Overload protection for an agricultural machine

An agricultural machine, comprises a frame, two or more processing members, which are movably attached to the frame, and a drive device which is configured to drive the processing members. The drive device comprises a main drive shaft, two drive sections connected to the main drive shaft, which drive sections are each configured to drive one or more of the two or more processing members, wherein each drive section comprises a drive shaft. The drive device further comprises a protective device which enables a certain angular rotation between the two drive shafts in the case of an overload of one of the two drive shafts, and a coupling brake device which is configured to effect, at the angular rotation between the two drive shafts, a coupling between the other drive shaft and a part of the agricultural machine that is not co-rotatable with the two drive shafts.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Dutch application number 1035973 filed on 24 Sep. 2008, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to agricultural machines, in particular agricultural machines having driven processing members such as mowers, tedders or rotary harrows. The invention further relates to a method of overload protection for such machines.

2. Description of the Related Art

In general, in agriculture there is a trend to enlarge the machines in use in order thus to enable more efficient agricultural operation on a piece of land. With agricultural machines comprising rotating processing members this leads typically to an increasingly larger working width, and an increasing number of processing members being arranged in one row.

During processing ground or a crop which is lying on the ground it is possible that one of the processing members becomes stuck on an obstacle, for example a pole, tree or large stone. It will then suddenly no longer be possible for the processing member in question to rotate, while it is still being driven by the associated drive shaft. As a result thereof, the load which is exerted on the drive mechanism of the agricultural machine will suddenly increase quickly, resulting in a great risk of damage of the drive. In particular, there may be a great risk of the gear wheels of a transmission being damaged as a result of an overload.

It is known to place an overload protection on the main drive mechanism of a drive device in order to disconnect, in response to a suddenly increasing load, the drive mechanism from the driving source, for example the motor of a tractor, in order to limit the maximum load and thus to prevent damage of the drive mechanism.

However, in particular in the case of larger agricultural machines, in which a larger number of processing members should be driven simultaneously, the mass moment of inertia of the drive device and the processing members is accordingly greater. Suddenly stopping such a large mass which is interconnected via the drive mechanism may still provide a momentary load which may lead to damage of the drive mechanism. Moreover, an increased number of processing members also results in an increased total load which is required for driving all processing members. As a result, the components of the drive mechanism should be designed to be stronger.

In order to prevent the drive mechanism from being damaged, it could be possible, by means of an overload coupling, to disconnect only the blocked processing member from the drive mechanism at a too great load, so that the other processing members would be free to continue to rotate. However, this is only possible if the paths of movement of the processing members do not overlap. In a number of cases however, such as embodiments of mowers, tedders or rotary harrows, the paths of movement of the processing members overlap. For example, in the case of a specific type of tedder, each of the processing members may comprise arms with tines fastened thereto. The arms are arranged with respect to each other in such a manner that the arms of adjacent processing members rotate between each other. The processing members thus have overlapping paths of movement and should, therefore, be moved synchronously.

If, in such an agricultural machine, only the processing member which is suddenly blocked is disconnected, the arms of the adjacent processing member will run into the arms of the blocked processing member. This may lead to damage of the arms, for example serious deformation or breakage of the arms or the tines attached thereto. In such an agricultural machine, such a single disconnection of a single processing member is therefore unwanted.

EP 1 258 187 A2 discloses a drive mechanism for agricultural machines comprising a main drive shaft and two drive sections for driving a group of processing members. Each drive section comprises a drive shaft and an overload coupling which, in the case of an overload, falls back to a minimum torque value. The drive shafts are in line. A catching device is provided between the drive shafts. The catching device is a device to allow a maximum angular rotation between the two drive shafts and consequently the groups of processing members.

When one of the processing members is blocked by an obstacle, the group of processing members in question is completely blocked and disconnected from the drive mechanism by means of the overload coupling. However, the other group of processing members continues to rotate until the catching device does not allow further relative rotation because a maximum angular rotation has been reached. Consequently, the second group of processing members will also be blocked and the overload coupling present in this drive section will be disengaged. By using this drive mechanism, the processing members will not be stopped in one go, but in two phases.

A further advantage of the drive mechanism according to EP 1 258 187 A2 is that the load is distributed over the two drive sections, in which case there is provided for each drive section an overload coupling with a maximum load of 50% of the maximum load of the main drive shaft. However, as a result of the fact that the two drive shafts of the two drive sections are in operative connection with each other by means of the catching device, two peak loads will occur in the blocked drive section.

