Innersprings

Leggett & Platt is the worldwide leader in developing and producing innerspring technology. Offering a unique blend of comfort and durability, innersprings are the most popular sleep core in the United States.

At Leggett & Platt, we produce a wide range of open coil and fabric-encased innerspring units, including our popular family of ComfortCore® and growing line of ActivEdge™ steel perimeter products. Innersprings with Leggett & Platt's Active Support Technology® respond to the individual contours of a sleeper's body. The process for creating innersprings relies on a deep understanding of science, engineering, materials, and production.

Hooke

Since the inception of springs, Hooke's Law has been the science behind supported sleep.

Hooke's Law says a spring pushes back with the same force applied. Our engineers know how to use materials and engineering to take full advantage of Hooke's Law, creating innersprings that place the body in a supported position, delivering proper spinal alignment and comfortable sleep.

Image by Svjo via Wikimedia Commons

Innerspring Materials and Engineering

For innerspring units, we draw high-carbon steel wire into specific gauges. That wire is malleable, allowing it to take the shape of our designs, and tough so it bounces back to its original shape. To create this combination, we specify a formula for steel. Once we create that combination, the steel is drawn into wire. Wire becomes a coil. Our coils, unlike many others, are heat-treated. It's a process that makes wire remember its shape. When a wire bounces back, it delivers consistent comfort, night after night.

Steel

To make steel, Leggett & Platt combines 98 parts iron and less than two parts carbon. Our team refines recipes that produce tough, formable wire. A strong and malleable piece of wire will feed through our world-class innerspring-making machines, while retaining its durability.

Steel comes from iron ore and recycled scrap. Iron ore is mined, crushed, concentrated, made into pig iron, and then impurities are removed. Leggett & Platt's recycled scrap, which is mainly comprised of automobiles, is separated, sized, mixed together, melted, and then refined.


Making Steel

Our metallurgists formulate a recipe and then add 95% recycled scrap steel to the largest electric furnace in North America. That steel recipe determines how the end product will turn out. We melt the scrap using an electrical arc created by passing a current through solid carbon electrodes. The material liquefies, then a technician checks its initial chemical composition and makes coarse adjustments. The molten steel is transferred into a ladle and then to a ladle-refining furnace where the temperature and chemicals are adjusted. Once those adjustments are made, we transfer the molten steel to the caster, where it's poured into water-jacketed molds and allowed to solidify into squares about the size of a large man's fist.

Wire

Leggett & Platt controls every aspect of our innerspring-making process. It begins at Sterling Steel in Illinois, home to the largest electric furnace in North America. Each year we melt nearly a million tons of recycled scrap steel into billets. From there we make rod and draw it into wire at our own mills.

The wire used to produce Leggett & Platt innersprings is made using high-carbon steel, a designation that requires a ratio at least .04% carbon and 99.6% iron. As you can see, a little carbon goes a long way when it comes to creating steel.


Making Wire

We pull the steel rod through a series of steel-jacked, tungsten carbide wire-drawing dies. The first wire die is slightly smaller in diameter than the steel rod. Each subsequent die the wire is pulled through gets smaller and smaller, like nesting dolls, until finally we pull the wire through the die that creates its resting diameter. In addition to elongating and reducing the diameter, drawing the wire down creates a smooth, bright surface and increases wire's hardness, tensile strength, and elastic limit.

Coils

The steel wire used in Leggett & Platt's high-quality innersprings makes each coil sturdy and resilient, even after years of use.


Making Coils

Unlike imitators, our coils are tempered using heat or electricity so they bounce back to their original shape. After a coil is turned and shaped into its final form, tempering the spring realigns the molecular structure to ensure the coil won't sag or settle.

Depending on a person's weight, coils yield as weight is applied, creating an initial feeling of comfort with a feeling of support as the person's body rides down into the unit. Different body types require unique levels of firmness and support to get the sleep they need. Engineering a variety of firmness and support characteristics is a process that can be controlled with changes to wire tensile, gauge, coil turns, and coil connectors.

