DESIGN MODIFICATION AND ANALYSIS OF SEMIELLIPTICAL LEAF SPRING WITH BALLS

Concept :

The conventional semi elliptical leaf spring assembly has higher frictional losses due to the material properties and mode of assembly. The leaves in the assembly have sliding contact between each other, which increases the losses due to friction. When the springs are used for longer period of time due to wear, the frictional losses increases considerably. Also to increase the load carrying capacity of the leaf springs and to obtain good suspension effect balls are introduced in between two leaves of SUP9 grade. Modeling is done by using ProE and Analysis is carried out by using Ansys 13.0 for better understanding. Analytical results compared with the FEA results. After analyzing the existing geometry a new concept design is proposed to increase the load carrying capacity of the leaf springs and also for good suspension effects.

Current trends involving improvement:
Parabolic springs:
A more modern implementation of old leaf springs is the parabolic leaf spring for automobiles. The new innovative design is characterized by the use of the less leaves whose thick varies from the centre to the outer side following a parabolic pattern.
This results in inter leaf friction, because of which the only contact point between the springs in vehicle is at the end and the centre where the axle is connected. Spacers used in the new design prevent the other parts collisions.
Besides being less in weight the main benefit of parabolic springs is their greater flexibility, which is translated as a high ride quality of the vehicle. It gives us the high ride quality which refers to the high degree of safety to the riders from the uneven road surfaces and gives high level of comfort.
Since there is no contact between the leaves the coefficient of friction is nearly equals to zero.
Disadvantages:
 Higher span requirement when compared with semi elliptical leaf spring, due to decreased rigidity.
 Higher winding up effect when compared with semi elliptical leaf spring.
 Due to decreased rigidity the roll stiffness of the vehicle needs to be compromised with additional factors like anti roll bars.
Mono composite leaf springs:
Mono-leaf springs consist of one plate of spring steel that is thick in the centre and tapers out to the ends. Being only one leaf there are no supporting leaves. The material used is of a higher grade material.
Disadvantages:
 Sudden shifting of the axle causes a change in the control of the vehicle. This change of control can vary from slight loss to a complete loss of control.
 Widely used only in light duty vehicle due to reduced load carrying capacity.
 Usage of higher grade composite material increases the material and manufacturing cost.
In this project, an attempt was made to develop an semi elliptical leaf spring assembly which reduces the inter leaf friction considerably when compared with the existing semi elliptical leaf spring assemblies.
Existing Tata LPT 912R front semi elliptical leaf spring assembly is taken as the base model for working out a new design. The major advantages of semi elliptical leaf spring assembly with balls are,
 Interleaf friction can be considerably decreased when compared with existing semi elliptical leaf spring assembly.
 The rigidity is not as like the semi elliptical leaf spring assembly since the contacts between the leaves are not lost.
 Since the rigidity is not lost, the windup of springs will be less compared to the parabolic leaf springs.
 The fatigue life will increase considerably when compared to the existing semi elliptical leaf spring.
 The weight of the spring assembly can be reduced considerably due to usage of lesser no of leaves.
 The vehicle handling parameters are improved considerably, because the vehicle using parabolic springs may require Anti Roll bar as an additional factor to increase the roll stiffness of the vehicle.

CONCEPTUAL STUDY OF SEMI- ELLIPTICAL LEAF SPRING ASSEMBLY WITH BALLS

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The coefficient of friction for the balls is very less compared with the coefficient of friction between the leaves as such. By suitable design and arrangement the balls can be introduced in between the leaves to convert the linear motion of the leaves into an rolling motion, which has lesser coefficient of friction.

CONCEPT DESIGN OF SPRING ASSEMBLY:
Leaf assembly with balls for reducing inter leaf friction comprises; plurality of leaf springs having plurality of balls disposed on said grooves of leaf springs, plurality of spacers disposed in between the said leaf springs at the spring seat, wherein spacers provided in between the leaves.
Are stacked and fastened with spring clips such that said balls in said grooves in between said leaves transfers vertical load in vertical direction and nullifies the inter leaf friction in longitudinal direction. The balls are the active component of the leaves which distributes all the loads among the leaves. The balls can spin on its axis when there is a relative movement between the adjacent leaves.
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1-1st Leaf Spring
2-2nd Leaf Spring
3-3rd Leaf Spring
4-Rolling balls between the leaves
5-Spacers
6-Spring Clamps

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EXISTING SEMI-ELLIPTICAL LEAF SPRING ASSEMBLY STUDY

The semi elliptical leaf spring assembly of the Tata LPT 912R vehicle is considered as the base model. As the driving torque and braking effects are more in the front of the vehicle, the spring assembly used in the front of the vehicle is only considered for design. The load details and spring details are collected to perform the study.
Spring details:

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Other dimensions:
Vehicle load W = 4000 N
Eye bush diameter d = 34 mm
Measured free camber =115 mm
For design complexity only two leaves are taken for design. So two full length are only considered for design.

Material properties:
The SUP9 material is used for leaf spring design.

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DESIGN OF LEAF SPRINGS WITH BALLS
Conventional Leaf Spring:

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Calculations:

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Modified design with balls:
Length of spacers = 40 mm
Width of spacers = 10 mm
Thickness of spacers = 2.5 mm
Diameter of spherical ball = 5mm
Number of balls = 7
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Calculations:

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Checking for crushing strength of balls:
Material for balls is taken as chrome steel.
Minimum crushing load for a 5 mm diameter ball is about 11000 N.
It is well above the applied load, so the design will be safe for balls.
The design calculations were carried out by finding the deflection and bending stress.
The values are further compared to the FEA results by using Ansys13.0 software tool.

ANSYS REPORT

The model of leaf spring now imported into ANSYS 13 the boundary conditions and material properties are specified as for the standards used in the practical application. The material used for the leaf spring for analysis is structure steel, which have approximately similar isotropic behaviour and properties as compared to SUP 9.
The conventional leaf spring and the modified leaf spring were analysed under similar conditions using ANSYS software and the results are presented in Table .
The two model of half leaf spring assembly is developed. The first model is developed only considering two full leaves. Both the model are developed using ANSYS software. To reduce the complexity for solution, centre band and clamp are not modelled together. The properties of material are provided and mesh model is developed. Contact conditions are formed where bodies meet. The load is applied on the free end i.e. on eye and constraints are provided at each end of the leaf at centre.

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RESULTS AND DISCUSSIONS
The conventional and modified leaf springs are modeled and imported in Ansys and the results are compared with the analytical results.
The results are tabulated below.

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CONCLUSION
Thus a new modified semi-elliptical leaf spring with balls is designed by using Pro-E and analyzed by using Ansys 13.0. Analytical calculations were also carried out and compared with the FEA results. Also load rating of chrome steel balls and stainless steel balls are studied.
By reducing the friction in leaf springs load carrying capacity is increased for the same stiffness of the leaves material.
By observing the above table, it is noted that for the same load, deflections of the leaf spring get reduced.
Also it is said that the stress induced in the modified leaf spring is carried away by the balls only, as the balls have very high load carrying capacity.
And by introducing the balls it will carry the lateral loads, driving torque, braking torque of the vehicle also when the vehicle bumps the suddenly applied load on the springs is easily dissipated on the balls, thus producing a good suspension effects on the vehicle.




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