AU705343B2 - Automatic pallet dispenser clamping system - Google Patents

Description

AUTOMATIC PALLET DISPENSER CLAMPING

SYSTEM

Background of the Invention The present invention relates to improvements to automatic pallet dispensers.

Pallet collating and dispensing machines are known. Some kinds of these machines are electromechanically operated and often use an inordinate amount of power; such machines are disclosed in, for example, US Patent No. 3,884,366 and Australian Patent Application No. 40,895/78. Other kinds use hydraulic systems, but are often massive and bulky, and therefore tend to be expensive. Generally, state-of-the-art devices are disclosed in US 4,624,626 and in the specifications relating to British patents 1,447,855 and 1,530,797.

One problem with the construction of existing pallet dispensing systems is that the dispenser has a tendency to jam up when there is an interruption to the supply of power to the dispenser, such as by reason of a supply failure, or as commonly occurs due to factories switching off their compressed air and power supplies at night. This latter practice necessitates the cycling and emptying of the dispenser at the end of the day, a time-consuming task for the operator.

A further problem with the construction of existing pallet dispensing systems is that, in the event of a power failure, or a pneumatic or hydraulic failure to the pallet 9 actuators, a potentially dangerous situation is likely, in that the stack of pallets may S 20 suddenly be dropped from a considerable height. When one appreciates that each pallet weighs between 20 and 30 kg, the stack will hit the ground with extreme force, in all likelihood causing damage to the dispenser, the pallets and the surrounding environment.

In US patent No. 3,388,817 there is disclosed a case unstacker for unstacking cases stacked upon a pallet. A pallet with cases thereon is raised and a clamping 9 device clamps the cases and holds them in a raised position whilst the pallet is lowered.

Whilst such a device could be modified to dispense pallets, the clamping arrangement is inherently unsafe and does not provide an ability to maintain a clamping force in the event of a power failure.

In US 3,388,817 a clamping pad is biased by a spring. However, the pad is not connected to the spring by a single linkage, but by two linkages, one of which is o:\patents\comp\661 "floating". The floating link has one end pivotably connected to the other link and the other end to the pad. The pad is also connected to a hydraulic cylinder which raises and lowers it. The effect of this is that the application of a clamping force to a case will depend on the vertical position of the pad, which in turn is dependent on the extension of a cylinder. This is best seen in Fig. 6 where it is apparent that the compression of the spring reduces as the ram retracts and so reduces the clamping force. Thus clamping force is dependent on the ram remaining extended.

Furthermore, the weight of the case will tend to compress the ram. Thus if there is a failure in hydraulic or pneumatic pressure, the weight of the case will compress the ram and so release the clamping arrangement. Thus there is no fail safe feature. Accordingly, such an arrangement could not safely be modified for use in pallet dispensing.

It is an object of the present invention to alleviate at least some of the aforementioned disadvantages of the prior art.

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0 15 Brief Summary of the Invention S In the present invention, a jaw is held against one or two pallets by a biasing mechanism, such as a spring or closed air bag. This biasing mechanism provides sufficient compression to the jaw to maintain the stack of pallets in position. The jaw is disengaged from the pallets by an actuator, such as a pneumatic ram.

20 Failure of the pneumatic ram will not result in failure of the jaw, but merely a failure to disengage.

•In all embodiments of the invention, the actuator does not form part of the connection between the biasing mechanism and the jaw or the biasing mechanism and its mounting point. Accordinly, in one broad form, there is provided an 25 automatic pallet dispenser for dispensing or receiving pallets including: support means for supporting a plurality of pallets placed one on top of each other so as to form a stack; lifting means for raising or lowering at least the lowermost pallet of the stack; o:\patents\comp\6615.doc clamping means operable independent of the position of the lifting means to clamp at least one selected pallet in the stack; said clamping means including: a jaw movable between a first position in which it engages and clamps at least the selected pallet in the stack and a second position in which it is disengaged from at least the selected pallet in the stack; a biasing means, biasing the jaw to the first position, and; drive means connected to the jaw and operable to move the jaw from the first to the second position against the force of the biasing means; wherein, when said jaw is in the first position, the biasing means applies a sufficient compressive force to the jaw to maintain the selected pallet in position independent of the drive means and the lifting means.

