"ARTICULATION SYSTEM FOR CHAIRS"

Technical field of the invention

The present invention relates to an articulation system for chairs, in particular of the type in which the movements of seat and backrest are synchronized with each other.

State of the art

There are known chairs, in particular for office, which comprise an articulation system comprising a rigid frame bound to a support base for resting the chair on the ground, a support for the seat and a support for the backrest for the chair. The support for the seat and the support for the backrest are distinct and separate from each other, and are both articulated with respect to the frame and furthermore, thanks to an articulation mechanism that connects them, they are mutually articulated so that a movement of one corresponds to a predetermined movement of the other. In some solutions, the articulation system that connects the two supports is adjustable so that the relationship between the two movements can be varied.

When one of the two supports moves under the action of a user's stress (typically exerted on the backrest), the system opposes an elastic reaction which tends to set back both supports in the at-rest configuration (i.e. without stress). Said reaction is typically obtained by means of an elastic element, for example at least one spring. In some solutions, in order to adapt the system's response to the weight and/or to the preferences of the user, the reaction of the system is adjustable for a given tilting, for example thanks to a user- operated adjustment system to vary the degree of preloading of the elastic element or the position of at least one anchorage point of the elastic element to the frame and/or to one of the two supports.

In some embodiments of the known articulation systems, as the user's weight varies, the reaction of the system to a given tilting is higher the heavier is the user (in the jargon this effect is called 'weighing-people'), regardless of the presence or absence of an adjustment of the reaction of the elastic element.

WO 2009/15381 1 A1 discloses an adjustment device for chairs comprising a main body, engageable on a support stem of a chair; a support for the seat mounted on the main body; a support for the backrest mounted on the main body; and articulation means for engaging the support for the backrest and the support for the seat to the main body, the articulation means comprising a lever rotating with respect to the main body and hinged to the support for the seat and the support for the backrest, in order to obtain a determined movement relationship between the support for the backrest and the support for the seat.

WO 2010/097818 A1 discloses a device for synchronizing the inclination of the backrest and of the seat of a chair, of the type comprising a backrest frame, a seat frame and a support firmly constrained to the base of the chair, a first crank firmly constrained to the frame of the backrest and hinged to the back of the seat frame and to the support, and a second crank hinged to the front part of the seat frame and to the support, to form, together with the seat frame, an articulated quadrilateral.

Summary of the invention

The Applicant has noted that the known articulation systems for chairs have some drawbacks and/or can be improved in some aspects.

For example, the Applicant has observed that the articulation systems of WO 2009/15381 1 A1 and WO 2010/097818 A1 , due to their own structure based on levers or cranks pivoted on the frame, determine a rototraslational movement of the seat in which the vertical and horizontal components of the translational motion of the front of the seat are fixedly determined.

Furthermore, the Applicant has observed that the front hinge point of the seat spans an arc of a circle with the centre facing the rear part of the system, and this implies that, given a certain relationship between the vertical component and the horizontal component of the translational motion of the front part of the seat in the initial part of the movement, this relationship changes during the movement progressively unbalancing towards the horizontal component to the detriment of the vertical one. In particular, the lifting rate of the front part of the seat per tilting unit of the backrest is progressively lower as the degree of tilting of the backrest increases with respect to the at-rest position. This means that the effect of the weight of the user on the seat, which tends to oppose to the inclination

('weighing-people'), tends to decrease as the tilting of the backrest increases, until it is cancelled and potentially changes orientation with wider tilting.

Again, according to the Applicant, this behaviour determines a response to the tilting perceived by the user that is not always optimal along the entire tilting range, and/or determines limitations in the tilting amplitude of the backrest and/or of the seat.

An object of the present invention is to provide an articulation system for chairs which solves one or more of the problems described above.

