Coil springs called compression springs can store mechanical energy when they are compressed. These open-coiled, helical springs provide resistance to compressive loading. When these springs are...
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This article takes an in depth look at coil springs and their materials.
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A coil spring is an elastic element made of metal or heavy plastic in the form of curls or ringlets of round wire or cord that is wrapped around a cylinder. The winding of a coil spring can be loose or tight in a helical shape depending on the application for which it is made.
Coil springs of varying sizes are used to reduce shock and stress on a surface by allowing extra give. They change their shape when an external force is applied to them but return to their original shape when the force is removed. The energy of a coil spring is stored and recovered when the spring returns to its normal shape, which depends on the amount of force applied.
Coil springs are available in a wide range of sizes and shapes, which include volute, arc, clock, constant force, conical, hourglass, barrel, and helical coil springs. The most common type of coil spring is compression springs. They are produced using hot or cold winding with both types shaped on a mandrel, arbor, or cylinder.
Rust and environmental factors cause stress on coil springs, which leads to early failure. As an extra measure of protection, they are powder, epoxy, or polyester coated, with some being electroplated with zinc or nickel.
Coil springs are elastic devices that store and release mechanical energy and are manufactured using a variety of materials with steel being the most common. The production of coil springs includes the use of a simple coiling mechanism, such as a mandrel, arbor, or cylinder, regardless of the shape, size, or final use of the springs.
Whether a coil spring cold or hot wound, they all begin long wires of varying diameters. The type of process used to manufacture coil springs depends on whether the production run is small or high volume with small runs being completed on a lathe while large volume runs are produced on automated coiling machines or computer numeric controlled (CNC) machines.
The materials used to manufacture coil springs include different forms and alloys of steel. The most common types of metals include high and medium carbon steel, chromium vanadium steel, chromium silicon steel, various grades of stainless steel, copper, and nickel. The choice of metal is dependent on the application where the coil spring will be used. In all instances, the metal comes in the form of a wire.
In some manufacturing instances, metals are delivered as metal bars that are heated and drawn into wire to the proper diameter.
Heat Treatment – Cold winding begins with heat treatment of the wire or working to reach its highest strength level. The process of cold winding can only work with wires that have a diameter of 0.75 inches or 18 mm or less.
Mandrel – A mandrel is one of two methods used to produce coil springs. The process can be completed using a lathe, winding machine, or hand crank machine. A guide mechanism is used to align the wire to the required pitch, the distance between the wires, as it wraps.
Computer Numerical Control (CNC) – CNC spring coiling machines are more complex and intricate than the traditional mandrel design and involves a set of components that efficiently feed, wind, and configure the coils.
The complete process and each of its components can be seen in the diagram below.
Wire – Wire for hot coil winding can be thicker, with thicknesses varying between 3 inches or 75 mm up to 6 inches or 150 mm. The wires for hot winding are heated to 1700o F, which is why manufacturers can work with larger diameter wire.
Mandrel – The heated metal is coiled around a mandrel in the same process as cold coiling but with greater care. A CNC machine controls the rotation of the mandrel and the pitch distance.
Cooling – The immediate next step for hot coil winding is to cool the wound coil as quickly as possible, a process known as quenching. Several different methods are used to quench steel parts with oil being one of the more popular. The purpose of cooling is to harden the coil’s steel and minimize the formation of thermal and transformational gradients that could lead to cracking.
The extreme cooling changes the crystalline structure of a metal part and freezes the changes, which makes the metal harder. When cooling begins, a vapor blanket forms around the part as the first stage of the process. The vapor blanket is removed by heating the quenchant, after which the convective stage further removes heat to cool the part.
The popularity of cooling or quenching oil is due to how rapidly it quenches a part compared to other methods. A great deal of care has to be taken during the quenching process, which is very closely monitored to ensure the produced coiled spring performs up to expectations.
Regardless of whether the spring is produced hot or cold, it has to be heat treated to relieve the stress on the metal and help it maintain its normal resilience. The coiling process produces stress in the material to prevent distortions such as cracking. The heat treatment process includes placing the coiled spring in an oven at a set temperature for a specified amount of time to relieve the stress. After being heated, it is allowed to cool slowly.
Stress relief temperatures are between 350o F up to 500o F. The temperature of the process varies depending on the material. Cooling can last for one or two hours to avoid tension related to temperature differences in the material. The intricacies of the cooling process become more critical with larger springs.
The finishing of a coiled spring is dependent on its design, which can vary according to how it should be shaped, coated, have its pitch set, and how it is strengthened.
