Linear drive with braking mechanism

Abstract

The invention relates to a linear drive comprising a screw drive (2), the screw drive parts (4, 5) of which are in screw engagement, wherein one of the screw drive parts (4) is mounted for longitudinal movement along the drive axis (10) and has a stroke part (11) for performing a stroke. A drum brake (18) is released when one rotationally driven screw drive part (5) is rotationally driven and is closed when the stroke part (11) is under external load. The drum brake (18) is provided with a plurality of brake shoes (24) and a brake drum (21) having a braking surface (23) for the brake shoes (24). A releasing actuation element (26) and a closing actuation element (27) are provided, which actuate the brake shoes (24) in opposite directions.

Description

[0001] Linear drive

[0002] The present invention relates to a linear drive. Applications include medical technology, for example, for moving a table of a CT scanner. The invention also generally relates to patient tables comprising a frame and a patient bed that can be moved by means of the linear drive.

[0003] From DE 2238632 C2 a linear drive according to the features of the preamble of claim 1 has become known.

[0004] This linear actuator comprises a screw drive driven by an electric motor via a spur gear. The screw drive's threaded nut is rotationally driven, and the threaded spindle is arranged for longitudinal displacement within a housing of the linear actuator and connected to a lifting part. When the electric motor is switched off, a braking device prevents unwanted rotation of the threaded nut in the direction of the spindle axis in the event of an external force acting on the lifting part. This braking device comprises a loop spring that rests against the stationary housing part and prevents unwanted rotation of the threaded nut.

[0005] Wrap band springs cause grinding noise, which is undesirable depending on the application of the linear drive. The conventional linear drive uses hydrostatic displacement units to reduce grinding noise, but these require increased design effort.

[0006] Furthermore, it was found that in the case of large external forces acting on the lifting part in the axial direction along the drive axis, the technical limits of the wrap band spring are reached, so that a reliable braking function of the braking device is not ensured.

[0007] The object of the invention was to provide a linear drive that is easy to manufacture and has an improved braking device. According to the invention, this object was achieved by the linear drive according to claim 1. Useful further developments are specified in the dependent claims.

[0008] The linear drive according to the invention comprises a screw drive whose screw drive parts are in screw engagement and convert a rotation of one screw drive part about a drive axis into a relative displacement of the screw drive parts along the drive axis. Preferably, the rotating screw drive part is fixed to the frame in the axial direction, and the other screw drive part is longitudinally displaceable in the axial direction and rotationally fixed with respect to the frame.

[0009] A ball screw drive is suitable for the invention, the balls of which roll in ball grooves of a threaded nut and a threaded spindle, which are wound helically around a drive axis.

[0010] Roller screws are also proven screw drives that can be used for the invention.

[0011] In special applications, it may be appropriate to use slide screws, such as those used in trapezoidal screw drives. In these cases, the threaded nut and the threaded spindle are each provided with a trapezoidal thread and are in threaded engagement with each other.

[0012] For particularly small thread pitches, a planetary roller screw drive may be required, the planetary rollers of which mesh with the thread of the lead screw on the one hand and the grooved profile of the nut on the other. If a pitch-accurate planetary roller screw drive is to be used, the planetary roller carrier, which accommodates planetary rollers distributed around the circumference, is often rotationally driven. In this case, the planetary roller carrier can also be referred to as the screw drive part. In this version, the lead screw can be arranged so that it is fixed in rotation and can be longitudinally displaceable and can be provided with a lifting part. One screw drive part is rotationally driven via a drive shaft, while the other screw drive part is arranged so that it can be longitudinally displaceable along the drive axis and is designed to perform a lifting movement.The rotationally driven screw drive part can preferably be formed by the aforementioned threaded spindle, which can be rotatably mounted in a housing. It is also possible to rotationally drive the aforementioned nut and, for example, to rotatably mount it in a housing. The other longitudinally displaceable screw drive part can preferably be formed by the nut, which, for example, is guided in a housing and is guided for longitudinal displacement. It is also conceivable to form the other screw drive part by the threaded spindle, which is guided in the housing and is guided for axial displacement.