This object is achieved by an articulation system for chairs in accordance with the attached claims and/or having the following characteristics. According to a first aspect the invention relates to an articulation system for chairs, comprising:

- a (rigid) frame designed to be associated with a base for a chair;

- a (rigid) support for a seat mounted on said frame;

- at least an articulation element, rigid and hinged to said frame so that it can rotate about an axis of rotation, wherein said support for the seat is hinged to said articulation element at a hinge point,

- a support for a backrest, distinct from said support for the seat, coupled to said articulation element;

where the articulation system further comprises, for each side, a pin that is firmly constrained to a front portion of said frame and a slot afforded in a front portion of the seat support, the pin slidably engaging the slot,

wherein said articulation element and said pin and slot are configured in such a manner that said articulation system takes on an at-rest configuration, in the absence of tilting forces acting upon the support for the seat and/or upon the support for the backrest, and a maximum tilting configuration in which said support for the seat is rotationally translated with respect to the at-rest configuration, said pin sliding along said slot during the transition of the articulation system from the at-rest configuration to the maximum tilting configuration and vice versa,

and wherein, in a reference plane perpendicular to said axis of rotation, an angle, defined as an acute angle formed between a tangent to the slot in an instant contact point between the slot and pin and a horizontal line, above the horizontal line, faces rearwards.

According to a further aspect, the present invention relates to a chair comprising the articulation system in accordance with the present invention.

The chair preferably comprises a seat for a user rigidly coupled to said support for the seat and/or a backrest rigidly coupled to said support for the backrest.

The terms vertical, horizontal, upper, lower and the like refer to a condition of normal use of a chair incorporating the articulation system of the present invention.

The terms front and rear refer to a normal use of a chair embodying the present invention, wherein the user's legs are located at the front portion of the system.

According to the Applicant, the synchronized articulation systems between backrest and seat of WO 2009/15381 1 A1 and WO 2010/097818 A1 , which use articulation rods between the seat and the frame, make the front hinge point of the seat be forced to span, on the reference plane, a circle arc trajectory with concavity facing the axis of rotation of the rear articulation rod. This movement of the front hinge point constitutes a mechanical constraint which limits the possibility of designing more ergonomic movements of the seat and/or backrest.

For example, such movement of the front hinge point of the prior art can create an upper dead spot of the trajectory of the hinge point, in which the latter moves almost horizontally, and/or a predetermined dynamic of variation of the relationship between the vertical and horizontal component of the translational motion of the front part of the seat, as explained above. This in turn causes functional limits to the overall articulation system, for example of the type described above.

The present solution, instead, thanks to the articulation between the front part of the seat and frame through pin and slot allows a great versatility in the design of the slot and therefore in the movement of the front part of the seat.

The present solution allows, for example, to avoid upper dead spots for the front part of the seat.

Furthermore, the present solution allows to maintain a desired lifting rate of the front part of the seat as a function of the rotation of the articulation element, throughout the entire rotation of the latter. The development of this lifting rate depends on the specific design of the slot.

According to the Applicant, moreover, the fact that the aforesaid angle faces rearwards gives the system the 'weighing people' effect described above. In fact, in use, the support is subject to the weight force of the user, which generates a substantially vertical weight force acting on the slot. This weight force can be instantaneously decomposed into a component tangential to the slot in the instant contact point and into a component perpendicular to it (which is cancelled by the constraining reaction which has application point in the instant contact point between pin and slot). The tangential component of the weight force tends to push down the support in opposition to the tilting motion and this component must be won to effect and maintain the tilting. Therefore, as the weight of the user varies, the reaction of the system to a tilting is more intense the heavier is the user, even in the absence of adjustments of the reaction of the elastic element.

When the support for the seat is stressed in rotationally translation (for example by exerting a tilting force on the backrest that is articulated to the seat), the tilting force generates an action force on the slot which produces the relative motion between pin and slot and therefore the rotationally translation of the seat. The action force can be decomposed into its two components: the one tangential to the slot in the reaction point and the one perpendicular to the first. The perpendicular component is cancelled by the constraining reaction of the slot-pin coupling (which has an application point in the point of instant contact between pin and slot, wherein the perpendicular component of the action force and the constraining reaction push the pin against the slot), while the tangential component is the one that produces the relative movement between pin and slot.

The pin-slot coupling involves a sliding friction (static and dynamic) between the pin and slot surfaces in mutual sliding. This friction, having an application point in the instant contact point, is generally proportional to the overall constraining reaction perpendicular to the contact surface, which tends to push the pin and the slot against each other. This overall constraining reaction comprises the aforesaid component of the weight force perpendicular to the tangential component (and possibly the perpendicular component of the action force). Therefore, as the user's weight varies, the sliding friction force is greater the greater is the weight of the user and this effect contributes to the variation of the reaction of the system according to the user's weight.

