Lifting apparatus having a toggle lever mechanism

A lifting apparatus includes a lifting part that can be moved in linear direction between a rest position and a working position. At least one drive device is disposed for moving the lifting part. Each drive device includes a toggle lever mechanism, having a first toggle supported pivotally on the lifting part and a second toggle supported on a base part. A positioning device prespecifies the position of the lifting part in lifting direction and in a direction transverse to lifting direction in working position. To do so, said toggle preferably has a stop surface against which the lifting part is pushed into working position by the at least one drive device. In this working position, the toggle joint angle α of the toggle lever mechanism is smaller than 180° so that the two toggles and are outside the extended position.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is the national phase of PCT/EP2013/063246 filed Jun. 25, 2013, which claims the benefit of German Patent Application No. DE 102012107611.1 filed Aug. 20, 2012.

TECHNICAL FIELD

The invention relates to a lifting apparatus having a lifting part that is movable in a linear manner between a working position and a rest position.

BACKGROUND

Provided on the lifting part is a support unit and/or a clamping unit for supporting or clamping in place a workpiece. The support unit, for example, may be a stay disposed for supporting a cylindrical workpiece. The clamping unit may comprise, for example, a tailstock or a similar clamping means in order to clamp in place a workpiece, in particular, in axial direction. Preferably, the lifting apparatus is disposed for use in grinding machines for grinding cylindrical workpieces.

For example, a lifting table is known from publication DE 36 709 U1. The lifting table comprises a table top, a base part and a lever arrangement with four levers that are hinged next to each other to form a closed circle. One of the joints of the lever arrangement is arranged on the table top, and the opposite joint is arranged on the base part. The other joints are free joints. At least one toggle lever mechanism is actuated via a drive in order to lift or lower the table top relative to the base part. A pushrod of the drive connects the toggle joint of the toggle lever mechanism with the free links of the lever arrangement.

In doing so, the drive is supported by the lever arrangement between the base part and the table top and moved along as the table top is being moved in lifting direction. This is not favorable, because the mass of the drive must also be completely moved along. Furthermore, it has been found that the start-up of the working position of such lifting tables is too inaccurate for many applications. In particular, when workpieces are supported or mounted in precision machines, the accurate adjustment of the working position of the lifting part of the lifting apparatus to a few micrometers must be possible.

SUMMARY

Therefore, it can be viewed as the object of the present invention to provide a lifting apparatus that achieves high accuracy at the start-up of the working position of the lifting part that is moved in a lifting direction.

The lifting apparatus comprises a drive device arranged on a base part. The drive device comprises a drive coupled with a toggle lever mechanism. A first toggle lever of the toggle lever mechanism is hinged to the lifting part, and the other toggle lever of the toggle lever mechanism is hinged to the base part. Preferably, the drive acts on the toggle joint of the toggle lever mechanism. The working position of the lifting part is prespecified by a positioning device. In particular, the positioning device defines the maximum distance between the lifting part and the base part, so that, when the lifting part is moved in lifting direction away from the base part, the working position can be exactly approached. In order to hold the lifting part exactly in the desired working position, the positioning device is adjusted or set relative to the toggle lever mechanism in such a manner that the toggle levers are outside their extended position when the lifting part has reached the working position. In particular, the extended position of the toggle levers is not reached during the entire movement between the rest position and the working position of the lifting part. The toggle joint angle between the two toggle levers is preferably 30° to 40° in rest position and 170° to 178° in working position.

As a result of the fact that the toggle levers of the toggle lever mechanism subtend an angle of less than 180° in working position, a sufficiently great force can be applied to the lifting part in lifting direction via the drive of the at least one drive device and via the toggle lever mechanism, so that the lifting part is pushed precisely into the position defined by the positioning device. In this manner, the working position of the lifting part is exactly achieved, independent of the play prevailing in the drive device.

Preferably, the toggle lever mechanism comprises the at least one drive device consisting of only two toggle levers, each of said toggle levers extending in particular in a straight line between two bearing points. Therefore, the toggle lever mechanism comprises, in addition to the toggle joint, preferably only two bearing points.

In a preferred embodiment, the positioning device has an stop surface facing the lifting part, said stop surface—in working position—contacting a contact surface of the lifting part. Preferably, the stop surface and the contact surface are in contact with one another only in working position. Outside the working position, the positioning device preferably does not affect the position of the lifting part. The positioning device and the lifting part are made of steel, in particular. In doing so, the working position can be defined particularly well.

