Patent Publication Number: US-8966853-B2

Title: Panel and fastening system for such panel

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a Continuation Application of U.S. patent application Ser. No. 13/242,244 filed Sep. 23, 2011, which was a Continuation Application of U.S. patent application Ser. No. 11/780,322 filed Jul. 19, 2007 (now U.S. Pat. No. 8,024,904) which is a Divisional Application of U.S. patent application Ser. No. 10/486,457 filed Mar. 29, 2004 (now U.S. Pat. No. 7,451,578), which was a National Stage Entry of International Application No. PCT/DE2002/02444 filed Jul. 4, 2002, which claimed the priority of DE 101 38 285.5-25 filed Aug. 10, 2001, the priorities of these Applications are claimed and these Applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a fastening system for rectangular, tabular panels, especially floor panels, comprising retaining profiles disposed on the small faces of said panels. Opposite retaining profiles match said retaining profiles in such a manner that similar panels can be interlinked. The retaining profiles are designed as first retaining profiles on opposite small faces and as second retaining profiles on the remaining small faces, in such a way that, on a panel in a first line, a new panel can be locked in a second line by attaching the new panel to the installed panel at a temporary angle relative to the installed panel and then swiveling it down into the plane of the installed panel, where the opposite second retaining elements display corresponding hook elements and where a hook connection can be established by means of one of the hook elements of the new panel and a hook element of a panel that is already installed in the second line by swiveling down the new panel. Each hook connection is associated with an additional, loose locking element that, in the hooked state of two panels, prevents release of the hook connection in a direction perpendicular to the plane of the installed panels, provided that the locking element is located in a locking groove of one of the hook elements of a first panel and the locking groove is provided on a surface of the hook element that, in the installed state of the panels, is oriented approximately perpendicular to the plane in which the panels are installed. 
     BACKGROUND OF THE INVENTION 
     A fastening system without an additional locking element is known from DE 199 29 896 A1. A characteristic feature of a fastening system of this kind is that the first and second retaining profiles used have very different geometries and that the joining methods of the different types of retaining profile differ greatly as a result. In particular, the second retaining profiles, which are designed as hook elements and joined to form a hook connection, involve a technical problem. While the familiar hook connection effectively prevents floor panels from being slid apart in the plane at right angles to the small faces of the connected panels, it does not afford satisfactory resistance to the hook elements being released in a direction perpendicular to the plane of the installed panels. 
     A fastening system of this kind is used with preference for so-called laminate flooring that displays a core made of a wood material, such as MDF, HDF or particleboard material. The mechanical retaining profiles are generally milled into the small faces of boards made of wood materials. 
     Laminate flooring is predominantly laid in floating fashion. To reduce footstep sound, a footstep sound-insulating intermediate layer is usually laid between the base surface and the laminate panels. It is also known for a footstep sound-insulating layer to be permanently attached to the underside of laminate panels that faces the base surface. 
     The hook connection of the known fastening system is particularly problematic if, in the region of a hook connection, only the panel whose hook element is at the bottom, i.e. facing the base surface, is exposed to a high load. The upper hook element of the adjacent panel, hooked into the bottom hook, is not exposed to loading, as a result of which the load only presses the panel with the hook element at the bottom into the generally soft footstep sound-insulating intermediate layer. The top hook element of the non-loaded panel releases itself from the bottom hook element of the adjacent panel in the process. The hook connection ceases to function, and the function usually cannot be restored. 
     According to the prior art, undercuts are integrated in the hook connection, by means of which releasing of the hook connection perpendicular to the installation plane of the panels is said to be prevented. These undercuts have, however, proven to be insufficient to afford this kind of fastening element sufficient strength. 
     A generic fastening system displaying an additional locking element in known from WO 01/51732 A1. Following interlocking of the hook elements, the locking element has to be subsequently inserted at the point of connection. This requires an additional work step. Moreover, fitting of the locking element causes problems if a row of panels is located very close in front of a wall. Insufficient space is then available for the locking element to be inserted at the point of connection. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to develop a fastening system that is equipped with a hook connection that does not release itself even when the panel with the bottom hook element is subjected to a load and the upper hook element of the adjacent panel is not loaded. 
     According to the invention, the object is solved in that each hook connection is associated with an additional locking element that prevents, in the hooked state of two panels, the hook connection from being released in a direction perpendicular to the plane of the installed panels. 
