Background of the Invention Canned or bottled products are typically packaged in cartons, shrink wrap plastic or similar pacages and stacked in rectanaular layers on pallets for transport, warehouse storage and supply. If a wholesale customer orders the product in full pallet loads, the product can be supplied to the customer on the pallets. If. however, the customer orders the product by layer such that only a partial pallet (or a number of full pallets plus a partial pallet) is required, it is necessary to remove the necessary layer (s) of product from a pallet of the product. This depalletising process is often carried out manually, removing a carton of product at a time. at great labour and time costs.

A hydraulic device has been proposed to remove the product from the pallet in layers by gripping opposing sides of a product layer and lifting the layer from the pallet. The device is attached to and operated from a fork lift truck, and has opposing arms hinged about axes parallel to and above the sides of the pallet. The lower ends of the arms are provided with a rubber gripping plate which engages the sides of the product layer when the arms are pivoted inwardly. As the hinge points are fixed, the angle of the gripping plate to the product layer side varies depending on the width of the product layer and the positioning of the device with respect to the pallet. Whilst pallets are typically a standard 1165 mm by 1165 mm, various stacking patterns for different products provide different product layer widths, some overhanging the edge of pallet and others being narrower than the pallet. Upper or lower edges of the gripping plates can hence damage the product depending on the angle at which the gripping plates engage the product, and the ability to successfully lift the product without dropping the same is limited.

Object of the Invention It is the object of the present invention to overcome or substantially ameliorate at least some of the above disadvantages.

Summary of the Invention In one broad form the present invention provides a palletiser and depalletiser apparatus for respectively adding or removing product, stackable in layers on a pallet, to or from the pallet by layer, the apparatus comprising: a first gripper assembly having a first gripper surface for gripping a first side of a product layer, said first gripper surface lying in a first gripper plane with a horizontally disposed lateral axis, a second gripper assembly opposing said first gripper assembly, said second gripper assembly having a second gripper surface for gripping a second side of said product layer opposing said first side, said second gripper surface lying in a second gripper plane with a horizontally disposed lateral axis parallel to said first gripper plane lateral axis, a third gripper assembly having a third gripper surface for gripping a third side of said product layer, said third gripper surface lying in a third gripper plane with a horizontally disposed lateral axis perpendicular to said first and second gripper plane lateral axes, a fourth gripper assembly opposing said third gripper assembly, said fourth gripper assembly having a fourth gripper surface for gripping a fourth side of said product layer opposing said third side, said fourth gripper surface lying in a fourth gripper plane with a horizontally disposed lateral axis parallel to said third gripper plane lateral axis, a frame on which said first, second, third and fourth gripper assemblies are each mounted for horizontal displacement in a direction of travel perpendicular to its respective said gripper plane lateral axis, means for said horizontal displacement of each of said gripper assembly, and means for vertical displacement of said frame.

Typically said first, second, third and fourth gripper planes are each inclined at an angle of inclination of 2 to 6 degrees from vertical, said first and second gripper planes being inclined such that they downwardly converge and said third and fourth gripper planes being inclined such that they downwardly converge.

Preferably said angle of inclination of each of said first, second, third and fourth gripper planes is approximately 4 degrees.

Preferably said angle of inclination of each of said first, second, third and fourth gripper planes is adjustable.

Preferably said means for horizontal displacement of each of said gripper assemblies comprises a first means for displacing said first and second gripper assemblies by equal and opposite amounts, and a second means for displacing said third and fourth gripper assemblies by equal and opposite amounts.

Preferably said first and second means for horizontal displacement are each rack and pinion arrangements driven by a motor, said first and second gripper assemblies respectively having first and second opposing racks driven by a first pinion coupled to a first motor, said third and fourth gripper assemblies respectively having third and fourth opposing racks driven by a second pinion coupled to a second motor.

Preferably said first, second, third and fourth gripper assemblies each comprise: a gripper assembly body, a gripper element mounting member attached to said gripper assembly body and having a gripper element mounting face parallel to the respective said gripper plane, at least one gripper element mounted on said gripper element mounting face and forming the respective said gripper surface an arm including the respective said rack, and a pair of wheels engaging said frame for supporting and guiding the respective said gripper assembly during said horizontal displacement thereof.

