Patent Grant
10399739
The present application claims priority to European Patent Application No. 17169002.7, filed on May 2, 2017, which said application is incorporated by reference in its entirety herein.
The invention relates to a plastic pallet which firstly comprises a deck for storing objects to be transported, as well as feet which are formed protruding from a deck underside. In addition, the plastic pallet comprises runners which are formed in each case connecting at least two feet to each other on their undersides, i.e. on the side opposite the deck. Finally, the plastic pallet also comprises at least one stiffening structure which, for its part, comprises lower side rails arranged in the runners and upper side rails lying precisely above the lower side rails, arranged spaced apart from, and running parallel to, the latter. The upper side rails can be arranged in the deck in the area between a deck upper side and the deck underside, or also below the deck underside.
In addition to the conventional wooden pallets, plastic pallets are today playing an ever-increasing role in the transport and storage of goods. For example the lower weight and the possibility of forming almost any desired pallet structure using injection-moulding techniques are advantageous, with the result that a high degree of individuality can be achieved here, and it is possible in particular to respond to customer-specific requests. In addition, unless particular hygiene regulations are to be complied with, recycled material can be used for producing many pallet types. The use of additives such as for example reinforcing fibers is also possible. The deck can comprise a continuous, closed load platform, however the load platform can also be formed by a grid or rib structure.
On the underside of the deck, i.e. facing the ground, feet are formed protruding downwards. They have a height which makes it possible for the pallet to be picked up with the fork of a forklift truck and transported; the fork enters into the spaces between the feet. At the same time, however, the feet must also be capable of bearing the permissible weight of the pallet with goods stored thereon, without this resulting in signs of fatigue of the material. Although it is possible to produce the feet separately from a material with a higher impact strength, this type of production is more expensive in comparison with one-piece production of a pallet, as more tools have to be kept ready and the pallet then has to be assembled.
For transport on roller and chain conveyors on the one hand, and for increasing stability on the other hand, plastic pallets often also comprise runners which are formed in each case connecting at least two feet to each other on their undersides. The runners are mostly arranged parallel to each other; in the case of rectangular pallets their longitudinal direction usually lies parallel to the narrower edge of the pallet, though not necessarily: a connection of the feet along the longer edge is also possible. Circumferential runners can also be used, i.e. runners which in addition also connect the feet to each other along the longer edge of the pallet.
However, plastic pallets also have disadvantages compared with wooden or metal pallets. One disadvantage is that, under load, plastic pallets tend towards greater deformations than wooden pallets. At worst, this can lead to irreversible deformations. If goods with a high, though still permissible, mass are placed on the pallets, this leads to a deflection of the deck, wherein the feet with runners formed thereon are also slightly deformed, or bear their share of the deflection, in that the feet are inclined inwards at the top, in the direction of the deck center; however they move outwards at the bottom. Thrust, bending and shear forces thus occur, which can only insufficiently reversibly be reabsorbed by the pallet.
In order to reduce the deformation under load, it is known in the state of the art to reinforce plastic pallets with stiffening structures in order to increase in particular the bending stiffness of the pallets.
For example, DE 20 2015 100 355 U1 describes a plastic pallet that can be assembled from several parts, into the deck of which metal rods are inserted in longitudinal direction to increase the bending stiffness. The metal rods are here arranged transverse to the longitudinal direction of the runners. They reinforce the deck structure and lie parallel to each other, without being interconnected.
DE 10 2014 007 079 A1 describes a two-part plastic pallet with reinforcing profiles which have the function of stiffening elements. The stiffening elements are rod-like and are slid separately into the runners. Here the runner structure is reinforced in the area of the ground level.
In DE 10 2011 103 359 A1,
DE 10 2011 052958 A1 describes a pallet assembled from several parts, in which foot elements are formed arched and arranged crosswise. On their side facing the deck, in the area of the apex of the arches, supporting rods which can also be manufactured from metal are inserted, extending over the length of the foot elements. The bearing capacity of the pallet is increased by the grid arrangement. DE 43 36 469 A1 also describes a plastic pallet in which the deck structure is reinforced with a framework of reinforcing tubes which can for example be manufactured from steel.
