Drive device and method for conveying at least one carriage along a conveyor track

Abstract

The invention relates to a drive device for conveying at least one carriage along a conveyor track, comprising at least one carriage which can be moved along a linearly extending conveyor track, characterized by at least two conveyor elements which extend parallel to one another along the conveyor track and can be driven for relative shifts in opposite directions along the conveyor track, and of which each conveyor element comprises a number of coupling members for coupling to the carriage, which coupling members are arranged spaced apart from one another along the respective conveyor element, whereby the carriage can be coupled selectively to a first or a second of the two conveyor elements.

Description

[0001] Drive device and method for transporting at least one wagon along a conveyor track

[0002] The present invention relates to a drive device for conveying at least one carriage along a conveyor track, according to the preamble of claim 1, and to a method for operating such a drive device.

[0003] Drive devices of the type described here are used, for example, to move large machines or machine parts from one production or processing station to the next in industrial manufacturing. The parts to be moved are arranged on carriages, which are usually moved along a conveyor track on floor rails. Such carriages are also known as industrial conveyors. They are low-profile and have a platform on their upper side for receiving the load.

[0004] The carriages do not have their own drives installed; instead, they are moved by drive devices that are embedded, for example, between the rails in the hall floor.

[0005] The transport of the wagons should be carried out with as few interruptions as possible. One problem is to continuously transport the wagon over longer distances using a stationary drive system. Usually, a change of drive unit is necessary. An example of such an arrangement is shown in DE 10 2016 125 132 B4. The drive device disclosed therein comprises drive units that are offset from one another in the conveying direction and overlap each other, so that a change of drive unit can be carried out in the overlapping sections of the conveying path. This change of coupling can be performed as a continuous transfer from one drive unit to the next while the wagon is in motion.

[0006] The drive system shown here has the disadvantage of being comparatively complex due to the large number of overlapping conveyor sections with individual coupling elements. Therefore, there is a need for a simplified drive system that allows for uninterrupted coupling and thus continuous carriage movement, but with a simpler design. Furthermore, the individual conveyor sections dictate a drive timing that is fixed by design and cannot be individually adapted to the specific conditions.

[0007] It is therefore an object of the present invention to provide a drive device for conveying at least one wagon along a conveyor track, which enables continuous wagon transport over a longer distance with individual adjustment of the conveying rate, but is constructed in a comparatively simple and cost-effective manner.

[0008] This problem is solved according to the invention by the drive device with the features of claim 1 and by a method for operating this drive device with the features of claim 17. Preferred embodiments of this device and this method are specified in the dependent claims.

[0009] The drive device according to the invention comprises at least a first and a second conveying element, which extend parallel to each other along the conveying path and can be driven to relative displacements in opposite directions along the conveying path. Each of these conveying elements comprises a number of coupling elements for coupling to the carriage, which are arranged at intervals from each other along the respective conveying element. This allows the carriage to be selectively coupled to either the first or the second conveying element.

[0010] The parallel conveyor elements are arranged in opposite directions, allowing each element coupled to the carriage to move a predetermined distance in the conveying direction, thus propelling the carriage, while the second conveyor element moves in the opposite direction. Eventually, the carriage reaches a position where the coupling can be changed, at which point the second conveyor element takes over conveying the carriage. This element then moves in the conveying direction together with the carriage, while the first conveyor element, now uncoupled, is pushed forward in the opposite direction. This movement pattern can be repeated continuously. The carriage is thus moved alternately by the two conveyor elements and can be coupled to the couplings of the two conveyor elements alternately.The key point here is that changing the coupling does not necessarily require an interruption of the wagon transport; instead, the wagon can be transferred seamlessly from one conveying element to another during the journey by appropriate synchronization of the drive(s).

[0011] An advantage of the drive device according to the invention is that it may suffice to use only two conveying elements, each equipped with its own drive. It may also be possible to drive both conveying elements with a common drive. Within the scope of the invention, it is also possible to provide more than two conveying elements.

[0012] According to a preferred embodiment of the present invention, the conveying elements can be driven to a movement in the opposite direction to the intermittent conveying of the carriage. In a first movement cycle, the first conveying element is coupled to the carriage and driven to move in the conveying direction, while the second conveying element is detached from the carriage and driven to move in the opposite feed direction. In a subsequent second movement cycle, the second conveying element is coupled to the carriage and driven to move in the conveying direction, and the first conveying element is detached from the carriage and driven to move in the feed direction. The conveying distances traveled by the carriage during the first and / or second movement cycle (conveying cycle) can be adjusted by appropriately controlling the drive(s) of the conveying elements.This allows for a more individualized and flexible scheduling than is possible with current technology.

