DK201770887A1

DK201770887A1 - Roof window with improved spring assembly

Info

Publication number: DK201770887A1
Application number: DKPA201770887A
Authority: DK
Inventors: Ryberg Jesper
Original assignee: Vkr Holding A/S
Priority date: 2017-11-24
Filing date: 2017-11-24
Publication date: 2019-06-12
Also published as: CN111373106B; EP3714116B1; ES2895371T3; CN111373106A; HUE056557T2; PL3714116T3; WO2019101279A1; EP3714116A1; DK180218B1; DK201770887A9

Abstract

In the roof window intended for installation in an inclined roof surface, the lifting device (10) includes a lifting arm (14) inserted between the primary frame (1) and the at least one secondary frame (2, 3) of the roof window. The lifting arm (14) has a first end (12) rotatably connected with a sledge system(30) slidably connected with the primary frame (1) in a sledge guidance (16) and a second end (13) rotatably connected with the at least one secondary frame (3). The lifting device (10) furthermore includes a spring assembly (20) configured to be coupled to the sledge system (30) and in turn the first end (12) of the lifting arm (14) by means of a coupling mechanism, such that the spring assembly (20) is able to assume an uncoupled condition and a coupled condition relative to the sledge system (30). In turn, the spring assembly comprises a main spring system (29) and a buffer spring system (28) inserted between the main spring system (29) and the lifting device (10). According to the invention, the buffer spring system (28) comprises an outer spring (281) and an inner spring (282) of substantially identical pre-defined lengths.

Description

Title of Invention

Roof window with improved spring assembly.

Technical Field

The present invention relates to a roof window, particularly for installation in an inclined roof surface, comprising a primary frame, at least one secondary frame, such as a sash and/or an intermediate frame, and a lifting device including a lifting arm inserted between the primary frame and the at least one secondary frame, the lifting arm having a first end rotatably connected with a sledge system slidably connected with the primary frame in a sledge guidance and a second end rotatably connected with the at least one secondary frame, the lifting device furthermore including a spring assembly configured to be coupled to the sledge system and in turn the first end of the lifting arm by means of a coupling mechanism, such that the spring assembly is able to assume an uncoupled condition and a coupled condition relative to the sledge system, the spring assembly comprising a main spring system and a buffer spring system inserted between the main spring system (29) and the lifting device.

Background Art

Windows for installation in an inclined roof surface include the pivoting type hinged at or near the centre, the top-hinged type, and finally the roof windows that are top-hinged during normal operation but which pivot for cleaning. Examples of top-hung windows that pivot for cleaning are for instance disclosed in Applicant’s WO-A-89/10460, EP 0 733 146 B1, EP 1 873 323 B1 and EP 2 762 665 A2. To make it possible to pivot the window sash approximately 180° to a convenient cleaning position, the sash structure is connected with an intermediate frame with frame arms, which in the closed position of the window are positioned between the upper parts of the frame and sash side members, and which during normal use of the window as a top-hung window follow the sash side members.

In roof windows in which the operation takes place either entirely or

DK 2017 70887 A1 partially about a hinge axis at the top, it is known to balance at least part of the weight of the movable components by means of a lifting device inserted between a pair of associated side members of the stationary frame and the sash element, respectively, and having one end pivotally connected with one of the side members while its other end is hinged to a sliding shoe displaceable along the other side member, said shoe being urged by a tension spring in a direction so as to exert an outwardly directed pressure through the lever on the sash element. The purpose of this arrangement is to facilitate opening the window, and the dimensions may be chosen so that the spring can retain the top-hinged frame in equilibrium in a desired opening position.

The provision of an auxiliary or buffer spring to modulate the spring force during the opening and closing movement of the sash relative to the frame is well-known from Applicant’s above-mentioned EP 0 733 146 B1. Between the slide shoe and the lifting spring is an auxiliary spring, the prestressing of which can be adjusted in dependence on the roof inclination, by a manually operated adjusting element, for supplementing the spring force from the lifting spring within a given range of the opening angle of the window. Although this arrangement has proven to function well, the ever-increasing need for better insulated windows has resulted in multi-layer panes and consequently considerably higher weight of the sash for a given window size.

