Abstract:
The invention relates to a fastening system for panels whose edges are provided complementary holding profiles which match one another in such a manner that further panels can be fastened to the free edges of a previously placed panel. The holding profiles of at least the long edges are configured as complementary positive-fit profiles, one of the profiles having a projection with a convex bottom edge, and the other profile having a recess with a concave bottom edge, such that the profiles may be interconnected by a pivoting motion. Such complementary design of the profiles enables the positive-fit profiles of the long edges of two panels to form a common joint which, when the panels are laid, enables bidirectional pivoting of the panels with respect to one another about a pivot axis that is parallel to the joined long edges of the panels.

Description:
The invention relates to a panel and a fastening system for panels, especially for floor panels, that are placed on a base and whose edges are provided with holding profiles, where the holding profile of a long edge and the holding profile of the opposite edge, as well as the holding profiles of the other two short edges of a panel, match one another in such a manner that further panels can be fastened to the free edges of one of the placed panels, where at least the holding profiles of the long edges of the panels are configured as complementary positive-fit profiles and the panels are interconnected by pivoting them to be joined, that the positive-fit profile of one of the long edges of a panel is provided with a recess and the opposite edge of this panel with a corresponding projection, that the wall of the recess facing the base has an inside cross-section with a concave curvature and that the associated positive-fit profile of the opposite edge of the panel has a projection, the underside of which facing the base has a cross-section with a convex curvature, and that the convex curvature of the projection and the concave curvature of the recess are essentially of complementary design. 
     Fastening systems of this kind hold installed panels together by means of a positive-fit connection. In the case of floor panels installed in floating fashion on a base, in particular, a positive-fit connection between the panels prevents the formation of gaps, which can form, for example, as the result of thermal expansion or contraction due to a drop in temperature. 
     German utility model G 79 28 703 U1 describes a generic fastening system. Floor panels with a positive-fit profile of this kind can be connected very easily by means of a pivoting movement. In principle, the connection is also suitable for repeated installation. The resulting positive-fit connection is very stiff and thus very reliably prevents the formation of gaps. 
     The disadvantage is that the known fastening system is only suitable for very even bases. If the base is uneven, rough and undulating, a panel floor adapts only very poorly to the shape of the uneven base when using the known fastening system. For example, if a panel is held a slight distance above an undulating base by adjacent panels when installed and is then pressed onto the base under load, the interconnected floor panels are deflected. This deflection particularly stresses the joints with the engaged positive-fit profiles. Depending on the load, the interconnected panels bend down or up and are thereby forced out of the normal plane of installation. Due to the great stiffness of the connection, a high load is exerted on the thin cross-sections of the positive-fit profiles, which are thus very quickly damaged. The damage progresses rapidly until a projection or a recess wall ruptures. 
     Panels can suffer from alternating deflection even on a level base, namely when a soft intermediate layer, such as an impact sound insulation film or the like, is laid on the base. The intermediate layer is compressed at the loaded point and the panels buckle at the joints. 
     Thus, the object of the invention is to modify the known fastening system such that the stiffness of the connection between two, interconnected positive-fit profiles is adapted to the stress the panels must bear when installed on an uneven base. 
     According to the invention, the object is solved in that the positive-fit profiles of the long edges of two panels form a common joint when laid, in that the upper side of the projection of a panel facing away from the base displays a bevel extending up to the free end of the projection, in that the bevel increasingly reduces the thickness of the projection towards the free end, and in that the bevel creates space for movement for the common joint. 
     The new design permits articulated movement of two connected panels. In particular, two connected panels can be bent up-wards at the point of connection. If, for example, one panel lies on a base with an elevation, with the result that one edge of the panel is pressed onto the base when loaded and the opposite edge rises, a second panel fastened to the rising edge is also moved upwards. However, the bending forces acting in this context do not damage the thin cross-sections of the positive-fit profiles. An articulated movement takes place instead. 
     A floor laid using the proposed fastening system thus displays an elasticity adapted to irregular, rough or undulating bases. The fastening system is thus particularly suitable for panels for renovating uneven floors in old buildings. Of course, it is also more suitable than the known fastening system when laying panels on a soft intermediate layer. 
     The design caters to the principle of “adapted deformability”. This principle is based on the knowledge that very stiff, and thus supposedly stable, points of connection cause high notch stresses and can easily fail as a result. In order to avoid this, components are to be designed in such a way that they display a degree of elasticity that is adapted to the application, or “adapted deformability”, and that notch stresses are reduced in this way. 
