Kütahya Dumlupinar University
Mechanical Engineering Department
Assistant Professor Feridun Karakoc
DESIGN AND ANALISYS OF
AN EXCAVATOR ARM
Nuria Sanchez Ontañon
Rafael Villalobos Torres
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INDEX
1.
INTRODUCTION .................................................................................................................................. 4
2.
REPORT .............................................................................................................................................. 6
2.1.
PARTS ................................................................................................................................................. 6
2.1.1.
Primary Arm ............................................................................................................................. 6
2.1.2.
Forward Arm ............................................................................................................................ 7
2.1.3.
Base for Primary Arm ............................................................................................................... 8
2.1.4.
Shovel....................................................................................................................................... 9
2.1.5.
Connector FA to Shovel .......................................................................................................... 10
2.1.6.
Connector FA to Connector FA to Shovel ............................................................................... 11
2.1.7.
Hydraulic Systems .................................................................................................................. 12
2.1.8.
Bolts ....................................................................................................................................... 13
2.1.9.
Nuts ....................................................................................................................................... 14
2.1.10.
Fixed Base .............................................................................................................................. 15
2.1.11.
Rotating Base......................................................................................................................... 15
2.2.
ASSEMBLY.......................................................................................................................................... 16
2.2.1.
Mates ..................................................................................................................................... 16
2.2.2.
Procedure ............................................................................................................................... 17
2.2.3.
Mates for Analysis ................................................................................................................. 19
2.3.
CINEMATIC ANALYSIS ........................................................................................................................... 20
2.3.1.
Objective ................................................................................................................................ 20
2.3.2.
Procedure ............................................................................................................................... 21
2.3.3.
Results.................................................................................................................................... 21
2.4.
STATIC SIMULATION............................................................................................................................. 22
2.4.1.
Objective ................................................................................................................................ 22
2.4.2.
Procedure ............................................................................................................................... 23
2.4.3.
Results.................................................................................................................................... 23
2.5.
STATIC SIMULATION ON SHOVEL ............................................................................................................ 23
2.5.1.
Objective ................................................................................................................................ 24
2.5.2.
Procedure ............................................................................................................................... 24
2.5.3.
Results.................................................................................................................................... 26
3.
CONCLUSION .....................................................................................................................................27
4.
ANNEXES ...........................................................................................................................................28
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1. INTRODUCTION
An excavator is a machine widely used for different purposes, from construction to
waste management. It consists of different pieces held together with links and moved with
hydraulic pistons for making it work as desired. For the situations that it is required, the
machine must be able to stand high forces on its parts and for that reason the materials
used must be carefully picked and the design must be in concordance with this issue.
For the designing of this excavator the geometry was inspired by pictures of a
model found in the web and the measurements were taken and optimized from different
models found online.
The software used for the whole project of the machine was SolidWorks by Dassault
Systems, it is a parametric design program specially design for mechanical design and
analysis, used for the design of the parts, the assembly of the excavator, the motion
analysis, and the static forces analysis.
The first step was to design each part, with the correspondent dimensions desired,
after that it was necessary to assign a material to each of them to ensure the correct
functioning of the machine.
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Once the parts were created, the following step was to assemble all of them, giving
the correct restrictions and movements specifications for the desired outcome. The
important thing of this step is that it will have a huge influence in the following analysis.
The analysis made where for ensuring that the most critical part, in this case the
shovel (for reasons explained later), would support the forces applied upon it and don’t fail
nor have undesired displacements. Also, it was studied the speed and acceleration of said
part when one of the hydraulic pistons acted upon the system.
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2. REPORT
2.1.
Parts
2.1.1. Primary Arm
It is an element of variable section composed of a steel beam in drawer that is
articulated in the front part of the tractor to the right of the operator cabin.
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2.1.2. Forward Arm
Like the primary arm, this element is also a variable section, one of its ends is
articulated at the tip of the primary arm while the other one is articulated in the shovel.
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2.1.3. Base for Primary Arm
It serves as a foot for our excavator, it allows us to fix the main arm on the ground
to carry out our study on the shovel.
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2.1.4. Shovel
This is the element that serves as a container to deposit the material excavated, it
consists of teeth on its edge with the aim of facilitating the removal of the materials.
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2.1.5. Connector FA to Shovel
As can be seen in the image, this part
tries to connect the forward arm to the
hydraulic arm.
