Used to absorb shock, upholstery springs also provide firmness and resilience to particular types of seats. They are found in couches and chairs. While the cushions are often only padded material, the springs are placed in the body of the seat. There are three basic types of spring used in upholstered furniture: coil, zig-zag or sinuous and marshall. Upholstered furniture uses either the first or second type, whereas marshall springs are found on mattresses.
The gauge of springs ranges from 8 to 11 with coil springs, and from 8 to 13 gauge for sinuous springs. For every gauge there are 3 levels of firmness: soft, regular and hard. Regular springs, also known as medium springs, are the most common type. Springs may be as short as 4¾ inches and as high as 14 inches. Anything more than 10 inches, though, may be too unstable and tends to fall over. As far as coil springs are concerned, the shorter the spring, the higher the gauge.
Coils springs are considered a superior product. They are most commonly found on antique furniture and new, high-end furniture. Depending on the height of the seat, the height of the spring will vary and with it the gauge. Seats are mostly fitted with regular or medium tension springs. Coil springs often outlive the seats into which they're installed.
To tie the springs together once they have been placed, a 4-way knot system using twine is employed. 8 knots are fastened to each spring: 2 front to back, 2 side to side, and 1 between each of those 4 knots. When all tied together, the springs are tied to the front, back and side rails of a piece of furniture in addition to being tied together. Only the spring in the middle (of a 9 spring configuration) is not tied to a wood rail. The twine is held to the wood frame by a series of evenly spaced tacks. Between the tacks and the springs, the twine is kept very taut.
Zig-zag or sinuous springs are not as long lasting as coil springs. Nonetheless, they are used in most modern furniture because they are less expensive to produce. Sinuous spring is produced in rolls and is cut to length as needed. They are called zig-zag springs because the single band of metal goes back and forth like a quick sine wave. Sinuous springs clamp to each of the front and back rails of a chair or couch in parallel. When linked together, they are able to adequately support the weight of those seated.
This type of spring is primarily used in mattresses. The spring itself is enclosed within fabric, while the steel is tempered. If pre-compressed, marshall springs allow for a mattress that is firmer, making either side of the bed somewhat independent of the other.
Upholstery springs for furniture are either coiled or produced in a zig-zag formation. They come in different gauges along with differing tension levels. While coil springs are found in old and antique furniture, they are primarily reserved for high end furniture nowadays, owing to their superiority over zig-zag or sinuous springs.