     This innovative locking element can be a very simple component, for which there are various designs. The locking element can be supplied loose with panels, so that the layer himself can attach it at the envisaged point on the hook element during installation, after hooking the hook elements together or it can be already premounted for the layer when he installs the panels. Consequently, the layer does not have to work with any loose connecting parts. Alternatively, a locking element can be integrated into the core material of the panel. 
     In a particularly simple embodiment, each of the hook elements of the opposite small faces of a panel displays a locking groove extending in the longitudinal direction of the small face. When the hook elements are connected, the locking grooves of two panels are adjacent to each other, forming a common locking recess. When profiling the hook elements with milling tools, the locking grooves can be included in the milling process very easily. Appropriate contours must be provided on the milling tools for this purpose. 
     If the locking recess has a round or rectangular cross-section, this has the advantage that particularly inexpensive standard material can be used for the locking elements. Any desired material is open to consideration for locking elements with a round or rectangular cross-section. Corresponding rod material can, for example, be purchased in ready-made form, or by the meter for cutting into locking elements of appropriate length. For locking grooves that together form a locking recess with a round cross-section, it is particularly favorable if a nail is used as the locking element, or if the locking element is designed in the manner of a nail. The head of a nail facilitates not only insertion of the locking element into the hook connection during installation, but also removal of the locking element if the hook connection needs to be released again for the purpose of disassembling the panels. 
     It is very useful if in the hooked state of two panels, the locking element can simply be inserted into the locking recess wherein the cross-section of the locking element at least partially protrudes into the cross-section of the locking groove of the one panel and partially into the cross-section of the locking groove of the other panel. The division of the cross-section between the locking grooves can be selected virtually at random. It can, for example, be made dependent on whether one of the hook elements in which the locking groove is located is designed more stably than the other. The locking element can be inserted into the locking recess by sliding or hammering. The tolerances of the locking element and the locking recess can be designed in such a way that the locking element can be inserted into the locking recess either loosely or tightly. 
     Preferably, when a simple locking element with a round or polygonal cross-section is involved, it is advantageous for the locking grooves to be provided on hook element surfaces that, in the installed state of the panels, are oriented roughly perpendicular to the plane in which the panels are installed. 
     An alternative design of a fastening system with hook elements provides for each panel, in installed state, to display undercut bottom recesses on the underside facing the base surface, at least one of which is located in the region of each hook element on the underside of the panel. 
     The handling can be improved if, in the connected state of two panels, the undercut of the bottom recess of a first panel is oriented in the opposite direction to the undercut of the bottom recess of a second panel. For the fastening system design with bottom recesses in the region of the hook elements, the locking element is expediently designed as a U-shaped clip that, in assembled state, engages the undercut of the bottom recess of the first panel and the undercut of the bottom recess of the second panel. 
     In a third alternative for a fastening system, the locking element is located in a locking groove of one of the hook elements of a first panel and displays a resilient snap tab. In this context, a locking groove of the associated hook element of the opposite small face of a second panel forms an undercut snap-in depression, into which the snap tab of the hook element of the first panel snaps automatically during installation. 
     This design can be provided straightforwardly with a locking element displaying a snap tab projecting far beyond the small face in relaxed state, which snap tab comes into contact with the hook element of the adjacent panel during downward swiveling of a new panel into the plane of the installed panels and is automatically bent so far back that the snap tab no longer projects beyond the outer end of the hook element on the small face. When the hook connection has almost reached its locking position, the snap tab automatically springs forwards into the snap-in depression of the hook element of the adjacent panel, locking the hook connection in the vertical direction, i.e. perpendicular to the plane of the installed panels. 
     An automatic snap element of this kind can be pre-mounted in one of the hook elements, or enclosed separately so that the layer can attach it to the envisaged hook element himself during installation of the panels. 
     The automatic locking element and the snap-in depression are expediently designed in such a way that the locking element can easily be pulled out of the hook connection in the longitudinal direction of the small faces at any time using a simple tool, such as pointed pliers, if the panels need to be disassembled. To this end, a free space is provided on either side of the snap tab, so that pliers can be applied. 
