Patent Publication Number: US-11047448-B2

Title: Telescopic column

Description:
CROSS-REFERENCE 
     This application claims priority to German patent application no. 10 2017 207 246.6 filed on Apr. 28, 2017, the contents of which are fully incorporated herein by reference. 
     TECHNOLOGICAL FIELD 
     The disclosure relates to a telescopic column. 
     BACKGROUND 
     Telescopic columns are used in numerous areas of technology. Using a telescopic column, an object or a device can be brought into different positions. In typical embodiments the telescopic column includes a plurality of round, or right-angled, or polygonal column elements lying one-inside-another, often configured tubular, which are retractable and extendable with respect to one another by a motor-driven mechanism. Respective directly adjacent tubes have a maximum extendability so that to maintain the stability a certain overlap of the tubes is always ensured. A distinction is made here between telescopic columns that are positioned on the ground and those that are attached to a ceiling. While the former are used, for example, in operating tables and make possible an upward and downward movement of the table, the latter are attached to the ceiling or a stand, for example, for movably securing X-ray machines. . . . The X-ray machine is thus movable in height. In addition, another multiple-member arm can be attached between the telescopic columns, which multiple-member arm additionally makes possible a horizontal movement. 
     In some embodiments, a stopper is mounted at an end of the tubes of the telescopic columns, so that the maximum travel is mechanically limited and a respective end position of the movement is fixed. When the tubes are moved relative to each other, contact with the stopper may occur at full speed, which on the one hand stresses the material and on the other hand causes a loud noise. The noise is disruptive in particular in medical applications. In addition, the impact generates a shock that compromises operator comfort, since it acts, for example, on a hand of an operator. 
     SUMMARY 
     It is therefore an aspect of the present disclosure to provide a telescopic column that avoids such disadvantages. 
     This is achieved by a telescopic column according to embodiments of the disclosure. 
     One embodiment of the disclosure relates to a telescopic column having the following features: 
     at least two telescopic elements movable with respect to one another between two end positions, 
     at least one damper unit that is configured such that prior to reaching at least one of the end positions a force slowing the relative movement of the telescopic elements is exertable on at least one of the telescopic elements. 
     In embodiments of the disclosure the telescopic elements are linearly movable with respect to one another. In addition, a rotary movement can simultaneously or alternatively be performed. The damper unit is preferably independent from a drive of the telescopic column. The constructive expense is thereby reduced. Due to the damper unit a braking process is reliably introduced by the slowing force prior to the reaching of the end position; the braking force reduces the relative speed of the telescopic elements with respect to one another. Thus any contact with the stopper in the respective end position is with significantly reduced speed. The material is thus spared and loud impact noises avoided. Operating comfort is increased. Alternatively the damper unit can be designed such that a separate stopper is no longer required. In this case the damper unit brakes the movement until stoppage. However, a separate stopper can also additionally be provided in this embodiment. In preferred embodiments the damper unit includes an absorption element, using which the kinetic energy is absorbable. Thus as known with shock absorbers per se, the movement can be efficiently damped, and a hard impact as well as vibrations can be avoided. 
     Preferably the damper unit cooperates with both telescopic elements in order to generate the force. Thus mechanical or electromechanical components of the damper unit can be connected to both telescopic elements and correspondingly designed to cooperate. If further telescopic elements are provided, then in preferred embodiments one or more damper units can respectively be disposed between two of the telescopic elements. 
     In one preferred embodiment of the disclosure the damper unit is configured such that due to the generated force the movement of the two telescopic elements is continuously slowed until reaching the end position. In this case stoppers are not mandatory. Alternatively stoppers can nevertheless be provided that are redundant, for example, to avoid an uncontrolled extending or retracting of the telescopic column or a separating of the telescopic elements, which could lead to an accident in the case of a failure of the damper unit. They can additionally relieve the damper unit in the event of extended persistence in the extended state. 
     In one preferred embodiment of the disclosure the damper unit comprises at least one damping element and a stop, wherein for generating the force the damping element is bringable into operating contact with the stop. This embodiment is mechanically simple and reliable. In addition, a particularly space-saving embodiment can thus be implemented. 
     The damping element is preferably secured on one of the telescopic elements. This can be effected, for example, using screws or adhesive, which is particularly easily executable. The stop is preferably secured on the other of the telescopic elements such that prior to reaching the end position the stop is bringable into operative contact with the damping element. Thus a reliable and cost-effective damping can be achieved in a constructively simple and space-saving manner. 
     In one preferred embodiment of the disclosure a further stop is secured on other of the telescopic elements such that prior to reaching the other end position the further stop is bringable into operative contact with the damping element. A damping near both end positions can thus be achieved in a simple manner. 
     The damping element is preferably retained in a hole-type opening of one of the telescopic elements. This is a particularly space-saving embodiment. It can thus be easy to assemble and disassemble. The stop is then preferably secured on the other of the telescopic elements such that prior to reaching the end position the stop is bringable into operative contact with the damping element. The movement can thus be reliably slowed. 
     In one preferred embodiment of the disclosure the damping element comprises the following features: 
     A housing, wherein a plunger is movably disposed, 
     A spring element by which a force is generatable between the plunger and the housing. 
     Comparable damping elements are known from other applications, for example in the damping of drawers when closing. They can be produced cost-effectively and compact and according to experience work reliably. 
     Preferably a cavity filled with a fluid is formed in the housing, in which cavity the spring element and the plunger are disposed. The fluid is pressed through a hole-type tapering, integrated in the plunger or present separately, into a further cavity. The kinetic energy can thereby be absorbed and the movement damped. This makes possible a particularly uniform slowing and damping of the movement. Preferably here the piston is bringable into operative contact with the stop. As soon as the telescopic elements are again moved away from one another, the spring element presses the plunger back into its initial position again so that in the next process a braking process can take place again. 
     Alternatively a gas compressible by the plunger can be introduced in the housing so that the kinetic energy can be converted. 
     In one preferred embodiment of the disclosure the telescopic elements have circumferential measurements adapted to one another and are disposed nested one-inside-the-other such that from the outer- to the inner-lying telescopic element an inner surface of the outer-lying telescopic element respectively corresponds to an outer surface of the next-inner-lying telescopic element via a damper unit, wherein a damping element of the damper unit is respectively secured on the outer surface and a stop of the damper unit is secured on the corresponding inner surface. This is advantageous in particular in telescopic columns having multiple nested telescopic elements, since the braking force on a plurality of the telescopic elements can thus be transmitted by a plurality of damper units. This assembly can also be embodied in reverse. 
     In particular, ceiling-hanging telescopic columns are often equipped with complex cable drives that occupy correspondingly large installation volumes. At the same time there are strict safety requirements especially in the medical field. It is a further aspect of the disclosure to provide a telescopic column that is compact and simple and that simultaneously satisfies strict safety standards. 
     This is achieved by a telescopic column according to embodiments of the disclosure. 
