Pallet having impact resisting plastic top

Abstract

A thermoplastic pallet has a top frame which comprises bridge parts that span the openings between columns into which forks may be inserted for lifting. The plastic bridges are designed for absorbing horizontal impact loads. Each bridge has several lengthwise impact absorber zones of differing compliance to horizontal loads. The zones are comprised of vertical ribs which define open or closed cells. The edge of the pallet deck, which is preferably corrugated metal, is spaced apart from the outer edge of the bridge. It is alternatively fastened to the inner edge of the bridge or to a subframe which spaced apart from the inner edge of the frame.

Description

TECHNICAL FIELD

The present invention relates to pallets used for transporting industrial and commercial goods by means of forklift devices and the like.

BACKGROUND OF THE INVENTION

Traditional wood pallets and newer thermoplastic pallets can be damaged when they are hit by an object, such as by part of a forklift device, or by being rammed against a stationary object. Impact from the heel of the tine of a forklift truck has been identified as a common cause of damage. The problem is exacerbated when the fork truck hits the pallet at an angle; and when the pallet is heavily loaded and on a slip-resistant surface.

The edges of the top of the pallet are particularly prone to damage. When the edge of a common wood pallet deck is damaged, a slat can be replaced. However, molded plastic pallets, having integral structures, are not so easily repaired. Thus, for plastic pallets there is a need for either lessening the likelihood of damage, or devising some easy repair procedure. The present invention uses the former approach.

A popular plastic pallet, which can be made in the present invention, is often referred in the U.S. to as a GMA pallet. It has a rectangular base, eight columns running up from the periphery of the base, and a rectangular deck. The pallet is in the shape of a 40 inch×48 inch rectangle. A comparable European pallet is a 1000 mm by 1200 mm pallet. For such a pallet to become accepted for widespread use in commerce, it must meet various technical and performance standards. The Grocery Manufacturers of America (GMA), Washington, D.C., U.S., in conjunction with other organizations, has published a document entitled “Recommendations on the Grocery Industry Pallet System” (1992). It describes requirements some of which have been added to or modified in the subsequent years.

Some manufacturers and users use tests to measure the resistance of a pallet to such kind of damage. For instance, one large industrial commercial user has what are called tine heel tests, and a typical test involves 10 simulated blows of an about 700 pounds weight, traveling at about 6 feet per seconds at a given location on the edge of the top part of the pallet. See also “Recommended Test Protocol for Plastic Pallet, Version 3” (1998) published by Virginia Tech, Blacksburg, Va., U.S., sets forth mechanical performance and test requirements for pallets including a warehouse racking test, as just mentioned.

Plastic pallets have been in commercial use for some time, but to date commercial pallets which meet important criteria, such as structural strength, cost, weight, or fire resistance, have in general been unsuccessful in passing the severity of the 10-blow tine heel test. The top decks of plastic pallets are particularly prone to tine heel damage; they tend to locally fracture and there is progressive failure from repeated blows. But since plastic pallets are molded and usually welded together, pallets are not easily field welded or otherwise repaired. So, if a pallet edge is damaged, the result can be costly discard of the pallet.

One of the limitations in making a pallet with a damage tolerant edge is to avoid adversely affecting the other strength characteristics of the pallet, or weight, fire resistance, and cost of manufacture. Thus, there are many constraints in addressing the problem structurally and by materials engineering. There is a need to make improvements which are at the same time economic, light weight, and otherwise do not undercut the other advantages attributed to plastic pallets.

SUMMARY

An object of the invention is to provide a pallet having a plastic top with resistance to damage from horizontal impact blows. A further object is provide a pallet having a metal deck with means for protecting the deck from permanent deformation for such kinds of blows. A still further object is to provide means for making wood pallets more durable.

In accord with the invention, a pallet has a plastic frame supported on columns running upwardly from a base, and a deck for carrying goods. Bridge portions of the frame, which span the fork openings between the columns, have impact absorbing structure for resisting horizontal impact loads. Preferably, the lengthwise portion of a bridge which is near the outer edge of the pallet has compliance to horizontal impact load which is different from the compliance of the lengthwise portion of the bridge which is near the inner edge. Compliance refers to the deformation and absorption of impact loads which characterizes a structure or portion thereof. The deck of the pallet may be attached in different ways; the outer edges of the deck are inset from, or spaced apart inwardly from, the outer edge of the frame.

In different embodiments, the compliance of a bridge may either increase or decrease or be uniform across the width of the bridge. The deck is attached to a low compliance, lengthwise portion of the bridge, near the inner edge of the bridge. The lengthwise zone near the outer edge of the bridge has greater compliance. When impacted, the outer zone is an impact absorber. It deforms elastically to accommodate the load. There is at least one, preferably at least two, lower compliance zones, compared to the inner zone where the deck is attached, and which has a structure characteristic of a common plastic structural member.

