LINEAR DRIVE WITH TWO REDUCTION STAGES

Description

[0001] The invention relates to a linear drive, preferably a motor-driven, more preferably an electric-driven linear drive with two reduction stages.

[0002] The linear actuators mentioned above are primarily used to convert the mostly rotary motion of a motor, usually an electric motor, into the linear motion of an actuator. They comprise a gear between the motor and the actuator, which usually – unless it is a manual transmission – only has one reduction ratio. The speed of the actuator can be adjusted via the motor speed, with the maximum actuating force being proportional to the applied torque of the motor at an unchanged gear ratio or reduction.

[0003] However, with manipulators, especially those designed to simulate humanoid or animal-like movements, the challenge of increasing the movement speed at the expense of the actuating force and vice versa—i.e., changing the gear ratio or reduction—repeatedly arises. This would enable good control of the force and movement sequences, particularly with manually controlled manipulators. Depending on the task to be performed, human muscles allow for high actuating forces at low movement speeds and low actuating forces at high movement speeds. Furthermore, the required maximum power and thus the size of a motor with an adjustable gear reduction would be significantly lower than for an electric motor-driven linear drive with a constant reduction.

[0004] While manually or automatically shiftable transmissions offer the advantage of being able to change the gear ratio, they are usually large, heavy, expensive, and complex. This represents a limiting factor, particularly when designing humanoid or animal-like robots with a large number of actuators and limited installation space.

[0005] In DE 94 13 740 U1 and EP 1 877 677 B1, the functional principle of a harmonic drive is applied to linear gears to achieve very high gear ratios. The radial sliding of inner teeth into the outer teeth is similar to a nut sliding radially into the spindle. This allows for high gear ratios to be achieved, saving space, but does not change the gear ratio.

[0006] In EP 0 529 521 B1, on the other hand, a two-stage rotation-translation gear is described in order to achieve very high reduction ratios without backlash and with low friction.

[0007] Furthermore, DD 211 768 A5 and EP 0 776 285 B1 disclose further solutions for a multi-stage reduction with several nested spindles.

[0008] DD 211 768 A5 describes a linear drive with two motors, one of which drives a nut and the other a threaded spindle. The motors are mounted at the outer end of two telescopically movable tubular housing sections secured against rotation about their axes, and the nut and threaded spindle are located inside these housing sections. The reduction ratio can be switched via the respective operation of the motors.

[0009] EP 0 776 285 B1 describes a multi-stage spindle drive for converting a rotary motion into a linear motion with different retraction and extension speeds. It consists of at least two telescopically intermeshing and screw-connected threaded spindles with different pitches. One threaded spindle is connected to a drive element, and another threaded spindle interacts with an output element via a nut. The threaded spindles and the retraction and extension speeds can be switched by friction. The reduction ratio can be switched via different friction ratios.

[0010] Furthermore, DE 01 057 411 B discloses a friction gear for converting a rotary motion into a feed motion with cylindrical bodies that are frictionally engaged and positioned at a variable angle to each other, in this example, a ring body around a shaft. The gear reduction is continuously adjustable via the angle between the ring body and the shaft.

[0011] DE 10 2013 015 257 B3 describes a rotation-translation gear with selectable reduction. It operates according to the differential spindle principle, allowing high reduction ratios. The reduction ratio can be changed by switching between normal spindle operation and differential spindle operation. When a certain force is reached, an additional gear stage is engaged automatically or manually. The force acts on a clampable system whose force-displacement characteristic curve shows a significant increase in force over the displacement.

[0012] CH 647306 A5 discloses a linear drive with two motors, one driving the nut and the other the spindle. By selecting the motor speeds, the feed rate can be varied over a wide range.

[0013] DE 10 2007 059 457 A1 further describes a load-sensitive power transmission device with a rotating threaded spindle and a rotating nut arranged eccentrically around it. The axial displacement is achieved solely by the nut on the axially fixed, but rotating threaded spindle. The gear reduction is adjusted by the rotational speed of the nut, with the rotational movement of the nut being damped by a viscous or frictional embedding in a slide mount, thus establishing a load-dependent rotational speed of the nut and thus an individual reduction ratio.