     One embodiment of the disclosure relates to a telescopic column, including at least two telescopic elements linearly movable with respect to each other, further comprising a drive system comprising the following features: 
     a drive unit including a first clutch element, 
     a drive unit including a shaft, a windable connecting element, and a second clutch element connected to the shaft such that the clutch element and the shaft rotate together, wherein the connecting element is connected to the shaft and at least one of the movable telescopic elements, 
     a brake unit that is configured such that a sufficiently strong holding force is transmissible onto the second clutch element that the telescopic elements are held in their relative position with respect to each other, 
     the brake unit is further configured such that applying a drive torque against the first clutch element reduces the holding force such that the telescopic elements are movable relative to each other, and 
     the brake unit is further configured such that applying an output-side drive torque against the second clutch element increases the holding force acting on the second clutch element. 
     The brake unit offers a high degree of safety, such as is required in particular in medical applications, with compact design. The two clutch elements are preferably manufactured from steel or a comparably durable material and can be made compact. A distinction is made in the disclosure between the drive side and the output side. In order to move the telescopic elements, i.e., for retracting or extending the telescopic column, a force generated by the drive unit, for example, by a motor, is transmitted to the first clutch element. This is, for example, set into rotation. The flow of force taking place up to now defines the drive side and a drive-side force or a drive-side torque. Due to the always-acting gravitational force, in the event of a lack of a corresponding counter-force there is the tendency in a telescopic column in most operating states for the relative positions of the telescopic elements to change with respect to one another. This is always the case in ceiling-hanging telescopic columns when the telescopic column is not fully extended. In the fully extended state, retaining elements usually stabilize the telescopic elements with respect to one another so that the telescopic column does not fall apart. As long as the telescopic column is not fully extended, the gravitational force must be compensated elsewhere. Gravity produces a force on the telescopic elements, which exert a force on the shaft through the connecting element and thus on the second clutch element connected thereto. This flow of force defines the output side of the telescopic column. If the output-side torque were not compensated, the telescopic column would move in an uncontrolled manner into its fully extended state. However, due to the permanently present retaining force, in the event of the absence of a drive-side torque the brake unit compensates the output-side torque, so that the telescopic column can be held in any position. For this purpose no drive-side force is required, so that the drive unit only need be activated for operation. The torque permanently acting on the drive-side in the not-fully-extended state additionally increases the retaining force, so that even with an increase of the load no uncontrolled extending occurs. In contrast to known systems, the drive system has extremely compact dimensions and a mechanically simple realizability. In addition, it meets the high safety standards in the medical field. However, embodiments of the disclosure are also usable in other areas, for example, in production facilities or workshops. 
     If the telescopic column is to be retracted, then a drive-side torque is generated, whereby the retaining force is reduced. Simultaneously the two clutch elements can preferably come into operative contact so that the torque is transmitted onto the shaft and the connecting element is wound up. The direction of the torque is to be chosen accordingly. The telescopic column is subsequently retracted. Conversely if the telescopic column is to be extended, then a drive-side torque is generated in the other direction, whereby the retaining force is also reduced. Simultaneously the connecting element is unwound and with assistance of the gravitational force the telescopic column is extended. 
     For telescopic columns located on the ground, often referred to as lifting columns, the embodiments apply correspondingly reversed with respect to the interaction of the drive with the gravitational force, so that here with extending of the telescopic column the gravitational force is worked against, and with retracting the gravitational force is worked with. Otherwise all advantages and constructive features mentioned in the embodiments of the disclosure described here can also be transferred to lifting columns and used analogously with slight modifications. 
     In preferred embodiments of the disclosure one of the telescopic elements is fixedly connected to or positioned with the ceiling or the floor of a space. It can be, for example, a treatment space or a workshop space. It is also possible that the telescopic element is connected to a frame. The second or the plurality of further telescopic elements are respectively linearly movable with respect to the first telescopic element and extendable therefrom or retractable thereinto. The telescopic elements lie nested one-inside-the-other with decreasing outer-diameter or -circumference. The drive unit is preferably connected to the supported or fixed telescopic element. The connecting element transmits a force generated by the drive unit to the second or the innermost of the telescopic elements and thus makes possible a retracting and extending of the telescopic column The innermost of the telescopic elements here causes a retracting or an extending of the remaining telescopic elements, if present. Alternatively the force transmission can also be effected directly on all further telescopic elements, so that they are movable directly by the drive unit. This would be advantageous, for example, in synchronized telescopic columns. 
     In one preferred embodiment of the disclosure the connecting element is embodied as a cable, belt, or chain. This makes possible a simple and cost-effective and, here, reliable and secure force transmission. The connecting element is preferably connected to the shaft such that with rotation of the shaft it is windable or unwindable. Embodiments including steel cables are particularly preferred. 
     In one preferred embodiment of the disclosure the brake unit includes a spring element and a brake surface, wherein the spring element is configured such that due to a preload it is bringable into frictional contact with the brake surface for generating the retaining force and such that it is bringable into operative contact with the clutch elements and thereby its spring tension is changeable. For example, the clutch elements, the spring element, and the brake unit form a coil spring coupling unit. These make possible a particularly compact design so that little space is to be provided for the drive unit and the brake unit. Simultaneously this embodiment of the disclosure can be adapted to various situations via the dimensioning and design of the spring element and of the housing. 
     The brake surfaces can be formed, for example, on the inner side of a housing surrounding the spring element. The preload for generating the retaining force presses the round spring element outward against the brake surface. Alternatively, the brake surfaces can be formed on the outer side of an element, for example, of a cylindrical mandrel, that is surrounded by the spring element. Here the preload acts conversely. The following embodiments and advantages are each based on a housing with a spring element lying therein. However, they are transferrable in a simple manner to an outer-lying spring element and a brake element lying therein. 
     In one preferred embodiment of the disclosure the operative contact between the clutch elements and the spring element is generatable by at least one coupling element formed on each of the clutch elements and at least one coupling element, corresponding thereto, of the spring element. These can be, for example, claw-type extensions, extending axially and lying inside the spring element, which correspond to an extension of the spring element. Rotating the clutch element and the coupling elements causes the latter to press on the extension of the spring element, and in the case of an embodiment as a coil- or wrap-spring, pushes it open or closed depending on the direction of rotation. The spring tension is thus increased or decreased in relation to the surrounding housing. Thus the retaining force of the brake unit can be influenced by the drive unit. 