Summary of Invention

It is therefore the object of the invention to provide a roof window, in which operation of the roof window is facilitated within a large range of roof inclinations also for windows with heavy-weight multi-layered panes.

This is achieved with a roof window of the kind mentioned in the introduction, which is furthermore characterised in that. the buffer spring system comprises an outer spring and an inner spring of substantially identical pre-defined lengths.

It is thereby possible to provide a spring assembly in which the spring characteristics of the buffer system are improved in that the outer and inner

DK 2017 70887 A1 springs together have a spring constant that is larger than a single spring of the same length. In turn, the spring assembly has larger stiffness meaning that easy operation of roof windows having a heavier sash is made possible. Conversely, the improved spring characteristics entail that the length dimensions of the lifting device may be kept, rather than providing the buffer spring as a single, longer spring.

The outer spring and the inner spring may have mutually different spring constants. This is in particular advantageous as the inner spring may easily be formed with a reduced thread dimension and diameter compared to the outer spring.

In a presently preferred embodiment, the outer spring and the inner spring of the buffer spring system have mutually opposite thread directions. This has surprisingly proven to render particularly satisfactory spring characteristics of the buffer spring system.

Other features and advantages will appear from the following description of preferred embodiments and appended drawings.

Brief Description of Drawings

In the following description embodiments of the invention will be described with reference to the schematic drawings, in which

Fig. 1 is a perspective view of a roof window, seen from the interior side and in an open position;

Fig. 2 is a perspective view of details of a roof window with a lifting device, in a first embodiment of the invention and corresponding to the open position of the roof window of Fig. 1;

Fig. 3 is an isometric view of the details of the roof window of Fig. 2, in a closed position of the window;

Fig. 4 is a partially exploded view of the details of Fig. 3;

Fig. 5 is an isometric view of the spring assembly shown in Fig. 3;

Fig. 6 is a partial isometric view, on a larger scale, of the spring assembly shown in Fig. 5;

Fig. 7 is a partial exploded isometric view of some elements of Fig. 6;

DK 2017 70887 A1

Figs 8a to 8e are isometric views of the elements of Fig. 7;

Fig. 9 is a partial isometric view, on a larger scale, of details shown in Fig. 3;

Figs 10 and 11 are partial isometric views of details of the lifting device of the roof window in a second embodiment of the invention;

Figs 12a to 12h are schematic partial side views of an embodiment of the roof window of the invention, during coupling and uncoupling of the lifting device;

Figs 13 to 18 are partial or full perspective views of details of the roof window in the second embodiment;

Fig. 19 shows a schematic perspective view of details of a roof window in a third embodiment of the invention;

Fig. 20 shows a schematic side view of details of a roof window in a fourth embodiment of the invention; and

Figs 21 to 24 show partial or full, exploded, or sectional views of details of a roof window in a fifth embodiment of the invention.

Description of Embodiments

Referring initially to Fig. 1, the general configuration of a roof window which is top-hinged during normal operation and which pivots for cleaning is shown. Such a window is shown and described in further detail in Applicant’s above-mentioned European patent No. 0 733 146 B1, the contents of which are hereby incorporated by reference. In Fig. 1, a lifting device 10 is indicated, which, referring now also to Fig. 2 to 9, forms part of a first embodiment of the roof window according to the present invention.

The roof window comprises a primary frame in the form of a stationary frame 1 configured for installation in an inclined roof surface. At least one secondary frame is connected to the stationary frame 1, in the embodiment shown a first secondary frame in the form of a sash 2 carrying a pane 4, and a second secondary frame in the form of an intermediate frame

3. The intermediate frame 3 is fastened to the stationary frame at a top mounting fitting 5, and the sash 2 is hinged at the top of the roof window, via

DK 2017 70887 A1 the intermediate frame 3 to the stationary frame 1, to render the roof window top-hung during normal operation. The sash 2 is also pivotally connected to the intermediate frame 3 in order to be able to rotate the sash 2 to provide access to the outside of the pane 4, for instance for cleaning purposes. To that end, the intermediate frame 3 is provided with a frame hinge part 6 of pivot hinge fitting. Although not shown in detail, it is clear to the skilled person that the sash 2 is provided with the counterpart sash hinge part of the pivot hinge fitting.

