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Construction
The consumer usually never sees the construction features of an upholstered
piece, but they are important. The overall quality of the materials and
construction techniques used dictate the comfort level of an upholstered piece
and its ability to satisfy the consumer over the long term. A basic
upholstered piece is composed of a frame, springs, foam, decking, cushioning,
padding, and a cover.
Frame
The frame gives structural support and determines the basic shape of any piece
of upholstered furniture. If a frame is unstable, an upholstered piece will
not be durable, no matter how fine or costly its design, padding, cushioning,
or cover. Quality frames are generally made of solid wood, but plywood,
engineered wood products, a variety of polymers and metals are also used.
Hardwood frames are usually constructed from kiln dried mixed hardwoods. White
ash is a superior frame making wood. White oak, red oak, and American elm are
good, and hard maple and birch are acceptable. Softwoods make poor frames.
Kiln drying reduces the moisture content of the lumber, a process which inhibits
checking, splitting, and strengthens the finished product.
Engineered wood products may look not strong, but can be stronger than hardwood
because the layers add to the strength. They are sometimes used at critical
stress areas when maximum strength is needed
Wooden frame joints are often double doweled, which means that round wooden pegs
are fitted into holes in two adjacent frame sections and glued. Epoxy coated
staples and gang nails are also commonly used. The gang nail is a metal plate
with saw teeth, which immobilizes the joint when it is pressed into the wood
with a hydraulic press. Major joints need the additional support of corner
blocks, which should be glued and screwed into place.
Since lumber costs increase rapidly with increasing board thickness, some
manufacturers may hold down frame coasts by skimping at the precise point where
ample strength is most important. The engineering principal involved is that
strength varies directly with rail width and with the cube of thickness. If we
assume that a certain 1î 1îeam will sustain a load of 100 pounds, then a
beam 1îhick and 2îide will sustain 200 pounds. An old rule of thumb
suggests that rails of 3îr more in width should be 1 1/8thîhick, while
rails less than 3îide should exceed 11/8thîn thickness.
Springs
Springs or webbing are attached to the frame to give the seat a desired amount
of elasticity. The most commonly used springs for furniture are coil springs,
sinuous (sag-less) springs, and the Flexsteel ribcage spring system.
Upholstery with coil springs originated in France during the reign of Louis XV,
replacing the method of stuffing hair or feathers over webbed frame covers.
Coil spring upholstery did not however come into widespread use until the
mid-1800íwhen innovations such as the spring edge and springs in upholstered
backs were popularized.
The bottoms of coil springs are normally clipped to the webbing base on which
they sit. These springs are then compressed and tied with twine to the frame
and each other. Eight-way hand tie (ties with eight knots) has been the
industry standard and is a retail and consumer ìzz word.îSprings are tied
to give them an initial tension sufficient to ensure an effect of order and
neatness. They are also tied to hold them in the perpendicular position
essential to proper spring action and to prevent shifting of the top of one
spring in relation to the others. Tied springs also help to distribute the
weight rather than placing it all on one or few springs and it bridges the open
spaces inside of and around the top coils to provide support for filling
materials.
Coil spring construction produces a firm, uniform seat, however, other methods
which combine different kinds of springs, only webbing, flexible fabric, or
soft cushioning materials and foams can produce high quality upholstered
products.
A common spring construction method, which produces a comfortable soft
upholstered front edge, is a spring edge. With the spring edge, the tops of
the springs are positioned well above the top rail of the frame to form a firm
but flexible floating edge.
Sagless springs, webbing, flexible foundation fabric, a drop-in unit, a simple
wood platform of other types of spring units can be used instead of the more
labor intensive hand tied coil spring constructions. Sagless springs are
attached to the frame with steel clips or tacks. Helical springs are then used
to connect them. The degree of comfort derived from this method depends on the
resiliency of the spring.
Webbing, which traditionally consisted of linen or jute band assembly, has now
been largely replaced by synthetic webbing materials. Webbing and solid panels
can also be used as the deck for top cushions instead of springs.
Another type of spring system is the Flexsteel blue ribcage spring system. In
the late 1800íthe founders of Flexsteel purchased Swiss watch spring steel to
create a new spring system. This steel is used in the spring movement of
watches and clocks because of its continuous flexibility and strength. The
steel gets stronger the more times it is flexed. This allows the Flexsteel
ribcage spring system to give everlasting comfort, durability, and reliability
for as long as you own your sofa.
Deck
In conventional (coil spring) constructions, layers of padding are added on top
of the springs or webbing to complete the deck (the area under the seat
cushions). In quality upholstery applications, the deck should be well padded,
with no bumps or lumps. The padding should extend to the front edge of the
deck, completely covering the spring edge or frame.
Cushions
Loose or attached cushions sit on top of the deck. The majority of cushions are
made up of polyurethane foam. A ìrshal unit,îade up of coil springs, which
are sewn in pockets and then attached. Down or synthetic down-like materials
may be used instead of foam in some upholstery applications.
Polyurethane foam seat cushions which contain high-density foam (2.0 pounds per
cubic foot polymer density or greater) are desirable. They offer better
support and durability than lower density foams. Back cushions do not need
foam that is as dense, but shredded foam which is cheaper to use, may
eventually shift, compact and settle, getting lumpy and uneven looking.
Cushions are normally wrapped or capped (crowned) with polyester, low-density
foam or other soft material for added surface plushness. This wrapping should
give the cushion a softly rounded look. It also helps prevent the fabric from
rubbing against the dense foam core and fills cushion corners.
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