Optimization of Steel Helical Spring by
Composite Spring
Journal: International
Journal of Multidisciplinary Sciences and Engineering, Vol. 3, No. 6, June 2012
Author:
Mehdi Bakhshesh and Majid Bakhshesh
Summary by:
Abhinav Jaiswal, 2
PGDIE 42
Abstract:
Springs that can reserve high level of
potential energy, have undeniable role in industries. Helical spring is the
most common element that has been used in car suspension system. In this research,
steel helical spring
related to light
vehicle suspension system under the effect of a uniform loading has been
studied and finite element analysis has been compared with analytical solution.
Afterwards, steel spring has been replaced by three different composite helical
springs including E-glass/Epoxy, Carbon/Epoxy and Kevlar/Epoxy. Spring
weight, maximum stress and deflection have been compared with steel
helical spring and factors of safety under the effect of applied loads have
been calculated. It has been shown that spring optimization by material
spring changing causes reduction of spring weight and maximum stress
considerably. In any case, with changing fiber angle relative to spring axial,
composite spring properties have been investigated.
Introduction
Helical springs are simple forms of springs,
commonly used for the suspension system in wheeled vehicles. Vehicle suspension
system is made out of springs that have basic role in power transfer, vehicle
motion and driving. Therefore, springs performance optimization plays important
role in improvement of car dynamic. The automobile industry tends to improve
the comfort of user and reach appropriate balance of comfort riding qualities
and economy. There is increased interest in the replacement of steel helical
spring with composite helical spring due to high strength to weight ratio. On
the other hand, there is a limitation at the amount of applied loads in
springs. The increase in applied load makes problem at geometrical alignment of
car height and erodes other parts of car. So, springs design in point of
strength and durability is very important. Reduction of spring weight is also
principal parameter in improvement of car dynamic. By replacement of steel
helical spring with composite helical spring will reduce spring weight in
addition to resistance raise under the effect of applied loads. In this research, a static analysis is
employed to investigate behaviour of steel and composite helical spring related
to light vehicle suspension system. Steel spring has been replaced by three
different composite helical in ANSYS software and results have been compared
with analytical solution. The objective is to compare the load carrying capacity, stiffness and weight savings of composite
helical spring with
that of steel
helical spring.
Advanced composite fibers such as glass,
carbon and Kevlar- reinforced suitable resins, are expected to be widely used
in vehicle suspension system
application so that
spring of different shapes
can be obtained.
This refers to the high specific strength (strength-to-density ratio)
and high specific modules (modules-to-density ratio) of this advanced composite
materials. The method used for the production of the springs is a variation of
the RTM (Resin Transfer Molding) process. Through this method, the dry braids
are positioned in the mold before being impregnated with the resin, making
production very clean. In this case, an open mold consisting of a helically
grooved mandrel is used, and the braids are impregnated by plunging in a bowl
filled with resin.
Many studies are carried out to investigate
the behaviour of composite springs. Senthil Kumart and Vijayarangan
investigated behaviour of composite leaf spring for light passenger vehicles.
Compared to steel spring, the composite leaf spring was found to have lesser
stress, higher stiffness and higher natural frequency than that of existing
steel leaf spring and weight of spring was reduced by using optimized composite
leaf spring. They also concluded that fatigue life of composite leaf spring was
more than that of conventional steel leaf spring.
Solid Modeling of
Metal Helical Spring
Helical springs have the characteristic
parameters that affect their behaviours. In addition to the physical properties
of its material, the wire diameter (d), loop diameter (D), number of loops (Na)
and the distance between two consecutive loops (P) are the parameters that
affect the behaviour of spring. These parameters have been illustrated in Fig. below:
Before analysis of helical spring, the rate of spring, shear
modulus and poison coefficient are needed to be calculated.
Simulation of
Steel Helical Spring
Spring Geometry is modeled
in SOLIDWORKS software and
then is analyzed
under uniform loading
condition in ANSYS Software.
Axial displacement and shear stress have been compared with analytical results.
