Spring is a flexible mechanical element that can undergo significant amount of elastic deformation under the action of external loading. While experiencing elastic deflection, spring can absorb mechanical energy and store it within itself, which is subsequently released in a controlled way when it regains original shape and size. It has wide area of application starting from low-duty requirements (such as spring-operated ball point pen, self-closing door hinge, bike or cycle stand spring, spring-loaded toys and watches, etc.) to heavy-duty requirements (such as cam-follower, brake, clutch, vehicle suspension, railway buffer, vibration damper, etc.).
There are different types of springs made of different materials to fulfil varying requirements. Helical spring, leaf spring, spiral spring, Belleville spring, volute spring, disk spring, etc. are notable one. Among them, helical spring has wide range of applications. Specialized springs are also frequently used to suite the intended purpose. In its service life, a spring may experience axial loading (tensile or compressive), torsion, bending, or a mixed type of loading. A particular type of spring is selected on the basis of certain parameters such as intended purpose, nature of force, durability requirement, available apace, working condition, etc. Both coil spring and leaf spring are suitable for vehicle suspension system and railway buffer system; however, no one can fulfil each and every requirement for such purpose.
Coil spring is made by winding a constant diameter wire in the form of helix
Leaf spring is made by clamping a series of flat plates of varying length one above another.
Although the coil is wound in helical path, basic shape of helical spring is cylindrical (hollow).
Basic shape of leaf spring is semi-elliptic.
Coil spring offers small amount of flexibility in transverse direction.
Leaf spring does not offer flexibility in transverse direction.
Load bearing capacity of this type of spring is comparatively lower.
Load bearing capacity of this type of spring is much higher.
Coil spring tends to absorb shocks and vibrations and thus provide a comfortable ride.
Leaf spring tends to lift of the vehicle body when it experiences bumps. So it cannot provide comfortable ride.
Helical springs are light weight and cheaper.
Generally leaf springs are heavy and costly.
Leaf spring totally dominates coil spring when load carrying capacity is the matter of interest. Heavy industrial trucks are fitted with leaf spring. Moreover, speed barkers (especially steep bumps) of the road exert enormous reactive force when vehicles, even in slow speed, run over it. Leaf spring can comfortably endure such jerk without much problem. Coil spring may fail by buckling if load exceeds certain limit.
Coil spring offers a comfortable and jerk free journey even when the vehicle is driving on in rough road. Thus it is preferred for passenger vehicles. However, in such road, leaf spring tends to lift up the vehicle body, especially when the vehicle is unloaded. So it is preferred in heavy trucks where comfortability is not prime factor.
Leaf spring is heavy and costly, although it is simple in construction and installation. It also requires larger space for installation. On the other hand, coil spring is of light weight and in majority of the cases it is cheaper.
Scientific comparison among coil spring and leaf spring in terms of vehicle suspension is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.