[0013] The drive shaft can be part of a traction drive, with the motor shaft of an electric motor driving the drive shaft via a traction mechanism—e.g., a toothed belt. The drive shaft is preferably arranged coaxially with the rotationally driven screw drive part.

[0014] Furthermore, the linear drive is provided with a braking device which is opened under the rotational drive of one of the rotationally driven screw drive parts and which is closed under external axial load of the lifting part and counteracts displacements of the lifting part.

[0015] The braking system is designed as a drum brake with multiple brake shoes and a brake drum with a braking surface for the brake shoes. The braking system comprises a release actuating element and a closing actuating element, with the actuating elements actuating the brake shoes in opposite directions.

[0016] The release actuating element of the braking device is located on the drive side of the linear actuator and engages the brake shoes beneath the rotary drive of the drive shaft. The release actuating element is connected to the drive shaft and follows the rotational movements of the drive shaft.

[0017] The closing actuating element of the braking device is actuated when the lifting part is subjected to external axial load. The closing actuating element is connected to the rotary-driven screw drive part, which is subjected to a torque due to the screw engagement when the lifting part is subjected to an external axial load. The closing actuating element is actuated independently of the rotary drive. For example, if the drive moves in the direction of the applied force, the closing actuating element is permanently actuated, but the opening actuating element is also actuated, so that the friction of the brake is just large enough to ensure that the speed specified by the motor is not exceeded.

[0018] In a preferred further development, the drive shaft actuates the releasing actuating element in both directions of rotation, i.e. independently of the direction of rotation of the drive shaft, and drives the rotationally driven screw drive part.

[0019] If the drive shaft is rotationally driven, i.e. rotationally driven via a motor, a drive-side torque acts on the drive shaft, under which the releasing actuating element is actuated.

[0020] When the drive shaft is unloaded, meaning a motor drive is switched off, the rotary drive is switched off. As mentioned at the beginning, one screw drive part experiences a torque due to the screw engagement with the other screw drive part when the other screw drive part is subjected to axial load. Under this torque, one screw drive part actuates the closing actuating element.

[0021] If the screw drive components are formed, for example, by a threaded nut and a threaded spindle in screw engagement with the threaded nut, and the threaded spindle is rotationally driven, the threaded nut, which is arranged in a rotationally fixed and axially displaceable manner, is subjected to an axial load due to the connected lifting part and its external axial load. In this case, this axial external load is transferred to the rotationally driven threaded spindle. The screw engagement generates a torque applied to the threaded spindle about the drive axis. Under this torque, the threaded spindle actuates the closing actuating element for the drum brake. For this purpose, the closing actuating element can have a coupling part that is attached to the threaded spindle. An arrangement with the brake drum and brake shoes arranged axially one behind the other is conceivable. In this case, the brake shoes are actuated in axially opposite directions.

[0022] Preferably, a known brake drum arrangement is provided, in which the brake shoes are actuated in opposite directions in radial directions. A cost-effective further development provides two brake shoes. However, it is conceivable to arrange more than two brake shoe segments distributed around the circumference and actuated in the manner described.

[0023] When the drive shaft is subjected to motor load, the release actuating element rotates and engages the brake shoes. Regardless of the direction of rotation, the brake cymbal is deflected in a direction away from the braking surface by the drive force of the actuating element.

[0024] Under external axial load of the lifting part when, for example, the drive is switched off, the axial force acting on the lifting part can generate a circumferential force component on the rotating screw drive part due to the screw engagement of the two screw drive parts, i.e. a torque. Undesired rotation of this screw drive part is prevented by the closing actuating element acting on the brake shoes and always deflecting them towards the braking surface of the brake drum, regardless of the direction of rotation of the acting external axial force. The force between the brake shoes and the brake drum depends on the magnitude of the acting torque, which is generated due to the external axial force acting on the lifting part. The greater the axial external load, the greater the acting braking force under which axial displacement of the lifting part in the direction of the acting load is prevented.

[0025] As a result, the brake shoes are deflected exclusively in one direction, away from the braking surface, by the releasing actuating element, and exclusively in the opposite direction, i.e., toward the braking surface, by the closing actuating element. In the preferred arrangement, the brake shoes are arranged in a brake drum in a known manner and are actuated in the radial directions.