A lifting device 10 is composed of a spring arrangement 20 and a sledge system 30 which cooperate to assist in the opening of the window, that is, bringing the secondary frame or frames to an angled position relative to the primary frame. Here, from a closed position, the user operates the operating device of the window in the form of a handle 7 at the bottom member of the sash 2. The bias of the spring arrangement 20 of the lifting device 10 acts on a lifting arm 14 inserted between the stationary frame 1 and the intermediate frame 3. In turn, the lifting arm 14 exerts a moment on the intermediate frame 3 and hence to the sash relative to an axis through a top hinge pin 11. In combination with the force exerted by the user on the handle 7, transferred to a lifting moment, the moment resulting from the weight of the sash 2 with pane 4 is overcome. Closing the window from the open position entails the opposite movements of the sash 2 and relevant parts of the lifting device 10. A similar lifting device may be provided at each side of the roof window. Finally, the roof window is provided with a ventilation device 8 acting to allow passage of air also in the closed position of the window.

In further detail, the lifting arm 14 having a first end 12 rotatably connected with a sledge system 30 slidably connected with the primary frame 1 in a sledge guidance 16 and a second end 13 rotatably connected with the at least one secondary frame 3, the lifting device 10 furthermore including a spring assembly 20 configured to be coupled to the sledge system 30 and in turn the first end 12 of the lifting arm 14 by means of a coupling mechanism, such that the spring assembly 20 is able to assume an uncoupled condition and a coupled condition relative to the sledge system 30.

DK 2017 70887 A1

Further details and advantages of the particular coupling mechanism are the subject of Applicant’s co-pending patent application filed on the same day as the present application. Here, the coupling mechanism comprises a first coupling member 21 associated with the spring assembly 20 and is adapted to cooperate with a second coupling member 31 associated with the sledge system 30, the first coupling member including a hook element 21 and the second coupling member including receiving means 31 formed in the sledge system 30 and configured to cooperate with said hook element 21 in the coupled condition, the hook element 21 being configured to assume at least a non-engagement position and an engagement position, and the coupling mechanism furthermore comprising a coupling plate 25 arranged to assume at least a first position corresponding to the coupled condition and a second position corresponding to the uncoupled condition, the coupling plate 25 allowing the hook element 21 to assume said non-engagement position. Operation of the presently preferred embodiments will be described in further detail in connection with Figs 12a to 12h.

In the embodiment shown, the hook element 21 of the first coupling member has at least one hook portion 214 facing the sledge system 30. This provides for a particularly easy and safe coupling procedure.

Even though the coupling mechanism could function well with only a single hook and receiving means pair, it is preferred that the hook element 21 as shown has a U-shaped configuration including a base plate portion 212 and two flange portions 213, one hook portion 214 being provided at each of the flange portions 213.

Turning now in particular to the exploded view of Fig. 7 and the detailed individual views of Figs 8a to 8e, the spring assembly 20 comprises a tension rod 22 including a rod portion 221 extending substantially along the length of the spring assembly 20, and a head portion 222 facing the hook element 21, the tension rod 22 being provided with at least one inclined portion 223, 224 at the transition between the head portion 222 and the rod portion 221.

In the present embodiment, the hook element 21 is provided with an

DK 2017 70887 A1 aperture 211 in the base plate portion 212 to allow passage of the tension rod 22 such that the head portion 222 of the tension rod 22 is retained within the U-shape at the front side of the base plate portion 212 and the rod portion

221 at the back side.

The coupling plate 25 of the coupling mechanism has a base portion 252 located at the back side of the base plate portion 212 of the hook element 21 and is provided with an oblong aperture 251 to allow passage of the rod portion 221 of the tension rod 22, and wherein the coupling plate 25 is slidable in the plane of the base portion 252 substantially perpendicularly to the rod portion 221 such that when in a bottom position, the hook portion 21 is capable of assuming its engagement position, and in a top position, the hook portion 21 is kept in its non-engagement position.

