The invention relates to an operating mechanism for a window covering comprising a drive shaft for driving and thereby raising and/or lowering a window covering and a drive wheel that engages with an engagement means for rotating the drive wheel. The invention further relates to a window covering coupled to the drive shaft of such an operating mechanism.

Numerous operating mechanisms for raising and/or lowering window coverings such as roller blinds, pleated blinds, Venetian blinds and Roman blinds are known, each of which offers a different user experience. One universally known variant is an operating mechanism provided with a cord or chain by means of which the window covering can be raised by (manually) pulling on the cord or chain, but for which the force of gravity is used for lowering the window covering, whereby the window covering comes down under its own weight. The advantage of this variant is that lowering the window covering requires no effort from the user.

To prevent the window covering from coming down unintentionally under the effect of gravity while it is being raised or is in the raised position, known operating mechanisms of the aforementioned variant are fitted with a drop prevention system. In principle, this system ensures that movement of the window covering in the downwards direction is prevented. However, if the lock is released through the action of a user, for example by moving the cord or chain sideways, the window covering will come down. So as additionally to prevent the cord or chain from moving along with the window covering upon lowering, known operating

mechanisms are also fitted with an uncoupling mechanism by means of which the window covering is allowed to run free.

A disadvantage of the aforementioned known operating mechanisms is that the drop prevention system and the uncoupling mechanism result in the operating mechanism often having a complicated structure, and in it being formed of a large number of parts. The complicated structure of the operating mechanism additionally contributes to the operating mechanism being of increased size, which is

undesirable from a practical and aesthetic point of view. The object of the present invention is therefore to provide an improved operating mechanism for a window covering which allows the window covering to run freely downwards and also overcomes at least one of the aforementioned drawbacks.

To this end, the invention provides an operating mechanism for a window covering, comprising: a stationary carrying structure; a drive shaft, preferably at least partially accommodated within the carrying structure, for driving and thereby raising and/or lowering the window covering; a drive wheel that engages with an engagement means, in particular a cord or chain, for rotating the drive wheel; a switchable coupling for switching between a coupled state in which the drive shaft is coupled to the drive wheel and an uncoupled state in which the drive shaft is uncoupled from the drive wheel; and an operating element for switching the coupling between the coupled state and the uncoupled state, wherein the coupling comprises a transmission element, which transmission element can be translated by means of the operating element between: a first position in which the transmission element is coupled to the drive wheel and the coupling adopts the coupled state; and a second position in which the transmission element is uncoupled from the drive wheel and the coupling adopts the uncoupled state, wherein the drive shaft is free to rotate in the uncoupled state.

As already mentioned above, the window covering may be formed by a roller blind, a pleated blind, a Venetian blind or a Roman blind, inter alia. In a typical case, the window covering engages directly with a winding reel that is connected to the drive shaft. Alternatively, the window covering may be hung by means of a cord hanging arrangement, in which case the cords run around winding reels that are connected to the drive shaft. The characteristics of the drive wheel usually depend on the engagement means used, in that the drive wheel is configured such that

engagement of the engagement means with a running surface of the drive wheel without slippage is possible. The drive wheel may be suitable for engaging with an endless engagement means such as an endless cord or an endless chain. To ensure child safety, the engagement means may however also be ended, in the form of a single cord or a single chain, such that there is no danger of strangulation. In the latter case, the engagement means is usually also of limited length. When an ended engagement means is used, the drive wheel is typically configured not only for guiding, but also for winding the engagement means. In this case, the drive wheel may be designed as a reel around which the engagement means is able to wind and/or unwind.

By using the switchable coupling in combination with the operating element, it is possible for a user to switch between the coupled state and the uncoupled state by means of the operating element. In the coupled state, it is possible for the drive shaft to be driven by the drive wheel, with the drive shaft being entrained by the drive wheel, while, in the uncoupled state, the drive shaft is free to rotate, as a result of which the drive shaft and hence the window covering is allowed to run free. In the latter state, the window covering will come down under its own weight due to the effect of gravity, resulting in a window located behind the window covering being at least partially covered thereby. Furthermore, the translatable transmission element ensures that the free rotation or otherwise of the drive shaft is dependent only on the state of the coupling. In particular, the drive shaft will necessarily be free to rotate when the transmission element is in the second position and uncoupled from the drive wheel, which corresponds to an uncoupled state of the coupling. In a typical case, the coupling is configured such that the transmission element is independently translatable relative to the drive shaft and the drive wheel. In such a case, the transmission element is then (continuously) coupled to the drive shaft for the transmission of a rotation from the transmission element to the drive shaft and vice versa.

