WO2010092086A2

WO2010092086A2 - Apparatus for comminuting animal residues

Info

Publication number: WO2010092086A2
Application number: PCT/EP2010/051657
Authority: WO
Inventors: Gudmunder Orn Jensson
Original assignee: Atall Ehf.
Priority date: 2009-02-11
Filing date: 2010-02-10
Publication date: 2010-08-19
Also published as: GB0902257D0; WO2010092086A3; GB2467744A

Abstract

Apparatus for comminuting animal residues wherein front and rear clamping plates are provided which engage respective ends of the two fixed arrays of support plates, the connectors being received in the undercut portion of respective elongate recesses and passing through openings in the two clamping plates and being retained in position with respect to the front clamping plate by respective releasable fasteners, that actuating means act on the rear clamping plate and are selectively actuable to urge the rear clamping plate and thus the support plates towards the front clamping plate and that a removal device is provided which is selectively engageable in the elongate recess of at least one of the arrays of support plates and is operable to support the weight of the array of support plates and the associated fixed blades and to move them to a position remote from the associated cylindrical array of movable blades.

Description

APPARATUS FOR COMMINUTING ANIMAL RESIDUES

The present invention relates to apparatus for comminuting animal residues and is concerned with that type of such apparatus which includes two cylindrical arrays of movable blades mounted on respective rotary shafts, which are connected to respective motors arranged to rotate the cylindrical arrays of blades about the axes of the shafts, each cylindrical array comprising a plurality of arcuate blades spaced apart in the direction of the axis of the associated rotary shaft, two spaced arrays of fixed arcuate blades carried by respective support plates, the support plates being arranged in two fixed arrays, the fixed and movable blades being so arranged that, as the cylindrical arrays of blades are rotated, the blades of each cylindrical array pass through the gaps between the blades of the other cylindrical array and through the gaps between the lower portions of the fixed blades of a respective one of the arrays of fixed blades and are spaced from the upper portions of the fixed blades by a distance which progressively increases in the upward direction, the support plates of each fixed array having an undercut recess remote from the fixed blades, the recesses of each fixed array being aligned and together defining an elongate recess, the support plates of each fixed array being fastened together by a respective connector.

After processing animal carcasses to remove meat for consumption by human or animals, there is a considerable volume of matter which is not required or inedible including fat, tendons, bones and the like. It is usual for these residues to be ground or comminuted to enable the ready removal of useful substances, such as oils and greases and to facilitate the ultimate disposal of the remaining matter.

Comminution apparatus of the type referred to above is disclosed in PCT/IS2006/000022. Apparatus of this type is found to be highly advantageous compared to the previously known devices which use, for instance, meshing screws or flailing weights because the earlier devices tended to pulverize the residues rather than comminute them and it is now appreciated that this results in destruction of the cellular structure of the residues which means that it is then more difficult for the fats in the residues to be removed by solvents. It is also found that the energy consumption of comminution apparatus of the type disclosed in the prior document has an energy consumption up to 90% less than that of the previously known devices.

However, the apparatus disclosed in the prior document does need to be cleaned on a regular basis and this requires disassembly of the apparatus to allow access to both the movable and fixed blades. Furthermore, it is not uncommon for the apparatus to become jammed due, for instance, to a large bone or a foreign object such as a horse shoe entering one of the downwardly convergent gaps between the rotating movable blades and the upper portion of one or other array of fixed blades. This can result in jamming of the machine which can only be rectified by largely disassembling the machine and then reassembling it.

However, disassembly and subsequent reassembly of a machine can be a very time-consuming task and this can therefore result in a significant reduction in the processing efficiency of the machine.

It is therefore the object of the present invention to provide apparatus for comminuting animal residues of the type referred to above which may be more easily and rapidly disassembled and reassembled than known machines to facilitate cleaning or maintenance and which therefore has a higher processing efficiency.

In accordance with the present invention, apparatus of the type referred to above is characterised in that front and rear clamping plates are provided which engage respective ends of the two fixed arrays of support plates, the connectors being received in the undercut portion of respective elongate recesses and passing through openings in the two clamping plates and being retained in position with respect to the front clamping plate by respective releasable fasteners, that actuating means act on the rear clamping plate and are selectively actuable to urge the rear clamping plate and thus the support plates towards the front clamping plate and that a removal device is provided which is selectively engageable in the elongate recess of at least one of the arrays of support plates and is operable to support the weight of the array of support plates and the associated fixed blades and to move them to a position remote from the associated cylindrical array of movable blades.