Furthermore, the coupling plate 25 is here provided with a bottom flange 253 formed on the base portion 252 to abut on the at least one inclined portion 223 in the non-engagement position via the base plate portion 212. This brings about the actual retention of the hook element 21 in its nonengagement position in the embodiment shown.

The coupling plate 25 is provided with a cut-out 254 at the top of the base portion 252 to form at least one, preferably two, upstanding lugs 255. This eases the application of tools and the re-entry into the coupling position. Furthermore, the cut-out 254 allows for accommodation of reinforcement or stiffening elements for the sash.

In principle the configuration of the spring assembly 20 may be carried out in any suitable manner as long as it fulfils the requirements due to the relatively large forces involved. A robust structure as in the present embodiment includes an angle element 23 provided near the head portion

222 of the tension rod 22, having an aperture 231 to allow passage of the rod portion 221 of the tension rod 22.

Here, an end piece 24 is provided, having an aperture 241 to allow passage of the rod portion 221 of the tension rod 22, the end piece 24 furthermore having an upstanding wall 242, on which a protrusion 243 is formed to surround the aperture 241 of the end piece 24, and wherein two

DK 2017 70887 A1 protruding flanges 244 are provided at either side of the upstanding wall 242 to define a track 245.

As shown, the angle element 23 is provided with an upstanding leg 232 and a bottom leg 233, the aperture 231 of the angle element 23 being provided in the upstanding leg 232, and wherein two protruding flanges 234 are provided substantially in parallel to the bottom leg 233 and each defining a gap 235 relative to the upstanding leg 232.

In the assembled state, the end piece 24 is positioned with the back side of its upstanding wall 242 abutting the front side of the upstanding leg 232 of the angle element 23, between the bottom leg 233 and the protruding flanges 234, and wherein the base portion 252 of the coupling plate 25 is received slidably in the track 245 of the end piece and the respective gaps 235 of the angle element 23.

In order to obtain a smooth movement during the coupling procedure, each flange portion 213 of the U-shaped the hook element 21 here includes an apex 210 at the front end, a rounded transition 215 between the apex 210 and the hook portion 214, a bottom edge portion 218 between the hook portion 214 and the base plate portion 212, an opposite top portion 217, and at least one inclined edge portion 216, 219 between the top portion 217 and the apex 210.

Further details of the sledge system 30 are shown in a second and presently preferred embodiment in Figs 10 to 11 and 13 to 16. Elements having the same or analogous function as in the first embodiment carry the same reference numerals.

In the preferred embodiments shown, a sledge 31 of the sledge system 30 comprises a bottom portion 310, a first wall portion 311 and a second wall portion 312, one receiving recess 314 being preferably provided at each of the transitions between the first wall portion 311 and the bottom portion 310 and between the second wall portion 312 and the bottom wall portion 310, and wherein a hole 315 is provided in the first wall portion 311 and a hole 316 is provided in the second wall portion 312 for receiving an axle 40 connected to the first end 12 of the lifting arm 14.

DK 2017 70887 A1

In the presently preferred embodiments described herein, the receiving means of the second coupling member include at least one receiving recess 314 in the sledge 31 of the sledge system 30. The second coupling member could also take other shapes. One conceivable, alternative solution would be to utilise the axle 40 as receiving means. This alternative solution is shown as the fourth embodiment shown in Fig. 20.

Further details of the axle 40 shown in Figs 17 to 19 are given for information purposes only and are the subject of Applicant’s co-pending patent application filed on the same day as the present application.

In order to control the frictional forces more precisely at the movement of the sledge system 30 in the sledge guidance 16, the sledge system 30 comprises a runner 32 in the embodiment shown, configured to accommodate the sledge 31 and being provided with at least one recess 324 opposite the respective receiving recess 314 of the sledge 31. Further details of the runner 32 are shown in Figs 15 to 17 for information purposes only; such details are the subject of Applicant’s co-pending patent application filed on the same day as the present application. It is noted that the present invention is applicable also to sledge systems in which there is no runner, or a runner of a different configuration. In the embodiment shown, the runner 32 is formed of a plastic material such as POM which may also be provided with a coating of for instance Teflon® or other treatment having the function to act as a lubricant to reduce the friction.