[0026] This braking device advantageously enables a useful refinement, with which, on the one hand, the drive load is transferred from the drive shaft via the brake shoes to the rotary screw drive part, and, on the other hand, an external load on the lifting part is transferred via the brake shoes to the brake drum. For example, if part of the linear drive is an electric motor that drives the drive shaft via a gear, a drive load is applied to the drive shaft.

[0027] The rotary connection between the drive shaft and the rotary-driven screw drive part is preferably achieved by means of brake shoes, which transfer the drive torque from the drive shaft to the rotary-driven screw drive part. In this embodiment, the power flow is via the brake shoes.

[0028] The drum brake therefore performs a dual function: On the one hand, the brake shoes transmit power between the input and output shaft when the input shaft is rotationally driven and the lifting part on the output side performs a stroke. On the other hand, the brake shoes generate a sufficiently high braking force when an external axial force acting on the lifting part acts on the rotationally driven screw drive part. Under this torque, the brake shoes are pressed against the braking surface of the brake drum. If this restoring torque is greater than the driving torque on the input side, the closing actuating element is activated.

[0029] A practical further development provides a braking readiness of the braking system in which the brake shoes are pressed against the braking surface when the linear drive is unloaded. This means that the brake shoes rest against the braking surface of the brake drum and prevent unwanted adjustment movements of the lifting part. Preferably, the brake shoes are spring-loaded against the braking surface.

[0030] Preferably, two brake shoes are provided, arranged opposite each other around the drive axle. The brake drum preferably surrounds the two brake shoes, so that contact is made with the braking surface of the brake drum when the brake shoes are radially displaced outward. The advantage of the radially outer brake drum is that large diameters are available with a correspondingly large circumferential segment for the braking surface, allowing sufficiently high braking forces to be generated.

[0031] Compression springs can be arranged between the two brake shoes, with their spring force acting in a radial direction. For example, helical compression springs can be used, with one end supported on one brake shoe and the other end on the other. These helical compression springs are under spring preload and press the two brake shoes away from each other radially outward against the braking surface of the brake drum. In this arrangement, the braking surface is located on the inner circumference of the brake drum.

[0032] The linear drive preferably has a housing which accommodates the screw drive, one part of which is designed as a rotationally driven threaded spindle and the other part of which is designed as a threaded nut which is guided in the housing in a rotationally fixed and longitudinally displaceable manner and is connected to a lifting part which can be displaced out of the housing.

[0033] In this preferred arrangement, the brake drum is fixed to the housing. The rotary drive shaft is connected to the threaded spindle via the brake shoes; the drive power is therefore transmitted via the brake shoes.

[0034] In a preferred design, the release actuating element comprises first stops and first counter-stops, which are assigned to the brake shoes and the drive shaft. For example, the drive shaft can carry stop parts in the form of pins, bolts, or claws, which engage in recesses in the brake shoes and abut positively against the walls of these recesses in order to initially introduce the drive torque into the brake shoes. Torques are transmitted through contact between the stop and counter-stop. The recesses can be formed by grooves, holes, or elongated holes. The first stops and the first counter-stops are arranged such that, regardless of the direction of rotation of the drive shaft, the brake shoes are directed radially toward each other and away from the braking surface of the brake drum.

[0035] The contact of the stop parts with the wall has a radial distance to the screw drive axis and therefore forms a lever arm.

[0036] In a preferred development, the closing actuating element is provided with a coupling part which is connected in a rotationally fixed manner to the one rotationally driven screw drive part and which has a lever forming the second stop which engages in a gap formed by the brake shoes, wherein the second counter-stop is formed by walls on the brake shoes which delimit the gap, one of which is formed on one brake shoe and another on the other brake shoe.

[0037] The coupling part can be an integral part of the threaded spindle, meaning it can be manufactured in one piece. For example, if the threaded spindle is made of solid material, one end can be machined to form a polygon that can be used as a lever for actuating the brake shoes.

[0038] In these further developments, the drive shaft transfers the drive power via the stop parts in a form-fitting manner to the brake shoes and via the brake shoes to the coupling part, and via the coupling part to the rotationally driven screw drive part.

