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FIELD OF THE INVENTION 
     The present invention relates, in a first aspect, to a membrane element for covering surfaces, more particularly for covering ceilings or walls. 
     The invention further relates to a method for covering surfaces, more particularly for covering ceilings or walls. 
     Finally, the invention relates to a revetment for a surface consisting of membrane elements of the invention. 
     A generic membrane element comprises a frame and a membrane material stretched over said frame. 
     In a generic method for covering a surface, more particularly for covering a wall or a ceiling, membrane elements comprising a frame and a membrane material stretched over said frame are attached side by side to the surface to be covered. 
     RELATED ART 
     In prior frame systems, the tensile forces necessarily arising from the stretched membrane elements are absorbed over the entire length of the frame edges resulting in bending of the membrane elements. Therefore, relatively thick profiles are required in order to minimize the deformation resulting from this stress. This results in high material usage, high weight, and high costs and is undesirable, particularly in the case of backlit membrane elements, since the thick frame profiles become visibly apparent. In an additional variant known from the prior art, the membrane tension is built up by a spring mechanism disposed inside the frame. This mechanism is technically complex and also takes up considerable space in terms of the width of the profile. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a membrane element and a method for covering surfaces, which membrane element and method permit a surface to be covered with the aid of much thinner frame structures. 
     The membrane element of the type cited above is further developed, according to the invention, in that at least one frame region is bent inwardly as a result of forces exerted on the frame by the membrane material and that tightening units are provided on the inwardly bent frame region to pull or press said frame region outwardly with respect to an abutment, towards the unbent state. 
     The method for covering surfaces of the type described above is further developed, according to the invention, in that the frames of the membrane elements are pre-tensioned by the membrane material, at least one frame region bending inwardly as a result of the forces exerted by the stretched covering material, that the pre-tensioned membrane elements comprising bent frame regions are fixed to the surface to be covered and that for closing the revetment, at least one or more of the membrane elements having inwardly bent frame regions are tightened or pressed together with frames of adjacent membrane elements. 
     One main concept of the invention relates to the fact that it is important to keep the visible width of the profiles as thin as possible. This provides the frame system with a decisive advantage, particularly when used in lighting systems. 
     The inventors then found that the bending occurring in thin frames or at least in some frame regions thereof can be eliminated in a simple manner by pushing or pulling the inwardly bent frame regions outwardly toward an adjacent abutment. 
     An important basic concept of the invention is thus to keep the frames sufficiently thin so as to allow at least parts of the frame to be bent inwardly due to forces exerted on the frame by the membrane material. 
     Finally, a central concept of the invention consists in the provision of tightening units in the region of the inwardly bent frame parts for the purpose of pulling or pushing these frame parts outwardly with respect to an abutment, towards the unbent state. 
     A first important advantage of the invention is that considerably thinner frames than in the prior art are made possible, providing much more freedom, for example in the field of light design. 
     A further important advantage and central concept of the invention is that the assembly, more particularly the opening and/or closing, of the revetment can be carried out from the front and especially from a face side thereof. This considerably facilitates mounting of the revetment of the invention. 
     The frame elements provided by the invention are suitable for covering plane surfaces or surfaces curved in one direction, preferably in the architectural sector, for example, for covering ceilings, walls, or panels for interior and exterior use. The frames described herein can be covered with films, fabrics, particularly coated fabrics, light scattering films or non-woven fabrics of all types. The membrane elements can thus be used for decorative purposes or for lighting and/or illuminating purposes in addition to their covering function. Furthermore, the revetment of the invention and the membrane elements of the invention can have an acoustic function; that is to say, they can serve for damping or reducing noise and sounds. Frequently, a combination of the aforementioned functions is desired. 
     The novelty of the invention thus consists in the manner in which the membranes are tightened, the thinness of the frame elements which is of advantage particularly in the case of backlit surfaces, and finally the ease of mounting and of partially or completely demounting membrane elements once installed. 
     In a particularly preferred variant, the frame of the membrane element is covered on both sides, in which case the membrane material on the first side can be a light scattering material and the membrane material on the second side can be a translucent material. The light scattering material ensures that the optical effect is not adversely affected by dirt, for example, dead insects, and that the contour of the lamps does not show. 