In the prior art solutions, in which the front coupling between the support for the seat and the frame consists of a connecting rod directly hinged to the frame and the support, the Applicant has found that the reaction of the system results, given the same spring, generally lower than the reaction of the present invention. According to the Applicant, this may be due to the sliding friction that develops between the pin and the slot and which, on the contrary, is negligible in the case of a connecting rod.

The present invention in one or more of the aforesaid aspects may have one or more of the following preferred characteristics, some of which refer to the aforesaid reference plane.

Preferably, said support for the seat is directly hinged to said articulation element. In this way the system is structurally simple and rational.

Typically, the system comprises an articulation element as described above for each side of the articulation system.

The present invention also provides for solutions in which said angle is equal to zero in at least one tilting configuration, for example in the at-rest configuration. For example, at least one portion of the slot can be horizontal.

In an embodiment, said slot has an extension at least partially rectilinear, preferably it is entirely rectilinear. In this way the system provides a substantially regular reaction along the entire tilting (for example, the tilting force applied at one point of the backrest is substantially linear with the resulting tilting angle). For example, the relationship between the horizontal and vertical components of the translational motion of the front part of the session remains substantially constant throughout the tilting excursion. According to the applicant, this embodiment is particularly advantageous in combination with a system for adjusting the reaction of the elastic element for a given tilting.

In the aforesaid embodiment, preferably said angle in said at-rest configuration is greater than or equal to 0°, more preferably greater than or equal to 3°, and/or less than or equal to 40°, more preferably less than or equal to 30°.

In an embodiment, said slot has an extension at least partially curvilinear, preferably it is entirely curvilinear. In this way it is possible to make the reaction of the system to vary according to the tilting.

According to the applicant, this embodiment is particularly advantageous in the absence of a system for adjusting the reaction of the elastic element for a given tilting.

Preferably in this embodiment, said angle in said at-rest configuration is greater than or equal to 10°, more preferably greater than or equal to 15°, and/or less than or equal to 50°, more preferably less than or equal to 40°.

Preferably, said angle in said maximum tilting configuration is greater than or equal to 40°, more preferably greater than or equal to 50°, even more preferably greater than or equal to 60°.

Preferably, said slot has concavity facing downwardly and rearwardly. In this way the relationship between horizontal and vertical components develops in an opposite way with respect to the development that is obtained in the case of a front lever or crank articulation (that is, a circular arc with concavity facing rearwards). For example, the lifting rate may increase as the rotation of the articulation element increases.

Preferably, said angle in said at-rest configuration is less than or equal to said angle in said maximum tilting configuration.

Preferably, said angle gradually increases (more preferably monotonically) throughout at least one portion of, more preferably throughout the entire, movement of the articulation system from the at-rest configuration to the maximum tilting configuration.

In this way the 'weighing people' effect described above is enlarged as the amplitude of the tilting increases. In fact, as the tilting angle increases, the tangential component of the action force is progressively reduced in favour of an increase in the perpendicular component.

Moreover, as the tilting angle increases, the tangential component of the weight force increases progressively to the detriment of a decrease in the perpendicular component of the weight force. As the tilting angle increases, the tilting force necessary for the tilting must overcome, in addition to any elastic spring reaction force (which substantially increases linearly with the tilting angle), also the aforesaid tangential component of the weight force, which increases progressively and the greater the heavier is the user, provided that the tangential component of the action force decreases. The result of these effects is a gradual hardening of the reaction as the tilting increases, additional to that of the spring's elastic return force, and proportional to the user's weight.

The Applicant observes that as the tilting increases, the contribution to the sliding friction force provided by the weight force decreases, while the contribution to the sliding friction force provided by the action force increases, thereby always ensuring the presence of friction.

In an embodiment the extension of the slot comprises both a rectilinear portion, preferably comprising a location point of the pin in the at-rest configuration, and a curvilinear portion, preferably comprising a location point of the pin in the maximum tilting configuration. Preferably said curvilinear extension is in the shape of a substantially circular arc having a centre placed in a lower-rear quadrant with respect to the slot.

Preferably, said circular arc has a radius greater than or equal to 50 mm, more preferably greater than or equal to 60 mm, and/or less than or equal to 200mm, more preferably less than or equal to 150mm.

The Applicant has verified that these shapes and/or values give the system good ergonomic and/or weighing-people characteristics.