It is of advantage if the positioning device comprises an adjustment device for adjusting the distance between the base part and the stop surface. In this manner, the working position of the lifting part can be adjusted and adapted to the requirements of the machine tool. For a precise adjustment, the adjustment device may comprise a differential screw. For example, it is possible with the use of this differential screw to adjust an abutment part of the positioning device, where the stop surface is provided, in the desired position relative to a pylon of the positioning device connected to the base part in lifting direction.

In order to fix the stop surfaces in place, a clamping screw arranged transversely to the differential screw can be tensioned and secured against inadvertent rotation.

It is also possible for the pylon of the positioning device to extend through a cutout in the lifting part, i.e., in particular, abut against the lifting part.

In a preferred embodiment, at least one positioning projection is provided on the stop surface and/or contact surface, said positioning projection interacting with an associate positioning indentation on the respectively other side. The positioning projection and/or the positioning indentation, viewed in cross-section, may be prism-shaped, for example, triangular or trapezoid. In doing so, the contact surface and the stop surface may only be the respectively inclined surfaces of the at least one positioning projection or the respectively associate positioning indentation.

In particular, the at least one positioning projection comes into engagement with the respectively associate positioning indentation when the lifting part is in its working position. As a result of this, the lifting part—viewed transversely or radially to lifting direction—can be positioned in a precisely defined position relative to the positioning device. By means of forces introduced into the lifting part via the drive device in transverse direction to lifting direction it is thus possible to support the positioning device. A guide arrangement that is optionally provided for guiding the lifting part in a sliding manner in lifting direction is thus not subjected to transverse forces when the lifting arrangement pushes the lifting part into the working position.

By means of the at least one positioning projection and the at least one associate positioning indentation, it is possible to achieve a play-free micrometer-accurate positioning of the lifting part in working position. In particular, in working position of the lifting part, it is only the inclined surfaces of the at least one positioning projection of the stop surface and/or the contact surface that abut against the associate inclined surfaces of the at least one positioning indentation of the respectively other surface. The working position of the lifting part can be repeatedly started up with micrometer-accuracy.

In one embodiment, several drive devices, for example two drive devices, may be present. Each drive device comprises a drive and a toggle lever mechanism, wherein the drive devices are configured the same, in principle. The toggle lever mechanisms of the two drive devices may be arranged, for example, on two opposite sides of the positioning device or of the pylon of the positioning device. As a result of this, it is possible to achieve a good distribution of forces or introduction of forces. The drives of the plurality of drive devices may be connected in parallel, so that the sum of the driving forces of all drives is transmitted over each and every toggle lever mechanism.

The drive device may either comprise a controllable drive or also a manually operable drive. The controllable drive has the advantage that the lifting part can be automatically moved between the rest position and the working position during the operation of the machine tool or while a workpiece is being machined. In a preferred exemplary embodiment, the controllable drive may be a double-acting fluid cylinder. Preferably, a pneumatic cylinder is used. The piston rod of the fluid cylinder may act on the toggle joint of the toggle lever mechanism. In doing so, the cylinder housing of the fluid cylinder may be pivotally supported on the base part. The use of air for the actuation of the fluid cylinder is not problematic because—while the air is compressible—the working position of the lifting part can be precisely maintained due to the positioning device when the pneumatic cylinder pushes the lifting part into the working position via the toggle lever mechanism. Due to the relatively large toggle joint angle of more than 170° and preferably more than 175°, the force with which the lifting part is moved into the working position and, in particular, against the stop surface, is sufficiently great.

When the lifting part is being moved out of the rest position into the working position, the speed of the lifting part decreases if the piston rod of the fluid cylinder is moved in a uniform manner. This has the advantage that the working position is reached at a reducing lifting speed, and a softer stop of the lifting part in working position is achieved by means of the positioning device. In addition, fluid lines connected to the fluid cylinder may comprise one or more throttles in order to limit the movement speed of the piston rod. The movement of the piston rod is controlled via the pressure on a fluid control unit that, for example, is a component of the machine control.

The at least one drive device may additionally comprise a fluid control unit that is disposed to apply a fluid pressure to the fluid cylinder or to the affected working chamber of a fluid cylinder, regardless of whether the lifting part is in working position or in rest position. As a result of this, it is ensured that the lifting part is held with sufficiently great force in working position or in rest position and that precise positioning is achieved.