[0014] As an alternative to electric motor drives, hydraulic drives are available, which can be used to drive an actuator in a space-saving manner, but require a hydraulic supply along with control valves and hydraulic connecting means to the actuator.

[0015] Examples of applications for linear drives of the type mentioned above can be found primarily in robotics, such as the humanoid robotics mentioned above, but also in everyday applications such as electric drives for windows, gates or roller shutters, where on the one hand low forces must be realized with high actuating speeds, but depending on the situation temporarily high forces must also be overcome, such as when starting up.

[0016] Based on this, there is a Object of the inventionto propose an electromotive linear drive with two reduction stages (or transmission stages) which does not have the aforementioned limitations, is characterized by a small size and is characterized by a load-dependent switching of the reduction.

[0017] The problem is solved by a linear drive having the features of claim 1. Subclaims referring back to this describe advantageous embodiments.

[0018] To achieve the object, a linear drive, preferably a motorized linear drive, more preferably an electric motorized linear drive with two reduction stages is proposed, comprising at least the following components: a) a drive spindle with an external thread; b) a housing, arranged concentrically around the drive spindle; c) a hollow shaft which is rotatably mounted concentrically around the drive spindle in the housing and can be driven by a first drive source, preferably a first motor; d) a drive element with an internal thread which is rotatably mounted in the hollow shaft with an eccentric offset to the drive spindle and can be driven by a second drive source, preferably a second motor; e) plain bearing elements for axially guiding the drive spindle, preferably in the housing, more preferably rotatably mounted as plain bearing shells in the hollow shaft.

[0019] The threads of the drive spindle (external thread) and the drive element (internal thread) have the same thread module; they preferably mesh over several tooth flanks, are thus positively coupled in the axial direction and are therefore suitable for transmitting an axial force and an axial displacement to the drive spindle. The concept provides that this axial force and displacement are transmitted only through these intermeshing threads. However, the internal thread diameter of the drive element is larger than the external thread diameter of the drive spindle, with the eccentric offset being set such that the internal thread of the drive element only engages positively on one side, i.e. only over a limited angular range (preferably less than 90°, more preferably less than 45°, depending on the ratio of the external to internal thread of the thread cross section) with the external thread of the drive spindle, i.e.only in this area an axial force can be transmitted.

[0020] Depending on the module, eccentricity, ratio of the external to internal thread of the thread cross-section and tooth shape, basically all angular ranges from a few degrees up to (almost) 360° can be realized. Even the manufacturing tolerances of a standard thread (e.g. M8) are generally large enough to realize the stated angular ranges for the proposed linear drive. Smaller angular ranges generally also result in lower friction between the tooth flanks, since the internal thread of the drive element rolls more over the external thread of the drive spindle. Larger overlaps generally have the advantage of a larger tooth flank area available for force transmission and therefore a greater load-bearing capacity. When the hollow shaft is driven by the first drive source, the friction losses are approximately proportional to 2 x tooth height x friction coefficient x axial force per revolution.When the drive element is driven directly via the second drive source with a transmission, the friction losses proportional to the circumference x coefficient of friction x axial force per revolution are significantly higher, but the transmittable axial forces are also lower.

[0021] As the hollow shaft rotates relative to the drive spindle, the orientation of the eccentricity of the drive element in the drive spindle in the housing also changes.

[0022] Accordingly, a linear drive with two drive sources, preferably two motors, more preferably electric motors, with two reduction ratios is proposed. With these two drive sources, a rotary motion is converted into a linear motion axially relative to the drive spindle in two operating mechanisms, which can generally be used individually or in combination. The two operating mechanisms differ particularly in the reduction ratio, with each of the two operating mechanisms preferably being powered by only one of the two drive sources.