Particularly advantageously, by means of the operating mechanism according to the invention fitted with only the abovementioned coupling, a user is able to choose when the drive shaft is entrained by the drive wheel and when the drive shaft is able to rotate freely. Because the free running of the drive shaft in the

aforementioned coupling is dependent on and is accompanied by the mutual uncoupling of the drive wheel and drive shaft, the functions of drop prevention system and uncoupling mechanism are combined. By dispensing with the need for a separate embodiment for the drop prevention system and for the uncoupling mechanism, it is therefore possible to simplify the structure of the operating mechanism and to decrease the size of the operating mechanism.

In one possible embodiment of the operating mechanism according to the invention, the transmission element is able to translate between the first and second position in the axial direction relative to the drive shaft. The axial translatability of the transmission element allows the operating mechanism to be made even more compact, since the various components of at least the coupling may thus be aligned axially relative to one another and may even be able to share the same axis of rotation. To allow the transmission element to be continuously rotationally coupled to the drive shaft, the transmission element is able to engage with the inside or outside of the sleeve surface of the drive shaft, wherein, in the first case, the drive shaft must be at least partially hollow and wherein, in the second case, the transmission element must be at least partially hollow, and wherein the transmission element and the drive shaft must overlap one another at least over the translatable distance.

In an advantageous case, the coupling may comprise a pressure element for forcing the transmission element in the direction of the first position. In this case, the pressure element may be a compression spring, in particular a coil spring. In such a case, a first end of the pressure element is adjacent to the drive shaft and a second, opposite end is adjacent to the transmission element. By forcing the transmission element in the direction of the first position, said pressure element ensures that the drive shaft is coupled to the drive wheel by default, by means of which it can be ensured that the drive shaft rotates freely only if a user actively switches the switchable coupling by means of the operating element to the uncoupled state. This prevents the window covering from dropping down unintentionally.

In one embodiment of the operating mechanism according to the invention, the operating element may be able to rotate about an axis of rotation between a first position in which the coupling adopts the coupled state and a second position in which the coupling adopts the uncoupled state. By allowing the operating element to move by rotation, a clear distinction can be drawn between the linear pulling movement on the engagement means, by means of which the drive wheel is generally driven and the window covering is raised, and the movement that is required to switch the coupling between the coupled and uncoupled state. In this way a user is prevented from unexpectedly dropping the window covering instead of raising it. To facilitate the operation of the operating element, the operating element may comprise a lever that is able to rotate about the axis of rotation. In this case, the lever may comprise a feedthrough channel for feeding the engagement means through from the drive wheel out of the operating mechanism. This allows the engagement means to be guided neatly from and to the drive wheel. This furthermore makes it possible for the lever and hence the operating element to be rotated in the direction of rotation of the operating element by means of a change in direction of the engagement means. Consequently, a user is able to operate both the drive wheel and the operating element only by using the engagement means. In an advantageous case, for this purpose, the feedthrough channel is oriented in the longitudinal direction of the lever. The lever may also be able to rotate about an additional axis of rotation that is perpendicular to the axis of rotation, for example by coupling the lever to the axis of rotation of the operating element by means of a universal coupling. This additional degree of freedom affords a user the possibility of also moving the lever in a direction that is perpendicular to the axis of rotation, as a result of which the risk of the operating mechanism being damaged due to incorrect handling of the operating element is decreased.

In another possible embodiment of the operating mechanism according to the invention, the operating element may be coupled to the transmission element via a transmission, wherein the transmission is configured to convert a rotation of the operating element into a translation of the transmission element. In this case, the transmission may be provided with a shaft provided with a cam, which shaft acts as the axis of rotation for the operating element and is coupled to the operating element; and a push rod coupled to the transmission element, wherein the shaft provided with a cam and the push rod are positioned relative to one another such that, upon rotation of the shaft provided with a cam, the cam shifts the push rod in the axial direction relative to the drive shaft. By means of the combination of the shaft provided with a cam and the push rod, a simple yet reliable transmission of forces between the operating element and the transmission element can be achieved, wherein a rotary movement can also effectively be converted into a translation.