Preferably said frame includes: a first rail extending along an axis parallel to said direction of travel of said first and second gripper assemblies, said first rail supporting said first and second gripper assembly wheels, and a second rail intersecting said first hollow beams and extending along an axis parallel to said direction of travel of said third and fourth gripper assemblies, said second rail supporting said third and fourth gripper assembly wheels.

Preferably said first rail is in the form of a first hollow beam housing said first and second gripper assembly wheels, said first and second gripper assembly racks and said first pinion, said second rail being in the form of a second hollow beam housing

said third and fourth gripper assembly wheels said third and fourth gripper assembly racks and said second pinion.

Preferably a plurality of gripper elements is mounted on each said gripper element mounting face, each said gripper element being in the form of a rubber block.

Preferably each said rubber block has parallel front and rear faces and side faces perpendicular to said front and rear faces, said front face forming part of said gripper surface and being defined by a substantially continuous front wall, said side faces being substantially continuous and defined by side walls, a honeycomb type arrangement of holes extending from said rear face to said front wall between said side walls so as to define a grid arrangement of webs extending from said front wall to said rear face.

Preferably each said rubber block is formed of 30 to 40 durometer rubber.

Preferably said front wall is approximately 7 mm thick.

Preferably each said rubber block has a depth of approximately 80 mm as measured between said front and rear faces.

Preferably each said web tapers in thickness toward said rear face.

Preferably each said rubber block is mounted on a said gripper mounting face by fasteners.

In another broad form the present invention provides a gripper element for gripping a side of a layer of product stacked on a pallet as defined above.

Brief Description of the Drawings Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a plan view of a gripper assembly arrangement.

Figure 2 is a plan view of the gripper assembly arrangement of Figure 2 with the second and fourth gripper assemblies retracted.

Figure 3 is a side elevation view of one form of the first gripper assembly of the arrangement of Figure 1.

Figure 4 is a cross sectional view of the first gripper assembly of Figure 3 at section 4-4 and including a portion of a frame.

Figure 5 is a cross sectional view of the first gripper assembly of Figure 3 at section 5-5.

Figure 6 is a rear elevation view of the first gripper assembly of Figure 3 and including a portion of the frame.

Figure 7 is a side elevation view of one form of the second gripper assembly of the arrangement of Figure 1.

Figure 8 is a side elevation view of one form of the third gripper assembly of the arrangement of Figure 1.

Figure 9 is a side elevation view of one form of the fourth gripper assembly of the arrangement of Figure 1.

Figure 10 is a plan view of one form of a frame.

Figure 11 is a front elevation view of the frame of Figure 10.

Figure 12 is a schematic front elevation view of a product layer gripper by a pair of gripper assemblies.

Figure 13 is a front elevation view of the frame of Figure 10 mounted on a support structure in turn mounted on a rail system.

Figure 14 is a front elevation view of one form of the third/fourth gripper mounting face with gripper elements mounted thereon.

Figure 15 is a front elevation view of one form of the first/second gripper mounting face with gripper elements mounted thereon.

Figure 16 is a rear elevation view of a gripper element.

Figure 17 is a cross sectional view of the gripper element of Figure 16 taken at section 17-17.

Figure 18 is a side elevation view of the gripper assembly of Figure 16.

Figure 19 is a side elevation view of another form of the first gripper assembly.

Figure 20 is a side elevation view of another form of the third gripper assembly.

Figure 21 is a front elevation view of another form of the third/fourth gripper mounting face with gripper elements mounted thereon.

Figure 22 is a front elevation view of another form of the first/second gripper mounting face with gripper elements mounted thereon.

Figure 23 is a partial plan view of another form of the frame.

Detailed Description of the Preferred Embodiments The preferred embodiment provides a palletiser and depalletiser apparatus for a pallet having product, such as cartonised beverage cans or bottles, stackable on the pallet in layers. When the apparatus is operated as a depalletiser it is able to remove the product by layer and will be described in detail with reference to this application.

However, it will be appreciated that when the apparatus is operated in the reverse fashion to that described below it functions as a palletiser and is able to add the product by layer.

The depalletiser has first, second, third and fourth gripper assemblies 100, 200,300 and 400 arranged as depicted in Figure 1. To aid with the current description, the arrangement of the gripper assemblies is further depicted in Figure 2 with the second and fourth gripper assemblies 200,400 retracted from their assembled position.