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2.1.6. Connector FA to Connector FA to Shovel
This piece tries to make a bridge with
the previous connector and join the
shovel to the hydraulic arm to allow
the movement in it.
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2.1.7. Hydraulic Systems
There are two hydraulic cylinders on the excavator, one connects the primary arm
with the base and the other, the primary arm with the shovel. Both are used to control the
movement, the first one moves the arm up or down in a linear displacement and the second
one allows the movement of the shovel.
They both are composed by two pieces: the hydraulic tube exterior and the interior.
2.1.7.1
Hydraulic Tube Exterior
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2.1.7.2
Hydraulic Tube Interior
2.1.8. Bolts
The screw is a metal piece whose function is to join two or more elements. This
piece is mainly composed of three parts: head, neck and thread.
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2.1.9. Nuts
The nuts are cylindrical elements that have a hole in the center, in the shape of a
thread, in which the bolts are attached. its most important function is to adjust and hold
the bolts in the assembly.
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2.1.10. Fixed Base
Just a base to simulate the fixed portion of the excavator.
2.1.11. Rotating Base
As the one before, a symbolic part to represent the rotatory attachment from the arm
to the fixed part.
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2.2.
Assembly
In Solidworks an assembly is a file on which all the pieces are put together, making the
machine, in this case the excavator, take form and be able to move properly. In this step
the whole project is assembled in a way that the pieces can work as desired and restrict
the degrees of freedom of each part that are not logical with the functioning of the
apparatus. For this, restrictions are used.
In our case few kinds of restrictions were used since the movement of the excavator is
not complicated, but it was important to make sure that parts like the hydraulic actuators
were able to move between the desired range and not disassemble in the process of
animating an analyzing its movements.
2.2.1. Mates
2.2.1.1
Concentric
A concentric mate is used to keep the axis of two cylindrical surfaces or circular lines
together, it also gives the option to restrict the turning motion between the two parts.
2.2.1.2
Coincident
The coincident mate is for bringing features, particularly faces/planes, together. It
allows for two pieces to share a planar face, those could be planes or faces of the specific
piece. It also allows to give a fixed distance between the two faces.
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2.2.1.3
Limit Distance
The limit distance mate allows two faces of two different parts to be able to move
in a specific range, the two inputs that rule that are the maximum and minimum distance
between said faces.
2.2.2. Procedure
When all the pieces desired to be assembled are ready for this step, one should
have in mind the kind of movement or position between them.
For the excavator the first step was to lock the intermediate planes of the arms and
shovel so they would be restricted to move only in the Y-Z plane between them, because a
displacement in the X axis direction is undesired, this also applies to the base. The base
should be fixed so it won’t move at all, then, the upper part of it (rotating base) is attached
to the fixed one with a concentric and a planar restriction, this allows a rotatory movement
with the center of the circles as the axis and only that.
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Following this, a concentric mate between the bolts and corresponding holes in
each part and a planar restriction between the interior face of each bolt’s head and the
outer face of the piece they are attached to, also the nuts would be put in place with the
same procedure. The concentric restriction would allow the pieces to rotate between each
other with the bolt as an axis and the planar restriction would make the bolt and nut stay
in place.
This give a realistic movement allowance to the piece as if the bolt was working as
proper. It’s important to notice that making this the first planar restriction on the planes of
the arms would be redundant.
The procedure explained before with the bolts is used the same way for every one
of these kinds of part, used to fix together all the arms, shovel, pistons and extra pieces.
For the hydraulic pistons, a limit distance restriction is applied with a concentric one
between the base of it and the part that moves in and out. For this, a minimum and
maximum distance is defined. The first is set to be one or two millimeters from the inner
base surface of the exterior part and the base of the inner piece, the maximum is set to be
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around an 80% of the total length of the movable part. This way the inner piece would
move between these distances and through the inner circle of the exterior tube, the
rotation would be restricted by the bolt connecting the pieces to the arms.
This way all movements that are not desired or natural to the machine would be
restricted. Some issues might arise, like the shovel moving to the opposite side of the arm
if moved intentionally in a “hard” way, this would be fixed by an angle limitation, but since
the movement is going to be dictated by the linear actuators in the motion analysis, this is
unnecessary since it will not happen with this kind of action.
Once all the assembly is correctly done, simulations can be performed with the
security that there are not going to be undesired results due to displacements that are not
natural to the excavator itself.