Load is in direction of spring axis and is exerted on the one end of spring and
other end is fixed in X, Y and Z directions. Meshes with different resolutions
are generated to insure grid independence. Element selected for this analysis is
SOLID45. SHELL element does not show stress variation in the course of
diameter. BEAM element represented stress along the length only and doesn't
show other information about stress. SOLID92 is a pyramid element that
increases time of calculations and it has error in nonlinear complex models. Therefore, a cubic SOLID45 element has been
used in the stress analysis. This element is defined by eight nodes having three degrees of freedom at
each node: translations
in nodal x,
y and z directions.
Replacement Steel
Spring with Composite Spring
Steel helical spring has been replaced by three different
composite helical springs including E-glass/Epoxy, Carbon/Epoxy and
Kevlar/Epoxy. The loading conditions are assumed to be static. Spring Shear stress
has been obtained using FEM and has been compared with steel helical spring.
Composite spring properties have been studied with changing fiber angle
relative to spring axial. The element is SOLID 46, which is a layered version
of the 8-nodes structural solid element to model layered thick shell or solids.
The element has three degree of freedom at each node and allows up to 250
different material layers.
A. Composite helical spring weight
Before modeling of composite helical spring, its weight has been
calculated and compared with steel helical spring. Helical spring weight can be
written as:
where, Na is no. of active loops, d
is wire diameter
and p is weight
per unit volume that can be calculated by
where; Vf , pm is fiber volume and its density,
Vm , pf
is resin volume and its density.
Results
for different percentage of fiber have been shown in Table below:
Compared to steel helical spring, Composite helical spring has
been found to have lesser weight. Also it is concluded that changing percentage
of fiber, especially at Carbon/Epoxy composite, does not affect spring weight.
B. Direction of Fiber in Composite Helical Spring
Spring strength must be calculated at fiber along and fiber
vertical direction and can be written as:
where, Ea is
strength of composite helical spring at along of fiber and Em is its strength in vertical direction of fiber.
Angle fiber has been changed so that fiber position has been
considered in direction of loading, perpendicular to loading and at angles of
30 and 45 degree relative to applied loads. In every case, three different
composite helical springs including E-glass/Epoxy, Carbon/Epoxy and
Kevlar/Epoxy have been considered and longitudinal displacement and shear
stress have been calculated to analyze the effect of spring material upon
spring behaviour. Longitudinal displacement under the effect of fiber angle has
been shown in Fig. below :
Spring has the least longitudinal displacement
when fiber position has been considered to be in direction of loading. With
changing fiber angle, spring longitudinal displacement increases so that it
reaches the greatest value when fiber position has been considered to be
perpendicular to loading. Also, it shows that E-glass/epoxy composite helical
spring has the most flexibility and Carbon/Epoxy composite helical spring has
the least displacement.
Shear stress under effect of fiber angle has been
shown in Fig. below:
Spring has the most Shear stress when fiber
position has been considered to be in direction of loading. With changing fiber
angle, Shear stress reduces so that it reaches the least value when fiber
position has been considered to be perpendicular to loading.
Factors of safety under the effect of applied loads have been
calculated with changing fiber angles. Results have been presented graphically in
Figure below:
Fig. shows that for a composite helical spring, the most safety
factor under the effect of applied loads is related to case that fiber position
has been considered to be perpendicular to loading. Also, Carbon/Epoxy
composite helical spring has more safety factor at any fiber angle in
comparison with other composite helical springs. Therefore, that composite
helical spring is more suitable at this aspect.
Conclusion
In this paper, a helical steel spring has been replaced by three different
composite helical springs. Numerical
results have been compared with theoretical results and found to be in good
agreement. Compared to steel spring, the composite helical spring has been
found to have lesser stress and has the most value when fiber position has been
considered to be in direction of loading. Weight of spring has been reduced and
has been shown that changing percentage of fiber, especially at Carbon/Epoxy
composite, does not affect spring weight. Longitudinal displacement in
composite helical spring is more than that of steel helical spring and has the
least value when fiber position has been considered to be in direction of
loading. The most safety factor is related to case that fiber position has been
considered to be perpendicular to loading and it is for Carbon/Epoxy composite
helical spring.