[0039] If the rotationally driven screw drive part is formed by the threaded spindle in the preferred design, the coupling part can be provided with a receiving bore into which the threaded spindle engages with one end face. Locking means are provided to connect the threaded spindle to the coupling part in a rotationally fixed manner.

[0040] The clutch part can engage with its lever in a gap formed by the two brake shoes. The lever can be designed as a two-flat element that positively engages with the brake shoes, which, under a rotary drive and the force of the first stops, press against the two-flat element or the polygonal element, thus rotating it. The two brake shoes can be flat or flat on their facing circumferential sides, and with these flat circumferential sides, they define the gap into which the lever engages.

[0041] Under the rotary drive, the lever can be clamped between the two brake shoes. The lever arm is preferably longer than the width of the gap, so the lever cannot rotate freely relative to the brake shoes.

[0042] The coupling part can be provided on the front side with a pin which engages in a bore of the drive shaft in order to allow sufficient pivoting of the coupling part and drive shaft around the drive axis for actuation of the drum brake.

[0043] In other words, the closing actuating element preferably has a second stop and a second counter-stop, which are assigned to the brake shoes and the one threaded drive part. The second stop can be formed by the aforementioned double-flat or polygonal element. The second counter-stop can be formed by walls of the brake shoes involved. If two brake shoes are preferably provided, the walls of the two brake shoes face each other and define a gap into which the second stop engages.

[0044] The second stop and the second counter-stop are therefore functional elements that enable the dual function of the braking device described above. On the one hand, they serve to transmit power when the drive shaft is rotationally driven. On the other hand, they serve to generate a sufficiently high braking force by applying a torque to the second stop of the rotationally driven screw drive part, preferably designed as a double-flat surface, about the drive axis and pressing against the two adjacent walls of the two brake shoes, which are deflected radially away from each other under this torque against the braking surface of the brake drum.

[0045] Preferably, the drive shaft and the one rotationally driven screw drive part are pivotally connected to each other about the screw drive axis via pivot bearings. This can be achieved simply by providing the drive shaft with a bore or hollow cylindrical receptacle arranged coaxially to the drive axis, into which a bearing pin of the rotationally driven screw drive part engages. In this way, a certain degree of pivoting mobility can be established for releasing and applying the drum brake.

[0046] A preferred development provides a linear drive whose stop parts are formed by bolts or claws projecting axially from the drive shaft and fastened to the drive shaft distributed over the circumference, wherein two bolts in each case engage in an elongated hole forming the recess in the two brake shoes, and wherein the elongated holes with the bolts are arranged at a radial distance from a plane in which the drive axis lies, and wherein each bolt on the one hand of this plane is arranged diametrically to a bolt on the other hand of this plane, and wherein under rotary drive a diametrically arranged pair of bolts displaces both brake shoes towards one another against the spring force.

[0047] The second stop, formed by a dihedral, preferably engages between the two brake shoes. When the drive shaft is rotationally driven, a diametrically arranged pair of bolts actuates the two brake shoes in a radial direction towards each other so that the dihedral between the two brake shoes is in sufficiently strong contact with the walls of the two brake shoes to reliably transmit the drive power. When an external axial load is applied backwards, a torque acts on the rotationally driven screw drive part. If this torque is sufficiently large, the dihedral levers the two brake shoes radially apart in the direction of the braking surface of the brake drum. Sufficiently large means that the rotationally driven screw drive part should be rotated backwards under the external axial load. This is prevented by the drum brake.

[0048] In other words, the rotationally driven screw drive part can have a lever on its end face which forms the second stop and engages in a gap formed by the brake shoes, wherein the second counter-stop is formed by walls on the brake shoes which delimit the gap, one of which is formed on one brake shoe and another on the other brake shoe. The lever can preferably be designed as a dihedral, the flat sides of which are in contact with both walls of the two brake shoes for the transmission of torque, wherein the dihedral can be arranged radially within the releasing actuating element, thus preferably arranged radially between the two above-mentioned recesses in the two brake shoes, wherein the bolts or claws of the drive shaft engage in these recesses.

