- Published 27 Feb 2024
- Last Modified 6 Mar 2024
- 12 min
Extension Springs: A Complete Guide
In our comprehensive guide, we look at the different types of extension springs that are available on the market, and what you can use them for.
What are extension springs?
An extension spring, also known as a tension spring, is a helical length of tightly coiled metal, usually fitted with steel hooks or loops at either end for easy attachment to various fixtures, equipment and components. Extension springs are very often considered an integral part of general door hardware kits, but they’re also found playing any number of important roles in other applications, from industrial to automotive, agricultural to electronics, and many more.
The main job of extension springs is to absorb and store energy. As the spring stretches out (extends) under force loads, it absorbs and stores the energy transferred by those loads in its many coiled loops of metal. When the force is removed or relaxed, it redistributes the force back into motion that allows the attached object to return to its original position.
Extension springs thus provide the exact opposite type of support that a compression spring does. The latter is designed to do a similar job - storing energy, essentially, to help objects move between positions - but a compression spring does so by being squashed down, as opposed to stretched out. Either way, the process involves energy being stored and subsequently released by the spring. The extension spring provides resistance to a pulling force by simultaneously attempting to drag the moving object(s) back into the original position. Familiar examples would be the type of springs we often see around the edges of a trampoline, on the rear of a wooden garden gate, or along the inside hinges of a car door.
The main visual difference between an extension spring and a compression spring is in the spacing of the individual coils themselves. Unlike a compression spring, which features widely spaced coils to allow for squashing forces, an extension spring typically has coils that are tightly wound enough to rest alongside one another when not under load. As forces are transferred through the extension spring, the coils will be pulled apart, and will subsequently return to their ‘closed’ position once the force is removed.
Other names for extension springs
Depending on the application, environment and precise role they’re intended to fill, extension springs might variously be known as:
- Tension springs
- Expansion springs
- High tension springs
- Coil tension springs
- Tension coil springs
- Helical extension springs
- Extension coil springs
- Coil extension springs
What are extension springs used for?
There are many and varied uses for extension springs across a wide array of industrial, workplace and domestic settings.
In addition to the familiar door and trampoline examples outlined above, extension springs are very commonly found in a diverse range of other panel and flap assemblies, as well as in various sorts of electrical equipment and white goods, and in all manner of applications where one or more elements requires tensioned support for counteracting a weight or other pulling force.
In this section, we’ll look more closely at some of the familiar everyday uses for extension springs.
Tension springs for window screens
Most domestic window and door screens typically ‘snap’ back to their original closed position once you let them go, and a significant proportion of them do so through the use of extension springs.
In most setups, the tensioned springs are attached at one end to the side of the window screen, and at the other to a non-moving window track or door frame.
When a pulling force is transferred through the extension spring by somebody opening the screen, the spring itself will typically extend outward and attempt to pull the screen back into its original position. When closed, the spring returns to its original tightly wound shape and holds the screen closed.
Tension springs for door and window screens are generally positioned above or to the side of the screen itself, such that no additional space allowance is needed for fitting them, making them a quick and easy addition to almost any door or window hardware kit.
Extension springs for garage doors
Garage doors - particularly sectional or rollover types - very often work with extension springs to help the heavy door open and close more smoothly, with less support and/or lifting force required from the operator.
One end of the tension spring is generally attached to a fixed bracket above the door, while the other end will be affixed to the door itself, each via a pulley mechanism. The tension in the spring counterbalances the weight of the garage door as it moves downward into position, attempting to pull it back up and thus stopping it from falling into place too quickly or heavily.
When the door is fully closed, the spring will therefore be at full stretch. When it’s time to open the door again, the extension spring trying to contract again helps lift the weight of the door back into position, making the job much easier for the operator. Once the garage door is fully open, the extension springs above it will have returned to their least stretched position, and the majority of the door’s weight will be supported by the runner tracks it slides onto.
Guitar tension springs
It’s very common for electric guitars to offer a vibrato or tremolo action, which allows the player to vary the pitch of a note quickly by applying downward force to a lever arm on the body of the instrument.
This lever is connected to the guitar’s bridge via a series of slim tension springs, which are set up such that they attempt to pull the bridge back to its original position when the player applies force to the tremolo arm. The tension on these springs must be balanced against the tension and gauge of the guitar’s strings, which continually attempt to pull the bridge of the instrument in the opposite direction.
In order for a stable tuning to be achieved, these two opposing forces must be in equilibrium - this will allow the bridge of the guitar to lie perfectly flat against the body of the instrument. Most guitars therefore offer multiple points of adjustment for each source of opposing tension, from the machine heads at the ends of the strings to the screws holding the claw attachments for the extension springs.
Pool cover tensions springs
Pool cover tension springs work much like they do on a trampoline - in other words, the springs positioned around the edge of the swimming pool safety cover can be stretched inward, allowing the cover to be pushed down some distance while attempting to drag it back into position.