Thus, there is a particular need for an alternative drive mechanism for an agricultural machine, which reduces the risk of damage of the drive mechanism or processing members in the case of an overload of one or more of the processing members.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses these problems by providing an agricultural machine, comprising: a frame, two or more processing members, movably attached to the frame, and a drive device configured to drive the processing members, wherein the drive device comprises: a main drive, two drive sections connected to the main drive, each configured to drive one or more of the two or more processing members, wherein each drive section comprises a drive shaft, a protective device which enables a defined angular rotation between the two drive shafts in the case of an overload of one of the two drive shafts, and a coupling brake device configured to effect, at the defined angular rotation, a coupling between the other drive shaft and a part of the agricultural machine that is not co-rotatable with the two drive shafts.

The drive mechanism according to the invention enables an angular rotation between the two drive shafts in the case of an overload of one of the drive shafts, i.e. that the load becomes higher than a maximum operative load level. The load level at which this happens is usually higher than the normal operative load of the drive shaft in question. It is possible that during normal use of the agricultural machine there already occurs an angular rotation between the two drive shafts, for example with flexible coupling elements in the drive mechanism, but this angular rotation is usually smaller than the angular rotation which occurs in the case of overload.

Subsequently, in the case of overload, the angular rotation between the two drive shafts, which are preferably in line, is used to slow down the not overloaded drive shaft. This slowing down is effected by creating, at the defined angular rotation, a coupling between the non-overloaded drive shaft and a part of the agricultural machine that is not co-rotatable with the drive shafts, for example the frame or a housing of the drive mechanism mounted on the frame.

This coupling can be effected in the manner of a clutch or brake by pushing a friction surface which co-rotates which the drive shaft against a friction surface which does not co-rotate with the drive shaft in question. In an alternative embodiment, there can be effected a positively locked coupling between the non-overloaded drive shaft and the part of the agricultural machine which is not capable of co-rotating with the drive shaft. For this purpose, there is provided a mechanism which, at the angular rotation, in the case of overload, brings the drive shaft from a clearance position in which the drive shaft can rotate freely, into a blocking position in which the drive shaft is coupled with a part of the agricultural machine which is not capable of co-rotating with the drive shaft and, as a result thereof, can no longer rotate freely with respect to the frame of the agricultural machine.

Through this coupling, the drive shaft which, initially, is not overloaded will also have a load which is higher than the maximum load level of the overload coupling and, as a result thereof, said drive shaft will also disengage.

Such a drive mechanism has the advantage that only a peak load will be exerted on the blocked drive section. The second drive section is slowed down by action against the frame. There is thus effected a disengagement of the drive of both drive sections without the risk of damage of the processing members resulting from asynchronous movement of the processing members of the different drive sections.

The drive mechanism has the additional advantage that the maximum load in each drive section is determined by the drive section itself, because the drive sections are not interconnected by means of a catching device or the like. It is thus possible to select a maximum load for each drive section by means of the overload coupling of the drive section in question. The maximum load per drive section can therefore differ to a considerable extent.

The defined angular rotation required for slowing down can, for example, be obtained by using a flexible coupling in the partial drive of the overloaded drive shaft, or an overload coupling. A flexible coupling is a coupling which, depending on the load, allows an angular rotation, for example by placing a torsion element in the drive mechanism.

An overload coupling disengages at least partially when the load comes above a certain level. This is, for example, a coupling which, in the case of a load above the maximum load, continuous to transfer a limited moment. Alternatively it may be a coupling which completely disengages in the case of overload, for example a coupling comprising elements which break in the case of load above a maximum load, for example breaking bolts, a cam switch coupling or a ratchet slip coupling. The advantage of an overload coupling is that the maximum load can be set individually for each drive section, for example 60 percent of the maximum output of the main drive shaft. It is then also possible to provide the different drive sections with different maximum loads, because the drive sections are disengaged.

In one embodiment, the overload coupling comprises a coupling which transfers the maximum load in the case of a load which is higher than the maximum load. Such a coupling does not fall back to a limit moment in the case of an overload, but the transferred moment remains equal to the moment in the case of a maximum load. Such an embodiment has the advantage that, if the sum of the maximum loads of the overload couplings of the different drive sections is larger than the maximum load of the main overload coupling in the drive shaft, the main overload coupling will also disengage in the case of overload of one of the drive sections.

An example of such a coupling is a friction coupling. A friction coupling comprises two coupling halves which are contiguous to each other and which transfer a moment by means of friction between the two coupling halves. The moment to be transferred is limited to a maximum moment. If one of the coupling halves is driven at a higher load than the maximum load, the coupling halves will slip with respect to each other, in which case a moment is transferred which is equal to the maximum moment.