If several drive devices are provided, the fluid control unit may be configured as a common fluid control unit for all drive devices.

Furthermore, it is advantageous if at least one drive device comprises a sensor unit for detecting the working position and/or the rest position. For example, the double-acting fluid cylinder may comprise end position switches.

Furthermore, it is of advantage if there is an auxiliary drive in addition to the at least one drive device. The auxiliary drive comprises, in particular, a deformable body that may be, for example, a helical spring, leaf spring or an elastomer body or the like. In rest position of the lifting part, the deformable body, for example the spring, can be deformed and preferably compressed, and thus applies an auxiliary force on the lifting part or an element that is connected to the lifting part. The auxiliary force acts on or pushes the lifting part out of the rest position into lifting direction. By means of this auxiliary force of the auxiliary drive, the movement of the lifting part out of the rest position is supported. Due to the small toggle lever mechanism angle in rest position, the lifting force effected by the at least one drive device in lifting direction is small so that the drive of the drive device would have to generate a considerable total force in order to reach sufficient lifting force. This, however, could lead to undesirably great loads. Total forces of the drive of such magnitude could be reduced by providing the auxiliary drive.

Preferably, the auxiliary force is available not only in rest position but in any position of the lifting part. In rest position, the auxiliary force may display its greatest strength, and said strength may decrease with the increasing distance or movement of the lifting part toward the working position.

In a preferred embodiment of the lifting device, the lifting part is linearly guided over a guide arrangement. For example, the guide arrangement may comprise one or several guide rails extending in a straight line in lifting direction. Preferably, the guide rails are arranged on the side of the lifting part opposite the drive device.

DETAILED DESCRIPTION

FIGS. 1 through 3show a lifting apparatus10comprising a lifting part11that can be moved in a lifting direction H. The lifting part11is plate-shaped. Said lifting part comprises a mounting surface12, to which can be mounted a support unit13—as in the exemplary embodiment shown herein—and/or a chuck unit. The support unit13comprises a support part14with a support indentation15whose cross-section may be arched or prism-shaped. A not specifically shown workpiece in the support recess15can be partially circumscribed and supported in its axial extension direction. In this manner, it is possible to support workpieces mounted on one side in a chuck with the use of a support unit13at an axial distance from the chuck in order to avoid bending while said workpieces are being machined.

In modification of the illustrated exemplary embodiment, it would also be possible to use a chuck with a tailstock or the like on the lifting part11. For example, it is possible with the use of a chuck to mount a free axial end of a workpiece in axial direction relative to a chuck. To accomplish this, the clamping unit can be supported on the lifting part so as to be drivable or slidable in a direction transverse to lifting direction H. Alternatively or additionally, it is also possible to support the entire lifting apparatus so as to be shiftable in a tensioning direction.

On the side opposite the mounting surface12, the lifting part11has a driving side16that faces the base part17. In the exemplary embodiment, the base part17is configured as a base plate. Arranged on the base part17, there is at least one drive device18for moving the lifting part11in lifting direction H. In the exemplary embodiment described here, two drive devices18constructed in an equal manner are provided. Each drive device18comprises a drive19that, in accordance with the example, consists of a double-acting fluid cylinder20. Each fluid cylinder20comprises two working chambers charged with pressurized air, said chambers being fluidically separated from one another by the piston. The piston is connected to a piston rod21, said piston rod projecting on one end from the cylinder housing. The free end of the piston rod21is coupled with a toggle lever mechanism22and is in contact, for example, with a toggle joint31of the toggle lever mechanism22. In this manner, by moving the piston rod21in and out via the toggle lever mechanism, it is possible for the fluid cylinder20to move the lifting part11into lifting direction H.

In order to act on the working chambers, each fluid cylinder20has two fluid ports23. Via the fluid lines24that are schematically shown inFIG. 3, the fluid ports are connected to a fluid control unit25. Via the fluid control unit25, the fluid pressure is supplied to one or the other working chamber of the double-acting fluid cylinder20. On the end opposite the free end of the piston rod21, the cylinder housing is pivotally supported on the base part17.

Instead of the pneumatic fluid cylinder20, it would also be possible to alternatively provide hydraulic cylinders. Alternatively to the cylinders described here, it would also be possible to use other controllable drives19, for example, electric motors, linear motors or the like, as the drive19for the drive device18. Furthermore, it is also possible to provide—instead of a controllable drive19—a manually actuated drive and couple it with the associate toggle lever mechanism22. This may be an actuating lever arrangement, for example.