Tensile Strength

Tensile strength is a wire's capacity to stretch without breaking and its ability to withstand compression. At Leggett & Platt, we use high-tensile wire to create coils that are supportive and longer lasting than our competitors and those offering cheap imports. Our testing data that proves our product's superiority is available to customers.

Gauge

Wire gauges typically run from 12 to 19. A lower number corresponds to thicker wire. That means 12-gauge wire is fatter than 19-gauge. You can increase firmness by lowering the wire gauge. One way mattress makers can create a softer sleep surface is by using higher-gauge wire, which is thinner and has more give. Units with thinner wire gauges are usually softer and springier.

Turns

If you take a straight piece of wire and create a turn, you will add a weak point. Turn that wire a few times and you'll see a coil spring begin to form. The more turns, the softer the coil. A coil turn is defined as a complete rotation. Most coils have between five and eight turns. If you have two separate pieces of wire and their gauges are the same, a coil with a higher number of turns should be softer because each turn creates a point of weakness, making the coil easier to compress. Fewer turns means fewer weak points, resulting in a firmer coil.

Connections

For innerspring units using Bonnell and offset innersprings, the coils are connected using a helical lacing technique that involves a long spiral of high-gauge wire binding the rows together and bordering the entire unit. This method keeps the unit together while allowing the coils to move independently and conform to the shape of the body.

Types of Innersprings

There are two major types of innersprings: open coil and fabric-encased coils.

Open Coil

Wire is fed through a coiler, where steel is shaped into springs. The strands of springs are moved into an assembler, where helical lacing is threaded through key parts of the springs to connect the coils and form a cassette. An operator then moves the spring core to a clipping station, where a Leggett & Platt partner adds clips, often connecting a border rod. Once the open coil innerspring unit is complete, it is flat-packed for shipping.

Fabric-Encased Coils

To create ComfortCore® units, fabric is fed into the machine, where the steel coil is inserted into partially formed pockets. Once a coil is placed inside, the pocket is ultrasonically welded and each spring is sealed inside an individual encasement. Coil strands are then fed into an assembler, where a precision glue head applies adhesive to bond the strands together and form a full unit. Once the ComfortCore® unit is complete, it is roll-packed for shipping.

Benefits of Innersprings

There's an art and science to turning steel into the most popular sleep core on the market. While the process is fascinating and complex, what matters is that people get a good night's sleep. Innersprings offer a unique blend of comfort and durability that delivers blissful rest, night after night.

Comfort

Innersprings support and conform to the contours of your body. Infinite pressure relief means your body will sag. When sinking occurs, your body loses proper spinal alignment, which can lead to muscles tensing and fighting for a stable position. Support is a critical factor in finding a relaxed position that leads to deep sleep.

Durability

Your mattress should feel the same way every day. It's well known in the mattress industry that cheap foams break down and lead to body impressions. Leggett & Platt innersprings are tested for durability because we want people to enjoy consistent sleep. Mattresses packed with low-density foam and other cheap materials often do not last. Leggett & Platt is tenacious with internal testing and has monitored the results of millions of units being sold and used in the real world. All that testing and caring about our quality means we can prove our products are durable and deliver great sleep, night after night.

Also, we've been roll-packing our products for years and have proven our innerspring's bounce-back characteristics. If you're building beds for shippable eCommerce programs, we have durable, comfortable components that are roll-pack-ready.

Cooler

Look at springs and then look at foam. Which do you think sleeps cooler? Innersprings are open to the air, whereas foam is a lock-cell material that traps heat. Kansas State University and the Institute of Environmental Research has even shown that innersprings transfer heat up to 28% more efficiently than all foam products. Gimmicks might create an initial cooling effect, but innersprings are proven to more efficiently move heat away from a sleeper.