     In one preferred embodiment of the disclosure each of the clutch elements includes at least two coupling elements and the spring element includes two coupling elements, each corresponding to one of the coupling elements of the clutch elements, wherein with rotation of the clutch element in the clockwise direction one of the coupling elements of the clutch element respectively enters into operative contact with one of the coupling elements of the spring element, and with rotation of the clutch element in the counterclockwise direction the respective other one of the coupling elements of the clutch elements enters into operative contact with the other coupling element of the spring element, so that independent of the respective direction of rotation of the respective coupling element a changing operative contact analogous to the spring tension is generatable. In this context “analogous” means that the operative contact changes equally or nearly equally, i.e., the retaining force increases or decreases. Thus a drive system for a telescopic column can be provided wherein the force transmission from the drive unit to the brake unit or from the output unit to the brake unit is effected independently of the respective direction of rotation. Consequently the direction of rotation need not be taken into consideration in the construction of a corresponding telescopic column. 
     This is in particular advantageous in preferred embodiments of the disclosure that comprise a further drive system that is constructed analogously to the first drive system and functions as a redundant drive system. The second drive system can be embodied completely separately. Alternatively and preferably both drive units can be driven via a common motor so that they work synchronously. In this case it can be constructively advantageous to operate both shafts in respectively opposing directions of rotation in order to achieve a compact- and simple-as-possible design. Thus both drive units can be supported, for example, parallel and closely spaced, and be rotated in opposite directions simultaneously via a drive wheel of a motor, which drive wheel lies between them. Alternatively other common drives, such as a bevel gear, spur gear, belt, etc. can be used. Due to the rotational-direction-independent transmission of the operative contact it is ensured that with the transmitting of forces from the respective drive unit to the output unit the same effect, i.e., retracting or extending of the telescopic column, is achieved. Thus one of the drive units can be configured, for example, as a main drive unit, and extend and retract the telescopic column in normal operation. The second drive unit is then embodied as a redundant safety system and performs the movements in parallel. In the case of a failure of the first drive system, for example, by breakage of the connecting element, the gravitational force is directly absorbed by the safety system and an uncontrolled retracting or extending is avoided. 
     In one preferred embodiment of the disclosure, each of the clutch elements respectively includes at least one third coupling element, wherein the coupling elements of the clutch elements are configured such that one of the coupling elements of both clutch elements is bringable into direct contact as soon as one of the first or second coupling elements of the clutch elements is in operative contact with one of the coupling elements of the spring element. A simultaneous reduction of the retaining force and a direct bringing-into-engagement of the coupling elements is thereby achieved, so that with the reducing of the retaining force the drive-and output-side of the drive are in direct operative contact via coupling elements and a torque of the motor is directly transmitted. The movable telescopic element is then directly held and moved by the motor. 
     In one alternative embodiment the coupling elements of the clutch elements each have an opening that is configured and disposed such that in the case of operative contact the coupling elements of the spring element are receivable therein such that one of the couple elements of each of the two clutch elements is bringable into operative contact. Analogously in this embodiment a simultaneous reduction of the retaining force and a direct bringing-into-engagement of the coupling elements is achieved, so that with reducing of the retaining force the drive- and output-side of the drive are in direct operative contact and a torque of the motor is directly transmitted. 
     With the use of a steel cable, a belt, or a chain for moving the telescopic column there is in principle the risk of a breakage and an uncontrolled lowering, i.e., extending, of the telescopic column. In this case there is an acute risk of injury or damage. In this respect a safety mechanism is to be provided that reliably stops movement in the case of a break. 
     It is therefore a further aspect of the present disclosure here to provide a simple and secure and relatively simply and compactly implementable solution. 
     In one embodiment of the disclosure a telescopic column comprising the following features is specified: 
     at least two telescopic elements moveable linearly with respect to one another, 
     at least one drive unit connected to a first of the telescopic elements, 
     at least one connecting element, by which a force is transmissible from the drive unit to a second of the telescopic elements, 
     a monitoring unit, which is configured such that an operating parameter of the connecting element is detectable, 
     wherein upon detecting of the operating parameter outside of a predefinable range a defined change of an operating state of the drive unit is performable. 
     By monitoring the connecting element, a failure, wear, or generally a fault of the system can be reliably recognized in a simple manner, and the operating state of the drive unit adapted accordingly. A switching-off of the drive unit could be provided, for example, when the connecting element fails or a failure is imminent. For this purpose it is advantageous to also provide a locking mechanism, by which a further extending of the telescopic column is prevented. Alternatively or additionally an alarm can also be issued. Depending on the use case, for example, the existence of the connection of drive unit and second telescopic element can be monitored as state of the connecting element, depending on its specific embodiment. In the case of an embodiment of the connecting element as a cable, belt, or chain, it could thus be monitored whether the cable or belt or the chain is broken. Alternatively an aging condition and wear could be concluded from a stress of cable, belt, or chain so that the connecting element can be preventively exchanged. 
     In one preferred embodiment of the disclosure the telescopic column comprises a second drive unit and a second connecting element by which a force is transmissible from the second work unit to the second of the telescopic elements. Such a redundant drive system can also prevent an accident in the event of a total failure of the first connecting element and make possible the planned change of the operating state. Alternatively a further operation of the telescopic column can also be ensured. In this case an alarm is preferably additionally emitted. 
     In one preferred embodiment of the disclosure the monitoring device includes the following features: 
     A detector element corresponding to at least one of the connecting elements and detecting its operating state, 
     a switch element interacting with the detecting element, by which switch element a switching operation is performable, by which the operating state of at least one of the drive units is influenceable. 
     This design ensures in a simple manner that with detection of the state of the connecting element outside definable parameters the drive unit is switched off, for example, by a simple switching operation. 
     In one preferred embodiment of the disclosure the telescopic column further includes the following features: 
     the detector element comprises a preloadable lever arm by which a defined clamping force is exertable on the connecting element, 
     a counter-force element by which a counter-force is exertable on the connecting element, 
     a basic tension of the connecting element, which basic tension is generated by a clamping force and counter-force, forms the operating parameter, 
     the clamping force and the counter-force are chosen such that in the normal operating state of the connecting element a balance of the two forces prevails and the lever arm is held in a balance position, 
     the connecting element is configured such that a change of the basic tension changes the balance such that the lever arm is deflectable out of its balance position, 
     the switching element is configured such that with deflection of the lever arm out of its balance position the switching operation is performed. 
     Using this embodiment the state of a cable or belt or a chain can be mechanically and/or electronically monitored in a simple and reliable manner In the case of a cable, with corresponding sensitivity an elongation caused by wear can also be detected and the cable exchanged prior to failure. 
     In one preferred embodiment of the disclosure the drive unit comprises a shaft on which the connecting element is disposed windable and unwindable, so that with winding or unwinding of the connecting element it is linearly movable with respect to the first telescopic element due to its connection to the second telescopic element. This embodiment is particularly compact and easy to manufacture, as well as particularly reliable. 
     In one preferred embodiment of the disclosure the connecting element is embodied such that with a failure of the connecting element the counter-force is fully or partially cancelled. This can be detected and evaluated in a particularly simple manner via a mechanical force meter as a change of the state of the connecting element. 