[0013] Preferably, the coupling elements are designed as drivers intended to engage a corresponding coupling element of the carriage in the conveying direction during the conveying movement of the conveying element to which the respective driver is attached. These drivers can operate without their own motor drive. In a simple embodiment, these drivers can be cams that project onto the conveying elements and engage at a corresponding engagement position on the carriage. More preferably, the drivers are designed to be able to slide over the corresponding coupling element of the carriage without engaging it during a forward movement of the respective conveying element relative to the carriage, in the opposite direction to the conveying movement of the carriage. Thus, the driver of a conveying element can pass the coupling element virtually without resistance in the opposite direction of conveying.

[0014] According to a further preferred embodiment, the drivers are designed such that they yield during sliding over a coupling element and can move out of the way. For example, a driver designed as a cam is spring-loaded upwards and, during the sliding movement over the coupling element, slides downwards against the spring tension and thus offers no resistance.

[0015] According to a further preferred embodiment, the coupling element(s) are designed to yield during the sliding of a driver over the coupling element and to be able to move out of the way of the driver. In this case, the coupling elements of the carriage can yield while the drivers remain fixed.

[0016] Preferably, the conveying elements comprise pull rods or push rods.

[0017] According to a further preferred embodiment, the drive device according to the invention comprises at least one drive for the conveying elements, which includes at least one spindle drive and at least one motor. The torque generated by the motor is then converted via the spindle drive into a linear movement of the conveying elements.

[0018] Preferably, each conveying element has its own individual drive, comprising a spindle drive and a motor for moving the spindle drive. Each motor thus drives one conveying element. For example, servo motors could be used. Furthermore, it is possible to control the existing drive motors and thus the movements of the conveying elements by a common control unit. According to a preferred embodiment of the present invention, the coupling elements of each conveying element are arranged at equal intervals on the element.

[0019] Preferably, the spacing intervals of the coupling elements on both conveying elements are the same.

[0020] According to another preferred embodiment, the carriage runs on rails, and at least the conveying elements are arranged between the rails.

[0021] Preferably in this case, the rails are floor rails, and the conveying elements are embedded in the floor between the rails and extend under the carriage.

[0022] According to a further preferred embodiment, the drive device according to the invention comprises a brake for holding the carriage in its position along the conveyor track when not driven. This can be one or more friction brakes that spring-loadedly press towards a guide rail of the carriage.

[0023] Preferably, the brake is designed to exert a continuous braking effect while the carriage moves along the conveyor track. The aforementioned friction brake can therefore maintain continuous contact with the guide rail. The braking effect is sufficient to hold the carriage in its position without impeding its conveying movement.

[0024] According to another embodiment, the brake is designed to exert a braking effect only at predetermined points along the conveyor track. In this case, it does not engage continuously, but rather, for example, runs onto predetermined braking points on or next to the rail, while otherwise maintaining no contact with the rail.

[0025] The present invention further relates to a method for operating a drive device of the type according to the invention, in which the conveying elements are driven to a movement in the opposite direction to the intermittent conveying of the carriage, wherein in a first movement cycle the first conveying element is coupled to the carriage and driven to move in the conveying direction, while the second conveying element is detached from the carriage and driven to move in an opposite feed direction, and in a subsequent second movement cycle the second conveying element is coupled to the carriage and driven to move in the conveying direction and the first conveying element is detached from the carriage and driven to move in the feed direction.

[0026] In each of the movement cycles, the carriage is conveyed by one of the conveying elements, while the other conveying element is moved in the opposite direction of conveying, while in a subsequent movement cycle the other conveying element transports the carriage.

[0027] A preferred embodiment of the present invention will be explained in more detail below with reference to the accompanying drawing.

[0028] Figs. 1A, 1B and IC show an embodiment of the drive device according to the invention for conveying at least one wagon along a conveyor track,

[0029] Figures 2A and 2B show detailed views of a second embodiment of this drive device.

[0030] Figures 3A and 3B show detailed views of a third embodiment of this drive device, and

[0031] Figs. 4A and 4B show detailed views of a fourth embodiment of this drive device.

