Introduction

This article takes an in depth look at coil springs and their materials.

Chapter One – What is a Coil Spring?

A coil spring is a mechanical component with a helical shape made from wound metal. It functions by storing potential mechanical energy and using that energy to absorb shock. Coil springs are created by winding a wire into a continuous helical coil, allowing the wire to transform into a spring capable of energy storage.

Coil springs come in various sizes and are designed to absorb shock and reduce stress on surfaces by providing flexibility. When subjected to an external force, they deform but revert to their original shape once the force is removed. The spring stores energy while compressed and releases it as it returns to its initial shape, with the amount of energy related to the applied force.

Under weight, coil springs compress, decreasing in size and storing mechanical energy in the process. Once the weight is removed, the spring expands and releases the stored energy as it returns to its original form.

Coil springs are categorized into three primary types: compression, extension, and torsion, each serving a distinct purpose. Among these, compression springs are the most widely used. They are typically made by hot or cold winding of spring steel and are designed to absorb forces and provide resistance when compressed.

Coil springs are susceptible to rust and other environmental factors, which can lead to premature failure. To enhance durability, they are often coated with powder, epoxy, or polyester, and some are electroplated with zinc or nickel for added protection.

Chapter Two – How Coil Springs are Made?

Coil springs are elastic components designed to store and release mechanical energy, and they can be made from various materials selected based on the spring's design. They come in two main types: close wound, where the coils are tightly spaced and touch each other, and open wound, where the coils are spaced apart at the ends.

The choice of material and manufacturing process for coil springs varies depending on the production scale. Small production runs are typically completed using a lathe, while high-volume production is done with automated coiling machines or computer numerical control (CNC) equipment.

The Manufacturing of Coil Springs

Coil Spring Materials

Coil springs are commonly made from various metals, including high and medium carbon steel, chromium vanadium steel, chromium silicon steel, various grades of stainless steel, copper alloys, and nickel. The selection of metal depends on the specific application of the coil spring. Additionally, since coil springs can be made from different forms of metal, the material used may vary to suit the type and design of the spring.

In some cases, metals are supplied as bars, which are then heated and drawn into wire of the required diameter for spring production.

Coil Spring Manufacturing Processes

Cold Winding Process

Heat Treatment –Cold winding begins withheat treatmentof 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 the two primary methods for manufacturing coil springs. This process can be carried out using a lathe, winding machine, or hand crank machine. During production, a guide mechanism ensures that the wire is aligned correctly to achieve the desired pitch, which is the spacing between the coils as they are wrapped.

Computer Numerical Control (CNC) –CNC spring coiling machines are advanced and sophisticated compared to traditional mandrel systems. These machines use a series of components to precisely feed, wind, and shape the coils with high efficiency and accuracy.

• Feed rollers pull the wire from the reel and feed it into the wire guides.
• Wire guides are flat with different sizes of grooves to match the feed rollers but do not exactly match the wire size but are within its range.
• Block guide ensures that wire continues to the coiling point. The groove of the block guide is the exact same diameter as the wire.
• The arbor is the portion of the machine that winds the wire as a mandrel does and has three points of contact.
• The pitch tool is programmed to position the wire at the proper pitch to meet the design. It slides up and down the arbor, moving along the sloped surface.
• The coiling point pushes the wire into its coiled shape by deflecting its trajectory. The deflection point is programmed into the CNC machine and determines the deflection angle.
• The cutter slices the wire when it has reached its desired length. It may be positioned above or below the arbor depending on the CNC machine’s design.

The entire process and its components are illustrated in the diagram below.

Hot Winding

Wire –For hot coil winding, the wire can be relatively thick, ranging from 3 inches (75 mm) to 6 inches (150 mm) in diameter. During hot winding, the wire is heated to around 1700°F, allowing manufacturers to work with larger diameter wire.

Mandrel –In the mandrel process, heated metal is coiled around a mandrel similarly to cold coiling, but with increased precision. The rotation of the mandrel and the pitch distance are controlled by a CNC machine.

Cooling –Following hot coil winding, the next critical step is to cool the wound coil rapidly through a process called quenching. Various methods are used for quenching steel parts, with oil being a common choice. This cooling process is essential for hardening the coil’s steel and preventing the development of thermal and transformational gradients that could cause cracking.