If a friction coupling is overloaded, it may be desirable to bring the drive mechanism back into the initial position. For this purpose, each drive section may comprise a reset device which is configured to disengage, if desired, the drive section. Said reset device may comprise a clearance coupling which enables to rotate a drive section, in the direction opposite to the drive device, to the initial position. As an alternative, the reset device may be a disengaging device of the friction coupling itself, for example a hydraulic disengagement. It should be noted here that such reset devices can also be applied for other types of slip couplings.

Another overload coupling which, at a load higher than the maximum load, transfers the maximum load, is an overrun coupling with pretension force. This overload coupling too comprises two coupling halves which rotate with respect to each other in the case of an overload and then transfer a moment which is equal to the moment in the case of maximum not disengaged load.

In one embodiment, the coupling brake device comprises: a first brake portion which is mounted, rotationally fixed and movably in axial direction, on an end of the first drive shaft, a second brake portion which is mounted rotationally fixed on an end of the second drive shaft, a brake mechanism between the first and the second brake portion, which, at an angular rotation equal to or greater than the minimum angular rotation, pushes the first and the second brake portion away from each other, and a first abutment face which is not co-rotatable with the two drive shafts and against which the first brake portion can abut as a result of the first and the second brake portion being pushed away from each other.

In such an embodiment, at the defined angular rotation of the first drive shaft with respect to the second drive shaft, as a result of overload, the first and the second brake portion will be pushed away from each other, so that the first brake portion will abut against the abutment face. As a result, the brake portion will be blocked with respect to the abutment face which is not co-rotatable with the drive shaft. By this coupling between the not blocked drive shaft and the abutment face the load in the not blocked drive section will increase and disengage the overload coupling of the second drive section and/or a main overload coupling, so that the second drive section too will no longer be driven. As a result, both groups of processing members will no longer be driven and come to a standstill without the risk of damage as a result of the asynchronous rotation of the two groups.

In one embodiment, the second brake portion is mounted movably in axial direction on the second drive shaft, and there is provided a second abutment face which is not co-rotatable with the two drive shafts, against which the second brake portion can abut when the first and the second brake portion are pushed away from each other. By providing two brake portions which are axially movable with respect to the respective drive shaft, the slowing down force, which is obtained by pushing the first and the second brake portion away from each other and thus pushing the first and the second brake portion against the abutment faces, can be increased.

In one embodiment, a friction element, for example an annular friction element, can be provided between the brake portions and the respective abutment face.

In certain embodiments, the brake mechanism comprises one or more balls which are each mounted in oppositely located ball tracks in the first brake portion and the second brake portion, wherein the ball tracks have a decreasing depth when viewed from a nominal or equilibrium angular position of the first brake portion or the second brake portion, respectively. The nominal angular position is the position of the drive shafts with respect to each other in the unloaded condition with no relative angular rotation between the two drive shafts.

In the equilibrium angular position, the depth of the ball tracks can be selected in such a manner that the ball present in the ball tracks has some clearance between the two brake portions. If angular rotation occurs between the first and the second brake portion, the first and the second brake portion will rotate with respect to each other. As a result, other parts of the ball track will be positioned opposite each other. Since the depth of the ball tracks from the nominal angular position decreases, the ball will be clamped between the two brake portions, and at further angular rotation, the ball will push the two brake portions away from each other. At the overload angular rotation, the ball pushes the two brake portions away from each other to such an extent that the brake portion in question will be pushed against the abutment face. As a result, a coupling is effected between the not overloaded drive shaft and a part of the agricultural machine, for example the frame, which is not co-rotatable with the two drive shafts. This coupling is maintained until the angular rotation between the two drive shafts is smaller than the overload angular rotation.

It should be noted that the ball can be positioned in the nominal angular position in a manner in which it is clamped between the two brake portions, in which case, however, the brake portions do not yet abut against the one or more abutment faces.

In one embodiment, each ball track in the upwardly tapering part thereof has an effective angle with respect to the plane perpendicular to the longitudinal centre line of the respective drive shaft which is smaller than 30 degrees, for example 25 degrees. In a further embodiment, the ball track has around the nominal position a substantially flat part over an angle of not more than 20 degrees, preferably not more than 10 degrees, with respect to the centre line of the respective drive shaft.