Preferably, said slot is afforded in a respective lateral wall of the support for the seat. Preferably, said pin is integral (that is, in a single piece) with said frame. Compared to a comparative solution in which the front articulation between the frame and the support for the seat is made with a slot afforded in the frame and a pin firmly constrained to the support, the present solution reduces production costs, since the pin can be moulded together to the frame and therefore does not require production and assembly as a separate element.

Preferably, in said at-rest configuration, the hinge point is located, with reference to said reference plane, above a horizontal line passing through said axis of rotation.

In an embodiment, in said at-rest configuration and/or in said maximum tilting configuration, the hinge point is located, with reference to said reference plane, in front of a vertical line passing through said axis of rotation

In this way, since the rear portion of the support tends to lift during the tilting, there is a "weighing-people" effect at least (and preferably also) at the hinge point. This solution is particularly advantageous in the absence of a system for adjusting the reaction of the elastic element for a given tilting of the supports.

In an embodiment, in said at-rest configuration and/or in said maximum tilting configuration, the hinge point is located, with reference to said reference plane, in back of a vertical line passing through said axis of rotation.

In this way it is limited the lifting of the legs (and in general the lifting of the whole seat), since the rear portion of the support tends to lower during the tilting. This solution is particularly advantageous in combination with a system for adjusting the reaction of the elastic element for a given tilting of the supports, which compensates for the substantial lack of 'weighing-people' effect.

In an embodiment, the support for the backrest is firmly constrained to said articulation element. For example, the articulation element may be a whole with said support for the backrest, or the support for the backrest can be rigidly fixed to the articulation element. In this way the system as a whole is constructively simple, reliable and economical in its construction.

In an alternative embodiment, the support for the backrest is connected to the articulation element in an articulated manner. In this way, further degrees of freedom are introduced between the backrest and the seat, for greater ergonomics and/or versatility and/or comfort of the chair.

For example, in the aforesaid alternative embodiment, the articulation system comprises a further articulation element, rigid and hinged to said frame to rotate about a further axis of rotation, wherein said support for the backrest is hinged to said articulation element at a side of the articulation element opposite to said hinge point of the seat with respect to said axis of rotation, and it is also hinged to said further articulation element.

Preferably, the articulation system comprises at least one elastic element (e.g. a spring) operatively interposed between said frame and said support for the seat and/or between said frame and said support for the backrest. In this way, advantageously, the elastic reaction force developed by the elastic element counteracts the tilting with respect to the at-rest configuration, typically all the greater the wider is the tilting.

Preferably, said elastic element is configured to operate in extension. Preferably in the at- rest configuration, said elastic element exerts a residual elastic return force between support and frame ('preload'). Preferably, said elastic element has a first end (directly) abutted to the frame and a second end (directly) abutted to the hinge point and/or to the support for the seat.

In an embodiment the first end of the elastic element is directly abutted to a pin firmly constrained to said frame and arranged in a front-lower portion of the frame.

Preferably, the second end of the elastic element is directly abutted to a hinge pin coinciding with said hinge point.

In an embodiment the articulation system comprises an adjustment system structured to move a first and/or a second end of said elastic element with respect to the frame and/or the support for the backrest, and/or to vary the elastic constant of the elastic element. In this way the user can adjust the 'hardness' of the response for a given tilting. For example, a first end of the elastic element is abutted to a pin movable with respect to said frame and, preferably, a second end of the elastic element is abutted to a hinge pin coinciding with said hinge point.

Preferably, the chair comprises a supporting base for resting it on the ground and a stem mounted on the supporting base, said frame being rigidly mounted at an upper end of said stem.

Brief description of the drawings

The characteristics and advantages of the present invention will be further clarified by the following detailed description of some embodiments, presented as a non-limiting example of the present invention, with reference to the attached figures, in which:

- figures 1 and 2 show a schematic side view of a system according to a first embodiment of the present invention respectively in an at-rest and maximum tilting configuration, with some parts in transparency;

- figures 3 and 4 show a schematic side view of a system according to a second embodiment of the present invention respectively in an at-rest and maximum tilting configuration, with some parts in transparency;

- figures 5A and 5B schematically show an example of decomposition of the forces according to the present invention.

Detailed description of some embodiments of the invention

The figures show an articulation system 1 for chairs according to the present invention.

For the sake of conciseness, the same reference numbers will be used for corresponding elements in the various embodiments. The articulation system 1 comprises a rigid frame 2, which is for example designed to be rigidly mounted at an upper end of a stem 3 mounted on a supporting base (not shown) for resting a chair on the ground.