[0023] The term "reduction" or "transmission" refers to the degree of conversion of the movement of another physical quantity, for example, a number of revolutions, into a similar movement with a different value. With a reduction or transmission of a number of revolutions, the number of revolutions of the driving shaft is reduced or increased relative to that of the driven shaft. The reduction or transmission ratio causes the torque to increase or decrease accordingly within the same ratio (minus any transmission losses).

[0024] In the present linear drive, a rotary motion of a drive source is converted into a linear feed motion, i.e., a longitudinal motion of the drive spindle. In this case, the conversion does not occur within the framework of similar physical quantities, which is why, in this case, i.e., the conversion of rotary motions by the first or second drive source into a translatory linear motion of the drive spindle, is referred to as a first or second pitch. However, in this case, a pitch can be mathematically divided into two sub-steps, namely a conversion of a physical quantity (in this case from a rotary to a translatory movement with, conceptually, a single gear pitch for both drive sources) and a reduction or transmission ratio between two similar physical quantities, each with a reduction or transmission ratio for the two drive sources as well as their transmission-side conversion ratios. The reduction or transmission ratio is defined as the quotient i of the speed between the gearbox input and the gearbox output. If i > 1, the speed is reduced (reduction), but the transmitted torque is increased. If the transmission ratio is i < 1, the speed is increased and the transmitted torque is reduced.

[0025] The terms reduction and transmission as well as reduction and transmission ratio are used in the context of the description of the invention in the above-mentioned sense and represent the different conversion ratios, ie the two different gear heights or gear height ratios for the two drive sources.

[0026] When the hollow shaft is driven by the first motor, a gear similar to a cycloidal gear is connected upstream, so that the effective pitch or reduction in the aforementioned sense of the gear is determined by the thread pitch multiplied by the gear ratio of the cycloidal gear (first operating mechanism). The cycloidal gear results from the rolling of the internal thread of the drive element, which does not rotate with the hollow shaft, on the external thread of the (non-rotating) drive spindle. A reduction or transmission ratio is calculated from the effective thread diameters of the drive element D and the drive spindle d: i = D / (Dd).

[0027] The second drive source rotates the drive element with internal thread in the hollow shaft with an eccentric offset to the drive spindle (second operating mechanism); this causes the gear to behave like a normal spindle drive, whereby the reduction ratio, in the assumed case of a hollow shaft fixed in the housing, is determined only by the thread pitch of the internal thread of the drive element.

[0028] In the aforementioned second mechanism, the pitch or reduction ratio in the aforementioned sense is determined by the pitch of the internal thread of the drive element. There is no rolling of the internal thread of the drive element on the external thread of the drive spindle, but rather a sliding of the tooth flanks of the internal thread of the drive element on the external thread of the drive spindle.

[0029] In the context of the description, the term "reduction" in connection with the conversion of a rotary movement by a drive source to a translatory movement of the drive spindle includes both a reduction and a transmission.

[0030] The second mechanism of action therefore serves to implement a fast gear, while the first mechanism of action serves to implement a slow reduction gear with a higher load capacity.

[0031] The concept further provides for the second drive source (i.e., the one driving the lower gear ratio) to have overload protection with a power limit or shutdown. If a positioning movement encounters mechanical resistance in the low-reduction overdrive gear, this drive is shut down, and the positioning movement is performed with a higher reduction using the first drive source, which preferably runs in parallel.

[0032] In a drive with stepper motors, these can preferably be driven at two speeds, the ratio of which corresponds to the gear ratio i. Overload protection of the second motor is not necessary in this case, but a speed limit for the first motor is desirable in many applications to avoid step losses.

[0033] The aforementioned drive sources preferably comprise motors with or without gears (geared motors) or stepper motors, each comprising a rotating drive shaft. Electrically driven motors are preferably used.