The first gripper assembly 100, depicted in more detail in Figures 3 through 6, has a first gripper surface 101 for gripping a first side of a product layer. The first gripper surface 101 lies in a first gripper plane with a horizontally disposed lateral axis G1.

The second gripper assembly 200, depicted in more detail in Figure 7, opposes the first gripper assembly 100 and has second gripper surface 201 for gripping a second side of the product layer which opposes the product layer first side. The second gripper surface 201 lies in a second gripper plane with a horizontally disposed lateral axis G2 which is parallel to the first gripper plane lateral axis G1.

The third gripper assembly 300 has a third gripper surface for gripping a third side of the product layer. The third gripper surface lies in a third gripper plane with a horizontally disposed lateral axis G3 perpendicular to the first and second gripper plane lateral axes G2, G4.

The fourth gripper assembly 400 opposes the third gripper assembly 300 and has a fourth gripper surface 401 for gripping a fourth side of the product layer opposing the third side. The fourth gripper surface 401 lies in a fourth gripper plane with a horizontally disposed lateral axis G4 parallel to the third gripper plane lateral axis G3.

The first, second, third and fourth gripper assemblies 100,200,300 and 400 are mounted on a frame 500 (not shown in Figures 1 and 2, but shown in detail in Figures 10 and 11 and described below) allowing horizontal displacement of each of the

gripper assemblies 100,200,300,400 in a direction of travel perpendicular to its respective gripper plane lateral axis G1, G2, G3, G4. Means are also provided for the horizontal displacement of the gripper assemblies, enabling the gripper surfaces to engage the product layer sides, as well as means for vertically displacing the frame 500, allowing a product layer which has been gripped by the gripping assemblies (and any further layers above) to be lifted away from the pallet.

The first, second, third and fourth gripper planes/gripper surfaces are preferably inclined at an angle of inclination of between 2 and 6 degrees from vertical.

Each gripper plane is inclined such that it downwardly converges with the gripper plane of the opposing gripper assembly. The effect of this inclination is best described with reference to Figure 12, schematically depicting a cross section through the centre of a product layer 10 of cartons 15 and opposing first and second gripping surfaces 101,201 gripping the first and second sides 11,12 of the product layer. The inclination of the gripping surfaces 101,201 (which is exaggerated in Figure 12 to display the principal) results in an increased gripping pressure being applied toward the base of the product layer sides 11,12 as opposed to the top of the product layer sides. This causes greater compression of the product layer toward the base which results in the product layer arching up slightly. Each of the cartons 15 of the layer is effectively wedged between the two neighbouring cartons 15 (or between a neighbouring carton and the neighbouring gripper surface), helping prevent the carton from sliding downwards out of the layer. With the third and fourth gripper planes also being inclined, a similar cross section could be taken perpendicularly to that depicted in Figure 12 showing a similar arched effect.

If the angle of inclination is insufficient, the wedging effect may not be sufficient to support the weight of the layer without providing an overall excessive gripping force by the gripper assemblies, whilst an excessive angle of inclination may provide excessive local compressive forces and crush the lower edge of the product layer. In the prior art hinged devices, the angle of inclination of the gripping plates is wholly dependent on the width of the product layer, which varies between product.

Various products are hence prone to excessive damage as a result of the lack of control over the angle of inclination in the prior art device.

The current inventor has found that the ideal angle of inclination of each of the first, second, third and fourth gripper planes is approximately 4 degrees for many products. This angle has been found to be effective for depalletising cartons or plastic shrink wrapped slabs of 24 beer cans or stubbies by layer. Several such layers can be lifted at a time. The preferred embodiment provides for adjustment of the angle of inclination of each of the gripper planes/gripper surfaces to allow for different angles which may be found to be more appropriate for different product layers.

The first gripper assembly 100 is generally formed as a gripper assembly body 110, a gripper element mounting member 120, at least one gripper element 600, an arm 140 and a pair of wheels 150,151. The gripper assembly body 110 is formed of two vertically disposed side plates 111,112 welded to and separated by a lower vertically disposed web 113 and an upper horizontally disposed web 114.

The gripper element mounting member 120 includes a mounting plate 121 having a gripper element mounting face 122 parallel to the first gripper plane and a lower portion bent back through a right angle to form a stiffening flange 123. Vertically disposed connecting plates 124,125 welded to the mounting plate 121, including to the stiffening flange 123, also serve to stiffen the gripper element mounting member 120.

