Seating furniture with independently free-swinging seat and backrest

An article of furniture, such as a chair, including a free-swinging seat and a free-swinging backrest. The chair includes a seat and backrest coupled to a chair frame, where the chair frame includes first and second laterally spaced apart components. The first and second components each have a floor strut and a pair of legs that may be arranged in a substantially triangular configuration for supporting the seat. The chair also includes a coupling device, which may include a spring element, coupled to the legs and configured to allow forward movement of the seat and backward movement of the backrest, where movement of the seat and backrest may occur independently of one another. In some embodiments, the maximum movement of the seat and backrest may be limited by restricting movement of the coupling device.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119 of German Utility Model Application No. DE 10 2010 054 887.1, filed Dec. 17, 2010, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The field of the disclosure generally relates to seating furniture, in particular, seating furniture having a free-swinging seat and a free-swinging backrest.

BACKGROUND

U.S. Pat. No. 4,787,672 of Werner describes a free-swinging chair having an angular seating portion having a seat and a backrest, both configured as one common component. A chair frame for supporting the seating portion has two lateral components having a tubular profile that is bent into an almost Z-type shape to achieve a spring-like effect. To avoid an excess swinging and/or rocking action of the chair, a bar-shaped reinforcement member is disposed on both lateral components, connecting the two end sections of the respective Z-shaped components. In this configuration, any excess sizing of the bent tubular profile can be avoided which, aside from an increase in weight, would also reduce the spring action.

German App. No. DE 41 35 488 A1 of Landgraf describes one means for avoiding excess sizing of the bent tubular profile. For example, Landgraf proposes inserting reinforcement members into the tubular profile before the bending process, which are then subsequently deformed together with the tubular profile and thus contribute to a reinforcement of the profile. However, a reinforcement of this kind runs counter to a desired spring action.

German App. No. DE 297 18 785 U1 of Erhard discloses a chair construction with an immobile seating area and a free-swinging backrest that swings independently of the seating area. The chair frame is constructed of square-steel-bar tubing including two arc-type constructs for holding the backrest in the bottom area of the base points and in the upper area of the backrest.

Accordingly, the present inventors have identified a need for seating furniture having a frame and a free-swinging seat and backrest, where both the seat and backrest swing freely and independently of each other. In addition, the present inventors have identified a need for such a seating furniture that helps avoid excess sizing of the frame.

SUMMARY

An article of seating furniture, such as a chair, including a seat, a backrest, and a chair frame. As used herein, the seat defines a main seating direction extending from a rear side of the seat to a front side of the seat. The chair frame includes two laterally spaced apart components, where each of the components includes: a floor strut having opposing front and rear ends, the floor strut arranged in a substantially horizontal orientation parallel to the seating direction; a first leg having opposing upper and lower ends, wherein the lower end adjoins to the rear end of the floor strut, and wherein the first leg extends vertically and in an angular orientation from the rear end of the floor strut; a first fastener disposed on the upper end of the first leg and configured to be coupled to the seat; a second leg having opposing upper and lower ends, wherein the lower end adjoins to the front end of the floor strut, and wherein the second leg extends vertically and in an angular orientation from the front end of the floor strut; and a second fastener disposed on the upper end of the second leg and configured to be coupled to the seat, wherein the upper end of the first leg is proximal to the front side of the seat and the upper end of the second leg is proximal to the rear side of the seat. In addition, the article includes a longitudinally changeable coupling device connected on a first end via at least one first connection site with the first leg and on a second end via at least one second connection site with the second leg, wherein the coupling device is configured in such a way that its maximum length is limited.

In one embodiment, the seat and the backrest of the seating furniture are configured as separate components that are separately fastened to the first and/or second legs of the chair. In such a configuration, it is possible for the seat and the backrest to have the ability of swinging freely and independently of each other. Alternatively, the seat and the backrest can also be configured as a single, unitary component or as separate components connected to each other. In such embodiments, the seat and backrest unit are movably coupled to each other so as to provide a free swinging action of the seat and backrest unit.