     The described aspects of the disclosure with reference to the drive system, the safety system, and the damping system can already in themselves bring significant advantages compared to known solutions. However, a particularly preferred embodiment of the disclosure comprises at least two of these systems, and in the ideal case, all three of these systems. Thus the drive system and the safety system according to embodiments of the disclosure work together and are optimally adapted to one another. The damping system likewise works together with the drive system in an adapted manner. 
     Further advantages, features and details of the disclosure arise from the exemplary embodiments of the disclosure described in the following with the assistance of the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows telescopic column, 
         FIG. 2  shows the telescopic column in a partially extended state, 
         FIG. 3  shows a sectional view of the telescopic column according to  FIG. 1 , 
         FIG. 4  shows a detail view of a drive unit of the telescopic column according to  FIG. 1 , 
         FIG. 5  shows a sectional view of the drive unit according to  FIG. 4 , 
         FIG. 6  shows an exploded view of the coupling unit of the drive unit according to  FIG. 4 , 
         FIG. 7  shows the coupling unit of  FIG. 6  from the viewing direction of the output side, 
         FIG. 8  shows the coupling unit of  FIG. 6  from the viewing direction of the drive side; 
         FIGS. 9 to 11  show the coupling unit of  FIG. 6  from both viewing directions in different operating states, 
         FIG. 12  shows a detail view of the coupling unit of  FIG. 6 , 
         FIG. 13  shows a shock absorber for a telescopic column, 
         FIG. 14  shows the shock absorber according to  FIG. 13  in a detail view, 
         FIGS. 15 to 17  show the shock absorber according to  FIG. 13  in a schematic sectional view in different operating states, 
         FIGS. 18 and 19  show the shock absorber according to  FIG. 13  in the telescopic column, 
         FIGS. 20 to 24  show a monitoring unit for a telescopic column, 
         FIGS. 25 to 27  schematically show the function of the monitoring unit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a telescopic column in one embodiment as ceiling stand  1 . A receiving plate  3  is disposed at its upper end, using which the ceiling stand  1  is attachable to the ceiling of a room via four screw holes  5 . This can be, for example, a medical examination room, an operating room, a workshop, or a manufacturing device. A column element  7  is attached to the receiving plate  3 , which column element  7  extends vertically downward in the installed state. Here the column element  7  is embodied nearly square with respect to its base area, with slightly chamfered edges. In other embodiments the base area can also have other geometries, i.e., for example, round or triangular. The base area can also change in the downward direction. A housing  9  is disposed laterally of the column element  7 . The functional elements of a drive unit are disposed partially in the housing  9  and partially in the upper region of the column element  7 . A motor  11  is disposed below the housing  9  that is designed to interact with the elements of the drive unit. The internal design of the drive unit is described in detail with reference to  FIGS. 4 and 5 . 
     Alternatively such embodiments are also possible for telescopic columns that are designed to stand on the floor. In this case the outermost, in other embodiments also the innermost column element is attached to the floor or to a movable or moving frame, and with actuation of the drive unit the inner- or outer-lying column elements are extended upward or retracted downward. 
     In  FIG. 2  the ceiling stand  1  is depicted in another operating state. Inside the column element  7  four further column elements  21 ,  23 ,  25  and  27  are disposed one-inside-the-other. The size of each base area respectively decreases here so that the column element  27  has the smallest base area. The four column elements  21 ,  23 ,  25 , and  27  are linearly movable with respect to the respective directly farther-outer-lying column element  7 ,  21 ,  23 , or  25 . Thus the column element  21  can move downward out of the column element  7 . Analogously the column element  23  can move linearly downward out of the column element  21 , etc. a receiving plate  31  is disposed at the lower end of the column element  27  on which a load to be lifted or lowered can be attached. Alternatively the load can also be directly connected through the receiving plate to the column element  27 . The load can be, for example, an X-ray apparatus. The drive unit and the motor  11  can be controlled via a not-depicted operating unit, which is embodied, for example, as a remote control or handle, and thus extend the column elements  21 ,  23 ,  25 , and  27  downward and retract them upward. Accordingly the attached load is raised or lowered. 
       FIG. 3  is a sectional view of the ceiling stand  1 . Inside the column element  7 , attached under the receiving plate  3 , four further column elements  21 ,  23 ,  25 , and  27  are depicted in the retracted state. A part of the drive unit is supported above the four column elements  21 ,  23 ,  25  and  27  in the column element  7 . The drive unit comprises a first cable drum  33  rotatably supported in the housing, on which cable drum  33  a cable  35  is wound. The cable  35  is connected to the receiving plate  31  on the other end by an attachment unit  37 . By rotating the cable drum  33 , in this depiction in the clockwise direction, the cable  35  is unwound from the cable drum  33 , whereby the column element  27 , and subsequently the column elements  21 ,  23 , and  25  continually lower due to gravity. Depending on the friction conditions present or the construction the lowering can also occur discontinuously, perhaps by the column elements  21 ,  23 ,  25 , and  27  extending sequentially. In this way the ceiling stand  1  can extend downward and lower the attached load. By rotating the cable drum  33  in the counterclockwise direction an upwardly directed force is exerted on the receiving plate  31  so that the column elements  21 ,  23 ,  25  and  27  retract again. 
     In the depicted embodiment according to the disclosure a further cable drum  43  is provided on which a cable  45  is wound. The cable  45  is in turn connected via an attachment unit  47 , and this via a spring  49 , to the receiving plate  31 . The spring serves to relieve the cable  45  if different speeds of the cables  35  and  45  arise in operation due to tolerance differences or expansions of the components. With respect to the direction of rotation, the cable  45  is wound around the cable drum  43  differently than the cable  35 , so that with counterclockwise rotation of the cable drum  43  it is unwound and with clockwise rotation it is wound. To lower the support plate  31  and thus to extend the ceiling stand  1  it is consequently necessary that according to the depiction of  FIG. 3  the cable drum  33  and the cable drum  43  are simultaneously rotated clockwise and counterclockwise respectively. Accordingly to retract the ceiling stand  1  the cable drum must be rotated counterclockwise and the cable drum  43  rotated clockwise. The cable drum  43  and the cable  45  wound onto it serve mainly as a redundant safety mechanism in case the cable  35  wears or breaks during use. In this case the receiving plate  31  and thus the column elements  21 ,  23 ,  25 , and  27  would be held by the cable  45  and also be movable further. 
     The ceiling stand  1  further includes a monitoring unit  51 , whose structure and function is described in detail with reference to  FIGS. 20 to 27 . 
     In  FIG. 4  essential parts of the drive unit are depicted in section. The drive unit comprises a main drive unit  61  as well as a safety unit  81 . The main drive unit  61  includes a shaft  63 , which is supported via three bearing points  65  in the drive housing  9  (not depicted) or in the column element  7 . On the shaft  63  the cable drum  33  is attached to the cable  35 . Drive-side the main drive unit  61  includes a worm gear  67  that can be set into rotational movement via a drive shaft  17  of the motor  11 . The worm gear  67  is radially and axially defined; however, it can rotate about the shaft  63 . The force transmission between the worm gear  67  and the shaft  63  is effected via a coupling unit  69  whose function is described in detail with respect to  FIGS. 5 to 12 . 