[0034] An optional embodiment provides a linear drive in which the first and second drive sources comprise a first and a second drive means, respectively, wherein both drive means can be driven by a common motor drive with only one motor and transmission means. According to the invention, a clutch (switching clutch or slip clutch) is proposed as overload protection for the second drive source, which, in the event of an overload, preferably above a torque threshold, disconnects at least the second drive means from the motor or transmission means and preferably transfers them to a freewheeling mode. Alternatively, the overload protection comprises an actively switchable clutch, at least for the aforementioned disconnection of at least the second drive means from the motor or transmission means.The actively switchable clutch is preferably controlled electromagnetically or electromechanically by means of external control signals, which are preferably received outside the linear drive as force or displacement signals.

[0035] According to the invention, the drive spindle is designed with an anti-rotation device relative to the housing, whereby the drive spindle is mounted for only axial movement within the housing. This anti-rotation device is preferably implemented by corresponding axially oriented guide fits on the drive spindle, preferably outside the threaded area, or alternatively via a torque support on the drive spindle relative to the housing. An anti-rotation device can also be implemented without separate components or guide fits, particularly if the drive receiver connected to the drive spindle is itself non-rotatable and has a connection to the housing (indirect anti-rotation device).

[0036] A further design of the linear drive provides for the drive element to comprise a nut or several nuts with an eccentric threaded bore that are firmly coupled in series. In particular, several firmly coupled nuts eliminate the need for separate guides for the drive spindle in plain bearings; the friction in these plain bearings is eliminated.

[0037] A further design of the linear drive provides for the plain bearing elements for axial guidance of the drive spindle to comprise two concentrically rotatably mounted plain bearing shells in the hollow shaft. The plain bearing elements are thus arranged outside the drive sources and as close as possible to the engagement of the internal thread of the drive element with the external thread of the drive spindle, which advantageously increases the axial guide rigidity and thus benefits the positioning accuracy of the actuating movement.

[0038] The tooth form of the external thread of the drive spindle and the internal thread of the drive element are coordinated. The tooth form requirements can be transferred to a two-dimensional model. The two-dimensional curves result from a section through the stationary spindle in the first operating mechanism. A stationary curve represents the tooth form of the spindle, while a curve moving in the axial and radial directions represents the tooth form of the drive element, including the rolling movement around the spindle. The threads must be designed so that the two curves touch but must not penetrate or overlap. Optimization is achieved by maximizing the contact times and length of the contacting curve sections, which results in low surface pressure (taking Hertzian pressure into account, a contact point also becomes a surface).The longer the contact duration in two-dimensional cases and the closer the two curves are to each other, the larger the contact area in three dimensions and thus the lower the surface pressure. However, a larger contact area also results in higher friction, since the conditions for rolling motion can only be met at one point. Depending on the application, a choice can be made between low friction and high load-bearing capacity.

[0039] For cost-effective production, the spindle is preferably realized with a standard thread, e.g., trapezoidal thread. With suitable drive elements / nuts (e.g., three-dimensionally printed), a large portion of the movement (two-dimensional section) occurs during which the curves touch. This corresponds to a large angular range in section with a plane whose normal is axially aligned with the spindle. There is no overlap during the movement.

[0040] Alternatively, the drive element / nuts can be designed with a standard thread and the spindle with an adapted thread.

[0041] The invention is explained in more detail using exemplary embodiments, the following figures, and descriptions. All features and their combinations presented are not limited to these exemplary embodiments and their configurations.

[0042] Rather, these should be considered as representative of other possible, but not explicitly presented as exemplary embodiments, further configurations that can be combined. Fig.1a and b two sectional views of a first embodiment with two axial motors, a drive element with internal thread and two plain bearing elements, Fig.2a and b two sectional views of a second embodiment with two axial motors and three drive elements with internal thread, Fig.3a and btwo sectional views of a third embodiment with only one drive source and Fig.4a and b two sectional views of a fourth embodiment in which both motors are arranged away from the drive spindle and outside the housing and are coupled to the hollow shaft or the drive element via gear drives.