In another embodiment in accord with the invention, the edge of the pallet deck is spaced apart from the inner edge of the bridge. For example, the deck is supported on a subframe mounted within the open rectangle of the frame. Preferably, the outer portion of the bridge has lengthwise zones of differing compliance; and, zone near the outer edge is lower in compliance, i.e., stiffer, than the inner zone or zones.

The higher compliance of the impact absorber zones or a bridge as a whole may be achieved in different ways. Preferably, they are achieved mechanically, as by having open or closed cells which are defined by transverse and lengthwise vertical running ribs. The shape and size of the cells will vary, to change compliance. Alternately, different property materials may be used in the bridge to vary compliance. Compliance may change step wise or progressively, across a bridge, from high to low.

When cells are used to effect higher compliance, the bulk density of the impact absorber portion of a bridge is about 30-60 percent of the bulk density of the thermoplastic material of the pallet, which is preferably a polyolefin. Preferably, each bridge has a metal stay running at or near the outer edge, to enhance its resistance to impact damage. Preferably, the invention is used in combination with an injection molded polypropylene GMA pallet, and the pallet has a metal deck attached to a subframe which is spaced apart from the inner edges of the frame and thus of the bridge parts which substantially deform when complying to an impact load. Preferably the pallet has a base which includes metal beams. The invention may be used with pallets which have plastic decks of the type known in the prior art.

In further accord with the invention, a pallet which is made predominately of non-plastic material, such as wood or metal, has a top deck which comprises at least one leader, or end board, that is made of thermoplastic and has features which characterize the bridges of the thermoplastic pallet.

In still further accord with the invention, a method of making a pallet having bridges with impact absorbing rib structure includes making a cover plate having a rib buttress which may or may not have stay attached, and welding that cover plate to a pocketed basic element.

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the corner of a pallet having an impact absorbing edge.

FIG. 2 is a more detail view of the pallet of FIG. 1

FIG. 3 is a vertical cross section view through the bridge spanning the columns of the pallet of FIG. 1.

FIG. 4 is an isometric view of another pallet with the cover plate of the frame removed to show the interior cell construction.

FIG. 5 is an isometric view of part of the frame of the pallet of FIG. 4.

FIG. 6 is a top view of part of the frame of the pallet shown in FIG. 5.

FIG. 7 is a cross section through a pallet frame like and deck, where the frame comprises a flat cover plate, like the frame shown in FIG. 4.

FIG. 8 is like FIG. 7, showing a frame formed by a cover plate which has integral ribbing.

FIG. 9 is a partial isometric view showing a vertical cross section like that shown in FIG. 8 and 9, where the bridge has through holes.

FIG. 10 is a top view of a portion of a pallet having a metal stays along the outer edges of the portions of the frames, or bridges, which span the space between the columns.

FIG. 11 is a vertical cross section through portion of the pallet shown in FIG. 10.

FIG. 12 is like FIG. 10, and shows how a bridge deforms under an impact load.

FIG. 13 shows alternative stay ends and anchors.

FIG. 14 is an exploded vertical cross section view of part of a welded bridge like that shown in FIG. 9.

FIG. 15 is an exploded isometric view of a bridge comprised of a base structure and a cover plate having ribbing and a serrated stay.

FIGS. 16A, 16B and 16C show different embodiments of stays.

FIG. 17 is an exploded view of a pallet which includes features shown in other Figures.

FIG. 18 is an isometric view of a portion of a wood pallet having an impact absorbing plastic leader board.

FIG. 19 is a vertical cross section through an alternative dual material plastic leader, having a metal stay for impact resistance.

DESCRIPTION

The present invention may be used with a predominately thermoplastic pallet which incorporates metal reinforcing beams, as described in U.S. Pat. No. 6,705,237 “Plastic Pallet Design” of Moore et al. Preferably, the pallet has a corrugated aluminum or other metal deck as described in a related application entitled “Plastic pallet having metal deck”, bearing Atty. No. EPC-2435, and filed on even date herewith by R. Brochu et al. The disclosures of the patent and patent application are hereby incorporated by reference.

The pallet may be molded in components and subassemblies, preferably by injection molding using gas assistance. See U.S. Pat. No. 5,401,459. The several parts or subassemblies may be joined together as an assembly by known thermoplastic fabrication methods, including welding, adhesive bonding, mechanical interlocking, and by fasteners. See for instance, U.S. Pat. Nos. 6,250,234 and 6,283,044. The thermoplastic parts of the pallet may be made of commercial grade polypropylene, high density polyethylene or other polyolefin. Other plastics may be used. It will be understood that regions which are referred to as solid, or which are shown as solid for convenience, are characterized as such in contrast to and distinction from other portions, such as impact absorbers, which have substantial size cells and low bulk density, or other compliance-lowering features. Any parts of the pallet which are called solid may have comparatively small interior open or closed hollows. For instance, the parts may have artifacts of the molding process or they may be intentional hollowed by recesses and the like, for purposes of reducing shrinkage, economizing on material, etc. Preferred pallets of the present invention preferably have metal beams in the cross rails of the base together with a corrugated metal deck. The pallets here are characterized as predominately thermoplastic, or simply as “thermoplastic”, because by weight they are more than 50 percent thermoplastic. Use of the term plastic or thermoplastic with respect to a pallet or part of a pallet does not exclude the presence of lesser fraction of metal, fibers, fillers, fire retardants, etc.