The narrowing 313 on the runner 32 is made for reasons of space availability. By forming the runner slenderer in one side, it is possible to make 322a thicker, and tolerances are accommodated in the side where 322 is - in that way the axle 40 is guided such that the end 402 does not protrude to wear on the side of the guidance.

An edge portion 328 is provided as a cut-out in order to allow space for other parts of the assembly. A flange portion 329 scrapes the guidance to the sides of the guidance.

Figs 5 and 6 show the general components of the spring assembly 20 of a roof window, and reference is in particular made to Figs 21 to 24.

DK 2017 70887 A1

The general components of the spring assembly are: a buffer spring system 28 acting as an auxiliary system to a main spring system 29. In order to adapt one and the same spring assembly to varying roof inclinations, an adjustment means is provided in the form of a movable adjustment plate 26 which is placed in one of several recesses 27a in a spring casing 27 accommodating the buffer spring system 28.

As shown, the buffer spring system 28 comprises an outer spring 281 and an inner spring 282 of substantially identical pre-defined lengths. The pre-defined length of the springs 281, 282 is typically chosen according to the dimensions of other parts of the lifting device. In the mounted condition, the inner spring 282 is accommodated inside the outer spring 281 so as the axes of the respective springs extend substantially co-axially.

In the embodiment shown, the outer spring 281 and the inner spring 282 of the buffer spring system 28 have mutually different spring constants, and the outer spring 281 and the inner spring 282 of the buffer spring system 28 have mutually opposite thread directions.

The spring assembly 20 here further comprises at least one spring plug 283, 284 configured to be in contact with the respective end of the inner spring 282 of the buffer spring system 28 in the mounted condition. That is, here the two spring plugs 283 and 284 form the respective ends of the buffer spring system 283 and 284. At the manufacture stage, it is advantageous that the spring plugs 283, 284 is provided with friction-increasing means at the surface configured to be in contact with the respective end of the inner spring 282 of the buffer spring system 28. In this way, the two springs are kept together in a safe manner.

In principle, the coupling mechanism may be designed in any suitable manner and operated accordingly, as long as the fundamentals as outlined initially are fulfilled. As will be understood from the following description of the operational steps performed during coupling and uncoupling of the spring assembly 20 from the sledge system 30 in the embodiments shown and described in the above with reference to the sequence of Figs 12a to 12h, the hook element 21 of the first coupling member is in the presently

DK 2017 70887 A1 preferred embodiments arranged to slidably contact the sledge system 30 in one direction and to engage said second coupling member 31 in another direction, namely by being forced in the opposite direction to ensure engagement with the first and second coupling members by the components of the lifting device itself. Not all elements are indicated by reference numerals in Figs 12a to 12h for ease of readability and it is referred to the above description of the remaining Figures for detailed explanations.

Turning first to Fig. 12a, the roof window is shown in its closed condition, that is the secondary frame or frames are lying substantially in parallel with the primary frame. Here, the intermediate frame 3 represents the secondary frames, and the position of the sledge guidance 16 and the spring assembly 20 indicate the corresponding position of the stationary frame 1 although not shown, i.e. here substantially horizontal for reasons of clarity, even though the stationary frame 1 will most often be installed in an inclined roof surface prior to coupling the spring assembly 20 to the sledge system 30. The coupling mechanism is not yet active, and the sledge system 30 is consequently located at a distance from the spring assembly 20. From the break-out enlarged view of Fig. 12a is apparent the position of the coupling plate 25 in its first position, i.e. bottom position, and corresponds to the engagement position of the hook element 21 in which coupling is possible.

When opening the window by rotating the intermediate frame 3 about the top hinge pin 11, the lifting arm 14 rotates as well, and the sledge system 30 moves in the direction of the spring assembly 20 and its associated first coupling member in the form of hook element 21. When approaching the position shown in Fig. 12b, the hook element 21 slides up on the sledge 31 and is able to rotate slightly on the tension rod 22, as the base plate portion 212 of the hook element 21 is able to move relative to the inclined portion 223 on the back side of the head portion 222. The coupling plate 25 is still in its first, or bottom position. During the continued movement in the opening direction of the intermediate frame 3 and the ensuing translational movement of the sledge system 30, the hook portions 214 of the hook element 21 will reach a position just above the receiving recesses 314. As the spring

DK 2017 70887 A1 assembly 20 is biased and exerts a pull in the tension rod 21, to the left in Fig. 12b, the hook element 21 will be subjected to a clockwise moment which in turn forces the hook portions 214 to enter the receiving recesses 314 of the sledge 31.