Thus in the apparatus in accordance with the present invention the arrays of support plates and thus the arrays of fixed blades are connected together and are urged towards one another by front and rear clamping plates. The support plates are connected by connectors which are received in the undercut portion of the elongate recesses and pass through openings in the two clamping plates and are retained in position with respect to the front clamping plate by releasable fasteners. Actuating means act on the rear clamping plate and are actuable to urge the rear clamping plate towards the front clamping plate. A removal device is provided which is selectively engageable in the elongate recess of at least one of the arrays of support plates and is operable to support the weight of the array of support plates and the associated fixed blades and to move them to a position remote from the associated cylindrical array of movable blades. Thus if it should be desired to clean the machine or to gain access to the sets of blades, for instance to replace a blade or in the event that jamming has occurred, the releasable fasteners are removed and the connectors are then retracted from the undercut recesses. The removal device is then engaged in the elongate recess of one of the arrays of support plates and is operated to support the weight of the support plates and then to move them to a position remote from the associated cylindrical array of movable blades. Access may therefore readily be had to the movable and fixed blades in order, for instance, to remove an article that has become jammed between them or to replace one or more of the blades.

A removal device may be connected to the machine adjacent each array of support plates and in this event the machine will of course have two removal devices. This is, however, not essential and it is also possible that only a single removal device is provided which is selectively connectable to the machine at two different positions, that is to say at positions which are adjacent the two arrays of support plates, respectively. The removal device may therefore be connected to one side of the machine and used to remove one set of support plates and it may then be disconnected from the machine and reconnected to it on the other side in order to remove the other array of support plates.

The recess in each support plate and thus in each array of support plates may have only a single undercut portion and in this event the undercut portion will be along the upper edge of the recess. It is, however, preferred that the recess in each support plate has two opposed undercut portions and that each connector is of elongate shape and received in the two undercuts and that the actuating means acts between one end of each connector and the rear clamping plate. The connectors are therefore securely located with respect to the arrays of support plates by virtue of being at least partially retained in the two undercuts and the actuating means, which urges the rear clamping plate towards the front clamping plate, acts between the connector and the rear clamping plate. It is convenient if each connector is of generally rectangular annular shape.

In order to enable the connectors to be removable from the undercut recesses, it is necessary that their connection to the front clamping plate is removable and in one simple embodiment each releasable fastener comprises a jamming member which extends through the associated annular connector and engages the surface of the front clamping plate.

It is preferred that each cylindrical array of movable blades comprises a plurality of separate blades which are separately removably connected to the exterior of a hollow drum connected to a respective rotary shaft. This means that if one of the movable blades should break or be otherwise damaged, it may be removed and replaced individually and it is not necessary to replace all of the blades in an array. The connection of the movable blades to a hollow drum means that if the blades are connected to the drum by a connector situated in the interior of the drum, access may readily be had to that connector through an open end of the drum.

In order to facilitate ready access to the movable blades, it is preferred that the front clamping plate is removably connected to a support frame of the machine. The apparatus also preferably includes a separate motor support, which is spaced from the rear clamping plate and on which the two motors are substantially supported. The weight of the two arrays of movable blades and the drums to which they are connected is also preferably substantially supported by that support. This will mean that the front clamping plate plays little or no part in supporting the weight of the two drums and the movable arrays of blades. Thus it is preferred that the weight of each motor and the associated gearbox or transmission system, if provided, is substantially the same as that of the drive shaft, drum and movable blades connected to it so that the whole assembly is balanced on the motor support, when the front clamping plate is removed.

It is, however, preferred that the two drums are positionally located at both ends and to this end it is preferred that each drum has a collar at its front end and that the drum is positionally located by the engagement of this collar with location means connected to the front clamping plate. In the preferred embodiment, the front clamping plate has apertures in it, around which locating collars are secured, each locating collar carrying a plurality of rotatable rollers which engage and locate the collar on a respective drum.

The weight of each motor is preferably transmitted to the support by a connection at the upper end of the motor but it is preferred that the motors are also connected to the support at their lower end by a resilient connection which permits limited relative movement in rotation. Thus it will be appreciated that if one or other drum should become jammed, the torque exerted by the motor will tend to rotate it about its connection with the support and such rotation is permitted by the resilient connections, though only to a limited extent.