2.2.3. Mates for Analysis
When making analysis, there are extra restrictions that might be used to be able to
analyze the movement of only certain pieces but without disassembly the whole machine
or creating a new assembly for it.
2.2.3.1
Mates for Cinematic Analysis
For the cinematic analysis that was required in this project, some restrictions were
applied so that the arms moved as desired, this included fixing the position of some of the
linear actuators so only the one applying a force would move. If that is not done, the
program will allow the rest of them to freely move while one actuates, this is something
not realistic since on hydraulic pistons if no pressure of oil is changed, they will stay in place.
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Also, the rotation of the base is restricted as would be in a real operating scenario
of the excavator. If not done, there would be one more degree of liberty to be analyzed by
the program, and even it would not result any movement in this part, the computational
requirements would, of course, be higher.
2.2.3.2
Mates for Static Simulation
There are two types of analysis in this section, one made with the whole machine
assembled and other one made only in the weakest piece, this been the shovel. The reason
to reach that conclusion is explained in section 2.4.
For the analysis on the shovel no mating restrictions were necessary, since it is not an
assembly.
In the case of the full assembly analysis, a few details had to be considered, the
hydraulic actuators would be applying force in the arms and shovel for it to move, for this
reason it could be assumed that they will not move, making the limit distance mate a fixed
one. The pistons would be bought with the specifications needed as the results of an
analysis, for that reason they are not taken into consideration in this analysis.
2.3.
Kinematic Analysis
Kinematics is the generic study of geometry, position, and movement of the solids that
make it up with respect to a reference system coordinated, independent of the causes or
forces that produce it. In this case, when doing the analysis on the arm of the excavator,
kinematics aims to design the desired movements of the set of elements considered in the
analysis through a previous simulation, and mathematically determine the positions,
velocities and accelerations that occur for a later analysis of forces of the shovel.
2.3.1. Objective
With this analysis we seek to know how the shovel behaves in a basic movement
such as raising it, if it follows a logical order for the structure to work and also design the
desired movements of the mechanical parts.
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2.3.2. Procedure
The first step in carrying out a kinematic analysis is to run a simulation. In this case,
the simulation consisted of raising the shovel, creating a linear motor in the lower hydraulic
arm and setting the foot of the shovel. Thus, we would achieve a linear movement upwards.
The second step would be to select the blade and calculate displacement and
acceleration at a constant speed of 20 mm/s.
2.3.3. Results
When a kinematic analysis is mentioned, it is necessary to talk about the resolution
of three problems that arise in mechanisms.
Position problem: this is a highly complex problem, since it deals with an analysis to
nonlinear behavior. In this problem they consist of determining the position and number
of elements, as well as the degree of freedom of the system.
Calculation of speeds and accelerations: this is easy to solve because it is a linear
problem. The resolution of this problem determines the speeds and accelerations of
different elements of the system when making a change of speed and/or acceleration of
one of the elements of the mechanism. This point is the main one we are interested in.
Analysis of successive positions: this last problem consists of evaluating the
variables of speed, acceleration in the different positions that the element is presented in
a cycle of displacement.
When working with solid works it is easy to solve these problems and finally at a
constant speed of 20mm/s we would obtain the following graphs:
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Green, acceleration. We see that it fluctuates in the graph, which means that the
acceleration in the shovel is not constant, although in least squares study it would follow a
constant regime.
In red, the displacement, which we can see is linear and increasing as we raise the
shovel.
Returning to the last-mentioned problem, the analysis of positions would come into
play the moments of inertia. In this simulation we will have two moments of inertia at two
points. The first at the base, in a negative sense, and the second, at the shovel in a positive
sense. Both equivalent, of the same value and in opposite directions, which indicates that
when moving the shovel, the structure follows a logical order so that it works and resists.
2.4.
Static Simulation
A static simulation refers to the analysis of the resultant forces and distribution of them
in one or a set of parts of an assembly. For this, the first thing to do is to generate the
desired fixtures on the analyzed piece, then the external forces that would be applied in a
real scenario are set, those can be either forces, pressure, or others. Then a mesh is
generated so the software analyzes small parts of each piece.
This is a really resumed explanation of the procedure to create an analysis, but since
this topic is not of interest in this project, this will suffice.
2.4.1. Objective
What is looked with this procedure is to find the weakest part on the excavator so
it can be analyzed separately in a simulation like this one.