Once the coupling has taken place, the hook element 21 withdraws further to the left due to the bias of the spring assembly, the hook portions 214 are drawn into stable engagement with the receiving recesses 314 in the sledge 31 as shown in Fig. 12c.

The window is now ready for normal operation, and during the subsequent closing of the intermediate frame 3, the movement of the lifting arm 14 pulls the sledge system 30 towards the top of the window, i.e. to the right as shown in Fig. 12d. The closing movement takes place during simultaneous further bias of the spring assembly 20 as is known per se.

Should the need for removal of the secondary frame or frames arise, the roof window is prepared for uncoupling by first rotating the intermediate frame 3 to an open position. Then, the coupling plate 25 is brought from its first or bottom position to its second or top position by an upwards movement, indicated by arrow A in Fig. 12f. In the embodiment shown, the window is not entirely open in the position of Fig. 12f, that is, the sledge system 30 is not in its left-most position but distanced from the spring assembly 20 by some millimetres. In practice, the sash is rotated to its fully open position and then moved slightly back in the closing direction such that the coupling plate 25 is free to be moved upwards. In this position the coupling plate is held in its upper most position by means of friction. The hook element 21 is still engaged with the sledge system 30 as the hook portions 214 are gripping the recesses 314 of the sledge 31 until the user grabs the sash and lifts it. During this movement, the hook element 21 is allowed to turn upwards in the counter-clockwise direction. This brings the hook element 21 to its nonengagement position in which coupling is not possible. This action is brought about in that the bottom flange 253 of the coupling plate 25 abuts on the back side of the base plate portion 212 of the hook element 21. The hook element 21 is thus affected by a counter-clockwise moment which keeps it in the nonDK 2017 70887 A1 engagement position. The secondary frame(s) may now be dismantled from the primary frame, here thus the intermediate frame 3 with sash 2 and pane 4 lifted off the connection with the stationary frame 1. The lifting of the coupling plate 25 may in principle take place in any suitable manner. However, in order 5 to ensure that unwarranted release does not take place, use of a suitable tool is preferably prescribed.

Bringing the roof window back to a position prepared for coupling takes place by simply rotating the intermediate frame 3 in the closing direction. Towards the end of the closing movement, the lifting arm 14 pushes 10 the coupling plate 25 downwards, in the direction of arrow B in Fig. 12h. Once the counter-clockwise moment previously acting on the hook element 21 is released, the hook element 21 is able to rotate in the clockwise direction to assume the position shown in Fig. 12a.

Above the invention has been described with reference to the sash 15 and frame of a roof window, but it will be understood that it also applies to other frame structures, such as for example door frames.

DK 2017 70887 A1 of reference numerals primary frame (stationary frame) first secondary frame (sash) second secondary frame (intemediate frame) pane top mounting fitting frame hinge part of pivot hinge fitting

Handle ventilation device lifting device hinge pin first end of lifting arm second end of lifting arm lifting arm top bearing fitting sledge guidance spring assembly first coupling member / hook element

210 apex

211 aperture

212 base plate portion

213 flange portion

214 hook portion

215 rounded transition edge portion

216 inclined edge portion

217 top portion

218 bottom edge portion

219 inclined edge portion tension rod

221 rod portion

222 head portion

223 first inclined portion

224 second inclined portion

225 adjustment ring angle element

231 aperture

232 upstanding leg

233 bottom leg

234 protruding flange

235 gap

DK 2017 70887 A1 end piece

241 aperture

242 upstanding wall

243 protrusion

244 protruding flange

245 track coupling plate

251 oblong aperture

252 base portion

253 bottom flange

254 cut-out

255 upstanding lug adjustment plate spring casing

27a recess for adjustment plate buffer spring system

281 outer spring

282 inner spring

283 spring plug

284 spring plug main spring system

291 spring

292 spacer tube sledge system sledge

310 bottom portion

311 first wall portion

312 second wall portion

313 narrowing

314 receiving recess

315 hole

316 hole

317 first opening

318 second opening runner

320 bottom portion

321 first wall portion

321a first journal section

322 second wall portion

322a second journal section

323 bowl portion

324 recess

325 cut-out

326 cut-out

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327 resilient upstanding portion