DE 20 2007 000 985 U1 describes a plastic pallet which is provided with reinforcements both in the area below the deck and in the area of the feet just above the ground. According to the embodiment shown in
WO 2007/019833 A1 describes a plastic pallet in which reinforcing elements are arranged below the base plate of the pallet in the area of the feet and within the deck. Here
Although such a structure of stiffening elements with recesses is very advantageous with respect to the weight and the connection to the plastic, and increases the stability with respect to direct loading from above, there is scarcely any increase in loading due to shear forces. In addition, the connection of the longitudinal or cross braces to the foot rails via the stays occurs only through the bond in the plastic, with the result that the pallet can withstand only low bending and shear forces.
The object of the invention is therefore to develop a pallet which, compared with the pallets known in the state of the art, has an increased resistance to bending and shear forces and consequently less deflection.
In the case of a plastic pallet of the type described at the outset, this object is achieved in that the at least one stiffening structure comprises rungs, in each case with a predominantly closed surface, which connect the lower side rails in the feet to the upper side rails. The rungs are formed in one piece on the side rails or preferably bonded thereto in each case via contact surfaces, or also connected in friction-locking or form-locking manner, wherein the types of connection can also be combined, and wherein both types of rungs can definitely be realized on a stiffening structure. Through these measures the bending stiffness of the pallet on the one hand and the shear strength of the pallet in a plane parallel to the upper side of the deck on the other hand are increased vis-à-vis pallets known in the state of the art. In the case of a predominantly closed surface, the proportion of openings in the rungs is less than 50%, mostly less than 25%. Recesses and openings are found only where this is necessary or advantageous for reasons of manufacturing technology. In fact the proportion of openings is therefore less than 10% of the surface as a rule.
The at least one stiffening structure is thus formed as a ladder-like structure with side rails and rungs, wherein the side rails are connected to the rungs and firmly and preferably inseparably interconnected, with the result that the ladder-like structure is capable of absorbing correspondingly high shear forces. The firm and preferably inseparable connection which is necessarily present in the case of one-piece formation of the rungs and the side rails and, in the case of designs in which the rungs are not formed on side rails, is preferably achieved by an extensive adhesive bond, for example by gluing, but particularly preferably by welding, is only a partial aspect. To increase the bending stiffness or shear strength it is equally essential that the rungs have a predominantly closed surface, in the case of plate-shaped rungs for example, this means that in the plate-shaped rung parts, as few openings or recesses as necessary are formed, which however in any case occupy less than 50% of the overall surface of the plate-shaped rung part, as a plurality of such recesses reduce the shear strength. If possible, such openings should be dispensed with. As a rule, the plate-shaped rung parts therefore comprise either no openings, or only one, two or three openings through which, for example, optional cross braces can be pushed to form a grid structure. Should no cross braces be used, the ladder-like stiffening structures therefore preferably comprise no openings.
There are various possibilities for connecting the stiffening structures to the pallet or inserting them therein. They can for example already be inserted into the mould, for example an injection mould, during production, with the result that the stiffening structure is almost completely enclosed by the hardened plastic. In this way a particularly firm fit can be guaranteed. In order to be able to exchange the stiffening structures in the event of wear, they can also be pushed into the pallet or the feet of a one-part pallet from below or above. The connection to the plastic of the pallet can then also be effected in friction- and/or form-locking manner. However, the pallet is preferably designed in several parts, and the stiffening structures—optionally connected via cross braces—are inserted into the runners before the deck is placed on the runners and connected thereto, for example via snap locks or in friction-locking, form-locking or bonded manner.
In a simple embodiment, the stiffening structure can for example be manufactured in one piece from strip steel, wherein the spaces between the rungs are punched out, milled or introduced into the stiffening structure in another manner that is suitable in terms of processing technology. The thicker the strip selected, the more the shear strength is also increased. At the same time, however, the mass of the plastic pallet is also increased and if the ladder-like stiffening structure is made of metal, in particular of steel—as is preferably the case—this can lead to the mass of the plastic pallet with stiffening structures becoming higher than the mass of a comparable wooden pallet, with the result that a substantial advantage of the plastic material is lost. On the other hand, too thin a sheet as ladder-like stiffening structure cannot produce the required shear strength. Instead of being made of metal, the ladder-like stiffening structure can also be produced from other materials which can provide the necessary bending and shear stiffness of the pallet. For example, glass-fiber- or carbon-fiber-reinforced plastics are also possible.
