WO2003093149A1

WO2003093149A1 - Belt conveyor

Info

Publication number: WO2003093149A1
Application number: PCT/SE2003/000544
Authority: WO
Inventors: Robert Johansson
Original assignee: Sicon Roulunds Ab
Priority date: 2002-05-03
Filing date: 2003-04-04
Publication date: 2003-11-13
Also published as: AU2003217132A1; SE0201332D0; SE0201332L; SE519886C2

Abstract

A belt conveyor comprises an endless, curved conveyor belt, which has at least one traction-force-absorbing member along the length of the conveyor belt and one belt-carrying member at each longitudinal edge of the conveyor belt, support rollers, which by way of the belt-carrying members support the conveyor belt over the path thereof, and at least one first and one second drive pack, which are each provided with at least one drive roller, which is adapted to transmit a traction force to the traction-force-absorbing member. The traction forces of the first and the second drive pack are individually controllable. The belt conveyor further comprises a detecting means provided at least at the first drive pack and adapted to detect the belt tension in the conveyor belt adjacent to the drive pack, and a control means for individually controlling the traction force of the first drive pack in dependence on a change in the belt tension detected with the aid of the detecting means in order to keep the belt tension on the exit side of the first drive pack above a predetermined minimum value.

Description

BELT CONVEYOR

Field of the Invention

The present invention relates to a belt conveyor, comprising an endless, curved conveyor belt, which has at least one traction-force-absorbing member along the length of the conveyor belt and one belt-carrying member at each longitudinal edge of the conveyor belt . Such a belt conveyor further comprises support rollers, which by way of the belt-carrying members support the conveyor belt over the path thereof, and at least one first and one second drive pack, which are each provided with at least one drive roller, which is adapted to transmit a traction force to the traction-force-absorbing member. The invention further relates to a method of controlling driving of a belt conveyor of the kind mentioned above ,

Background Art

A belt conveyor of the kind stated above is disclosed in SE 8505793-3, which describes a belt conveyor having an endless, curved belt and two traction-force- absorbing members which extend over the whole length of the belt conveyor and which are each connected to an associated edge portion of the belt and project from this plane. The belt conveyor has rollers, such as guide rollers, support rollers and backing rollers, on which the belt runs, and pressure rollers for directing the traction-force-absorbing members towards, for example, a drive or guide roller. A belt conveyor of the kind mentioned above is also disclosed in SE 8802367-6. The conveying belts of the belt conveyors according to the above specifications carry the goods in a bag shape suspended from the support rollers, which support the belt. Such conveyor belts are particularly suitable for transporting a load along a winding or inclined path, i.e. the belt is curved. In curves, these conveyor belts are not subjected to inner tensions, since the difference in distance between the portion of the conveyor belt fol- lowing the shortest path through the curve and the portion of the conveyor belt following the longest path through the curve is insignificant.

In conveying paths along which the load varies, for example long paths or paths with varying elevation, a plurality of motor-driven rollers, also called drive rollers, have to be arranged along the conveying path for driving the conveyor belt. If a load conveyed by the conveyor belt is unevenly distributed over the belt, which is the case, for example, during the initial loading of the belt and during emptying thereof, there is a risk of the friction between the belt and the drive roller getting so small that the belt starts to slip on the drive roller. The longer the conveyor belt, the greater the risk of the load being unevenly distributed. This risk is particularly large in connection with two-way transport, i.e. when the belt conveyor carries a load also on a return path, and in the case of a belt path with elevations. Slippage causes wear, and also a risk of the belt derailing, since the weight of the belt will cause it to start gliding off the roller if the friction is too small. The problem of derailment can be solved to some extent by means of pressure rollers as described in the previous paragraph. However, mounting such pressure rollers entails additional costs and does not prevent the belt from slacking, which makes it harder to drive the belt. Thus, more energy is required while at the same time the wear problem remains. Finally, the maximum conveying path of existing conveyor belts of the kind mentioned by way of introduction is limited by the risk of increased wear and derailment.

US-A-3, 923,151 discloses a belt conveyor with only one driving motor. The traction force of the driving motor is controlled in dependence on a detected belt tension in the conveyor belt . The control operation is carried out to limit the belt tension during starting and stopping of the belt. However, US-A-3 , 923 , 151 does not solve the problem of slippage between the conveyor belt and a drive roller. The control operation is carried out to ensure that the traction force acting on the conveyor belt does not generate excessive tension in the belt.

