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What Determines the Strength and Flexibility of Coil Springs
Coil springs find their way to use in myriad applications, starting with the car that you drive to the chair that you are sitting on. Their storage and release capabilities of energy, shock absorption and holding of force are remarkable. But have you ever paused to think what is powerful and hard in one, and soft and yielding in another? The solution to this question is a most interesting physics, materials science, and engineering phenomenon.
The Critical Role of Material Choice
By definition, a spring is a storage of energy. The nature of the material where it is manufactured determines its performance. All metals may not be turned into efficient spring.
One of them is the modulus of elasticity of the material or the Youngs Modulus. This is a measure of the natural stiffness of the material. In the majority of spring steels, the value is fairly constant. It implies that with a certain design of springs, a high-grade steel will not really make it springier, but will mostly make it stronger and more durable.
The final tensile strength of the material is important. A spring has to be able to endure the cycles of loading and unloading repeatedly without changing its shape. This demands high elastic limit i.e., there is always a definite level of stress below which the material will self-consciously assume its original shape. Popular steels include high-carbon steels and alloy steels such as chromium-vanadium or silicon-manganese which can be processed in order to achieve this high strength.
The Power of Spring Design and Geometry
Although the material gives the potential, the physical dimensions of the spring give the final characteristics. The physics of a coil spring are a master work and a number of factors are in harmony.
A major factor is the diameter of the wire. This is possibly the most natural relation. A spring constructed of a heavy duty and thick wire will be far more resistant to compression and be much stronger than a spring constructed of a thin wire. The strength of the wire is enhanced tremendously with the diameter of the wire; in reality, it is proportional to the diameter of the wire to the fourth power. This implies that a slight increment in wire thickness leads to a very huge increment in stiffness.
Of great importance is the coil diameter as well. Suppose that we bend a thick rod and a thin rod. The thinner rod bends easily. This is the same thing which should be applied here. The spring having a large mean coil diameter will be highly flexible as compared to that having a small coil diameter provided all factors remain constant. The stiffness is directly proportional to the cubes of the mean coil diameter.
Last but not least, the active number of the coils is crucial. The spring with more coils will be much softer and will be able to deflect a load much easier than a short spring with less coils. The reason behind this is that the load is shared among a larger number of coils, and this means that each of the coils will have to bend a lesser amount. Increased coils activity implies increased flexibility.
The Influence of Manufacturing Processes
It is a critical process which entraps the strength of a spring between the time it was a plain piece of wire and it is a high performance spring.
Even the coiling process may cause stresses in the material. The actual change is however brought about through heat treatment. Springs are hardened and tempered after they have been wound. This thermal operation removes the internal stresses of coiling and maximizes the microstructure of the metal providing it with that much needed high elastic limit and durability. A spring that is untreated in terms of heat treatment will deteriorate or permanently set itself with time.
Shot peening is another process. This is by subjecting the surface of the spring to small spherical media bombardment. This produces a dimpled surface that causes compressive stresses which drastically enhances the fatigue life of the spring. It will eliminate cracks starting and propagating and enable the spring to undergo millions of compression cycles.
To sum up, there is no magic ingredient of strength and flexibility of a coil spring. They are the cautious and calculated finding of the correct material, drawing of the exact geometry, and the use of certain manufacturing treatments. It is the orderly togetherness of these three pillars, which enable such a complicated and necessary task to be carried out by a simple coil of wire.