However, it has become clear that a sufficiently high shear strength can be produced if in particular the side rails have a corresponding thickness, whereas the rungs can be designed with a smaller thickness. In a preferred embodiment, the side rails therefore have a predetermined thickness, which can be established for example with reference to the required shear strength. By the thickness of the side rails is meant the extent of the side rails perpendicular to their longitudinal direction and perpendicular to the longitudinal direction of the rungs in the ladder-like structure. By designing only the side rails thicker, significant savings can be made in terms of material and thus weight, without resulting in loss of shear strength.
If the rungs are designed in one piece on the side rails, side rails and rungs merge; the rungs can therefore be produced thinner. If the rungs are connected to the side rails via contact surfaces in bonded, friction-locking and/or form-locking manner, the contact surfaces are selected as large as possible in their extent, namely both in height—i.e. in the longitudinal direction of the side rails—and perpendicular thereto, wherein curved surfaces are also possible, basically perpendicular to the height.
In order to guarantee a high degree of stability with respect to bending and shear strength, the rungs have a predetermined height in the longitudinal direction of the side rails—the width in the view in the case of lying ladder-like stiffening structures—which corresponds to at least 80% of the width of the respective foot receiving the rung. For the sake of clarity, the term “height” relates to a standing ladder-like structure; in the case of lying ladder-like structures, this corresponds to the width in the view. The height of the rungs is preferably selected such that the maximum available installation space in the respective foot—which can be different for different feet on the same pallet—is utilized, i.e. in the case of a bonded, friction-locking or form-locking connection, the extent of the contact surfaces in the longitudinal direction of the side rails preferably corresponds to the predetermined height.
The side rails need not be made of solid material over the entire thickness; the side rails can also be formed as hollow structures with different cross-sections. Particularly advantageously, the hollow structure is assembled from various surfaces, wherein in each case at least one of the surfaces of a side rail is aligned parallel to the deck upper side—i.e. perpendicular to the longitudinal direction of the side rails and the rungs, which also contributes to the increase in stability. In the case of the use of hollow structures, the side rails are for example formed as tubes with the cross-section of a quadrilateral, for example a trapezium, rectangle or square, and then correspondingly comprise four surfaces. Alternatively they can also be formed as T-beams or as double T-beams; here too at least one surface—that of the crossbeam of the “T”—lies parallel to the deck upper side.
In this way it is possible to achieve a high degree of stability of the stiffening structure with respect to bending and shearing in the pallet, perpendicular to the direction of the runners, i.e. perpendicular to a plane in which the ladder-like structure lies.
If the rungs are connected via contact surfaces in bonded manner to the side rails formed as tubes with a quadrilateral cross-section, these contact surfaces preferably lie parallel to the deck surface and the extent of the contact surface in the direction of the thickness of the side rails is at least one-quarter of the thickness, but preferably at least half the thickness. Particularly preferably, the extent of the contact surface in the direction of the thickness however corresponds to the entire thickness; this guarantees the best possible stability of the bonded and extensive connection.
The contact surfaces can, however, also lie perpendicular to the deck surface in the plane spanned by rungs and side rails; in the case of tubes with a rectangular cross-section, for example, small plates can then be welded to the side rails, without the plates having to be bent. Depending on the shape of the side rails, the contact surfaces can also have any other shape, or protrude at a different angle; it is important that the contact surfaces are selected so large that they guarantee a secure connection of rungs and side rails up to a predetermined maximum shear and bending load.
This also applies in the case of a friction-locking or form-locking connection. The latter can for example be designed as a snap lock wherein the contact surfaces, in the case of rungs and side rails, then correspond to the surfaces of the lock which lie next to each other in the connected state. A correspondingly stable connection can, for example, be achieved if the snap lock is aligned along the longitudinal direction of the side rails and extends over the predetermined height.