[0032] According to Figures 1A, 1B and IC, the drive device 10 according to the invention comprises two parallel rails 12a, 12b on which a carriage 14 can be moved. For this purpose, the carriage is equipped with four wheelsets 16a, 16b, 16c and 16d, which are arranged in pairs on the rails, such that two wheelsets 16a and 16b run on the upper rail 12a shown in the figures, and the two remaining wheelsets 16c and 16d run on the lower rail 12b.

[0033] Rails 12a and 12b extend linearly and together form a conveyor track along which carriage 14 can move. In the movement sequence described below, carriage 14 moves along rails 12a and 12b from the right side of the drawing to the left. This direction will be referred to here as conveying direction A.

[0034] Rails 12a and 12b are floor rails embedded in the ground, for example, in the floor of a production hall. Car 14 is a material handling vehicle for transporting workpieces.

[0035] In the present embodiment, the carriage is driven by two conveying elements 18a, 18b, which extend parallel to each other centrally between the rails 12a, 12b along the conveyor track. The conveying elements 18a, 18b are embedded in the floor between the rails 12a, 12b and extend under the carriage 14. Both conveying elements 18a, 18b are of the same length.

[0036] In this case, the conveying elements 18a and 18b are drawbars. They can be moved relative to each other in opposite directions along the conveying path. They are essentially identical in construction and each has a number of coupling elements for coupling to the wagon 14, which are arranged at intervals along the respective conveying element 18a and 18b.

[0037] The conveying element 18a shown at the top of the drawings shall be referred to below as the first conveying element. Along the first conveying element 18a, eight coupling elements 18a.1, 18a.2, 18a.3, 18a.4, 18a.5, 18a.6, 18a.7 and 18a.8 (hereinafter: first coupling elements 18a.1 to 18a.8) are arranged at equal intervals, in the aforementioned order starting from the free (right in the figures) end 20 of the first conveying element 18a in the direction of a drive of the first conveying element 18a, which will be described in more detail below. Similarly, the second conveying element 18b comprises eight coupling elements 18b.1, 18b.2, 18b.3, 18b.4, 18b.5, 18b.6, 18b.7 and 18b.8 (hereinafter: second coupling elements 18b.1 to 18b.8), which are arranged in this order from the free end 22 of the conveying element towards its drive at equal intervals. The standing intervals between the coupling elements 18b.1 to 18b.88 are the same as the standing intervals between the coupling elements 18a.l to 18a.8 of the first conveying element 18a.

[0038] Car 14 is equipped with a coupling device 24, which is designed to interact with the first coupling elements 18a.1 to 18a.8 and the second coupling elements 18b.1 to 18b.8. For this purpose, the coupling device 24 comprises coupling elements designed so that the coupling elements 18a.1 to 18a.8 and 18b.1 to 18b.8 can engage with them, so that car 14 can be selectively coupled to either the first conveying element 18a or the second conveying element 18b, and car 14, in the coupled state, is carried along by the respective conveying element 18a or 18b. The coupling elements 18a.1 to 18a.8 and 18b.1 to 18b.8 are designed for this purpose as drivers, which are cams that project from the respective conveying elements 18a and 18b.

[0039] The interaction between the coupling elements 18a.1 to 18a.8, 18b.1 to 18b.8 and the coupling device 24 will be explained in more detail with reference to Figs. 2A and 2B.

[0040] Figures 2A and 2B show an example of the first conveying element 18a. Its free end 20 is located on the left in the figure, while the conveying direction A points to the right. An embodiment of the rail carriage 14 is also shown, with the coupling device 24 located on its left side.

[0041] Two coupling elements 18a.1 and 18a.2 are shown as examples on the coupling element 18a. These are projecting drivers, namely cams, which protrude from the top surface of the conveying element 18a.

[0042] The coupling elements 18a.l, 18a.2 are designed to engage a coupling element 24a of the coupling device 24 of the carriage 14 during movement of the conveying element 18a relative to the carriage 14 in conveying direction A. This situation is illustrated in Fig. 2A. Here, the coupling element 18a.l engages the coupling element 24a and thus pulls the carriage 14 in conveying direction A on the rails (not shown in detail). If, however, the conveying element 18a is moved in the opposite direction to the left, i.e., advanced under the carriage 14, as shown in Fig. 2B, the coupling element 18a.l can slide under the coupling element 24a. This is because the coupling element 24a is designed to yield during this sliding movement of the coupling element 18a.l over the coupling element 24a during the forward movement and to move away from the driver 18a.l.For this purpose, the coupling element 24a is designed as a pivotable element which can be pushed upwards by the coupling element 18a.1 during its feed movement (Fig. 2B), but remains rigid in the opposite direction during a movement in conveying direction A (Fig. 2A) against the carriage 14 and cannot move out of the way.