     Moreover, the positive-fit profiles are designed in such a way that a load applied to the upper side of the floor panels in laid condition is transmitted from the upper-side wall of the recess of a first panel to the projection of the second panel and from the projection of the second panel into the lower-side wall of the first panel. When laid, the walls of the recess of the first panel are in contact with the upper and lower side of the projection of the second panel. However, the upper wall of the recess is only in contact with the projection of the second panel in a short area on the free end of the upper wall of the recess. In this way, the design permits articulated movement between the panel with the recess and the panel with the projection, with only slight elastic deformation of the walls of the recess. In this way, the stiffness of the connection is optimally adapted to an irregular base which inevitably leads to a bending movement between panels connected to each other. 
     Another advantage is that panels with the fastening system according to the invention are more suitable for repeated installation than panels with the known fastening system, because the panels with the fastening system according to the invention display no damage to the positive-fit profiles even after long-term use on an uneven base. The positive-fit profiles are dimensionally stable and durable. They can be used for a substantially longer period and re-laid more frequently during their life cycle. 
     Advantageously, the convex curvature of the projection and the concave curvature of the recess each essentially form a segment of a circle where, in laid condition, the centre of the circle of the segments of the circle is located on the upper side of the projection or below the upper side of the projection. In the latter case, the centre of the circle is located within the cross-section of the projection. 
     This simple design results in a joint where the convex curvature of the projection is designed similarly to the ball, and the concave curvature of the recess similarly to the sokket, of a ball-and-socket joint, where, of course, in contrast to a ball-and-socket joint, only planar rotary movement is possible and not spherical rotary movement. 
     In a favourable configuration, the point of the convex curvature of the projection of a panel that protrudes farthest is positioned in such a way that it is located roughly below the top edge of the panel. This results in a relatively thick cross-section of the projection in relation to the overall thickness of the panel. Moreover, the concave curvature of the recess offers a sufficiently large undercut for the convex curvature of the projection, so that they can hardly be moved apart by tensile forces acting in the installation plane. 
     The articulation properties of two panels connected to each other can be further improved if the inside of the wall of the recess of a panel that faces the base displays a bevel extending up to the free end of the wall and the thickness of this wall becomes increasingly thin towards the free end. In this context, when two panels are laid, the bevel creates space for movement of the common joint. This improvement further reduces the amount of elastic deformation of the walls of the recess when bending the laid panels upwards. 
     It is also expedient if the recess of a panel for connecting to the projection of a second panel can be expanded by resilient deformation of its lower wall and the resilient deformation of the lower wall occurring during connection is eliminated again when connection of the two panels is complete. As a result, the positive-fit profiles are only elastically deformed for the connection operation and during joint movement, not being subjected to any elastic stress when not loaded. 
     It is practical if the holding profiles of the short edges of a panel are likewise designed as complementary positive-fit profiles and can be connected to one another by a linear connecting movement. 
     For the sake of simplicity, the holding profiles of the short edges of a panel are provided with conventional, roughly rectangular tongue-and-groove cross-sections. They are very simple and inexpensive to manufacture and, after connecting the long edges of panel, they can be joined very easily by being laterally slid into one another. The long edges of the panels can also be slid into one another in the parallel direction along their entire length. 
     In another configuration of the short edge of a panel, the cross-sections of the positive-fit profiles essentially correspond to the cross-sections of the positive-fit profiles of the long edges of the panel. The ability to also connect two panels in articulated fashion on their short edges benefits the flexibility of a floor covering. 
     The positive-fit profiles preferably form an integral part of the edges of the panels. The panels can be manufactured very easily and with little waste. 
     The positive-fit profiles according to the invention are particularly suitable if the panels consist essentially of MDF (medium-density fibreboard), HDF (high-density fibreboard) or a particle board material. These materials are easy to process and can be given a sufficient surface quality by means of cutting processes, for example. In addition, these materials display good dimensional stability of the milled profiles. 
     Another benefit results if the spaces for movement of the common joints are provided with a filler that remains flexible after curing when the panels are installed. This filler preferably seals all joints, particularly the top-side joint, such that no moisture or dirt can enter. During articulated movement of the connected panels, the flexible filler is compressed or expanded, depending on the rotational direction of the articulated movement. In this context, it always adheres to the contact surfaces of the edges of the panels and reverts to its initial shape when the articulated movement is reversed. The filler helps return the joint to its original position due to its elastic, internal deformation. 