328 edge portion

329 flange portion axle

401 rifled end

402 other end

403 markings

404 location mark friction brake device friction brake element first cone second cone spring plug

Claims

Claims

1. A roof window, particularly for installation in an inclined roof surface, comprising a primary frame (1), at least one secondary frame (2, 3), such as a sash (2) and/or an intermediate frame (3), and a lifting device (10) including a lifting arm (14) inserted between the primary frame (1) and the at least one secondary frame (2, 3), the lifting arm (14) having a first end (12) rotatably connected with a sledge system (30) slidably connected with the primary frame (1) in a sledge guidance (16) and a second end (13) rotatably connected with the at least one secondary frame (3), the lifting device (10) furthermore including a spring assembly (20) configured to be coupled to the sledge system (30) and in turn the first end (12) of the lifting arm (14) by means of a coupling mechanism, such that the spring assembly (20) is able to assume an uncoupled condition and a coupled condition relative to the sledge system (30), the spring assembly comprising a main spring system (29) and a buffer spring system (28) inserted between the main spring system (29) and the lifting device (10), c h a r a c t e r i s e d in that the buffer spring system (28) comprises an outer spring (281) and an inner spring (282) of substantially identical pre-defined lengths.
2. A roof window according to claim 1, wherein the outer spring (281) and the inner spring (282) of the buffer spring system (28) have mutually different spring constants.
3. A roof window according to claim 1 or 2, wherein the outer spring (281) and the inner spring (282) of the buffer spring system (28) have mutually opposite thread directions.
4. A roof window according to any one of the preceding claims,

DK 2017 70887 A1 wherein the spring assembly (20) further comprises a spring casing (27).
5. A roof window according to claim 4, wherein said spring casing (27) further includes at least one recess (27a) for receiving an adjustment plate (26).
6. A roof window according to any one of the preceding claims, wherein the spring assembly (20) further comprises at least one spring plug (283, 284) configured to be in contact with the respective end of the inner spring (282) of the buffer spring system (28) in the mounted condition.
7. A roof window according to claim 6, wherein said at least one spring plug (283, 284) is provided with friction-increasing means at the surface configured to be in contact with the respective end of the inner spring (282) of the buffer spring system (28).
8. A roof window according to any one of the preceding claims, wherein the coupling mechanism comprises a first coupling member (21) associated with the spring assembly (20) and adapted to cooperate with a second coupling member (31) associated with the sledge system (30), in that the first coupling member includes a hook element (21) and the second coupling member includes receiving means (31) formed in the sledge system (30) and configured to cooperate with said hook element (21) in the coupled condition, wherein the hook element (21) is configured to assume at least a non-engagement position and an engagement position, and wherein the coupling mechanism furthermore comprises a coupling plate (25) arranged to assume at least a first position corresponding to the coupled condition and a second position corresponding to the uncoupled condition, the coupling plate (25) allowing the hook element (21) to assume said non-engagement position.
9. A roof window according to claim 8, wherein the spring assembly

DK 2017 70887 A1 (20) comprises a tension rod (22) including a rod portion (221) extending substantially along the length of the spring assembly (20), and a head portion (222) facing the hook element (21), the tension rod (22) being provided with at least one inclined portion (223, 224) at the transition between the head

5 portion (222) and the rod portion (221).
10. A roof window according to claim 9, wherein the hook element (21) is provided with an aperture (211) in a base plate portion (212) of the hook element (21) to allow passage of the tension rod (22) such that the head

10 portion (222) of the tension rod (22) is retained within the U-shape at the front side of the base plate portion (212) and the rod portion (221) at the back side.