The two motors will in practice be connected to a control system and this control system will in practice be programmed to cause the two motors to rotate in opposite directions such that the two arrays of movable blades move upwardly in the region in which they pass through the gaps between the other array of movable blades and thus move downwardly in the region in which they pass between the gaps between the associated fixed blades. This means that when animal residues are placed on top of the two arrays of movable blades, the residues will be carried outwardly and into the gaps of downwardly decreasing width defined between the movable blades and the upper portions of the fixed blades. If a very large bone or a foreign object such as a horse shoe should be present in the residue, this will enter one or other of the gaps of downwardly decreasing width and cause the associated array of movable blades to become jammed. The preferred embodiment of the apparatus in accordance with the invention includes a torque sensor associated with each motor and connected to a control system, the control system being programmed to cause the two arrays of cylindrical blades normally to rotate such that the movable blades move upwardly in the region in which each array of blades moves between the gaps between the other array of blades and the control system is programmed also to reverse the direction of rotation of each of the cylindrical arrays of blades on receipt of a signal from the associated torque sensor indicating that the torque produced by the associated motor has exceeded a predetermined threshold value. Thus if one of the arrays of movable blades should become jammed, this results in an increase in the torque exerted on it by the associated motor and this increase is sensed by the associated torque sensor. On receipt of this signal, the control system causes the cylindrical array of blades in question to rotate in the opposite direction for a limited period of time.

As mentioned above, if one of the arrays of movable blades should become jammed, the increased torque exerted by the associated motor will result in practice in the motor in question rotating through a small angle against the resilience of its resilient connection. The control system is preferably programmed, on receipt of the said signal, to stop the associated motor and this will enable the motor to return to its normal position under the action of the force exerted on it by the deformed resilient mounting. This slight rotary movement of the motor and thus the drum connected to it may of itself result in the unjamming of the drum and this will mean that when the motor is reactivated, the drum will not have to start from a jammed position. The control system is preferably programmed then to cause the motor to rotate in the opposite direction after a predetermined period of time has elapsed. The control system is preferably programmed also to cause the associated motor to rotate in the opposite direction at a speed which is less than that of the other motor. This will mean that the object which has become caught in a gap of downwardly decreasing width and caused the associated array of movable blades to become jammed will then be caused to move out of the gap and transferred to the other array of movable blades. It will then be transported into the other gap of downwardly decreasing width defined by the other array of movable blades and the set of fixed blades associated with it. If those cooperating blades are again unable to comminute the object in question, jamming will again occur and the direction of that set of movable blades will then also be reversed. The article which caused the jamming will then be transferred back to the first set of movable blades, which has by this time recommenced rotation in the original direction. An object such as a bone may thus be transferred from one set of movable blades to the other on several occasions. It may be that it will be comminuted on the second, third or even fourth attempt and in this event all is well and operation of the machine will continue. If, however, the object in question is, for instance, a horse shoe which can of course not be comminuted, there is a risk that it would be transferred from one set of movable blades to the other and then back again indefinitely. It is therefore preferred that the control system is programmed to produce an alarm signal if the direction of rotation of one of the cylindrical arrays of blades has reversed more than a predetermined number of times, for instance five times or even ten times. Once such an alarm signal is produced, e.g. audibly, it will be necessary for an operator to terminate operation of the machine and then to at least partially disassemble it to enable him to remove the article in question manually.

Further features and details of the invention will be apparent from the following description of one specific embodiment which is given by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a view of the front end of apparatus for comminuting animal residues in accordance with the present invention;

Figure 2 is a perspective view of the apparatus of Figure 1 ;

Figure 3 is a side view of a single spacer plate and a single support plate with a fixed blade connected to it;

Figure 4 is an end view of two support plates carrying fixed blades and spacer plates on each side of the support plates; Figure 5 is a perspective exploded view of a portion of one of the fixed arrays of support plates and associated fixed blades and spacer plates;

Figure 6 is a perspective view of only the two arrays of support plates and associated fixed blades and the two rotatable drums carrying respective arrays of movable blades;

Figure 7 is an axial sectional view of one of the drums and a portion of the front clamping plate;

Figure 8 is a perspective view from below of the two motors and their associated gearboxes connected to the motor support;

Figure 9 is a horizontal sectional view of a single motor and the associated gearbox, bearing housing, drive shaft and rotatable drum;

Figure 10 is a perspective view of the apparatus showing a first stage in the removal of one of the arrays of support plates and associated fixed blades;

Figure 11 is a plan view of the apparatus showing the two fixed arrays of support plates and associated fixed blades in the removed state; and

Figure 12 is a perspective view of the apparatus in the state shown in Figure 11.