Summary of the Invention

The object of the present invention therefore is to provide a belt conveyor which allows the risk of slippage and derailment to be reduced. According to the invention, this object is achieved by a belt conveyor of the kind stated by way of introduction, which is characterised in that the traction forces of the first and the second drive pack are individually controllable, and that the belt conveyor further com- prises a detecting means provided at the first drive pack and adapted to detect the belt tension in the conveyor belt adjacent to the drive pack, and a control means for individually controlling the traction force of the first drive pack in dependence on a change in the belt tension detected with the aid of the detecting means in order to keep the belt tension on the exit side of the first drive pack above a predetermined minimum value .

Drive pack here means one or more adjoining drive rollers arranged along the length of the conveyor belt. Between the drive rollers of such a drive pack, non-driving support rollers may be provided.

The object is also achieved by means of a method of controlling driving of a belt conveyor of the kind stated above, which method is characterised in that the tensile force of the conveyor belt adjacent to the first drive pack is detected, and that the traction force of the first drive pack is individually controlled in dependence on a change in the detected belt tension in order to keep the belt tension on the exit side of the first drive pack above a predetermined minimum value .

The drive packs in belt conveyors of the present kind are mechanically interconnected through the conveyor belt which they drive. To drive a conveyor belt at a certain speed each drive roller must rotate at a corresponding speed. According to conventional technology, the drive rollers are therefore each designed to exert the same traction force so as to achieve a constant speed of the conveyor belt. The invention is based on the understanding that it is not necessary to cause all the drive rollers to transmit the same traction force to the conveyor belt. Each drive roller will still rotate at the same speed, since they are mechanically interconnected through the conveyor belt. However, the contribution of the drive rollers to the propulsion of the belt will be different for each drive roller.

The invention is further based on the understanding that individual control can be used to solve the problem of drive roller slippage. The control operation is carried out in order to keep the belt tension adjacent to each drive roller above a minimum value. By the belt conveyor according to the invention having a detecting means which is arranged at the first drive pack and adapted to detect the belt tension of the conveyor belt adjacent to the drive pack, it is possible to determine, at an early stage, if the belt tension is getting too low or is already too low. If this is the case, the traction force of the drive pack can be controlled to keep the belt tension above a predetermined minimum value. In this way, a belt tension that is temporarily too low can be rapidly adjusted to prevent the occurrence of slippage. This control operation is carried out separately for each drive pack or each drive roller, so that the belt tension adjacent to a drive roller is maintained essentially independently of the belt tension adjacent to other drive rollers along the conveyor belt. However, traction force control is required only in connection with drive packs where there is a risk of slippage. Owing to the reduced risk of slippage and derailment obtained by means of the invention, it is also possible to design conveyor belts of the kind stated by way of introduction which can handle long conveying paths and considerable load variations .

Belt tension here means the inner tension in the longitudinal direction of the conveyor belt, which is generated by the conveyor belt being suspended, by it carrying a load and by it being driven along its path. Thus, the belt tension is dependent on the traction force used to drive the conveyor belt . The reason for keeping the belt tension on the exit side of the drive pack above a minimum value is that the traction force, and, thus, the belt tension, is always lowest on this side of the drive pack. The traction force has the same direction as the driving direction of the conveyor belt. A drive pack influences the conveyor belt in this direction, which means that the tensile force on the exit side of the drive pack equals the difference between the tensile force on the entry side and the traction force exerted by the drive pack to drive the conveyor belt.

In one embodiment, the belt conveyor further comprises a plurality of detecting means, which are each adapted to detect the belt tension of the conveyor belt adjacent to an associated drive pack. Owing to these de- tecting means, the belt tension adjacent to a plurality of drive packs can be controlled. This allows each position of the conveyor belt in which there is a risk of slippage to be monitored with the aid of a detecting means . Furthermore, the belt conveyor preferably comprises a plurality of control means, which are each adapted to control the traction force of an associated drive pack in dependence on a change in the belt tension adjacent to the drive pack, detected with the aid of a detecting means. As a result, the traction force is controlled at a plurality of drive packs to prevent the belt tension from getting too low at each of these drive packs.

The detecting means of the belt conveyor are preferably arranged to detect the belt tension on the exit side of the respective drive packs. Since the belt tension on the exit side is to be kept above a minimum value, it is convenient to detect the belt tension on the exit side. Consequently, the detected belt tension is precisely the value that is to be kept above a minimum value .