The articulation system 1 further comprises a support for the seat 4, rigid, and a support for the backrest 5, rigid and distinct from the support for the seat, mounted on the frame in an articulated manner. In the shown examples, the support for the seat consists of two separate parts, one for each side of the system.

The chair typically comprises a seat 6 rigidly coupled to the support for the seat 4 and a backrest 7 rigidly coupled to the support for the backrest 5. The stem, the seat and the backrest are shown schematically in dashed lines only in fig. 1 , as for example of a known type.

The articulation system further comprises, for each side of the articulation system, an articulation element 14, rigid and hinged to the frame to rotate about an axis of rotation 15. In the embodiments shown in figures 1 -4, the support for the backrest 5 is a whole (i.e. a single piece) with the articulation element 14. Alternatively (not shown) the support for the backrest can be distinct from the articulation element, but rigidly fixed to the latter. In both cases, during the tilting, the backrest simply rotates around the axis of rotation 15, for example for a maximum tilting amplitude of 18°.

In an alternative embodiment, not shown, the support for the backrest is connected to the articulation element in an articulated manner, similarly to the articulation of the support for the backrest with respect to the frame and to the support for the seat described in the aforesaid document WO 2009/15381 1 A1 , to which reference is made for details.

The support for the seat 4 is directly hinged to the articulation element in a hinge point 16. For example, as shown, the hinge can be made with a single pin passing through the entire system, or a pair of pins, one for each side, each pin being firmly constrained to the support for the seat 4 and the articulation element 14.

Typically, the articulation system 1 comprises at least one elastic element 20 (shown partially or schematically), for example a spring, operatively interposed between the frame 2 and the support for the seat 4, to counteract the rotationally translation of the seat with respect to the at-rest configuration, all the greater the wider is the rotationally translation.

The articulation system further comprises, for each side, a pin 9 firmly constrained to a front portion of the frame 2 and a slot 8 afforded in a front portion of the support for the seat 4, the pin engaging the slot slidably.

In the shown examples, each slot 8 is afforded in a side and vertical wall of the respective part of the support for the seat and each pin 9 protrudes from a side and vertical wall of the frame towards the outside.

Preferably, the pin 9 is integral (that is, in a single piece) with the frame.

The articulation element 14 and the pin-slot coupling allow the articulation system to assume an at-rest configuration (shown in figures 1 and 3), in the absence of tilting forces acting on the support for the seat and/or on the support for the backrest (or when said tilting forces do not exceed the possible preloading force of the elastic element 20), and a maximum tilting configuration (shown in figures 2 and 4), where the support for the seat is rotationally translated with respect to its position in the at-rest configuration and the support for the backrest assumes the maximum possible tilting with respect to the at-rest position. During the rotationally translation the slot slides with respect to the pin. Typically, in the at- rest configuration, the seat 6 is arranged horizontally and/or the backrest 7 is substantially vertical.

It is observed that figures 1 -4 show a view of one side of the system 1 , the side being defined with respect to the cinematic axes. In other words, by the sides of the system it is meant the two opposite portions of the system with respect to a median plane of the system perpendicular to the cinematic axes (e.g. axes 15, 16 etc). Typically, the articulation system is specular with respect to the median plane, possibly with the exception of the elastic element which may be present only on one side or may have differences between the two sides. Therefore, for example, the elements illustrated are symmetrically reproduced also on the side of the system opposite to that shown.

In an embodiment, as for example shown in figures 1 -4, in a reference plane perpendicular to the axis of rotation 15 (for example the lying plane of the figures), the slot 8 has an extension at least partially curvilinear with concavity facing substantially inferiorly and rearwardly.

In an embodiment, not shown, the slot can have an extension at least partially rectilinear. Conventionally, in the reference plane, an angle 18 is defined as the acute angle formed between a tangent 1 1 to the slot in an instant contact point 12 between the slot and pin and a horizontal line 13, the angle being above the horizontal line.

Preferably the angle faces rearwards.

In the embodiment shown in figures 1 and 2, the slot 8 has entirely curvilinear extension, wherein the angle 18 increases monotonically throughout the movement of the articulation system from the at-rest configuration to the maximum tilting configuration.

Exemplarily the curvilinear extension follows a spline and the acute angle in the at-rest configuration is equal to 45° and in the maximum tilting configuration it is equal to 80°. Exemplarily the curvilinear extension can be approximated to a circular arc having radius, at the centre of the slot, of 18.5 mm.