     The safety unit  81  is constructed analogously to the main drive unit  61 . It also comprises a shaft  83  that is supported via three bearings  85 . On the shaft  83  the cable drum  43  is attached to the cable  45 . Also analogously a worm gear  87  is provided that is also in engagement with the drive shaft  17 . Due to the opposite arrangement of the two worm gears  67  and  87  with respect to the drive shaft  17 , it is ensured that with rotation of the drive shaft  17  the two worm gears  67  and  87  each rotate in opposite directions. Due to the opposing winding directions of the two cables  35  and  45 , with rotation of the drive shaft  17  in one direction both cables  35  and  45  are unwound, and with opposite rotation wound up. Also with the safety unit  81  the force transfer between the worm gear  87  and the shaft  83  is effected via a coupling unit  89  that is constructed analogously to the coupling unit  69 . 
     In  FIG. 5  a sectional view of the drive unit is depicted. The shafts  63  and  83  of the main drive unit  61  and of the safety unit  81  are depicted. A renewed explanation of the elements already described with respect to  FIG. 4  is omitted here; rather, the mode of action of the coupling units  69  and  89  shall be described. The inner diameters of the worm gears  67  and  69  are larger than the outer diameter of the shafts  63  and  83 , respectively. In this respect a rotational movement of the worm gears  67  and  69  is not transmitted to the shafts  63  and  83  by a direct contact. The transmission of the force is exemplarily explained on the basis of the coupling unit  69 . The coupling unit  89  functions analogously. 
     Drive-side the coupling unit  69  comprises a first clutch element  91  that is fixedly connected to the worm gear via screws  92 , and pins (not depicted). The clutch element  91  also has a larger inner diameter than the outer diameter of the shaft  63 , so that also here no direct torque transfer to the shaft  63  takes place. Output-side the coupling unit  69  includes a second clutch element  93  that is disposed axially opposing the clutch element  91 . At its inner diameter the clutch element  93  is disposed on the shaft  63  in a fixedly seating manner A rotational movement of the clutch element  93  consequently exerts a torque on the shaft  63 , by which the shaft  63  is also set into rotation in the same direction. A rotational movement of the shaft  63  generated output-side also exerts a torque on the coupling unit  93 . The coupling unit  69  further includes a coil spring  94  that serves as a retaining mechanism. The detailed design and the mode of action of the coupling unit  69  is described in detail in  FIGS. 6 to 12 . 
     In  FIG. 6  the two clutch elements  91  and  93 , as well as the coil spring  94 , are depicted pulled apart axially in an exploded view. The clutch element  91  includes a ring-type base element  101  in which screw holes  102  are provided for receiving the screws  92  not depicted here (see  FIG. 5 ) and/or pins. Radially outward on the base element  101  three claw-type coupling elements  103   a ,  103   b , and  103   c  are disposed that extend axially toward the clutch element  93 . The clutch element  93  includes a shaft seat  105  that resembles the base element  101  with respect to its external design. However, at the inner diameter the shaft seat  105  is adapted to the diameter of the shaft  63  or  83  such that after the installation there is a friction-fit or keyed connection. For this purpose at its inner diameter the shaft seat  105  includes an axially extending groove  106  that is in engagement with a corresponding projection on the shaft  63  or  83  or a key and supports the coupling for conjoint rotation. In comparison thereto the base element  101  sits in contrast only loosely on the shaft  63  or  83 ; it is thus not connected thereto such that they rotate together. Radially outwardly lying the clutch element  93  includes three claw-type coupling elements  107   a ,  107   b , and  107   c  that extend axially toward the clutch element  91 . 
     The coupling elements  103   a ,  103   b , and  103   c  and  107   a ,  107   b , and  107   c  can be considered as sections of a cylindrical shell; thus they have a curved configuration in the circumferential direction. With respect to their dimensions they are chosen small in the circumferential direction such that a relatively large spacing respectively remains between them. With axial joining of the two clutch elements  91  and  93 , each one of the coupling elements  103   a ,  103   b , and  103   c  comes to rest between two of the coupling elements  107   a ,  107   b , and  107   c . Analogously each one of the coupling elements  107   a ,  107   b , and  107   c  lies between two of the coupling elements  103   a ,  103   b , and  103   c . End-side the coupling elements  103   a ,  103   b , and  103   c  then lie slightly radially spaced outside the outer circumference of the shaft seat  105 . Analogously the coupling elements  107   a ,  107   b , and  107   c  lie slightly radially spaced outside the outer circumference of the base element  101 . The clutch elements  103   a ,  103   b , and  103   c  and  107   a ,  107   b , and  107   c  are also chosen small with respect to their dimensions in the circumferential direction such that there is a defined distance respectively between one of the coupling elements  103   a ,  103   b , and  103   c  and the adjacent two coupling elements  107   a ,  107   b , and  107   c , i.e., even in the assembled form a complete cylindrical shell is not formed. Thus, for example, with initially central orientation of the coupling elements  103   a ,  103   b , and  103   c  and  107   a ,  107   b , and  107   c  with respect to one another the clutch element  93  can be rotated about a defined angle until it comes into contact with another of the components. 
     Radially outside the coupling elements  103   a ,  103   b , and  103   c  and  107   a ,  107   b , and  107   c  the helically shaped coil spring  94  wraps around these. End-side the coil spring  94  respectively includes radially inwardly curved ends  109  (output side) and  109 ′ (drive side) on which each one of the coupling elements  103   a  and  103   b  or  107   b  and  107   c  can exert a force acting on the coil spring  94  in the circumferential direction. The end  109  lies between the coupling elements  103   b  and  107   b ; thus depending on the rotation direction it can enter into operative contact with these. The end  109 ′ lies between the coupling elements  103   a  and  107   c  and can enter into operative contact with these in an analogous manner Due to their arrangement the coupling elements  103   c  and  107   a  do not enter into contact with the ends  109  and  109 ′ in any operating state. For axial fixing of the coil spring  94 , the coupling elements  103   a ,  103   b , and  103   c  and  107   a ,  107   b , and  107   c  each include an end-side, radially outer-lying projection  111 . 
     In  FIG. 7  the coupling unit  69  is shown in a three-dimensional depiction in the assembled state from the viewing direction of the output side. The outer part of the coupling unit  69  comprises a brake housing  113 , which is attached in the housing of the ceiling stand  1  such that it is secured against rotation. In each of the depictions of  FIGS. 4, 5 and 6  the brake housing has not been shown for better overview. In  FIG. 7  it can be seen how the coupling elements  103   a ,  103   b , and  103   c  lie end-side outside the outer circumference of the shaft seat  105 , and due to the distance of the clutch elements  103   a ,  103   b , and  103   c  and  107   a ,  107   b , and  107   c  a limited, independent rotation of the clutch elements  91  and  93  with respect to each other is possible. 