The drive means compresses the biasing means, either directly or indirectly, to l•.withdraw the jaw. Failure of the drive means will allow the jaw to return or remain in 15 the first position. Preferably the drive means and biasing means are arranged in parallel but other arrangements may be utilised.

The jaw may be mounted on a rotatable support, with the biasing means biasing the support for rotation in one direction and the drive means operable to cause rotation in the other direction.

0 V, A further embodiment utilises a linkage between the biasing means and the jaw which moves linearly and the drive means connects with this linkage to overcome the biasing means and withdraw the jaw. Where an air bag is utilised as the drive means, preferably it is torus shaped with the linkage passing through its centre.

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o:\patents\comp\661 The invention is predicated on the basis that the clamping means principally and preferably relies on the drive means for engagement with and disengagement from the pallet only and not for the continuous compressive force, which is supplied by the biasing means. In preferred forms the drive means is only utilised for disengagement of the jaw and engagement of the jaw relies on the biasing means alone.

The novel construction of the present invention obviates the need for the dispenser to be cycled and emptied overnight, since the drive means and biasing means function independently of each other. The construction is such that even if the actuator malfunctions in some way, the pallet will be maintained at a predetermined height due to the independent compressive force applied to the jaw by the biasing means.

Preferably the means to support the pallet stack comprises a hopper to accommodate the pallets.

S

The clamping means is preferably operable to clamp at least one of the pallets at a predetermined height above floor level. The clamping means may clamp two or more pallets at a time.

Suitably, the dispenser may also include sensing means to sense the presence or non-presence of a pallet at the bottom of a stack thereof.

The clamping means may be actuated in any suitable manner such as electromechanically, hydraulically or pneumatically. It is presently preferred that the clamping means be actuated by a pneumatic cylinder.

The clamping means of the present invention is preferably disposed at the side, as opposed to the rear, of the hopper. That is, the clamping means is disposed at a side or sides adjacent the point where the pallets are dispensed. Preferably, two clamping means are disposed on one side of the dispenser.

The biasing means may be any means which translates a sufficient force to the pallet to maintain the pallet at a predetermined height. It is presently preferred that the biasing means take the form of a compression spring but other devices such as a closed air bag or a leaf spring may be used.

The biasing means may suitably be connected to the jaw, preferably at or near one end thereof, to bias the jaw towards the pallet, such that in the case where the *clamping means engages the pallet at a predetermined height, the force applied to the jaw by the biasing means is sufficient to bias the jaw to maintain the pallet in its S. predetermined disposition. In this way, the jaw is prevented from disengaging from the pallet in the event of a power failure, or a pneumatic or hydraulic failure.

It will be appreciated that where the drive means and biasing means are interconnected, the drive means is adapted to override the biasing force of the biasing S 25 means, such as for example during the operation of disengaging or releasing the S. pallet for dispensing.

;A support member for the jaw may advantageously be pivotably connected to the jaw, to assist in engaging and disengaging the jaw from the pallet, as will be later described in greater detail with reference to one embodiment of the invention. The support member for the jaw is also suitably pivotably connected to the support surface.

Preferably, the hopper may be mounted on a housing which includes a pair of lifting cylinders of the kind known in the art as being "back-to-back".

A back-to-back lifting cylinder arrangement is preferably disposed at each side of a pallet-holding frame, the lifting cylinders being actuable to either raise or lower associated lift arms which, in turn, are adapted to either lift or drop a pallet off the stack.

A back-to-back lifting cylinder arrangement is one in which two cylinders are joined together with their respective piston rods extending in opposite directions. In this application one of the piston rods is affixed to the housing or framework and the two cylinders "float". The other piston is attached to the lift arms. By extending one, both or neither of the two pistons relative to their respective cylinder, three separate lift arm heights can be achieved.

Ideally, the clamping means is enclosed within the housing, a control console such as a pneumatic control console being provided on the outside of the housing.

Brief Description of the Drawings Preferred non-limiting embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a general perspective view of a pallet dispenser according to one embodiment of the present invention; *Figure 2 is a schematic cross-section of one embodiment of the dispenser of the present invention; S 20 Figure 3 is a diagram of the unloading circuit which may be used in a dispenser of any of Figs. 1 to 4. For clarity manual override switches have been omitted; Figure 4 is a diagram of the loading circuit which may be used in a dispenser according to Figs. 1 to 4. For clarity manual override switches have been omitted.

Figure 5 is a schematic cross-section of a fifth embodiment of the invention.