     In an alternative configuration of the fastening system, one short edge of a panel has a first hook element and the opposite short edge of the panel has a hook element that complements the first hook element, the hook elements being provided with holding surfaces that, when assembled, hold the panels together in such a way that the surfaces of the panels abut without gaps at the short edges. 
     In order to install the panels, the positive-fit profiles on the long edges of the panels must be connected first. To this end, a panel is positioned at an angle and the projection of one long edge is inserted into the recess of the long edge of a laid panel. The common joint is formed in this way. The panel is then held in the angled position and slid in its longitudinal direction until it hits the short edge of an adjacent panel. In this position, the hook elements of the short edges of adjacent panels overlap. If the angled panel is now swung down by means of the joint, the overlapping hook elements engage. They catch behind one another, preventing the panels from being pulled apart in their longitudinal direction. Due to the hook elements, an overlap can be achieved that is roughly equal to one-third of the entire panel thickness. This method for locking the short edges of the panels is similar to the lateral overlap of roofing tiles. 
     For the sake of simplicity, the first hook element is formed by a web protruding roughly perpendicularly from the short edge and located on the upper side of the panel, where a hook projection facing the lower side of the panel is provided on the free end of the web, and the second hook element is formed by a web protruding from the opposite short edge and located on the lower side of the panel, where a hook projection facing the upper side of the panel is provided on the free end of this web. 
     The upper side of the panel merges with a reduction in thickness from the area with the thickness of the full panel into the web. The thickness of the web is roughly equal to one-third the panel thickness. The same applies to the lower side of the panel. Opposite the upper-side hook element, the lower-side web merges with a reduction in thickness from the area with the thickness of the full panel into the web, which is again roughly one-third the thickness of the panel. 
     The webs and the hook projections are thus of relatively solid design. This improves the strength and durability of the fastening system according to the invention. 
     The hook projection of the lower-side web advantageously contacts the upper-side web of a second panel when a panel is installed. In addition, a space is provided between the hook projection of the upper-side web of the second panel and the lower-side web of the first panel. 
     Of course, this can also be reversed, so that a space is provided between the hook projection of the lower-side web of the first panel and the upper-side web of the second panel. It is important that one web/hook projection pair of connected hook elements is in definite contact when laid and that the other web/hook projection pair of the same hook elements has a space. If the fastening system were designed such that both web/hook projection pairs were in contact at all times, no definite contact would be achieved due to the tolerances involved in manufacturing the holding profiles, the result being that one web/hook projection pair would sometimes be in contact and sometimes the other. 
     One configuration of the fastening systems provides that the holding surfaces of the hook projections engage in such a way that they can only be hooked together by means of elastic deformation. This can prevent the hook elements from moving apart under load, for example due to an uneven base. If one panel is loaded, the connected panel moves in the same direction as the loaded panel. The joint stays together. 
     For the sake of simplicity, the holding surfaces of the hook projections are inclined and the hook projections taper from their free ends towards the webs. In addition, the holding surfaces of complementary hook projections contact one another, at least in some areas. This is a simple design of the hook projections provided with an undercut, because a plane holding surface that is easy to manufacture is provided as the undercut. 
     Another benefit results if the front side of the upper-side hook projection of one panel at least contacts the second panel in the region of the upper side of the panel when the panels are installed, and if a space is provided between the lower-side hook projection of the second panel and the front side of the first panel. This measure in turn serves to ensure the definite contact of two connected panels at all times by means of the structural design. 
     On the underside of the panels, which is laid on a base, such as screed, an air gap can be tolerated between the panels in the region of the joint. 
     An alternative configuration with hook elements on the short edges of the panel is designed such that at least one of the front sides of one of the hook elements of the panels has a protruding snap element on its free end, which engages an undercut recess of the other hook element of the panel. This design has proven to be particularly practical, because the holding profiles can be snapped together by applying slight pressure, thus undergoing elastic deformation. In addition, the holding elements display good wear resistance, which favours multiple installation. The wear resistance is good because the various locking functions are carried out by different areas of the holding element and the load on the holding element is thus distributed. For example, the panels are lokked perpendicular to the installation plane by the snap element and the recess. In contrast, the holding surfaces of the hook projections lock the panels in order to prevent them from being pulled apart in their longitudinal direction. 