     The coil spring  94  is wound and dimensioned such that with installation in the brake housing  113  it must be radially contracted, i.e. radially reduced. The coil spring  94  is held under tension radially by the brake housing  113  and axially by the projections  111 , so that the coil spring  94  cannot directly relax again and is held in the preloaded state. In this state the coil spring  94  is therefore not rotatable in the brake housing  113  and develops a defined retaining force depending on the design. The clutch element  93  is rotatable about a small angle until, depending on the direction of rotation, either the coupling element  107   b  presses against the end  109  (as depicted in  FIG. 7 ) or the coupling element  107   c  presses against the end  109 ′ (see  FIG. 8 ). The end  109  of the coil spring  94  lies between coupling elements  103   b  and  107   b.    
     In  FIG. 8  the coupling unit  69  is depicted in a depiction analogous to  FIG. 7 , but here from the viewing direction of the drive side. Here it can analogously be seen how the coil spring  94  is held axially by the projections  111  and is thus axially fixed overall. The second end  109 ′ of the coil spring  94  lies between the coupling elements  103   a  and  107   c.    
     After installation of the coupling unit  69  or  89  on the shaft  63  or  83  and the receiving of the load, due to the gravitational force an output-side torque generated by the column elements  21 ,  23 ,  25  and  27  via the cables  35  and  45  and the cable drums  33  and  43  permanently acts on the respective coupling element  93 . This is then rotated in a manner depending on the direction of rotation so far until either the coupling element  107   b  presses against the end  109  or the couple element  107   c  presses against the end  109 ′. The respective end  109  or  109 ′ consequently receives a force acting in the circumferential direction. Due to the winding direction of the coil spring  94 , in both cases this force effects a force on the coil spring  94 , since this tries to widen. Due to the surrounding brake housing  113  the force causes no actual widening, but rather a strengthening of the frictional operative contact of the coil spring  94  with the brake housing  113 . Due to the initial retaining force due to the preload and this increased friction, the output-side torque is fully compensated and a further rotating is prevented. An unwinding of the coil  35  or  45  and an extending of the column elements  21 ,  23 ,  25  and  27  is thus prevented; the arrangement consequently retained. This is the initial state of the ceiling stand  1 . An increase of the output-side torque, for example, by enlarging of the received load, increases the pressure on the coil spring  94  and thus the friction with the brake housing  113  so that even then the position is held. 
     In  FIGS. 9, 10, and 11  the coupling unit  69  is depicted in various operating states, each from the viewing direction of the output side (left) and of the drive side (right). Here the position of the coupling elements  103   a ,  103   b , and  103   c  or  107   a ,  107   b , and  107   c  with respect to the ends  109  and  109 ′ is different. 
     In  FIG. 9  an intermediate state is depicted wherein none of the coupling elements  103   a ,  103   b , and  103   c  or  107   a ,  107   b , and  107   c  are in contact with the ends  109  and  109 ′. Accordingly the end  109  lies free between the coupling elements  103   b  and  107   b . The end  109 ′ lies free between the coupling elements  103   a  and  107   c.    
     In  FIG. 10  after a rotating of the drive-side clutch element  91  the coupling element  103   a  is in contact with the end  109 ′. With a continuing of the rotation in the same direction the coupling element  103   a  will exert a force on the end  109 ′ and move it in the clockwise direction (right depiction). The coil spring  94  is thus radially contracted, and the retaining force with respect to the brake housing  113  is reduced. After brief further turning the end  109 ′ enters into contact with the coupling element  107   c , which is depicted in  FIG. 11 . The clutch element  93  is then moved along in the same direction. This rotating is then transmitted to the cable drum  33  or  43  and correspondingly the cables  35  or  45  are wound or unwound. Depending on the direction of rotation a retracting or extending of the ceiling stand  1  thus results. 
     In order that the transfer of the drive torque does not only take place via the contact between the coupling element  103   a , the end  109 ′, and the coupling element  107   c , the dimensions of the components in the circumferential direction are chosen such that with the production of this effect chain the coupling elements  107   b  and  103  are also in contact by their mutually facing side surfaces and additionally transfer the drive torque. The end  109 ′ is consequently relieved. 
     In  FIG. 12  the size ratios are illustrated schematically. The coupling elements  103   a ,  103   b ,  107   b  and  107   c  each span an angle γ, the coupling elements  107   a  and  103   c  an angle β. An angle α falls between the coupling elements  107   a  and  107   b  or  107   c . The same angle α falls between the coupling elements  103   c  and  103   a  or  103   b . The sizes of the angles α, β and γ are chosen such that the coupling elements  107   b  and  103   c  enter into contact simultaneously when the couple element  103   a  enters into contact with the end  109 ′ and the end  109 ′ enters into contact with the coupling element  107   c . A reliable torque transmission is thus ensured. 
     A drive-side rotating of the clutch element  93  in the other direction of rotation works in a completely analogous manner, wherein the end  109  then forms an effect chain with the coupling elements  103   b  and  107   b , and the coupling elements  103   c  and  107   c  are in direct operative contact. The ceiling stand  1  is extended or retracted accordingly. 
     With a rotating of the drive shaft of the motor  11 , the worm gear  67  coupled thereto receives a torque and set into rotation. This is transmitted by the screw connection to the clutch element  91 . With a rotating in the clockwise direction from the viewing direction of the drive side as in  FIG. 8 , for example, the coupling elements  103   a ,  103   b , and  103   c  rotate in the clockwise direction toward the coupling elements  107   a ,  107   b , and  107   c , and here the space located between them initially decreases. Here an increased pressure results of the coupling element  103   a  with the end  109 ′ of the coil spring  94 . Due to the built-up pressure on the end  109 ′ of the coil spring  94  is subjected to a force acting in the circumferential direction in the clockwise direction. With sufficiently large drive-side torque a radial contracting of the coil spring  94  results, so that the friction-fit connection to the brake housing  113  is relaxed. Consequently the output-side torque is no longer completely compensated, with the result that the shaft is set into rotation and the cable  35  or  45  unwinds and the column elements  21 ,  23 ,  25 , and  27  are extended. This leads to a lowering of the load. Due to the existing output-side torque the coupling element  107   c  remains in constant contact with the end  109 ′ of the coil spring  94 ; it is thus also rotated along in the clockwise direction. As soon as the drive-side torque is omitted due stoppage of the rotational movement of the motor  11 , the then no longer compensated or overcompensated output-side torque immediately leads to a widening of the coil spring  94  again and a friction-fit contact with the brake housing  113  and thus production of the retaining force, with the result that the load is held at the new level. 
