Concept and manufacture of a hollow crankshaft forming tool
Sara Tavares Luzia Melo Gamboa
Department of Mechanical Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001,
Lisbon, Portugal, 2014.
Abstract
The concept of lightweight structures has been evolving throughout the years, being
incorporated in various branches of engineering and manufacturing technologies, due to the
growing concern of the ecological impact. The use of lightweight structures has many inherent
advantages, with the most relevant being weight reduction and the use of less material.
Although the crankshaft is a component that has its characteristics and manufacturing
processes well defined, the idea of applying the concept of lightweight structures to this
component, through an innovative process, emerged as a relevant possibility.
The main goal of this paper was to establish a numerical and experimental study about the
viability of manufacturing a hollow crankshaft, based on the column instability principle, by
applying plastic deformation on the material. To achieve this goal, prototype tools were
developed and since the original concept didn’t work there was the need to rethink the concept
so three distinct deformation modes were used. The process was studied, through experimental
trials and numerical simulations, to verify if it was possible to manufacture this component and
to define the multiple field variables that define this process. To assure the viability of the
process a comparison was made between the numerical and experimental results.
The manufacturing of hollow crankshafts was possible, allowing the outcome that was
expected, which means that the proposed goals, in this paper, were accomplished. It is also
relevant to refer that the numerical-experimental correlations, turned out to be quite satisfactory.
Keywords: Hollow crankshafts; Lightweight structures; Prototype tool; Deformation modes;
Plastic deformation; Numerical simulations.
Introduction
With the development of human society and the increase of industrialization, the need for a
global awareness is more crucial, due to the ecological impact on the environment and natural
resources.
In modern transportation engineering, the application of lightweight components and structures
is rapidly growing, due to the innumerable economical and ecological advantages that their use
brings. The main advantage of these components is the mass reduction, and their use and
manufacturing is considered a central challenge that involves approaches from different
engineering areas. Therefore, lightweight construction can be defined as “an integrative
construction technique using all available means from the fields of design, material science, and
manufacturing in a combined way to reduce the mass of a whole structure and its single
elements while at the same time the functional quality is increased.”[1].
Due to the innumerable advantages mentioned before, the idea of applying the concept of
lightweight structures to the manufacturing of a hollow crankshaft emerged. Although the
crankshaft is a component that has its characteristics and manufacturing processes well
defined, very few studies have been made in the manufacturing of hollow crankshafts.
Crankshafts are employed to convert circular into reciprocating motion or reciprocating into
circular motion. They have several applications in diverse branches of engineering, such as
aerospace, aeronautic and automobile engineering and are manufactured in several
configurations and sizes, allowing their application in motors of diverse dimensions. They are
1
mainly used in internal combustion engines, but can also be found in air compressors, electric
energy generators and turbines.
They are typically manufactured by forging, casting or machining and each of these processes
has advantages over the others [2]. Forged crankshafts have a more homogeneous
composition than the cast crankshafts, but their overall production cost is higher. These two
processes are suitable for high volume production, while the machining process is only viable
for low production, due to the enormous material waste and machining time that elevates
tremendously the overall cost of this process.
Some studies have been made in the domain of lightweight crankshafts. One of these studies
was based on the manufacturing of hollow crankshafts through the process of hydroforming [3].
The study was able to achieve expansion coefficients of about 50%, and little alteration of the
initial thickness. Other studies were centered on reducing the weight of the structures by
designing and machining crankshafts, achieving weight reductions of 30 to 50%. [4,5].
The main goal of this paper is to produce a hollow crankshaft, through the plastic deformation of
the material. Originally, this process was based on the column instability phenomenon (Euler’s
instability), and by applying an axial force on the tube, that had been previously filled with a
mandrel, the plastic deformation of the material occurred. This mode of deformation was based
on a study on the manufacturing of crankshafts by forming process [6]. The original mode was
not successful and after rethinking and improving the concept, two additional modes of
deformation were tested in this paper.