     For the sake of simplicity, the protruding snap element of the first panel is designed as a ridge that extends over the entire length of the edge, and the undercut recess of the second panel is designed as an elongated groove that receives the ridge in the connected position. In order to make the connection, the ridge and the groove must be inserted into one another by elastically deforming the hook elements. 
     This configuration of the fastening system is suitable for use in cases where no glue is to be used, particularly for multiple installation. In order to take up laid panels, one row of adjacent panels is expediently raised such that they rotate upwards at an angle in the joint. The projections are then pulled out of the recesses at an angle and the joint dismantled. The panels are then only connected at the short edges. It is recommendable to pull apart the joined holding elements of the short edges along their longitudinal extension, in order to avoid material-fatiguing deformation of the hook elements in this way during dismantling. 
     Another improvement is that the air-filled spaces existing when two panels are installed form glue pockets. In addition to using the proposed fastening system for glueless laying of floor panels, it is also particularly suitable for connection with glue. For this purpose, the points on the holding profiles that must be glued can, for example, be indicated in the instructions or designated by markings on the holding profile itself. In this way, the user can apply glue exactly at the points where glue pockets are formed when two panels are installed. 
     In most applications of the floor panels, installation with glue is considered to be the most expedient method for laying the panels. This is because it significantly improves the durability of the panels. The gluing of the holding profiles almost completely prevents the ingress of dirt and moisture into the joints. This minimises moisture absorption and the swelling of the panels in the joint region of the holding profiles. 
     Of course, applications may arise in which glueless installation is preferable. For example, if a floor covering frequently has to be installed, taken up again and re-installed, e.g. for floor coverings on exhibition stands. 
     The panels are preferably made of a coated substrate material and the holding profiles form an integral part of the edges of the panels. It has become apparent that the strength of modern substrate materials, such as medium-density fibreboard (MDF) or high-density fibreboard (HDF), which are provided with a wear-resistant wear layer, makes them particularly suitable for the use of the proposed fastening system. Even after multiple installation, the holding profiles are still in such good condition that reliable connection is possible even on an uneven base. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of the invention is illustrated in a drawing and described in detail below on the basis of FIGS. 1 to  12 . The figures show the following: 
     FIG. 1 Part of a fastening system on the basis of the cross-sections of two panels prior to connection, 
     FIG. 2 The fastening system as per FIG. 1 in assembled condition, 
     FIG. 3 A connecting procedure, where the projection of one panel is inserted into the recess of a second panel in the direction of the arrow and the first panel is subsequently locked in place by a pivoting movement, 
     FIG. 4 A further connecting procedure, where the projection of a first panel is slid into the recess of a second panel parallel to the installation plane, 
     FIG. 5 The fastening system in assembled condition as per FIG. 2, where the common joint is moved upwards out of the installation plane and the two panels form a bend, 
     FIG. 6 The fastening system in laid condition as per FIG. 2, where the joint is moved downwards out of the installation plane and the two panels form a bend, 
     FIG. 7 A fastening system in the laid condition of two panels, with a filler material between the positive-fit profiles of the edges, 
     FIG. 8 Special holding profiles for the short edges of a panel in connected condition, 
     FIG. 9 Another configuration of special holding profiles for the short edges of a panel in connected condition, 
     FIG. 10 A schematic diagram of a holding profile with a lower-side web and a drawing of two cutting tools for machining the undercut, 
     FIG. 11 A third configuration of the special holding profiles for the short edges of a panel in connected condition, 
     FIG. 12 A configuration according to FIG. 11, to which an additional snap element has been added. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the drawing, fastening system  1  is explained based on oblong, rectangular panels  2  and  3 , a section of which is illustrated in FIG.  1 . Fastening system  1  displays holding profiles, which are located on the edges of the panels and designed as complementary positive-fit profiles  4  and  5 . The opposite positive-fit profiles of a panel are of complementary design in each case. In this way, a further panel  3  can be attached to every previously laid panel  2 . Positive-fit profiles  4  and  5  are based on the prior art according to German utility model G 79 28 703 U1, particularly on the positive-fit profiles of the practical example disclosed in FIGS. 14, 15 and 16 and the associated descriptive part of G 79 28 703 U1. 
     The positive-fit profiles according to the invention are developed in such a way that they permit the articulated and resilient connection of panels. 
