WO1996031420A1

WO1996031420A1 - A method and an apparatus for stacking and de-stacking pallets

Info

Publication number: WO1996031420A1
Application number: PCT/DK1996/000160
Authority: WO
Inventors: Sven Ole Christian Olesen
Original assignee: Soco System A/S
Priority date: 1995-04-03
Filing date: 1996-04-02
Publication date: 1996-10-10
Also published as: AU5332396A

Abstract

Two parallel, elongate base members (8), each having two pallet support levers (3) for supporting one or more pallets (2) are fixedly arranged on a subsurface (7). The levers (3) are pivotable about axes (4) such that lifting of a pallet (2) vertically past the levers (3), for instance by means of a fork-lift truck, will pivot the levers (3) such that a pallet (2) to be inserted in a stack of pallets from below may pass by the levers (3). When the pallet (2) has passed, the levers (3) will pivot back to the pallet support position under the influence of gravity whereafter the inserted pallet (2) and any pallets in the stack thereabove may be lowered onto the levers. Hereby, the apparatus may be used to stack pallets by inserting one or more pallets in a stack of pallets from below. The levers (3) may be retained in their raised position by means of magnets or mechanical means while the stack of pallets is lowered past the levers (3). Thereby, the apparatus may also be used to de-stack pallets by removing one or more pallets from the stack from below.

Description

A METHOD AND AN APPARATUS

FOR STACKING AND DE-STACKING PALLETS

This invention relates to an apparatus for stacking substan¬ tially rectangular pallets in a vertical stack of single pallets by inserting one or more pallets into the stack from a position substantially vertically below the stack.

In situations where many pallets are utilized it is necessary to stack the empty pallets to save space. Normally, the pallets are stacked by fork-lift transport mechanisms such as fork-lift trucks by placing each new pallet on top of the topmost pallet in the stack. This is time-consuming and requires that the trucks stacking the pallets have a lifting capability allowing stacking of the desired number of pallets in each stack. Therefore, mechanisms allowing inserting a pallet into the stack from below have been developed, e.g. as disclosed in Swedish examined patent application No. 451 130 where a fork-lift truck moves the new pallet horizontally to a position below a pallet supported by four levers each arranged on a respective one of four vertical stanchions each located near a respective one of the corners of the supported pallet. The levers are pivotable from a horizontal position in which they support the pallet and a raised position in which they do not engage the pallet, and they are biased by a spring towards their support position.

The erection of this known apparatus requires precisely positioning the four stanchions vertically and horizontally and fastening them to a support surface which is relatively time-consuming. The stanchions are moreover relatively exposed to being bent by the fork-lift trucks operating around them. Finally, the stack of pallets is relatively poorly protected against being accidentally tilted and even overturned by accidental contact with the trucks.

An object of the invention is to alleviate or overcome these disadvantages and this is achieved in that the apparatus according to the invention comprises pallet supporting means including at least two pallet engaging means each movable to and fro between a pallet release position and a pallet sup¬ port position for engagement in said support position with one of two opposed side edges of a substantially horizontal pallet constituting the lowest pallet in the stack, the bottom surface of said pallet in the supported position thereof defining a horizontal support level, said support level being at a distance above a substantially horizontal surface at least equal to the height of a pallet, the pallet supporting means further comprising two spaced unitary base members adjacent to and extending along said opposed side edges, all of the pallet engaging means being supported by the two base members and each of the two base members sup- porting at least one of the pallet engaging means, and all the pallet engaging means being arranged for substantially simultaneous movement from the support position to the release position thereof by substantially vertically raising a pallet located in a pre-stacking position between the base members and vertically below the lowest pallet in the stack.