In order to produce a sufficiently stable friction-locking connection, the rungs can, for example, be formed wedge-shaped on their sides facing the side rails—here too preferably over the entire height—and the side rails can comprise corresponding receptacles.
The ladder-like stiffening structure can be realized in a different way; particularly advantageous embodiments are described below.
In a particularly preferred design, particularly suitable for very high quantities, the stiffening structure is formed as an extruded aluminium profile. In this case, the rungs are formed in one piece on the side rails. Between the rungs, openings are made, for example by punching or milling, through which the forks of a forklift truck can enter. Aluminium has the advantage that it is a light metal; in addition no protection against corrosion is necessary.
In a further preferred embodiment which is suitable in particular for smaller and medium quantities of less than 10,000, the stiffening structure is formed in one piece as a tube with a square cross-section, which is bent into the shape of two side rails with rungs lying in between. In this way it is possible to design a stiffening structure with a maximum of three rungs which are formed in one piece on the side rails. Such a stiffening structure can be realized in different ways which differ from each other especially with respect to where the two tube ends are arranged in the stiffening structure. For example, through seven-fold bending, in each case by 90°, it is possible to produce an “8”-shaped structure. In a preferred embodiment which requires only six bends, the two ends of the tube are bent from one of the side rails to the other, opposite side rail and form the middle rung. The tube ends are connected to each other and to the other, opposite side rail in bonded manner. The connection is particularly preferably effected over the entire thickness of the side rail. This type of production makes it possible to provide the tube ends with a further bend to increase the stability, with the result that the effective height of the rung, corresponding to the width in the case of a lying ladder-like structure, grows. This increases the stability with respect to bending and shear strength, when forces act in the area of the middle foot. The bonded connection is particularly preferably produced by welding; the welding points are then protected against corrosion, for example by galvanizing. This profile is in principle relatively inexpensive to produce, as tubes with a square cross-section, for example with a cross-section of 20×20 mm and a wall thickness of 2 mm, are available in large quantities on the market. When profiles are produced, approximately a quarter of the costs arise through sawing the square tubes in order to cut them to length. Through the use of a single, bent tube these costs can be minimized.
In another embodiment which is somewhat expensive in production and more expensive because of the more time-consuming manufacture, the side rails are also formed as tubes with a square cross-section, however at least the inner rungs are formed as plate-shaped connection elements, in the case of which contact surfaces are formed as standing seams on two opposite sides. These are one-piece elements which are also commercially available as so-called C-profiles with a wall thickness of 2 mm for example; alternatively, production by cutting and bending from a flat sheet is also possible. Steel sheet is in particular possible as material, but all other metals and metal alloys which fulfil the requirements can also be used.
By a standing seam is meant a bending-up of the edge of the plate-shaped connection element by 90°. The bent-up surface of the plate-shaped connection element then forms the contact surface. The extent of the contact surface in the direction of the thickness of the side rail is at least one-quarter of the thickness. In the case of a tube diameter of the square tube of approximately 2 cm, the bending edge then lies at a distance of at least 5 mm from the edge of the plate-shaped connection element. However, for a stable connection it is advantageous to make the contact surface as large as possible, with the result that the bending edge lies at a distance of at least half, i.e. 10 mm, at best even the thickness of the tube corresponding to 20 mm from the edge of the plate-shaped connection element, parallel thereto.
A particularly stable, however also production-intensive variant is obtained if all of the rungs are designed as such plate-shaped connection elements, including the outer rungs. At the contact surfaces the plate-shaped connection elements are welded to the tubes, then the welding points have to be galvanized. Depending on the choice of material, it can also be necessary to galvanize the entire stiffening structure.
A somewhat less production-intensive variant, in which the high degree of stability with respect to bending and shear strength in the case of a stiffening structure with three rungs is retained for the middle rung—on which experience shows that the greatest forces act—consists of designing the middle, inner rung as a plate-shaped connection element with contact surfaces formed as standing seams, as described above, but bending the two outer rungs from a tube with a rectangular or square cross-section. The two side rails and the two outer rungs are in this case formed in one piece from a bent tube.