     One of the positive-fit profiles  4  of the present invention is provided with a projection  6  protruding from one edge. For the purpose of articulated connection, the underside of projection  6 , which faces the base in laid condition, displays a cross-section with a convex curvature  7 . Convex curvature  7  is mounted in rotating fashion in complementary positive-fit profile  5 . In the practical example shown, convex curvature  7  is designed as a segment of a circle. Part  8  of the edge of panel  3 , which is located below projection  6  and faces the base in laid condition, stands farther back from the free end of projection  6  as part  9  of the edge, which is located above projection  6 . In the practical example shown, part  8  of the edge, located below projection  6 , recedes roughly twice as far from the free end of projection  6  as part  9  of the edge, located above projection  6 . The reason for this is that the segment of a circle of convex curvature  7  is of relatively broad design. As a result, the point of convex curvature  7  of projection  6  that projects farthest (shown at Point A of FIG. 1) is positioned in such a way that it is located roughly below top edge  10  of panel  3 . 
     Part  9  of the edge, located above projection  6 , protrudes from the edge on the top side of panel  3 , forming abutting joint surface  9   a.  Part  9  of the edge recedes between this abutting joint surface  9   a  and projection  6  of panel  3 . This ensures that part  9  of the edge always forms a closed, top-side joint with the complementary edge of a second panel  2 . 
     The upper side of projection  6  opposite convex curvature  7  of projection  6  displays a short, straight section  11  that is likewise positioned parallel to base U in laid condition. From this short section  11  to the free end, the upper side of projection  6  displays a bevel  12 , which extends up to the free end of projection  6 . 
     Positive-fit profile  5  of an edge, which is complementary to positive-fit profile  4  described, displays a recess  20 . This is essentially bordered by a lower wall  21 , which faces base U in laid condition, and an upper wall  22 . On the inside of recess  20 , lower wall  21  is provided with a concave curvature  23 , which has the function of a bearing shell. Concave curvature  23  is likewise designed in the form of a segment of a circle. In order for there to be sufficient space for the relatively broad concave curvature  23  on lower wall  21  of recess  20 , lower wall  21  projects farther from the edge of panel  2  than upper wall  22 . Concave curvature  23  forms an undercut at the free end of lower wall  21 . In finish-laid condition of two panels  2  and  3 , this undercut is engaged by projection  6  of associated positive-fit profile  4  of adjacent panel  3 . The degree of engagement, meaning the difference between the thickest point of the free end of the lower wall and the thickness of the lower wall at the lowest point of concave curvature  23 , is such that a good compromise is obtained between flexible resilience of two panels  2  and  3  and good retention to prevent positive-fit profiles  4  and  5  being pulled apart in the installation plane. 
     In comparison, the fastening system of the prior art according to FIGS. 14, 15 and 16 of utility model G 79 28 703 U1 displays a considerably greater degree of undercut. This results in extraordinarily stiff points of connection, which cause high notch stresses when subjected to stress on an uneven base U. 
     According to the practical example, the inner side of upper wall  22  of recess  20  of panel  2  is positioned parallel to base U in laid condition. 
     On lower wall  21  of recess  20  of panel  2 , which faces base U, the inner side of wall  21  has a bevel  24 , which extends up to the free end of lower wall  21 . As a result, the wall thickness of this wall becomes increasingly thin towards the free end. According to the practical example, bevel  24  follows on from one end of concave curvature  23 . 
     Projection  6  of panel  3  and recess  20  of panel  2  form a common joint G, as illustrated in FIG.  2 . When panels  2  and  3  are laid, the previously described bevel  12  on the upper side of projection  6  of panel  3  and bevel  24  of lower wall  21  of recess  20  of panel  2  create spaces for movement  13  and  25 , which allow joint G to pivot over a small angular range. 
     In laid condition, short straight section  11  of the upper side of projection  6  of panel  3  is in contact with the inner side of upper wall  22  of recess  20  of panel  2 . Moreover, convex curvature  7  of projection  6  lies against concave curvature  23  of lower wall  21  of recess  20  of panel  2 . 
     Lateral abutting joint surfaces  9   a  and  26  of two connected panels  2  and  3 , which face the upper side, are always in definite contact. In practice, simultaneous exact positioning of convex curvature  7  of projection  6  of panel  3  against concave curvature  23  of recess  20  of panel  2  is impossible. Manufacturing tolerances would lead to a situation where either abutting joint surfaces  9   a  and  26  are positioned exactly against each other or projection  6 /recess  20  are positioned exactly against each other. In practice, the positive-fit profiles are thus designed in such a way that abutting joint surfaces  9   a  and  26  are always exactly positioned against each other and projection  6 /recess  20  cannot be moved far enough into each other to achieve an exact fit. However, as the manufacturing tolerances are in the region of hundredths of a millimetre, projection  6 /recess  20  also fit almost exactly. 