In some embodiments, the floor struts and the two legs of each laterally spaced apart component are arranged in a substantially triangular configuration. In such embodiments, bending moments occur mainly in connecting areas between the floor struts and the first and/or second leg (s) inside the chair frame. In particular, in the front, the seat is able to swing downward by the first legs bending forward in the area of the rear end section and the backrest is able to swing rearward by the second legs bending to the rear in the area of the front end section of the floor strut. The floor struts each extend substantially horizontally and are substantially parallel in relation to the floor on which the seating furniture is located. The floor struts can lie directly on the floor, or can be arranged at a distance from the floor supported by base elements and/or base gliding elements. In some embodiments, the floor struts may be connected to each other by at least one cross-member to improve the stability of the chair frame.

In some embodiments, the first legs and the second legs of the chair frame extend, respectively, from the rear and/or front end section of the floor strut. The connection therein can be configured as angular or bent. The angle between the floor strut and the two legs is, as a matter of principle, freely selectable and can be selected as different or identical for the first and second legs in relation to each other. In some embodiments, the two lateral components of the chair frame are configured and disposed in a substantially symmetrical configuration in relation to the main seating direction. In addition, the floor struts may extend essentially parallel in relation to the main seating direction, or may be in an angular orientation having an angular range of approximately plus or minus 15 degrees in relation to the main seating direction to increase the stability of the chair frame and thereby of the entire piece of seating furniture.

In some embodiments, the connection area between the floor struts and the legs, are constructed from a single, unitary component to absorb the bending moments at that location. Alternatively, the floor struts and legs can optionally be configured in one or in multiple pieces. In other embodiments, the first leg is spaced apart from the second leg by a minimum distance of 25 millimeters to ensure that there are no nips on the seating furniture that may cause injury to users as the chair swings back and forth.

In some embodiments, the upper end sections of the first legs are positioned in front of the upper end sections of the second legs, meaning the first legs of the chair frame intersect with the second legs, thus allowing the seat to be mounted in front of the back rest at the first fastening section of the first legs. The first legs of the chair frame therein can extend diagonally relative to the main seating direction, optionally inside or outside of the chair frame. Furthermore, the upper end sections of the first legs may be at least, in part, taken up in the seat, meaning the underside of the seat. In some embodiments, the first fastening section may extend essentially horizontally to provide easier mounting and alignment of the seat on the first fastening section. In other embodiments, the fastening sections of each of the two lateral components of the chair frame may be connected to each other to improve the stability of the chair frame and seating furniture.

In such embodiments, at least one first connecting site of the coupling device may be disposed and arranged in the connection area, which allows for the at least a first connection site to be positioned as closely as possible in relation to the center axis of the seating furniture in the main seating direction and provides a stable and central connection of the first and second legs.

In some embodiments, the seating furniture includes a longitudinally changeable coupling device configured and operable to prevent excess swinging and/or rocking of the seat and the backrest. This coupling device connects the two legs of the lateral components of the chair frame in such a way that the seat is prevented from moving too far away from the backrest.

In some embodiments, the coupling device may be structurally sound such that additional, stronger sizing of the chair frame, which would counteract the free swinging action and also result in a higher weight, is unnecessary. By limiting the free swinging motion of the seat and/or the backrest, the bending moments acting at the bent connections between the floor strut and the respective leg are also limited, whereby the material stresses on the chair frame are also reduced.

In some embodiments, the coupling device has at least one connection site with the first legs of the lateral parts of the chair frame as well as at least one second connection site with the second legs of the lateral parts of the chair frame, meaning the coupling device may include one or more coupling lines that correspondingly require one or several first and/or second connection sites with the legs of the chair frame. The number of first and second connecting sites of the coupling device on the first and second legs, respectively, can optionally be identical or different in relation to each other. In embodiments having two or more coupling lines, the coupling lines may be disposed and/or configured symmetrically with regard to the main seating direction.

In some embodiments, the coupling device is configured and/or disposed in such a way that its connecting direction is substantially parallel in relation to the main seating direction. Alternatively, the coupling device may be angularly aligned in relation to the main seating direction. In embodiments having more than one coupling line, in particular, at least a portion of the coupling lines may be oriented at an angle relative to the main seating direction. The totality of the coupling lines may be configured and disposed symmetrically in relation to the main seating direction, resulting overall in a coupling device with a direction of action in the main seating direction.