FIG. 1 shows one quarter section of a rectangular pallet 20 having an impact absorbing rim and a textured metal deck 22. FIG. 2 is a more detailed view of the impact absorbing bridge part of the pallet. Pallet 20 comprises formed sheet metal deck 22, preferably made from Type 5052 aluminum alloy, which is attached to injection molded thermoplastic frame 24. In general, the pallet has a configuration which is familiar in GMA plastic pallets. The frame is by definition an open-center rectangle, i.e., it runs around and encloses a space. The deck spans the interior space and is attached to the center column. The deck, and such portion of the frame which may not be covered by the deck, provide the top surface of the pallet for receiving goods.

The frame is mounted on eight vertical columns, namely outer or peripheral columns 28 (comprising corner columns and midpoint columns) and center column 29, which run upwardly from pallet base 30. The columns may be fabricated as integral elements of the injection molded frame. See part 45 in FIG. 17. Base 30 is comprised of outer rails 31 which form a rectangle. Cross rails 33 interconnect the centers of the outer rails and intersect at the location of center column 29. Metal beams 74 run within the base rails 31, 33, as taught by the art, including by U.S. Pat. Nos. 5,868,080, 6,705,237, and 6,955,128. Additional beams of metal or other high elastic modulus and high strength material (compared to the plastic) may be used, including around the periphery of the frame or across the center of the pallet at the elevation of the deck, although doing so will tend to compromise other advantages of the present invention which are attained with the use of a corrugated metal deck.

The invention is described in terms of a rectangular GMA pallet, referred to in the foregoing Background, which pallet is about 40 inch×48 inch×5 to 6 inches high pallet, and has a 12 to 13 inch wide and 3 to 4 inch high opening 34 between each pair of outer columns for entry of lifting forks. Bridge portions 25, hereafter called simply bridges, are those portions of the rectangular frame 24 which span the spaces between the columns. In the FIG. 1 embodiment, bridges 25 are the means of support for deck 22. Bridges 25 comprise portions 26 with impact absorbing characteristics, which portions are hereafter referred to as impact absorbers 26, which enable bridges to resist damage from horizontal impact loads, such as those imposed by the heels of forklift tines.

FIG. 3 is a vertical cross section through the outer part of pallet 20. The top surface of deck 22 lies in essentially the same plane as the top of frame 24. The deck is fastened to the frame by hold-down plate 60, which is pinned to the frame. The edge of the deck is inset from, or horizontally spaced apart from, the outer edge of the frame. That enables horizontal elastic deformation of the outer portion of the bridge without causing permanent deformation of the deck. Near its outer edges, the deck has corrugations which run parallel to the edges.

Compared to the inner solid portion 62 of the frame, and to the portions of the frame which overlie the columns, the impact absorber portion 26 of the bridge has lower bulk density and lower section modulus about a vertical axis and greater compliance to a horizontal load, when the pallet is hit laterally by a fork truck part. Why this is so, and the advantage, are now described for the pallet of FIG. 1 and another style GMA pallet, and for a predominately wood pallet.

As illustrated by the embodiment of FIG. 1 through FIG. 3, an exemplary bridge 25 of pallet 20 has three different compliance zones, 79, 81, 62, running lengthwise. Two of those compliance zones, 79, 81 comprise the impact absorber 26. The innermost solid portion, zone 62, supports the deck. The impact absorber comprises a combination of lengthwise ribs 52, 27 and transverse arcuate ribs 54. Outermost lengthwise rib 27 provides the outer edge of the bridge. Arcuate ribs 54 run transversely, to interconnect the lengthwise ribs and the inner portion 62 of the bridge. Referring to the outer zone 79, the lengthwise ribs and arcuate ribs define a multiplicity of small nominally circular bores 58. The ends of the circular bores 58 are flattened, as shown. Between each bore is an hour-glass shape bore 56. The bores will hereafter be mostly referred to as open cells. The almost circular and hour-glass cells which comprise the outer zone 79 are larger than the corresponding cells which comprise the middle zone 81. The arcuate and lengthwise ribs 52 have nominally the same thickness. Thus, under a horizontal plane load applied at the outer edge of the pallet, the outer zone 79 will be less stiff, and will have more compliance, than the middle zone 81. The innermost solid zone 62 will be the most stiff and least compliant. When a horizontal impact load hits outer edge rail 27, bending of the arcuate ribs causes them to deform into the hour glass bore spaces.