     In preferred embodiments of the revetment of the invention, light sources are present for backlighting the membrane elements. Numerous new designs are possible for this purpose, due to the considerably thinner frames that can be realized by virtue of the invention. 
     In principle, the membrane material can be attached to the frame in any desired way, for example, by implementing a plurality of separate connection points, e.g. clamps. An easily obtained and crease-free connection of the membrane material to the frame is realized by attaching the membrane material to the frame using a Keder. 
     Basically, the frame can be made of known materials suitable for this purpose in their mechanical properties, such as plastics materials. The frame is very advantageously made of aluminum profiles, particularly profiles having a semi-elliptical cross-section. 
     Basically, the abutment can be formed by any mechanically suitable object. Inwardly bent frame regions of an adjacent membrane element serve particularly well as abutments. These inwardly bent frame regions are then pulled or pressed together in the manner proposed by the invention so that the forces occurring in the frames are counter-balanced. The number of tightening units provided on the frame regions is advantageously governed by the length of the frame regions to be pulled or pressed together and additionally by the elasticity of the membrane material. 
     In an alternative variant, edge profiles that may contain, in particular, installatory components such as electrical supply lines or data lines, can also serve as the abutment. 
     In a particularly preferred variant of the membrane element of the invention, the tightening units comprise bolted joints for pulling the bent frame regions toward the abutment. Tightening units of such type are easy and economical to produce and perform their function reliably. In order to keep the production costs for the frame profiles within limits, slot nuts for the bolted joints can be secured in the frame and in addition threaded bolts may be present for screwing the slot nuts together, particularly in opposing frame regions. These slot nuts are basically threaded nuts. 
     In order to pull opposing frame regions together, in a simple exemplary embodiment, the threaded bolt can have a right-hand thread over one end region thereof and a left-hand thread over the other, the slot nuts being provided with corresponding threads. 
     Screwing of the threaded bolts into the slot nuts is easily accomplished when the slot nuts are secured in the frame with a certain degree of play. 
     In a further preferred alternative characterized by particularly reliable operability, a drive wheel, in particular with a toothed wheel, is provided in the frame for turning the threaded bolt. 
     A functionality particularly designed for closing the tightening units is achieved when the threaded bolt has a tapered region for passage through the slot nut, a rear stop for securing it from displacement against the drive wheel and a front stop for securing it from displacement against the slot nut. 
     A recess is advantageously formed in the frame in the region of the tightening units to allow for access to the tightening units with the aid of a tool. This can be achieved in aluminum profiles in an uncomplicated manner known per se. 
     Particularly well-controlled and reliable work is possible when stop means, particularly screws, are provided in the frame in the region of the recess for defined positioning of the tool. 
     Alternatively, the tightening units may comprise clamp-connections. In a simple variant, a clamp-connection is achieved by a separate clamping member that can be fitted on the frame and, in particular, is displaceable. 
     In accordance with the method of the invention, the corner regions of the inwardly bent frame regions of adjacent membrane elements are pushed together by means of at least one clamping piece that can have, in particular, a U-shaped or H-shaped profile by fitting the clamping piece on the corner regions of both frames and then pushing the clamping piece away from the corner region, as a result of which the inwardly bent frame regions of the adjacent membrane elements are pushed together. If appropriate, several such clamping pieces can be provided and fitted on the two frame regions to be joined. 
