Springs are fundamental mechanical components found in many mechanical systems. Developments in material, design procedures and manufacturing processes permit springs to be made with longer fatigue life, reduced complexity, and higher production rate. Most springs are linear which means the resisting force is linearly proportional to its displacement.
Linear springs obey the Hooke’s Law,
F = k × Dx
Where F is the resisting force, k is the spring constant,
and Dx is the displacement.
Depending on load characteristics spring may be classified as
Most springs are made with iron- based alloy( high-carbon spring steels, alloy spring steels, stainless spring steels), copper base spring alloys and nickel base spring alloys.
Iron- based alloy
i) High Carbon Spring Steel
(C 0.7-1.0,Mn 0.3-0.6& remaining Fe) These spring steels are the most commonly used of all spring materials because they are the least expense, are easily worked, and are readily available. They are not suitable for springs operating at high or low temperature or for shock or impact loading.
ii) Alloy Spring Steel
EN-45(C 0.5,Mn 1.0,Cr 0.2-0.9,V0.12 &remaining Fe),EN-60(C0.5-0.75,Mn0.6- 1.2&remaning Fe). These spring steels are used for conditions of high stress, and shock or impact loadings. They can withstand a wider temperature variation than high carbon spring steel and are available in either the annealed or pre-tempered conditions.
iii) Stainless Spring Steel
(Cr18,Ni8,C 0.1-0.2&remaining Fe )The use of stainless spring steels has increased and there are compositions available that may be used for temperatures up to 288°C.
They are all corrosion resistant but only the stainless 18-8 compositions should be used at sub-zero temperatures. They are suitable for valve springs.
Copper Base Spring Alloys
Copper base alloys are more expensive than high carbon and alloy steels spring material. However they are frequently used in electrical components because of their good electrical properties and resistance to corrosion. They are suitable to use in sub-zero temperatures.
i) Brasses(Cu67,Zn33): Switch control, electrical application.
ii) Nickle Silver(Cu56,Ni18,Zn18): Electrical relays.
iii) Pb Bronze (Cu92,Sn8,Pb 0.1): Bushes.
Iv) Beryllium Copper(Cu98,Be2.0): Relays and Bushes
Nickel Base Spring Alloys
Nickel base alloys are corrosion resistant, and they can withstand a wide temperature fluctuation.
The material is suitable to use in precise instruments because of their non-magnetic characteristic, but they also poses a high electrical resistance and therefore should not be used as an electrical conductor.
i) Monels (Ni68,Cu27 &remaining Fe and Mn
ii) Inconels (Ni76,Cr16&Fe8)
iii) Chromels (Ni80,Cr20)
iv) Nichrome (Ni60,Cr16 &Fe24)
v) Elinver (Ni36,Cr12 &restFe)
vi) Inver (Ni35,Fe65)
Properties of Spring Materials
1. It should possess high modulus of elasticity.
2. It should have high elastic limit
3. It should have high fatigue strength
4. It should have high creep strength
5. It should have high notch toughness
6. It should have good resistance to corrosion
7. It should have high electrical conductivity
The dynamic behaviors of springs have to be analyzed when they are used in a moving mechanism.
The nominal frequency of operation should be well under the spring’s first resonant frequency; typically about 15-20 times lower for safety reason. The force the spring exerts as it approaches its resonant frequency will tend to decrease, which could have disastrous implications for the spring assembly.