The coupling device may be configured and/or disposed in such a way that its connecting direction extends essentially horizontally. In this embodiment, the coupling device may be disposed in very close vicinity to the seat and take up minimal space. In such configurations, the seating furniture may be arranged on a table without the coupling device interfering with the suspension action or becoming damaged due to the suspension action. Alternatively, the coupling device can also be configured and/or disposed in such a way that its connecting direction extends essentially vertically and angularly in relation to the horizontal line. In some embodiments, the coupling device may be taken up at least, in part, in the seat, more specifically in the underside of the seat for additional protection. In other embodiments, the coupling device may be positioned completely underneath the seat.

In some embodiments, the length of the coupling device can be changed between the first and the second legs of the two lateral components of the chair frame and it may be configured to have a limited maximum length. The coupling device may connect the first and second legs of two lateral components with each other and allow—starting with a relaxed resting position of the seating furniture and/or the chair frame (e.g., without any load acting on the piece of seating furniture, for example due to a person sitting thereon)—for a widening of the distance between the first and second connecting sites on the first and/or second legs (“longitudinally changeable”) to the point of a preset maximum distance. Naturally, this applies for the intended use of the seating furniture involving the related typically occurring forces and weights. The disclosure comprises coupling devices that counteract this distance change with a force, as well as coupling devices that freely allow this distance change (to the maximum distance).

In some embodiments, the at least one connecting site of the coupling device is arranged with the first legs of the chair frame in front of and/or above the at least one second connecting site of the coupling device with the second legs of the chair frame. In this positioning of the coupling device, the longitudinal limitation of the coupling device may limit the movement of the seat and/or the backrest.

In some embodiments, the coupling device includes at least one elastic spring element, such as a helical tension spring. In such embodiments, the elastic spring element may dampen the motion of the seat and/or backrest and/or counteract it by the force created by the spring element.

In other embodiments, the coupling device may include at least one rope, which may comprise a steel rope, on which at least one end is equipped with a stop element. While the seating furniture and/or the chair frame is in a resting state, this rope may be loosely routed between the first and the second connection sites. In addition, when the seating furniture and/or chair frame experiences a load, the rope, via the stop element, delimits the length of the coupling device and thereby the distance between the first and second legs of the lateral parts.

In other embodiments, the coupling device may include, in the alternative or in addition to the at least one elastic spring element, at least two rigid elements that engage with each other and are movable in relation to each other. The two rigid elements may be bolts that are guided inside corresponding oblong holes.

In some embodiments, the maximum length of the coupling device may be variably adjustable to provide for adjustment depending on, for example, expected load (such as for a child, adolescent, adult) without having to change or switch out the coupling device. In other embodiments, the chair may be provided with a set of multiple coupling devices (e.g. one for a child, one for an adolescent, and for an adult) that are easily interchangeable depending on the intended use of the chair. In yet another embodiment, the coupling device may be configured in such a way that its minimum length is limited and/or variably adjusted as well. Depending on the coupling device between the two legs of the lateral part of the chair frame, such a minimum length of the coupling device may restrict the motion of the seat and/or backrest.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to the drawings, this section describes particular embodiments and their detailed construction and operation. The embodiments described herein are set forth by way of illustration only and not limitation. The described features, structures, and characteristics may be combined in any suitable manner in one or more embodiments. In view of the disclosure herein, those skilled in the art will recognize that the various embodiments can be practiced without one or more of the specific details or with other methods, components, materials, or the like. For the sake of clarity and conciseness, certain aspects of components are presented without undue detail where such detail would be apparent to those skilled in the art in light of the teachings herein and/or where such detail would obfuscate an understanding of more pertinent aspects of the embodiments.

FIGS. 1-4illustrates an embodiment of an article of seating furniture, such as a chair100, having a coupling device for providing independent free-swinging motion for a seat and a backrest of the seating furniture. For convenience, the following description of the seating furniture100uses a chair as an example of the seating furniture100. It should be understood that the description specifically identifying a chair is for illustration purposes only and not meant to limit the seating furniture to a chair. With reference toFIGS. 1-4, the chair100includes a seat10and a backrest12. The seat10and the backrest12may comprise separate structures coupled or connected together by a supporting structure in an angular relationship as desired to provide a comfortable and/or ergonomic seating position. In other embodiments, the seat10and the backrest12may be constructed from a single, unitary component.