     For the purpose of cleaning or maintenance, the membrane elements can be removed entirely or merely swiveled out by loosening the tightening units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Additional characteristics and features of the membrane element of the invention, the method of the invention and the revetment of the invention are described below with reference to the accompanying diagrammatic figures, in which: 
         FIG. 1 : is a diagrammatic view of a membrane element of the invention; 
         FIG. 2 : is a sectional view of an exemplary embodiment of a frame profile for a membrane element of the invention; 
         FIG. 3 : is a partially cutout view of the frame profile shown in  FIG. 2  comprising machined sections for accommodating the components of the tightening units; 
         FIG. 4 : is a perspective view of a corner region of a membrane element of the invention; 
         FIG. 5 : shows the frame profile shown in  FIG. 2  comprising attached membrane materials, Keders and slot nuts together with means for positively securing the same; 
         FIG. 6 : shows a revetment of the invention before the membrane elements are pulled together; 
         FIG. 7 : shows a revetment of the invention comprising diagrammatically indicated tightening units in the state in which the membrane elements are pulled together; 
         FIG. 8 : shows an additional exemplary embodiment of a revetment of the invention comprising an edge profile; 
         FIG. 9 : is a detailed view of the connection of the membrane elements to the edge profile; 
         FIG. 10 : shows several views of a drive wheel for a tightening unit; 
         FIG. 11 : shows the arrangement of the drive wheel shown in  FIG. 10  in the frame profile shown in  FIG. 2 ; 
         FIG. 12 : is a perspective view of a tightening bolt for a tightening unit; 
         FIGS. 13 and 14 : show the positioning of the tightening bolt shown in  FIG. 12  in the drive wheel shown in  FIG. 10 ; 
         FIGS. 15 and 16 : show a first method variant for tightening adjacent frame regions together; 
         FIGS. 17 and 18 : show a second method variant for tightening adjacent frame regions together; 
         FIGS. 19 to 21 : show the mode of operation of a tool for actuating the tightening units; 
         FIGS. 22 to 27 : show variants for removing or swiveling out membrane elements for maintenance and/or cleaning purposes; and 
         FIG. 28 : shows an alternative exemplary embodiment of a tightening unit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The basic concept of the invention will now be described with reference to the exemplary embodiments shown diagrammatically in  FIGS. 1 ,  6 , and  7 . In all figures, like components are designated by like reference numerals. 
       FIG. 6  shows diagrammatically a revetment  100  for a surface such as a ceiling, Substantially comprising a plurality of membrane elements  10  of the invention. A membrane element  10  of the invention, shown diagrammatically in a partial sectional view in  FIG. 1  with a light source L and comprises a frame  12  and a membrane material  14  stretched over both sides thereof. The membrane material  14  stretched over the frames  12  of the membrane elements  10  exerts inwardly directed forces on the frames  12  so that frame regions  56  are bent inwardly, as indicated diagrammatically in  FIG. 6 . 
     The central concept of the invention is to provide the inwardly bent frame regions  56  with tightening units  60 , with the aid of which the inwardly bent frame regions  56  of adjacent membrane elements  10  are pulled together or pressed on each other. This is shown diagrammatically in  FIG. 7 , where three tightening units  60  are provided on each of the long sides of the membrane elements  10  and one tightening unit is provided on each of the short sides.  FIG. 7  shows the membrane elements  10  in the state in which they have been pulled or pressed together. The outer membrane elements  10  shown in  FIG. 7  are screwed to an edge profile  62  with the aid of tightening units  60 . 
     The frame profiles of the individual membrane elements are thus screwed together at one or more points so that the tensile forces arising from the cover are absorbed over a short span width. The profiles can thus be designed as thin units, this being an important advantage gained by the present invention. The bolted joint is designed in such a way that it can be activated in spite of the inaccessibility of the frame interior when the frame has been covered on both sides. This activation, for example, can be carried out with the aid of a special tool described in detail below, in a narrow seam between adjacent frames by way of a toothed wheel disposed there. Additional springs or other mechanisms for stretching the membrane are not required since the membrane material is itself elastic and acts as a spring. Stretching of the membrane, as necessary to attain a smooth appearance under realistically occurring external loads and temperature differences, and the necessary dimensional stability are imparted to the assembly when the individual frames are covered. The frame edge, still unsupported at this stage, is deformed as a result of such stretching and bends inwardly. The inwardly curved frame edges, for example of two frames lying opposite to one another, are bolted together during assembly and so the desired linearly stretched shape is achieved. This increases the tension in the membranes to a planned extent. By virtue of the fact that the membrane forces are transferred from one frame to the other by way of a number of distributed bolted joints, these forces are counter-balanced and thus make it possible to maintain the desired thinness of the frame profile. 
     An alternative variant for a revetment  100  is shown in  FIG. 8 . Here, in addition to the outer frame profiles  62 , a supporting and/or frame profile  62  is provided at the center, which is likewise connected to the membrane elements  10  with the aid of tightening devices  60 . 
     The general architecture of the membrane elements  10  will now be described with reference to  FIGS. 1 to 5 . 