25 Figure 6 is a schematic plan view of a sixth embodiment of the invention.

Figure 7 is a schematic cross-section of the fig. 6 embodiment.

Detailed Description of Preferred Embodiments of the Invention The automatic pallet dispenser which may be operated by compressed air, hydraulic or other means, in accordance with the present invention, includes a hopper for pallets 1 mounted above a housing, indicated at 2 in Figure 1. Housing 2 encloses 2 "back-to-back" lifting cylinders 30, as previously described. The lifting cylinders 30 are disposed one at each side of the housing 2, these lifting cylinders being actuable, preferably pneumatically, to either raise or lower associated lift arms 3 which, in turn, lift and drop a pallet 5 as desired or required. Housing 2 also encloses a clamping mechanism, generally referenced 6 disposed to one side of the housing 2, and a pneumatic control console 7; a pair of fixed guides (not shown) aid in lining up the pallets as they are placed one atop another in the hopper 1. It should be noted that the pallet dispenser of the present invention is well adapted to handle non-standard pallets of various dimensions.

It is to be understood that the dispenser may include more than one clamping mechanism 6 depending on, for example, the number and size of pallets desired to be stacked and/or dispensed.

A first embodiment of the present invention is shown at Figure 2.

Clamping mechanism 6 includes a spring 202 and clamp cylinder 203 connected in parallel at their ends to, respectively, fixed support 204 and clamping jaw 201.

On extension of cylinder 203, clamping jaw 201 engages the pallet 5 and clamps it against pallet holding frame 230. Compression spring 202 ensures that the clamping jaw is biased into engagement with the pallet or pallets 5, such that in the 20 event that the cylinder 203 fails, the clamping plate will remain engaged with the .".pallets, thus preventing them from falling. It will be appreciated that the clamp *".cylinder 203 need not be mounted directly to the jaw but that a mechanical linkage may be used. On retraction of the cylinder 203, the spring 202 is compressed, drawing the clamping jaw 201 away from the pallet or pallets 5. The pallet(s) may 25 then be moved vertically by the lift arms 3.

0 The dispensing mode of operation of the automatic pallet dispenser of the present invention will now be described with reference to Figure 2 and 3 of the drawings.

Fig. 3 shows the pneumatic circuit for the automatic pallet dispenser of the present invention. For clarity only the valves and switches used for dispensing pallets are shown. The dispenser may be controlled to load pallets by way of similar valves and switches, which are described with reference to Fig. 4.

In fig. 3, the valves are at the positions when the lift arms 3 are being lowered but are not yet fully down.

High pressure air is supplied by supply line 18 to pallet sensor switch 19 via bleed valve 20. Pallet sensor switch 19 is an open/close valve. When a pallet is present at the dispensing position the valve 19 is open and line 21 communicates via valve 19 to atmosphere. When no pallet is present at the dispensing position the switch 19 is closed and line 21 is blocked. Input line 21 is a control to pallet valve 22 which is spring biased to the position shown, in which input line 23 is blocked and output line 24 is discharged to atmosphere. When switch 19 is closed, pressure builds in line 21 and switches valve 22 to communicate input line 23 with output line 24. When switch 19 is open, the valve 22 returns to the position shown.

Output line 24 communicates with lift arm sensor switch 25 which senses when lift arms 3 are fully down. In the position shown, the switch 25 is at the position when the lift arms 3 are not fully down and line 24 is blocked and output line 26 is discharged to atmosphere. When the lift arms 3 are fully down, lines 24 and 26 communicate.

Thus valves 22 and 25 act as an AND gate, in that high pressure is only supplied to line 26 from supply line 18 if the lift arms 3 are fully down and there is no pallet in the dispensing position, ie just after a pallet has been removed from the dispensing position.

Line 26 is a control for lift cylinder valves 27 and 28. Valve 27 controls the upper cylinder 30a of each back-to-back lifting cylinders 30 whilst valve 28 controls the lower cylinders The two valves 27 and 28 receive high pressure air via input lines 31, 32 and are switchable to either raise or lower the cylinders. As an example, to lower cylinders input line 31 is connected to line 33 and line 34 discharges to atmosphere. To 25 raise the cylinders 30a, input line 31 is connected to line 34 and line 33 discharges to atmosphere.