However, the detecting means may alternatively be adapted to detect the belt tension on the entry side of a drive pack in order to keep the belt tension on the exit side of the drive pack above said predetermined minimum value. The detected measurement value is then used to derive the value of the belt tension on the exit side. According to a second alternative, the detecting means may be adapted to detect the belt tension on both the entry side and the exit side of a drive pack.

According to one embodiment, the belt conveyor has at least one pair of support rollers, which are arranged on both sides of the conveyor belt and which support the conveyor belt by clamping the belt-carrying members together. Such a device helps avoid derailment. Alternatively, at least one of the support rollers in such a pair may also be a drive roller. This allows driving of the conveyor belt along a straight section. For long straight sections, it may be advantageous to add a drive unit in order to avoid the occurrence of excessive load variations in the conveyor belt.

Moreover, a plurality of drive rollers in a drive pack of the belt conveyor may be arranged along a curve in the path of the conveyor belt. This embodiment is advantageous in curves of a large radius, since each drive roller in the drive pack only contributes to part of the deflection. Owing to the above solution involving a plurality of drive rollers in a curve, the radius of each drive roller does not have to be particularly large. In another embodiment, a drive pack of the belt conveyor comprises only one drive roller. In this case, detecting means may be provided on the exit side of each drive roller, which ensures more reliable driving.

When a drive pack comprises only one drive roller, the bend of the curve along which the drive roller is driving the conveyor belt is suitably at least 90°. At this angle, satisfactory transmission of force to the conveyor belt is obtained.

The detecting means of the belt conveyor preferably comprises sensors, which are arranged to measure the force which the belt tension on the relevant side of the respective drive packs exerts on a shaft of the drive roller whose traction force is to be controlled. This is a straightforward way of measuring the belt tension. Preferably, the sensors of the belt conveyor are load cells. These sensors are characterised by high measurement precision and are commonly used in similar applications in the industry.

In a preferred embodiment of the belt conveyor, at least one of the belt-carrying members forms the traction-force-absorbing member. This allows a simple construction of the conveyor belt, since the belt-carrying and traction-force-absorbing functions can be combined in one and the same member. Suitably, the drive rollers of the belt conveyor also serve as support rollers to avoid the need for a separate support roller to be mounted adjacent to each drive roller.

In the method of driving a belt conveyor according to the invention, the traction force of the first drive pack is preferably reduced in dependence on a detected drop in the belt tension on the exit side of the first drive pack below a predetermined value, which is greater than the predetermined minimum value. This gives a safety margin which ensures that the value never falls below the minimum value . According to the alternative arrangement of the detecting means of the belt conveyor, an alternative method may be to reduce the traction force of the first drive pack in dependence on a detected drop in the belt tension on the entry side of the first drive pack below a pre- determined value, which is greater than the predetermined minimum value.

A preferred method of driving a belt conveyor comprises the step of increasing the traction force of the first drive pack in dependence on an increase in the detected belt tension. This allows the belt tension on the exit side of the drive pack to be kept below a predetermined maximum value, so as to avoid excessive stress on the belt and rollers.

Preferably, the belt tension of the conveyor belt is further detected adjacent to a plurality of drive packs and the traction force of a drive pack is individually controlled in dependence on a change in the detected belt tension adjacent to the drive pack. This allows the traction forces of a plurality of drive packs to be controlled independently of each other.

Brief Description of the Drawings

The invention will be described in more detail below with reference to the accompanying schematic draw- ings, which for the purpose of exemplification illustrate a currently preferred embodiment of the invention.

Fig. 1 is a schematic plan view of a curved belt conveyor.

Fig. 2 is a cross-sectional view of a conveyor belt supported by a drive roller.

Fig. 3 is a schematic plan view of a drive pack in which a plurality of drive rollers contribute to the deflection of the conveying direction of the conveyor belt.

Fig. 4 is a schematic plan view of a drive roller with a load cell. Fig. 5 is a schematic plan view of a drive roller with the load cell in an alternative position.

Fig. 6 is a graphic representation illustrating the belt tension variation along the transport path of the belt conveyor. Fig. 7 is a schematic block diagram illustrating how the torque of the drive motor is controlled in reference to the detected belt tension.

Description of a Preferred Embodiment Fig. 1 is a plan view of a belt conveyor 1. The belt conveyor 1 comprises an endless conveyor belt 2. The conveyor belt 2 is supported by support rollers 3 and driven along a path by drive rollers 4. The drive rollers 4 may be arranged in drive packs 5 consisting of adjacent drive rollers 4 or, alternatively, the drive rollers 4 may be arranged individually. The conveyor belt 2 has a charging zone 6, in which a load 7 is received by the conveyor belt 2 to be conveyed thereby. The conveyor belt 2 further has a discharging zone 8, in which the load 7 is discharged from the conveyor belt 2. Thus, the conveyor belt 2 conveys a load 7 from the charging zone 6 to the discharging zone 8 and returns from the discharging zone 8 to the charging zone 6 without a load. Alternatively, the conveyor belt may be adapted for two-way transport, which allows it to carry another load during its return.