     Panels  2  and  3 , with described complementary positive-fit profiles  4  and  5 , can be fastened to each other in a variety of ways. According to FIG. 3, one panel  2  with a recess  20  has already been laid, while a second panel  3 , with a complementary projection  6 , is being inserted into recess  20  of first panel  2  at an angle in the direction of arrow P. After this, second panel  3  is pivoted about the common centre of circle K of the segments of a circle of convex curvature  7  of projection  6  and concave curvature  23  of recess  20  until second panel  3  lies on base U. 
     Another way of joining the previously described panels  2  and  3  is illustrated in FIG. 4, according to which first panel  2  with recess  20  has been laid and a second panel  3  with projection  6  is slid in the installation plane and perpendicular to positive-fit profiles  4  and  5  in the direction of arrow P until walls  21  and  22  of recess  20  expand elastically to a small extent and convex curvature  7  of projection  6  has overcome the undercut at the front end of concave curvature  23  of the lower wall and the final laying position is reached. 
     The latter joining method is preferably used for the short edges of a panel if these are provided with the same complementary positive-fit profiles  4  and  5  as the long edges of the panels. 
     FIG. 5 illustrates fastening system  1  in use. Panels  2  and  3  are laid on an uneven base U. A load has been applied to the upper side of first panel  2  with positive-fit profile  5 . The edge of panel  2  with positive-fit profile  5  has been lifted as a result. Positive-fit profile  4  of panel  3 , which is connected to positive-fit profile  5 , has also been lifted. Joint G results in a bend between the two panels  2  and  3 . The spaces for movement  13  and  25  create room for the pivoting movement of the joint. Joint G, formed by the two panels  2  and  3 , has been moved slightly upwards out of the installation plane. Space for movement  13  has been utilised to the full for pivoting, meaning that the area of bevel  12  on the upper side of projection  6  of panel  3  is in contact with the inner side of wall  22  of panel  2 . The point of connection is inherently flexible and does not impose any unnecessary, material-fatiguing bending loads on the involved positive-fit profiles  4  and  5 . 
     The damage soon occurring in positive-fit profiles according to the prior art, owing to the breaking of the projection or the walls of the positive-fit profiles, is avoided in this way. 
     Another advantage results in the event of movement of the joint in accordance with FIG.  5 . This can be seen in the fact that, upon relief of the load, the two panels drop back into the installation plane under their own weight. Slight elastic deformation of the walls of the recess is also present in this case. This elastic deformation supports the panels in dropping back into the installation plane. Only very slight elastic deformation occurs because the pivot of the joint, which is defined by curvatures  7  and  23  with the form of a segment of a circle, is located within the cross-section of projection  6  of panel  3 . 
     FIG. 6 illustrates articulated movement of two laid panels  2  and  3  in the opposite sense of rotation. Panels  2  and  3 , laid on uneven base U, are bent downwards. The design is such that, in the event of downward bending of the point of connection out of the installation plane towards base U, far more pronounced elastic deformation of lower wall  21  of recess  20  occurs than during upward bending out of the installation plane. This measure is necessary because downward-bent panels  2  and  3  cannot return to the installation plane as a result of their own weight when the load is relieved. However, the greater elastic deformation of lower wall  21  of recess  20  generates an elastic force which immediately moves panels  2  and  3  back into the installation plane in the manner of a spring when the load is relieved. 
     In the present form, the previously described positive-fit profiles  4  and  5  are integrally moulded on the edges of panels  2  and  3 . This is preferably achieved by means of a so-called formatting operation, where the shape of positive-fit profiles  4  and  5  is milled into the edges of panels  2  and  3  in a single pass by a number of milling tools connected in series. Panels  2  and  3  of the practical example described essentially consist of MDF board with a thickness of  8  mm. The MDF board has a wear-resistant and decorative coating on the upper side. A so-called counteracting layer is applied to the underside in order to compensate for the internal stresses caused by the coating on the upper side. 