The chair100and all of its components may be constructed from any suitable materials. For example, the seat10and the backrest12may be constructed from plastic or wood. In some embodiments, the seat10and/or backrest12may include a fabric or cushioned covering to provide additional comfort. In other embodiments, the chair100and its components may be constructed from other materials, which may depend on various factors such as weight, cost, durability, and availability of materials. Moreover, the shape, size and material of the seat10and the backrest12may be selectable by the manufacturer, or by the end consumer, or may be made of standard sizes, shape, and materials common in the industry.

The seat10defines a main seating direction14that extends from a generally rear side14ato a generally front side14bof the chair100. In the figures, the main seating direction14is marked, respectively, at a central axis of the chair100. The main seating direction14results from normal use of the piece of seating furniture in a straight upright sitting position in which the user is seated in the chair10at any given time, supported in the back via the backrest12and with the gaze straight ahead to the front.

The seat10and backrest12are fastened or coupled to a chair frame16. The chair frame16includes two laterally spaced apart components or parts,16a,16b, configured and disposed symmetrically in relation to the center axis of the chair100in the main seating direction14. In some embodiments, the chair frame16and the two components16a,16bmay be constructed of a rigid metal or metal alloy material, such as high-tenacity steel or other structurally stable materials.

Each of the two lateral components16a,16b, include a floor strut18that extends essentially horizontally (i.e., parallel to the floor). In such embodiment, the floor struts18are aligned parallel or substantially in relation with the main seating direction14(seeFIG. 3). In some embodiments, the floor struts18may be arranged in an angular configuration in relation to the main seating direction14, where the angle may range from approximately plus or minus 15 degrees. In other embodiments, foot elements, gliding elements or something structures (not shown) may be mounted on the floor struts18. The floor struts18of each lateral component16a,16bmay be connected to each other by a cross-member20extended therebetween.

Referring now toFIG. 2, each lateral part16a,16bfurther comprises a first leg22that extends at a first angle33cand in a vertical direction from a rear end section18aof the floor strut18and upward toward the seat10. This first leg22may bend around the first connection area24between the floor strut18and the first leg22allowing the first leg22to swing freely to the front when the chair100experiences a load. The phantom lines inFIG. 2illustrate the first leg22′ swinging forward in a loaded position and at a second angle33din relation to the floor strut18.

The chair100further includes a first fastening section26(seeFIGS. 2 and 3) on each lateral part16a,16b, where the fastening section26is disposed on an upper end section22aof the first leg22that is directed away from the floor strut18. The first fastening section26extends parallel or substantially parallel to strut18and may be connected to a second fastening section (i.e., on the mirror side of the chair100) by a connection section28to increase the stability of the chair frame16. A mounting plate30may be attached on the first fastening section26and connected, such as by a screw or other similar means, to an underside of the seat10.

Moreover, each lateral part16a,16bincludes a second leg32that extends at a third angle33aand in a vertical direction from a rear end section18bof the floor strut18and upward toward the seat10. This second leg32bends around the front connection area34between the floor strut18and the second leg32allowing the second leg32to swing freely to the rear when the chair100experiences a load. The phantom lines inFIG. 2illustrate the second leg32′ swinging backward in a loaded position and at a fourth angle33bin relation to the floor strut18.

A second fastening section36is disposed, respectively, on an upper end section of the second leg32that is directed away from the floor strut18. These second fastening sections36may extend essentially horizontally toward the rear14a(i.e., parallel or substantially parallel to the floor struts18) of the chair100, then upward in an arc. The backrest12may be fastened on the second fastening sections36of the second legs32. In some embodiments, the second fastening sections36of the lateral parts16a,16bmay be coupled to each other as well to further increase the stability of the chair frame16.

As illustrated inFIG. 2, the floor struts18, the first leg22, and the second leg32of each lateral part16a,16bform a substantially triangular shape. The first fastening sections26of the first legs22therein are positioned along the main seating direction14, at least in part, in front of the second fastening sections36of the second leg32. As illustrated inFIGS. 1 and 3, the first legs22that extend to the front are guided along the outside of the second legs32that extend toward the rear of the lateral parts16a,16b.