In yet another possible embodiment of the operating mechanism according to the invention, the operating mechanism may comprise a rotor, which rotor is coupled to the drive shaft in the second position of the transmission element. This rotor, which rotates together with the drive shaft when the drive shaft rotates freely, increases the moment of inertia of the drive shaft, thereby further increasing the resistance of the drive shaft to a change in the rotational speed of the drive shaft. When the drive shaft rotates freely, the drive shaft will therefore accelerate less upon

lowering/unwinding of the window covering, resulting in the window covering being able to be lowered in a more controlled manner.

So as effectively to increase the mass moment of inertia of the rotor, the rotor may be provided with at least one weight that is connected to the rotor at a distance from the axis of rotation of the rotor. In such a case, the rotor is then provided with a plurality of weights that are connected to the rotor rotationally symmetrically relative to the axis of rotation of the rotor. Furthermore, the rotor may have a mass moment of inertia that is dependent on the speed of rotation of the rotor, which mass moment of inertia increases with increasing speed of rotation. An advantage of this variable mass moment of inertia is that the resistance of the rotor and hence of the coupled drive shaft to angular acceleration further increases as the rotational speed of the drive shaft increases. This provides a greater degree of assurance that the lowering of the window covering will occur in a controlled manner and at an acceptable speed. To provide the aforementioned dependence between speed of rotation and moment of inertia of the rotor, the rotor may comprise at least one weight, which weight is able to be moved in the radial direction of the rotor and which weight is configured to move away from the axis of rotation of the rotor in said radial direction at a predetermined speed of rotation under its own mass inertia. In layman's terms, the aforementioned movement of the at least one weight outwards in the radial direction is caused by the centrifugal force, which, from the weight's perspective, acts on the weight upon rotation.

The rotor may be coupled to the drive shaft via a step-up transmission in the uncoupled state of the coupling, which step-up transmission is configured to make the rotor perform more than one complete rotation for each complete rotation of the drive shaft. By means of this stepping-up effect of the transmission on the rotor, the speed with which the window covering comes down under the effect of gravity in the uncoupled state of the coupling is limited further still. The step-up transmission may comprise a central gear that is connected to the rotor and a rotatable planet gear carrier that is engaged by the transmission element in the second position of the transmission element, wherein the planet gear carrier comprises at least one planet gear that is connected to the planet gear carrier such that it is free to rotate, which at least one planet gear engages with the central gear that is connected to the rotor and with an internal toothing provided on the stationary carrying structure. In this case, the central gear acts as the sun gear, while the internal toothing on the stationary carrying structure acts as the ring gear. The rotational speed of the planet gear carrier will, as already mentioned above, be lower than the rotational speed of the central gear and hence the rotor. Using a planetary gear train as the step-up transmission has the advantage of compact dimensions for a relatively high transmission ratio.

In one possible particular form of embodiment of the operating mechanism according to the invention provided with a rotor, the axes of rotation of the rotor and of the drive wheel, and preferably also the axis of rotation of the transmission element, may be coincident with the axis of rotation of the drive shaft. In this way the operating mechanism may be made especially compact, which facilitates installation in a stationary head rail of a window covering and may also limit the dimensions of this head rail, which is desirable from a practical and aesthetic point of view.

It is possible for the coupling to comprise a gear that can be coupled to the drive wheel and for the transmission element to comprise a gear that, in the first position, engages with the gear that can be coupled to the drive wheel and, in the second position, disengages from the gear that can be coupled to the drive wheel. If the operating mechanism is provided with a rotor, the coupling may furthermore comprise a gear that is coupled to the rotor, wherein the gear of the transmission element is configured such that, in the second position, it engages with the gear that is coupled to the rotor and, in the first position, disengages from the gear that is coupled to the rotor. By means of this gear coupling between the transmission element and the drive wheel (or rotor), transmission of forces between the rotary parts relative to one another that is resistant to wear and without slippage can be achieved. In a further embodiment of the operating mechanism according to the invention, the drive wheel may be provided with an entraining spring that is connected to the drive wheel, which entraining spring deforms upon rotation of the drive wheel and makes contact with the gear that can be coupled to the drive wheel such that the gear that can be coupled to the drive wheel is coupled to the drive wheel and a rotation of the drive wheel is transmitted to the gear that can be coupled to the drive wheel. The coupling between the gear that can be coupled to the drive wheel and the entraining spring then usually takes place on the basis of friction due to the clamping contact with the gear that can be coupled to the drive wheel. The deformation of the entraining spring, which is typically formed by a torsion spring (spiral spring), is dependent on the direction of rotation of the drive wheel, which causes the entraining spring to twist around the longitudinal axis. Via this twisting, the turns of the entraining spring can be brought closer together or moved further apart, resulting in the diameter within the turns decreasing or increasing, respectively. Depending on the positions of the drive wheel and gear that can be coupled to the drive wheel relative to one another, in one of these situations clamping contact is usually made with the gear that can be coupled to the drive wheel, wherein the gear that can be coupled to the drive wheel is carried along with the rotation of the drive wheel. In the other direction of rotation of the drive wheel, the gear that can be coupled to the drive wheel is not engaged by the entraining spring, in which case the gear that can be coupled to the drive wheel will not be carried along with rotation of the drive wheel. This unidirectional rotational coupling is particularly advantageous in the case that the engagement means that engages with the drive wheel is ended, as it is in the case of a single cord or a single chain.