Valve 27 is switchable between its two states depending on the pressure in input lines 35 and 36 whilst valve 28 is switchable depending on the pressure in input 444o lines 37 and 38. Input lines 35 and 37 are connected to line 26 and application of a 30 high pressure at line 26 when lines 36 and 38 are at a low pressure causes valves 4444 27 and 28 to expand both cylinders. Conversely, application of a high pressure to lines 36 and 38 when lines 35 and 37 are at a low pressure causes the valves 27, 28 to contract their respective cylinders.

A lift arm sensor switch 40 senses when the lift arms are fully raised, at which point they are immediately below the lowermost pallet of the stack. When the lift arms are fully raised, sensor switch 40 connects line 36 to supply line 18. At other times switch 40 is biased to a position in which it connects line 36 to atmosphere.

A second lift arm sensor switch 41 senses when the lift arms have moved downwards from a fully raised position to a position one pallet height below that fully raised position. Sensor switch 41 has a spring loaded pivoting arm 42 which is not activated when the lift arms move upwards through that position. When the lift arms move down to the lower position, switch 41 connects line 43 to supply line 18.

At other positions of the lift arms, switch 41 is biased to connect line 43 to atmosphere. It will be noted that line 38 is connected to line 43.

The clamping cylinders 10 are controlled by clamp valve 44 which is switchable between two positions to extend or contract the clamping cylinders. The valve 44 is controlled by the relative pressures in lines 36 and 43. Conversely when line 36 is at high pressure and line 43 is at low pressure, the clamping cylinders 10 contract.

When line 36 is at low pressure and line 43 is at high pressure, they expand.

In use the lift arms 3 will be lowered and a pallet will be in the dispensing position.

The clamp cylinders 10 will be extended to allow the clamp to clamp the stack of pallets. The pallet is removed and switch 19 causes valve 22 to connect lines 23 20 and 24. Because the lift arms are fully lowered lines 24 and 26 are already oOOQ connected. High pressure air is thus supplied to input lines 35 and 37. Because SW lines 36 and 38 are connected to atmosphere, the valves 27 and 28 switch to extend both sets of cylinders 30a and 30b. The lift arms 3 commence to rise and switch 25 disconnects lines 24 and 26 and connects line 26 to atmosphere. The valves 27 and 28 remain in their positions. The cylinders 30 extend until the lift arms 3 are fully raised, at which point switch 40 is triggered to connect supply line o 18 to line 36. This has two effects. Firstly, valve 44 switches so as to contract clamp cylinders 10 and release the clamp. Secondly, it switches valve 27 to contract cylinders 30a. Cylinder 30b remains fully extended. At this point the stack of pallets is not clamped and is only supported by the lift arms 3. Thus as the cylinder 30a contracts, the stack lowers. When the stack has lowered from the fully raised position, switch 40 returns to its usual position, connecting line 36 to atmosphere.

When the stack has lowered one pallet height, switch 41 is triggered, applying high pressure to line 43 and line 38. The clamp valve 44 is thus switched to now extend the clamping cylinders and hence clamp the stack again, except for the lowermost pallet. Valve 28 is also switched to cause cylinder 30b to contract. Thus the lift arms 3 continue descending, supporting a pallet. Once the lift arms have passed switch 41, it returns to its usual position, in which line 43 is connected to atmosphere.

When the lift arms are fully lowered switch 25 is activated to connect lines 24 and 26. Simultaneously switch 19 is activated to connect line 21 to atmosphere. This allows valve 22 to switch to disconnect lines 23 and 24 and connect line 24 to atmosphere. The device is thus reset to await removal of the pallet.

The dispenser may also operate in reverse to receive individual pallets and stack them in the hopper. The circuitry for this operation is shown in Figure 4. Referring to Fig. 4, there are shown the valves for loading a pallet to the bottom of the stack.

This function is useful when articles are being removed from pallets and the empty pallets need to be stacked.

The circuit includes clamping cylinders 10 controlled by clamp valve 44 and lifting cylinders 30 controlled by valves 27 and 28. These valves operate as in the Fig. 3 embodiment, but the states they assume and how they are controlled are different.

Pallet valve 122 has input line 123 and output line 124. The state of the valve 122 is controlled by sensor switch 119. Valve 122 only connects lines 123 and 124 o when a pallet is present at other times line 123 is blocked and line 124 vents to 20 atmosphere. A lift arm sensor switch is not necessary as a pallet can only be loaded when arms 3 are fully down.

