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 our growing line of ActivEdge™ steel perimeter products.

Making Innersprings

From steel to sleep, our company controls the entire process. Leggett & Platt owns North America’s largest electric furnace, located in Sterling, Illinois. There we melt scrap and transform the molten liquid into rod. Rod is drawn into wire. Our high-carbon wire is formed into a variety of gauges we can shape into unique coil designs.


1

STEEL

To make steel, Leggett & Platt begins by combining 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, 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.

To make 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.

Next, 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.

Each of these squares, called a billet, is about the same length as a Tyrannosaurus Rex.

Each of these squares, called billets, is about the same length as a Tyrannosaurus Rex. Once poured, billets are cooled for handling and transferred to the rolling mill. At the rolling mills, the billets are reheated, then pass through 27 sets of rolls to reduce the size, creating rod as small as 7/32 of an inch.

2

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.

To make 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.

Our heat-treated wire is malleable enough to take the shape of our designs, yet tough so it bounces back to its original shape night after night.

3

COILS

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

hookes law diagram

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.

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 connecters.

Tensile Strength

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 competitor’s and those offering cheap imports. Testing data that proves our product’s superiority is available to customers.

Turns

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.

Gauge

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.

Connections

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.