In the following, a cabinet with a vertically moveable section at its rear side is used as an example for an application of the linear actuator.

When, for example, a cabinet is mounted on a wall at a given height, it can be difficult to reach goods stored at the topmost shelves in the back of the cabinet. Therefore, in many cases this part of the cabinet is not used for storing goods. It is therefore advantageous to use this space for a separate part or section of the cabinet which is moveable downwards at the rear of the cabinet. In this case the cabinet itself has a smaller depth, i.e. the space within the cabinet is smaller in a direction perpendicular to the wall on which the cabinet is mounted. The remaining space is used to accommodate the moveable part. When the moveable part is in the lowered position, it is open and goods can be stored in this part. Thereafter, the moveable part can again be moved upwards into an upper position where it is closed again.

The moveable section of the cabinet is moved using a linear actuator. The linear actuator comprises a spindle which is rotated by drive means. The spindle nut arrangement is threaded on the spindle and connected in a non- rotatable manner to the moveable section of the cabinet. When the spindle is rotated, the spindle nut arrangement is moved upwards and downwards, depending on the direction of rotation of the spindle. However, as items can be positioned in the moving path of the moveable part of the cabinet there is a risk of the moveable section hitting an item during its movement in the downwards direction, with the resulting risk that the cabinet or the item is damaged. It is an even greater concern that a person's fingers are squeezed between the cabinet and the item.

It is an object of the invention to provide a linear actuator with a simple squeeze protection. This object is solved with a linear actuator comprising a spindle, drive means for driving the spindle rotatable about an axis, a spindle nut arrangement in threaded engagement with the spindle and a driven element connected to the spindle nut arrangement, and the linear actuator further comprises a spindle nut arrangement comprising a first part and a second part, the first part being in engagement with the spindle and the second part being connected to the driven element, wherein the first part and the second part are moveable relative to each other in a direction along the axis between a driving position in which the second part prevents rotation of the first part and a squeeze protection position in which the first part is released from the second part.

In the driving position the first part and the second part are connected in a rotational interlocking manner. This means that rotation of the spindle causes a movement of the spindle nut arrangement along the axis of the spindle, and thereby also movement of the driven element. When, however, the driven element, for example, the moveable section of the cabinet, touches an obstacle, the first part of the spindle nut arrangement upon rotation of the spindle moves further while the second part of the spindle nut arrangement does not. The result is that the rotational interlocking connection between the first part and the second part is released and the first part can rotate together with the spindle without generating a driving force. As soon as the obstacle is removed, the second part is moved downwardly by gravity and the rotational interlocking connection between the first part and the second part is established again. Rotation of the spindle now again causes a movement of the spindle nut arrangement and thereby the driven element. Such a squeeze protection does not require any sensor. It operates exclusively mechanically. By adding the spindle nut arrangement to the linear actuator, a simple and compact solution is obtained. In an embodiment of the invention in the driving position the first part and the second part are in form-fit engagement. The form-fit engagement is stable enough to transmit the necessary forces or torques, in particular when moving the driven part against gravity. In an embodiment of the invention the first part and the second part of the spindle nut arrangement comprises a teeth and groove arrangement having teeth and grooves adapted to receive the teeth. When the teeth arrangement is moved out of the groove arrangement there is no rotational interlocking connection between the first part and the second part. When the teeth arrangement has been inserted into the groove arrangement, the rotational interlocking connection is established.

In an embodiment of the invention at least one of the teeth arrangement and the groove arrangement comprises curved walls. The curved walls facilitate the separation of the first part and the second part when the driven part is stopped by an obstacle.

In an embodiment of the invention in a circumferential direction around the axis, the teeth have a first width and the grooves have a second width, wherein the first width is smaller than the second width. This as well facilitates the separation of the first part and the second part when the driven element comes into contact with an obstacle.

In an embodiment of the invention at least one of the teeth and the grooves comprise a length in a direction parallel to the axis which is at most equal to a quarter of a thread pitch of the thread of the spindle. In other words, when the movement of the second part of the spindle nut arrangement has been stopped and the spindle has been rotated by 90° or less, the rotational interlocking connection between the first part and the second part of the spindle nut arrangement is released and is thereby in the squeeze protection position. In a preferred embodiment the rotational interlocking connection is released after stopping of the second part when the spindle has been rotated by 40° or by 50°. In an embodiment of the invention the second part forms a housing accommodating the first part. Such a construction facilitates mounting of the linear actuator.

In an embodiment of the invention the housing of the second part of the spindle nut arrangement limits the movement of the first part of the spindle nut arrangement along the axis of the spindle in the squeeze protection position, where the first part is released from the second part.

In an embodiment of the invention in a circumferential direction around the axis, the housing of the second part comprises only one opening large enough for the first part to pass through. For mounting it is possible to position the second part so that the opening is directed upwardly. The first part can then be inserted into the second part via the opening and cannot fall out of the second part since there is no further opening which is large enough to allow the first part to pass through. ln an embodiment of the invention in a circumferential direction of the axis, the housing comprises an opening for each groove. In this way, the second part can be formed (for example) by injection molding wherein the mold can have a simple form.

In an embodiment of the invention the second part of the spindle nut arrangement comprises an outer contour in form of a polygon surrounding the axis. The polygon form can cover the total height of the second part, i.e. the polygon form can extend parallel to the axis over the entire second part. It is, however, also possible that the polygon form is present only over a part of the height of the second part. The polygon form is a simple way to achieve a non-rotatable connection between the second part and the driven element.

In an embodiment of the invention the first part comprises at least one flat surface between two teeth. This saves material and furthermore allows easy gripping of the first part e.g. with automatic tools. The invention furthermore comprises a cabinet as described at the outset. A vertically moveable section of the cabinet is connected to a linear actuator as described above.