°ooo° o' SLine 124 is connected to only valve 28 which is switched to extend bottom cylinders 30b, so the pallet is only raised until lift arm sensor switch 141 senses when the lift arms are at a position to place the pallet just below the clamped stack.

25 Sensor switch 141 is triggered on upwards movement, not downwards movement.

On triggering of switch 141, clamp valve 44 is switched to retract clamping cylinders 10. Switch 141 is also connected to valve 27, which is switched to extend upper cylinders 30a. Lift cylinders continue to rise, lifting the stack which is now c..

resting on the lift arms 3. When the lift cylinders 30a, 30b are fully extended, lift 30 arm sensor switch 140 is triggered, which in turn causes valve 44 to extend clamp cylinders 10. This clamps the lowermost pallet of the stack, which is now the pallet raised by the lift arms. Simultaneously, sensor switch 140 causes valves 27 and 28 to both switch to cause lift cylinders 30a and 30b to retract. Thus the lift arms 3 are lowered to the fully down position, to await receipt of another pallet.

The dispensers according to the invention may either unload only, load only or be capable of loading and unloading. Where loading and unloading is provided, it will be appreciated that the pneumatic circuitry of Figs. 5 and 6 will be combined.

Figure 5 shows a variation of the figure 2 embodiment.

Referring to figure 5, there is shown a schematic front view of a dispenser 300. For clarity the lifting mechanism is not shown. The dispenser 300 includes a clamping mechanism 308 for clamping one or more 310 pallets in position. The clamping mechanism 310 comprises a clamping jaw 312 which is connected to a rod 330 which bears on the spring 334. The spring 334 or rod 330 is provided with a pressure plate 332 to aid transfer of the forces. The rod 330 passes through a torus shaped air bag 336 which is provided with a second pressure plate 338 on its leftward side with an aperture through which the rod 330 passes. The rod is stepped in diameter with the larger diameter portion to the left of the air bag 336 and with the smaller diameter portion passing through the aperture in the pressure plate. The larger diameter portion may not pass through the pressure plate .**oaperture. Alternatively, the air bag may be located to one side of the rod 330 to avoid the need for a torus shaped air bag.

The air bag is sandwiched between the pressure plate 338 and part of the :o dispenser frame 340 and is connected to a supply of high pressure air. Supply of high pressure air to the air bag 336 will cause it to expand toward the left and cause the pressure plate 338 to bear on rod 330. This retracts the clamping jaw from the pallet 310. On removal of the high pressure supply, either as a result of normal operations or as a result of a power failure, the spring 324 will urge the piston 316 and jaw 312 back into clamping engagement with the pallet 310. Thus removal of the high pressure supply, for whatever reason, causes automatic clamping of a pallet due to the action of the spring 334.

Figures 6 and 7 show a further embodiment in which the clamping jaw 312 is mounted on a pivoting structure 340. The structure is pivotably mounted about a horizontal axis 342 which extends parallel to the side of the pallet 310 and at a position above the clamping position. The structure 340 comprises first and second arms 344, 346 which extend substantially vertically in opposite directions from the axis 342. The clamping jaw 312 is mounted on the free end of arm 346. Pivoting of the structure 340 clockwise about the axis (as seen in fig. 7) causes the clamping jaw 312 to be retracted from the pallet 310, whilst anti-clockwise rotation causes clamping of the pallet 310.

Sandwiched between the free end of the second arm 346 and part of the dispenser frame is a spring 350 which is in compression, thereby always biasing the structure 340 to rotate in an anti- clockwise direction and so clamp the pallet 310. An air bag 352 is sandwiched between the free end of the first arm 344 and the dispenser frame, on the same side as spring 350. The air bag is connected to a supply of high pressure air and when so supplied, may overcome the spring 350 and cause the structure 340 to rotate clockwise, thereby releasing the pallet 310. It will be noted that because the axis 342 is above the jaw 312, the weight of the pallet or pallets will tend to cause a clockwise rotation of the structure 340. This increases the compression of the spring 350 and so the clamping force is increased, so resisting further movement.

Fig. 6 shows a plan view of the device of fig. 7 and it will be seen that two clamping devices are utilised, one to clamp the front and one to clamp the rear of the pallet.