Preferably the base members are elongate and in their longitudinal direction extend along substantially the entire length of the corresponding one of said opposed side edges of the lowest pallet in the stack, and, preferably, at least one of the base members at a longitudinal end thereof comprises blocking means to prevent a pallet located in the pre-stack¬ ing position from being moved out from between the base members while being substantially horizontally displaced in a direction substantially parallel to the longitudinal direc- tion of the base members. Furthermore, each of the base members at a longitudinal end thereof preferably comprises guiding means cooperating with the corresponding guiding means of the other base member for guiding a pallet into the pre-stacking position between the base members while being substantially horizontally displaced in a direction substan¬ tially parallel to the longitudinal direction of the base members. In a preferred embodiment of the apparatus according to the invention each of the pallet engaging means comprises a lever pivotable about a substantially horizontal axis substantially parallel to said opposed side edges of the lowest pallet in the stack, the lever being pivotable between a higher pallet release position in which the arm of the lever does not intersect a vertical plane containing the corresponding one of said opposed edges and a lower pallet support position in which the arm intersects said vertical plane and abuts a fixed element preventing the lever from pivoting further away from the pallet release position. Advantageously, the loca¬ tion of the center of gravity of each lever in the pallet release position thereof is such with relation to the co¬ rresponding horizontal axis that the lever will pivot to its pallet support position when solely influenced by the force of gravity. A particularly simple releasable support of the stack of pallets is hereby obtained.

Preferably, the upper surface of the arm of each lever in the pallet support position thereof is substantially planar and substantially horizontal, and the lower surface of the arm of each lever in the pallet support position is advantageously substantially planar with the plane of said lower surface being parallel to the axis of the lever and inclined upwards in the direction from the base member on which the lever is supported to the side edge of the pallet being supported by the arm.

In a further preferred embodiment each base member comprises an elongate generally box shaped body having a substantially vertical lateral surface extending parallel to the longitudi- nal direction of said body and facing the corresponding substantially vertical lateral surface of the other base member, each body supporting at least two spaced pallet engaging means arranged adjacent to said lateral surface, at least two of said pallet engaging means on each base member being located to support the lowest pallet in the stack at a respective position along the corresponding edge of said pallet closer to the respective end of said edge than to the center of the said edge, and the blocking means of each base member advantageously comprises a vertical plate extending substantially towards the other base member substantially at right angles to the longitudinal direction of the base mem¬ bers, and the guiding means of each base member comprises a guiding member with a substantially planar vertical surface extending from the end of said base member opposite the end thereof comprising said blocking means, the vertical surfaces of the two guiding members tapering outwards relative to each other to constitute a funnel for a pallet being horizontally displaced into the pre-stacking position.

In another aspect of the invention it relates to an apparatus for stacking and de-stacking substantially rectangular pal- lets in and from a vertical stack of single pallets by respe¬ ctively inserting and removing one or more pallets into and from the stack by inserting and removing said one or more pallets into and from, respectively, the bottom of the stack, the pallets being of the type having two vertically spaced plate elements extending along each of two opposed edges thereof. In industrial operations utilizing pallets of this type, for instance the standard Europallets, it is constantly necessary to retrieve an empty pallet from a stack thereof for use at various work-stations. To avoid the relatively time-consuming operation of raising the forks of a fork-lift truck to the top of the stack and, furthermore, to be able to work with trucks with a relatively small lifting height it is necessary to remove an empty pallet from the bottom of the stack.

An apparatus of this type is disclosed in Norwegian examined patent application No. 155,928 which apparatus comprises support means which are moved vertically up and down by means of driven ratchet mechanisms and pivot to and fro from a pallet supporting position to a pallet releasing position and thereby can place the bottommost pallet of a stack of pallets on a floor and thereafter lift the rest of the stack away from the thus separated pallet which thereafter is removed by fix a fork-lift truck. This apparatus is relatively compli¬ cated and therefore relatively expensive and requires a relatively large amount of maintenance.

A further object of the invention is to provide an apparatus of this type that is relatively simple and has few or no driven elements and therefore is relatively inexpensive and simple to maintain. This object is achieved in that the apparatus according to the invention comprises pallet sup- porting means including at least two pallet engaging means each movable to and fro between a pallet release position and a pallet support position for engagement in said support position with the upper plate element of one of said two opposed side edges of a substantially horizontal pallet constituting the lowest pallet in the stack, the bottom surface of said pallet in the supported position thereof defining a support level, said support level being at a distance above a substantially horizontal surface at least equal to twice the height of a pallet less the thickness of the upper plate elements, the pallet supporting means further comprising spaced base members adjacent to said opposed side edges, at least one of the pallet engaging means being sup¬ ported by each of the base members, all the pallet engaging means being arranged for substantially simultaneous movement from the support position to the release position thereof by substantially vertically raising the lowest pallet in the stack, the apparatus further comprising retention means for preventing all the pallet engaging means from returning from the release position thereof to the support position thereof until a predetermined event has occurred.