[0043] The Fig.1 and 2 The embodiments of linear drives with two reduction stages shown in (a and b) each comprise a housing 1 with a coaxially inserted and axially movable drive spindle 4 with external thread, a hollow shaft mounted in a housing around the drive spindle with ball bearings 2 with two bearings eccentrically inserted into the hollow shaft 3 for a drive element 7. Furthermore, a first and a second motor are provided as drive sources, each with a stator 9 or 11 and rotor8 or 10, around the drive spindle 4 The drive spindle is located in the rotors in all three designs 8 and 10 mounted axially displaceably in the housing by means of plain bearings. The stators 9 and 11 In the example, each is attached to a cover plate with screws 12 of the housing.

[0044] The first engine (in the longitudinal sections AA in Fig.1a and 2a right) drives with its rotor 8 the hollow shaft 2. The second motor (in the longitudinal sections AA in Fig.1a , 2a and 3a left) drives with its rotor 10 one concentric on the drive spindle 4 sliding bearing clutch 5 which in turn transmits the rotating movement via driver elements 13 each eccentric to the drive spindle 4in the hollow shaft 2 with warehouse 3 mounted drive element 7 forwards.

[0045] In the example, the power transmission takes place via the screw heads of preferably at least two screws screwed axially into the rotor, whereby the screw heads 14 positively (optionally also with additional elastic intermediate elements for a backlash-free fit) in corresponding frontal recesses of the hollow shaft 2 or the clutch 5 engage positively.

[0046] A connection between two joining partners is called a positive fit if the joining partners are connected by an interlocking surface structure and are preferably fixed to each other. In the context of this application, the term is also used for a positively acting coupling, whereby the fixation can also be achieved via a clearance fit, as in the present case, in the sense of a driver coupling, including the aforementioned driver element.

[0047] Close to the drive element on both sides 7 on the drive spindle 4 acting plain bearings in the clutch 5 (Coupling is here as a plain bearing element 6) and a (further) plain bearing element 6 serve to provide additional guidance for the drive spindle in the hollow shaft, which advantageously makes the aforementioned power transmission means between the drive sources and the drive spindle more rigid.

[0048] The drive element 7 is through the bearings 3 Eccentrically mounted outside a central axis defined by the drive spindle. As a result, the drive element rolls with its internal thread during one rotation of the hollow shaft. 2 on the drive spindle 4 and rotates by a fraction of a total revolution that corresponds to the difference between the diameters of the shaft and drive spindle in relation to the drive spindle diameter.

[0049] Fig.1a and b shows an embodiment with two hollow-shaft motors arranged around the drive spindle. The screw heads 14 form a coupling with corresponding recesses in the plain bearing element without an axial offset, in which the rotor 10 transmits its rotating movement in a form-fitting manner. The cylindrical driver elements 13With correspondingly larger recesses in the plain bearing element, they form a clutch with compensation for axial misalignment. This mechanism of a clutch with axial misalignment is preferably backlash-free. The rotor 8 also transmits its rotating movement via screw heads 14 into the corresponding recess in the hollow shaft 2. Fig.1a and b thus represent an embodiment with two axial motors, a drive element with internal thread and two plain bearing elements and thus a structurally simple embodiment of the invention.

[0050] Fig.2a and b represents an embodiment with two motors with hollow shaft arranged around the drive spindle, in which the plain bearing elements 6 Instead of plain bearing surfaces, they are provided with an internal thread matching the drive spindle. A coupling mechanism with the aforementioned driver elements 13To compensate for the axis offset, both plain bearing elements are then coupled with the drive element arranged between them 7 and with the aforementioned screw heads 14 with the second motor. The screw heads 14 form with corresponding recesses in the plain bearing element 6 the clutch 5 in contrast to the Fig.1a and b illustrated embodiment with an axial offset. The cylindrical driver elements 13 in the drive element 7 form with correspondingly larger recesses in the plain bearing element 6also a (further) coupling with compensation for axial offset. A coupling is also used between the drive element and the second plain bearing element. This mechanism of a coupling with axial offset is preferably free of play. For this purpose, the hollow shaft 2 preferably has four bores for the bearings offset over the circumference of the plain bearing elements, so that the drive element and the two plain bearing elements, which in this embodiment perform the same function as the drive element, are located on opposite sides of the drive spindle. 4 intervention. Fig.2a and bThis represents a design with two axial motors and three drive elements (including the two plain bearing elements) with internal threads. The two plain bearing elements are replaced by elements with internal threads whose rotation is coupled. This design is somewhat more complex in terms of construction, but the plain bearing elements can be omitted, and both inertial and drive forces act symmetrically to the drive spindle, which can prevent vibrations at high speeds. One advantage is the distribution of the axial force transmission to additional elements with internal threads, which, however, requires a more complex coupling mechanism.