In an example of bridge 25 of pallet 20, the diameter of the outer bore 56 is nominally 1.25 inches; the inner bore hole 58 diameter is about 0.875 inches; the segments 54 and rib 52 are about 0.187 inches thick; and, the outer edge rail 27 is about 0.225 inches thick. The bridge is about 5 inches wide and the impact absorber zones are 2 to 3 inches wide. The bulk density of the impact absorber portion is preferably about 30 to 60 percent of the density of bulk density of thermoplastic material of the pallet.

Compliance is the mechanical shape change response of a fixed object to an applied load. When a body is said to be have more compliance or to be more compliant, it means that for a given force or load, a structure or portion of structure, has a greater deformation or accommodation of the load. In the case of a pallet frame and bridge, the predominant interest here is the compliance to a horizontal load, one which acts parallel to the plane of the pallet top. The whole of a structure, e.g., a bridge, or different zones in a structure, e.g., portions of a bridge, can be treated and analyzed with respect to compliance. Compliance can be determined through structural analysis, in particular by finite element analysis; or by experimental testing, as by using strain gages. Stiffness is a property of a structure or portion which is the inverse of compliance; it may alternatively be used in characterizing the invention.

When it is said here that the compliance of one zone is different from that of another, that means the difference is significant to mechanical behavior of the item. Generally, it will mean there is a difference of least about ten percent or more. In this description all portions of a structure are presumed to have some degree of compliance; and, all materials used in a pallet will exhibit plastic behavior beyond an elastic limit. When there is compliance by a portion of the pallet, it means the portion first deforms elastically in response to an applied force. Compliance deformation in a pallet of the invention is primarily resilient; i.e., within the elastic limit of the structure. However, within the realm of elastic compliance, there may be small regions of stress concentrations and resultant plastic deformation. A part of the pallet may suffer some minor “set” or permanent deformation. It is undesirable in a pallet to have a large amount of permanent deformation, although obviously some can be accepted, while the pallet remains useful.

Since in bridge 25 of pallet 20 the open cells run vertically, the impact absorber portion, and the bridge as a whole, has much lesser compliance to vertical loads than to horizontal loads. Thus bridge can bear the load of a deck carrying goods, without undue deformation. The difference in directional behavior of the impact absorber of bridge 25 can be characterized in terms of unit section modulus, that is, the section modulus of a square cross section portion, one in which the height of the section is equal to the height of the bridge. The unit section modulus of each bridge is substantially greater in the vertical direction than in the horizontal direction.

When it is desired to have increased stiffness at the innermost portion of the bridge, that portion may be made thicker; or, it may include a lengthwise running member made of metal or other material having elastic modulus higher than the plastic. (Such a stiffener is different from the stay described below.) While the three compliance zones just described are preferred, in the generality of the invention, there may be more or less. For example, three may be only an inner zone 62 having high stiffness and low compliance, to which the deck will be attached, and an outer zone comprising the rest of the bridge width, which will be the impact absorber. There may be more zones, as next described. The stay may be used with a bridge having a uniform compliance across its width, that is, a bridge comprised of one zone, when the bridge is not used to support the deck and is spaced apart from the edge of the deck.

FIG. 4 through 9 show another embodiment pallet, 20A, which has a frame/bridge having some different and additional features. FIG. 17 is an exploded view of the pallet 20A. Numbers with suffixes denote elements which correspond with those previously mentioned. FIG. 4 shows a partial cutaway quadrant of rectangular pallet 20A. Part of the cover plate of the frame is cut away to show the interiors of the closed cells.

Pallet 20A has a frame 24A with an impact absorbing bridge 25A. The columns 28A are molded integrally with the frame and part of the base, in the form of subassembly part 45. See the exploded view in FIG. 17. Pallet 20A is different from pallet 20 in several respects, including the direction in which the compliance of the bridge varies across its width and the manner in which the outer edge of the deck is supported with respect to the frame. There are other differences which will become apparent.

In the FIG. 4 embodiment, frame 24A is constructed by welding cover plate 39 to the injection molded basic frame element 70, which is a portion of a larger injection molded part 45. The molded part 45 includes basic frame element, a subframe and brackets and eight peripheral columns 28A. The columns 28A, which are hollow (and which are preferably filled with thermoset foam upon final assembly), have vertical internal ribs and shallow vertical external surface grooves, for impact resistance, as described in a related patent application.