Grinding – Some springs require grinding to flatten their ends to match their design. The grinding process is completed manually or automatically depending on the manufacturing process. For manual grinding, the coil spring is held in place by a jig and ground using an abrasive tool. With automated grinding, the spring is held in place while both ends are ground according to machine programming. Some lubrication is necessary for the grinding process to keep the metal cool and carry away the waste.
In many cases, the grinding of a spring is necessary so that it can fit into an application or be able to sit flat. The process makes it possible for the spring to stand upright.
Shot Peening – Shot peening consists of impacting the surface of the coiled spring with shot, small round metal balls, glass, or ceramic particles. The impact of the many shots produces a compressive stress layer and changes the coil springs mechanical properties. The barrage of shots smooths the surface of the spring and compresses it.
Set Pitch and Length – To set the pitch and length of a spring, it is compressed until the wires touch. This may be repeated until the required dimensions are achieved. Measuring the pitch can be difficult. Initially, the pitch can be estimated by measuring the distance between the loops of the spring. It is an easy method but not the most accurate. The best method for getting the true pitch is to use a spring pitch formula, which is available in computer programs.
The length and the pitch can be seen in the diagram below. There are different views regarding the pitch of coil springs. Many experts believe that specifying the number of active coils is a better measure than pitch.
Applying Coatings – Coatings are applied as a form of protection against corrosion since most of the metals used to produce springs are vulnerable to the effects of the elements. The entire surface of the spring is protected using various methods, including spray painting, dipping in rubber, or electroplating with zinc or chromium.
There are four common forms of coil spring ends: closed, open, square, and pigtail. It is important to choose the right end to fit a coil springs application.
The closed end version of coil springs is the most common form. In closed end coil springs, the pitch of the end of the spring is flat with its tip in contact with the adjacent coil. The close end design provides a stable coil spring that can stand flat on its own.
Square end coil springs are a type of closed end coil spring where the end of the spring has been ground. They have greater deflection than open end coil springs.
Open end coil springs are continuous springs where the coil continues on its pitch. They are sometimes referred to as tangential coil springs. Open end coil springs are used in applications where the spring will sit on a seat, which is common in automobile applications. In order for an open end coil spring to be used, it must have a seat upon which it can stand.
In a pigtail coil spring, the spring’s last coil has a smaller diameter than the rest of the coils. The smaller diameter of the pigtail end of a coil spring makes it possible to attach it to a mechanism using a screw or bolt.
The first step in the manufacture of coil springs is the selection of the appropriate metal from which to fabricate the springs. Critical to the selection process is the tensile strength of the metal such that it conforms to the demands of the application. The majority of springs are made from steel alloys due to their abundance and cost. Copper and nickel alloys are also used for specific applications.
The type of metal chosen to produce a metal spring has to be able to hold and release the necessary mechanical energy for a long period of time without failure. The main properties of well made coil springs is their ability to supply force between contacting surfaces and being able to absorb shock and stress.
High carbon steel is the most popular type of steel for the manufacture of coil springs. It is strong, durable, and economical and is available in 1045 and 1095 alloys. High carbon steel has a 1% carbon content with a manganese content of 0.9% and a very fine pearlite layer structure that makes the steel very hard, brittle, and less ductile.
Music Wire – Music wire, also known as piano wire is considered the toughest of the coil spring wires. It is a high carbon spring steel that is uniform, has high tensile strength, and has the ability to withstand the stress produced by multiple loadings. Music wire has exceptional temperature resistance and is often used in foundries and refractories with extremely hot internal temperatures.
Chromium vanadium steel has exceptional strength with flexibility. When used to produce coil springs, it has a medium carbon content that gives it toughness and wear resistance. The addition of vanadium to steel makes it easier to shape without the fear of it breaking. In processing chromium vanadium steel, it is heat treated to increase its resistance to wear and fatigue. It is designed to be used in high stress environments where there are moderately elevated temperatures.
Chromium silicon steel is known for its high hardness and ability to withstand high stress. Its hardness and heat resistance make it ideal for use at temperatures exceeding 275o F. Chromium silicon steel was developed as coil springs for anti-aircraft weapons that have high shock loads.
When speaking of stainless steel, it is important to specify the grade of stainless steel since each grade has properties and characteristics designed to meet the needs of select applications.
Stainless Steel 302 is the most common stainless steel wire used to manufacture coil springs. It is especially useful if the coil spring is going to be exposed to moisture or a corrosive environment. Stainless steel 302 can resist the effects of high temperatures up to 550o F or 228o C. It is the softest and most flexible of the various stainless steel with high tensile strength.