[0051] Fig.3a and b shows a design with only one motor around the drive spindle as the drive source and a coupling element for load-dependent switching between the two operating modes. The rotor 8transmits its rotating movement via screw heads 14 into the corresponding recess in the hollow shaft 2. Instead of the second engine, switchable clutches are 15 and 16 ( Fig.3b ) provided, which the rotations of the hollow shaft 2 and the drive element 7 and switch between the two operating modes, i.e. between different reduction ratios, solely via the switching state. The coupling is implemented here with magnets, which act as drivers between the drive element until the desired torque is reached. 7 with internal thread and hollow shaft 2 The magnets are arranged in a ring 19 around the hollow shaft 2 They are integrated and can be moved via an axial movement of this ring. This changes the distance between the magnets and thus the torque at which the clutch engages. Fig.3a and bIn a schematic representation, they represent an embodiment with only one drive source and a coupling element for load-dependent switching between the two operating modes.

[0052] Preferably, the aforementioned couplings are 15 or 16 They are designed as automatic clutches that switch at a specified torque. If this clutch is a directional freewheel, for example, you get a mechanism with fast movement in one direction and powerful movement in the other.

[0053] Fig.4a and b represents again an embodiment with two motors, whereby the first and / or second motor is mounted away from the drive spindle 4 and preferably outside the housing 1 The rotors 8 and 10 transmit their rotational movement via gear drives 17 or 18(alternatively belt drives) on the hollow shaft 2 or the clutch 5. This design is particularly suitable for motors without a hollow shaft. By separating the motor and housing, the drive spindle can be mounted directly in the housing, preferably via plain bearings, as shown.

[0054] In machines with high demands on dynamics and installation space (e.g. humanoid robots), the motors of the Fig.4a and b illustrated embodiment preferably arranged decoupled from the linear drive mechanism, ie for the realization of a particularly light and compact linear drive, laid outside the linear drive mechanism, wherein the power transmission can optionally be realized via flexible shafts instead of the illustrated gear transmissions.

[0055] The drive spindle 4must be secured against rotation during operation. The gearbox has two operating modes with two reduction ratios and both automatic and targeted switching between operating modes: Operating mode 1: If only the second motor (in the longitudinal sections AA in Fig.1a , 2a and 4a , left) is switched on, the gearbox behaves like a normal spindle drive, ie the drive element 7 rotates in the fixed hollow shaft 2, wherein the internal thread area of ​​the drive element, which is in engagement, is connected to the external thread of the drive spindle 4 The transmission ratio is determined solely by the drive spindle pitch. Operating mode 2: If only the first motor (in the longitudinal sections AA in Fig.1a , 2a and 4a , right) is switched on, the hollow shaft rotates2 around the drive spindle, while the drive element 7 and the second motor rotate in the ratio of the cycloidal transmission. The axis of rotation of the drive element rotates with the hollow shaft around the drive spindle. The drive element rolls 7 with its internal thread like in a cycloidal gear with a cycloidal transmission via the external thread of the drive spindle 4. The drive spindle ratio is multiplied by the cycloidal gear ratio. This ratio is i = d / (Dd), where d is the effective diameter of the threaded rod and D is the effective diameter of the drive element.

[0056] An operating mode can be set manually or controlled by switching one of the motors on and off.