Advantageously the predetermined event is constituted by the passage of a predetermined period of time or the predeter¬ mined event is constituted by the bottom surface of the lowest pallet attaining the support level or a level at distance below said support level equal to the height of one or more pallets when being vertically lowered. In the first case a very simple control of the de-stacking is achieved where the operator of the fork-lift truck only has to be instructed as to the time period elapsing before operations can continue, and in the second case the operations are continuous.

In an advantageous embodiment of the apparatus according to the invention at least a portion of each of the pallet engag¬ ing means consists of a magnetic material such as iron and the retention means comprises a displaceable magnet or a stationary electric coil for inducing a magnetic force field to retain the engagement means in the pallet release position thereof.

Preferably, the base members are elongate and in their longi¬ tudinal direction extend along substantially the entire length of the corresponding one of said opposed side edges of the lowest pallet in the stack, and in a preferred embodiment of the apparatus according to the invention each of the pallet engaging means comprises a lever pivotable about a substantially horizontal axis substantially parallel to said opposed side edges of the lowest pallet in the stack, the lever being pivotable between a higher pallet release posi¬ tion in which the arm of the lever does not intersect a vertical plane containing the corresponding one of said opposed edges and a lower pallet support position in which the arm intersects said vertical plane and abuts a fixed element preventing the lever from pivoting further away from the pallet release position.

In a simple and advantageous embodiment of the apparatus according to the invention the location of the center of gravity of each lever in the pallet release position thereof is such with relation to the corresponding horizontal axis that the lever will pivot to its pallet support position when solely influenced by the force of gravity, and preferably the upper surface of the arm of each lever in the pallet support position thereof is substantially planar and substantially horizontal. Advantageously, the lower surface of the arm of each lever in the pallet support position is substantially planar with the plane of said lower surface being parallel to the axis of the lever and inclined upwards in the direction from the base member on which the lever is supported to the side edge of the pallet being supported by the arm.

In a preferred embodiment of the apparatus according to the invention the pallet supporting means only comprises two base members, each base member comprising an elongate generally box shaped body having a substantially vertical lateral surface extending parallel to the longitudinal direction of said body and facing the corresponding substantially vertical lateral surface of the other base member, each body support¬ ing at least two spaced pallet engaging means arranged adja- cent to said lateral surface, at least two of said pallet engaging means on each base member being located to support the lowest pallet in the stack at a respective position along the corresponding edge of said pallet closer to the respec¬ tive end of said edge than to the center of the said edge.

In order to facilitate the operation of the apparatus accord¬ ing to the invention both for de-stacking and stacking Euro- pallets or the like the apparatus according to the invention advantageously further comprises control means for the acti¬ vation and de-activation of the retention means in a pre- determined manner and mode-switching means for activating and de-activating said control means corresponding to a pallet de-stacking mode and a pallet stacking mode, respectively.

This invention furthermore relates to a method for stacking substantially rectangular pallets in a vertical stack of single pallets by inserting one or more pallets into the stack from a position vertically below the stack, and accord¬ ing to the invention the method comprises the steps of:

- providing two spaced unitary base members, each bese member supporting at least one pallet engaging means arranged for substantially simultaneous movement from the support position to the release position thereof by substantially vertically raising a pallet located in a pre-stacking position between the base members and vertically below the lowest pallet in the stack,

- placing the one or more pallets in the pre-stacking posi¬ tion,

- lifting the one or more pallets such that the pallet en¬ gaging means are moved into the relase position thereof,

- causing the pallet engagement means to return to the pallet support postion thereof,

- lowering the one or more pallets until it or they are supported by the pallet engagement means.