Further possibilities for keeping the material consumption as low as possible in the case of a high degree of stability with respect to bending and shearing consist of using thinner sheets instead of thick sheets or thick stiffening structures, in which case the side rails are formed by bending along the longitudinal direction of the side rails. In this way, seams can be formed on the side rails. The introduction of beading as a special form of the bending is also possible as a reshaping likewise serving for the stiffening; beading can be introduced into the side rails at any point in the longitudinal direction thereof. In this case the stiffening structure is formed as a rolled and/or bent metal profile, with openings made between the rungs which are formed in one piece on the side rails. The bending is effected in the longitudinal direction of the side rails. Sheets of different thickness can be used here, depending on the required load-bearing capacity, for example sheets with thicknesses of from 1 mm to 4 mm. The stability of the stiffening structure is therefore not achieved through the material thickness here, but through the formation of the side rails by bending, whereby they can also be impressed, in particular, with a predetermined thickness. When metal profiles are used, the side rails can be formed on the profile edges as standing seams in the simplest case. A higher degree of stability is achieved by double standing seams, i.e. by two 90° bends in the same orientation following each other at short distances in the transverse direction of the profile—with bending edges along the longitudinal direction of the side rails. The side rails can also be formed as foldovers, i.e. 180° bends. To further increase the stability it can be advantageous to combine standing seams and foldovers with each other. Between the rungs, the openings are made; this can be effected for example by punching, cutting or milling out. The rungs are preferably formed plate-shaped; i.e. in the longitudinal direction of the side rails, they have a predetermined height which almost reaches the dimensions of the feet in the longitudinal direction of the side rails. In the case of tapering feet, the shape of the plate forming the rung can also be correspondingly adapted, for example into a trapezium shape.
It goes without saying that the above-named features, and those still to be explained below, can be used not only in the stated combinations, but also in other combinations or alone, without departing from the scope of the present invention.
The invention is explained in more detail below, for example with reference to the attached drawings which also disclose features essential to the invention, in which:
The stiffening structure 5 is formed ladder-like and therefore comprises rungs 8 which connect the lower side rails 6 in the feet 3 to the upper side rails 7. The surface of the rungs is predominantly closed, i.e. it comprises no openings or recesses and if it does, then the surface of the openings or recesses is less than 50%, as a rule less than 10%, as a proportion of the entire surface of the rungs 8. Recesses and openings are made only where this is necessary or appropriate for reasons of manufacturing technology.
The rungs 8 are formed either in one piece on the lower side rails 6 or the upper side rails 7, or they are connected thereto, in each case in bonded manner via contact surfaces. Depending on the embodiment, some of the rungs 8 can also be formed in one piece on one or both side rails and other rungs can be connected to the side rails 6, 7 in bonded manner. The type of adhesive bond is selected depending on the material. In the case of metal stiffening structures 5, welding in particular is a possibility here. Depending on the material—for example carbon-fiber- and glass-fiber-reinforced plastics can also be used for the stiffening structure—other types of connection can also prove appropriate, for example friction- or form-locking connections, wherein all types of form locking can also be combined with each other.
Through the one-piece formation of the rungs 8 on the side rails 6 and 7, or through the bonded connection via larger contact surfaces on the one hand and through the predominantly closed surface of the rungs 8 on the other hand, the bending stiffness of the plastic pallet and in particular the shear strength of the plastic pallet in a plane parallel to the deck upper side 2 are increased.
Through the use of stiffening structures 5 formed in such a way, it is possible to reduce the deflection of the plastic pallet when supporting a load in the middle, for example from 22 mm to less than 10 mm in the case of a plastic pallet with the dimensions 1200 mm×800 mm and with 3 feet connected to runners. The shear stiffness is increased as shear forces are diverted via or absorbed by the stiffening structures 5 which can in particular be made of metal.