     With a rotating of the worm gear  67  and thus of the coupling element  91  in the counterclockwise direction, a convergence of the coupling elements  103   a ,  103   b , and  103   c  and  107   a ,  107   b , and  107   c  analogously results. In this case the coupling element  103   b  interacts with the end  109  of the coil spring  94  and the coupling element  107   c  with the end  109 ′ of the coil spring  94  in an analogous manner, with the result that the rotational movement is in turn transmitted to the shaft  63 . In this case the cable  35  is wound and the load is lifted. 
     In a fully analogous manner the coupling unit  89  acts between worm gear  87  and shaft  83 . Due to the construction of the coupling unit  69  or  89  it does not matter in which direction of rotation the torques each act. A drive-side torque always leads to a contracting of the coil spring  94  and a loosening of the brake, while an output-side torque widens the coil spring  94  and increases the brake effect. 
     In  FIG. 13  one of the column elements  21  is sectionally depicted. The other column elements  23 ,  25  and  27  are analogously constructed. On a side surface  500  it includes a flat and wide groove-shaped recess  501 , wherein a slot  502  is formed. A shock absorber  503  is inserted and secured in the slot  502 . The construction of the shock absorber  503  is described in detail in  FIGS. 14 to 17 , its mode of operation on the basis of  FIGS. 18 and 19 . 
     In  FIG. 14  the shock absorber  503  is depicted. It includes a receiving element  505  on each end side, each of which receiving elements  505  includes a center part  507  and two guide parts  509 . The guide parts  509  are wider than the center part  507 , so that a groove  511  arises laterally in each case. In the installed state, the housing of the column element  21 , which housing borders the slot  502 , engages into the grooves  511 , with the result that the shock absorbers  503  are secured and guided. End-side the guide parts  509  have a round shape  513 . Between the center parts  507  a damping element  515  is disposed that comprises a cylinder  517  and a plunger  519 . With exerting of axial pressure on the plunger  519  and the cylinder  517  the plunger  519  enters the cylinder  517  in a damped manner Thus the spacing of the two center parts  517  can decrease. With decreasing spacing the force to be expended is greater due to the damping properties of the damping element  515 , until the plunger  519  is completely received in the cylinder  517 . The damping element  515  can be embodied, for example, as is known from drawer- or door-dampers. As can be seen in  FIG. 13  the width of the slot  502  is not constant over its length, but rather is wider in the center. The central width is chosen such that the guide parts  509  are completely received there and can be successively pushed toward the axial end of the slot  502  so that the housing can engage into the grooves  511  and the shock absorber is secured. For installation it is necessary to axially compress the shock absorber  503  so that the second guide part  509  is placed in the wide part of the slot and can then be pushed to the axial end by relaxing of the shock absorber  503 . 
     In  FIGS. 15 to 17  the inner construction of the shock absorber is schematically depicted. The cylinder  517  is filled with a liquid  521 . Alternatively a gas can be used. Furthermore in the cylinder  517  a spring  523  is disposed that presses a piston  525  upward. The piston  525  is in turn connected to the plunger  519 ; it can thus be acted upon with a force. The piston  525  can move against the force of the spring  523  in the cylinder  517 . The piston  525  comprises a channel  527 , through which during movement the liquid  521  can flow between the two half-spaces of the cylinder  517 , which half-spaces are defined by the piston  525 . Otherwise the piston  525  would be blocked due to the incompressible liquid  521 . The piston  525  comprises a further channel  529  that is embodied significantly wider than the channel  527 . The channel  529  is provided with a valve  531  that makes possible a passage of liquid  521  only in a movement direction of the piston  525 , namely with an upward-directed movement. 
     In  FIG. 16  due to an impinging of the plunger  519  with a force the piston is moved upward, whereby the spring  523  is compressed. Due to the relatively narrow channel  527  a flow specifically of the liquid  521  is made possible, but the upward movement of the piston  525  is strongly damped and thus slowed. The kinetic energy is correspondingly absorbed. The valve  531  is closed, with the result that no liquid  521  can flow through the channel  529 . 
     In  FIG. 17  the plunger  519  is no longer acted upon by a force, with the result that the piston  525  is moved upward again by the spring  523  into its initial position. In this movement direction the valve  531  opens and the liquid  521  can flow through the channels  527  and  529 . The upward-directed movement of the piston  525  is therefore significantly less strongly damped than the previous downward-directed movement. 
     In  FIGS. 18 and 19  the mode of operation of the shock absorber  503  in the installed state is illustrated. Two of the column elements  7 ,  21 ,  23 ,  25 , and  27  (here  7  and  21 ) are exemplarily shown. The slot  502  with shock absorber  503  lying therein is disposed in the upper region of the respective inner-lying of the two column elements  21 ,  23 ,  25  and  27  (here  21 ). For this purpose a round stopper  532  is correspondingly attached in the lower region of the respective outer-lying of the column elements  7 ,  21 ,  23 , and  25  (here  7 ). The rounding of the stopper  532  is adapted to the round shape  513 . With extending of the telescopic column, near the maximum extension length an engaging of the lower receiving element  505  of the shock absorber  503  with the stopper  532  results, so that the plunger  519  is pressed in the cylinder  517 . As described above the movement is then damped by the liquid  521 , and the column elements are slowly and gently braked and stopped. The end of the movement is depicted in  FIG. 19 . No hard impacts arise at the end of the movement, whereby the wear is minimized and undesired noises are avoided. For safety a stopper  533  is also attached in column element  7 , which stopper  533  comes into engagement with a further stopper  535  in column element  21  if a shock absorber should fail. In addition the shock absorber  503  is relieved in the extended state. In the upper region not depicted here the column element  7  also comprises stoppers that during retracting of the column element  21  come into engagement towards the end of the retracting with the upper receiving element  505  of the shock absorber  503 , and brake and stop the movement. Depending on the loads and speeds a plurality of shock absorbers can be used between each two of the column elements; this plurality of shock absorbers can also be used in different side surfaces. 
     In  FIGS. 20 and 21  the monitoring unit  51  is shown in two operating states. It comprises a frame  52  attached inwardly to the column element  7 , on which frame  52  a bracket  53  is rotatably secured. End-side a roller  55  is rotatably or fixedly secured to the bracket  53 . The cables  35  and  45  are guided downward through the bracket, wherein the cable  45  is in contact with the roller  55 , while the cable  35  does not contact the bracket. Between the frame  52  and the bracket  53  a spring  56  is disposed that presses on the bracket with a force below the axis of rotation  57  toward the frame  52 . The part of the frame  53  carrying the roller  55  is thereby pulled downward in addition to the weight force of the roller  55 , until a state of equilibrium is reached due to the clamping force of the cable  45  against which the roller  55  presses. In the normal state depicted in  FIG. 3 , i.e., with intact cable  35 , the clamping force is generated by the spring  49 , via which the cable  45  is connected to the receiving plate  31 . Consequently the cable  45  does not then extend vertically downward, but rather is pulled somewhat to the side guided via the roller  55 . This normal state is also maintained during extending or retracting of the ceiling stand  1 , since the nearly full load hangs on the cable  35 . The cable  45  serves primarily as safety- and detector-cable. Should the cable  35  break, the full weight force of the column elements  21 ,  23 ,  25 , and  27  would be on the receiving plate  31 , and the load located thereon would hang on cable  45 . Consequently another force would pull on the end of the spring  49 , which disrupts the balance. The cable  45  would be more tightly drawn and the bracket  53  impinged with a stronger force, which is thereby deflected upward. The spring  56  is thereby stretched. This state is depicted in  FIG. 20 , where the cable  45  is stretched nearly vertically downward by the increased downwardly acting force, and the roller  55  is thereby pressed upward. 