In another embodiment, each of the ball tracks is bowl shaped and each ball track extends over an angle of 65 to 85 degrees with respect to the centre line of the respective drive shaft. While the use of a ball and track arrangement may be a preferred manner of indicating a relative rotation between two drive shafts, it will be understood that other alternative indicators of relative movement may be provided as well as other alternative elements for initiating a braking force on the not overloaded drive shaft.

In certain embodiments, the brake device may comprise one or more pretensioning elements to pretension the first and the second brake portion towards each other. By means of such pretensioning elements, for example spring elements or rings of flexible material, it can be prevented that in normal operation the brake portions come into contact with the abutment faces and thus cause unintended braking or coupling.

Another aspect of the invention comprises a coupling brake device for an agricultural machine having a drive device, the coupling brake device being configured to effect, in the case of a defined relative angular rotation between two drive shafts of the drive device as a result of an overload of one of the two drive shafts, a coupling between the other drive shaft and a component which is not co-rotatable with the two drive shafts, in order thus to limit the angular rotation between the two drive shafts. The brake device may be as described above or may be any other suitable arrangement capable of exerting a brake force on one of the drive sections or shafts in response to a relative rotation between the two drive shafts. In particular, friction braking devices acting on any part of the drive between an overload protection and a processing member.

According to a yet further aspect of the invention there is provided a method of protecting a drive device for two or more overlapping groups of processing members of an agricultural machine, comprising: providing a main drive mechanism and at least two drive sections, wherein each drive section has a drive shaft for driving one or more of the two or more processing members; providing an overload protection in the first and the second drive sections; in the case of overload of the first drive shaft, enabling an angular rotation of the first drive shaft with respect to the second drive shaft, and applying a braking force to the second drive shaft in response to a defined angular rotation.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following is a description of certain embodiments of the invention, given by way of example only and with reference to the drawings.FIGS. 1 and 2show a tedder denoted as a whole by reference numeral1. The tedder comprises a frame2comprising a front end which is configured to be coupled to a pulling vehicle in order to be moved in the direction of travel R. Near the rear end of the frame2there is provided a wheel set3to support the frame2. At the rear end of the frame2there are mounted two carrier arms4which extend at opposite sides of the frame2.

Each of the carrier arms4supports eight rotatable processing members5. The processing members5comprise a number of arms6having a number of tines7. A drive mechanism8, shown in more detail inFIG. 3, is provided for driving the processing members5in a rotating manner. The carrier arms4are configured to be folded up to a transport position in which the carrier arms4with processing members5will be positioned on the supports9. There is provided a hydraulic circuit with actuators for folding in the carrier arms4to the transport position and folding them out to the operative position.

The drive mechanism8comprises a main drive shaft10which, at one end, is configured to be connected to a drive source, and which, at the other end, is connected to a gear box11. In the main drive shaft there is provided a main overload coupling30which maximises the drive capacity of the entire drive mechanism8. The gear box11is further operatively connected to two drive sections8a,8bwhich each comprise a drive shaft14a,14b. There are provided drive couplings12which, via a first transmission13, are connected to a drive shaft14a,14b. Each drive shaft14a,14bcomprises for each of the eight processing members5a second transmission15and a shaft16by means of which the processing member5in question is drivable in a rotating manner.

The processing members5rotate in overlapping paths of movement with respect to the adjacent processing members5. The processing members5are driven in turn in opposite directions of rotation so that the arms6with tines7of different processing members5does not prevent each other from rotating.

Despite the overlapping paths of movement the processing members5do have a free angle of rotation, i.e. an angle over which a processing member5can rotate with respect to an adjacent processing member5without abutting against the adjacent processing member5. Said free angle of rotation depends on the design of the processing members5. In an embodiment of a tedder as shown inFIGS. 1and2, this free angle of rotation of the processing members5is between 5 degrees and 20 degrees, for example 10 degrees, in both directions. In another embodiment, the processing members are of a rotary harrow, where the rotary harrow has two arms per processing member, and the free angle of rotation can be maximally 90 degrees in both directions.

Since the second transmission15has a speed reduction of approximately a factor three of the drive shaft14a,14bto the shaft16, a free angle of rotation of 10 degrees for the processing members5is a free angle of rotation of 30 degrees for the drive shafts14with respect to each other.

According to the invention it is possible to use this free rotation space to slow down a not overloaded drive section if the other drive section would be stopped as a result of overload. This makes it possible to prevent damage to the agricultural machine as a result of not synchronously moving processing members.