     The membrane elements  10  consist of a stabilizing frame  12  and one or two membrane materials stretched over said frame. 
     The frame  12  comprises thin semi-elliptical frame profiles  20  preferably made of extruded aluminum.  FIG. 2  shows a sectional view of one such profile or frame profile  20 . The frame  12  preferably has a rectangular or square shape. In principle, the frame  12  can have virtually any desired shape, in particular the shape of any desired polygon. As can be seen from  FIGS. 6 ,  7 , and  8 , the frames  12  do not have additional reinforcing elements of any kind for transverse or longitudinal reinforcement. 
     The frame profile shown in  FIG. 2  is preferably made of extruded aluminum, the ratio of profile height to profile width preferably ranging from approximately 4:1 to 5:1. The wall thickness in a profile  20  having a height of 80 millimeters is about 2 millimeters. All cavities, bores, and milled portions required for producing, mounting, and carrying out restoration of the membrane elements  10  are provided in the profile  20 . The inwardly oriented side of the profile  20  has a semi-elliptical contour  24 . Cavities  26  are provided in the upper and lower regions for the insertion of corner angular members. A cavity  28  is provided at the center for a slot nut, which is part of a tightening unit described in detail below. Recesses  32  serve to accommodate holding screws for an actuating tool which is to be used on the tightening units and is likewise described below. The upper and lower edge regions of that side of the profile  20  that faces outwardly in the assembled membrane element  10  of the invention are each provided with grooves  22  for a Keder or welt of membrane material, which is yet to be inserted. At points nearer the center, angular members  34  are formed which serve as stops for an adjacent profile. Between these angular members  34  there is a groove  38 , in which a drive wheel for a tightening unit can be accommodated. In the region of the lower stop  34 , milled slots  44  are additionally provided for the accommodation of the drive wheels of the tightening units. Furthermore, a central bore  40  is provided in the profile  20  for the passage of a tightening bolt into a slot nut, and bores  42  are further provided for securing the slot nuts with the aid of suitable pins. 
     The corners of the frame profiles  20  are miter-cut and connected to each other with the aid of angular members in the frame corners to be accommodated in the cavities  26  of the profile. A slot  48  is provided in each of the corner regions of the upper and lower sides of the profiles to allow for folding of overlapping membrane material  14  at the corners after the covering. This is shown diagrammatically in  FIG. 4 . 
     The fixation of the membrane materials on the profiles  20  is shown in  FIG. 5 . Here, a sealing membrane  16  is affixed to the upper side of the profile  20  with the aid of a welt  54  inserted into the groove  22  provided therefor in the profile  20 . In the same way, a face membrane  18  is likewise affixed to the lower side with the aid of a welt  54  inserted into the lower groove  22  of the profile  20 .  FIG. 5  further shows a slot nut  50  which has been inserted into the cavity  28  and is secured therein with the aid of pins  52  that are retained in the holes  42 . 
     The membrane materials used can be textile fabrics, films, and non-woven fabrics of all types depending on the application. The term “face membrane” refers to the membrane that is oriented toward the viewer, for example, inside a room and forms the visible boundary of the room. As shown in  FIG. 5 , a sealing membrane  16  can be disposed on the rear side of a frame  20 , which sealing membrane  16  serves mainly to cover the interior of the membrane elements  10  and protect the same from pollution and the permeation of moisture. Furthermore, a sealing membrane  16  of such type can serve as a diffuser in the case of backlit membranes in order to considerably reduce, though not entirely prevent, the effect of the lamp and the shadows of dirt particles becoming visible. Films or coated fabrics are mainly suitable for a purely covering function. Preferably light scattering films, fabrics and non-woven fabrics are suitable for use as diffusers. Acoustically effective elements, for example sound absorbing elements, can preferably be realized by the use of a perforated face membrane. 
     In order to produce the membrane elements, the frame profiles  20  are first miter-cut according to the desired frame sizes and connected to form a frame  12  with the aid of angle profiles. The face membrane  18  and optionally the sealing membrane  16  are then stretched over the finished frame  12  to form smooth surfaces. The face membrane  18  and the sealing membrane  16  are connected to the frame profile  20 , as shown in  FIG. 5 , by a clamp-connection by a welt  54  or a welt cord preferably made of a soft plastics material such as PVC or TPE or any other suitable material. In lieu thereof, a clamping strip or a clamping rail made of extruded aluminum may be used. 