Stainless Steel 316 is one of the cleanest stainless steels and has all of the other properties of resistance to moisture and can resist the effects of high temperatures. Although it has positive qualities, it is the weakest of the three types of wire used to produce coil springs.
Stainless Steel 17-7 is resistant to the effects of moisture and corrosion but is stronger and harder than 302 and 316. The hardness of stainless steel 17-7 makes it possible to produce coil springs with greater force and stiffness. Stainless steel 17-7 is able to resist the effects of temperatures up to 650o F or 343o C.
The silicon in silicon manganese steel increases the steel's hardness, controls its decomposition in tempering, and strengthens its ferrite. Manganese also adds to the hardness of the steel and reduces its decarburization. Coil springs made using silicon manganese steel are hot-formed at 900o C to produce a coil spring with a hardness of 225 BHN. Silicon manganese coil springs are used for torsion bars, railway cars, leaf springs for automobiles, and stabilizers.
The popular use of copper based spring alloys is due to their electrical properties and high corrosion resistance. The additional and attractive properties of copper based alloys make them more expensive. They are frequently used in electrical components and subzero temperatures.
Beryllium Copper has electrical conductivity, corrosion resistance, and high mechanical strength with higher tensile strength compared to other copper alloys and good fatigue strength. It is able to retain its ductility even when placed in cryogenic conditions. The non-magnetic properties of beryllium copper make it an ideal choice for use in the medical field in MRI machines and directional drilling equipment for industrial use.
Phosphorus Bronze contains copper, tin, and phosphorus and has greater corrosion resistance and strength than other copper alloys due to the addition of tin. The phosphorus content increases its wear resistance and the stiffness of coil springs made from it. Phosphorus bronze coil springs have excellent strength, good fatigue resistance, and good machinability. They are used for marine fuel pumps, oil rigs, and oil pipeline valves.
The use of nickel based alloys for the production of coil springs is due to nickel's high corrosion resistance and its ability to withstand extremely high temperatures and temperatures well below zero. Since nickel based alloys are non-magnetic, they can be used in gyroscopes, chronoscopes, and indicator instruments. They are highly resistant to the effects of electrical currents and are very poor electrical conductors. Nickel based spring alloys have a Rockwell Hardness of C35 up to C48.
Described below are a few common nickel based spring alloys.
Inconel X750 is a hardened nickel chromium alloy with high strength at temperatures up to 1300o F or 740o C and oxidation resistance up to 1800o F or 982o C and resistance to relaxation.
Inconel 600 is a nickel chromium iron alloy with corrosion and oxidation resistance, excellent strength in high temperatures, and resistance to chloride stress cracking because of its high nickel content. Additionally, it is resistant to attacks by inorganic and organic chemicals and performs well at temperatures below –300o F. Spring tempered Inconel 600 is used in applications that require a tensile strength of 220 Ksi. Coil springs produced using Inconel 600 are cold worked.
Inconel 625 contains nickel, chromium, molybdenum, and niobium, which provide it with excellent corrosion and oxidation resistance and high strength at high temperatures. The high strength of Inconel 625 is due to its molybdenum and niobium content. Additionally, as with Inconel 600, Inconel 625 is resistant to chlorine stress corrosion cracking and pitting. This property makes it suitable for use in ocean and seawater applications.
Inconel 718 is age hardened nickel, chromium, niobium, and molybdenum with excellent corrosion resistant properties. It is a high strength alloy for use in applications with extremes of high and low temperatures. Inconel 718 is ideal for the manufacture of springs due to its exceptional fatigue, creep, and rupture strength. The many positive characteristics of Inconel 718 make it the perfect material for coil springs used in a wide variety of industrial projects and operations.
The metals that are listed above are a few of the types of metals and materials used to produce coil springs. Every day, researchers and engineers are finding other metals, such as aluminum, to be used as coil spring materials.
Springs can be coil and non-coil, which is determined by their design. Coil springs have a coiled appearance and are made from a single strand of metal that follows a helical path to create the coil. The most common method used to manufacture coil springs is to wrap wire of varying diameters around a mandrel, arbor, or cylindrical tubing.
Each type of coil spring has features that distinguish it from the other types. The general rule for coil springs is that they are designed to endure some form of stress, which can be compression or extension.
A volute coil spring is shaped like a cone and used in applications involving heavy loads. They can withstand a greater amount of pressure due to their configuration. The ends of volute coil springs, when exposed to a load, stiffen at their edges. As the pressure of the load increases, the coils move past each other, which enables the spring to compress to a very short compact length that is not possible with helical coil springs.