As is particularly obvious fromFIG. 1, the two drives19are connected in parallel, so to speak, so that the driving force of each drive19acts on both toggle lever mechanisms22, respectively. In accordance with the example, there is a connecting piece30which connects the two free ends of the piston rods21with one another and which is coupled with both toggle joints31of the toggle lever mechanism.

The toggle lever mechanism22of a drive device18comprises a first toggle lever22ahinged to the lifting part11as well as a second toggle lever22bhinged to the base part17. The two toggle levers22a,22bof a toggle lever mechanism22are supported in a hinged manner next to each other on the toggle joint31. Consequently, each toggle lever mechanism22comprises three bearing points. A first shaft32mounted to the lifting part11and being accessible at least in sections from the driving side16of the lifting part11is disposed for bearing the first toggle lever22a. Preferably, the first shaft32is disposed for the hinged bearing of two first toggle levers22aof the two toggle lever mechanisms22. In lifting direction H below the first shaft32, a second shaft33is supported on the base part17and is accessible at least in sections. In the exemplary embodiment, the second shaft33is disposed for the hinged support of the two second toggle levers22bon the base part17.FIG. 4shows the three bearing points of the toggle lever mechanism22. The toggle lever mechanism22has a needle bearing34at each bearing point. The needle bearings34are sealed with respect to the environment, for example by radial shaft seal rings.

Each of the toggle levers22a,22bconnects its two bearing points in a straight line. Essentially, they have the shape of a bone.

The two drive devices18move the lifting part11between a rest position R (FIG. 3) and a working position A (FIG. 2). In rest position R, the lifting part11is at the smallest distance from the base part17and contacts the base part17, as in the example. In this rest position R, the toggle joint angle α subtended by the toggle levers22a,22bis approximately 30° to 35°. In rest position R, the longitudinal axis of the piston rod21forms the bisector of the toggle joint angle α. In rest position R of the lifting part11, the piston rod21extends approximately at a right angle with respect to lifting direction H.

Inasmuch as the distance of the toggle joint31increases relative to the base part17during the movement of the lifting part11out of the rest position R into the working position A, the cylinder housings of the fluid cylinders20are pivotally supported on the base part17. By moving out the piston rod21, the toggle levers22a,22berect and the subtended toggle joint angle α becomes larger. In working position A, the toggle joint angle α has its maximum value and is smaller than 180°. Consequently, the toggle levers22a,22balso do not assume their extended position in working position (toggle joint angle α=180°). In the entire range of motion of the lifting part11from the rest position R into the working position A, the toggle joint angle α is at all times smaller than 180°. In the exemplary embodiment, the toggle joint angle α is approximately 175° to 178° in the working position A.

The working position A of the lifting part11is prespecified by a positioning device40. In so doing, the positioning device40defines the maximum distance between the lifting part11and the base part17.

In the exemplary embodiment described here, the positioning device comprises a stop surface41facing the lifting part11and, as in the example, facing the mounting surface12. In the exemplary embodiment, the stop surface41is provided on an abutment part42. The abutment part42is arranged on a pylon43of the positioning device40.

By means of an adjustment device44, the abutment part42is adjustably supported on the pylon43in the adjustment direction H. A differential screw45shown inFIG. 7belongs to the adjustment device44. This differential screw45connects the abutment part42with the pylon43so that a highly precise adjustment of the distance between the stop surface of the abutment part and the base part17is possible. The abutment part42is guided over two pins46extending parallel to the differential screw45in the guide cutouts on the pylon43. A clamping screw extending transversely to lifting direction H and fixing the position of the abutment part42relative to the pylon43, for example, by preventing any adjustment or torquing of the differential screw45, is used for locking the adjusted position of the abutment part42in place.

The illustration ofFIG. 6is incomplete and does not show the abutment part42. The differential screw45and the pins46of the adjustment device44are shown inFIG. 7.

The abutment part42comprises a parallelepipedal section47in extension of the pylon43. On the end facing away from the pylon43, the two transverse parts48extend transversely to lifting direction H in opposite directions away from this parallelepipedal section47. As a result of this, the abutment part42is provided with an overall T-shape. Each transverse part48is provided, on the side facing the base part17or the lifting part11, with a section41aof the stop surface41. Consequently, the stop surface41is not cohesive but consists of two spaced apart sections41aon respectively one transverse part48.

