket rod and working arm. The load is composed of gravity and tangential force. Operating Condition 3: Cylinders of the working arm and the bucket rod have the greatest force arm; cylinder of the bucket operates at maximum equivalent force arm. The load is composed of gravity and tangential force. Operating Condition 4: Cylinder of the working arm retracts completely, the tips of teeth are on the extended line of the line segment passing through the joint of bucket and bucket rod, and the joint of bucket rod and working arm; the three points are all on the plumb line. The load is composed of gravity, tangential force and side force. Operating Condition 5: Cylinder of the bucket rod retracts completely; cylinder of the working arm ensures that the working arm and bucket rod operate at a position furthest away from the axis of rotation. Cylinder of the bucket extends to its maximum length, enabling the bucket to take up loads. The load is composed of gravity and inertial braking moment [8]. 2.4. Material Initially the backhoe excavator working device was made up of steel alloy which is heavier. So in order to reduce the weight of the device, an aluminum alloy was used instead of steel alloy. This change lightens the components of the arm, allows to increase the load capacity of the bucket and so it is possible to increase the excavator productivity per hour [11, 12]. 2.5. Load Conditions For the purpose of increasing the productivity of excavator, many different load conditions have been studied numerically on the original excavator in order to estimate a safety factor and the deformability or flexibility of each component. These parameters have been used in order to design a new arm. Five different load conditions have been checked in order to establish the stress conditions in each component of the excavator arm. The first load condition concerns the leveling operation which allows starting the bucket at the maximum and minimum distance from the axle of rotation. The distance of the bucket from the surface does not change in this roto-translation (Figure 4).

(a)

(b)

Figure 4. First Load Condition: (a) Initial Position and (b) Final Position[11]. The second and third load conditions were similar. The second concerns the lifting operation with the maximum load at the minimum distance from the axle of rotation. The Vol. 2, No. 1, January 2013

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third load condition concerns the lifting operation at the maximum distance from the axle of rotation (Figure 5).

Figure 5. Second and Third Load Conditions: Lifting at the Maxima and Minima Distance from Axle of Rotation [11]. The fourth load condition is a usual operation which concerns the leveling in the orthogonal direction as regards to the axle of the arm. This load condition is more important in order to evaluate the torsional behavior of the components (Figure 6).

Figure 6. Fourth Load Condition [11] The last load condition examined is an exceptional condition. In this case the force applied to each component of the excavator arm is the maximum force generated by the hydraulic cylinders both in tension and both in compression [11,12]. 3. Finite Element Method (FEM) FEM is a computational approach that allows manufacturers to virtually test products and systems under different conditions such as changes in temperature and stress. The Finite Element Method (FEM), also known as Finite Element Analysis (FEA), is a numerical Vol. 2, No. 1, January 2013

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technique that has become an essential component for the design of products and engineering systems, as well as for the analysis of a broad variety of physical problems. As it is suggested in its name, finite element discretizes a continuum problem into a number of finite elements. Generally, design geometry is complex and the discretization allows for a consistent treatment of such complex geometries and boundary conditions. The engineer might find situations on which the creation of a prototype is not possible. Virtual engineering and in particular the finite element method (FEM) help in the development process by allowing the engineer to create an extensive variety of scenarios for a specific model [15]. The modeling approach involves the following steps: 1) Material Properties: The first step on analyzing the model with Unigraphics NX was to set material properties and conditions. The material properties in the design of the excavator arm will evidently determine to a great extent its reaction under any condition. Only those materials should be chosen which stand firm under the application of big loads given that the mechanism we are designing will experience and develop continuous amounts of stress. 2) Boundary Condition Enforcement: The second step when utilizing FEM in Unigraphics is to define Boundary conditions. Basically, when setting boundary conditions, we are adding constraints to the system. These are then translated into conditions in the analysis such as fixed points, individual force lines of action and points of contact, among others. 3) Simulation and Discretization: Once the properties and concepts previously defined are understood for the design of the excavator arm, the next step is to discretize. The process of discretization, which entails the subdivision of a body into more simple bodies, consists of geometry modeling, and meshing that will conclude as a set of discrete simultaneous equations describing the system. 4) Convergence and Solution: Now, the remaining part is to approximate a solution. Among the several classical variation methods for the approximation of solutions for the study of a physical problem, the most utilized in FEM include the Ritz method, the Galerkin method, the least square method, and the collocation method. All of these methods seek an approximate solution in the form of a linear combination of suitable approximation functions. 4. Conclusion The existence of commercial software packages has allowed engineers and analysts to narrow the development process in either products or systems since the past few decades. Virtual engineering and finite element method (FEM) provides detail analysis of a conceptual design. These tools allow us to determine forces and stresses that are developed in critical points. This helps us to determine modifications for the purpose of meeting the established criterion of the design. The study builds a complete, integrated and refined 3D FE model of the mechanism, and examines stiffness, strength and deformation at five typical operating conditions. Integrated FE analysis for a whole working mechanism greatly reduces the computational errors arising from simplification of components, among other sources. It improves design of the current product, and provides a new approach for design and analysis of similar products. Thus, the study aims at increasing the volumetric capacity of the bucket after considering different load conditions and this can be achieved by reducing the weight of the arm. So, Vol. 2, No. 1, January 2013