     The fundamental advantage of locking by means of a snap-in locking element, as opposed to a locking element to be slid in, is that no space is required in front of the small face of a row of panels for positioning the locking element against a locking recess and sliding it into said recess. Close to a wall, a locking element that needs to be slid in can no longer be inserted into a locking recess, whereas the snap-in locking element can easily be attached laterally to one of the hook elements and locked by swiveling down a new panel. 
     A fourth alternative for the fastening system provides for the locking element to be designed as a claw component which, in installed state, is located between interlocking hook surfaces of the hook elements. The claw component has claw elements that grip into the surface of the hook surfaces and prevent vertical separation of the hook elements. 
     Preferably, a space for the claw component is provided between the interlocking hook surfaces in order to avoid excessive constraining forces between the hook elements. 
     Furthermore, it is useful if the claw component is locked in a recess provided in the hook element for this purpose and, in the assembled state of the hook connection, adapts to the hook element, starting at the recess and extending over the hook surface. 
     Straightforwardly, the recess for the claw component can be located on the part of the hook element that engages the corresponding hook element, where the opening of the recess is located on a surface of the hook element that faces the base surface and where, in the assembled state of the hook connection, the claw component is bent in such a way that it projects between the engaging hook surfaces. 
     Handling of the claw component is facilitated by the fact that it is of L-shaped design and that a first leg of the L-shaped claw component can be inserted in the recess of the hook element and a second leg, provided with the claw elements, points in the direction of the hook surface of the same hook element. In this context, the latter leg is automatically bent into the space between the engaging hook surfaces during installation. 
     The purpose of a further useful improvement is that one and the same locking recess formed by locking grooves of two hook elements can serve to accommodate different locking elements that display different geometries and afford the hook connection the necessary strength by means of different locking mechanisms. The locking grooves and locking elements are specially coordinated with each other to this end. In this context, either a rod-shaped locking element can be slid into the locking recess in its longitudinal direction or, alternatively, a locking element displaying a resilient snap tab can be accommodated in the same locking recess, where one of the locking grooves then forms a retaining mount for the locking element provided with the snap tab, and the associated locking groove forms an undercut snap-in depression that the resilient snap tab engages automatically during assembly of the hook connection. 
     A panel with a fastening system according to the invention displays two different types of interacting retaining profiles. The retaining profiles via which the individual rows of a laid floor are interlocked display retaining profiles that are interlocked according to the principle of inclined attachment of a new panel with subsequent downward swiveling of the same. The type of retaining profile required for this makes it possible to mechanically interlock a new panel on a row of installed panels by means of a hinge-like swiveling movement. As a result, the individual panel rows are protected against being pulled apart in the plane in a direction perpendicular to the interlocked retaining profiles. 
     On the remaining two small faces of the panel, retaining profiles are provided in the form of hook elements, where a first hook element projects from the small face and, in the installed state, faces the base surface, and the second hook element projects from the small face and faces the decorative top side of the panel. Both hook elements of a hook connection are protected against moving apart in a direction perpendicular to the plane of the installed panels by an additional locking element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of the invention is illustrated below in a drawing and described in detail on the basis of the Figures. The Figures show the following: 
         FIG. 1  illustrates a perspective view of a retaining profile that can be mechanically interlocked by attaching a new panel at an angle and subsequently swiveling it down into the plane of the installed panels; 
         FIG. 2  illustrates the attachment at an angle of the retaining profiles according to  FIG. 1 ; 
         FIG. 3  illustrates the retaining profiles according to  FIG. 1  in the interlocked state; 
         FIG. 4  illustrates retaining profiles in the form of hook elements according to the prior art; 
         FIGS. 5-10  illustrate embodiments of a hook connection with one or more additional locking elements with a rectangular cross-section; 
         FIGS. 11-14  illustrate a design of a hook connection with one or more additional locking elements displaying a round cross-section; 
       FIGS.  15 / 16  illustrate embodiments of a hook connection with locking elements that, in the installed state of the panels, are recessed into hook element surfaces that lie in a roughly horizontal position; 
         FIG. 17-20  illustrates an embodiment of a hook connection with a locking element with a resilient snap tab that automatically engages an associated snap-in depression during assembly of the hook connection; 
         FIG. 21  illustrates a hook connection with a locking element designed as a claw component; 
         FIG. 22  illustrates a hook connection with a locking element designed as a clip on the underside of the panels; 
         FIG. 23  illustrates a further locking element with a resilient snap tab and a locking groove adapted to accommodate the locking element; 
         FIG. 24  illustrates a hook connection with the locking element according to  FIG. 23  during the connecting procedure; 
         FIG. 25  illustrates a hook connection with the locking element according to  FIG. 23  in engaged state; and 
         FIG. 26  illustrates a hook connection with the same locking grooves and the same locking recess as according to  FIG. 25 , where the snap tab locking element is replaced by a locking element with a round cross-section. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
       FIG. 1  of the drawing shows a perspective view of one type of retaining profile for fastening system  1  according to the invention. The opposite small faces of panels  2  and  3  are provided with corresponding retaining profiles, so that adjacent panels  2  and  3  can be connected to each other. This type of retaining profile is a modified tongue-and-groove joint, where tongue  4  engages an undercut in the lower groove wall of groove  5 , so that, in the installed state, the two panels  2  and  3  are protected against being pulled apart in the plane of installed panels  2  and  3  and perpendicular to the direction of the: interlocked small faces. 