     Finally, FIG. 7 shows two panels  2  and  3  in laid condition, where fastening system  1  is used with a filler  30  that remains flexible after curing. Filler  30  is provided between all adjacent parts of the positively connected edges. In particular, top-side joint  31  is sealed with the filler to prevent the ingress of any moisture or dirt. In addition, the elasticity of filler  30 , which is itself deformed when two panels  2  and  3  are bent, brings about the return of panels  2  and  3  to the installation plane. 
     FIG. 8 shows special holding profiles, which are provided for the short edges of panels  40  and  41 . The opposite, short sides of each panel have matching holding profiles  42  and  43  with complementary hook elements  44  and  45 . In this way, a right-hand holding profile  42  of a first panel  40  can always be connected to a left-hand holding profile  43  of a second panel  41 . FIG. 8 shows the short edges of panels  40  and  41  in connected position. Hook element  44  is formed by a web  46 , which protrudes roughly perpendicularly from the short edge and is located on the upper side of the panel O. In this context, the free end of web  46  is provided with a hook projection  47  facing the underside V of panels  40  and  41 . Hook projection  47  is engaged in a hook projection  48  of second panel  41 . Hook element  45  of second panel  41  is formed by a web  49 , which protrudes from the edge of second panel  41  and is located on the underside V of second panel  41 . Hook projection  48  is located on the free end of web  49  and faces the upper side O of panel  40 . Hook projections  47  and  48  of the two panels  40  and  41  are hooked into one another. 
     When the second panel  41  is installed, hook projection  48  of second panel  41  with lower-side web  49  contacts upper-side web  46  of first panel  40 . For the purpose of definite contact, a space L 1  is provided in the present configuration between hook projection  47  of upper-side web  46  of first panel  40  and lower-side web  49  of second panel  41 . 
     According to FIG. 8, holding surfaces  50  and  51  of hook projections  47  and  48  engage one another in such a way that hook projections  47  and  48  can only hook into one another by elastic deformation. An opening, which is formed between inside surface  52  of holding profile  43  of second panel  41  and the opposite holding surface  50  of hook projection  48 , has a width a at its narrowest point. This width is less than width b of hook projection  47  of first panel  40  at its widest point. Due to this design, and due to the elastic deformation during connection of hook projections  47  and  48 , complementary hook projections  47  and  48  snap together into a defined end position. In the present configuration, holding surfaces  50  and  51  of hook projections  47  and  48  are of simple form and designed as angled, plane surfaces. Hook projections  47  and  48  taper from the free ends towards webs  46  and  49 . In the present practical example, holding surface  51  of hook projection  47  of first panel  40  is rounded on the upper and lower end, as shown in FIG.  8 . The same applies to holding surface  50  of hook projection  48  of second panel  41 . This facilitates the insertion of hook projections  47  and  48 , in that hook profiles  42  and  43  are slowly expanded in elastic fashion during a connecting movement that is perpendicular to the plane of installation. This facilitates installation and spares holding profiles  42  and  43 . 
     Abutting holding surfaces  50  and  51  of interacting panels  40  and  41  thus press against one another in certain areas. The resulting spaces can advantageously serve as glue pockets  53 . Furthermore, a space L 2  is provided between front side  54  of lower-side hook projection  48  of second panel  41  and inside surface  55  of first panel  40 . The resulting intermediate space can likewise serve as glue pocket  53 . The same applies to front side  56  of upper-side hook projection  47  of first panel  40 , which, when assembled, contacts second panel  41  at least in the region of the upper side of the panel O. In the present practical example, an intermediate space, which is likewise designed as a glue pocket  53 , expands from below upper side of the panel O towards the inside of the connection. 
     A second configuration of a fastening system is illustrated in FIG.  9 . It shows the same technical features with the same reference numbers as in FIG.  8 . The configuration according to FIG. 9 differs from the practical example in FIG. 8 in that, of the two pairs of web  49 /hook projection  47  and web  46 /hook projection  48 , the pair in contact and the pair with a space L 1  are reversed. The basic function of the fastening system remains the same. Hook projection  47  is again in definite contact and the surface of the floor covering has no gaps. 
     FIG. 10 shows a schematic diagram of a panel  41  with a holding profile  43  according to the invention. It shows schematically how the undercut contour of hook projection  48  can be manufactured with the help of two cutting tools W 1  and W 2 , which rotate about axes X 1  and X 2 . Tools W 1  and W 2  create recess  57 , into which a complementary hook projection of another panel (not shown) can be snapped. 