[0049] Linear drives according to the invention are particularly suitable for patient tables, such as those known for computer tomography tables. Such a patient table comprises a frame and a patient bed movably connected to the frame. The housing of the linear drive is movably mounted on the frame, and the lifting part of the linear drive is movably mounted on the bed. The housing and the lifting part can each be provided with a bearing eye. The frame and the bed can be provided with connection points for the bearing eyes. The bearing eyes can be pivotally connected to the frame and the bed, for example, via pins.

[0050] In the case of a CT scanner, the frame may have a carriage on which the table—the section on which the patient lies or sits—is mounted. The table can be moved around several axes to reach a desired position: displacement along a longitudinal axis of the table, as well as along a vertical axis, and along a transverse axis.

[0051] With the linear drive according to the invention, a set position of the table can be reliably maintained, preventing unintentional retraction of the table, for example, due to the force exerted by gravity. After the table or table has reached a desired position, the linear drive can be de-energized without the table sinking due to gravity. The braking device according to the invention reliably prevents such unwanted sinking.

[0052] The invention is explained in more detail below with reference to an exemplary embodiment illustrated in a total of 11 figures. Figure 1 shows a linear drive in longitudinal section,

[0053] Figure 2 shows the linear drive from Figure 1 in a cross-section along the line AA,

[0054] Figure 3 shows the linear drive in two perspective views,

[0055] Figure 4 shows a braking device of the linear drive in two perspective views,

[0056] Figure 5 shows a cross section through the braking device from Figure 4,

[0057] Figure 6 shows a longitudinal section through the braking device of Figure 4 along the line cc,

[0058] Figure 7 shows a drive shaft of the linear drive in perspective view,

[0059] Figure 8 shows a perspective view of the output part of the linear drive,

[0060] Figure 9 shows a brake shoe of the braking device in a view,

[0061] Figure 10 shows the braking device as shown in Figure 4 with the directions of action of drive forces and

[0062] Figure 11 shows the braking device as in Figure 4, with the directions of braking forces indicated. Figures 1 to 3 show a linear drive in longitudinal and cross-sectional views, as well as in perspective views. A screw drive 2 is installed in a housing 1 of the linear drive, which in the exemplary embodiment is formed by a known ball screw drive 3. The screw drive 2 comprises two screw drive parts 4, 5, which are in screw engagement with one another. In the exemplary embodiment, a threaded nut 6 forms one screw drive part 4, and a threaded spindle 7 forms the other screw drive part 5.

[0063] The threaded spindle 7 is rotationally driven and mounted in the housing 1 by means of a double-row ball bearing 8. The threaded nut 6 is arranged in the housing 1 in a rotationally fixed manner and guided for longitudinal displacement. For this purpose, guide grooves 9 are provided in the housing 1, which are arranged axially parallel to a drive axis 10 of the screw drive 2. In the exemplary embodiment, the drive axis is the spindle axis of the threaded spindle 7. Springs 19 firmly attached to the outer circumference of the threaded nut 6 engage in the guide grooves 9.

[0064] An electric motor 13 is mounted on the housing 1 and drives with its motor shaft 14 via a traction drive 15 a drive shaft 16 which is arranged coaxially to the threaded spindle 7.

[0065] Between the drive shaft 16 and the screw drive 2, a braking device 17 is arranged, which is formed by a drum brake 18 (Figures 5 to 11), via which a power transmission between the rotary drive and the lifting part 11 takes place and via which a braking power is transmitted when an external axial load acts on the threaded nut 6, and under this load the threaded spindle 7 is exposed to a sufficiently large retroactive torque.

[0066] An approximately tubular lifting part 11 is firmly connected to the threaded nut 6 at one end. The lifting part 11 penetrates the housing 1 at one end and is provided with a connecting pin 20 at the other end, to which external machine components (not shown) can be connected. The tubular lifting part 11 surrounds the threaded spindle 7 in the housing 1 and moves axially relative to the housing 1 under a rotary drive of the screw drive 2. The threaded spindle 7 is provided with a coupling part 12 (Figure 8) at its end facing the drive shaft 8. The coupling part 12 connects the threaded spindle 7 to the drive shaft 16 via the drum brake 18.

[0067] Figure 4 shows the drum brake 18 in two perspective views from the outside. A brake drum 21, as well as the drive shaft 8 and the clutch part 12, can be seen. In this embodiment, the drive shaft 8 and the clutch part 12 are both functional components of the drum brake 18.