       FIG. 2  shows the attachment of a new panel  2  at an angle. In this context, tongue  4  of new panel  2  is always engaged with groove  5  of installed panel  3  in the direction of arrow P 1 , and new panel  2  is subsequently swiveled down onto base surface V until the position illustrated in  FIG. 3  is reached. It is easily comprehensible that a curved area  4   a  of the cross-section of tongue  4  engages a depression  5   a  with a curved cross-section in bottom groove wall  5   b  of groove  5  in such a way that panels  2  and  3  are prevented from being slid apart in the plane perpendicular to the interlocked small faces. 
     The remaining small faces of a panel  2  or  3 , equipped with fastening system  1  according to the invention, are provided with corresponding retaining profiles with hook elements  6  and  7 . These have the advantage that they hook into each other simultaneously, as it were, with the interlocking of the retaining profiles described according to  FIGS. 1 to 3 , tongue  4  and groove  5 , following attachment at an angle, as a result of new panel  2  being swiveled down onto base surface V. No lateral joining movement of any kind whatsoever is necessary to establish resultant hook connection  8 . 
     Hook connection  8 , illustrated in  FIG. 4 , disengages. This occurs, for example, on uneven substrates, where there is air between the panels and base surface V, and also when a soft, footstep sound-insulating intermediate layer  9  is located between the panels and base surface V. The symbolically represented weight  11  in  FIG. 4  illustrates how a panel whose hook element faces the base surface sinks into soft, footstep sound-insulating intermediate layer  9  when exposed to the load of weight  11 . This results in vertical offset  12  at the surface of panels  2  and  10 . 
       FIGS. 5 to 10  illustrate different embodiments of hook connections  8 , all of which are interlocked with an additional locking element  13 . Locking element  13  prevents hook connection  8  moving apart in a direction perpendicular to the plane of installed panels  2  and  10 . Even when loaded in accordance with  FIG. 4 , additional locking element  13  prevents vertical offset of hooked panels  2  and  10 . Locking element  13  has a rectangular cross-section in the embodiments in  FIGS. 5 to 10 . Locking grooves  14  and  15  are provided to accommodate locking element  13  and, in the hooked state of hook elements  6  and  7 , are positioned exactly opposite each other in such a way that a common locking recess  16  is formed, into which locking element  13  is inserted in a direction perpendicular to the drawing plane shown. The embodiment according to  FIG. 5  shows a free space  17  between the free end of hook element  7 , which faces base surface V, and the small face of associated panel  2 . 
     In  FIG. 6 , on the other hand, no clearance is provided at the same point. Instead, an undercut connection  18  is provided here, which likewise interlocks in a direction perpendicular to the plane of installed panels  2  and  10 . On panel  2 , whose hook element  6  faces the surface, a freely projecting surface of the small face of hook element  6  displays locking groove  14 , while locking groove  15  of opposite hook element  7  of hook connection  8  is provided on a receding surface  19  of hook element  7 . The same applies to the embodiment according to  FIG. 6 . 
       FIGS. 7 and 8  show examples of a hook connection  8 , where the small face is provided with additional locking element  22  accommodated in locking groove  15  on freely projecting surface  20  of hook: element  7 , which faces base surface V. Accordingly, locking groove  14  on corresponding hook element  6  is located on receding surface  21  of hook element  6  on the small face of panel  2 .  FIG. 7  shows an example where hook elements  6  and  7  display a free space  17  in the region of locking element  22 . In contrast, according to  FIG. 8 , no free space is provided between hook elements  6  and  7  in the region of locking element  22 . Instead, undercut connection  18  increases the resistance of hook connection  8  to being pushed apart in a direction perpendicular to the plane of installed panels  2  and  10 . 