     The monitoring unit  51  is furthermore in the position to detect a break of the cable  45 . Also in this case the balance for the spring  49  is disturbed since due to the latter more tension can be transmitted to the cable  45 . This state is depicted in  FIG. 21 . The cable  45  is no longer pulled downward and is therefore correspondingly slack. In comparison to the normal state the force of the spring  56  pulls the bracket  53  downward, whereby the roller  55  is also moved downward. 
     With respect to their loadability the cables  35  and  45  are designed such that they are each in the position to carry the permitted total load alone. In this respect in the normal state only the cable  35  is loaded, while the cable  45  serves only as a safety cable. However, in the case of the breakage of one of the cables  35  or  45  there is a potential safety risk, with the result that the drive is to be shut off or at least an alarm is to be issued. As already explained, the monitoring unit  51  is also in the position to detect a break of each of the cables  35  and  45  by a change of the position of the bracket. 
     In  FIG. 22  the monitoring unit  51  is depicted separately from the other viewing direction. On the frame  52  a switch is mounted that comprises a switch housing  54  and can be switched via a movable arm  58 . Here the arm  58  is formed from a curved metal plate that resiliently pushes the arm  58  away from the switch housing  54 . The arm  58  is in contact with a semicircular formation  59  formed on the bracket  53 , which formation  59  in turn includes a projection  59 ′. With rotating of the bracket  53  the spacing between the arm  58  and the switch housing  54  is changed. With contact of the arm  58  with the projection  59 ′ the arm  58  is pushed relatively close to the switch housing  54  and thus closes the switch. This position is shown in  FIG. 22  and corresponds to the already explained normal state. With rupturing of one of the cables  35  or  45  the bracket  53  is moved upward or downward, whereby the arm  58  comes into contact with the formation  59  above or below the projection  59 ′. These states are depicted in  FIGS. 23  (cable  35  broken) and  24  (cable  45  broken). The arm  58  therefore moves away from the switch housing  54  and the switch thereby opens. If one of the cables  35  or  45  breaks, an alarm can thus be issued via connected electronics on the basis of the state of the switch. Likewise the system can be switched off. 
     The normal state is schematically depicted in  FIG. 25 . The receiving plate  31  is connected with both cables  35  and  45 , wherein the connection to the cable  45  is effected via the spring  49 . The load is distributed on the cables  35  and  45 , which is depicted by arrows  601  and  603 . The main load acts on the cable  35 . The cable  45  is deflected to the left from the vertical position by the spring  56 , with the result that a balance to the acting force of the spring  49  arises, which is depicted by arrows  605  and  607 . Here the spring  56  is tensioned between a relaxed and a maximally tensed state, which is illustrated by a display  600  only depicted here for illustration. 
     In  FIG. 26  the case of a broken cable  45  is depicted. The full load hangs on the cable  35 , which is depicted by the long arrow  601 ′. On the part of the cable  45  a very slight or even no force thereby acts on the spring  65 , which is depicted by the shorter arrows  605 ′ and  607 ′. Thus the spring  56  can relax, which is illustrated by a deflection of the display  600 . In this case the switch is opened and an alarm is issued. 
     In  FIG. 27  the case of the broken cable  35  is depicted. The full load thereby hangs on cable  45 , which is represented by the longer arrow  603 ″. Accordingly the spring  49  is stretched and the cable  45  tightened. Thus a greater force acts on the spring  56 , which is represented by the longer arrows  605 ″ and  607 ″. The spring  56  is thus also stretched, which in turn is illustrated by the deflection of the display  600 . Also in this case the switch is opened and an alarm is issued. 
     However, using the monitoring unit  51  not only the previously described rupturing of the cables  35  or  45  can be detected. Rather, wear of the primarily loaded cable  35  can also be detected. With the use of steel cables, with continuous use fatigue results by breakage or stretching of individual steel fibers. The cable  35  thereby becomes slightly longer. In this respect the balance of the forces on the cable  45  is changed, which is also detectable by use of a force meter. Thus a breakage of a cable is often avoided beforehand by timely exchange. 
     Using the described safety system, in combination with the drive concept described, an extremely compact and reliable as well as safe telescopic column can be provided that in particular satisfies medical requirements. 
     Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above may be utilized separately or in conjunction with other features and teachings to provide improved telescopic column. 
     Moreover, combinations of features and steps disclosed in the above detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. 
     All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. 
     REFERENCE NUMBER LIST 
       1  Ceiling stand 
       3 ,  31  Receiving plate 
       5  Screw hole 
       7 ,  21 ,  23 ,  25 ,  27  Column element 
       9  Housing 
       11  Motor 
       17  Drive shaft 
       33 ,  43  Cable drum 
       35 ,  45  Cable 
       37 ,  47  Attachment unit 
       49 ,  56  Spring 
       51  Monitoring unit 
       52  Frame 
       53  Bracket 
       54  Switch housing 
       55  Roller 
       57  Axis of rotation 
       58  Arm 
       59  Formation 
       59 ′ Projection 
       61  Main drive unit 
       63 ,  83  Shaft 
       65 ,  85  Bearing point 
       67 ,  87  Worm gear 
       69 ,  89  Coupling unit 
       81  Safety unit 
       91 ,  93  Clutch element 
       92  Screw 
       94  Coil spring 
       101  Base element 
       102  Screw hole 
       103   a ,  103   b ,  103   c ,  107   a ,  107   b ,  107   c  Coupling element 
       105  Shaft seat 
       106  Groove 
       109 ,  109 ′ End 
       111  Projection 
       113  Brake housing 
       500  Side surface 
       501  Recess 
       502  Slot 
       503  Shock absorber 
       505  Receiving element 
       507  Center part 
       509  Guide part 
       511  Groove 
       513  Round shape 
       515  Damping element 
       517  Cylinder 
       519  Plunger 
       521  Liquid 
       523  Spring 
       525  Piston 
       527 ,  529  Channel 
       531  Valve 
       532 ,  533 ,  535  Stopper 
       600  Display 
       601 ,  601 ′,  603 ,  603 ″ Arrow 
       605 ,  605 ′,  605 ″ Arrow 
       607 ,  607 ′,  607 ″ Arrow