Springs are fundamental mechanical components found in many mechanical systems. Steel is an important engineering material for the manufacture of high strength components of any mechanical system. Technically spring is an elastic component which is able to store an applied force effect. A very high degree of quality, reliability and service life is expected in springs since it is vital for the functioning of the mechanical system. The spring’s capacity to take on static and dynamic loads over an extended period of time depends on the steel that goes into its making. Steel material for the production of the springs is generally called spring steel. Spring steels feature the unique characteristic of being able to withstand considerable twisting or bending forces without any distortion. Products made from these steels can be bent, compressed, extended, or twisted continuously, and they will return to their original shape without suffering any deformation. This characteristic is defined as high yield strength and is the result of the specific composition and hardening of the steels. Spring steels are stainless, low alloy, medium or carbon steels manufactured to very high yield strengths. Spring steels are also used when there are special requirements on rigidity or abrasion resistance.
Spring steels are to meet the following requirements from the technical point of view.
- High elastic limit – It is the tension that can be applied on the material without a plastic deformation.
- High ultimate strain – It is the value of the extension until rupture in relation to the original length.
- High contraction at fracture – It is the change of the original cross section in comparison to the cross section at rupture.
- Good creep rupture strength – It is a kind of tensile strength taking in account temperature and time.
- Good endurance limit –It is the reaction of the material on constantly changing maximum stresses till the plastic deformation begins.
- Low surface decarburization and a clean, free from fracture surface makes the outer shell of the material soft hence it should be avoided.
The above special requirements of spring steels are being met by adding different alloying elements in the steels. These are silicon, manganese, chromium, vanadium, molybdenum and nickel (in case of stainless steels). Most of the spring steels are hardened and tempered to about 45 Rockwell C. Generally springs are made with high carbon spring steels, alloy spring steels and stainless spring steels.
- High carbon spring steels – High carbon spring steels are the most commonly used material because it is less expensive, it can be easily worked and it is readily available. These steels are not suitable for springs operating at high or low temperatures or for shock or impact loading.
- Alloy spring steels – These spring steels are used for conditions of high stress and shock or impact loadings. These steels can withstand a wider temperature variation than high carbon spring steels and are used in either the annealed or pre tempered conditions. Silicon is the key component in most of the alloy spring steels. A typical example of alloy spring steel contains 1.50 %-1.80 % silicon, 0.70 %-1.00 % manganese and 0.52 %-0.60 % carbon.
- Stainless spring steels – The use of stainless spring steels has increased in recent times. There are compositions available which can be used for temperatures up to 288 deg C. All these steels are corrosion resistant but only the stainless steel of 18-8 composition should be used at sub zero temperatures.
Spring steel is manufactured as bars with round or flat cross section, as well as wire, sheets or strips. 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 follow Hooke’s Law, F = k x Dx, where F is the resisting force, k is the spring constant and Dx is the displacement. The most commonly used springs are coil springs, leaf springs, clock springs and clips. Coil springs can be compression springs, tension springs or torsion springs. These types of springs are shown in Fig 1.
Fig 1 Types of springs
The selection of materials used by spring designers is often met with decisions about specification requirements, cost, availability, reliability and performance. The vast majority of spring requirements are normally met by the common high carbon spring wire materials such as piano wire, hard drawn or oil tempered carbon steels etc. These raw materials can meet many of the requirements of strength, reliability, cost and availability requirements. The starting materials for manufacture of most of the springs are described below.
- Piano wire – It is the most widely used of all spring materials for small springs since this wire is the toughest. It has the highest tensile strength and can withstand higher stresses under repeated loading conditions than any other spring material. It can be obtained in diameters from 0.12 mm to 3 mm. It has a usable temperature range from 0 to 120 deg C. Piano wire contracts under heat and can be plated.
- Oil tempered wire – This is a general purpose spring material used for springs where the cost of piano wire is prohibitive and for sizes outside the range of piano wire. This material is not suitable for shock or impact loading. This material is available in diameters from 3 mm to 12 mm. The temperature range for this material is 0 to 180 deg C. This wire does not generally change dimensions under heat and can be plated. It is also available in square and rectangular sections.
- Hard drawn wire – This is the cheapest general purpose spring steel and normally used where life, accuracy and deflection are not very important. This material is available in sizes 0.8 mm to 12 mm. It has an operating range 0 to 120deg C.
- Chrome vanadium steel – This is the most popular alloy spring steel for improved stress, fatigue and long endurance life conditions as compared to high carbon steel materials. This material is also suitable for impact and shock loading conditions. This steel wire is available in annealed and tempered sizes from 0.8 mm to 12 mm. It can be used for temperatures up to 220 deg C. It does not generally change dimensions under heat. It can be plated.
- Chrome silicon steel – This is an excellent spring material for highly stressed springs requiring long life and/or shock loading resistance. It is available in diameters 0.8 mm to 12 mm and can be used from temperatures up to 250 deg C. This material also does not normally change dimensions under heat. It can be plated.
- Martensitic stainless steel – This is corrosion, resisting steel which is unsuitable for sub zero conditions.
- Austenitic stainless steel – It is a good corrosion, acid and heat resisting steel for springs with good strength. It is useful in moderate temperature conditions. It has low stress relaxation.