Arc coil springs, also known as bow springs, have an arched shape that does not run straight from end to end but has a curve in the middle in the shape of the bow for a bow and arrow. They were originally designed for use in dual mass flywheels for drive trains. Force is applied at either end of the spring to pull objects together.
Variable coil springs have different distances between the turns of the spring. The variations in the distance between the turns change the spring’s compression at each of its turns. When a variable coil spring is compressed, the pounds of energy change with each compression or at an exponential curve. The design of variable springs results in easier unlocking, improved recoil energy storage, dampening, feeding, breaching, and lockup.
Torsion coil springs are a flat spiral coil that is used to apply torque or store rotational energy. They store and release rotational energy via torque, which causes the spring to rotate on its axis to apply or resist a load. Torsion springs are attached to components that will rotate around their axis. In most cases, they are very tightly wound but also have versions that have pitch to reduce friction between the coils.
As with most springs, torsion springs can be positioned at angles, horizontally or vertically to be rotated along or against their axis. When a torsion spring is rotated, its coil is wound tighter to store or release energy. Common uses for torsion springs are mouse traps or self closing screen doors.
Compression coil springs are wound helically in order to resist axially applied force. They are a very powerful spring that are manufactured in a wide variety of sizes from ones for retractable pens to ones used on automobile suspension systems. There are various types of compression springs, which include helical, conical or volute, barrel, and hourglass.
Straight Coil compression springs have coils with the same diameter throughout their length. They are the most common form of compression spring.
Hourglass compression springs are tapered in the middle to a smaller diameter than the rest of the coils. The design of hourglass springs keeps them centered in a larger diameter hole. They have some of the advantages of conical springs with the added benefit of being symmetrical.
Barrel compression springs have small diameters on their ends and larger diameters in the middle. A problem with common compression springs is their tendency to buckle or bow when pressure is applied. Barrel compression springs prevent buckling because their shape makes them more flexible, take up less space, and be capable of fitting into different designs. Since a barrel compression spring is wider in the middle, pressure is distributed differently, creating better stability.
Extension springs are like compression springs but are tightly wound and loaded with tension. They have hooks or loops at either end to apply pull force. Extension springs are attached at both ends to components. When the components move apart, the extension spring attempts to bring them back together. The initial tension in the spring determines how tightly coiled it is and can be changed to meet the needs of an application. The tight winding of extension springs is designed to oppose extension. In essence, they provide return force for components that have extended to the actuated position.
Coil springs are a very widely used tool that are found in a never ending array of components, machines, devices, instruments, and vehicles. They are a central part of medical equipment, cars, heavy lifting equipment, and small delicate tools. Their ability to store and release energy is necessary for the support and efficient operation of applications.
In trucks and cars, coil springs are an essential part of the suspension system coordinated with the struts and shocks. They absorb impact from the road, support the vehicle’s weight, ensure a smooth ride, and keep the wheels in contact with the road. In smaller varieties, they hold the hood open and control the movement of the valves of the engine. Coil springs are the reason that car seats are comfortable.
Coil springs are widely used in doors to hold a door open or close it automatically. They are a critical part of the opening and closing of garage doors since they bear the weight of the door and hold it in place until it is pulled or moved to be closed. Coil springs for doors are designed to match the weight and size of the door to avoid failure. In automatic doors, they exert resistance to keep doors closed but release their tension when the open sensor is activated.
Coil springs are an essential part of all construction projects from being anchors for new construction to reinforcing existing foundations. They provide support for sidewalks to control movement and motion. Coil springs are included in safety release valves to open and close pipelines and release pressure in a system. They are used as support for railroad cars and engines as well as supports for bridges and overpasses. In conditions where vibrations or motion can damage an application, they are used as dampeners and sound absorbing devices.
Coil springs are a necessary part of the function of a firearm in the form of magazine springs, trigger springs, and recoil springs. When used in detonators, they are flattened and held in a compressed state until triggered. Coil springs are used due to their reliability and ability to withstand the constant use required from a loaded weapon.
The average home has a huge number of coil springs in nearly every room. They are found in mattresses, chairs, and couches to provide support and reduce the impact on the furnishings. Modern children’s toys use springs to activate them and keep them running. They are an important part of switches, garage door openers, cabinets, appliances, and an endless number of applications. In many ways, homes are very dependent on coil springs for smooth and effortless living.
Compression coil springs are used to push the ballpoint of a pen out and return it. Springs for ballpoint pens must have sufficient elasticity to compress to a solid height. For it to work properly, the spring must have a loose index and enough coil in proportion to its free length and wire diameter. The more open its diameter and more coils, the lighter the spring will be.
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