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This chapter describes the methodology adopted for static finite element analysis of the
boom, arm, bucket and swing link of the backhoe excavator in ANSYS. Furthermore;
boundary conditions, meshed models, and results of boom, arm, bucket and swing link
are presented and discussed. FEA of all four parts of backhoe excavator attachment are
done by considering the maximum breakout force configuration as calculated and
presented in chapter 7.
8.1 Introduction to finite element method
The finite element method is an approximate numerical procedure for analyzing large
structures and continua. The finite element method became popular with the
advancements in digital computers since they allow engineers to solve large systems of
equations quickly and efficiently. The finite element method is a very useful tool for the
solution of many types of engineering problems such as the analysis of the structures,
heat transfer and fluid flow. The method is also used in the design of air frames, ships,
electric motors, heat engines and spacecraft. The finite element method is also used for
analyzing the behavior of components of biological systems. In most structural analysis
applications it is necessary to compute displacements and stresses at various points of
interest. The finite element method is a very valuable tool for studying the behavior of
structures. In the finite element method, the finite element model is created by dividing
the structure in to a number of finite elements. Each element is interconnected by nodes.
The selection of elements for modeling the structure depends upon the behavior and
geometry of the structure being analyzed. The modeling pattern, which is generally called
mesh for the finite element method, is a very important part of the modeling process. The
results obtained from the analysis depend upon the selection of the finite elements and the
mesh size. Although the finite element model does not behave exactly like the actual
structure, it is possible to obtain sufficiently accurate results for most practical
applications. Once the finite element model has been created, the equilibrium equations
can easily be solved using digital computers without having to solve a large number of
partial differential equations by hand. The deflections at each node of the finite element
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respect to the overall model dimensions and do not play a substantial role in the
simulation results can be avoided or suppressed.
8.3 Analysis assumptions The stress analysis that ANSYS simulations provides is appropriate only for linear
material properties. These properties are where the stresses are directly proportional
to the strain in the material (no permanent yielding of the material). Linear behavior
results when the slope of the material stresses-strain curve in the elastic region
(measured as the Modulus of Elasticity) is constant.
The total deformation is assumed to be small in comparison to the part thickness. Asfor an example, while studying the deflection of the beam, the calculateddisplacement must be less than the minimum cross-section of the beam.
The results are temperature independent. The temperature is assumed not to affectthe material properties.
Inertia of the component is neglected as the acceleration of the parts is less than 0.05m/sec2in the field.
Minimum element size allows automatic refinement in small areas.
8.4 FEA of backhoe excavator parts
Here, intention to perform static FEA is to know the developed stresses are within the safe
stress limit of the material used for backhoe excavator attachments. Based on this analysis
one can identify that the optimization of backhoe mechanism is possible or not.
The presented work includes the static FEA of bucket, arm, boom and swing link by
applying boundary conditions as calculated in chapter 7 using ANSYS workbench software,
in which the elements generated by the software itself and they are tetrahedron in shape. The
material properties are considered as discussed in chapter 5. Fine meshing is performed for
analysis of all the parts of the backhoe attachment which provides accurate results. The post
processing includes the results of Von Misses stresses and displacement developed with the
application of known boundaryconditions.
Von Misses stresses is used as a criterion in determining the onset of failure in
ductile materials, and the materials in the presented study for the parts of the bucket, arm,
boom and swing link are of ductile materials, so the design of all parts should be on the
basis of Von Misses stresses acting on the parts.
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The failure criterion states that the Von Misses stresses VM should be less than the
yield stresses y of the material by taking appropriate safety factor into consideration.
This indicates for the design of a part to be safe, the following condition must be satisfied
(Tirupathi R. Chandrupatla et al., 2005).Design stress for ductile material,
VM y
Safety factor
. (8.1)
The backhoe parts gets deformation as the load applied on them. The deformation or
displacement of the backhoe part should be less than that of the minimum thickness of plate
used in the parts of backhoe attachment to be analyzed for safe stress condition. The next
section presents the FE analysis of the bucket.
8.4.1 FEA of the bucket
Materials used for the parts of the bucket assembly are listed in table 5.1 of chapter 5.
Properties of the materials used to model the bucket are listed in table 5.2 and table 5.3.