The concept presented in this paper was tested by numerical and experimental analysis. After
the results were obtained, a numerical-experimental comparison was made to, not only validate
the process but also to validate the numerical software used. The numerical simulations were
only used in the cases corresponding to the third mode of deformation, due to the high
complexity involving each simulation, which were the ones that obtained success
experimentally.
Methods
1. Prototype tool
The basis of the tools used in the three different modes of deformation, began with the design of
a prototype tool and the development of dies to constrain the material flow on the crankpins and
main journals of the crankshaft.
a)
b)
c)
Figure 1 – a) Prototype tool; b) Main journals forming dies; c) Crankpins forming dies.
This prototype tool was designed for the production of a crankshaft with a single crankpin. The
crankpin was formed by the arm that connected from the tube to the tool column, and the main
journals were formed by the discs (figure 1.b). The crankpin forming dies (figure 1.c) were not
equal to the arm presented in the prototype tool, but the material flow and mode of deformation
is equal to both of them.
2
1.1. First and second modes of deformation
The first and second modes of deformation used the dies presented in figure 1 and a vertical
tool to transmit the axial applied force, just like the one presented in figure 1. In the first mode,
an axial force was applied and due to the column instability phenomenon, the plastic
deformation occurred in the areas that were not constrained by the dies. The second mode, was
developed in two phases. In the first phase a transversal force was applied in the crankpin
forming die, to force the instability of the preform, and the second phase consisted in applying
an axial force.
a)
b)
c)
d)
Figure 2 – a) First mode of deformation; b) Crankshafts produced by the two modes with the arrows indicating
the place where the crack propagated; c) 1st fase of the 2nd deformation mode; d) 2nd fase of the 2nd deformation
mode
Both of these deformation modes were not successful, due to the appearance of cracks in the
produced crankshafts. So, the original concept was altered and the third mode of deformation
was tested, using the same dies, used in the original two modes, but with a different tool.
1.2. Third mode of deformation
In the experimental tests performed for the third mode of deformation, a different tool was used
that could apply simultaneously an axial force and a transversal force. This mode was divided
into three distinct phases:
1- Applying a transversal force
2- Applying an axial and a transversal force, simultaneously.
3- Applying an axial force
a)
b)
c)
Figure 3 - Solidworks modeling of the deformation of the preform corresponding to the third deformation mode:
a) 1st phase; b) 2nd phase; c) 3rd phase.
3
2. Experimental background
2.1. Material characterization
Throughout the experimental work, the raw materials used were commercial S460MC (carbon
steel) for the tube and commercial lead for the mandrel. The steel was characterized in a
previous study [7], so in this paper the lead was the only characterized material because the
mechanical behavior is essential in what concerns numerical testing. Therefore the stress-strain
behavior was determined by means of a compression test carried out at room temperature, with
a cross-head speed of 10 mm/min, in order to reproduce quasi-static conditions, thus
eliminating dynamic effects. The test was performed on a preform of lead with 29.69 mm of
diameter and 30 mm of height. The relationship between effective stress-strain was
approximated by a power law given by Ludwik-Hollomon’s equations:
(1)
True Stress (MPa)
100
80
60
40
20
0
0
0.2
0.4
0.6
0.8
1
1.2
True Strain
Figure 4 – Evolution of the stress–strain curve obtained from uniaxial compression tests for the lead
2.2. Experimentation
The experimental tests were performed at room temperature and under quasi-static condition by
enforcing the upper-table of the universal testing machine to a 10 mm/min displacement rate. At
total, 5 experimental tests were performed for the three deformation modes. In table 1, the
experimental plan of the tests is presented, including the vertical displacement made in each
phase.