     Finally, FIG. 11 shows an alternative configuration with special complementary holding profiles  60  and  61  on the short edges of panels  62  and  63 . Hook elements  64  and  67  are again provided, which have webs and hook projections as in the configurations above. The configuration according to FIG. 11 is designed such that front side  75  of lower-side hook element  64  of second panel  63  has a protruding snap element  65  on its free end, which engages an undercut recess  66  of upper-side hook element  67  of first panel  62 . Hook elements  64  and  67  can be snapped together by applying slight pressure and undergoing elastic deformation. Panels  62  and  63  are locked perpendicular to the installation plane by snap element  65  that engages recess  66 . The locking of panels  62  and  63  to prevent them from being pulled apart in their longitudinal direction is achieved by holding surfaces  68  and  69 , which are provided on hook projections  70  and  71  of hook elements  64  and  67 . 
     In the configuration shown, protruding snap element  65  of second panel  63  is designed as a ridge that extends over the entire length of the edge. Undercut recess  66  of first panel  62  is designed as an elongated groove, which receives the ridge in the connected position. The ridge and the groove can be milled in a single manufacturing step by a process known as formatting. In order to connect panels  62  and  63 , the ridge and the groove must be inserted into one another by elastically deforming hook elements  64  and  67 . 
     FIG. 12 shows another configuration, which is based on the configuration in FIG.  11 . In this context, the same features in the two figures are designated by the same reference numbers. Compared to the configuration in FIG. 11, the configuration according to FIG. 12 is designed such that front side  72  of upper-side hook element  67  of first panel  62  also has a protruding snap element  73  on its free end, which engages an undercut recess  74  of lower-side hook element  64  of second panel  63 . In order for hook elements  67  and  64  to snap together, somewhat greater pressure must be exerted than in the practical example according to FIG.  11 . Panels  62  and  63  are locked together more firmly than in the configuration according to FIG. 11 due to snap element  65  engaging recess  66  and the additional snap element  73  engaging recess  74 . Protruding snap elements  65  and  73  of panels  62  and  63 , respectively, are designed as ridges that extend over the entire length of an edge. Of course, the ridge on a hook projection  64  or  67  can also be replaced, for example, by a protruding nose with a bevel (not shown), where the bevel of the nose is oriented such that the corresponding hook element is gently expanded as the connection procedure progresses. Undercut recesses  66  and  74  of panels  62  and  63  are designed as elongated grooves, which receive the ridges in the connected position. The ridges and the grooves can be milled in a single manufacturing step by a process known as formatting. In order to connect panels  62  and  63 , the ridge and the groove must be inserted into one another by elastically deforming hook elements  67  and  64 . The practical examples in FIGS. 11 and 12 also differ in reference to the interaction of webs  46 ,  49  with hook projections  71 ,  70 . According to FIG. 11, web  46  contacts hook projection  71  and a space is provided between hook projection  70  and web  49 . According to FIG. 12, a space is provided between web  46  and hook projection  71  and hook projection  70  contacts web  49 . 
     LIST OF REFERENCE NUMBERS 
       1  Fastening system 
       2  Panel 
       3  Panel 
       4  Positive-fit profile 
       5  Positive-fit profile 
       6  Projection 
       7  Convex curvature 
       8  Part of the edge 
       9  Part of the edge 
       9   a  Abutting joint surface 
       10  Top edge 
       11  Section 
       12  Bevel 
       13  Space for movement 
       20  Recess 
       21  Lower wall 
       22  Upper wall 
       23  Concave curvature 
       24  Bevel 
       25  Space for movement 
       26  Abutting joint surface 
       30  Filler 
       31  Top-side joint 
       40  Panel 
       41  Panel 
       42  Holding profile 
       43  Holding profile 
       44  Hook element 
       45  Hook element 
       46  Web 
       47  Hook projection 
       48  Hook projection 
       49  Web 
       50  Holding surface 
       51  Holding surface 
       52  Inside surface 
       53  Glue pocket 
       54  Front side 
       55  Inside surface 
       56  Front side 
       57  Recess 
       60  Holding profile 
       61  Holding profile 
       62  Panel 
       63  Panel 
       64  Hook element 
       65  Snap element 
       66  Recess 
       67  Hook element 
       68  Holding surface 
       69  Holding surface 
       70  Hook projection 
       71  Hook projection 
       72  Front side 
       73  Snap element 
       74  Recess 
       75  Front side 
     G Joint 
     K Centre of circle 
     O Upper side of the panel 
     P Arrow 
     U Base 
     V Underside