In that case, a rotation of the drive wheel in a first direction is used to drive the drive shaft and thereby raise the window covering, whereby the engagement means for the drive wheel is unwound. A rotation of the drive wheel in a second, opposite direction may also be used to wind the engagement means back onto the drive wheel.

Following on from the above, although not necessarily accompanied by the abovementioned measures, the drive wheel may be coupled to a torsion spring, in particular a spiral spring, such that, upon rotation of the drive wheel, a moment of force is exerted on the drive wheel in a direction opposite the direction of rotation. The torsion spring therefore ensures that the engagement means, usually an ended, single cord or ended, single chain, which engages with and winds around the drive wheel, is, after being unwound by a rotation of the drive wheel in a first direction of rotation, automatically wound back onto the drive wheel as the torsion spring rotates the drive wheel in a second direction of rotation, opposite the first direction of rotation, under the effect of said moment of force. An advantage of using such a torsion spring is that a cord of limited length may be used, which retracts automatically after being pulled out. Additionally, the cord may be single (ended instead of endless). The latter aspects contribute to eliminating or greatly mitigating the strangulation risk for children.

Furthermore, the gear that can be coupled to the drive wheel may make contact with a portion of the stationary carrying structure via a stabilizing spring, at least in a stationary position. In such a case, the gear that can be connected to the drive wheel is, to this end, connected to the stabilizing spring, which stabilizing spring makes contact with a portion of the stationary housing in the stationary position of the gear that can be coupled to the drive wheel so as to subject the gear that can be coupled to the drive wheel to a frictional force. This frictional force must ensure that, in the coupled state of the coupling, the drive shaft does not lower by itself (under the weight of the window covering that can be driven by the drive shaft) but can be maintained in any position desired by the user. In this case, the stabilizing spring is preferably configured such that, upon a sufficiently large rotation of the gear that can be coupled to the drive wheel in the direction of rotation in which the window covering is raised, it is deformed such that the clamping force on the stationary carrying structure is released or decreased such that a rotation of the gear that can be coupled to the drive wheel is possible (without preventative friction).

The invention further relates to a window covering coupled to the drive shaft of an operating mechanism according to the invention. The advantages and possible embodiments of such a window covering have already been described above in the context of the operating mechanism according to the invention.

The invention will be explained by means of non-limiting exemplary embodiments which are illustrated in the following figures, in which: - Figure 1 is a front view of an operating mechanism according to the invention;

- Figure 2 is a side view of the operating mechanism as shown in Figure 1 ;

- Figure 3 is an exploded representation of the operating mechanism as

shown in Figures 1 and 2;

- Figure 4 is an axial cross section along surface C-C through the operating mechanism shown in Figure 1 , in a coupled state; and

- Figure 5 is an axial cross section along surface C-C through the operating mechanism shown in Figure 1 , in an uncoupled state.

In the following figures, identical elements of the operating mechanism are denoted by the same reference numbers. It will become clear that the invention is not limited to the exemplary embodiments illustrated and described in the following figures, but that numerous variants are possible within the framework of the attached claims which will be obvious to a person skilled in the art.