The gripper element mounting member connecting plates 124,125 are located either side of the gripper assembly body 110 immediately adjacent the gripper assembly body side plates 111,112.

A threaded rod 126 passes through the gripper assembly body side plates 111, 112 and the gripper element mounting member connecting plates 124,125 toward the bottom thereof to pivotably fasten the gripper element mounting member 120 to the gripper assembly body 110. Nuts 127 and a spacer tube 128 are used to secure the threaded rod 126 with respect to the plates 111,112,124,125. Another threaded rod 129 passes through the gripper element mounting member connecting plates 124,125 toward the top thereof, passing through a notch 115 in each of the gripper assembly body side plates 111,112 such that the threaded rod 129 is not attached thereto. The threaded rod 129 is secured to the gripper element mounting member connecting plates 124,125 by nuts 130 and a spacer element 131. Two threaded adjusting rods 132 are secured to opposing ends of the spacer element 131 (or the threaded rod 129) and to the vertically disposed web 113 of the gripper assembly body 110. The length of the

adjusting rods 132 separating the web 113 and the spacer element 131/threaded rod 129 can be adjusted by means of adjusting nuts 133 and associated lock nuts 134. A slot 116 is provided in each of the gripper body assembly side plates 111,112 to allow access to the lock nuts 134.

Provision of adjustability in the adjustment rod 132 lengths enables the gripper element mounting member 120 to be pivoted about the lower threaded rod 126, thereby adjusting the angle of inclination of the first gripper plane/first gripper surface 101.

In the preferred embodiment a plurality of gripper elements 600 are mounted on the gripper element mounting face 122 to form the first gripper surface 101. The gripper elements 600, which are described in detail below, are mounted on the gripper element mounting face 122 by fasteners 135 passing through the mounting plate 121.

Upper and lower end plates 136,137 are welded to the upper and lower surfaces of the gripper element mounting member 124,125 and project forward of the gripper element mounting face 122 adjacent the gripper elements 600, helping to support the gripper elements 600 against any shear deformation. The upper and lower end plates 136,137 are set back approximately 20 mm from the first gripper surface 101 to allow for some deformation of the gripper elements 600, particularly when a single carton protrudes beyond the end of the rest of the side of a product layer as a result of non-uniform stacking of the pallet.

The arm 140 and wheels 150,151 are fastened to the gripper assembly body 110 via threaded rods 152,153 and nuts 154,155 toward the top of the gripper assembly body side plates 111,112. The arm 140 is formed of two plates 141,142, one disposed on either side of each of the wheels 150,151 and separated therefrom by "Ralloy"ultra high molecular weight polyethylene washers, and an upwardly facing rack 143 with teeth extending between the arm plates 141,142 perpendicular to the gripper assembly axis of travel. The wheels 150,151 are cast iron and are mounted onto the threaded rods 152,153 via bearings for supporting and guiding the first gripper assembly whilst it is being horizontally displaced by a first pinion 702 engaging the rack 143.

The second, third and fourth gripper assemblies 200,300,400, depicted in Figures 7,8 and 9 respectively, are each identical to the first gripper assembly except for the lateral width of the gripper element mounting member 220,320,420 and

number of gripper elements 600 and the arrangement of the arms 240,340,440. As is best seen in Figure 1, the first and second gripper element mounting members 120,220 are narrower than the third and fourth gripper element mounting members 320,420.

Here the first and second gripper element mounting members 120,220 are approximately 994 mm wide whilst the third and fourth gripper element mounting members 320,420 are approximately 1240 mm side. The shorter first and second gripper element mounting members 120,220 can thus be displaced within the lateral extent of the third and fourth gripper elements 320,420 to enable the gripper elements 600 to engage the sides of a narrow product layer whilst still allowing for the third and fourth gripper element mounting members 320,420 and associated gripper elements 600 to span a short distance to engage the ends of the product layer if it is also short in length. As the standard pallet is 1165 x 1165 mm, the minimum achievable span between opposing gripper elements of 994 mm is sufficient to grip most typical product layers which typically either slightly overhang or are slightly narrower than the pallet.