The lifting part11has a cutout50shown inFIG. 6. Through this cutout50extends the pylon43in rest position R and the parallelepipedal section47of the abutment part42in working position A. For better illustration,FIG. 6does not show the pylon43and the abutment part42. Pylon43and the parallelepipedal section47of the positioning device40, respectively, are not disposed for guiding the lifting part11in lifting direction H but only extend through it through the cutout50. A gap may exist in the region of the cutout50between the positioning device40and the lifting part11, so that, in rest position R of the lifting part11and during the movement of the lifting part11in lifting direction H, there is no guide contact between the positioning device40and the lifting part11.

A guide arrangement55that, in the exemplary embodiment comprises two guide rails56extending parallel to one another in lifting direction H, is disposed for guiding a lifting part11in lifting direction H. A guide part57extends around said rails, said guide part being rigidly connected to the lifting part11.

The stop surface41of the positioning device40and, as in the example of the abutment part42, is associated with a contact surface60on the lifting part11, wherein the contact surface60in the exemplary embodiment is represented by a section of the mounting surface12.FIGS. 5 and 6show the contact surface60particularly well. Corresponding to the stop surface41, the contact surface60also comprises two spaced apart, separate surface sections60a, each being associated with a section41aof the stop surface41. If the lifting part11is in its working position A, the contact surface60abuts against the stop surface41.

Referring to the preferred exemplary embodiment shown herein, neither the contact surface60nor the stop surface41is configured as a flat surface, this also being possible in modification of the exemplary embodiment. For example, the contact surface60of the lifting part11is arranged on several positioning projections61. Each positioning projection61is associated with a positioning indentation62, in which case the stop surface41is provided on these positioning indentations62. In working position A of the lifting part11, each positioning projection61comes into engagement with the associate positioning indentation62.

In modification of the depicted embodiment, it would also be possible to provide the stop surface41either on the positioning projections61or on the positioning projections61and the positioning indentations62, in which case the contact surface60may be provided corresponding to the associate positioning indentations62or on the positioning indentations62and the positioning indentations61.

In the exemplary embodiment shown here, the positioning projections61have a prismatic and, for example, trapezoidal shape—when viewed in cross-section. They taper away from the lifting part11. The associate positioning indentations62have a cross-section adapted to the cross-sectional form of the positioning projections61, said cross-section having a triangular form in the exemplary embodiment. Due to the triangular cross-section of the positioning indentations62and the trapezoidal cross-section of the positioning projections61, a small free space remains with respect to the engaging positioning projection61in working position A of the lifting part11at the bottom of each positioning indentation62, as is obvious fromFIG. 5. The abutment part42and the lifting part11abut against one another only on the respective inclined surfaces of the positioning projections61and the positioning indentations62, the latter thus forming the stop surface41or the contact surface60.

Due to this design of the contact surface60and the stop surface41, it is possible to achieve a highly precise positioning of the lifting part11relative to the positioning device40and, as in the example, relative to the abutment part42. The prism-shaped positioning projections61in the exemplary embodiment extend at a right angle relative to lifting direction H. In the exemplary embodiment at least one of the prism-shaped positioning projections61extends at a right angle to the other positioning extensions61. As a result of this, the position of the lifting part11is exactly prespecified by the positioning device40in working position A radially with respect to lifting direction H.

A fluid pressure is applied to each fluid cylinder20of the two drive devices18by means of the fluid control unit25, so that the contact surface60of the lifting part11is pushed via the piston rod21and the two toggle lever mechanisms22against the stop surface41of the positioning device40. The transverse forces transverse to lifting direction H acting on the lifting part11, in so doing, are supported by the positioning indentations62in the abutment part42and thus do not act on the guide arrangement55. In lifting direction H, as well as in a direction transverse to lifting direction H, an exact positioning of the lifting part11in working position A is achieved. This working position A can be approached with extremely high accuracy. As a result of this, it is ensured that, when the lifting part11is automatically moved between its rest position R and its working position A, the support unit13located on the lifting part11repeatedly assumes the prespecified desired position for supporting the associate workpiece in a highly exact manner. In the exemplary embodiment described here, the lifting part11and thus also the supporting unit13can be positioned with micrometer-accuracy in lifting direction H and in a direction transverse thereto.