FIG. 4 through FIG. 8 show the internal rib structure of the frame along with other features and alternatives. With particular reference to FIG. 4, and FIG. 7 to 9, deck 22A rests on a support means, namely rectangular subframe 64 which runs around the interior of frame 24A. The subframe is spaced apart from the inside edge 72 of the frame. Subframe 64 is mounted on cantilever supports 68 which extend inwardly from the frame, at locations where there are columns. Optionally, the subframe and supports may include metal members for strength. Thus, the spaces 30 between the frame and the outer edge of the deck and subframe enables the bridges of the frame to deform substantially inwardly under impact load, without deforming the deck. A space 30 is about 0.3 to 2 inches wide, most preferably about 1.3 to 1.5 inches. For more details of the deck suspension system of pallet 20A, see the related application entitled “Plastic pallet having deck suspension system”, bearing Atty. No. EPC-2439, filed on even date herewith by R. Moore et al., the disclosure of which is hereby incorporated by reference. In the present invention, other deck supporting means may be used, to space the deck apart from most or all of the inner edge of the bridges. The inner edge of the deck may be inset about 5 inches from the outer edge of the frame. In the FIG. 1 embodiment the inset may be 1 to 3 or more inches. For a 40×48 pallet, the deck will have an area which is at least 75 percent of the area of the whole of the top of the pallet. While the invention is described and in some respects may be claimed in terms of a 40 inch×48 inch U.S. GMA pallet, those dimensions may vary within a few percentages; and thus, the dimensions will comprehend a 1000 mm×1200 mm European pallet.

FIGS. 5, 6 and 7 show the shapes and arrangement of the interior cells which provide a bridge 25A of the frame 24A with special impact absorbing performance. In this embodiment, the bridge has lengthwise impact absorber zones which have compliances that progressively increase with distance from the outer edge. In comparison to a bridge of the pallet 20 shown in FIG. 1, a bridge of pallet 20A has no solid inner most portion for supporting the deck. The outermost, not innermost, zone has the greatest stiffness and least compliance. In this embodiment, that helps the outer edge resist cracking due to very concentrated loads; and the design cooperates with the use of a stay, which is described below. This is enabled since the deck is mounted on the subframe. FIG. 5 shows a portion of the bridge 26A with the cover plate 39 removed. FIG. 6 shows the vertical running rib construction of the bridge in more detail. FIG. 7 shows the cover plate 39 in place. FIG. 8 shows a different configuration of cover plate and basic element; that is a different way of constructing a bridge. FIG. 9 is like FIG. 8, showing a perforated bridge cover plate, which in the bridge creates some open cells or through-holes. See also FIG. 15.

Bridge 25A can be considered as a beam, for resisting horizontal loads and for supporting vertical loads carried on the top of the pallet. The cells are bounded or defined by ribs. All or most of the cells are closed. The special configuration of the ribs and cells provides the desired shock absorber properties. As shown in FIG. 7, in the finished assembly, all the ribs of the bridge are connected to the top plate 39 and bottom plate 37, and the construction provides good support for vertical loads, better than when the cells are open. See also FIGS. 8 and 9.

The frame may be constructed by injection molding a pocketed basic frame element 70 which part of the larger element 45, and then attaching a flat cover plate 39 to make a unitary structure, as shown in FIG. 7. The cover plate may be attached to the element 70 by hot plate welding, vibratory welding, ultrasonic welding, and other known processes, less preferably including mechanical fastening and snap fits. The cells of element 70 thus become closed cells inside the impact absorber. With reference to FIG. 5 through FIG. 7, element 70 of the frame has a multiplicity of cells 29, 41, 33, 35 which are defined by lengthwise ribs 40 and cross ribs 43. The bottom surface of the bridge and impact absorber is provided by integral plate 37. In an example of the invention, the bottom plate 37 is about 0.09 in. thick, the top plate is about 0.10 inch thick and the cells are about 0.5 inch high. The ribs are typically about 0.2 inch thick. Thus, the heights of the cells are about 70% of the height (thickness) of the frame.

The rib arrangement, which is configured to distribute load in a special way, can be characterized by referring to the shape of the cells or pockets which the ribs define and bound. With reference to FIG. 5 through FIG. 7, the size of the cells increases from the outer edge to the inner edge of the bridge. As a corollary, the density of rails is greater near the outer edge. Thus the stiffness is higher and the compliance is lower at the outer part of the bridge than it is at the inner part. Preferably, there are four rows 82, 84, 86 and 88 of cells, which comprise four different lengthwise zones of the bridge. Their shape and dimensions can be ascertained from FIGS. 5 and 6.

More specifically, the cells 29 of row or zone 82, which is nearest the outer edge rail 27A of the bridge, have a first size. All cells have the same width, where width corresponds with bridge width. The cells in the different rows have different shapes. Cells 33 of the third zone 86 and fourth row 88 are the same size length, and their lengths are twice that of the cells 29, 41. As can be seen, rails 43 which define the innermost cells are continuous across the width of the bridge. Also, inside radii of the corners of the cells decrease with cell distance from edge 27A, to also help increase compliance. As shown in the variation of FIG. 6, radii at the outer ends of the cells 29A of the outer row 82A are great enough to provide a strong arch shape at the outer rail 27A.