[0043] In this way it is possible to advance the conveying element 18a under the carriage 14 against the conveying direction A until the coupling element 18a.l has passed the coupling element 24a, and then to reverse this movement so that the conveying element 18a is pulled in the conveying direction A and engages the coupling device 24 of the carriage 14 with the coupling element 18a.l and takes it along.

[0044] In this case, the carriage 14 is also equipped with a brake 40 designed to hold the carriage 14 along the conveyor track when unpowered. This prevents the carriage 14 from rolling away when unpowered. In the embodiment shown in Figures 2A and 2B, the brake 40 is designed to continuously exert a braking effect while the carriage is moving along the conveyor track. For this purpose, the brake 40 is designed as a friction brake, which uses spring pressure to continuously press a brake shoe against the respective rail 12a, 12b. In the example shown, the brake shoe is pressed against the respective rail 12a, 12b. In other variants, the brake shoe can be pressed laterally against a respective rail 12a or 12b, for example, from between the rails 12a, 12b or from outside (in Figure 1A, therefore, above the rail 12a or below the rail 12b).Accordingly, the brake 40 can be arranged above the respective rail 12a, 12b or in another position, e.g. between the rails 12a, 12b or laterally outside the relevant rail 12a, 12b.

[0045] Figures 3A and 3B show an alternative embodiment of the brake 42, designed to exert braking action only at predetermined points along the conveyor track. At these points, brake cams 46 are provided next to one of the rails 12a, 12b. If the car 14 makes contact with the brake cam 46 at such a braking point as shown in Figure 3B via the brake shoe 44, it will not leave its position on the rail when not under power. The brake cams 46 can be arranged, for example, between the rails 12a, 12b or laterally outside the respective rail 12a, 12b.

[0046] The design of the fixed coupling elements 18a.1, 18a.2 on the conveying element 18a and the coupling element 24a which is flexible in one direction is the same as shown in Figs. 2A and 2B.

[0047] Figures 4A and 4B show a further embodiment of the drive device according to the invention, in which the coupling elements 24a are fixed, but the coupling elements 18a.1, 18a.2 are flexibly designed. The coupling elements 18a.1, 18a.2 are pressed upwards by spring pressure. If the conveying element 18a is pulled in conveying direction A relative to the carriage 14 (Fig. 4A), the coupling element 18a.1 engages the coupling element 24a and guides the carriage 14 during further movement. If the conveying element 18a is moved in the opposite direction (Fig. 4B), the coupling element 18a.1 is pressed downwards by the coupling element 24a, so that the coupling element 24a can slide over and pass through the coupling element 18a.1. The conveying element 18a can therefore glide under the carriage 14 with virtually no resistance.

[0048] In the embodiment shown in Figs. 4A and 4B, a constantly active friction brake 40 is also shown according to Figs. 2A and 2B.

[0049] As shown in Figures 1A, 1B and IC, each of the first and second conveying elements 18a, 18b is driven by an individual drive. This drive comprises a motor 26a, 26b, which drives a spindle 28a, 28b to rotate it. The spindle 28a, 28b is in turn coupled to the respective conveying element 18a, 18b via a carriage 30a, 30b running on it, such that the rotation of the spindle 28a, 28b moves the carriage 30a, 30b linearly and, during this movement, displaces the conveying element 18a, 18b. The two spindles 28a, 28b are positioned side by side and parallel below the conveyor elements 18a, 18b, and the motors 26a, 26b are arranged at opposite ends of this spindle pair, with each motor 26a, 26b driving one of the spindles 28a, 28b to rotate. The motors 26a, 26b are controlled by a common controller.

[0050] These drives allow the two conveyor elements 18a and 18b to be driven in opposite directions, i.e., in the direction of conveyance A, so that they move in opposite directions along the conveyor track. This enables the carriage 14 to be conveyed intermittently by the two conveyor elements 18a and 18b.

