An extension spring, also known as an expansion or tension spring, is a helical-shaped spring that stretches or elongates under load applied on either end of the device. An extension spring is an important component that is utilized in a wide range of devices and mechanisms. This comprehensive article explores everything you need to know about extension springs.
An extension spring features a helical shape made from closely coiled metal wire. This type of spring is typically a close wound, meaning the diameter of the wire is the same as that of the spring pitch. As such, the spring coils touch each other when the spring is in a resting state.
The spring offers resistance when a force is applied through extending or stretching. It returns to its original state when the force is withdrawn. Extension springs have hooks or hoops at either end, where components are attached.
There are two key mechanisms at work at these ends. More tension is present at the attachment ends of the spring than the spring’s loops (coils). When the device is under a tension load that elongates the spring, it absorbs and stores energy.
When the force from the load is relaxed or removed, the energy is converted into kinetic energy, which makes the attached components return to their original positions.
An extension spring is designed to work opposite to the way a compression spring works. Both types of springs are designed for the same purpose - to store mechanical energy. The difference is that an extension spring does so by stretching, while a compression spring achieves the same by being squashed down.
Another feature of extension springs is no spacing between the individual coils. The tightly wound coils rest alongside one another when at rest and have initial tension.
Through the transfer of force, the coils are pulled apart and return to their closed state once the extending force is removed. The design of extension springs makes them excellent devices for absorbing and storing energy and resisting pulling forces.
While all extension springs have the same basic design and working mechanisms, there are several variances in the devices, including:
This type is a basic version of the extension spring design with typically helical metal coils wound together with loops or hooks at either end. You can easily attach stock extension springs to other components to make a force transfer and load bearing mechanism.
These are extension springs produced under tight and strict tolerances and are made from particularly high-quality materials. The physical dimensions of precision extension springs are more exact, with the devices designed to support specific amounts of load.
In heavy cycles and some dynamic applications such as prototyping, equipment repair, and maintenance, precision springs can extend the life cycles of important equipment and mechanisms.
Many manufacturers design and produce extension springs based on customer specifications. These may include specific sizes, materials, load requirements, metal gauges, and finishes.
Custom extension springs are crucial for various highly specific applications. In such cases, the stock configurations may be challenging or unfeasible to work with, either due to demanding environments or space restrictions.
These are high quality springs constructed from thicker metal widths and have significantly wider spring diameters. They are capable of supporting much higher load weights compared to standard extension springs.
The springs employed in mechanical industrial applications are usually heavy-duty extension springs.
A drawbar extension spring features a metal loop that goes through the middle of the spring, referred to as a drawbar. The loads attached to either end of the spring are connected to the drawbar. The spring is a mix of extension and compression spring mechanisms in one piece of hardware.
This unique assembly enables the main spring to compress when force is applied to extend the drawbar. It simultaneously provides a fixed stopping point, which prevents hyperextension (overstretching).
In case of a spring failure, the drawbar takes over the load bearing function. This type of extension spring is used to provide additional safety to potentially dangerous load bearing mechanisms such as platforms, heavy doors, and suspended seats. The drawbar holds the components until you can replace or repair the assembly.
This is a type of extension spring consisting of flat metal stripping. The spring is formed into a tight coil that lengthens and straightens in a linear manner when it is deflected. The spring's attempt to return to its original coiled form provides the return force.
Manufacturers use a variety of materials to produce extension springs. The material used is largely determined by the intended application. The performance of an extension spring is highly dependent on the type of material used to construct the device.
Manufacturers consider several factors when determining the materials for spring production. These factors include spring parameters, operating temperatures, and corrosion resistance. Below is an overview of the material used to manufacture extension springs:
Carbon steel contains a percentage of carbon that gives the steel exceptional strength. It also allows manufacturers to heat-treat the material for extra hardness and durability.
Manufacturers may alloy carbon steel with various elements or materials to imbue with the material with additional useful properties. The alloys also increase the operating temperatures of the springs. These alloys may include nickel, chromium, silicon, manganese, vanadium, and molybdenum.
Stainless steel is widely used to manufacture extension springs. It is cost-effective, highly corrosion resistant, and has magnetic properties. The various types of steel used to manufacture extension springs include stainless steel 17-7 PH, 316, 304, and 302.
Copper alloys are used in the manufacture of extension springs since they provide electrical conductivity and excellent corrosion resistance. Extension springs made from copper alloys are used in extreme temperature, marine, chemical process, and electric spring applications. Types of copper alloys include phosphor bronze, brass (zinc copper), beryllium copper, and nickel copper alloys.
Nickel is a highly versatile material that can be alloyed with many metals. Nickel is used in spring production for its high heat resistance. Extension springs made from nickel alloys are strong, highly durable, and reliable.
Nickel alloys used in spring production include nickel chromium iron alloy, nickel chromium molybdenum alloy, nickel cobalt chromium, and Hastelloy. The extension springs function well in chemical environments.