In this case, a second charging zone is arranged just after the discharging zone 8 and a second discharging zone is arranged just before the charging zone 6.

Fig. 2 is a cross-sectional view of the conveyor belt 2. The conveyor belt 2 is bag-shaped in cross section and the load 7 is filled into the bottom of the bag. Two traction-force-absorbing members 9, 10 are arranged along the longitudinal edges of the conveyor belt 2. One of these members 9 is arranged above the other 10, so that the conveyor belt 2 forms a closed bag. The longitudinal edges of the conveyor belt 2 can be separated to open the bag shape during charging and discharging. The traction-force-absorbing members 9, 10 are .wedge-shaped and adapted to engage with grooves in the support rollers 3 and the drive rollers 4. The support rollers 3 and the drive rollers 4 support the con- veyor belt 2 by way of the traction-force-absorbing members 9, 10. This means that the traction-force- absorbing members 9, 10 are also belt-carrying members. Moreover, through the engagement with the rollers 3, 4, the traction-force-absorbing members 9, 10 also take up the load force and are therefore load-carrying members. A support roller 3 supports the conveyor belt 2 whereas a drive roller 4 also drives the conveyor belt 2 along its path. A support roller 3 is arranged in a static manner to support the conveyor belt 2. The support roller 3 may also act to guide the conveyor belt 2 in a deflection of direction. The drive roller 4 comprises a pulley 11, which is rotatably arranged on a shaft 12 supporting the pulley 11 at the appropriate height. The drive roller 4 further comprises a motor (not shown) , which rotates the disc 11. When the disc 11 is rotated, the conveyor belt 2, which engages the disc 11, is trained in the forward direction. A support roller 3 may have the same appearance as the drive roller 4 in Fig. 2. The drive rollers 4 drive the conveyor belt 2 along its path. Accordingly, the pulleys 11 of the drive rollers 4 rotate at a speed which corresponds to the speed of the conveyor belt 2. The motor of each drive roller 4 drives the pulley 11. If the speed at which the motor drives the pulley 11 is lower than the speed of the con- veyor belt 2, the conveyor belt 2 will cause the pulley 11 to follow its speed. Thus, the motion of the pulleys 11 of the different drive rollers 4 is synchronised due to the fact they are mechanically interconnected through the conveyor belt 2. The drive rollers 4 may contribute to different degrees to the driving of the conveyor belt 2. Accordingly, the traction force of each drive roller 4 is individually controllable without this resulting in different speeds in different portions of the conveyor belt 2.

By rotating a pulley 11 for the purpose of driving the conveyor belt 2, the direction of the conveyor belt 2 will be changed adjacent to a drive roller 4. To obtain straight driving of the conveyor belt 2 , two or more drive rollers 4 may be provided in succession for the purpose of redirecting the conveyor belt 2 into the correct direction. However, to obtain a driving effect on a straight portion of the path, it is also possible to arrange a pressure roller 13 on the opposite side of the engagement of the drive roller 4 with the conveyor belt 2. The conveyor belt 2 is tangent to the drive roller 4, and in this tangent point on the drive roller 4, a driving force can be transmitted to the conveyor belt 2, since the pressure roller 13 presses the conveyor belt 2 against the drive roller 4. Alternatively, both rollers on opposite sides of the conveyor belt 2 may be drive rollers 4. Driving of the conveyor belt 2 is thereby ob- tained without any change in the direction of the path followed by the conveyor belt 2.

Fig. 3 illustrates a drive pack 5, which guides the conveyor belt 2 ^'in a deflection of direction. The drive pack 5 comprises a plurality of rollers 3, 4, which may be both support and drive rollers. All rollers 3, 4 contribute to the deflection, which means that each roller 3, 4 only causes a small deflection of the conveyor belt 2. This means that the radius of each roller 3, 4 does not have to be particularly large compared with a design involving only one roller causing the whole deflection of the conveyor belt 2. A drive pack 5 according to Fig. 3 is particularly useful in conveyor belts 2 that are adapted to carry large goods, since such conveyor belts 2 cannot negotiate tight corners and, thus, needs a large deflection radius.