[0068] Figure 5 shows a cross-section of the drum brake 18. The brake drum 21 is lined on its cylindrical inner circumference with a brake lining 22, the inner circumferential surface of which forms a braking surface 23.

[0069] Two brake shoes 24 arranged around the drive axle 10 engage the brake drum 21. Both brake shoes 24 are provided with friction surfaces 25 on their outer circumference for braking contact with the braking surface 23.

[0070] The drum brake 18 has a release actuating element 26 and a closing actuating element 27. The actuating elements 26, 27 actuate the brake shoes 24 in opposite radial directions, depending on whether the rotary drive is activated or whether an external axial load acts on the lifting part 11 (not shown here). The release actuating element 26 releases the drum brake 18 and enables unbraked drive by the electric motor 13, regardless of the direction of rotation. The closing actuating element 27 closes the drum brake 18 and counteracts any undesired axial displacement of the lifting part 11.

[0071] Figure 5 further shows that the drum brake 17 is provided with a pretensioning device 28 which ensures braking readiness when the linear drive is load-free and presses the brake shoes 24 against the braking surface 23.

[0072] The pretensioning device 28 has helical compression springs 29, which are supported under resilient axial preload on both brake shoes 24 and spring these radially outward in the direction of the braking surface 23. For proper guidance of the two brake shoes 24 relative to one another, guide pins 30 are provided, which are arranged axially parallel to the helical compression springs 29 and which are fastened on the one hand to one brake shoe 24 and on the other hand are slidably guided along guide bores 31 of the other brake shoe 24.

[0073] For the explanation of the actuating elements 26, 27, reference is made to the further figures 6 to 11, which are referenced in the following description.

[0074] The releasing actuating element 26 comprises first stops 32 and first counter-stops 33, which are assigned to the two brake shoes 24 and the drive shaft 16.

[0075] Figure 5 shows the first stops 32, which are formed by two stop parts 34, in the exemplary embodiment by a total of four bolts 35, which are arranged axially parallel to one another and are firmly connected to the drive shaft 16.

[0076] Figure 7 shows the drive shaft 16 as a single part, on whose end face facing the drum brake 18 the bolts 35 project axially (one bolt 35 is indicated by dashed lines).

[0077] Both brake shoes 24 are each provided with an elongated hole 37 formed by a recess 36, into which two bolts 35 engage (Figure 5). From Figure 5 it can be seen that a certain amount of rotational play of the drive shaft 16 relative to the brake shoes 24 is possible. It can be clearly seen that the bolt 35 shown on the left in the figure rests in the upper elongated hole 37 against the radially inner wall 38, which forms the first counter-stop 33. The second bolt 35 engaging in this elongated hole 37, however, is spaced from this wall 38. Of the two bolts 35 engaging in the lower elongated hole 37, only the bolt 35 shown on the right rests against the radially inner wall 38. If one imagines the drive shaft 16 rotated clockwise relative to the two brake shoes 24, the other bolt 35 rests on the radially inner wall 38, while the other - previously resting - bolt 35 is without contact with the wall 38.The elongated holes 37 with the bolts 35 are arranged at a radial distance from a plane in which the drive axis 10 lies, and wherein each bolt 35 on the one hand is arranged diametrically opposite a bolt 35 on the other hand of this plane, and wherein a diametrically arranged pair of bolts displaces both brake shoes 24 towards one another counter to the spring force when driven by rotation.

[0078] Figure 6 shows a longitudinal section of the drive shaft 16 and the axially adjacent coupling part 12, which is non-rotatably connected to the threaded spindle 7. The drive shaft 16 and the coupling part 12 are arranged pivotably relative to each other around the drive axis 10. A pivot bearing 41 has a cylindrical pin 42 formed on the end face of the coupling part 12, and a bearing opening 43 formed on the drive shaft 16, into which the pin 42 engages and enables pivoting movements.

[0079] Figure 5 also shows the closing actuating element 27, which has a second stop 39 and a second counter-stop 40. The second stop 39 is formed on the coupling part 12. The second counter-stop 40 is formed on both brake shoes 24.