       FIGS. 9 and 10  show designs in which each hook connection  8  is equipped with two locking elements  13  and  22 . According to  FIG. 9 , the positions of locking elements  13  and  22  are taken from a combination of  FIG. 5  and  FIG. 7 . In  FIG. 10 , the positions of locking elements  13  and  22  are taken from a combination of  FIGS. 6 and 8 .  FIG. 9  is a further example of a hook connection  8  where the free end of hook element  7  facing base surface V displays a space  17  between it and the small face of hook element  6  of adjacent panel  2 , whereas  FIG. 10  provides an undercut connection  18  at the same point. 
     According to  FIGS. 11 to 14 , locking elements with a round cross-section are provided. According to  FIG. 11 , an integral locking groove  23  with a semi-circular cross-section is provided on the outer free end of hook element  6 , which faces the upper side of panel  2 . On corresponding hook element  7 , a locking groove  24  is provided accordingly on receding surface  19  of hook element  7 , meaning that the two locking grooves  23  and  24  together produce locking recess  25  with a circular cross-section, in which locking element  26  is located. The same applies to the embodiment according to  FIG. 12 . In  FIG. 11 , a free space  17  is provided between hook element  7  of the one panel  10 , which faces base surface V, and a receding surface  21  of hook element  6  on the small face of adjacent panel  2 , whereas an undercut connection  18  is provided at the same point according to  FIG. 12 . 
     According to  FIG. 13 , the location for circular locking element  27  is, in contrast to  FIG. 12 , shifted to the free end of hook element  7  of panel  10 . Locking groove  28  of corresponding hook element  6  is accordingly provided on receding surface  21  on the small face of adjacent panel  2 .  FIG. 13  is an example illustrating that locking element  27  can be provided at a location where a free space  17  is provided between the outer free end of bottom hook element  7  and opposite surface  21  of corresponding hook element  6 . An embodiment without free space  17 , with plane surfaces lying flat against each other, and an embodiment with an undercut connection  18  according to bottom hook element  7  in  FIG. 12 , can likewise be equipped with a locking recess and a locking element  27 . 
       FIG. 14  illustrates a particularly stable hook connection  8 , where two locking elements  26  and  27  with a circular cross-section are used. The locations of locking elements  26  and  27  constitute a combination of the embodiments according to  FIG. 11  and  FIG. 13 . 
       FIGS. 15 and 16  show the installed state of panels  2  and  10  with completed hook connection  8 . In this context, locking grooves  30 ,  31 ,  32 , and  33  are provided in surfaces that lie roughly parallel to the plane of the installed panels. Again, locking grooves  30  and  32  of the one hook element  6 , and locking grooves  31  and  33  of corresponding hook element  7 , are positioned in such a way that they are exactly opposite each other and together each form a locking recess in which a locking element  34  or  35  is located. According to both  FIG. 15  and  FIG. 16 , it is possible to dispense with one of locking elements  34  and  35 , and the corresponding locking recess, in order to simplify the geometry. Since locking elements  34  and  35  have to prevent hook connection  8  moving apart in a direction perpendicular to the plane of panels  2  and  10 , locking elements  34  and  35  are designed in such a way that they jam themselves laterally into the groove walls of locking grooves  30 ,  31 ,  32  and  33 . In the practical examples illustrated, claw elements  34   a  and  35   a  are provided for this purpose, projecting from the surface of the locking elements. These claw elements can also be designed in the manner of barbs, where the barbs in the one locking groove  30  and the barbs in the opposite locking groove  31  of the same locking recess are oriented in opposite directions. The same applies to the barbs in locking grooves  32  and  33 . 