As a result of the fact that the working position A is prespecified by the positioning device40in such a manner that the two toggle levers22a,22bof a toggle lever mechanism22are outside their extended position, it is possible to apply sufficient force to the lifting part11via the associate drive19, said force being applied for pushing the lifting part11against the stop surface41of the positioning device40in working position A. Inasmuch as the toggle joint angle α in working position A is large, the usual pneumatic operating pressures in the range of 5 to 6 bar are sufficient to hold the lifting part11in working position A, said operating pressures being usually available anyhow in factory buildings.

When the lifting part11is being moved out of the rest position R into the working position A, the speed of the lifting part11in lifting direction H decreases as the toggle joint angle α increases, while the piston rod21is uniformly moved out. This is advantageous because the lifting part11, despite the uniformly moved piston rod21, abuts at a lower speed against the stop surface41of the positioning device40. In the exemplary embodiment described here, the lifting speed of the lifting part11is further limited in that the fluid throttles24aare inserted in the fluid lines24(FIG. 3).

Each drive device18, or at least one of the drive devices18may comprise a sensor unit65for detecting the working position A and the rest position R of the lifting part11. In the exemplary embodiment described here, the sensor unit65comprises two end-position switches66in the respective fluid cylinder20, by means of which the position of the piston rod21or the piston in the cylinder housing can be detected, and thus the working position A and the rest position R of the lifting part11can be indirectly determined. When the working position A is reached, one of the end-position switches66delivers a first sensor signal S1. When the rest position R is reached, the respectively other end-position switch66of the fluid cylinder20delivers a second sensor signal S2.

In rest position R, the toggle joint angle α is relatively small so that the force exerted by the drive19on the toggle joint31of the toggle lever mechanism22generates only a minimal force component in lifting direction H. This may have the effect that a relatively great driving force is necessary in order to be able to move the lifting part11out of the rest position R. In order to be able to avoid the demand for relatively great driving forces via the driving unit18in rest position R of the lifting part11, an auxiliary drive70is provided in the lifting apparatus10described here. The auxiliary drive70makes available an auxiliary force in lifting direction H, said auxiliary force—at least in rest position R—acting indirectly or directly on the lifting part11. Consequently, the lifting motion of the lifting part11out of the rest position R into the working position A is supported by the auxiliary force of the auxiliary drive70.

The auxiliary drive70comprises a deformable body71that is deformed and, in accordance with the example, compressed in rest position R, thus providing the auxiliary force in lifting direction H in rest position R. In the exemplary embodiment, the deformable body71is a compression spring or a helical spring72having a longitudinal axis extending in lifting direction H. The compression spring or helical spring72is arranged in the region of the guide arrangement55parallel to the guide rails56. Its lower end is supported by the base part17. The opposite end abuts against a support part73that, in accordance with the example, is directly mounted to the lifting part11. The spring force applied by the compression spring or helical spring72in lifting direction H thus acts on the lifting part11via the support part73.

In the exemplary embodiment described here, the support part73is in contact with the compression spring or helical spring72along the entire travel path of the lifting part11. Therefore, the spring force or auxiliary force in lifting direction H increases or decreases over the entire lifting path of the lifting part11. Consequently, any force surges caused by the auxiliary drive70in lifting direction H are avoided.

As an alternative to the exemplary embodiment described here, the deformable body71could also be a leaf spring or an elastomer body. It would also be possible for the deformable body71to be in contact with the support part73or another element connected to the lifting part11in only a specific toggle joint angle range starting from the rest position R. However, in this case, it would be possible for force surges of the auxiliary force to occur in lifting direction H.

The invention relates to a lifting apparatus10comprising a lifting part11that can be moved in linear direction between a rest position R and a working position A. The lifting part11carries a support unit13for a workpiece and/or a clamping unit for said workpiece. At least one drive device18and, in particular, two drive devices18, are disposed for moving the lifting part11. Each drive device18comprises a toggle lever mechanism22, said toggle lever mechanism comprising a first toggle lever22asupported pivotally on the lifting part11and a second toggle lever22bsupported on a base part17. The two toggle levers22a,22bare pivotally supported next to one another on a common toggle joint31. A drive19of the drive device18is in contact with the toggle joint31. A positioning device40prespecifies the position of the lifting part11in lifting direction H and, in particular, also in a direction transverse to lifting direction H in working position A. To do so, said toggle lever preferably has an stop surface41against which the lifting part11is pushed into working position A by means of the at least one drive device18. In this working position A, the toggle joint angle α of the toggle lever mechanism22is smaller than 180° so that the two toggle levers22aand22bof the toggle lever mechanism22are outside the extended position.

LIST OF REFERENCE SIGNS