Using the stiffening structures 5 it is possible to reduce the deflection to the degree that is considered permissible in the case of wooden pallets of a comparable size, or to an even lower degree. The thicker the stiffening structures—by thickness is meant the extent perpendicular to the longitudinal direction of the side rails and perpendicular to the longitudinal direction of the rungs—the higher the shear and bending stiffness, which is however associated with a higher mass. Although plastic pallets are per se lighter than wooden pallets of the same size, in the case of correspondingly thick stiffening structures 5 the weight of comparable wooden pallets can be exceeded, thereby losing a substantial advantage of plastic pallets.
However, if on the other hand the thickness of the lower side rails 6, the upper side rails 7 and the rungs 8 is selected too small, for example as pure sheet with a constant thickness, in the case of too small a thickness, the necessary shear stiffness cannot be realized. For this reason, at least the upper side rails 7 and the lower side rails 6 have a predetermined thickness.
In the case of a bonded connection of the rungs 8 to the side rails 6, 7 via contact surfaces, and also in the case of a friction- or form-locking connection, the size of the contact surfaces is selected or predetermined depending on a predetermined maximum bending and shear load of the plastic pallet; as a rule the contact surfaces should be selected as large as structurally possible.
In the longitudinal direction of the side rails 6, 7 the rungs 8 have a predetermined height for increasing the shear stiffness and bending stiffness in the longitudinal direction of the side rails 6, 7, which is based on the width of the feet; it should be at least 80% of the width of the respective foot receiving the rung. Here the term “height” is used on the basis of a standing ladder, for a lying structure it corresponds to the width. In the case of a connection of the rungs 8 to the side rails 6, 7 via contact surfaces, the extent of the contact surfaces in the longitudinal direction of the side rails 6, 7 preferably corresponds to the predetermined height.
For the embodiment of the side rails 6 and 7, many design variants are possible, for example the lower side rail 6 and/or the upper side rail 7 can be assembled as hollow structures from various surfaces, for example they can be formed as tubes with the cross-section of a quadrilateral, in particular a trapezium, rectangle or square, which facilitates the connection of the contact surfaces; but an embodiment as a T-beam or as a double T-beam is also conceivable. At least one of the surfaces in each case of one side rail (6, 7) is then preferably aligned perpendicular to the longitudinal direction of the respective side rail 6, 7 and perpendicular to the longitudinal direction of the rungs 8. Contact surfaces can then be formed on these surfaces, in particular for the adhesive bond.
In the case of a bonded connection of the rungs 8 to the side rails 6, 7, the contact surface therefore preferably lies in a plane perpendicular to the longitudinal direction of the rungs 8 and the side rails 6, 7. The extent of the contact surface in the direction of the thickness should then as a rule be more than half the thickness. Depending on the embodiment, the rungs 8 can also have a smaller thickness, in the case of formation from a sheet, for example, a thickness corresponding to the sheet thickness.
Various embodiments of stiffening structures 5 are explained below with reference to
A third embodiment is shown in
A modification of this embodiment is shown in
A further design for a stiffening structure is shown in
This fourth embodiment of a stiffening structure has a particularly good cost-benefit ratio, for one thing as the square tube 14 has to be cut to length only once and bent only four times. However, due to the plate-shaped connection element which can have a C- or S-shape in cross-section, the shear strength and bending stiffness are further increased compared with the designs shown in
A further—particularly stable—fifth embodiment of a stiffening structure for a plastic pallet is shown in
Although the designs described in
The metal profile shown in perspective in
An eighth design of a stiffening structure is finally shown in
All the profiles are characterized by the fact that, with relatively low mass, they are capable of giving a plastic pallet the required bending and shear stiffness, with the result that the deflection in the middle is no greater than in the case of wooden pallets; on the other hand however, the mass of the plastic pallet with stiffening structures is even lower than in the case of conventional wooden pallets of the same size. Whereas the latter, with dimensions of 1200×800 mm, have a weight of 20-25 kg, it is possible with the invention presented here, to keep the weight of the plastic pallets significantly below this, at approximately 15-20 kg.
| Number | Date | Country | Kind |
|---|---|---|---|
| 17 169 002 | May 2017 | EP | regional |
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| 20180339802 A1 | Nov 2018 | US |