The apparatus comprises a support frame 10, removably connected to which by fasteners 12 is a front clamping plate 14. Also supported by the support frame 10 but movable with respect to it is a rear clamping plate 16, which is parallel to the front clamping plate 14 but spaced from it. Spaced yet further from the front clamping plate 14 beyond the rear clamping plate 16 is a motor support 18. Removably supported by the support frame 10 are two spaced arrays of support plates 4, which may be seen in Figures 4 and 5. Each adjacent pair of support plates 4 is spaced apart by a similarly shaped plate 2, which constitutes a spacer plate. Each support plate 4 and spacer plate 2 affords an arcuate surface on one side which is not of circular shape but is of increasing radius of curvature in the upward direction. On the other side of each plate 2, 4 it affords a rectangular recess 20, at each end of which there is an undercut portion 22. Each recess 20 may therefore be considered to have two different widths, the width at the bottom of the recess being greater than that at the top, that is to say at its opening to the exterior. Connected to the arcuate surface of each support plate 4 is an arcuate fixed blade 5 of corresponding shape. The blades are connected in position to the plates 4 by pegs 1 which pass through openings in the plates. Received in a recess in the arcuate surface of each spacer plate 2 is a tooth or blade 3, which projects beyond the arcuate surface. As mentioned above, the plates 4 are spaced apart by spacer plates 2 and there is thus a gap between each adjacent pair of stationary blades 5.

Received in the space between the two arrays of support plates 4 are two rotatable drums 24. Each drum 24 is hollow and connected to its surface is an array of movable blades 26. Each blade 26 has a circular external surface and extends over only a short distance in the circumferential direction. The blades 26 are arranged in a number of sub-arrays, each of which is of helical shape and consists of a number of elongate blades extending in the circumferential direction and spaced apart in the axial direction. The fixed blades and movable blades are arranged so that when the drums are rotated, the movable blades on each drum pass through the gaps between the movable blades on the other drum and through the gaps between the lower portions of the fixed blades of an associated one of the two arrays of fixed blades. The non-circular shape of the fixed blades is such, however, that at their upper portions they define with the movable blades a gap whose width increases in the upward direction.

At its front end, each drum 24 has an annular collar 28. This collar is received within a respective annular retaining collar 30 integral with the front clamping plate 14. The retaining collar 30 carries a plurality of rotatable retaining rollers 32, whose surface is in rolling contact with the internal surface of the collar 28. The drums 24 can therefore rotate with respect to the collars 30 but are positionally located by them.

The rear ends of the two drums are connected to respective drive flanges 34 to be rotated thereby. The drive flanges 34 are connected to respective drive shafts 36, which are also connected to the outputs of respective gearboxes 38. The drive shafts are supported by taper roller bearings 37 within a bearing housing 39. The gearboxes 38 are suspended from a motor support 40 and their inputs are connected to respective electric motors 42. The drive shafts 36 are of massive construction and are supported by the motor support 40, whereby the drums 24 are substantially supported by the motor support 40 in cantilever fashion. The gearboxes 38 and motors 42 are therefore suspended at their upper end from the motor support 40 but the motors 42 are also connected, in this case via portions of the gearboxes 38, to the motor support 40 by means of respective bolts or the like 44 which pass through openings in portions of the gearboxes 38 and through resilient bushes 46 and through holes in brackets 48 connected to the motor support 40. The resilient bushes 46 therefore permit limited rotation of the motors and gearboxes relative to the motor support 40.

Each array of support plates 4 and the associated spacer plates 2 are connected together by a respective connector 50 of rectangular annular shape, the elongate limbs of which are received in the undercuts 22 in the elongate recesses 20. Each elongate connector 50 passes through a rectangular opening 52 in the front clamping plate 14 and is releasably connected to it by a jamming member in the form of a small plate 54 which is positioned within the aperture defined by the annulus and engages the front surface of the plate 14. The connector 50 also passes through a rectangular aperture in the rear clamping plate 16 and through a further aperture 56 in the motor support 40. An actuator constituted by a hydraulic cylinder 52 acts between the rear end of the connector 50 and the opposed rear surface of the rear clamping plate 16. When the actuator 52 is actuated, it urges the rear clamping plate 16 towards the front clamping plate 14 and thus securely clamps the support plates against one another between the front and rear clamping plates.