Table 1 – Experimental plans
nd
rd
1 Phase
(mm)
2
Phase
(mm)
3
Phase
(mm)
Sucess
200
60
-
-
No
2
200
3.675
100
-
No
3
2
250
15
57
-
No
4
3
170
3.5
8
28
Yes
5
3
191
1
8
34
Yes
Case
Deformation
mode
1
1
2
st
(mm)
4
3. Numerical Background
3.1. Finite element modelling conditions
The numerical modelling of the plastic deformation occurred in the crankshaft was performed by
means of the in-house computer program I-FORM, that was developed in the 80’s in the
department of Mechanical Technology of Instituto Superior Técnico. This program has been
extensively validated against experimental measurements of metal forming processes [8,9], and
is built upon the irreducible finite element flow formulation which is based on the following
extended variational principle to account frictional effects,
(2)
Where represents the control volume limited by the surfaces,
and , where velocity and
traction are, respectively, prescribed and
is a large positive constant enforcing the
incompressibility constrain. Friction at the contact interface
between tube and tooling is
assumed to be a traction boundary condition and the additional power consumption term is
modelled through the utilization of the law of constant friction
[8].
The preforms discretizations were personalized in a zone, in which the plastic deformation
occurred, this is, the zone that was not constrained by any dies. The standard discretization
offered a good refinement, but it implied an extremely large number of elements that would
increase severely the computation times. This way, the number of elements was able of being
reduced, and only refined in the zone that needed the most. The preforms were discretized by
means of linear hexahedral elements with 8 nodes, and the dies were discretized by means of
contact-friction spatial triangles. The resulting meshes were 7225 elements for the smaller
perform and 8875 for the larger preform.
The simulations were also performed in three different stages, each corresponding to a phase
of the deformation mode used.
a)
b)
c)
d)
Figure 5 – a) Mesh of Case 4 (170 mm) preform; a) Mesh of Case 5 (191 mm) preform; c) Assembly of mesh dies
and preform (case 4); d) Assembly of mesh dies and preform (case 5);
Results and discussion
1. First and second modes of deformation
The first and second modes of deformation that were presented above in this paper, didn’t
achieve success in the experimental tests performed. The obtained crankshafts didn’t
correspond to the expected geometry and all of them presented cracks in different places. In the
load- displacement curves studied for these modes of deformation, the points where the cracks
5
occurred, for each curve, are shown in figure 6. Also the curves, that correspond to the second
deformation mode are divided by a vertical line in phase 1 (i) and phase 2 (ii).
140
100
i
ii
120
80
Crack
80
Crack
Load (kN)
Load (kN)
100
60
60
40
40
20
Exp case 1
20
Exp Case 2
0
0
0
a)
20
40
60
Displacement (mm)
80
b)
0
15
30
45
60
75
90
Displacement (mm)
105
100
i
ii
80
Load (kN)
Crack
60
d)
40
f)
20
Exp Case 3
0
0
c)
20
40
60
Displacement (mm)
80
e)
Figure 6 – a) Evolution of the load-displacement curve for Case 1; b) Evolution of the load-displacement curve
for Case 2; c) Evolution of the load-displacement curve for Case 3; d) Crack in the crankshaft corresponding to
Case 1; e) Crack in the crankshaft corresponding to Case 2; f) Crack in the crankshaft corresponding to Case 3.
2. Third deformation mode
2.1.
Experimental results
In the experimental work performed for this mode of deformation, the results were successful,
allowing for the purpose of this paper to be fulfilled. It was possible to achieve the
manufacturing of single crankpin hollow crankshafts that are presented in figure 7.
Figure 7 – Crankshafts manufactured by the third deformation mode, corresponding to Cases 4 and 5.
6
2.2.
Numerical results
The purpose of studying numerical simulations is to analyze the main field variables that
command this process of forming. Besides that, if the numerical-experimental correlation
achieves good results, once again the validation of this software is made. This means that one
can take advantage in studying a forming process only by numerical simulations, without the
need for developing dies and experimental procedures.
To evaluate the probability of failure, normally ductile damage criteria are analyzed to discover
which is the most probable zone to fracture. No ductile damage criteria was applied in this
paper, due to the fact that this deformation is exceptionally complex and has several crack
separation modes associated to it. To analyze the zone with the most probability of cracks
occurring, a study was performed on the evolution of the average stress and the effective strain
rate. This indicated that the zones with the most tendency to fracture were the ones that
suffered the most plastic deformation, exteriorly and interiorly, and the zones that were in
contact with the forming dies.
a)
b)
d)
c)
Figure 8- a) Average stress distribution on Case 4; b) Effective strain rate distribution for Case 4; c) Average
stress distribution on Case 5; d) Effective strain rate distribution for Case 5.