In use, when there is a deforming concentrated impact load applied to edge 27A of the frame which comprises bridge 25A, the bridge deforms as a complex beam, to distribute the load and resist deformation. At the point of impact, the closely spaced ribs of the first row of cells transfer local compressive and bending loads to the ribs of the second row of cells, as well as to the attached upper and lower plates 37, 39. The close spacing of the ribs and the curved corners of the cells in the zones nearer to the outer edge of the bridge provide good strength, to help the outermost rib or edge 27A avoid fracturing. The portion of the bridge which is nearest the interior of the pallet has larger cells and fewer ribs. It is largely subjected to tensile and bending stresses, and has sufficient structure to carry the less concentrated loads transferred to it. It another way of characterizing the bridge of this invention embodiment, the inner region of the bridge has lower bulk density and thus lower section modulus than does the outer region.

FIG. 4 through 7 imply a certain method of construction, i.e., forming a basic element 70 having open pockets and then attaching a flat cover plate to make the pockets into closed cells. Another method of constructing the bridge is illustrated by means of FIGS. 8, 9, 14, and 15, and it is described below, since it facilitates the inclusion of a metal stay 55 when that is desired. The method is described further below.

An impact absorbing bridge of the type just described, when made of industrial polypropylene, is capable of resiliently deforming laterally at least 0.5 to 1.5 inches. That is up to about 12 percent of the about 12 inch open span of the bridge. When pallet 22A of FIG. 4 is compared to pallet 22 of FIG. 1 and to pallets of the prior art in a test simulating the blow of the heel of the tine of a lifting fork, pallet 22A excels. The special rib and cavity configuration minimizes the tendency of the bridge to tear. Since the deck is spaced apart from the bridge a sufficient distance, when deforming under load, the bridge will not contact and damage the edge of the deck. When the load is within the design limit, the maximum deformation of the impact absorber does not exceed the elastic limit of the plastic elements. And even if the deformation exceeds the elastic or plastic limit of the impact absorber, the deck may still be protected from permanent deformation, since it is spaced apart from the bridge in the preferred embodiment of FIG. 4.

As shown by FIG. 9, and also in FIG. 15, an alternate embodiment bridge 25C may have some open cells amongst the preponderant multiplicity of closed cells. Lengthwise spaced apart open cells are formed by means of through-holes 53 in the top plate and corresponding holes in the bottom plates. Open cells allow drainage and in particular, when appropriately placed, they enable fire sprinkler water to preferentially fall onto the outer rails. While the closed cell construction of the bridge is preferred, in the generality of the invention, bridges for a pallet of the type which includes a subframe or other suspension system may alternatively have some or all open cells, like those in the bridges 25 of pallet 20. Likewise, pallet 20 may have all or partially closed cells.

Fewer or more than the preferred four compliance zones may be used in the invention relating to pallet 20A, and they may less preferably change from low compliance at the outer edge In another embodiment, a bridge has uniform impact absorbing structure across its width, when the deck is not attached to the bridge, but spaced part as by the subframe mounting or other means.

A typical distinct property compliance zone in the invention will have a width, or horizontal plane measurement, which is a significant fraction, for example at least 20 percent, of the width of the bridge. In pallet 20 of FIG. 1, the nominal widths of the zones 79, 81, 62, as they run from exterior to interior of the frame are respectively about 30, 20 and 40 percent of the whole width of a bridge, for instance, an exemplary about 5 inch wide bridge. In pallet 20A, the width of each of the zones 82, 84, 86, 88 is about 20-25 percent of the width of the bridge, for instance in an exemplary 4.3 inch wide bridge.

While the embodiments described have largely step changes in compliance across the width of the bridge, more sophisticated design may be used to obtain more gradual progression across the width. In such instance, the term zone will be applied to mean a lengthwise portion of the bridge which is at least 20 percent of the bridge width. While the impact absorbing bridge has been described in terms of substantially constant thickness vertical ribs which define cells of different dimension and shape, within the generality of the invention, the rib thickness and orientation may vary along with, or independently of, variation in the cell configurations. In another alternative, the bridge can have changing compliance across its width as a result of parameter change other than rib and cell size. For instance, at least a portion of the desired change in compliance may be obtained by changing the density of the material, as by incorporating a multiplicity of relatively of small closed cells formed during the molding process. However, doing that will not achieve the directional structure properties achieved by vertical running ribs, particularly for open cell construction. In another alternative, different property plastic material zones may be used, as may be achieved by dual-shot injection molding of polypropylene in combination with an elastomer such as Santoprene™ elastomer While the invention has been described in terms of a metal deck, the invention can be applied to pallets having corrugated decks of other materials, and to other decks which are taught by the art, including wholly plastic injection molded decks, blow molded decks, decks made by pultrusion, etc.

A stay may be used with the foregoing impact absorbing bridge, to make it even more effective in resisting impact blows. A stay is a band of metal or other high modulus material (compared to the plastic) which runs near the outer edge of the bridge. FIG. 10 is a partial top view, and FIG. 11 is a partial vertical cross section, of pallet 120 which is like pallet 20. In FIG. 10 through FIG. 12, numbers for elements which are preceded by the digit 1 to denote elements corresponding to prior two digit number elements. Each side of pallet 120 has two metal stays 155A, 155B. Each stay runs along the front edge of one of the bridges 125. The ends of the stays are anchored in the plastic frame where it runs over columns 132.