For this, the aspects that were most analyzed were the Von Misses maximal tension,
the displacement, and the security factors. With a combination of these parameters, it is
possible to determine the part that will fail first and make a more detailed analysis on it to
find out the maximum values for the excavator to work with.
Also, it’s important to make sure that the part that will fail first is one that would be
easily replaceable so that users don’t lose the machine due to that reason.
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2.4.2. Procedure
As explained in section 2.2.3.2 restrictions must be applied so the movement is
restricted in a way that the machine won´t move.
Once the machine is prepared, the fixtures must be applied so that the forces that
we apply later will not generate movement. In this case the reaction applied is a
combination of three surface pressure on the inner faces of the shovel. With this, we will
be simulating the force applied on the shovel while excavating.
The magnitudes used were: 160
𝑁
𝑚2
, 40
𝑁
𝑚2
and 20
𝑁
𝑚2
all perpendicular to the
application surface. The mesh, another important aspect, was set to triangular and with a
relatively small triangles size.
2.4.3. Results
When the system is subjected to these forces, we can see that the weakest part is the
𝑁
“Connector FA to Connector” with a maximum Von Misses strain of 7.91 ∗ 105 𝑚2 . Even
though this is the part that will probably fail first, it will not be separately examined because
of its simplicity, for further analysis the shovel will be used.
2.5.
Static Simulation on Shovel
As it was said previously, the weakest part is the “Connector FA to Connector” part,
but since is such a simple geometry, it is not in our interest to analyze it and therefore, the
study will be centered in the shovel.
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2.5.1. Objective
When analyzing the whole shovel, we seek to comprehend the way the stress is
going to be spreading all over the part, this is crucial to know how to improve the design of
a part or approve it as a functional part of the machine.
2.5.2. Procedure
For this test a force distributed throughout the inner surface of the shovel was
applied with a specific angle between its action line and the surface where they are applied.
In that way we can simulate three different states of work of the part an analyze the results
with a bigger amount of data.
In all cases, the holes where the shovel connects with the rest of the mechanism are
fixed so that the shovel won’t move and simulating reactions as if they were made by a
screw holding it together.
2.5.2.1
Force at 60 Degrees to Forward Surface
𝑁
In this case of study, a pressure of 100 𝑚2 was applied on the inner most forward
face of the shovel, in an angle of 60 degrees to that surface. This state simulates the
moment when the shovel is being pressed towards the ground before start making the hole
in the ground.
2.5.2.2
Force at 90 degrees to Forward Surface
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𝑁
In this case of study, a pressure of 100 𝑚2 was also applied on the inner most
forward face of the shovel, but perpendicular to this surface. This state simulates the
moment when the shovel is dragging the dirt to recollect it and make the hole.
2.5.2.3
Force Normal to Curved Surface
In this set up, three different pressures were applied, one on the inner most forward
𝑁
𝑁
𝑁
face of 100 𝑚2 , one on the inner biggest surface of 350 𝑚2 and one of 50 𝑚2 in the face that
connects the previous two. In this case we simulate digging with the shovel full of material.
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2.5.3. Results
With the data obtained from the three cases, we can say that the most stressful of them
all was the case with the full bucket, generating a maximum Von Misses stress value of
𝑀𝑁
2.04 𝑚2 in a single point. This point was recurrently the weakest part of the shovel on the
tests.
60 degrees
90 degrees
Full shovel
These results are in between the expected but can be fixed with simply adding a fillet
in the corner so the tension spreads more uniformly than with a 90-degree angle.
Also, for the displacement, the biggest value was showed in the full shove case, with a
total of 2.35 ∗ 10−2 𝑚𝑚 which represents almost nothing and does not mean a threat to
our part.
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3. CONCLUSION
The design of a machine such as this, requires quite a lot of time and preparation,
because of the number of details that need to be taken into consideration while on it.
Nevertheless, the results obtained from this investigation and deign procedure are very
satisfactory. Focused mostly on the analysis of the machine, it is proven that the structure,
with the measurements used, is functional.
We can also point that with the results obtained from the analysis, it is safe to say that
a decrease in the amount of material used is an effective way to improve the design. It is
due to be analyzed in a further investigation with tools for topologic optimization.
As stated before, the focus was on the analytic part of the project, since the modeling
part was somehow familiar for us, the things that where most newly learned were the static
and kinematic analysis. Both were done in a satisfactory way, with good results obtained,
for that reason we can say that the excavator is ready to a further step of optimization.
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4. ANNEXES
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