[0051] In the situation shown in Fig. 1A, the conveying elements 18a and 18b are displaced relative to each other such that the first conveying element 18a is displaced further against the conveying direction A than the second conveying element 18b by a distance interval between the two coupling elements 18a.1 and 18a.2. The carriage 14 is coupled to the conveying element 18a via its coupling device 24 and the coupling element 18a.2 in such a way that it can be moved in the conveying direction A by driving the conveying element 18a. The coupling between the carriage 14 and the conveying element 18a is carried out as described above in connection with Figs. 2A, 3A, or 4A.

[0052] During this movement of the first conveying element 18a, the second conveying element 18b is simultaneously moved in the opposite direction, i.e., against the conveying direction A, by two distance intervals until the situation shown in Fig. 1B is reached. During this movement, the coupling elements 18b.2 and 18b.3 slide under the coupling element 24a of the coupling device 24, so that the coupling element 18b.3 can engage the coupling element 24a during a reversal of the movement of the conveying elements 18a, 18b and move the carriage 14 further in the conveying direction A by one distance interval, while the first conveying element 18a moves forward against the conveying direction A until the situation shown in Fig. IC is reached.

[0053] The movement from Fig. 1A to Fig. 1B corresponds to a first movement cycle in which the first conveying element 18a is coupled to the carriage 14 and driven to move in conveying direction A, while the second conveying element 18b is detached from the carriage and driven to move in the opposite feed direction B. In contrast, the movement from Fig. 1B to Fig. 1IC corresponds to a second movement cycle in which the second conveying element 18b is coupled to the carriage 14 and driven to move in conveying direction A, and the first conveying element 18a is detached from the carriage 14 and driven to move in feed direction B.

[0054] In the present embodiment, the coupling elements are aligned with respect to the conveying direction A when changing the coupling from the first conveying element 18a to the second conveying element 18b and vice versa, i.e., approximately in the situation shown in Fig. 1B. Here, the coupling of the carriage 14 is released from the first conveying element 18a and engaged by the conveying element 18b via the coupling element 18b.3, which then moves the carriage 14 forward. This coupling change can be achieved without interrupting the movement of the carriage 14 by appropriately synchronizing the motors 26a and 26b.

[0055] The arrangement of the coupling elements 18a.1 to 18a.8 and 18b.1 to 18b.8 on the conveyor elements 18a, 18b is shown in Figures 1A, 1B and IC as merely an example in the present embodiment. The coupling elements can also be arranged at irregular intervals and need not be at equal intervals on both conveyor elements 18a, 18b. Additional conveyor elements can also be arranged between the rails 12a, 12b, allowing for a more detailed implementation of the carriage's movement cycle.

[0056] It is also by no means necessary to shift the carriage 14 by exactly one stop interval between the coupling elements during a movement cycle; rather, a shift by different distances can also be carried out, i.e., even across other coupling elements of the counter-rotating conveying element, before the coupling change and the reversal of motion take place. Finally, the conveying distances of the carriage 14 do not have to be covered in every movement cycle; these can be individually controlled. Furthermore, the spindle stroke of the spindle drive of a motor 26a, 26b can be dimensioned larger than is required for the movement of the carriage 14 within the conveying direction A during a movement cycle.

[0057] Finally, it is possible to transport the wagon 14 further at its end in the direction of transport A by means of at least one of the conveying elements 18a, 18b, i.e. to transfer it, for example, to another section of a track not shown in detail, a turntable or the like.

[0058]

[0059] 10 Drive unit direction

[0060] 12a, 12b rails

[0061] 14 cars

[0062] 16a, 16b, 16c, 16d Wheelsets

[0063] 18a first conveying element (first pull rod)

[0064] 18b second conveying element (second pull rod)

[0065] 18a.1 to 18a.8 Coupling elements (drivers) of the first conveying element

[0066] 18b.1 to 18b.8 Coupling elements (drivers) of the second conveying element

[0067] 20 free end of the first conveying element

[0068] 22 free end of the second conveying element

[0069] 24 Coupling device

[0070] 24a Coupling element

[0071] 26a, 26b Motors

[0072] 28a, 28b Spindles

[0073] 30a, 30b Sled

[0074] 40, 42 brakes

[0075] 44 brake shoe

[0076] 46 brake cams

[0077] A Direction of conveyance

[0078] B Feed direction

Claims

PATENT CLAIMS 1. Drive device (10) for conveying at least one carriage (14) along a conveyor track, comprising at least one carriage (14) movable along a linearly extending conveyor track, characterized by at least one first and one second conveying element (18a, 18b) extending parallel to each other along the conveyor track and capable of being driven to relative displacements in opposite directions along the conveyor track, each conveying element (18a, 18b) comprising a number of coupling elements (18a.1 to 18a.8, 18b.1 to 18b.8) for coupling to the carriage (14), arranged at intervals from each other along the respective conveying element (18a, 18b), whereby the carriage (14) can be selectively coupled to the first or the second conveying element (18a, 18b).