The drive device8comprises in each of the drive sections8a,8ban overload coupling17. The overload coupling17comprises a breaking bolt18which breaks in the case of a load greater than the maximally desired operative load, as a result of which the drive shaft14a,14bin question is disconnected from the main drive shaft10.

Each of the overload couplings17is set at a maximum operative load which is lower than the maximum load of the main overload coupling30in the main drive shaft10. For example, the maximum load of each of the overload couplings17can be 17.55% to 75% of the maximum load of the main overload coupling30in the main drive shaft10. This has the advantage that the maximum load of the components of the individual drive sections8a,8bis lower than the maximum load of the entire drive mechanism8.

The drive device8further comprises a coupling brake device19comprising a first brake portion19aand a second brake portion19b.

FIG. 4shows the coupling brake device19in more detail. The first brake portion19aand the second brake portion19bare each rotationally fixed by means of a key22, but mounted movably in axial direction on the first drive shaft14a, the second drive shaft14b, respectively. In each of the circular surfaces located towards each other of the first brake portion19aand the second brake portion19bthere are provided four bowl-shaped ball tracks20distributed over the circumference of the surfaces. The coupling brake device19further comprises four balls21which are provided in the oppositely located ball tracks20.

InFIGS. 5,6and7, the first brake portion19ais shown in more detail. The second brake portion19bis designed analogously.

With reference toFIG. 4, the two brake portions19a,19bare pretensioned towards each other by annular spring elements23. As a result, the two surfaces facing each other of the first brake portion19aand the second brake portion19bwill be closest possible to each other. In the nominal angular position of the first brake portion19aand the second brake portion19b, the bowl-shaped ball tracks20are located opposite each other and the distance between the bottoms of the ball tracks20is greater than the diameter of the ball21.

The brake portions19aand19beach have an annular abutment face24which is located opposite annular abutment faces which are provided on a housing26of the drive mechanism8. The housing26is rigidly connected to the frame2of the agricultural machine1and cannot rotate with the drive shafts14a,14b. Between the annular abutment faces24and25of the brake portions19a,19b, the housing26, respectively, there are provided friction rings27of friction material to improve the braking function between the abutment faces24and25. The friction rings27are loose elements. As an alternative, it is also possible to apply friction material on one of the abutment faces24,25or both of them.

At an angular rotation between the first drive shaft14aand the second drive shaft14band, consequently, between the first brake portion19aand the second brake portion19b, the bowl-shaped ball tracks20will move with respect to each other and the distance between the bottoms located opposite each other of the ball tracks20will decrease. As a result, the balls21will be clamped between the two brake portions19a,19band at a further angular rotation the two brake portions19a,19bwill be moved from each other by the balls21against pretension of the spring elements23. As a result, the abutment faces24,25will be pushed against the friction rings27so that there will be created a coupling between the brake portions19a,19band the housing26. This coupling will make it impossible for the brake portions19a,19bto rotate further with respect to each other.

An overload of the drive sections8a,8bis counterbalanced as follows by means of the above described drive device8.

At a simultaneous overload of both drive sections8a,8b, the main overload coupling30will disengage so that both drive sections8a,8bwill substantially synchronously come to a standstill.

If an overload occurs in one of the drive sections8a,8b, for example in the first drive section8a, the overload coupling17in the drive section8aconcerned will disengage by breakage of the breaking bolt18. As a result, the disengaged drive section8awill no longer be driven by the main drive shaft10, while the other drive section8bis still being driven. This results in an angular rotation between the first brake portion19aof the disengaged drive shaft14aand the second brake portion19bof the drive shaft14bwhich is still being driven. By the angular rotation the two brake portions19a,19bwill be pushed away from each other, as described above, so that each friction ring27will be clamped between the abutment faces24and25. This results in a coupling between the drive shaft14bof the second drive section8band the housing26, and the load of the second drive section8bwill increase until it reaches the maximum load of the overload coupling17of the second drive section8band disengages same by breaking the breaking bolt18. As a result, both drive sections8a,8bare disengaged from the main drive shaft10.

FIG. 8shows an alternative embodiment of a drive device118according to the invention.

Analogously to the drive device described above, the drive device108comprises a main drive shaft110which, at one end, is configured to be connected to a drive source, and which, at the other end, is connected to a gear box111. In the main drive shaft there is provided a main overload coupling130which maximises the drive capacity of the entire drive mechanism108. The gear box111is further operatively connected to two drive sections108a,108bwhich each comprise a drive shaft114a,114b. There are provided drive couplings112which, via a first transmission13, are connected to a drive shaft114a,114b. Each drive shaft114a,114bcomprises for each of the eight processing members5a second transmission115and a shaft116by means of which the processing member5in question is drivable in a rotating manner.