     A further embodiment of hook connection  8  is illustrated in  FIGS. 17 to 20 .  FIG. 17  shows a loose locking element  36  with a resilient snap tab  37 , which is wide open in the relaxed state illustrated.  FIG. 18  shows the gradual joining of hook connection  8  according to the direction of arrow P 2 . In this context, locking element  36  according to  FIG. 17  is inserted in a groove  38  in the freely projecting surface  38   a  of upper hook element  6 . Resilient snap tab  37  is bent back by the joining movement itself. As soon as hook connection  8  almost reaches the locking position shown in  FIG. 19 , snap tab  37  of locking element  36  automatically springs into locking depression  39  of corresponding hook element  7 . In the position shown, snap tab  37  is less wide open than in the relaxed position illustrated in  FIG. 17 , meaning that it permanently exerts spring pressure on locking depression  39 , thus securely locking hook connection  8 . 
     A floor layer can insert locking element  36  as a loose element into groove  38 , provided in upper hook element  6 , for this purpose, or it can be pre-mounted on hook element  6  by the manufacturer. Locking element  36  can extend over the entire length of the small face of a panel, or over only part of the length of the small face. In the practical example, it starts at one end of the small face and extends over half its length. 
       FIG. 19  shows that free spaces are present on both sides of snap tab  37 . These can be used, for example, to pull locking element  36  out of hook connection  8  with the help of pointed pliers, and thus to release hook connection  8 , for the purpose of disassembling panels  2  and  10 . 
       FIGS. 18 and 19  show a design where the outer end of hook element  7 , which faces base surface V, displays a free space  17  between it and corresponding hook element  6 . 
     A further embodiment of the hook connection with a locking element  36  displaying an automatic snap tab  37  is illustrated in  FIG. 20 . The only difference compared to the embodiment according to  FIG. 18  and  FIG. 19  is that the free outer end of hook element  7  of panel  10 , which faces base surface V, does not display a free space  17  between it and corresponding hook element  6  of connected panel  2 . Instead, an undercut connection  16  is again provided, which, just like locking element  36 , prevents hook connection  8  from moving apart in a direction perpendicular to the plane of installed panels  2  and  10 . 
     According to  FIG. 21 , a different design of fastening system  1  provides a locking element in the form of a claw component  40 , which, in assembled state, is located between interlocking hook surfaces  41  and  42  of hook elements  6  and  7 . Claw component  40  displays claw elements  40   a , which grip into the surface of hook surfaces  41  and  42 , preventing vertical separation of hook elements  6  and  7 . To create space for claw component  40  and to avoid constraining forces between hook elements  6  and  7 , a free space  43  is formed between interlocking hook surfaces  41  and  42 .  FIG. 21  shows claw component  40  in the assembled state of hook connection  8 . Claw component  40  is secured in recess  44  provided for this purpose in hook element  6  and adapts to hook element  6 , starting at recess  44  and extending over hook surface  41 . Recess  44  for claw component  40  is located on the part of hook element  6  that engages corresponding hook element  7 , where the opening of recess  44  is located on a surface  45  of hook element  6  that faces the base surface. In this context, claw component  40  is bent in such a way that it projects into space  43  formed by interlocking hook surfaces  41  and  42 . 
     Claw component  40  is of L-shaped design prior to assembly. A first leg of the L-shaped claw component is inserted in recess  44  of hook element  6 . The second leg is provided with the claw elements and points roughly perpendicularly away from the small face of panel  10  prior to assembly. During assembly, the latter leg is automatically bent into space  43  between interlocking hook surfaces  41  and  42 . 
     The final design of the fastening system according to the invention, illustrated in  FIG. 22 , makes use of a locking element in the form of clip  46 . For this purpose, every panel  2  and  10  has, on its underside facing base surface V, undercut bottom recesses  47  and  48 , one of which is located in the region of each hook element  6  or  7  on the underside of panel  2  or  10 . One clip  46  engages one bottom recess  47  and  46  in each of two adjacent panels  2  and  10 . So that clip  46  prevents hook connection moving apart in a direction perpendicular to the plane of installed panels  2  and  10 , each bottom recess  47  and  48  has an undercut. According to  FIG. 22 , the undercut of bottom recess  47  of a first panel  2  is, in the assembled state of two panels, oriented in the opposite direction to the undercut of bottom recess  48  of a second panel  10 . Clip  46  is of U-shaped design. It is obvious that clip  46  also prevents plane, lateral separation perpendicular to hook elements  6  and  7  of the small faces of panels  2  and  10 , thus supporting the function of hook connection  8 . 