An embodiment of the invention will now be described in more detail with reference to a drawing, wherein:

Fig. 1 shows a front perceptive of a cabinet,

Fig. 2 shows a perspective of the cabinet from the rear, Fig. 3 shows a side view of a spindle nut arrangement,

Fig. 4 shows a perspective view of the spindle nut arrangement in a driving position,

Fig. 5 shows a perspective view of the spindle nut arrangement in a squeeze protection position, and

Fig. 6 shows a perspective view of the first part of the spindle nut arrangement.

Fig. 1 schematically shows a cabinet 1 comprising a fixed section 2 and a vertically moveable section 3. The vertically moveable section 3 is positioned at the rear 4 of the cabinet. A rear wall of the fixed section 2 of the cabinet 1 has been removed in order to show parts of a linear actuator 5 which is used to drive the moveable section 3 up and down. In the position shown in Fig. 1 the moveable section 3 is in its lowermost position. In Fig. 2 the moveable section 3 of the schematically shown cabinet 1 is in its uppermost position.

The linear actuator 5 comprises a spindle 6 which is used to drive a driven element 7. In the present case the driven element 7 is a tube having a square or rectangular cross-section. The spindle 6 itself is rotated by means of drive means 26, for example, by means of an electric motor. If necessary, a gear or transmission can be arranged between the electric motor and the spindle 6.

A spindle nut arrangement 8 is mounted in the driven element 7. The spindle nut arrangement 8 comprises a first part 9 and a second part 10. The first part 9 (Fig. 6) is a spindle nut, which comprises a through-going bore 1 1 having an inner thread 12. The inner thread 12 is engaging the outer thread of the spindle 6. When the spindle 6 is rotated and the first part 9 is fixed against rotation, the first part 9 is moved in a direction along the axis of the spindle 6, wherein the direction of movement depends on the direction of rotation of the spindle 6.

As can be seen in Fig. 6, the first part 9 comprises a teeth arrangement having four teeth 13 which are evenly distributed in the circumferential direction. A flat surface 14 is arranged between two teeth 13. In the present embodiment, four flat surfaces 14 are present, i.e. a flat surface between each pair of teeth 13. Further, the first part 9 comprises a circumferential surface 15 in the form of a circular cylinder.

The second part 10 (cf. Fig. 4 and 5) is a spindle nut housing having side walls 16, 17 and a rear wall 18. An opening 19 is provided at the front side of the second part 10. The opening 19 is large enough for the first part 9 to pass through so that the first part 9 can be inserted into a hollow 20 of the second part 10.

The second part 10 comprises a groove arrangement having four grooves 21 . The number of grooves 21 of the groove arrangement may correspond to the number of teeth 13 of the teeth arrangement of the first part 9.

As can be seen in Fig. 2, in circumferential direction the teeth 13 have a first width and the grooves 21 have a second width, wherein the first width is smaller than the second width. Furthermore, the teeth 13 comprise curved walls 22 and the grooves 21 comprise curved walls 23. The teeth 13 and the grooves 21 comprise a length in a direction parallel to the axis of the spindle 6 which is at most equal to a quarter of a thread pitch of the thread 12 of the spindle 6. Preferably, the height is smaller and corresponds to 40° to 50° of the rotation of the spindle 6.

In the position shown in Fig. 3 and 4, (the driving position), the teeth 13 have entered the grooves 21 so that a rotational interlocking connection between the first part 9 and the second part 10 is established. The second part 10 is fixed against rotation by means of the driven element 7. When therefore the spindle 6 is rotated the first part 9 is moved up or down depending on the direction of rotation of the spindle 6. The second part 10 is held in contact with the first part 9 by means of gravity. Alternatively or additionally, a spring could be used. When the spindle 6 is rotated, the moveable part 3 of the cabinet 1 is moved up or down depending on the direction of rotation of the spindle. When during such a movement the moveable part 3 of the cabinet 1 touches an obstacle, it is stopped. However, the rotation of spindle 6 can continue. Such a rotation causes a further downward movement of the first part 9 so that the teeth 13 are moved out of the grooves 21 . Since the height, i.e. the length in a direction parallel to the axis, of the teeth 13 and of the grooves 21 is limited, as outlined above, the teeth 13 are released from the grooves 21 after a quarter of a rotation of the spindle 6.

When the teeth 13 are free from the grooves 21 , the first part 9 can rotate freely in the second part 10. Consequently, no driving force is transmitted to the driven element 7 and the risk of squeezing something between the moveable section 3 of the cabinet 1 and an obstacle is avoided or at least minimized.

When the obstacle is removed, the moveable section 3 of the cabinet 1 moves downwardly under the effect of gravity or any other force, like a spring-force, and the teeth 13 will again reach into the grooves 21 so that the first part 9 is held against rotation by the second part 10. Further rotation of the spindle 6 causes a movement of the moveable section 3 of the cabinet 1 as previously described.

Each of the walls 16, 17, 18 comprises an opening 24. The opening 24 is related to the corresponding groove 21 . When the second part 10 is formed by injection molding (for example), a mounting tool can be passed through the opening 24.

It is possible to make the width of the opening 19 a little smaller than the diameter of the cylindrical wall 15 of the first part 9. The hollow 20 comprises a width enlargement 25 which enlargement 25 is wide enough to accommodate the cylindrical part 15 so that the first part 9 can be pressed into the second part 10 via the opening 19.

Fig. 5 shows a second position of the first part 9 relative to the second part 10, i.e. the squeeze protection position. It can be seen that the teeth 13 have been released from engagement with the grooves 21 .