Studies confirm that pressure in healthy blood vessels of the skin amounts to 32 mmHg (4.3 kPa) and enables proper blood circulation (Krutul 2004). By examining the stresses on muscle tissue, it was found that stresses up to 34.6 kPa lasting for 35 min cause its stiffening, which leads to pressure pain (Gefen et al. 2005). Adverse loads on soft tissues of the human body, caused by lying down on a base that is too hard, can be reduced by proper support of the user’s torso using a soft and flexible material.
The conditions of the effect of the base on the human body in a lying down position can be illustrated using numerical calculations, using the algorithm of the finite elements method... >
The development of mechanical models of interaction of the human-bed systems aims to determine the distribution of forces of mutual effect of the technical item and the human body. By using mesh phantoms, which are a reflection of the human body of the 5th, 50th and 95th centile (Fig. 8.55), the distribution of forces working in its support points can be determined. The calculation scheme of the mechanical human-bed system can be reduced to the form of a multi-joint beam with a length that corresponds to the height of the user’s body... >
Bedsores are a major problem for people who are physically handicapped and forced to stay lying down or in a reclining position permanently. The practice of dermatology shows that ulceration begins in the deeper tissues and spreads from >
there outwards to the surface of the skin (Krutul 2004). At the same time, cracking of the skin and necrosis of adipose tissue is observed. The finite elements method is an excellent tool to simulate the phenomena in the scope of physical engineering, and with it, one can calculate and present anatomical processes that occur in the human body. Numerical modelling of the human body’s tissues or its individual organs consists in discretization, using any flat or spatial elements that are the basic unit of algorithm of the finite elements method...
Due to human physiology, maintaining sitting position for a couple of hours is not beneficial for the nervous and musculoskeletal systems. Despite the fact that maintaining such a position is physically less tiring compared to the standing position, then with an incorrect position of the body it can cause a much greater (by approx. 40 %) load of the lumbar part of the spine. In addition, the wrong distribution of weight on a seat can cause point loads on the cardiovascular system. As a consequence, long-term positioning of the body in the wrong position, on an
improperly fitted base, often causes pains, changes in degenerative arthritis, blood clots, as well as superficial inflammation of the venous system of lower limbs (Kaminska 2001).
Each day, one can see that the sitting position has... >
Most furniture pieces designed for sitting, especially office chairs, cafe chairs, cinema chairs or house chairs, have soft, upholstered seats and/or backrests. Usually these parts of the furniture, on an industrial scale, are made from flexible polyurethane foams, latex foams and spring systems covered by layers of polyurethane foam or coconut mats, as well as using the technology of embedding conical or cylindrical springs in polyurethane or latex foams. The selection of materials, as well as design solutions, in many cases, is coincidental. Designers, based on their own experience and feeling of comfort, often erroneously decide on a design and technology of making a seat... >
When designing furniture for lying down of sitting (with sleep and relaxation functions), anthropometric and physiological rules should be taken into account arising from their use. Mattresses, especially those of orthopaedic character, constitute one of the essential factors of the quality of life for people with musculoskeletal dysfunction. Rehabilitation of such patients is an ongoing process, and the level of daily activity and wellness determines the behaviour of previously achieved effects or conditions progress in the improvement of abilities... >
In upholstered furniture, there are several construction solutions used for spring units, which constitute the main spring layer. Biconical springs and cylindrical springs (Fig. 8.33a, b) are used in many designs of mattresses. However, the latest solutions tend towards a serial or parallel connection of springs of varying geometry (Fig. 8.33c). Such a compilation of springs enables to obtain nonlinear characteristics of deformations of spring systems of upholstered furniture and also to match the stiffness of the bed to the individual needs of the user.
The analysis of the softness of spring systems in upholstered furniture proves that these layers, from an engineering point of view, should consist of elements of nonlinear and progressive compression characteristics... >
The variable stiffness of the spring during operational loads should ensure high softness of the system at surface loads and significant stiffness when exposed to concentrated forces or forces of high intensity. For such exploitation conditions, a construction minimum is a biconical spring consisting of two conical springs differing in stiffness coefficients, but made from a single piece of wire.
The differentiation of the stiffness coefficient value should be forced by the selection of a suitable geometry of each part of the spring, which has been schematically shown in Fig. 8.27.
In the course of designing the shape of such a spring, it should be ensured that the coils of the lower cone are first settled on a hard base, therefore ensuring the exhaustion of the border of the largest sof... >
For biconical springs, which are applied in the Bonnell-type spring systems, the torsion moment Ms is not a value dependent only on the load, but a function of the changing length of the coil radius Ms = f(R). This radius depends on the angle of the unstretching of the spring a, R = f(a) (Fig. 8.26). The angle a is the angle between the intermediate radius R (variable) and the upper radius of the spring coil R1 depending on the number of the spring coils n. Therefore, the increase in the length of the intermediate radius can be written as:
R1 the largest radius of the coil,
R2 the smallest radius of the coil, a the angle of the unstretching of the spring and n number of the spring coils.
We calculate the strength of the wire in the conical spring by entering into the equation for max... >
8.4.1 Stiffness of Cylindrical Springs
The comfort of the use of upholstered furniture is connected, to a large extent, with the softness of the spring layer. Its quality can be adjusted by choosing or designing the appropriate springs and spring units.
First, let us imagine a cylindrical spring stretched with two forces P acting in its axis (Fig. 8.22.). By cutting this spring in a plane perpendicular to the axis of the wire, which the spring is made of, a balance of cross sections will be conducted (Fig. 8.23). The transverse force P and the resulting torsion moment Ms = P • R must be differentiated by tangential stresses in this cross section, therefore
t1 tangential stresses resulting from transverse forces, t2 tangential stresses resulting from torsion moment,
P load stretchin... >