Figure 1 shows a front view of an operating mechanism (1 ) according to the invention, accommodated within a head rail (2) of a window covering. To this end, the operating mechanism (1 ) is clasped by the head rail (2), for which the head rail (2) and the stationary carrying structure (3) of the operating mechanism (1 ) engage with one another and partially interlock. In the front view it is also possible to see that the operating mechanism (1 ) comprises a drive shaft (4) which partially protrudes from the stationary carrying structure (3) so as to engage with a further transmission structure (see Figures 3-5) from which the window covering is hung. The operating mechanism (1 ) further comprises an operating element (5) provided with a lever (35) by means of which the coupling (again see Figures 3-5) accommodated within the stationary carrying structure (3) can be operated. The operating element (5) is connected to a shaft (6), which also protrudes from the stationary carrying structure (3) and the head rail (2) on a side of the operating mechanism (1 ) opposite the operating element (5) so that the operating element (5) may be mounted on either side of the operating mechanism (1 ). In this way the operating mechanism (1 ) is suitable for use at either end of the head rail (2).

Figure 2 shows a side view of the operating mechanism (1 ) as shown in Figure 1. The head rail (2) within which the operating mechanism (1 ) is accommodated is illustrated only partially here. The stationary carrying structure (3) of the operating mechanism (1 ) and the drive shaft (4) which partially protrudes from the stationary carrying structure (3) can thus be seen. It can also be seen that the head rail (2) is provided with an end cover (7) on the end side, which end cover (7) closes off the end side of the head rail (2) and may also serve as an attachment point for the head rail (2) to a fixed surface, such as a window frame. The operating element (5) is attached on the outside of the head rail (2) to the operating mechanism (1 ) by said shaft (6) which also protrudes from the head rail (2).

Figure 3 shows an exploded representation of the operating mechanism (1 ) as shown in Figures 1 and 2. The operating mechanism (1 ) comprises a stationary carrying structure (3), of which only a portion is illustrated in Figure 1 . This stationary carrying structure (3) serves as an attachment structure for the other parts of the operating mechanism (1 ) and is typically designed to be fixed to an inner side of a head rail (2) of a window covering, as already shown in Figures 1 and 2. A drive shaft (4) is partially accommodated within the carrying structure (3), which, in the embodiment shown, consists of two portions (9, 10) that are connected to one another by a connecting portion (1 1 ). One end of the first portion (9) of the drive shaft (4) protrudes from the stationary carrying structure (3) and serves to drive a window covering (not shown), for example by engaging with a winding reel around which a window covering or a hanging cord for the window covering is wound. The second portion (10) of the drive shaft (4) connects the connecting portion (1 1 ), and thus the first portion (9) of the drive shaft (4), to a translatable transmission element (12) that is provided with a gear (13). Since the transmission element (12) is provided with a hollow shaft (14) which is provided with profiling on the inside that matches and engages with the profile of the second portion of the drive shaft (10), a rotation of the transmission element (12) is transmitted to the drive shaft (4). Because the hollow shaft (14) of the transmission element (12) fits over the drive shaft (4) over a certain distance in the longitudinal direction, the transmission element (12) will remain coupled to the drive shaft (4) during a translation in the axial direction as well. The transmission element (12) can be translated between a first position (see Figure 4) in which it engages with a gear (16) that can be coupled to a drive wheel (15) and a second position (see Figure 5) in which it engages with a gear (18) that is coupled to a rotor (17). By using a pressure element (19), the transmission element (12) is forced in the direction of the first position, and thus the gear (16) that can be coupled to the drive wheel (15), by default. The transmission element (12), the gear (16) that can be coupled to the drive wheel (15) and the gear (18) that is coupled to the rotor (17) form part of a switchable coupling (20) by means of which the drive wheel (15) or the rotor (17) can be coupled to the drive shaft (4), corresponding to a coupled or an uncoupled state of the coupling (20), respectively. By means of the drive wheel (15), which, in the embodiment shown, is configured to engage with and to wind an engagement means (not shown) in the form of a chain or cord, the drive shaft (4) can be driven in the coupled position of the coupling (20). To this end, the drive wheel (15) can be coupled, via an entraining spring (21 ), to the gear (16) that can be coupled to the drive wheel (15). The latter gear (16) is also coupled to a stabilizing spring (22) which, at least in a stationary position of the gear (16) that can be coupled to the drive wheel (15), makes contact with a portion (23) of the stationary carrying structure (3). The drive wheel (15) is furthermore coupled to a torsion spring (24) which, in the embodiment shown, is in the form of a spiral spring. By means of this torsion spring (24), the drive wheel (15), after rotation in a first direction, is forced back in a second, opposite direction, resulting in an engagement means unwound from the drive wheel (15) being wound back onto the drive wheel (15). In the uncoupled position of the coupling (20), in which the transmission element (12) is in the second position, the drive shaft (4) is coupled to the rotor (17) and the transmission element (12) engages with the gear (18) that is coupled to the rotor (17). In the embodiment shown here, the rotor (17) is provided with four weights (25) that are positioned around the rotor (17) rotationally symmetrically. These weights (25) are provided with a notch (26) in the radial direction which engages with a corresponding protrusion (27) in the radial direction provided on a central portion (28) of the rotor (17). By means of this attachment, the weights (25) are able to move in the radial direction of the rotor (17) along said protrusions (27), resulting in a change in the mass moment of inertia of the rotor (17). The rotor (17) is connected to the gear (18) that is coupled to the rotor (17) via a planetary gear train (29), forming a step-up transmission between the drive shaft (4) and the rotor (17). The gear (18) that is coupled to the rotor (17) acts here as the planet gear carrier (30), to which a plurality of (in the present embodiment three, of which two can be seen in Figure 2) planet gears (31 ) are rotatably attached. The planet gears (31 ) engage, via their toothing, both with a central gear (32) that is connected to the rotor (17), which gear (32) acts as the sun gear, and with an internal toothing (33, see Figures 4 and 5) provided on a portion (34, see Figures 4 and 5) of the stationary carrying structure (3), which internal toothing (33) acts as the ring gear. The operating mechanism (1 ) also comprises an operating element (5) for switching the coupling (20), which operating element (5) is provided with a lever (35). To this end, the operating element (5) is coupled to the transmission element (12) via a transmission (36), wherein the transmission (36) comprises a shaft (6) that is provided with a cam (37) and a push rod (38). The shaft (6) that is provided with a cam (36) is here engaged by the lever (35) of the operating element (5), while the push rod (30) makes contact with the transmission element (12). Figures 4 and 5 illustrate the operation of the latter transmission (36).