Finally, the invention also relates to a method for stacking and de-stacking substantially rectangular pallets in and from a vertical stack of single pallets by respectively inserting and removing one or more pallets into and from the stack by inserting and removing said one or more pallets into and from the bottom of the stack, the pallets being of the type having two vertically spaced plate elements extending along each of two opposed edges thereof, the pallets being of the type having two vertically spaced plate elements extending along each of two opposed edges thereof, and according to the invention the method comprises the steps of:

- providing pallet supporting means including at least two pallet engaging means each movable to and fro between a pallet release position and a pallet support position for engagement in said support position with the upper plate element of one of said two opposed side edges of a substan¬ tially horizontal pallet constituting the lowest pallet in the stack, the bottom surface of said pallet in the supported position thereof defining a support level, said support level being at a distance above a substantially horizontal surface at least equal to twice the height of a pallet less the thickness of the upper plate elements,

providing spaced base members adjacent to said opposed side edges, at least one of the pallet engaging means being supported by each of the base members, all the pallet en¬ gaging means being arranged for substantially simultaneous movement from the support position to the release position thereof by substantially vertically raising the lowest pallet in the stack,

providing retention means for preventing all the pallet engaging means from returning from the release position thereof to the support position thereof until a predetermined event has occurred,

- lifting the lowest pallet such that the pallet engaging means are moved into the relase position thereof,

- causing or allowing the predetermined event to take place,

- lowering the lowest pallet to a support postion thereof or to an extraction position thereof depending on whether a stacking or de-stacking operation is being carried out.

In the following, various embodiments of the apparatus accor¬ ding to the invention will be described with reference to the accompanying drawings where

Figure 1 is a pictorial representation of a stack of pallets stacked on an a first embodiment of an apparatus according to the invention with the pallet engaging means in their support position,

Figure 2 is a pictorial representation of the apparatus shown in Fig. 1 with the pallet engaging means in their release position and an empty pallet being inserted below the stack, Figure 3 is a top view of a second embodiment of an apparatus according to the invention with a pallet supported by the pallet engagement means,

Figure 4 is an end view of the apparatus shown if Fig. 3 seen from the pallet insertion end thereof with a pallet supported on the pallet engagement means and an empty pallet arranged in the pre-stacking position thereof,

Figure 5 is an enlarged fragmentary sectional view of the pallet engagement means enclosed in the circle designated by the letter C in Fig. 4 showing the pallet engagement means in its support and release positions, and

Figure 6 is a view similar to the view shown in Fig. 5 of an embodiment of an apparatus according to the invention for stacking and de-stacking pallets.

Referring to Figs. 1 - 5, a stack 1 of pallets is supported on a pallet 2 constituting the bottom of the stack 1. The pallet 2 is supported on four pallet engagement means 3 in the form of pivotable levers each pivotable about an axle or a pin 4 with a horizontal axis 5. The pins 4 are fixedly supported by two elongate, generally box-shaped base members 6 arranged on a horizontal surface 7. At one end, the pallet insertion end, each base member is provided with a guide member 8 having a triangular horizontal cross-section. At the other end each base member 6 is provided with a vertical plate 9 extending towards the other base member 6.

In Fig. 5 the lever 3 is shown in continuous lines in its support position with its pallet support surface 10 horizon¬ tal and abutting a fixed element 11 fixedly attached to the corresponding base member 6. The lever 3 is shown in dotted lines in its release position abutting a pallet 2a shown in dotted lines, the pallet 2a being in a raised position with respect to the pallet 2 and in its most proximate position to the corresponding base member 6. In Fig. 2, a pallet 12 is being inserted into a position directly below the pallet 2 by means of a fork-lift mechanism (not shown) prior to being lifted vertically by the fork- lift. In Fig. 1 the levers 3 are shown in their horizontal pallet support position in which they support the pallet 2. In Fig. 2 they are shown in their raised release position for illustrative purposes even though they will not be in this position until the bottom surface of the pallet 2 has been lifted by the fork-lift to a position corresponding to the position of the pallet 2a in Fig. 5. When the pallet 12 is inserted by the fork-lift, the guide members 8 cooperate like a funnel to guide the pallet 12 into the correct position below the pallet 2 and to reduce the risk of damaging the pallet and the base members 6 by collision with the ends of the base members 6. The vertical plates 9 shown in Fig. 3 serve to stop the pallet 12 during its insertion. The posi¬ tion of the pallet 12 is defined by the base members 6 and the plates 9 so that the stack 1 will be uniformly vertical and well-aligned.