The arm 140 of the first gripper assembly 100 extends from the body above a horizontal axis H1 passing through the axes of the wheels 150,151 in the direction of travel of the first and second gripper assemblies with the rack 143 at the end of the arm 140 facing downwardly. The arm 240 of the second gripper assembly 200 extends below the same horizontal axis H1 (which also passes through the second gripper assembly wheel axes) with the rack 243 facing upwardly. The first gripper assembly arm 140 is longer than the second gripper assembly arm 240 such that the two racks 143,243 are positioned with the first rack 143 directly below the second rack 243 and offset from the centre of the gripper assembly arrangement as depicted in Figure 1. The configuration enables the first and second racks 143,243 to engage the common first pinion 702 disposed therebetween to drive the first and second gripper assemblies 100, 200 toward each other at an equal rate, the offset ensuring the first pinion 702 and its associated helical worm gearbox and motor 701 driving the first and second gripper assemblies 100,200 is offset from the centre of the arrangement and the third and fourth gripper assembly arms 340,440.

The third and fourth gripper assembly arms 340,440 are arranged in a similar way, with the third gripper assembly arm 340 extending above a horizontal axis H3 passing through the axes of the wheels 350,351 in the direction of travel of the third

and fourth gripper assemblies with the rack 343 at the end of the arm 340 facing downwardly. The arm 440 of the second gripper assembly 400 extends above horizontal axis H3 with the rack 443 facing upwardly. The fourth gripper assembly arm 440 is longer than the third gripper assembly arm 340. Having the longer arms 240,440, one extending above the axis of the wheels and the other below, allows for the two arms 240,440 to pass one above the other at the centre of the gripper assembly arrangement whilst having all of the wheel axes in a single plane.

The frame 500, depicted in Figures 10 and 11, has a first rail, here in the form of a first hollow RHS beam 501 extending along the axis H1 parallel to the direction of travel of the first and second gripper assemblies. The first hollow beam 501 houses and engages the first and second gripper assembly wheels 150,151,250,251 at opposing ends of the first beam. The first and second gripper assembly arms 140,240 are also housed within the first beam 501. The first motor and gearbox 701 is mounted on the exterior of the first hollow beam 501 adjacent the first and second racks 143,243, with the first pinion 702, mounted on bearings 703, protruding into the interior of the beam between the first and second racks 143,243 to engage the same.

A second rail, again in the form of a second hollow RHS beam 511, intersects the first beam 501 at the centre of the gripper assembly arrangement. The third and fourth gripper assembly wheels 350,351,450,451 and arms 340,440 are housed in the second beam 511 in a similar manner to the first and second gripper assemblies with the first beam 501, with the second motor and gearbox 711 mounted on the second beam 502 and the second pinion 712, mounted on bearings 713, engaging the third and fourth racks 343,443. In use, the first and second motors 701,711 can be controlled independently to drive the first and second gripper assemblies 100,200 and the third and fourth gripper assemblies 300,400 toward each other independently.

Slots 502,503 and 512,513 are respectively provided in the side walls of each of the hollow beams 501,511 toward each end adjacent the gripper assembly wheels for reception of the threaded rods 152,153,252,253,352,353,452,453 forming the axles of the gripper assembly wheels 150,151,250,251,350,351,450,451. Each slot has a length allowing for displacement of each of the gripper assemblies over a distance of approximately 170 mm. The relative location of the various components of the gripper assemblies and the frame hollow beams is best shown in Figures 4 and 6,

showing the first gripper assembly 100 mounted on the first hollow beam 501. Taking one side of the wheel 501, a washer abuts the side of the wheel, followed by the first gripper arm plate 141, another washer, then the wall of the first hollow beam 501. The threaded rod 153 passes through the slot 511, with a"Ralloy"low friction spacer plate 800 abutting the outside of the first beam 501 side wall. The first gripper assembly side plate 111 abuts the spacer plate 800, with a washer and nut 155 threaded onto the end of the rod 153 to secure the assembly. Use of the spacer plate 800 and washers keeps the assembly tight whilst allowing free movement along the first hollow beam 501.

The frame 500 also includes several struts 510 securing the beams 501,502 to opposing vertical beams 511. The vertical beams each include a pair of guide members 512 for engaging a vertical rail 901 to guide the frame 500 during vertical displacement thereof toward and away from a pallet disposed therebeneath. Mounting eyes 513 are provided on each vertical beam 511 for connection to roller chains 902 which are coupled to a further motor and gearbox 903 via sprockets 904 mounted on a support structure 900 including the vertical rails as depicted in Figure 13. The support structure 900 can in turn be mounted on a horizontal rail system 1000, again as depicted in Figure 13, for horizontal displacement of the entire depalletiser including the support structure, enabling product layers 10 removed from a pallet 1 by the depalletiser to be moved about a warehouse as required.

