Optimization of Spring

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 Afterwards, steel spring has been replaced by three different composite helical springs including E-glass/Epoxy, Carbon/Epoxy and Kevlar/Epoxy.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; V_f , p_m is fiber volume and its density, V_m , p_f  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, E_a is strength of composite helical spring at along of fiber and E_m 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.