FIG. 12 schematically illustrates how a bridge having a stay deforms when impacted by an object 42 like a tine heel, which has traveled along the angled arrow-path shown. The deformation position of the stay 155 (and the position of the outer edge of the bridge along which it lies) is schematically indicated in phantom by stay 155P. When performing within its design limit, the inner most zone of bridge 125 will not deform substantially, and thus the edge of deck 122 will not be damaged by the impact.

When impacted within its design limit, the stay and bridge deform elastically; and, when the load is removed they return to their original positions. The impact creates a substantial tension in the stay, and the resultant resolved vector force at the impact point opposes the load of the impact. In doing that, the stay transfers significant parts of the load to the stay anchor means. In experiments, deformation of an about 12 inch span bridge was largely concentrated in the center 4 to 6 inches of the bridge, when the bridge was hit in the exact center.

Stays 155 may be applied to pallets in different ways. In FIG. 10, each bridge has a separate stay. In an alternative, the stay runs from one corner of the pallet to the other, spanning two bridges. See the leader board of the wood pallet in FIG. 18. In another alternative, not shown, the stay runs circumscribes the pallet and attaches to itself. Separate stays with separate anchors are preferred because there will be a lesser extent of failure if there is failure of the stay at one bridge location. Preferably, the stay contained within plastic material, as described below, to protect it from possible corrosive environment.

FIG. 13 shows alternative stays 155M, 155N, 155P, 155Q, 155R with associated anchoring point structures 190M-R. After the ends of the stays are engaged with the anchoring structure, the cover of the frame is put in place. The plastic anchoring structures optionally include metal or other reinforcement. The stays 155M and 155N have holes so they attach to projections 190M, 190N. The stay 155P has a serpentine end which is captured within a plurality of closely spaced plastic pins 190P. The stay 155Q has a welded loop end which engages a pin. The stay 155R has a curved and angled end which is captured within the space between two close fitting buttresses 190R. In FIG. 16C, a tab 191 is formed at each end of stay 155V, by punching and bending out a small window of material. When installed, each end of the stay runs between the closely spaced pillars of the anchor 190V, so that tab 191 acts as a barb and prevents lengthwise motion. Other anchor end configurations may be used, including combinations of the foregoing with the perforations referred to below. After the ends of the stays are engaged with the anchoring points in the plastic basic element of the frame, the cover of the frame is put in place to enclose and protect the stay. Alternatively, stays may run along the exposed outer edge of the bridge, and the stays end can then enter frame interior to engage the anchors.

FIGS. 14 and 15 have relation to what is illustrated by FIGS. 8 and 9 namely a design and method of assembly. A bridge assembly comprised two main pieces: a basic element 170 and a cover plate 139. Cover plate 139 has a buttress 177, or male portion, comprised of downward projecting ribs. The buttress has slightly tapered opposing sides, for instance running at an angle from vertical of about 5 degrees. Rib buttress 177 mates with a lengthwise female cavity near the other edge of basic element 170, as indicated by the arrows connecting the two pieces in the Figures. Before the parts are welded together, the stay is laid into a groove on the vertical face of the outermost lengthwise rib of the buttress; or it is otherwise temporarily attached. When the parts 139, 170 are locally heated so they will weld, they are pressed together. The tapers provide good fit and the stay is thus captured inside the plastic assembly.

FIG. 15 also shows another feature of the invention: Stay 155U is anchored in two ways: First, by anchors at its ends, like stay 155R in FIG. 13. Second, by perforations in the stay which manifest as opposing side scalloped stay edges 79. The face of the rib buttress may have a molded in groove which is plain or has detail features, to receive the stay scalloped shape. After placement of the stay, the parts 139, 170 are mated and welded. Molten plastic will tend to flow about the stay and its edge perforations, thus anchoring it along its length, when the plastic cools and hardens. Thus, when there is an impact, stretching of the stay will be resisted by the engagement of the perforations as well as by the anchors. Thus, the tensile force generated in the stay is distributed along the length of the frame and away from the point of impact.

When a stay engages the plastic by means of a multiplicity of small mechanical features, such as by means of edge serrations or other perforations, it is here called “distributed anchoring”. As another example, a nominally one-half inch high scalloped stay 155V, shown in FIG. 16A, has offset opposing side scallop perforations. The scallop perforations have radii of about 0.5 inch and edge-depths of about 0.05 inch. When used with a nominal 12.5 inch span bridge, such a steel stay will be about 14.5 inch long; and, it will have bent ends of about 1.7 inch length L. Various other shapes of edge perforations may be used, including those which are sinusoidal or triangle shape. In another example, shown in FIG. 16B, stay 155W has a plurality of small diameter perforations. FIG. 16B also illustrates how a distributed anchor stay may be straight and have no shaped ends for anchors. The plastic material of the bridge which engages the stay perforations may be molded into a subassembly when it is made or may be formed by localized melting and flow of the plastic, as described. The distributed anchoring features may be non-uniformly distributed along the length of the stay. In substitution of some or all of the perforations, a stay may have raised portions such as tabs. Alternately, there may be pins or other fasteners through the stay. In still another alternative, adhesive bonding may be used, with or without a mechanical engagement of the type described.