[0057] If both motors are switched on, a load-dependent change occurs between the two operating modes and thus reductions. If the speed of the second motor decreases due to a higher required torque at a higher required actuating force, the speed of the first motor, reduced by the gear ratio in the cycloidal gear, exceeds the speed of the second motor at a certain point, resulting in a change from operating mode 1 to operating mode 2. The torque then converted with the cycloidal gear ratio is transferred to the drive element. 7and converted into an actuating force via the thread pitch. The operating mechanism changes from the drive element sliding on the drive spindle with high relative friction (but low absolute friction due to low axial force, operating mode 1) to the drive element rolling on the drive spindle with low relative friction (operating mode 2). If both motors remain switched on, the switching point therefore depends on the external force acting on the drive spindle – at high force, the gear stage with the high ratio is active, and at low force, the gear stage with the low ratio is active.

[0058] Controlling both motors therefore results in the following: In operating mode 1: At low force / torque, the low-reduction mechanism tends to overtake the high-reduction mechanism due to sliding movement in the spindle. The hollow shaft drive only reduces friction by reducing the relative speed through the rolling component. In operating mode 2: Both drives are connected in parallel, which adds the drive torques, but the feed rates generated are necessarily the same. The drive element converts this added torque into force. A self-locking mechanism prevents the drive spindle from sliding in the drive element.

[0059] The maximum actuating speed of the drive spindle 4is always proportional to the speed of the second motor (operating mode 1, lower reduction ratio). If different control parameters are to be used for different switching states, the switching state can be determined from the ratio of the speed of the first motor to the second motor.

[0060] Switching between the operating modes is advantageously not abrupt but smooth, i.e. in a transition area between rolling and sliding of the internal thread of the drive element on the external thread of the drive spindle.

[0061] In practice, it is proposed to control both motors together, whereby the desired speeds of each motor can be set and controlled separately or jointly using individual voltage or current control (e.g. with a PID controller). When switching between the operating modes, however, the controlled system and thus the control parameters change. It is proposed to have two different sets of parameters for the two operating modes, which are then used, for example, depending on the speed or the load of the two motors in relation to one another. If the motors rotate in the ratio of the cycloidal transmission, operating mode 2 is set. Conversely, operating mode 1 is set when the second motor rotates faster than the first motor. If the motors are controlled identically, the operating state can be detected based on the speed ratio.

[0062] With stepper motors, it is preferable to specify the speeds of both motors in the ratio of the cycloidal ratio. Once the faster motor has reached its maximum speed, only the slower motor can be increased in speed, which is in operating mode 1 (and the force is limited).

[0063] The selection of the drive spindle pitch, friction and required motor torque is made under the condition that the counteracting force on the actuating force of the drive spindle is not so great that the gear self-locking between the drive spindle and motors, in particular to the second motor connected with a lower reduction ratio, is overcome.

[0064] The aforementioned combination of a cycloidal gear with a drive spindle and two motors thus allows switching between two gear stages with both automatic and controlled switching options. The reduction ratio in the second operating mode is higher than in the first operating mode.

[0065] The linear drive is advantageously characterized by its simple design with a high proportion of standard components (bearings, drive spindle, motors, etc.). The remaining components, such as the drive element, the hollow shaft, and plain bearings, can be manufactured from friction-optimized plastic using simple means, such as 3D printing, since the accuracy requirements are relatively low. Due to the extensive surfaces available for power transmission, high forces comparable to spindle drives with nuts made of tribological plastics are still possible. This is very convenient for the production of customized, inexpensive, and lightweight multi-speed linear drives.