2. Drive device according to claim 1, characterized in that the conveying elements (18a, 18b) can be driven to a counter-rotating movement for the intermittent conveying of the carriage (14), wherein in a first movement cycle the first conveying element (18a) is coupled to the carriage (14) and driven to move in the conveying direction (A), while the second conveying element (18b) is detached from the carriage (14) and driven to move in an opposite feed direction (B), and in a subsequent second movement cycle the second conveying element (18b) is coupled to the carriage (14) and driven to move in the conveying direction (A) and the first conveying element (18a) is detached from the carriage (14) and driven to move in the feed direction (B).

3. Drive device according to claim 1 or 2, characterized in that the coupling elements (18a. 1 to 18a.8, 18b.1 to 18b.8) are designed as drivers which are provided to engage during a conveying movement of the conveying element (18a, 18b), at which- where the respective carrier is attached, to engage in the conveying direction (A) on a corresponding coupling element (24a) of the carriage (14).

4. Drive device according to claim 3, characterized in that the drivers are designed to be able to slide over a coupling element (24a) of the carriage (14) during a feed movement of the respective conveying element (18a, 18b) relative to the carriage (14) in the opposite direction of conveying (A), without engaging it.

5. Drive device according to claim 4, characterized in that the drivers are designed such that they can yield during sliding over a coupling element and avoid the coupling element (24a).

6. Drive device according to claim 4, characterized in that the coupling elements (24a) are designed to yield during the sliding of a driver over the respective coupling element (24a) and to be able to avoid the driver.

7. Drive device according to one of the preceding claims, characterized in that the conveying elements (18a, 18b) comprise pull rods or push rods.

8. Drive device according to one of the preceding claims, characterized by at least one drive for the conveying elements (18a, 18b) comprising at least one spindle drive (28a, 28b) and at least one motor (26a, 26b).

9. Drive device according to claim 8, characterized in that an individual drive is provided for each conveying element (18a, 18b), comprising a spindle drive (28a, 28b) and a motor (26a, 26b) for moving the spindle drive (28a, 28b).

10. Drive device according to one of the preceding claims, characterized in that the coupling elements (18a. 1 to 18a.8, 18b. 1 to 18b.8) of each of a conveying element (18a, 18b) are arranged at equal intervals on it.

11. Drive device according to one of the preceding claims, characterized in that the spacing intervals of the coupling elements (18a.1 to 18a.8, 18b.1 to 18b.8) are the same on both conveying elements (18a, 18b).

12. Drive device according to one of the preceding claims, characterized in that the carriage (14) runs on rails (12a, 12b) and at least the conveying elements (18a, 18b) are arranged between the rails (12a, 12b).

13. Drive device according to claim 12, characterized in that the rails (12a, 12b) are floor rails and the conveying elements (18a, 18b) are embedded in the floor between the rails (12a, 12b) and extend under the carriage (14).

14. Drive device according to one of the preceding claims, characterized by a brake (40, 42) for holding the carriage (14) in its position along the conveyor track in the unpowered state.

15. Drive device according to claim 14, characterized in that the brake (40) is designed to exert a braking effect continuously during the movement of the carriage (14) along the conveyor track.

16. Drive device according to claim 14, characterized in that the brake (42) is designed to exert a braking effect only at predetermined points along the conveyor path.

17. Method for operating a drive device according to one of the preceding claims, characterized in that the conveying elements are driven to a counter-rotating movement to the intermittent conveying of the carriage (14), wherein in a first movement cycle the first conveying element (18a) is coupled to the carriage (14) and driven to move in the conveying direction (A), while the second conveying element (18b) is detached from the carriage (14) and driven to move in an opposite feed direction (B), and in a subsequent second movement cycle the second conveying element (18b) is coupled to the carriage (14) and driven to move in the conveying direction (A) and the first conveying element (18a) is detached from the carriage (14) and driven to move in the feed direction (B).

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