In order to effect, in the case of overload of one of the drive sections108a,108b, blockage of the other drive section108b,108a, there is provided a coupling brake device119according to the invention. Said coupling brake device119comprising brake portions119a,119band balls121is designed analogously to the above described coupling brake device19and will not be explained here in further detail.

In order to achieve the angular rotation required for the operation of the coupling brake device119, there is provided a flexible coupling150in each of the drive sections108a,108b. Such a flexible coupling150, for example a torsion element, allows an angular rotation in a drive mechanism as a result of the load in said drive section108a,108b. Consequently, also in the case of normal load, the flexible coupling150will allow an angular rotation in the drive section. However, when use is made of corresponding flexible couplings150in both drive sections108a,108b, and the load on these drive sections108a,108bis substantially the same, the same angular rotation will occur in both flexible couplings150. It is also possible that different loads will result in different angular rotations. The flexibility of the flexible couplings150is selected in such a manner that, in the case of a normal operative load, the angular rotation between the two drive shafts114a,114bwill not become so great that the processing members5of the different drive sections108a,108bcan touch each other.

However, in the case of overload of one of the drive sections108a,108b, the angular rotation between the first drive shaft14aand the second drive shaft14bdo differ substantially. This difference in angular rotation is used, according to the invention, to couple the drive shaft of the not overloaded drive section to the frame of the agricultural machine by means of the coupling brake device119.

Subsequently, the load in the drive section blocked by the coupling brake device119will increase. If the sum of the loads in both drive sections108a,108bbecomes greater than the maximum load of the main overload coupling130, the main overload coupling130will disengage, and the entire drive mechanism will no longer be driven.

It is possible to make the change in angular rotation of the flexible coupling dependent on the load level. In particular, it is possible to make the change in angular rotation small in the case of loads below the maximum load level of the drive section108a,108bin question, and to make the change in angular rotation great in the case of loads above the maximum load level of the drive section108a,108bin question. This makes it possible, when there is no overload, to prevent unintended coupling of the coupling brake device119, while, in the case of overload, the coupling brake device119will quickly couple the not overloaded drive section to the housing26as a result of the angular rotation.

For the protective device, as an alternative for the overload coupling17ofFIG. 3comprising breaking bolts18, or the flexible couplings150ofFIG. 8, it is also possible to use other couplings, depending on the desired characteristics of the drive device.

It is, for example, possible to provide in each of the drive sections a coupling which transfers the maximum load in the case of a load higher than the maximum load. Such a coupling does not fall back to a rest moment in the case of an overload, but the transferred moment remains equal to the moment in the case of a maximum load. In such an embodiment, if the sum of the maximum loads of the maximum loads of the overload couplings of the different drive sections is larger than the maximum load of the main overload coupling in the drive shaft, the main overload coupling will also disengage in the case of overload of one of the drive sections. In such a case, the overload couplings of the drive sections will be overloaded only for a very short period of time.

An example of such a coupling is a friction coupling. A friction coupling comprises two coupling halves which are contiguous to each other and which transfer a moment by means of friction between the two coupling halves. The moment to be transferred is limited to a maximum moment. If one of the coupling halves is driven at a higher load than the maximum load, the coupling halves will slip with respect to each other, in which case a moment is transferred which is equal to the maximum moment.

If a friction coupling is overloaded, it may be desirable to bring the drive mechanism back into the initial position. For this purpose, each drive section may comprise a reset device which is configured to disengage, if desired, the drive section. Said reset device may comprise a clearance coupling which enables to rotate a drive section, in the direction opposite to the drive device, to the initial position. As an alternative, the reset device may be a disengaging device of the friction coupling itself, for example a hydraulic disengagement. It should be noted here that such reset devices can also be applied for other types of slip couplings.

In one embodiment, it is possible that, in the case of disengagement of the drive sections, the latter automatically move to the normal initial position under the influence of the spring elements23which are mounted in the coupling brake device. Said spring elements23can move the brake portions19a,19bto the nominal positions by pushing the brake portions towards each other. By this pushing, the balls21will bring the brake portions19a,19bback to the nominal position.

Another overload coupling which, in the case of a load higher than the maximum load, transfers the maximum load, is an overrun coupling with pretension force. This overload coupling too comprises two coupling halves which rotate with respect to each other in the case of an overload and then transfer a moment which is equal to the moment in the case of maximum not disengaged load.

Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. Further modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.