fatigue analysis and design optimization … int. j. mech. eng. & rob. res. 2014 p mahesh babu...

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Int. J. Mech. Eng. & Rob. Res. 2014 P Mahesh Babu and K Sreenivas, 2014

FATIGUE ANALYSIS AND DESIGN OPTIMIZATIONOF A DIGGER ARMP Mahesh Babu1* and K Sreenivas2

*Corresponding Author: P Mahesh Babu,[email protected]

In this project a detailed Fatigue Analysis of the digger arm under the worst loading conditionwas carried out. During the part of project a static and fatigue analysis of digger arm was carriedout using finite element analysis package. The 3 dimensional model of the digger arm wasdesigned using NX-CAD. Then the 3D model is imported into ANSYS using the parasolid format.The finite element idealization of this model was then produced using the 10 node tetrahedronsolid element. The analysis was performed in a static condition. From the analysis results totaldeformation, alternative stress and shear stresses are documented by using FEA software. Inthis project we will also find out the life, safety factor and damage of digger arm by using Goodmantool. S-N curve is given as input for the material used for digger arm. Finally design optimizationof the digger arm was done to increase the life of the digger arm component. NX-CAD softwarewas used for 3D modeling of the digger arm and ANSYS software was used to do the fatigueanalysis of the digger arm.

Keywords: Digger arm, FEA, Fatigue, Goodman curve

INTRODUCTIONA digger arm used in excavator is a typicalhydraulic heavy duty human operated machineused in general versatile constructionoperations, such as digging, ground levelling,carrying loads, dumping loads and straighttraction. The digger arm operates in worstworking conditions. Due to its worst workingconditions, it is subjected to huge loads. Thisdigging task is also repetitive in nature and

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Int. J. Mech. Eng. & Rob. Res. 2014

1 M.Tech Student, Department of Mechanical Engineering, Krishnachaitanya Institute of Technology & Sciences, Markapur, AndhraPradesh, India.

2 Professor, Department of Mechanical Engineering, Krishnachaitanya Institute of Technology & Sciences, Markapur, Andhra Pradesh,India.

during the operation the entire link mechanismworks under the dynamical condition. Becauseof this reason the digger arm fails veryfrequently and the entire system becomes idleand leads to a commercial loss to the owner.

The hydraulic excavator is most commonlyused for digging rocks and soil, but with itsmany attachments it can also be used forcutting steel, breaking concrete, drilling holesin the earth, laying gravel onto the road prior

Research Paper

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to paving, crushing rocks, steel, and concrete,and even mowing landscapes. Hydraulicexcavators have an operating weight of 20,000pounds (9,072 kg) or higher.

The invention of the hydraulic excavator, withits easier operation and cheaper productionhas replaced the excavator. Excavators arealso called diggers, JCBs (a proprietaryname, in an example of a generic tradework), mechanical shovels, or 360-degreeexcavators (sometimes abbreviated simplyto 360). Tracked excavators are sometimescalled “track hoes” by analogy to thebackhoe. In the UK, wheeled excavators aresometimes known as “rubber ducks.

PROBLEM DEFINITIONDigger arm has been designed and optimizedfor worst loading condition. Initially staticAnalysis is done by applying the worst loadingconditions generated due to static anddynamic conditions. Maximum principlestresses are calculated from the analysis.Fatigue life of the digger arm is estimated byusing the Goodman diagram tool by giving theinputs like principle stresses obtained from thestatic analysis for both static and dynamic loadconditions. In this project we will find out thelife, safety factor and damage of digger armby using Goodman diagram tool. S-N curvefor the material is given as input for the materialused for digger arm. NX-CAD software is usedfor 3D modelling of the digger arm and ANSYSsoftware is used to do the fatigue analysis ofthe digger arm.

METHODOLOGYThe methodology followed in the project is asfollows:

• Perform the Design calculations of theDigger arm.

• Create a 3D model of the digger arm usingNX-CAD software.

• Perform static analysis of the digger armfor static and dynamic load conditions usingANSYS software and calculate themaximum principle stresses. Estimate thelife, safety factor and damage of digger armby using Goodman diagram tool.

• Optimize the original model to increase thelife of the digger arm.

3D MODELLING3D model of the digger arm was developed inNX-CAD from the design calculations done.The model was then converted into a parasolidto import into ANSYS. A Finite Element modelwas developed with solid elements. Theelements that are used for idealizing the diggerarm were described below.

Initially static Analysis is done by applyingthe worst loading conditions generated due tostatic and dynamic conditions. Maximumprinciple stresses are calculated from the

Figure 1: Shows the 3D Model of DiggerArm

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analysis. Fatigue life of the digger arm isestimated by using the Goodman diagram toolby giving the inputs like principle stressesobtained from the static analysis for both staticand dynamic load conditions.