A detecting means 14 is provided adjacent to each drive roller 4, said means 14 being adapted to detect the belt tension of the conveyor belt 2. Figs 4-5 show two alternative ways of arranging the detecting means 14. The detecting means 14 consists of a load cell, which is adapted to detect the force exerted on the shaft about which the pulley 11 of the drive roller 4 is rotated. Suitably, the detecting means 14 is -adapted to measure the forces acting on the motor mount. In this case, the torque generated by the rotation of the pulley 11 is not detected. The load cell 14 is arranged in parallel to the belt tension either on the exit side of the drive roller 4, as shown in Fig. 4, or on the entry side thereof, as shown in Fig. 5. The load cell 14 then only detects the belt tension on the other side of the drive roller . The detected belt tension is used as an input value to control the motor driving the pulley 11 of the drive roller 4. Thus, the motor is controlled so as to keep the belt tension above a predetermined minimum value. Said minimum value is selected so that, if the belt tension is kept above this value, the risk of slip- page will be non-existent. The detected belt tension may be adapted to control a plurality of drive rollers 4 in a drive pack 5. According to a preferred embodiment, the detecting means 14 is adapted, in this case, to detect the belt tension on the exit side of the last drive roller 4 in the drive pack 5 as seen in the conveying direction. However, it will be obvious to a person skilled in the art that the detecting means 14 may, alternatively, be arranged to detect the belt tension adjacent to any drive roller 4 in the drive pack 5. Fig. 6 is a graphic representation of the belt tension value in different points in a belt conveyor 1 having the configuration as shown in Fig. 1. When the conveyor belt 2 is fully loaded (unbroken line) or empty (dashed line) , the belt tension is within the allowable limits. During emptying of the belt, a situation occurs in which only the last fourth of the conveyor belt 2 carries a load 7 (dash and dot line) . In this situation, the belt tension is too low on the exit side of the drive roller 4 at C. Thus, there is a considerable risk of the conveyor belt 2 starting to slip relative to said drive roller 4. A detecting means 14 provided at said drive roller 4 would indicate when the belt tension is getting too low, which could be used to individually control the driving force of the drive roller 4 at C to prevent the belt tension from getting too low. In this case, a reduction of the driving force of the drive roller 4 would lead to an increase in the belt tension on the exit side of the drive roller 4, thereby avoiding any slippage or derailment . The method of achieving said control will be described below with reference to Fig. 7.

A method of driving a belt conveyor according to the invention will now be described in more detail. The driving force of the motor exerted on the drive roller pulley is controlled in dependence on the detected belt tension, as illustrated in Fig. 7. By regulating the driving force of the motor, the motor torque M acting on the drive roller is controlled. This in turn controls the traction force F exerted by the drive roller on the conveyor belt. The belt tension of the conveyor belt is thereby affected, said belt tension being continuously detected for control purposes. Said control is effected by setting a desired value X for the belt tension. The difference e between the desired value X and the detected belt tension y is used as the input value to control the driving force of the motor. This difference is a measurement indicating the difference between the belt tension and the desired minimum value. When the detected belt tension falls below the desired value, the control operation has to be carried out. If the input value e for the motor driving force fed to the regulator is positive, the driving force of the motor is reduced while the speed of the conveyor belt is maintained. This results in an increase in the belt tension on the exit side of the drive roller. The driving force of the motor is reduced in this way until the belt tension again is greater than the desired value.

Suitably, the desired value is set so as to be greater than the predetermined minimum allowable value of the belt tension. Thus, if the belt tension falls below the desired value, it is still higher than the minimum allowable value. Control is effected when the belt tension falls below the desired value, which means that the belt tension is continuously kept above the predetermined minimum allowable value. Each drive roller is individually controlled. This allows the belt tension adjacent to each drive roller to be controlled individually and systems comprising a large number of drive rollers to be used without any slippage problems.

It will be appreciated that various modifications of the embodiment described above are conceivable within the scope of the invention, as defined by the appended claims. Thus, the conveyor belt may be designed in a number of ways depending on the intended application. For example, the belt may be hooked in cross section. The longitudinal edges of the conveyor belt do not have to be arranged one above the other. According to a further embodiment, they may be arranged side by side. Nor do the longitudinal edges need to have a traction-force-absorbing function. Traction-force-absorbing members may be provided along any other part of the conveyor belt.

Detecting means do not necessarily have to be provided at all drive packs. It may be sufficient to provide detecting means at the drive packs in the belt conveyor where the risk of the conveyor belt beginning to slip is particularly great.