     The initial tension in the membranes  16 ,  18  as builds up automatically when the membrane materials are stretched over the frames, produces an inward deformation of the thin frame profiles  20 , as shown diagrammatically in  FIG. 6 . The degree of deformation is determined by the pre-tensioning forces introduced and is precisely defined. This deformation is intentional and forms part of the assembly concept described below. 
     According to the invention, means are provided for re-tightening the frames deformed as a result of the membrane pre-tension so as to return them to the originally non-deformed state with the aid of special tightening mechanisms. For this purpose, the adjacent membrane elements  10  or, depending on the arrangement used, a suitable edge profile  62  can serve as the abutment in preferred variants. The membrane pre-tensioning forces are thus counter-balanced and neutralized. After being re-tightened, the frame is restored to its original shape prior to being covered with the membrane material. This is apparent from  FIGS. 7 and 8 . The membrane elements  10  can be arranged in different ways. In a first variant shown in  FIGS. 6 and 7 , an edge profile  62  surrounds an entire array of a plurality of membrane elements  10 . The edge profile  62  serves as an abutment to absorb the tension forces of the membrane elements  10  at the edges of the array. At the same time, the edge profile  62  can also serve as a terminating element or a connecting element for joining one array of membrane elements to an adjacent construction. The edge profile  62  is also preferably produced from commercially available aluminum profiles or canted aluminum or steel sheeting. 
     In the second embodiment shown diagrammatically in  FIGS. 8 and 9 , a supporting profile  62  is additionally provided in the inner region of the overall revetment, which supporting profile can have a U-shape, for example, and is advantageously also made of aluminum. This inner edge profile  62  can serve for accommodating and routing of installation-wirings such as cable looms and water pipes and the integration of sprinkler systems and other technical building installations. It may be advantageous, for this purpose, to provide the channel with a cover  64 . The edge profile  62  here mainly performs the function of absorbing the tension forces of the adjacent membrane elements  10 . Furthermore, this edge profile can also accommodate hinge bolts for pivoting the membrane elements  10 , as is described below in detail. 
     The tightening mechanism and the tightening method will now be explained with reference to  FIGS. 10 to 21 . 
     The tightening units  60 , with the aid of which the pre-tensioning forces of the membrane material  14  are counter-balanced, substantially comprise a threaded bolt or tightening bolt  80 , a drive wheel  70  and a slot nut to be inserted into the edge profiles  20 . A tightening bolt  80 , a drive wheel  70 , and two slot nuts  50  having female threads form a tightening unit  60 . The number of tightening units per unit length or per frame side is governed by the tension forces and the flexural rigidity of the frame profiles  20 . 
     The slot nuts  50  have a thread, into which the tightening bolts  80  can be screwed. The slot nuts  50  are inserted into the cavities  28  of the frame profile  20 , pushed into the correct position and positively secured in position therein on the left and right sides, for example by means of simple metal pins. The slot nuts are preferably mounted with a certain degree of play, which facilitates screwing thereof into the tightening bolt  80 . The slot nuts are preferably made of galvanized steel. 
     The membrane elements  10  are pulled together with the aid of the tightening bolts  80 , by virtue of which the pre-tensioning forces of the membrane elements  10  are counter-balanced and the inwardly bent frame regions  56  of the membrane element  10  are again aligned parallel to each other. A tightening bolt  80 , which is produced in a manner known per se, is shown diagrammatically in  FIG. 12 . At one end, the tightening bolt  80  has a stop  82 , which prevents the drive wheel  70 , described below, from slipping or falling out. Away from this end, the tightening bolt  80  has a flattened region  86 , this being a flat cut of a male thread  84  adapted to engage the slot nuts  50 . The tightening bolt  80  then merges via a taper  88  into a tapered region  87 , the axial length and radial thickness of which are such that this tapered region  87  can freely pass through the opening in the slot nuts  50 . This functionality is explained in more detail below. Finally, the end of the tightening bolt  80  is terminated by a cap nut  89 , which is screwed onto a thread not shown in  FIG. 12  and serves as a stop for a slot nut  50 . 
