OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
Most commonly used suspension springs are the open coil helical springs [3] (Fig. 1). They offer resistance to
compressive force. Round wire is the material is commonly used to make the springs. This is because it is readily
available and adaptable to standard manufacturing methods. Square, rectangular or special shaped wires are used
in some cases. When the diameter of the spring is constant throughout, it is called straight helical. There are
types where the diameter of the spring varies. These are tapered or conical springs.
2. THE UNCOILING PHENOMENON
In the design of helical compression springs, the coiling/uncoiling of active coils during compression of the
spring is not accounted for [4, 5, 6]. Although this concept forms the basis for the function of torsional springs, it
is ignored during the design of helical compression springs. The uncoiling effect is evident when motion of one
end of the spring is completely constrained and the other end is allowed to rotate freely during compression. This
also results in torsional forces on supporting structures of the spring. The free rotation results in a change in
spring attributes like rate of the spring and stresses generated within the spring.This has an obvious impact on the
working and life of the spring.
FIG. 2 UNCOILING ANGLE θ
Green line: Position of free end with rotation constrained
Pink line: Position of free end without constraintment
This paper showcases the work in a project [7], carried out to determine the extent of variation in load rates between
helical suspension springs operating under both the cases (coiling restricted and coiling unrestricted). The
work involved FEM analysis and experimental testing on springs, carried out at a leading spring manufacturer’s
premises. A fixture was designed and built to allow and measure the extent of uncoiling in springs. The experimental
and FEM data showed a variation in the load rate as well as rotational movement in the end coil. Uncoiling
angles ranging between 10º ~ 14º and load variation of up to 10% were noted at same compression height
experimentally.
When a compressive force (compression load) is applied on helical springs, they undergo plastic deformation and
store the energy. This deformation is a decrease in the pitch distance and hence the length of the spring. It also
causes a little observed angular movement of the coils around the axis of the spring or uncoiling of the helix (Fig.
2). In automobiles, this effect is given a free reign by providing a thrust bearing at one of the ends to allow free
rotation of the spring end [8].
By comparing the same suspension spring under different conditions – uncoiling restricted and uncoiling unrestricted,
it was determined that uncoiling has a significant effect on the functioning of the spring.
Currently, manufacturers ignore this effect while designing the spring and testing it. The project work has been to
determine:
• The variation in load between free and restricted springs
• The variation in stresses produced
• The extent to which the end coil turns during free uncoiling compression
Five different springs were chosen. Experimental analysis was done on all 5 springs. And FEM analysis was
done on one spring. The experimental analysis was carried on a Larson testing machine. The FEM analysis was
carried out on software called NASTRAN. The 3D modeling was done on SOLID WORKS 2006.
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