Only the bushes used in the bucket assembly are made up of the material IS 2062 (Yield
strength, y = 250 MPa), and rest of the parts of the bucket are made up of HARDOX 400
(Yield strength, y = 1000 MPa). Fig. 8.1 shows the boundary conditions applied to
bucket as calculated for maximum breakout force configuration and presented in Fig. 7.7
(section 7.4.1) of chapter 7.
Fig. 8.1 Boundary conditions applied to the bucket
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Fig. 8.2 Mesh view of the bucket
Fig. 8.3 Maximum Von Misses stresses of the bucket
Fig. 8.2 shows the mesh view of the bucket with 52219 nodes and 9636 elements.
Fig. 8.3 shows the results of the static force analysis as Von Misses stresses on the
bucket. Analysis shows the maximum Von Misses stresses (VM ) is acting at the leap
plate near teeth and it is 203.67 MPa. Minimum Von Misses stresses are acting at the
bush of the mounting lug and it is 0.51909 MPa.
All parts of the bucket are made up of HARDOX 400 material except the bushes
used in mounting lugs are made up of IS 2062 material. The maximum Von Misses
stresses are acting at the leap plate made up of HARDOX 400 material with the yield
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strength of 1000 MPa, by taking safety factor as 2, equation (8.1) yields the safe stress of
= 500 MPa, this clearly indicates VM
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Fig. 8.6 Mesh view of the arm
Materials used for the parts of the arm assembly are listed in table 5.4 of chapter 5.
Properties of the materials used to model the arm assembly are listed in table 5.2, 5.3 and
table 5.5. The bushes and the collar used in the arm assembly are made up of the material IS
2062 (Yield strength, y = 250 MPa), mounting lugs of the cylinders are made of HAROX
400 (Yield strength, y = 1000 MPa), and rest of the parts of the arm are made up of
SAILMA 450HI (Yield strength, y = 450 MPa). Fig. 8.5 shows the boundary conditions
calculated for maximum breakout configuration to carry out the static analysis of arm as
presented in Fig. 7.8 (section 7.4.2) of chapter 7.
Fig. 8.7 Maximum Von Misses stresses of the arm
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Fig. 8.8 Enlarged view of maximum stresses of the arm
Fig. 8.6 shows the mesh view of the arm with 44126 nodes and 8670 elements. Fig.
8.7 shows the results of the Von Misses stresses on arm assembly. As it can be seen from
the Fig. 8.7 that the maximum Von Misses stresses (VM ) is acting at the arm cylinder
mounting lug and it is 239.39 MPa. Fig 8.8 shows enlarge view of arm at which the
maximum stresses are produced.
The arm cylinder mounting lugs are made up from HARDOX 400 with yield
strength of 1000 MPa, by taking safety factor as 2, equation (8.1) yields the safe stress of
= 500 MPa, this clearly indicates VM
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Fig 8.9 shows the maximum displacement on arm is 0.35945 mm which is very less
compare to minimum thickness of plate used in the arm. Therefore, the arm is safe in
deformation for applied loading condition.
8.4.3 FEA of the boom
Fig. 8.10 Boundary conditions applied to the boom
Fig. 8.11 Mesh view of the boom
Materials used for the parts of the boom assembly are listed in table 5.6 of chapter
5. Properties of the materials used to model the boom assembly are listed in table 5.2, 5.3
and table 5.5. The bushes, and the collar used in the boom assembly are made up of the
material IS 2062 (Yield strength = y = 250 MPa), the mounting lugs are made of the
HARDOX 400 material with the yield strength of 1000 MPa and rest of the parts of the
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boom are made up of SAILMA 450HI (Yield strength = y = 450 MPa). Fig. 8.10 shows
the boundary conditions of the boom as calculated for maximum breakout configuration
to carry out the static analysis of boom as presented in Fig. 7.9 (section 7.4.3) of chapter
7.
Fig. 8.12 Maximum Von Misses stresses of the boom
Fig. 8.13 Enlarged view of maximum stresses of the boom
Fig. 8.11 shows the mesh view of the boom with 31855 nodes and 4647 elements. Fig.
8.12 shows the results of the Von Misses stresses on boom assembly. As it can be seen
from the Fig. 8.12 that the maximum Von Misses stress VM is acting on the mounting
lug and it is 246.71 MPa. Fig 8.13 shows enlarged view at which maximum stresses are
produced.