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using an aluminum alloy instead of steel alloy in a design of a new geometry for elements of the excavator structure can be proved more effective. Reference [1] Yahya H. Zweiri, Lakmal D. Seneviratne and Kaspar Althoefer, ‘A Generalized Newton Method for Identification of Closed-chain Excavator Arm Parameters’, Proceedings of the 2003 IEEE lnlernstional Coorerenee on Robotics & Automation Taipei, Taiwan, September 14-19, 1003. [2] Shahram Tafazoli, Peter D. Lawrence, and S. E. Salcudean, ‘Identification of Inertial and Friction Parameters for Excavator Arms’, IEEE TRANSACTIONS ON ROBOTICS AND AUTOMATION, VOL. 15, NO. 5, OCTOBER 1999. [3] Pao-Hwa Yang, Shiu-PingWang, Jiunn-Shean Chiang and Chung-Shing Wang, ‘The Adaptive Control Applied to the Analysis of the Excavator Dynamics’, The 3rd Intetnational Conference on Innovative Computing Information and Control (ICICIC'08) 2008 IEEE. [4] Jun Gu, ‘Design of Robotic Excavator Arm Control Utilizing Proportional-IntegralPlus’, IEEE 2008. [5] Chang Lv and Zhang Jihong, ‘Excavating force analysis and calculation of dipper handle’, IEEE 2011. [6] Samuel Frimpong *, Yafei Hu, Kwame Awuah-Offei, ‘Mechanics of cable shovelformation interactions in surface mining excavations’, Journal of Terramechanics 42 (2005) 15–33. [7] Dongmok Kim a, Jongwon Kim a, Kyouhee Lee a, Cheolgyu Park b, Jinsuk Song b, Deuksoo Kang a, ‘Excavator tele-operation system using a human arm’, Automation in Construction 18 (2009) 173–182. [8] Guohua Cui and Yanwei Zhang, ‘Integrated Finite Element Analysis and Experimental Validation of an Excavator Working Equipment’, IEEE 2009. [9] Jongwon Seo, Seungsoo Lee, Jeonghwan Kim, Sung-Keun Kim, ‘Task planner design for an automated excavation system’, Automation in Construction 20 (2011) 954–966. [10] Pyung Hun Chang, Soo-Jin Lee, ‘A straight-line motion tracking control of hydraulic excavation system’, Mechatronics 12 (2002) 119-138. [11] Luigi Solazzi, ‘Design of aluminium boom and arm for an excavator’, Journal of Terramechanics 47 (2010) 201–207. [12] Srdan M. Bosˇnjak, ‘Comments on ‘‘Design of aluminium boom and arm for an excavator”, Journal of Terramechanics 48 (2011) 459–462. [13] Sung-Uk Lee*, Pyung Hun Chang, ‘Control of a heavy-duty robotic excavator using time delay control with integral sliding surface’, Control Engineering Practice 10 (2002) 697–711. [14] Q.P. Ha, Q.H. Nguyen, D.C. Rye, H.F. Durrant-Whyte, ‘Impedance control of a hydraulically actuated robotic excavator’, Automation in Construction 9 _2000. 421– 435. [15] Enrique Busquets, ‘Finite Element Method Applied to a conceptual design of a Hydraulic Excavator arm’, The University of Texas at El Paso International Test and Evaluation Association.

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