Connected to each side of the support frame 14 adjacent a respective array of support plates 4 is a removal device 60. The removal device 60 constitutes two parallel hydraulic actuators 64 connected to move an upstanding hydraulic actuator 66 in the horizontal direction. Connected to the upper end of the hydraulic actuator 66 is a rectangular plate 68 whose upper surface carries an upstanding ledge 70, whose height and width correspond to those of the upper undercut 22 in the elongate recess in the outer side surface of the associated array of support plates. If it should be desired to gain access to the interior of the machine, the hydraulic actuators 52 are deactivated, thereby removing the clamping pressure from the arrays of support plates. The jamming members 54 are then removed and the connectors 50 together with the actuators 52 are then slid out rearwardly. The front clamping plate 14 is then disconnected from the support frame and then moved in the direction of the axes to disengage the collars 28 from the retaining collars 30. The drums 24 remain in position since they are supported in the manner of cantilevers. The actuators 64 are then actuated to move the hydraulic actuator 66 and support member 68 from the position shown in Figures 1 and 2 to a position in which the support plate 68 is within the recess 20. The actuator 66 is then actuated to move the upstanding ledge 70 into the upper undercut 22 and the removal device supports the weight of the support plates. This is the position shown in Figure 10. The actuators 64 are then actuated in the opposite direction to move the actuator 66 and the support plates 4 supported by it outwardly away from the machine. The actuator 66 may then be rotated through 90° to the position shown in Figures 1 1 and 12 to make the fixed blades 5 readily accessible. If any of the fixed blades 5 needs to be replaced, this may now be achieved relatively simply and rapidly and they may of course also be cleaned, e.g. with a high pressure hose and/or a stiff brush. The movable teeth may also be similarly cleaned and if any of them is damaged its connection to the associated drum may be released from the interior of the drum and a new blade connected at its position. The above procedure is of course reversed to reassemble the machine.

The two motors 42 are connected to be controlled by a control system and the control system is programmed so that, in normal operation, the two drums are rotated in the opposite direction with those portions of the drums which are directed towards one another moving in the upward direction. Expressed in other words, the drum seen on the left in Figure 1 will rotate anticlockwise whilst that seen on the right will rotate clockwise. Animal residues to be comminuted are introduced manually or mechanically into the machine through the gap defined by the upper surfaces of the two fixed arrays of support plates. The direction of rotation of the two arrays of movable blades result in the residues being carried outwardly in both directions into the gaps of reducing width defined between the two arrays of movable blades and the associated arrays of fixed blades. The residues are then comminuted, that is to say cut into small pieces, by the cooperation of these blades and the comminuted residues fall out downwardly through the support frame 10. If an object which cannot be readily comminuted is introduced into the machine, it will be carried by one or other rotating array of movable blades into the gap which they define with the opposed array of fixed blades and that array of movable blades will then become jammed. As jamming occurs, the torque exerted by the associated motor will rise rapidly and this increase in torque is detected by one of the two torque sensors which are associated with the two motors. The increased torque exerted by the motor in question will result in that motor and its associated gearbox rotating slightly about its upper mounting on the motor support 40 against the resilience of the associated resilient sleeve 46. The torque sensor sends a signal to the controller indicating the presence of a torque above a predetermined threshold and the controller is so programmed that it then stops the motor in question, though the other motor continues its rotation in the same direction and at the same speed. After a brief period of time of, perhaps, one second has elapsed and the motor which has stopped has returned to its normal operating position under the restoring force exerted on it by the associated resilient sleeve 46, the controller then causes the stationary motor to rotate again but in the opposite direction and at a reduced speed. This reverse rotation will cause the object responsible for the jamming to move back out of the gap between the arrays of fixed and movable blades and it will then be picked up by the other rotating array of movable blades and carried into the gap on the other side of the machine. If the object in question is then comminuted, all is well and operation of the machine continues. If, however, the other array of movable blades should now jam, the same procedure is repeated, that is to say the array of movable blades in question is stopped and, after a short period of time, is caused to move in the opposite direction at a reduced speed. The object which caused the jamming is then picked up by the other array of movable blades, which in the meantime has recommenced rotation at its original speed and in its original direction, and the process is repeated. The object may be, for instance, a large bone and in this event it may be comminuted on, say, the third or fourth attempt. If, however, comminution has not occurred after a predetermined number of attempts, say five or even ten attempts, this is an indication that it cannot be comminuted and that it may be an object such as a horse shoe. The controller is therefore programmed to issue an alarm signal, e.g. in audible or visible form, after a predetermined number of direction reversals has taken place in order to alert the operator to the fact that attention is required. It is then necessary for the operator to terminate operation of the machine and to remove the object in question, either manually through the opening at the top of the machine or, if necessary, after partial disassembly of the machine, as described above.