[0066] In all embodiments shown, the parts to be manufactured individually, in particular the housing, the hollow shaft, plain bearing elements and / or the drive element, can be produced in the 3D printer and are predominantly (except for the housing in Fig.4a , b) Can also be manufactured without support material using FDM 3D printing. This makes this design particularly suitable for cost-effective and adaptable production in various sizes and gear ratios. List of reference symbols:

[0067] 1 Housing 2 Hollow shaft 3 Eccentric bearing 4 Drive spindle 5 Coupling 6 Plain bearing / plain bearing element 7 Drive element 8 Rotor of the first motor as the first drive source 9 Stator of the first motor as the first drive source 10 Rotor of the second motor as the second drive source 11 Stator of the second motor as the second drive source 12 End plate of the housing 13 Driver element 14 Screw head 15 Coupling between hollow shaft and drive element 16 Coupling between hollow shaft and plain bearing element 17 Gear drive between rotor and hollow shaft 18 Gear drive between rotor and plain bearing element 19 Ring with magnets

Claims

1. Linear drive having two reduction stages, comprising: a) a drive spindle (4) for an axial positioning movement having an external screw thread, b) a housing (1) which is concentric around the drive spindle, c) a hollow shaft (2) which is rotatably mounted in the housing such that the hollow shaft is concentric around the drive spindle and which can be driven by means of a first drive source (8,9), d) a drive element (7) which is rotatably mounted in the hollow shaft such that the drive element has an eccentric offset to the drive spindle and which can be driven by means of a second drive source (10, 11), the drive element having an internal screw thread engaging in the external screw thread, and e) plain bearing elements (6) for axially guiding the drive spindle in the housing, the drive spindle being secured against rotation relative to the housing, wherein f) the screw threads of the drive spindle and of the drive element have the same gear module, g) the screw-thread inside diameter of the drive element is greater than the screw-thread outside diameter of the drive spindle, h) the eccentric offset is set such that the internal screw thread of the drive element engages in the external screw thread of the drive spindle only on one side, and i) the second drive source has overload protection with a switching or slipping clutch for power limitation or switch-off, a first reduction stage being applied without and a second reduction stage with power limitation or switch-off.

2. Linear drive according to claim 1, characterised in that the first and second drive sources (8, 9, 10, 11) each comprise a first and second motor respectively.

3. Linear drive according to claim 2, characterised in that the first or second motor comprises an electric motor, geared motor or stepper motor.

4. Linear drive according to claim 2 or 3, characterised in that the first and second motors are each arranged concentrically around the drive spindle (4).

5. Linear drive according to claim 4, characterised in that each motor has a stator (9, 11) fixedly mounted in the housing (1) and a rotor (8, 10) arranged around the drive spindle (4).

6. Linear drive according to claim 5, characterised in that plain bearing elements for axially guiding the drive spindle (4) are arranged concentrically in the rotors (8, 10) in a respective bore or are formed by the inner walls of the bores.

7. Linear drive according to claim 2 or 3, characterised in that the first and / or second motors are arranged away from the drive spindle (4).

8. Linear drive according to claim 7, characterised in that the first and / or second motors are arranged outside the housing (1).

9. Linear drive according to claim 7 or 8, characterised in that the two motors are coupled to the hollow shaft (2) or the drive element (7) via gear or belt drives (17, 18) and / or by means of flexible shafts.

10. Linear drive according to any of claims 2 to 9, characterised in that the rotors (8, 10) of the first and second motors are each coupled to the hollow shaft (2) or to the drive element (7) by transmission means (5, 13, 14) so as to be rotatable about the drive spindle (4).

11. Linear drive according to claim 1, characterised in that the first and the second drive sources comprise a first and a second drive means respectively, both drive means being driveable by a common motor drive with only one motor and transmission means, the overload protection of the second drive source comprising a clutch.

12. Linear drive according to claim 11, characterised in that the clutch has a freewheel or an actively engageable clutch.

13. Linear drive according to any of the preceding claims, characterised in that the drive spindle (4) is secured against rotation relative to the housing (1) and can only be moved axially within the housing.

14. Linear drive according to any of the preceding claims, characterised in that the drive element comprises a nut or a plurality of nuts firmly coupled to one another in series, or other elements having an internal thread with an eccentric threaded bore.

15. Linear drive according to any of the preceding claims, characterised in that the plain bearing elements (6) for axially guiding the drive spindle (4) comprise two plain bearing shells concentrically rotatably mounted in the hollow shaft (2).