[0080] Figure 8 shows a perspective view of the coupling part 12 and, for example, in Figure 10, the interaction with the second counter-stop 40. The second stop 39 is formed by a lever 44 in the shape of a dihedral 45 (see also Figure 10). The lever 44 engages in a gap 46 formed by the brake shoes 24, wherein the second counter-stop 40 is formed by walls 47 on the brake shoes 24 that delimit the gap 46, one of which is formed on one brake shoe 24 and another on the other brake shoe 24. The flat sides of the lever 44 are in contact with both walls 47 of the two brake shoes 24 when a torque is introduced to actuate the drum brake 18.

[0081] Figure 8 shows that the coupling part is provided with the above-described pin 42 on one end face and with an axial bore 48 on the other end face for receiving one end of the threaded spindle 7. Securing means (not shown) are provided for securing the coupling part 12 to the threaded spindle 7. Figure 10 clearly shows that the lever 44 is arranged centrally between the two elongated holes 37 of the brake shoes 24.

[0082] Finally, Figure 9 shows one of the two brake shoes 24, which are identical in the illustrated embodiment. The elongated hole 37, the wall 47, and part of the gap 46, into which the lever 44 engages, are clearly visible.

[0083] The operation of the drum brake 18 is explained below.

[0084] When the electric motor 13 actuates the linear drive, the drive shaft 16 rotates and its bolts 35 strike the walls 40 of the brake shoes 24 in the manner described above, pressing them toward each other, i.e., away from the braking surface 23. The two brake shoes 24 press, with their walls 47 (second counter-stop 40) defining the gap 46, against the flat sides of the lever 44 and transmit the drive power of the drive shaft 16 to the coupling part 12, which is firmly connected to the threaded spindle 7. This situation is illustrated in Figure 9, with the arrows indicating the direction of rotation of the drive shaft 16.

[0085] If the rotary drive - i.e. the electric motor 13 - is switched off, for example, and an external axial load acts on the lifting part 11, a retroactive torque is exerted on the threaded spindle 7. This situation is depicted in Figure 11. It can clearly be seen that the lever 44 of the coupling part 12, which is connected in a rotationally fixed manner to the threaded spindle 7, presses against the walls 47 of the two brake shoes 24 at diagonal points and levers them away from each other, i.e. radially away from each other, in the direction of the braking surface 23 of the brake drum 21, which is firmly connected to the housing 1. The arrows show the lines of action of the forces. In braking contact, twisting of the threaded spindle 7 and thus unintentional displacement of the lifting part 11 is impossible.

[0086] The drum brake 18 therefore takes over the power transmission between the drive shaft 16 and the screw drive 2 with the brake shoes 24 and the bolts 35 of the releasing actuating element 26 as well as with the coupling part 12 and its lever 44 of the closing actuating element 27. The drum brake 18 also takes over the braking power with the coupling part 12 of the closing actuating element 27 as well as with the brake shoes 24 and with the brake drum 21, which ensures a reliable locking of the screw drive 2 when the lifting part 11 is subjected to an external axial force.