Claims

1. Apparatus for comminuting animal residues including two cylindrical arrays of movable blades mounted on respective rotary shafts, which are connected to respective motors arranged to rotate the cylindrical arrays of blades about the axes of the shafts, each cylindrical array comprising a plurality of arcuate blades spaced apart in the direction of the axis of the associated rotary shaft, two spaced arrays of fixed arcuate blades carried by respective support plates, the support plates being arranged in two fixed arrays, the fixed and movable blades being so arranged that, as the cylindrical arrays of blades are rotated, the blades of each cylindrical array pass through the gaps between the blades of the other cylindrical array and through the gaps between the lower portions of the fixed blades of a respective one of the arrays of fixed blades and are spaced from the upper portions of the fixed blades by a distance which progressively increases in the upward direction, the support plates of each fixed array having an undercut recess remote from the fixed blades, the recesses of each fixed array being aligned and together defining an elongate recess, the support plates of each fixed array being fastened together by a respective connector, characterised in that front and rear damping plates are provided which engage respective ends of the two fixed arrays of support plates, the connectors being received in the undercut portion of respective elongate recesses and passing through openings in the two clamping plates and being retained in position with respect to the front clamping plate by respective releasable fasteners, that actuating means act on the rear clamping plate and are selectively actuable to urge the rear clamping plate and thus the support plates towards the front clamping plate and that a removal device is provided which is selectively engageable in the elongate recess of at least one of the arrays of support plates and is operable to support the weight of the array of support plates and the associated fixed blades and to move them to a position remote from the associated cylindrical array of movable blades.

2. Apparatus as claimed in Claim 1 including a support frame on which the arrays of fixed and movable blades are supported, wherein a single removal device is provided which is removably connectable to the frame adjacent each of the arrays of support plates.

3. Apparatus as claimed in Claim 1 or 2 in which the recess in each support plate has two opposed undercut portions, each connector being of elongate shape and received in the undercuts, the actuating means acting between one end of each connector and the rear clamping plate.

4. Apparatus as claimed in Claim 3 in which each connector is of generally rectangular annular shape.

5. Apparatus as claimed in Claim 4 in which each releasable fastener comprises a jamming member which extends through the associated annular connector and engages the surface of the front clamping plate.

6. Apparatus as claimed in any one of the preceding claims in which each cylindrical array of movable blades comprises a plurality of separate blades separately removably connected to the exterior of a hollow drum connected to a respective rotary shaft.

7. Apparatus as claimed in Claim 2 in which the front clamping plate is removably connected to the support frame.

8. Apparatus as claimed in any one of the preceding claims in which the two motors are substantially supported on a support spaced from the rear end plate.

9. Apparatus as claimed in Claims 6 to 8 in which each drum has a collar at one end and the drum is positionally located by the engagement of the collar with location means connected to the front clamping plate.

10. Apparatus as claimed in Claim 9 in which the front clamping plate has apertures in it, in which locating collars are secured, each locating collar carrying a plurality of rotatable rollers which engage and locate the collar on a respective drum.

11. Apparatus as claimed in Claim 8 in which the two motors are connected to the support plate at their lower end by a resilient connection which permits limited relative movement in rotation.

12. Apparatus as claimed in any one of the preceding claims including a torque sensor associated with each motor and connected to a control system, the control system being programmed to cause the two arrays of cylindrical blades normally to rotate such that the movable blades move upwardly in the region in which each array of blades moves between the gaps between the other array of blades and the control system being programmed also to reverse the direction of rotation of each of the cylindrical arrays of blades on receipt of a signal from the associated torque sensor indicating that the torque produced by the associated motor has exceeded a predetermined threshold value.

13. Apparatus as claimed in Claim 12 in which the control system is programmed, on receipt of the said signal, to stop the associated motor, and to cause it to stop the associated motor and then to rotate in the opposite direction after a predetermined period of time has elapsed.

14. Apparatus as claimed in Claim 3 in which the control system is programmed to cause the associated motor to rotate in the opposite direction at speed which is less than that of the other motor.

15. Apparatus as claimed in Claim 12, 13 or 14 in which the control system is programmed to produce an alarm signal if the direction of rotation of one of the cylindrical arrays of blades has reversed more than a predetermined number of times.