Plastic composite is used to produce extension springs with great thermal properties and chemical resistance. The most common type of plastic composite used is Ultem.
The production process of extension springs takes into account a number of factors regarding specifications and parameters.
Careful consideration of these factors ensures a manufacturer design and produces effective, high-quality extension springs. Among the most important parameters, include:
● Material used
● Free length - The length of the spring when it is at rest (length of the inner surface of the spring ends).
● Wire diameter - The width of the metal wire used to construct the spring coil.
● Coil diameter - The total width of the extension spring when at rest, excluding the wire diameter.
●Spring rate - The amount of force (pounds) required to extend the spring by an inch. Aspects such as spring material and wire diameter influence the spring rate.
●Maximum extended length - The furthest distance an extension spring can stretch before it deforms or fails.
●Maximum load - The weight or amount of stress the spring can handle at full extension.
●Spring pitch - The distance between the centers of the wires in adjacent spring coils. The spring pitch is equal to the wire diameter in closely wound coils.
●Initial tension - The force needed to overcome the standard pitch of the extension spring at rest.
Other factors that manufacturers consider are initial spring tension the spring can handle, the type of spring ends, and more.
The manufacturing process for extension springs is relatively simple and straightforward. While there may be variations depending on the type of spring being made, the process involves three main steps - winding, heat treatment, and finishing.
Winding is the first step, where manufacturers feed the spring wire into a mechanical machine known as a Computer Numerical Control (CNC) spring coiler.
The machine straightens the wire and molds it into the helical spring shape. The spring coiler features several tooling heads that form different spring shapes.
Following the formation of the extension spring, manufacturers feed it to an oven, heating it at a set temperature for a specific period of time. The heat treatment process depends on the type of material and extension spring. The spring is then cooled down in preparation for the finishing process.
The final step involves coating and finishing. Manufacturers coat the springs in layers of material, which enhance some of its physical and chemical properties. These include durability, corrosion resistance, and heat resistance.
The common coating processes include powder coating, plating, and shot peening. There is a variety of finishes for extension springs, including:
●Zinc galvanization - Manufacturers coat the extension springs with a galvanized zinc layer to help prevent corrosion.
●Black oxide – A cost-effective spring finish that affords extension springs a durable corrosion-resistant finish.
●Passivation - A process whereby manufacturers coat the extension springs with a layer of nitride or oxide. It improves the spring’s corrosion resistance.
●Gold Iridite coating – This coating improves the durability and conductivity of extension springs.
Loads or sources of force are attached to the ends of extension springs via a wide variety of spring ends. Manufacturers carefully consider the type of spring end to ensure the device can handle mechanical stress.
They design the spring hooks or loops based on the intended application. The most common types of extension spring ends include crossover loops and machine loops. Other types include:
●Double twisted loop
●Full twisted loop
● Half loop open
● Rectangular hook
Extension springs are a widely used type of spring. They are applied in a wide range of industries to support the function of numerous products used in everyday life. The various applications include:
●Garage doors - Extension springs are attached to the sides or top of garage doors to help in the opening and closing mechanism. The spring helps minimize the strength used to lift and close garage doors.
●Toys - Toy manufacturers utilize extension springs to produce toys that require projectile motion or throwing actions like cars and shooting toys.
●Medical devices - A wide variety of medical devices, such as surgical lights, stretchers, and surgical instruments, employ extension springs.
●Firearms - The magazine springs in firearms use extension springs to push rounds or cartridges into the firearm's magazine. The spring provides the force needed to push the rounds into the firing chamber. It enables fast feeding of cartridges. These springs are typically made of high carbon steel to withstand continuous cycles without sacrificing strength. In addition, the recoil spring used to absorb recoil shock in firearms is an extension spring.
●Window and door screens - Window and door screens snap back to their original closed positions by using extension springs. When a force is applied by opening the screen, the spring extends outwards and pulls the screen to its original position. When you close the window or door screen, the spring snaps back to its original state, keeping the screen shut.
Extension springs are also used in home appliances such as washing machines, foldout furniture like sofa beds and foldable tables, upholstery, and DIY tools.
An extension spring is unique in that its failure is likely to be more catastrophic compared to other types of springs.
When you compare an extension spring and a compression spring of similar material and size, the safe working stress of the extension spring is about ¾ of the load, while the compression spring can safely handle 100 percent of the load weight.
Further, while the stretch on the spring may be able to handle the working load, the spring ends are considerably weaker than the spring coils. As such, this is where the stress points will be concentrated.
High stress can snap or wrap the spring ends. For this reason, the application of an extension spring must account for these aspects. Exceeding the load capabilities of the springs often reduces their usable life cycles significantly.
All in all, extension springs are important devices designed to offer support to a range of consumer products, systems, and machinery. As such, it is important to source high-quality springs from reputable manufacturers.
GL Metal is one of the leading manufacturers and suppliers of quality extension springs. We use high-grade materials and refined production processes to produce best-in-class metal products.
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