[0087] List of reference symbols

[0088] Housing

[0089] screw drive

[0090] Ball screw drive

[0091] Screw drive part

[0092] Screw drive part

[0093] threaded nut

[0094] Threaded spindle double row ball bearing

[0095] Guide grooves

[0096] Drive axle

[0097] Lifting part

[0098] Coupling part

[0099] electric motor

[0100] Motor shaft

[0101] traction drive

[0102] drive shaft

[0103] braking device

[0104] drum brake

[0105] Feather

[0106] connecting pin

[0107] brake drum

[0108] brake pad

[0109] Braking surface

[0110] brake shoe

[0111] Friction surface releasing actuating element closing actuating element

[0112] Pretensioning device

[0113] Helical compression spring

[0114] guide pin

[0115] Guide hole first stop first counter stop

[0116] Stop part

[0117] bolt

[0118] recess

[0119] slot

[0120] Wall second stop second counter stop

[0121] swivel bearing

[0122] cones

[0123] Warehouse opening

[0124] lever

[0125] Two-flat

[0126] gap

[0127] wall

[0128] axial bore

Claims

Patent claims 1. Linear drive, with a screw drive (2), the screw drive parts (4, 5) of which are in screw engagement and convert a rotation of one screw drive part (5) about a drive axis (10) into a relative displacement of the screw drive parts (4, 5) along the drive axis (10), wherein one screw drive part (5) is rotationally driven via a drive shaft (16), and wherein the other screw drive part (4) is arranged so as to be longitudinally displaceable along the drive axis (10) and has a lifting part (11) for performing a stroke, and with a braking device (17) which is released under rotational drive of the one rotationally driven screw drive part (5) and which is closed under external load of the lifting part (11) and counteracts displacements of the lifting part (11), characterized in that the braking device (17) is designed as a drum brake (18) with a plurality of brake shoes (24) and a brake drum (21) with a braking surface (23) for the brake shoes (24),and a releasing actuating element (26) and a closing actuating element (27), wherein the actuating elements (26, 27) actuate the brake shoes (24) in opposite directions., 2. Linear drive according to claim 1, whose drive shaft (16) actuates the releasing actuating element (26) in both directions of rotation, and whose rotationally driven screw drive part (5) actuates the closing actuating element (27) in both directions of rotation under external load of the lifting part (11).

3. Linear drive according to claim 1 or claim 2, the drive load of which is transmitted from the drive shaft (16) via the brake shoes (24) to the rotationally driven screw drive part (5), and the external load of the lifting part (11) is transmitted via the brake shoes (24) to the brake drum (21).

4. Linear drive according to one of claims 1 to 3, whose drum brake (18) is in braking readiness and presses the brake shoes (24) against the braking surface (23).

5. Linear drive according to one of claims 1 to 4, whose releasing actuating element (26) has first stops (32) and first counter-stops (33) which are assigned to the brake shoes (24) and the drive shaft (16).

6. Linear drive according to one of claims 1 to 5, whose closing actuating element (27) has a second stop (39) and a second Counter-stop (40) which are assigned to the brake shoes (24) and to the one screw drive part (5).

7. Linear drive according to claim 5, whose drive shaft (16) and the one screw drive part (5) are pivotally mounted with one another about the screw drive axis (20) via a pivot bearing (41), wherein the first stops (32) are designed as stop parts (34) firmly connected to the drive shaft (16), and wherein the first counter stops (33) are formed by walls (38) of recesses (36) in the brake shoe (24), in which the stop parts (34) engage.

8. Linear drive according to claim 7, whose stop parts (34) are formed by bolts (35) or claws projecting axially from the drive shaft (16) and which are fastened to the drive shaft (16) distributed over the circumference, wherein two bolts (35) in each case engage in the recess (36) of the two brake shoes (24), and wherein the recesses (36) with the bolts (35) are arranged at a radial distance from a plane in which the drive axis (20) lies, and wherein a bolt pair is formed from two bolts (35), of which one bolt (35) is arranged on the one hand on this plane, and of which the other bolt (35) is arranged on the other hand on this plane diametrically to the one bolt (35).

9. Linear drive according to claim 6, whose closing actuating element (27) has a coupling part (12) which is connected in a rotationally fixed manner to the one rotationally driven screw drive part (5), which coupling part has a lever (44) forming the second stop (39) which engages in a gap (46) formed by the brake shoes (24), wherein the second counter-stop (40) is formed by walls (47) on the brake shoes (24) which delimit the gap (46), one of which walls is formed on one brake shoe (24) and another on the other brake shoe (24).

10. Linear drive according to one of claims 1 to 9, the housing (1) of which accommodates the screw drive (2), one of the screw drive parts (5) of which is designed as a rotationally driven threaded spindle (7), and the other screw drive part (4) of which is designed as a threaded nut (6) which is guided in the housing (1) in a rotationally fixed and longitudinally displaceable manner and is connected to the lifting part (11) which can be displaced out of the housing (1), wherein a brake drum (21) of the drum brake (18) is arranged fixed to the housing, and wherein an electric motor (13) drives the drive shaft (16) via a traction drive (15).

11. A patient table with a linear drive according to claim 10, comprising a frame and a patient bed movably connected to the frame, wherein the housing (1) of the linear drive is movably mounted on the frame and its lifting part (11) is movably mounted on the bed.

Images