Unlike a trampoline, they’re not tensioned to provide excessive bounce, but rather to allow for additional weight to be applied on top of the pool cover - either due to pool water ingress, precipitation, or objects/people falling onto it - without the cover or its support straps immediately breaking or sinking beneath the surface.
Pool cover extension springs are generally anchored directly into the ground or brickwork surrounding the water at one end, and attached to the cover via s-hooks passed through loops at the other.
Tension springs for sofa beds
Sofa beds and other sorts of fold-out furniture - certain spring-loaded tables or fold-away surfaces, for example - very often feature extension springs to help support heavier sections and make the job of setting them up and putting them away again much easier for the user.
In a typical fold-out sofa bed setup, the tension springs work much as they do for a garage door: as the sofa bed is fully opened out, the spring will near its full stretch position, attempting to pull the open section of the bed back into a folded configuration.
To ensure that this doesn’t happen until deliberately moved, most sofa bed springs will be arranged with a series of springs providing pulling forces in different directions. This allows a second tension spring to assume a majority of the supporting role once the fold-out sections move beyond a certain point in their arc.
Upholstery tension springs
Extension springs for upholstery - especially common on to many types of armchairs and settees - again work much as they do for a trampoline.
One end of the tension spring is attached to the base of the seat, while the other is affixed to the non-moving frame of the sofa or chair itself, allowing the seat to depress under the weight of the sitter before returning it to its original position once the force is removed.
As well as offering far more comfort for the sitter, this also helps to prolong the useful life of the furniture, reducing overall wear and strain on fixed parts of the frame. Because tension springs are generally cheap to replace and fairly quick and easy to fit, they can be swapped out multiple times to keep the same sofa or armchair feeling comfortable and cushioned even over very long periods of daily use.
What are extension springs made from?
Extension springs are almost always made from metal, which is necessary to achieve the high tension and support the heavy weights or pulling forces they’re often subject to. Most commonly, this metal will be steel - it can be either regular, zinc-plated or stainless, depending on the exact application in mind.
Depending on where and how a given spring is going to be attached and used, steel and stainless steel tension springs are also commonly sold pre-coated with various hard-wearing protective finishes for use outdoors or in more challenging workplace environments. Where a tension spring might be highly visible in certain hardware setups around the home, they can easily be designed, manufactured and installed with aesthetics in mind, and thus sold painted or coated accordingly.
Types of extension springs
There are numerous different types of tension springs sold on the UK and global markets. In the section below, we’ll outline some of the most widely available sorts of extension spring you’re likely to see being offered by suppliers and manufacturers on a day-to-day basis.
Stock extension springs
- Stock extension springs are the ‘basic’ version of the classic extension spring design outlined at the start of this guide.
- Generally speaking, they’ll feature a series of helical steel coils wound tightly together, with a hook or loop at either end for easy attachment to other components as part of a load-bearing and force transferral mechanism.
Precision extension springs
- Precision tension springs are generally manufactured under particularly tight tolerances, and from especially high quality materials, in order for the resulting product to be more exact in terms of its physical dimensions and the forces it’s designed to support.
- In certain dynamic or heavy cycle applications, such as maintenance, prototyping or equipment repair, fitting precision extension springs can significantly extend the life of key equipment and mechanisms before replacement is needed.
Custom extension springs
- Certain manufacturers and suppliers allow for precise sizes, steel gauges, load requirements and material/coatings of extension springs to be specified by the customer on ordering.
- This can be useful in certain highly specific applications where more standard configurations won’t work due to space constraints or especially demanding environments.
Safety drawbar extension springs
- Safety drawbar springs are effectively a combination of extension and compression spring in a single piece of hardware.
- They feature a special assembly that, when a load is applied, allows the main spring to compress as the drawbar extends, while simultaneously providing a fixed stopping point that prevents overstretching (hyperextension).
- In the event of a complete failure and breakage of the spring, the drawbar feature also takes over load-bearing duties, meaning certain types of otherwise potentially hazardous equipment - such as suspended seats, platforms and doors - remain supported until the assembly can be replaced.
Heavy duty tension springs
- Extension springs for heavy duty uses typically come in thicker steel widths, often have wider overall diameters, and will be able to support considerably higher load forces than standard smaller gauge tension springs.
- This is important for many industrial extension springs commonly used in mechanical applications where a great deal of pulling force needs to be applied safely, such as is often required in heavy machinery components for the agricultural, petrochemical and automotive sectors.
Extension springs with hook ends
- Most extension springs are sold either with hook or loop ends, each of which offer different types of anchoring points and can be more convenient to attach to particular sorts of hardware and equipment.
- Hook ends, usually provided in the form of an s-hook, are generally the easiest type to quickly attach and reattach to various items; loop ends require more work to attach, but can usually provide more robust anchoring once installed (since the loop has no break in it to provide a potential fail point during hyperextension).
- Both extension springs with hook ends and loop ends can be configured as crossover, machine style or side style - the best and most cost-effective choice will generally depend on the intended application, and how much force you’ll need the spring to support.