The gripper elements 600 are mounted on the gripper element mounting faces 122,222,322,422 in three horizontal rows, as depicted in Figure 14 and 15. For the narrower first and second gripper element mounting faces 122,222, each horizontal row has eight gripper elements 600 disposed side by side, whilst ten gripper elements 600 are provided in each horizontal row of the wider third and fourth gripper element mounting faces.

Each gripper element 600, depicted in detail in Figures 16 through 18, formed as a rubber block, has parallel front and rear faces 601,602 and side faces 603 perpendicular to the front and rear faces 601,602. The front face 601 of each gripper element forms part of the gripper surface 101,201,301,401 and is defined by a substantially continuous front wall 611 approximately 7 mm thick.. The side faces 603 are also substantially continuous and defined by side walls 613. A honeycomb type arrangement of holes 604 extends from the rear face 602 to the front wall 611 between

the side walls 612. The honeycomb hole arrangement defines a grid arrangement of vertical and horizontal webs 605 extending from the front wall 611 to the rear face 602.

Each of the webs 605 tapers in thickness toward the rear face 602 at an included angle of approximately 2° to a minimum thickness of 3.5 mm at the rear face between the centres of adjacent holes which have a 20 mm diameter at the rear face. Each of the holes 604 terminates at the front wall 611 in a generally hemispherical shape.

Each rubber block is preferably formed of 30 to 40 durometer rubber, with the current preferred embodiment being a 35 durometer rubber. Each block 600 is approximately 80 mm deep, 168 mm long and 121 mm wide. Fastener holes 606 extend from the front to rear face toward opposing ends of the block 600 for mounting the block 600 to a mounting face via the fasteners 135.

Provision of the honeycomb hole 604 arrangement creating a grid of webs 605 maintains shear stiffness of the rubber block, preventing excessive deformation from the downward shear force applied to the block front face 602 by the weight of a product layer being gripper and lifted, whilst reducing the effective compressive stiffness of the block allowing for deformation of the front face 602 toward the rear face 603 as the product layer is gripped. This reduces the risk of damage to any product which is protruding from the rest of the product layer side as a result of irregular loading, and allows the block front face 602 to deform to take up the generally sinusoidal form of the side of the product layer formed by the curved side of individual bottles or cans of product at the side of the layer. This more even distribution of load across the side of the product layer will again help to reduce the risk of any product breakage.

Whilst it is primarily the vertical webs 605 which act to give the block shear stiffness, the horizontal webs 605 support the vertical webs 605 against buckling.

Figures 19 and 20 show another version of the first/third and second/fourth gripper element mounting members 120-2/220-2 and 320-2/420-2 respectively.

These items are similar to the mounting members 120,220,320 and 420 previously described with reference to Figures 3,7,8 and 9 respectively except for the changes noted below.

The assemblies 120-2,220-2,320-2 and 420-2 have only two rows of gripper elements 600-2 (see also Figures 21 and 22). The gripper elements 600-2 are similar to the gripper elements 600 except they are each mounted at a slight angle to the mounting

member due to the inclusion of a metal spacer 1100 between their lower most end and the mounting member.

The angling of the gripping elements 600-2 further enhances the arching/ wedging effect described with reference to Figure 12.

The gripper element mounting members 120-2,220-2,320-2 and 420-2 are also themselves mounted at an angle to vertical by adjusting the upper threaded adjusting rods 132 and spacer element 131 previously described.

Figure 23 shows another version of the palletiser and depalletiser apparatus in which the first and second motor gearbox assemblies 701-2 and 702-2 both have parallel shaft helical gearboxes and not the helical worm gearboxes of the motor and gearbox assemblies 701 and 702 previously described with reference to Figure 10.

The assemblies 701-2 and 702-2 also include absolute reference type rotary shaft position encoders 1110 which issue signals indicative of the absolute rotation of the motor shaft (and thus the gripper assembly wheels) to a PLC (not shown) for use in determining the position of the gripper assemblies along their respective beams.

Whilst the present invention has been described with reference to one preferred embodiment, various modifications and alternatives embodying the invention will be apparent to the person skilled in the art.