In this embodiment, in the at-rest and maximum tilting configuration, the hinge point 16 is located, with respect to the reference plane, in front of a vertical line 30 passing through the axis of rotation 15, and above a horizontal line 13 passing through the axis of rotation. In this embodiment, in the transition from the at-rest configuration to that of maximum tilting, the seat rotationally translates in such a way that both the rear and the front part (the latter in greater measure) lift and retract (giving the 'weighing people' effect). Advantageously, the relative ratio between the lift and the retraction of the front part varies as a function of the tilting, wherein in the final part of the movement the lift has a higher prevalence than in the beginning of the movement.

This embodiment can therefore be advantageously free of a system for adjusting the reaction of the elastic element. For example, a first end of the elastic element 20 is directly abutted to a fixed pin 17 firmly constrained to the frame and arranged in a front-lower portion of the frame and a second end of the elastic element is directly abutted to the hinge pin coinciding with the hinge point 16.

In the embodiment of figures 3 and 4, the extension of the slot is entirely curvilinear and can be exemplarily approximated to a circular arc having radius, at the center of the slot, of 157,5mm. Exemplarily the angle 18 in the at-rest configuration is equal to 3°.

In this embodiment, in the at-rest configuration and in the maximum tilting configuration, the hinge point 16 is preferably located in back of the vertical line 30 with respect to the axis of rotation 15, and above the axis of rotation. In this embodiment, in the transition from the at-rest configuration to that of maximum tilting, the seat rotationally translates moving rearwards and in such a way that the front part remains substantially at the same height

(the 'weighing-people' effect is therefore negligible). Therefore, preferably, this embodiment comprises an adjusting system of the reaction of the elastic element. For example, in addition to a first elastic element 20, placed on one side of the system 1 and having a first end directly abutted to a fixed pin 22 firmly constrained to the frame and a second end directly abutted to the pin of the hinge point 16, it is provided a second elastic element 21 having a first end abutted to a pin 23 movable with respect to the frame 2 (by an adjustment system operated by the user and not shown in detail being, for example, of the known type) and a second end abutted to the pin of the hinge point 16. Figures 5A and 5B schematically show, for purely illustrative purposes, the slot-pin coupling, respectively in the at-rest configuration and in the maximum tilting configuration. The number 50 indicates the weight force and the number 51 indicates the action force, these forces acting on the slot 8 of the support. The ratio between the magnitudes of the weight force and the action force is totally arbitrary, for illustrative purposes only.

For the sake of descriptive clarity, the application point of these forces is assumed to be the instants contact point 12 between the slot and the pin.

The numbers 50 'and 51 ' indicate the respective component of the forces along the tangent 1 1 at the slot in the contact point 12 and the numbers 50 "and 51 " indicate the respective component of the forces along the direction perpendicular to the tangent 1 1 .

The vector 52 indicates the constraining reaction between pin and slot, perpendicular to the tangent 1 1 , having magnitude equal to the sum of the components 50 "and 51 ". The components 50 "and 51 " and the reaction 52 push the pin and the slot one against each other and are responsible for the sliding friction.

Although in the shown examples the side walls of the support for the seat are externally with respect to the frame, the present invention also provides for solutions (not shown) in which said side walls are located internally. In this case the pins (which can also be only one) protrude from the respective wall of the frame towards the inside, instead of towards the outside, as in the shown examples. The connecting element, which in the shown examples is interposed between frame and support for the seat, can alternatively be placed internally to the frame or externally to the support for the seat. The present invention also provides for solutions (not shown) wherein the slots have an extension comprising either a rectilinear portion, preferably comprising the point where the pin is located in the at-rest configuration, and a curvilinear portion, preferably comprising the point in which the pin is located in the maximum tilting configuration.

The present invention also provides for solutions (not shown) wherein the rectilinear extension of the slot is arranged horizontally, or vertically or with an inclination opposite to that shown in figures 3 and 4 (frontally facing angle).

The present invention also provides for solutions (not shown) wherein the rectilinear slot is combined with a location of the hinge point in front of the vertical line 30 with respect to the axis of rotation.

Furthermore, the present invention also provides for solutions (not shown) wherein the curvilinear slot is combined with a location of the hinge point in back of the vertical line 30 with respect to the axis of rotation.