After the pallet 12 has been positioned correctly below the pallet 2 it is raised vertically by the fork-lift until it abuts the bottom surface of the pallet 2. The fork-lift continues raising the pallet 12 thereby lifting the pallet 2 until one of the bottom boards 14 thereof engages an inclined bottom surface 15 of each of the levers 3 thereby pivoting each of the levers approximately into its raised position indicated by dotted lines in Fig.5. As the pallet 12 con¬ tinues to be raised the levers 3 will pivot into their sup¬ port position under the influence of the force of gravity once the lower boards 14 of pallet 2 and the top boards 16 of pallet 12 have been raised past the levers 3. The fork-lift thereafter lowers the pallet 12 until the top boards 16 abut the upper surfaces 10 of the levers. The pallet 12 and thus the whole stack 1 rests on the levers 3 abutting the fixed elements 11 and the fork-lift can pull out of engagement with the pallet 12. The pallet 12 has thus been inserted in the bottom of the stack l. The apparatus may also be used at a work-station where goods are loaded on pallets. A pallet is supported on the levers and is loaded with objects. When the loaded pallet is to be replaced by an empty pallet a fork-lift inserts the empty pallet bellow the full one and the operation described above is carried out. Thereafter the fork-lift removes the full pallet. Compared to conventional replacement of a full pallet with an empty pallet where the full pallet is removed and transported to its next station whereafter the empty pallet is picked up somewhere else and thereafter transported to the work-station, the work-station is out of commission for a much shorter period of time in the procedure utilizing the apparatus according to the invention, and the fork-lift saves a good deal of travel time.

The base members 6 are bolted to the support surface 7 at a mutual distance corresponding to the width of the pallets. Europallets have a standard length and have three different standard widths. The support surface 7 may have embedded nuts at three different locations for one of the base members 6 so that the distance between the base members 6 can be changed in a simple manner to accommodate any change in pallet widths.

This can also be accomplished by embedding two rails with U- shaped section in the support surface, said rails extending at right angles to the longitudinal direction of the base members and serving to retain the fastening bolts of one of the base members in a displaceable manner whereby the alter¬ ation of the distance between the base members can be accom¬ plished simply and quickly.

So as to allow de-stacking of the pallets by the method according to the invention by removing the bottommost pallet of the stack the apparatus described above with reference to Figs. 1-5 is further provided with four electrical coils 20 each associated with one of the levers 3 as shown dia- grammatically in Fig. 6. The levers 3 consist of a magnetic material such as normal steel and will be attracted by the magnetic field generated by the electrical coils 20 when the latter are activated. The coils 20 are connected to an elec¬ trical power source (not shown) and to control means (not shown) for activating and de-activating the coils 20 by connecting and disconnecting the power source from the coils 20 in accordance with certain input signals as described in the following.

The basic mode of the apparatus is the stacking mode with the coils 20 de-activated. When it is desired to initiate an operation to remove the bottommost pallet with its upper boards supported on the levers 3 the control means receives a first input signal causing the apparatus to enter the de- stacking mode by activating the coils 20. A fork-lift there¬ after raises the bottommost pallet so that its bottom boards pass the levers 3 and move said levers into the position shown by dotted lines in Fig. 6. The electric coils 20 will retain the levers in this position until they are de-acti¬ vated. The fork-lift thereafter lowers the bottommost pallet until its top boards and the bottom boards of the pallet supported by the bottommost pallet have passed by the levers 3 whereafter the control means receives a second input signal causing it to de-activate the coils 20 thereby releasing the levers 3 so that they pivot down to their support position. The fork-lift lowers the bottommost pallet further until the top boards of the pallet supported by the bottommost pallet abut the surface 10 of each of the levers 3 whereafter the bottommost pallet is lowered further leaving the rest of the stack supported on the levers 3 whereafter the bottommost pallet is removed from between the base members 6 and the apparatus is once again in its basic stacking mode.

The first signal initiating the de-stacking mode is given by, for instance, the fork-lift operator either by means of a push-button (not shown) on one of the base members or a remote signal (infra-red, radio or the like) to a receiver or sensor (not shown) arranged on one of the base members 6 and electrically connected to the control means. The second input signal is generated by a time-delay switch incorporated in the control means, said time-delay switch being activated by the first input signal and emitting the second input signal after a certain predetermined and adjustable period of time sufficient for the fork-lift operator to lift and lower the bottommost pallet to the position corresponding to the release of the levers 3. Upon receipt of the second input signal the control means may activate a "coil de-activated" signal (not shown) visible or audible to the fork-lift oper- ator indicating that the levers 3 have been released by the coils 20. The first input signal causes the "coil de-acti¬ vated" signal to be switched off.

