How Coil Springs are Made: The Manufacturing Process of Springs
Coil springs are elastic devices that store and release mechanical energy and are manufactured using a variety of materials, which are chosen in accordance with the design of the spring. They can be close wound or open wound with close wound coil springs having coils that touch while open wound coil springs have open ends.
The type of material and 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 Manufacturing of Coil Springs
Coil Spring Materials
The most common metals used to produce coil springs include high and medium carbon steel, chromium vanadium steel, chromium silicon steel, various grades of stainless steel, copper alloys, and nickel. The choice of metal is dependent on the application where the coil spring will be used. Since not all springs are made of wire, the metal for the manufacture of coil springs can come in different forms to fit the type and style of spring.
In some manufacturing instances, metals are delivered as metal bars that are heated and drawn into wire to the proper diameter.
Coil Spring Manufacturing Processes
Cold Winding Process
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.
a. Feed rollers pull the wire from the reel and feed it into the wire guides.
b. 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.
c. Block guide ensures that wire continues to the coiling point. The groove of the block guide is the exact same diameter as the wire.
d. The arbor is the portion of the machine that winds the wire as a mandrel does and has three points of contact.
e. 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.
f. 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.
g. 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 complete process and each of its components can be seen in the diagram below.
Hot Winding
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 1700° 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.
Stress Relief
When a spring is manufactured, its wire is stretched and coiled with force to make it take the shape of a spring. During the coiling process, the molecules of the metal have their natural balance disturbed leaving the spring with residual stress. If the residual stress is not relieved, a spring can have defects, cracks, and a short lifespan. Stress relief brings the molecules of the wires of a spring back into equilibrium.
Stress relief is performed by heating a spring to a level below its deformation level such that wire becomes malleable but does not melt. Once the heating process is completed, the spring is allowed to very slowly return to room temperature, and the molecules rearrange.
Alloy steels, such as chrome silicon or chrome vanadium, are stress relieved between 700° F and 800° F (371° C and 426° C). The high tensile strength of chrome silicon requires that it endure stress relief for one hour or more. Stainless steel 17-7 is stress relieved at 900° F (482° C) for over an hour, as well. Stress relief at higher temperatures and over longer periods of time significantly lowers the internal stress of a spring.
Finishing a Coiled Spring
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.
1. 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.
2. 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.
3. 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.
4. 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.
a. 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.
b. 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.
c. 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.
d. 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.
e. 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.