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The boom cylinder mounting lugs are made up of HARDOX 400 with yield
strength of 1000 MPa, by taking safety factor as 2, equation (8.1) yields the safe stress of
= 500 MPa, this clearly indicates VM
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Materials used for the parts of the swing link assembly are listed in table 5.8 of
chapter 5. Properties of the materials used to model the swing link assembly are listed in
table 5.2, 5.3 and table 5.5. The bushes are made up of the material IS 2062 (Yield
strength, y = 250 MPa), the mounting lugs are made of the material HARDOX 400
(yield strength, y 1000 MPa) and rest of the parts of the swing link are made up of
SAILMA 450HI (Yield strength, y = 450 MPa). Fig. 8.15 shows the boundary
conditions of the swing link as calculated for maximum breakout configuration to carry
out the static analysis of swing link as presented in Fig. 7.10 (section 7.4.4) of chapter 7.
Fig. 8.16 Mesh view of the swing link
Fig. 8.17 Maximum Von Misses stresses of the swing link
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Fig. 8.18 Enlarged view of maximum stresses of the swing link
Fig. 8.16 shows the mesh view of the boom with 47376 nodes and 25387 elements.
Fig. 8.17 shows the results of the Von Misses stresses on swing link. As it can be seen
from the Fig. 8.17 that the maximum Von Misses stresses VM is acting on the cylinder
mounting lug and it is 119.83 MPa. Fig. 8.18 shows the enlarged view of maximum
stresses of swing link.
The boom cylinder mounting lugs are made up of HARDOX 400 with yield
strength of 1000 MPa, by taking safety factor as 2, equation (8.1) yields the safe stress of
= 500 MPa, this clearly indicates VM
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in the swing link. Therefore, the swing link is safe in deformation for applied loading
condition.
8.4.5 FEA of the backhoe assembly
Fig. 8.20 Boundary conditions applied to the backhoe assembly
Fig. 8.21 Mesh view of the backhoe assembly
In the earlier sections, the Finite Element Analysis of each part of backhoe
excavator is carried out, now in this section the Finite Element Analysis of full assembly
is performed. As we can see in the Fig 8.20 the whole assembly acts as the cantilever
beam with one free end and one fixed end. Fig 8.20 shows the load applied to the full
assembly of backhoe attachment with appropriate constaints.
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Fig. 8.22 Maximum Von Misses stresses of the backhoe assembly
Fig.8.23 Enlarged view of maximum stresses of the backhoe assembly
Fig. 8.21 shows the mesh view of the boom with 151910 nodes and 47749 elements.Fig 8.22 shows the maximum Von Misses stresses produced in the backhoe attachment
assembly. Maximum Von Misses stresses acting on the bucket is 252.3 MPa. The
maximum Von Misses stresses is acting at the intersection of mounting lug and top
bucket plate as shown in Fig. 8.23, which is made up of HARDOX 400 material with the
yield strength of 1000 MPa. By taking safety factor as 2, equation (5.1) yields = 500
MPa, this clearly indicates VM
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Fig. 8.24 Maximum displacement of the backhoe assembly
Fig. 8.24 shows the maximum displacement in the excavator assembly reported is
3.41 mm at bucket end. The maximum displacement of the excavator assembly is less
than the minimum thickness of plate used in backhoe assembly. Therefore, the backhoe
attachment is safe in deformation for applied loading condition.
8.5 Stress analysis of backhoe parts with consideration of welding
In this setion the Finite Element Analysis is carried out for each part and the full assembly
of the backhoe excavator with welding consideraton. Now, introducing welding to the
each part of the excavator with the help of WLDMENT tool of the Autodesk Inventor
2011.
Welding proportions considered as per the thickness of the parts to be weld based
on industry practice. Then the parts with welding is analysed with FEA approach using
ANSYS. After this analysis, the effect of generated stresses on the backhoe parts with
welding consideration can be evaluated. The residual stresses generated in the welding at
weld joints. Here it is considered that the generated residual stresses will be removed with
the application of stress relieving process performed after the welding provided at each
weld joint of the backhoe parts.
Here, the boundary conditions and material properties used for the purpose of
analysis are remains same as the used in previous sections in which FEA of backhoe
attachment parts carried out without welding consideration.
