INTRODUCTION TO LEAF SPRINGS
In order to conserve natural resources and economize energy, weight reduction has been the main focus of automobile manufacturer in the present scenario. Weight reduction can be achieved primarily by the introduction of better material, design optimization and better manufacturing processes .The suspension leaf spring is one of the potential items for weight reduction in automobile as it accounts for ten to twenty percent of the un-sprung weight.
This helps in achieving the vehicle with improved riding qualities. It is well known that springs, are designed to absorb and store energy and then release it. Hence, the coefficient of friction and frictional losses of the material becomes a major factor in designing the springs. It can be easily observed that material having lower coefficient of friction will have a greater load carrying capacity at same spring rate.
Therefore by reducing the frictional losses between the leaves, the load carrying capacity can be increased and weight of the assembly can be reduced further without any modification in stiffness. The advantage of leaf spring is that it carries lateral loads, braking torque, driving torque in addition to shock absorbing.
The ends of the spring may be guided along a definite path as it deflects to act as a structural member in addition to energy absorbing capability.
Generally conventional leaf springs are stacked and clamped together with pins and clips. The stacked leaf spring generally has high inter leaf frictional losses at various vehicle dynamic load conditions. In order to compromise the inter leaf frictional losses and to withstand vehicle static and dynamic load conditions, more no of leaves are currently used in semi elliptical leaf spring assemblies.
The increase in number of leaves leads to the massive leaf spring assembly structure with increase weight. And due to the inter leaf frictions the fatigue life of the semi elliptical leaf spring assembly is considerably reduced.
Currently during the development of the existing semi elliptical leaf spring assembly following parameters are introduced to reduce the friction in spring steel materials (spring steels of SUP 9 and SUP 11 are currently used for manufacturing). The shot peening is done on the tension side of the leaves to reduce material wear rate and to reduce residual stress.
And as an additional aspect a lubricating substance of graphite grease or graphite primer is coated on the tension side of the leaves to reduce inter leaf friction. But due to the operating conditions and weather the coating materials degrades leading to the increased friction between the spring leaves.