FINITE ELEMENT ANALYSISMaterial Properties

Steel properties (A36)

Young’s Modulus (Ex) = 2e5N/mm2

Poisson’s Ratio = 0.3

Density = 7850 Kg/mm3

Yield Strength – 240 N/mm2

Ultimate strength – 475 N/mm2

Endurance Limit = 0.5 of ultimate strength= 475/2 = 237.5 N/mm2

Element Type Used

Element type: Solid92

No. of nodes: 10

Degrees of freedom: 3 (UX, UY, UZ)

Boundary Conditions1. Bucket cylinder (Fb) load is applied on arm

teeth in X-Direction.

2. Arm cylinder (Fs) load is applied on armteeth with cylindrical angle in X-Direction.

3. Digger arm is arrested in all Dof at hydrauliccylinder joints.

ANSYS RESULTS

1. 1.9 153 124

Table 1: Max. Deflection and Von MisesStress

S.No.

Deflection(mm)

VonMisesStress (Mpa)

PrincipleStress (Mpa)

Figure 2: Total Deflection of Digger Arm

Figure 3: Von Mises Stress on the DiggerArm

FATIGUE ANALYSIS

Figure 4: Goodman Diagram with Details

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FOR DYNAMIC LOADCONDITIONS

RESULTS

Figure 5: Von Mises Stress of Digger Armfor Dynamic Loading

MODIFIED MODEL

Figure 7: Change#1 Made on the DiggerArm

Figure 6: Goodman Diagram with Details

Figure 8: Von Mises of Modified DiggerArm for Dynamic Loading

FATIGUE LIFE CYCLE

Figure 9: Goodman Diagram with ModifiedModel

RESULTS AND DISCUSSIONDigger arm has been designed and optimizedfor worst loading condition. Initially staticAnalysis is done by applying the worst loading

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conditions generated due to static anddynamic conditions. Maximum principlestresses are calculated from the analysis.Fatigue life of the digger arm is estimated byusing the Goodman diagram tool by giving theinputs like principle stresses obtained from thestatic analysis for both static and dynamic loadconditions. After finding the fatigue life cycleof the original digger arm, design changeswere implemented to increase the fatigue lifecycle. The following observations were madefrom the analysis.

• The fatigue life cycle of the digger armobserved for the original model of the diggerarm from Goodman tool is 5420439 cycles.

• The fatigue life cycle of the digger armobserved for the modified model of thedigger arm from Goodman tool is 9459459cycles.

• It is observed that the fatigue life cycle isincreased by 42.6% by implementing thedesign changes as mentioned above.

CONCLUSIONThe digger arm is developed to performexcavation task for light duty construction work.Based on static force and dynamic force loads,finite element analysis is carried out for diggerarm. It is clearly depicted that the stressesproduced in the component of the digger armare within the safe limit of the material stressesfor the case of static and dynamic loadconditions. It is also clearly depicted that thefatigue life cycle of the digger arm is more by42.6% for modified digger arm compared tooriginal digger arm. Based on results we canconclude that optimization can help to reducethe initial cost of the digger arm as well as toimprove the functionality and life cycle as the

digger arm operates in worst workingconditions. The optimization also helps toavoid frequent failure of digger arm which maycause the entire system become idle and leadto a commercial loss to the owner.

REFERENCES1. Andrew Hall (2002), “Characterizing the

Operation of a Large HydraulicExcavator”, p. 1, Thesis of Master ofPhilosophy, School of Engineering, TheUniversity of Queensland Brisbane,Australia.

2. Bhaveshkumar P Patel and Prajapati J M(2011a), “A Review on FEA andOptimization of Backhoe Attachment inHydraulic Excavator”, IACSITInternational Journal of Engineering andTechnology, Vol. 3, No. 5, pp. 505-511.

3. Bhaveshkumar P Patel and Prajapati J M(2011b), “Soil-Tool Interaction as a Reviewfor Digging Operation of Mini HydraulicExcavator”, International Journal ofEngineering Science and Technology,Vol. 3, No. 2, pp. 894-901.

4. Bhaveshkumar P Patel and Prajapati J M(2012), “Evaluation of Bucket Capacity,Digging Force Calculations and StaticForce Analysis of Mini Hydraulic BackhoeExcavator”, Machine Design—TheJournal of Faculty of Technical Sciences,Vol. 4, No. 1, pp. 59-66.

5. Bhaveshkumar P Patel, Prajapati J M andSandipkumar P Parmar (2011), “A Reviewon FEA and Genetic Algorithm Approachfor Structural Optimization”, GlobalJournal of Mechanical Engineering andComputational Science, Vol. 1, No. 4,pp. 29-32.

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6. Mehmet Yener (2005), “Design of aComputer Interface for Automatic FiniteElement Analysis of an Excavator Boom”,pp. 1-4 and 68-69, MS Thesis, TheGraduate School of Natural and AppliedSciences of Middle East TechnicalUniversity.

7. Mehta Gaurav K (2008), “Design andDevelopment of an Excavator

Attachment”, p. 1, M.Tech DissertationThesis, Nirma University, Institute ofTechnology, Ahmedabad.

8. Mittal R K and Nagtath I J (2008),“Robotics and Control”, 9th Reprint, pp. 70and 190, Tata McGraw-Hill PublishingCompany Limited.

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