Alternatively, the time-delay switch is, in the de-stacking mode, activated by sensing means (not shown) arranged adjac- ent one of the levers 3 when said sensing means senses the presence of said lever in its retained position indicated by dotted lines in Fig. 6, the period of time being correspon¬ dingly shorter than in the foregoing arrangement as the operations to be performed by the fork-lift during said shorter period of time are less time-consuming.

In a further alternative embodiment of the invention the second input signal to the control means is generated by pallet position sensing means (not shown) arranged on one of the base members 6 when said pallet position sensing means senses that the bottommost pallet has been lowered past a support level defined by the bottom surface of a pallet supported by the levers 3 or a lower level if more than one pallet are to be de-stacked at a time. The pallet position sensing means may comprise two photosensitive cells arranged vertically one above the other immediately above and below said level, the second input signal being generated by the lowest cell and emitted to the control means when said lowest cell is activated after the highest cell has been activated.

In yet a further embodiment of the apparatus according to the invention the basic mode of the apparatus is the de-stacking mode and a first input signal de-activates the control means whereby the apparatus is brought into its stacking mode. In the de-stacking mode a second input signal may be generated in the same manners and has the same effects as in the embodiments described in the foregoing. The first input signal may be generated by a push button or a remote signal as described in the foregoing or by pallet insertion sensing means (not shown) arranged on one or both of the base members 6 or on one or both of the vertical plates 9 when said pallet insertion means has sensed that a pallet has been inserted in a pre-stacking position between the base members 6 and verti¬ cally below the lowest pallet in the stack. The first input signal activates a time-delay switch incorporated in the control means, said time-delay switch emitting a third input signal to the control means after a certain predetermined and adjustable period of time sufficient for the fork-lift opera¬ tor to lift and lower the inserted pallet into the position in which it is supported by the levers 3. The third input signal activates the control means so that the apparatus again is in its basic de-stacking mode.

The electrical coils 20 may be substituted by mechanical means displaceable from a position in which they engage each of the levers 3 to retain said levers in the position shown by dotted lines in Fig. 6 to a position in which the mechani- cal means are disengaged from the levers 3. The levers 3 may have protuberances engageable by said mechanical means.

The levers 3 may be substituted by other mechanical means arranged to be movable between a pallet release position and a pallet support position under the influence of the force of gravity or under the influence of other means arranged to influence them for such movement, and between said pallet support position and said pallet release position by substan¬ tially vertically raising a pallet located in said pre-stack¬ ing position. The embodiments of the apparatus shown in Figs. 1-5 may be provided with apertures and attachment means for inserting and/or attaching the elements required for the de-stacking mode, said elements being constructed as units for attachment to and/or insertion in said embodiments either before or after delivery to the end-user thereof.

For carrying out the method of stacking and de-stacking pallets according to the invention, the levers 3 or other elements with similar functions may be supported in many different manners allowing pallets to be stacked there¬ between. They may for instance be arranged on each its own base member, for instance a discrete pillar or cantilevered beam, or on a variety of frame structures suitable for the purpose.

Claims

1. An apparatus for stacking substantially rectangular pal¬ lets in a vertical stack of single pallets by inserting one or more pallets into the stack from a position vertically below the stack, the apparatus comprising pallet supporting means including at least two pallet engaging means each movable to and fro between a pallet release position and a pallet support position for engagement in said support posi¬ tion with one of two opposed side edges of a substantially horizontal pallet constituting the lowest pallet in the stack, the bottom surface of said pallet in the supported position thereof defining a horizontal support level, said support level being at a distance above a substantially horizontal surface at least equal to the height of a pallet, the pallet supporting means further comprising two spaced unitary base members adjacent to and extending along said opposed side edges, all of the pallet engaging means being supported by the two base members and each of the two base members supporting at least one of the pallet engaging means, and all the pallet engaging means being arranged for substan¬ tially simultaneous movement from the support position to the release position thereof by substantially vertically raising a pallet located in a pre-stacking position between the base members and vertically below the lowest pallet in the stack.