Quenching alters the crystalline structure of the metal, locking in changes and increasing hardness. Initially, a vapor blanket forms around the part as the first cooling stage. This blanket is then displaced by heating the quenching medium, followed by the convective stage, which continues to remove heat and cool the part.

Quenching oil is favored for its rapid cooling capabilities compared to other methods. The quenching process requires meticulous monitoring to ensure that the final coiled spring meets performance expectations.

Stress Relief

During spring manufacturing, the wire is stretched and coiled under force to form the spring shape. This process disrupts the natural balance of the metal's molecules, leaving the spring with residual stress. If not properly managed, this residual stress can lead to defects, cracks, and a reduced lifespan. Stress relief is used to restore the equilibrium of the metal molecules in the spring.

Stress relief is achieved by heating the spring to a temperature below its deformation point, making the wire malleable without melting it. After heating, the spring is gradually cooled back to room temperature, allowing the molecules to realign.

Alloy steels like chrome silicon or chrome vanadium undergo stress relief between 700°F and 800°F (371°C and 426°C). Chrome silicon, due to its high tensile strength, requires a stress relief duration of one hour or more. Similarly, stainless steel 17-7 is stress relieved at 900°F (482°C) for over an hour. Stress relief at higher temperatures and for extended periods significantly reduces internal stress in the spring.

Finishing a Coiled Spring

The finishing process for a coiled spring depends on its design requirements, including its shape, coating, pitch, and strength.

Grinding –Grinding is often needed to flatten the ends of a spring to meet its design specifications. This can be done either manually or automatically. In manual grinding, the spring is secured in a jig and ground with an abrasive tool. Automated grinding involves the spring being held in place while both ends are ground according to programmed settings. Lubrication is used during grinding to keep the metal cool and remove debris.

Grinding is frequently necessary to ensure the spring fits properly into its application and can sit flat. This process allows the spring to be positioned upright as required.

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 adjust the pitch and length of a spring, it is compressed until the coils make contact. This process may need to be repeated until the desired dimensions are achieved. Measuring the pitch can be challenging. A preliminary estimate can be made by measuring the distance between the spring's loops, but this method is less accurate. For a more precise measurement, a spring pitch formula, available through computer programs, should be used.

The length and pitch are illustrated in the diagram below. There are differing opinions on measuring pitch for coil springs. Some experts argue that specifying the number of active coils provides a more accurate representation than pitch alone.

Applying Coatings –Coatings are used to protect springs from corrosion, as the metals commonly used in spring manufacturing are susceptible to environmental damage. Various methods are employed to cover the entire surface of the spring, including spray painting, rubber dipping, or electroplating with zinc or chromium.

• Zinc coatings provide excellent corrosion resistance without the risk of hydrogen embrittlement. The zinc is applied in a solution of resin that is sprayed, dipped, or spun onto the surface of the coil spring.
• Electroplating is one of the most common forms for applying a coating since it is low cost and very effective. Materials that can be applied by electroplating include zinc, chromium, tin, and nickel, with tin and nickel used where electrical conductivity is important.
• Powder coatings come in a variety of colors for corrosion resistance and are typically used on large springs since the thickness of smaller springs can be too great.
• Pre-plated wire is used to manufacture coil springs as the raw material. Galvanized wire is the most common form of pre-plated wire because it has high corrosion resistance. Pre-plated wire guarantees that the complete surface of the spring has been fully treated.
• Plastic coating is a flexible material coated on springs for increased corrosion protection. The unfortunate aspect of plastic coating is that it can be damaged by constant compression and other factors.

Types of Coil Spring Ends

Coil spring ends come in four common forms: closed, open, square, and pigtail. Selecting the appropriate end type is crucial for fitting the spring to its specific application.

Closed End

The closed end variant is the most prevalent. In this design, the spring’s end pitch is flat, with the tip making contact with the adjacent coil. This design ensures that the coil spring remains stable and can stand upright independently.

Square End

Square end coil springs are a subtype of closed end springs, where the end is ground to a square shape. These springs typically offer greater deflection compared to open end springs.

Open End

Open end coil springs feature a continuous pitch, extending to the end. Often called tangential coil springs, they are used in applications where the spring needs to rest on a surface, such as in automotive settings. For an open end coil spring to be functional, it must have a designated seat to support it.

Pigtail Coil Spring

A pigtail coil spring has a last coil with a smaller diameter than the other coils. This smaller diameter allows the pigtail end to be attached to mechanisms using screws or bolts.