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8.5.1 FEA of the bucket with welding
Fig. 8.25 Maximum Von Misses stresses of the bucket with welding
Fig. 8.26 Maximum displacement of the bucket with welding
Fig. 8.25 shows the maximum Von Misses stresses acting at the leap plate near
teeth on the bucket with welding is of 178.33 MPa, moreover; all parts of the bucket are
made up of HARDOX 400 material except the bushes used in mounting lugs are made up
of IS 2062 material. The leap is made up of HARDOX 400 material with the yield
strength of 1000 MPa. By taking safety factor as 2, equation (8.1) yields = 500 MPa,
this clearly indicates that VM
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Fig. 8.26 shows the maximum displacement produced in the bucket with welding,
which is 2.245 mm and it is less than the thickness of the plate used in bucket, so the
design against deflection is safe for applied loading conditions. Maximum displacement
of the bucket with welding is less than the displacement of the bucket without welding.
8.5.2 FEA of the Arm with welding
Fig. 8.27 Maximum Von Misses stresses of the arm with welding
Fig. 8.28 Maximum displacement of the arm with welding
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Fig. 8.27 shows the results of the Von Misses stresses on arm with welding. As it
can be seen from Fig. 8.27 that the maximum Von Misses stresses is acting at the arm
cylinder mounting lug and it is 212.46 MPa. The arm cylinder mounting lug is made up of
HARDOX 400 material with the yield strength of 1000 MPa. By taking safety factor as 2,
equation (8.1) yields = 500 MPa, this clearly indicates that VM
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Fig. 8.30 Enlarge view of maximum Von Misses stresses of the boom with welding
Fig. 8.31 Maximum displacement of the arm with welding
Fig. 8.30 shows enlarge view of maximum Von Misses stresses of the boom with
welding. Fig. 8.31 shows the maximum displacement in the boom reported is 1.968 mm
at the boom to arm joint. The maximum displacement is very less compare to minimum
thickness of plate used in the boom so the boom is safe in deflection for given loading
conditions. Maximum displacement of the boom after welding is less than the
displacement of the boom without welding.
8.5.4 FEA of the swing link with welding
Fig. 8.32 shows the results of the Von Misses stresses on swing link with welding. As it
can be seen from the Fig. 8.32 that the maximum Von Misses stresses is acting on the
boom cylinder mounting lug and it is 119.29 MPa on swing link.
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Fig. 8.32 Maximum Von Misses stresses of the swing link with welding
Fig. 8.33 Enlarged view of maximum Von Misses stresses of the swing link
Fig. 8.34 Maximum displacement of the swing link with welding
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The boom cylinder mounting lug welded on swing link is made up of HARDOX
400 material with the yield strength of 1000 MPa. By taking safety factor as 2, equation
(8.1) yields = 500 MPa, this clearly indicates that VM
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Fig. 8.36 Enlarged view of maximum stresses of the backhoe assembly with welding
Fig. 8.37 Maximum displacement of the backhoe assembly with welding
Fig. 8.36 shows the enlarged view of excavator assembly where maximum stresses
are produced. Fig. 8.37 shows the maximum displacement in the excavator assembly
reported is 3.2302 mm at bucket end. The maximum displacement of the excavator
assembly is less than the minimum thickness of plate used in the backhoe attachment, so
the design is safe for deflection.
8.6 Summary
In the first section of this chapter, the introduction of Finite Element Method is explained,
second section explains the FEA procedure in ANSYS software, analysis assumptions are
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included in section three, in the fourth section FEA of backhoe excavator parts is carried
out without welding consideration and in the fifth section stress analysis of backhoe parts
performed with welding consideration. FEA results of backhoe attachment parts and of
backhoe assembly shows the stresses developed are 203.67 MPa, 239.39 MPa, 246.71
MPa, 119.83, and 252.3 MPa for bucket, arm, boom, swing link and backhoe assembly
respectively without welding consideration, where as with welding consideration the
stresses developed are 178.33 MPa, 212.46 MPa, 242.41 MPa, 119.29 MPa and 241.87
MPa respectively. The produced stresses in the various parts of the backhoe attachement
are within stress limit, therefor all parts of backhoe attachment are safe in strength for
applied known boundary conditions. The results clearly depict that the developed stresses
in backhoe parts without welding are less than than the stresses developed in backhoe
parts with welding, therefore we can say that the welding strengthen the backhoe parts
and its assembly.
The developed backhoe mechanism should be weight optimized for better
controlling during excavation task as well as to reduce the initial cost of the mechanism.
The next chapter 9 includes the structural optimiaztion of the backhoe attachment without
and with consideration of welding. The weight optimized backhoe excavator attachment
also checked for strength using FE analysis approach.