2. An apparatus according to claim 1, wherein the base mem¬ bers are elongate and in their longitudinal direction extend along substantially the entire length of the corresponding one of said opposed side edges of the lowest pallet in the stack.

3. An apparatus according to claim 2, wherein at least one of the base members at a longitudinal end thereof comprises blocking means to prevent a pallet located in the pre-stack¬ ing position from being moved out from between the base members while being substantially horizontally displaced in a direction substantially parallel to the longitudinal direc¬ tion of the base members.

4. An apparatus according to claim 2 or 3, wherein each of the base members at a longitudinal end thereof comprises guiding means cooperating with the corresponding guiding means of the other base member for guiding a pallet into the pre-stacking position between the base members while being substantially horizontally displaced in a direction substan¬ tially parallel to the longitudinal direction of the base members.

5. An apparatus according to any of the preceding claims, wherein each of the pallet engaging means comprises a lever pivotable about a substantially horizontal axis substantially parallel to said opposed side edges of the lowest pallet in the stack, the lever being pivotable between a higher pallet release position in which the arm of the lever does not intersect a vertical plane containing the corresponding one of said opposed edges and a lower pallet support position in which the arm intersects said vertical plane and abuts a fixed element preventing the lever from pivoting further away from the pallet release position.

6. An apparatus according to claim 5, wherein the location of the center of gravity of each lever in the pallet release position thereof is such with relation to the corresponding horizontal axis that the lever will pivot to its pallet support position when solely influenced by the force of gravity.

7. An apparatus according to claim 5 or 6, wherein the upper surface of the arm of each lever in the pallet support posi- tion thereof is substantially planar and substantially hori¬ zontal.

8. An apparatus according to any of the claims 5-7, wherein the lower surface of the arm of each lever in the pallet support position is substantially planar with the plane of said lower surface being parallel to the axis of the lever and inclined upwards in the direction from the base member on which the lever is supported to the side edge of the pallet being supported by the arm.

9. An apparatus according to any of the preceding claims, wherein each base member comprises an elongate generally box shaped body having a substantially vertical lateral surface extending parallel to the longitudinal direction of said body and facing the corresponding substantially vertical lateral surface of the other base member, each body supporting at least two spaced pallet engaging means arranged adjacent to said lateral surface, at least two of said pallet engaging means on each base member being located to support the lowest pallet in the stack at a respective position along the co¬ rresponding edge of said pallet closer to the respective end of said edge than to the center of the said edge.

10. An apparatus according to any of the claims 3-9, wherein the blocking means of each base member comprises a vertical plate extending substantially towards the other base member substantially at right angles to the longitudinal direction of the base members, and the guiding means of each base member comprises a guiding member with a substantially planar vertical surface extending from the end of said base member opposite the end thereof comprising said blocking means, the vertical surfaces of the two guiding members tapering out¬ wards relative to each other to constitute a funnel for a pallet being horizontally displaced into the pre-stacking position.

11. An apparatus for stacking and de-stacking substantially rectangular pallets in and from a vertical stack of single pallets by respectively inserting and removing one or more pallets into and from the stack by inserting and removing said one or more pallets into and from the bottom of the stack, the pallets being of the type having two vertically spaced plate elements extending along each of two opposed edges thereof and the apparatus comprising pallet supporting means including at least two pallet engaging means each movable to and fro between a pallet release position and a pallet support position for engagement in said support posi¬ tion with the upper plate element of one of said two opposed side edges of a substantially horizontal pallet constituting the lowest pallet in the stack, the bottom surface of said pallet in the supported position thereof defining a support level, said support level being at a distance above a sub¬ stantially horizontal -surface at least equal to twice the height of a pallet less the thickness of the upper plate elements, the pallet supporting means further comprising spaced base members adjacent to said opposed side edges, at least one of the pallet engaging means being supported by each of the base members, all the pallet engaging means being arranged for substantially simultaneous movement from the support position to the release position thereof by substan¬ tially vertically raising the lowest pallet in the stack, the apparatus further comprising retention means for preventing all the pallet engaging means from returning from the release position thereof to the support position thereof until a predetermined event has occurred.

12. Apparatus according to claim 11, wherein the predeter- mined event is constituted by the passage of a predetermined period of time.