     The structure of a drive wheel  70  and its cooperation with the tightening bolt  80  and the edge profile  20  will now be described with reference to  FIGS. 10 to 14 . The drive wheel  70 , the front view of which is shown in  FIG. 10   a ), the side view of which is shown in  FIG. 10   b ), and the rear view of which is shown in  FIG. 10   c ), comprises, as its main components, a circular disk-shaped plate  74 , a toothed wheel  72 , which is attached thereto or integrally molded therewith and which serves to drive the drive wheel, and an opening  76  for a tightening bolt  80  to pass therethrough. One special feature of this opening  76 , as shown in  FIG. 10   c ), is that the corner regions thereof are each provided with a carefully designed recess  78 , which prevents damage to the thread of the tightening bolt  18 . 
     The position of the drive wheel  70  relative to the frame profile  20  is shown in  FIGS. 11   a ) and  11   b ). An edge region of the plate  74  engages in a groove  36  behind the angular stop members  34  of the frame profile and is in this way secured from falling out, see  FIG. 2 . Furthermore,  FIG. 11   b ) shows the manner in which the drive wheel  70  is positioned centrally in relation to the recess  44  so that this recess  44  allows for access to the drive wheel  70  by a tool, which is described below. In the example illustrated, a toothed wheel  72  is used for driving the drive wheel  70 . The drive wheel  70  is preferably made of stainless steel. The drive wheel  70  has, at its center, an opening  76  matching the flattened region  86  of the tightening bolt  80  and is pushed over the latter, as shown in  FIGS. 13 and 14 . When the drive wheel  70  is rotated, the torque is transferred via the opening  76  to the tightening bolt  80 . The engagement of the plate  74  in the groove  36  behind the angular stop members  34  of the frame profile, see  FIG. 2 , additionally prevents the drive wheel  70  from moving back and forth on the tightening bolt in an uncontrolled manner. A turning tool described in detail below is used for turning the drive wheel  70 . 
     When carrying out the tightening procedure, the drive wheel  70  is positioned in front of the slot nut  50  on the frame profile  20 , as shown in  FIG. 11 . A tightening bolt  80  is then inserted through the drive wheel  70  and screwed forward into the slot nut  50  disposed inside the frame profile  20  until the stop  82  prevents further screwing. This situation is shown in  FIG. 15 . The stop  82  serves to hold the tightening bolt  80  when the membrane elements are loosened and to keep it from accidentally falling into the interior of the membrane element. A cap nut  89 , which is then screwed on from the rear onto that end of the tightening bolt  80  that is provided with a smaller thread, and is secured, for example adhesively, from undesired loosening. This cap nut  89  serves as the rear stop. 
     A first method of tightening the frame profiles together will now be described with reference to  FIGS. 15 and 16 . After establishing the initial state shown in  FIG. 15 , the regions of the frame that are to be connected are pressed together, for example with the aid of suitable pliers, such that the threaded part  84  of the tightening bolt  80  can be screwed into the slot nut  50  of the adjacent membrane element  10  with the aid of the drive wheel  70  activated by a turning tool. The pliers used for pressing the opposing frame regions together can, in particular, engage in the groove  36  of the profiles  20 —see  FIG. 3 . When the tightening bolt  80  has been turned far enough through the slot nut of the right-hand frame profile  20  in  FIG. 16 , there is no longer any thread engagement between the slot nut  50  and the tightening bolt  80  so that the tightening bolt  80 , when turned further, is held only by the thread of the slot nut  50  of the left-hand frame profile  20  in  FIG. 16 . When the tightening bolt has been screwed in far enough, the cap nut  89  will bear against the slot nut  50  of the right-hand frame profile  20  in  FIG. 16  and further turning of the tightening bolt  80  will bring the frame profiles  20  to be connected closer together. 
     Alternatively, the tightening bolt  80  can be unscrewed from the right-hand profile  20  with the help of the turning tool, already at the commencement of the tightening procedure, until stopped at the rear, that is to say, until the cap nut  89  comes into contact with the slot nut  50 . This is shown in  FIG. 17 . The left-hand profile in  FIG. 17  is then brought close to the tightening bolt  80  with the aid of pliers and the profiles can then be screwed together. Lastly, the final state, shown in  FIG. 18 , is achieved in both variants, in which the profiles  20  are in contact with each other via their angular stop members  34  and have consequently reached their end position. It is further apparent from  FIG. 18  that between the frame profiles  20  there exists a gap  66 , through which access to the toothed wheel  72  of the drive wheel  70  is made possible. 