The evolution of the module velocity vectors is shown in figure 9 and it evolved the way it was
expected, having vectors with transversal direction to the crankshaft in the first two phases, due
to the applied transversal force in these phases. In the second and third phase, the vectors also
have an axial direction due to the axial force applied in theses stages. The direction of the
vectors is from the inside to the outside, proving that the numerical deformation occurred as
expected.
a)
b)
c)
Figure 9 – Evolution of the module velocity vectors for the Case 4: a) End of the 1st phase; b) In the middle of
the 2nd phase; c) End of the 3rd phase.
7
2.3.
Numerical-Experimental comparison
The comparison between experimental and numerical results was quite satisfactory. In terms of
geometry both results were quite similar, having just small differences that supposedly occurred
due to the fact that in the numerical simulations no porosities in the lead were accounted for,
which made the deformation more rigid than in the experimental work. Besides that, some
clearances were also not accounted for in the numerical simulations that existed in the
experimental procedures.
b)
a)
d)
c)
Figure 10 – Geometry of the crankshafts: a) Numerical Case 4; b) Experimental Case 4; c) Numerical Case 5; b)
Experimental Case 5.
300
a)
Load (kN)
250
200
150
100
FEM Case 1
50
Exp Case 4
0
0
5
10
15
20
25
30
35
20
25
30
35
Displacement (mm)
300
b)
Load (kN)
250
200
150
100
FEM Case 2
Exp Case 5
50
0
0
5
10
15
Displacement(mm)
Figure 11 – Evolution of the load- displacement curves for numerical and experimental cases: a) Case 4; b)
Case 5.
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In the comparison of the load- displacement curves presented on figure 11, it is possible to see
that the numerical curves reproduce with some precision, the curves of the experimental results.
In both curves, the same fundamental behaviors are visible, such as the presence of all the
three distinct phases that correspond to the third mode of deformation.
Although the fundamental behaviors are present, some discrepancy is visible in the
correspondence of the curves in some zones. This occurred mainly due to two distinct reasons.
The first is that in the experimental results, the deformation is smoother due to the porosities
present in the lead, which leads to a more stable curve. In the numerical results the lead was
considered all rigid, making the corresponding deformation also more rigid.
The second reason has to do with the fact that not all clearances that were assumed in the
experimental procedures were defined in the numerical simulations. This contributes to the
difference in values in the load, on several zones. Besides that, in figure 11.b) the last part of
the curves is not in conformity, due to the lack of defining a damage criteria in the numerical
simulation. It is of the authors understanding that because of the great deformation that
occurred in that area, internal cracks in the lead existed, that would cause the numerical curve
to descend if they had been accounted for.
Conclusions
The growing global concern with the environmental impact, leads the way to the pursuit of new
manufacturing processes and more use of lightweight structures. The process presented in this
paper, fits in these two categories, revealing that it is a viable process in the manufacturing of
lightweight crankshafts.
The experimental results presented in this paper, allowed the validation of the studied process.
It was possible to obtain hollow crankshafts with a single crankpin, with the dimensions and the
desired geometry. These crankshafts presented a weight reduction of about 80%, when
compared to similar crankshafts that are not hollow, which is considered a very big reduction.
Although, they were not manufactured by the original concept presented in this paper (first
deformation mode), their viability was still assured by studying a different concept (third
deformation mode).
The numerical-experimental comparison presented good results, due to the fact that the loaddisplacement curves had the same fundamental behaviors and the obtained geometries were
very similar. Therefore it is possible to conclude that the software validation was once again
made, and that in complex deformation problems, it is possible to take advantage of only
numerical simulations, to understand the process without the need to build dies or perform
experimental tryouts.
Acknowledgements
The author expresses her gratitude to Dr. Luís Alves and Dr. Paulo Martins for all their support
in this work.
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