Claims

1. A belt conveyor, comprising an endless, curved conveyor belt, which has at least one traction-force- absorbing member along the length of the conveyor belt and one belt-carrying member at each longitudinal edge of the conveyor belt, support rollers, which by way of the belt-carrying members support the conveyor belt over the path thereof, and at least one first and one second drive pack, which are each provided with at least one drive roller, which is adapted to transmit a traction force to the traction-force-absorbing member, char a c t e r i s e d in that the traction forces of the first and the second drive pack are individually controllable, and that the belt conveyor further comprises a detecting means provided at the first drive pack and adapted to detect the belt tension in the conveyor belt adjacent to the drive pack, and a control means for individually controlling the traction force of the first drive pack in dependence on a change in the belt tension detected with the aid of the detecting means in order to keep the belt tension on the exit side of the first drive pack above a predetermined minimum value.

2. The belt conveyor as claimed in claim 1, further comprising a plurality of detecting means, which are each adapted to detect the belt tension in the conveyor belt adjacent to an associated drive pack. 3. The belt conveyor as claimed in claim 2, further comprising a plurality of control means, which are each adapted to control the traction force of an associated drive pack in dependence on a change in the belt tension, detected with the aid of a detecting means, adj cent to the drive pack.

. The belt conveyor as claimed in any one of claims 1-3, wherein the detecting means is adapted to detect the belt tension on the exit side of the first drive pack in order to keep said belt tension above the predetermined minimum value .

5. The belt conveyor as claimed in any one of claims 1-3, wherein the detecting means is adapted to detect the belt tension on the entry side of the first drive pack in order to keep the belt tension on the exit side of the drive pack above the predetermined minimum value.

6. The belt conveyor as claimed in any one of claims 1-3, wherein the detecting means is adapted to detect the belt tension on the entry side as well as the exit side of the first drive pack. 7. The belt conveyor as claimed in any one of the preceding claims, wherein drive rollers included in a drive pack are arranged along a curve along the conveyor belt in which the conveying direction of the belt is changed. 8. The belt conveyor as claimed in any one of claims 1-6, wherein a drive pack comprises only one drive roller arranged along a curve along the conveyor belt .

9. The belt conveyor as claimed in claim 8, wherein the bend of the curve is at least essentially 90°. 10. The belt conveyor as claimed in any one of the preceding claims, wherein the detecting means comprises a sensor, which is adapted to measure the force exerted by the belt tension on the associated side of the first drive pack on a shaft of the drive roller whose traction force is to be controlled.

11. The belt conveyor as claimed in claim 10, wherein the sensor is a load cell .

12. The belt conveyor as claimed in any one of the preceding claims, wherein at least one of the belt-carry- ing members constitutes the traction-force-absorbing member .

13. The belt conveyor as claimed in any one of the preceding claims, wherein said drive roller constitutes a support roller.

14. A method of controlling driving of a belt con- veyor, which comprises an endless, curved conveyor belt, which has at least one traction-force-absorbing member along the length of the conveyor belt and one belt-carrying member at each longitudinal edge of the conveyor belt, support rollers, which by way of the belt-carrying members support the conveyor belt over the path thereof, and at least one first and one second drive pack, which each comprise at least one drive roller, which is adapted to transmit a traction force to the traction-force- absorbing member, chara c t e r i s e d in that the traction force of the conveyor belt adjacent to the first drive pack is detected, and that the traction force of the first drive pack is individually controlled in dependence on a change in the detected belt tension in order to keep the belt tension on the exit side of the first drive pack above a predetermined minimum value .

15. The method as claimed in claim 14, wherein the traction force of the first drive pack is reduced in dependence on a detected drop in the belt tension on the exit side of the first drive pack below a predetermined value, which is greater than the predetermined minimum value .

16. The method as claimed in claim 14, wherein the traction force of the first drive pack is reduced in de- pendence on a detected drop in the belt tension on the entry side of the first drive pack below a predetermined value, which is greater than the predetermined minimum value .

17. The method as claimed in any one of claims 14-16, wherein the traction force of the first drive pack is increased in dependence on an increase in the detected belt tension in order to keep the belt tension on the exit side of the first drive pack below a predetermined maximum value .

18. The method as claimed in any one of claims 14-17, further comprising the steps of detecting the belt i tension in the conveyor belt adjacent to a plurality of drive packs and individually controlling the traction force of a drive pack in dependence on a change in the detected belt tension adjacent to the drive pack.