Figure 4 shows an axial cross section along surface C-C through the operating mechanism (1 ) shown in Figure 1 , in a coupled state. In the coupled state, the transmission element (12) is in the first position and the gear (13) of the

transmission element (12) engages with the gear (16) that can be coupled to the drive wheel (15). By means of the pressure element (19), which, in the embodiment shown, is formed by a coil spring, the transmission element (12) is forced into this first position. The shaft that is provided with a cam (37) with which the operating element (5) engages is, in the coupled state of the coupling, positioned in a direction facing away from the push rod (38) (downwards in the case shown). The drive wheel (15) is, upon rotation, coupled, via the entraining spring (21 ), to the gear (16) that can be coupled to the drive wheel, wherein, in the first position of the transmission element (12), the latter gear (16) is once again coupled to the transmission element (12). Since the transmission element (12) is coupled to the drive shaft (4), a rotation of the drive wheel (15) in the illustrated, first position of the transmission element (12) will lead to a rotation of the drive shaft (4).

Figure 5 shows an axial cross section along surface C-C through the operating mechanism (1 ) shown in Figure 1 , in an uncoupled state. In the uncoupled state, the transmission element (12) is in the second position and the gear (13) of the transmission element (12) engages with the gear (18) that is coupled to the rotor (17). The gear (18) that is coupled to the rotor (17) forms part of the planetary gear train (29), of which a central gear (13) that is connected to the rotor (17) also forms part. Consequently, the gear (18) that is coupled to the rotor (17) is coupled to the rotor (17), as a result of which, by means of the planetary gear train (29), step-up transmission is produced between the rotor (17) and the drive shaft (4). To move the transmission element (12) into the second position, the shaft (6) that is provided with a cam (37) is rotated by means of the operating element (5), resulting in the cam (37) being shifted in the direction of the push rod (38) and the push rod (38) being forced by the cam (37) in the axial direction relative to the drive shaft (4), against the force exerted on the transmission element (12) by the pressure element (19). In the illustrated uncoupled state of the coupling (20), the drive shaft (4) is able to rotate freely, in which case the rotor (17) rotates together therewith and a window covering (not shown) that is coupled to the drive shaft (4) will come down under its own weight.