       FIG. 23  shows a locking element  50  with a special cross-section, which can, in practice, be replaced by locking element  51  shown in  FIG. 26 . The latter locking element  51  displays a simple, round cross-section. In addition,  FIG. 23  shows an empty locking groove  52 , in which locking element  50  can be accommodated in captive fashion. This captive design ensures that locking element  50  cannot fall out of locking groove  52  during handling of a panel  2 , and during interlocking of hook connection  8 , according to the direction of arrow P 3 . To permit the exchange of locking elements  50  and  51 , locking grooves  52  and  53 , provided in hook elements  6  and  7 , are specially adapted to the geometry of the different locking elements  50  and  51 . 
     Locking element  50  is a development of locking element  36 , shown in  FIG. 17 . It displays a snap tab  54 , which is shown in wide-open, relaxed state in  FIG. 23 . On back  55 , locking element  50  displays a round form that, according to  FIG. 24 , matches and fits into locking groove  52  of hook element  6 . Locking element  50  is provided with retaining elements  56  and  57 , via which it can be fixed in locking groove  52  of hook element  6  in captive fashion. Retaining elements  56  and  57  additionally serve to prevent slipping or rotation of locking element  50  in locking groove  52  or in locking recess  58 , formed by locking grooves  52  and  53 . In the present embodiment, retaining elements  56  and  57  are designed as blunt toes. It can be seen on empty locking groove  52  in  FIG. 23  that it has material recesses  56   a  and  57   a  at the edges of its semi-circular cross-section, these serving to accommodate retaining elements  56  and  57 . Dimension A indicated on locking groove  52  is slightly smaller than dimension B indicated on locking element  50 . This results in captive clamping of locking element  50  in locking groove  52 . In a different embodiment, the retaining elements of locking element  50  are designed as barbs or claw elements (not shown), which can be fixed in part of the groove wall of locking groove  52  and hold locking element  50  in captive fashion on hook element  6 . The material recesses  56   a  and  57   a  in locking groove  52  are not necessary with this design. 
       FIG. 24  Shows the joining procedure for hook connection  8 . Panel  2  is swiveled down onto base surface V according to the direction of arrow P 3 , as a result of which hook elements  6  and  7  of panels  2  and  10  interlock. It is easily recognizable that locking element  50  is securely retained in locking groove  52  while the hook elements are connected in the manner described. As soon as the free end of snap tab  54  has passed upper edge  53   a  of locking groove  53 , snap tab  54  automatically springs into locking groove  53 , which serves as a locking depression for it and locks the hook connection, as illustrated in  FIG. 25 . 
     LIST OF REFERENCE NUMBERS 
     
         
           1  Fastening system 
           2  Panel 
           3  Panel 
           4  Tongue 
           4   a  Curved area 
           5  Groove 
           5   a  Curved depression 
           5   b  Lower groove wall 
           6  Hook element 
           7  Hook element 
           8  Hook connection 
           9  Footstep sound-insulating intermediate layer 
           10  Panel 
           11  Weight 
           12  Vertical offset 
           13  Locking element 
           14  Locking groove 
           15  Locking groove 
           16  Locking recess 
           17  Space 
           18  Undercut connection 
           19  Receding surface 
           20  Projecting surface 
           21  Receding surface 
           22  Locking element 
           23  Locking groove 
           24  Locking groove 
           25  Locking recess 
           26  Locking element 
           27  Locking element 
           28  Locking element 
           39  Locking groove 
           31  Locking groove 
           32  Locking groove 
           33  Locking groove 
           34  Locking element 
           34   a  Claw element 
           35  Locking element 
           35   a  Claw element 
           36  Locking element 
           37  Snap tab 
           38  Groove 
           38   a  Projecting surface 
           39  Snap-in depression 
           40  Claw component 
           40   a  Claw element 
           41  Hook surface 
           42  Hook surface 
           43  Space 
           44  Recess 
           45  Surface 
           46  Clip 
           47  Bottom recess 
           48  Bottom recess 
           50  Locking element 
           51  Locking element 
           52  Locking groove 
           53  Locking groove 
           53   a  Upper edge 
           54  Snap tab 
           55  Back 
           56  Retaining element 
           56   a  Material recess 
           57  Retaining element 
           57   a  Material recess 
           58  Locking recess 
         A Dimension 
         B Dimension 
         P 1  Direction of arrow 
         P 2  Direction of arrow 
         P 3  Direction of arrow 
         V Base surface