The chair frame16further includes a coupling device38. As illustrated inFIG. 2, the coupling device38is, on one end, connected with the first fastening sections26of the first legs22and, on another end, with the second fastening sections36of the second legs32of the lateral parts16a,16b. The coupling device38thus extends essentially horizontally (i.e., parallel or substantially parallel to the floor struts18and may be in the same plane or a plane above or below a plane of the floor struts18) between the two connection sites. As used herein, the connection site is defined as the connection between the coupling device38and the chair frame16, whereas the first and second fastening sections26,36are defined as the connection between the chair frame16and the seat10and/or backrest12. For instance, the first fastening section26is configured to be coupled to the seat10and the second fastening section36is configured to be coupled to the backrest12.

As shown inFIG. 2, a free space14cis maintained below the seat10, allowing for the chair to be, for example, suspended by the seat10on a table or other furniture. The coupling device38may also be, at least in part, integrated to the underside of the seat10for additional protection to the coupling device38and to reduce injury risk to the user and third parties. In some embodiments, the upper end sections22aof the first legs22may also, at least in part, be integrated into the seat10to further improve the stability of the chair100.

Referring now toFIG. 3, the coupling device38is attached such that it is substantially aligned along a center axis of the chair in the main seating direction14. The coupling device38includes a spring element40that is fastened, via a first fastening element42, to the connection section28of the first legs22and, via a second fastening element44, with the two second fastening sections36of the second legs32. In such configurations, the first connection site at the first fastening element42is positioned toward the front side14band the second connection site at the second fastening element44is positioned toward the rear side14arelative to the main seating direction14.

The spring element40of the coupling device38may counteract any excess distance D (seeFIG. 3) between the upper end areas of the first and second legs22,32. The maximum distance therein may be limited by a maximum length of the spring element40, and wherein this limitation is achieved by a dampening action during the stretching of the spring element40by the spring force ratios. If desired, the maximum length of the coupling device38can be adjusted for the respective user of the chair, such as to compensate for a user's body weight. This can be achieved, for example, by selecting a spring element40having an adjusted spring constant for the coupling device38. In another embodiment, the second fastening element44of the coupling device38may be variably adjustable to increase or decrease length of the spring element40. In other embodiments, the coupling device38may include two or more elastic spring elements40disposed in parallel or series relative to each other.

In one embodiment, the coupling device38may include two or more rigid components that movably engage with each other in order to specify a minimum and/or maximum length of the coupling device38. For example, in one embodiment illustrated inFIG. 5, the chair100may include an oblong hole62formed in a first rigid component60(e.g., a plate) and extending in the main seating direction14with a bolt66formed at or fastened to a second rigid component64(e.g., a rod) that is movably guided therein and engaging the hole62to restrict the coupling device38once it has reached a maximum distance. In another embodiment, the features of the coupling device38illustrated inFIG. 5may be combined with the features of the coupling device38inFIG. 4by, for example, replacing the steel rope50or a combination of the steel rope50and spring element40with the rigid components60,64.

In example operation, the coupling device38may behave as follows (seeFIG. 2). When a load is placed in the front side14bof the seat10, the coupling device38may extend to an extended position of the coupling device38′, and the seat10may tilt toward the front14bto a second position of the seat10′, because the first legs22of the lateral parts16a,16bare able to swing forward around the rear connection areas24with the floor struts18. However, this free swinging action of the chair10may be limited by the coupling device38that restricts the distance of the forward-moving first fastening sections26of the first legs22from the second fastening sections36of the second legs32that remain in their resting position. In this manner, excess swinging and/or rocking of the seat10toward the front side14bmay be limited, whereby any possible load application is also restricted to rear connection areas24, which allows for selecting a smaller sizing of the tubular profile and/or additional stiffening of the tubular profile may be entirely omitted.

Similarly, the backrest12may rock toward the rear side14ato a second position for the backrest12′ when the user places weight toward the rear side14aand presses against the backrest12, because the second legs32of the lateral parts16a,16bare able to swing around the front connection areas34with the floor struts18toward the rear side14a. This free swinging action by the backrest12may be limited by the coupling device38that restricts the distance of the second fastening sections36of the second legs32to the rear side14afrom the first fastening sections26of the first legs22that essentially remain in the resting position. Since excess swinging and/or rocking of the backrest12to the rear side14ais limited, the possible load with regard to the front connection areas34of the chair frame is also limited.

In some embodiments, to exclude and/or minimize injury risk, the first legs22are in every position spaced apart in relation to the second legs32of the chair frame16such that the first and second legs22,32are prevented from touch. An appropriate separation distance between the first and second legs22,32may be selected in such a way that no nips of any kind are created in any position of the chair frame16, even when the seat10tilts forward or the backrest12tilts rearward. In some embodiments, the first and second legs22,32may be separated by a minimum distance of approximately 25 mm.