     This will now be explained with reference to  FIGS. 19 to 21 , in which an actuating tool is shown diagrammatically and is provided with the reference numeral  90 . The actuating tool substantially consists of a plate  92 , on which, in the example illustrated, two additional toothed wheels  94 ,  96  are mounted for rotation, with the aid of which the toothed wheel  72  of the drive wheel  70  for the tightening bolt  80  can be rotated. For this purpose, the tool  90  engages through the recess  44  provided in the profile  20  and is accurately positioned with the aid of screws  98  screwed into bores  46 , see  FIG. 3 , by virtue of which all additional forces occurring when turning the tightening bolt  80  are absorbed by the frame profile  20 . This advantageously enables the operator to work in a force-free and thus comfortable manner. The toothed wheels are preferably turned with the aid of a motor, such as an electric motor for example. 
     An alternative example of a tightening unit is indicated diagrammatically in  FIG. 29 . Here, opposing frame profiles  20  are held together with the aid of a tightening piece  68  having an H-shaped profile. This profile can be inserted between the frame profiles  20 , for example through recesses provided at an appropriate place. Possibly, U-shaped profiles could be fitted on the outside along outwardly oriented angular members similar to the inwardly oriented angular members  34 . 
     Restoration, cleaning, and accessibility of the revetment of the invention and of the membrane elements of the invention will now be explained with reference to  FIGS. 22 to 27 . The term “restoration” is to be understood to mean mainly the cleaning and maintenance of the upper and lower membrane surfaces or of the materials used, respectively, as well as of the seams between the membrane elements  10 . The restoration of the membrane elements  10  or lamps possibly located behind the same can be carried out in different ways. In a first variant, the individual membrane elements  10  can, due to the special assembly system implemented by the tightening units  60 , be removed independently of each other and, on account of their low weight, be disassembled, for example, by two persons. This is shown diagrammatically in  FIG. 22 . This membrane element  10  can then be cleaned, replaced or modified in any other way possible, as desired. 
     In a second variant shown diagrammatically in  FIGS. 23 to 27 , the membrane elements  10  can be swiveled about tightening bolts  80  or hinge pins provided specifically for this purpose. The hinge pins can be provided, for example, between the membrane elements  10  and edge profiles  62 . All other tightening units  60  are undone before the membrane element  10  is swiveled. A swiveling procedure, in which the axis of rotation is located at approximately a quarter of the short side of a membrane element  10 , is shown in  FIGS. 23 to 25 . Here, after swiveling, a portion of the membrane elements  10  is located above the edge profiles  62 . The membrane element  10  is thus swiveled about the pin axis and then moved into an opening position. The location of the pivot axis and the direction of swiveling are governed by the space available on site and the individual requests of the client or architect. 
     For example, in the variant shown in  FIGS. 26 and 27 , the pivot axis is located in a corner region of the membrane elements and the latter are swiveled downwardly about their long sides.  FIG. 26  shows the various stages of the way of swiveling an individual membrane element.  FIG. 27  shows a situation referring to a total of six downwardly swiveled membrane elements  10  in a revetment  100  of the invention. 
     The features mentioned herein also permit easy and rapid accessibility to parts of the building and installed components such as lighting systems and other installations disposed behind the membrane elements. 
     The present invention provides a novel membrane element and a novel method for covering surfaces, which both allow for the supporting structures to be designed with a much thinner configuration and thus for considerable advantages to be achieved in terms of material usage and possible configurations.

Summary:
The invention relates to a membrane element for covering surfaces, more particularly for covering ceilings or walls, comprising a frame and a membrane material stretched over said frame. The membrane element is characterized in that at least one frame region is bent inwardly due to forces exerted on the frame by the membrane material and that the inwardly bent frame region has tightening units for pulling or pushing said frame region outwardly with respect to an abutment, towards the unbent state. The invention further relates to a method for covering surfaces, more particularly for covering ceilings or walls.