The two clamping jaws 312 are positioned to leave sufficient space for the lift arms 360 of the dispenser to engage the pallet between them. With this configuration two stationary clamping jaws, 362 are positioned opposite each clamping jaw 312 i."tfor engaging the other side of the pallet 310. This configuration may be utilised with and is not limited to this embodiment.

It will be appreciated that with all the embodiments described, the springs may take 20 the form of coil springs, leaf springs, air springs, or any other suitable biasing means. The particular choice will depend on the particular requirements for the device. Similarly, the choice of drive mechanism may be between hydraulic or pneumatic operated cylinders or air bags, again depending on the requirements of the device.

25 From the foregoing, it will be realised by one skilled in the art that numerous modifications and variations may be made to the invention without departing from the spirit and scope thereof.

o:\patents\comp\6615.doc The claims defining the invention are as follows: 1. An automatic pallet dispenser for dispensing or receiving pallets including: support means for supporting a plurality of pallets placed one on top of each other so as to form a stack; lifting means for raising or lowering at least the lowermost pallet of the stack; clamping means operable independent of the position of the lifting means to clamp at least one selected pallet in the stack; said clamping means including: a jaw movable between a first position in which it engages and clamps at least the selected pallet in the stack and a second position in which it is edisengaged from at least the selected pallet in the stack; i' a biasing means, biasing the jaw to the first position, and; drive means connected to the jaw and operable to move the jaw from the first to the second position against the force of the biasing means; wherein, when said jaw is in the first position, the biasing means applies a 0 sufficient compressive force to the jaw to maintain the selected pallet in position independent of the drive means and the lifting means.

S. 0 2. The pallet dispenser of claim 1 wherein the drive means overcomes the biasing 0 means and simultaneously withdraws the jaw to the second position.

3. The pallet dispenser of claim 1 or claim 2 wherein the jaw moves in a linear "0 "path between the two positions.

4. The pallet dispenser of claim 3 wherein the biasing means biases the jaw for movement in a first linear direction, and the drive means is operable to move the jaw in a second linear direction, opposite the first linear direction.

5. The pallet dispenser of any one of claims 1 to 2 wherein the jaw rotates 12

Claims (6)

1. 6. The pallet dispenser of any one claims 1 to 2 or claim 5 wherein: the clamping means includes a support for the jaw rotatably mounted about an axis; the biasing means biases the support for rotation about the axis in a first direction; and, the drive means is operable to rotate the support, against the action of the biasing means, in the other direction.
2. 7. The pallet dispenser of any one of claims 1 to 6 wherein the jaw is moved from the second to the first position by the biasing means alone.
3. 8. The pallet dispenser of any one of claims 1 to 7 wherein the biasing means and the drive means are connected in parallel between the jaw and a support surface. The pallet dispenser of any one of claims 1 to 8 wherein the weight of the pallet 15 stack on the clamp cannot drive the jaw to the second position from the first position. The pallet dispenser of any one of claims 1 to 9 operable to remove a pallet from the bottom of the stack. 1 1. The pallet dispenser of any one of claims 1 to 10 operable to add a pallet to the 20 bottom of the stack.
4. 12. The pallet dispenser of any one of claims 1 to 11 wherein the drive means comprises an air bag or a pneumatic cylinder.
5. 13. The pallet dispenser of any one of claims 1 to 12 wherein the biasing means comprises a closed air bag or a metal spring.
6. 14. A pallet dispenser substantially as herein described with reference to the drawings. oApatents\comp\661 Dated this 4th day of March, 1999 McNEALL INDUSTRIAL TECHNOLOGIES PTY LIMITED By its Patent Attorneys CHRYSILIOU MOORE MARTIN S S 5* S. S S. S 5* 55 S S S Se. S S. 55 S S *5 5555 5 5 55 4* 55 S S 5 S. S S S *S555q S Abstract The invention provides an automatic pallet dispenser for dispensing or receiving pallets comprising support means for supporting a plurality of pallets placed one on top of each other so as to form a stack; lifting means for raising or lowering at least the lowermost pallet of the stack; clamping means operable to clamp at least one selected pallet in the stack; said clamping means (6) including a jaw (201) movable between a first position in which it engages and clamps at least the selected pallet in the stack and a second position in which it is disengaged from at least the selected pallet in the stack; a biasing means (202), biasing the jaw to the first position and; a drive means (203) connected to the jaw (201) and operable to move the jaw from the first to the second position against the force of the biasing means (202). .i o• o *o *o.