While the coupling device38may have a maximum length (e.g., when not deformed or stretched beyond its yield point) to limit free swinging action of the seat10and the backrest12as previously described, the coupling device38may also have a minimum length, which may be preset by arranging and configuring the fastening elements42,44and/or additional elements to a preselected length.

FIG. 4illustrates a view of the coupling device38according to one embodiment. With reference toFIG. 4, the coupling device38is fastened to a fastening element42that is mounted or rigidly attached, for example by welding, on the connection section28. The coupling device38includes an elastic spring element40, which may be in the form of a helical tension spring. This spring element40is configured having a hook-shaped extension46on its end that engages with a first opening48in the fastening element42. In this configuration, the spring element40counteracts by its spring force any widening of the distance between the fastening elements42,44and limits the distance to a maximum measure.

The coupling device38may also include a steel rope50that is routed through an axial hollow space (not shown) of the spring element40. The steel rope50is routed on the end side through a second opening54in the fastening element42. The steel rope50includes at its end a stop sleeve52having dimensions larger than the second opening54so that the stop sleeve52cannot pass through the second opening54.

When the chair100is in its resting state, meaning without load, the steel rope50runs through the second opening54inside the fastening element54lying loosely in the spring element40, whereby the steel rope50is in an inactive state. When a load is placed on the chair100, such as when a user sits on the chair100, the first and second legs22,32of the chair frame16move, and the connection sites between the legs22,32and the coupling device38also move apart countering the force of the spring element40. This widening of the distance and/or elongation of the coupling device38may continue until a point where the stop sleeve52makes contact with the fastening element42at the end of the steel rope50. At this point, the spring element40can no longer continue stretching, thereby limiting the maximum length of the coupling device38.

In another embodiment, the coupling device38may include the steel rope50, but not the spring element40. In such embodiments, the steel rope50may similarly limit the maximum length of the coupling device38via the stop sleeve52abutting against the second opening54of the fastening element42.

FIG. 6illustrates a second arrangement of the coupling device38according to another embodiment. The embodiment illustrated inFIG. 6may include identical or substantially similar components and relationships as described with reference to the embodiments illustrated inFIGS. 1-4. As such, these components will not be further discussed, except to highlight differences between the embodiments.

With reference toFIG. 6, the coupling device38may extend along a substantially vertical axis and substantially perpendicular in relation to the main seating direction14from the first connection sites on the fastening sections26of the first legs22downward to the second connection sites on the second fastening sections36of the second legs32. In such embodiments, the coupling device38may include one spring element40or two parallel spring elements40aligned laterally offset and substantially symmetrically relative to the center axis in the main seating direction14.

In such configurations, the first connection sites with the first legs22are thus located at a higher position in relation to the struts18as compared to the second connection sites with the second legs32, which are located at a lower position above the struts18.

In another embodiment, the chair100may include the coupling device38as described inFIGS. 1-4(i.e., attached beneath the seat10and substantially aligned with the central axis of the main seating direction14) and another coupling device38as described inFIG. 6.

In yet another embodiment, the chair100may include a seat10having a main seating direction14, which extends from a rear side14aof the seat10to a front side14bof the seat10. The chair100further includes a backrest12proximal to the rear side14aof the seat10. a chair frame16having two lateral parts16a,16bthat each include: an essentially horizontally extending floor strut18; a first leg22extending from a rear end18asection of the floor strut18diagonally to the front and upward toward the seat10; a first fastening section26disposed at an upper end section of the first leg22that is directed away from the floor strut18and configured for fastening the seat10; a second leg32extending from a front end section of the floor strut18diagonally to the rear and upward toward the seat10; and a second fastening section36disposed at an upper end section of the second leg32that is directed away from the floor strut18and configured for fastening the backrest12; and wherein the upper end sections of the first legs22in the main seating direction14are positioned in front of the upper end sections of the second legs32; and a longitudinally changeable coupling device38that is connected on one end via at least one first connection site with the first legs22of the two lateral parts16a,16band another end via at least one second connection site with the second legs32of the two lateral parts16a,16b, wherein the coupling device38is configured in such a way that its maximum length is limited.