13. Apparatus according to claim 11, wherein the predeter¬ mined event is constituted by the bottom surface of the lowest pallet attaining the support level or a level at a distance below said support level equal to the height of one or more pallets when being vertically lowered.

14. An apparatus according to any of the claims 11-13, where¬ in at least a portion of each of the pallet engaging means consists of a magnetic material such as iron and the re- tention means comprises a displaceable magnet or a stationary electric coil for inducing a magnetic force field to retain the engagement means in the pallet release position thereof.

15. An apparatus according to any of the claims 11-14, where- in the base members are elongate and in their longitudinal direction extend along substantially the entire length of the corresponding one of said opposed side edges of the lowest pallet in the stack.

16. An apparatus according to any of the claims 11-15, where- in each of the pallet engaging means comprises a lever pivot¬ able about a substantially horizontal axis substantially parallel to said opposed side edges of the lowest pallet in the stack, the lever being pivotable between a higher pallet release position in which the arm of the lever does not intersect a vertical plane containing the corresponding one of said opposed edges and a lower pallet support position in which the arm intersects said vertical plane and abuts a fixed element preventing the lever from pivoting further away from the pallet release position.

17. An apparatus according to claim 16, wherein the location of the center of gravity of each lever in the pallet release position thereof is such with relation to the corresponding horizontal axis that the lever will pivot to its pallet support position when solely influenced by the force of gravity.

18. An apparatus according to claim 16 or 17, wherein the upper surface of the arm of each lever in the pallet support position thereof is substantially planar and substantially horizontal.

19. An apparatus according to any of the claims 16-18, where¬ in the lower surface of the arm of each lever in the pallet support position is substantially planar with the plane of said lower surface being parallel to the axis of the lever and inclined upwards in the direction from the base member on which the lever is supported to the side edge of the pallet being supported by the arm.

20. An apparatus according to any of the claims 11-19, where¬ in the pallet supporting means only comprises two base mem- bers, each base member comprising an elongate generally box shaped body having a substantially vertical lateral surface extending parallel to the longitudinal direction of said body and facing the corresponding substantially vertical lateral surface of the other base member, each body supporting at least two spaced pallet engaging means arranged adjacent to said lateral surface, at least two of said pallet engaging means on each base member being located to support the lowest pallet in the stack at a respective position along the corresponding edge of said pallet closer to the respective end of said edge than to the center of the said edge.

21. An apparatus according to any of the claims 11-20, where¬ in the apparatus further comprises control means for the activation and de-activation of the retention means in a predetermined manner and mode-switching means for activating and de-activating said control means corresponding to a pallet de-stacking mode and a pallet stacking mode, re¬ spectively.

22. A method for stacking substantially rectangular pallets in a vertical stack of single pallets by inserting one or more pallets into the stack from a position vertically below the stack, the method comprising the steps of:

- providing two spaced unitary base members, each bese member supporting at least one pallet engaging means arranged for substantially simultaneous movement from the support position to the release position thereof by substantially vertically raising a pallet located in a pre-stacking position between the base members and vertically below the lowest pallet in the stack, - placing a pallet in the pre-stacking position,

- lifting the pallet such that the pallet engaging means are moved into the relase position thereof,

- lowering the pallet until it is supported by the pallet engagement means.

23. A method for stacking and de-stacking substantially rectangular pallets in and from a vertical stack of single pallets by respectively inserting and removing one or more pallets into and from the stack by inserting and removing said one or more pallets into and from the bottom of the stack, the pallets being of the type having two vertically spaced plate elements extending along each of two opposed edges thereof, the pallets being of the type having two vertically spaced plate elements extending along each of two opposed edges thereof, the method comprising the steps of:

- providing spaced base members adjacent to said opposed side edges, at least one of the pallet engaging means being supported by each of the base members, all the pallet en¬ gaging means being arranged for substantially simultaneous movement from the support position to the release position thereof by substantially vertically raising the lowest pallet in the stack,

- lifting the lowest pallet such that the pallet engaging means are moved into the release position thereof,

- causing or allowing the predetermined event to take place,

- lowering the lowest palletss to a support postion thereof or to an extraction position thereof depending on whether a stacking or de-stacking operation is being carried out.