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. This is how idyllic designs come about, spellbinding with the finesse of the shape, designed for a unique form or colour, but not complying with the essential requirements of ergonomics and functionality. Therefore, it was valuable to gather reliable information about the stiffness of seats depending on the applied design and material solutions.
Experimental studies were conducted on three models of seats of chairs manufactured by one of the reputable factories of house furniture on the polish market at the time of conducting the studies. These models significantly differed among themselves both in terms of the materials used and the design solutions adopted (Fig. 8.46).
The force and movement values recorded during axial compression tests have been provided in Fig. 8.47. On the basis of this chart, it can be seen that the seats selected for the studies have much more different stiffness characteristics. The seat described as model A has a strongly progressive stiffness characteristic, while model S is characterised by an almost linear stiffness. The characteristics of the seats of model B are intermediate between the two previous ones, also with a tendency of progressive increases of forces in relation to movements.
The highest stiffness was shown by type A seats. In linear deformations, the force required to obtain deformations of the seat, the same as in model A, constituted only 86 % of the load value of the seat of model A in model B, while in model S it is 68 % of this load value. In nonlinear deformations, these differences clearly increase. Thus, the force required to obtain deformations of the seat, the same as in model A, constituted 57 % of the load value of the seat of model A in model B, while in model S only 29 % of this load value. This is a result of the fact that for the user, seats of lesser stiffness are more ergonomic and comfortable to use, made from
1. Fabric 2 Cotton
3- PUR foam ■ soft
4 PUR foam – hard
5 Upholstery belts 6- Felt
1. Fabric 2 Cotton
3■ PUR foam ■ soft 4 Felt 5. Sorinos
systems of springs and foam, because smaller load values produce the desired deflection of the seat.
It needs to be noted that constant stiffness of the springs is not the decisive factor for full comfort of use of the furniture piece for sitting, but the size of unit pressures and form of their distribution on the contact surface between the human body and the seat.
Figure 8.48 shows that the constant and unfavourable value of the local stiffness coefficient LWS are only featured in seats of model S. Constant stiffness of the seat does not ensure the adjustment of the values of contact stresses to the user’s body weight. The proportional deflection of spring layers leads to their overall compression and, consequently, forces the user, with a large weight, to sit on a hard and undeformed base. The system of the seat of model B is the best. The average and gently increasing stiffness of the system provides the ability to model the hardness of the seat and adjust it to the individual needs of the user.
From the point of view of the designer and manufacturer of furniture for sitting, the best characteristics of use, in terms of stiffness of the seat, are characterised by models made using upholstery belts and two layers of polyurethane foams of varying stiffness.