A representative stay for this and other embodiments, where the edge of the plastic frame is about 0.7 inch high, is a 0.5 inch wide AISI 1018 carbon steel strip, having a hardness of about R_B 65-70, a rectangular cross section, a thickness of about 0.10 to 0.30 inches, preferably about 0.015 to 0.021 inches. When the stay is placed within the plastic of the bridge, there is preferably an about 0.13 inch layer of plastic covering the stay. Although the stay is thin, and even when it is covered with plastic, experiments with steel stays shown that the stays combination of higher hardness and elastic modulus substantially help protect the plastic portions of the bridge in resisting localized damage and fracture, particularly when there are repeated blows at the same spot.

In the invention, the modulus of elasticity, hardness, and tensile strength of the stay material are substantially greater than the comparable properties of the plastic material of the frame. For instance, a steel stay has an elastic modulus of about 30×10⁶ pounds per square inch whereas polypropylene is about 0.22×10⁶ pounds per square inch. Stays of may be comprised of other metals, such as iron base, nickel base and cobalt base alloys. Stays may be made of non-metals, such as engineered plastic composites with continuous carbon fibers.

The section modulus about a vertical midpoint axis of a stay is very small. For instance a 0.10 to 0.20 inch by 0.5 inch rectangular stay, has a section modulus which is less than about 0.0001 inch⁴, which is about one-twentieth of the stay section modulus about a horizontal axis midpoint. It is also more than a magnitude less than the vertical axis section modulus of a typical bridge thermoplastic portion. These aspects emphasize how the thin flexible band nature of the stay, and its use in the present invention, distinguishes it from metal beams which have been used in pallets for other purposes. The aforementioned preferred stay has an aspect ratio (height to width) of greater than 10 to 1, preferably about 25 to 1 or greater. Of course the horizontal thinness of the band type of stay also enables it to deflect horizontally without plastic or permanent deformation. Even though it will compromise some of the foregoing benefits, in the generality of the invention, a stay may have cross section shape other than the preferred flat stay, for example, a C-shape. More than one stay may be used on one bridge. For instance, two stays may be placed in parallel, either above one another, or one in front of the other.

The stay invention may be used with pallet frames and bridges which have other types cross section structures that those have been are described, with bridges which have no different compliance zones, and to those pallets, the tops of which have plastic structures which are not particularly intended for impact absorption. Stays may be applied to pallets which have wholly plastic decks, as in the prior art, including decks which are detachable or integral with respect to the other parts of the pallet. While the invention has been described in terms of a pallet having a base with eight peripheral columns and a center column, the invention may be applied to pallets having fewer or more columns, to pallets having plate bases, and to pallets which have a top supported on columns, more properly called feet, which run downwardly without any interconnecting base.

The technology of impact absorbing structures and stays may be used with predominately wood pallets, by forming a plastic plank which is attachable to a wood pallet in substitution of a wood leader board. A leader board is a slat (more simply called “leader” hereafter) which is at the end of the deck of a wood pallet. FIG. 18 shows a portion of a predominately wood pallet 220 which is configured mostly like a common commercial pallet. The pallet has wood top deck slats 222 which are fastened to three parallel wood rails 293. The bottoms of the rails are connected by bottom plates or cross pieces 233. The cross pieces 233 at ends are sometimes called bottom leader boards. The invention herein will also be applicable to bottom leader boards, although it is described only in terms of a top leader. In the invention, the leader at either or both ends is of pallet 220 made of thermoplastic with impact absorbing characteristics.

A leader for a wood pallet is essentially constructed like one side of the top frame of one of the plastic pallets which are described above. FIG. 18 shows such a leader 299 fastened by nailing or screwing to wood rails 293. The leader has impact absorbing zones 226 at the outer edge in combination with a steel stay 255. The stay is fastened to the leader as previously described. The impact absorbing zones will be comprised of open or closed cells, as previously described. The impact absorbing zone or zones may comprise all the width of the leader board, in accord with the construction of bridge 25A of pallet 20A in FIG. 4, or it may comprise one or more impact absorbing zones in combination with an inner zone which is less compliant, in accord with the construction of bridge 25 of pallet 20 in FIG. 1. The stay may be exposed or buried within plastic, as previously described.

The plastic leader may have only a stay and no impact absorbing cell structure. FIG. 19 shows such a leader 399 having stay 255 in vertical cross section. Leader 399 is constructed of two different property thermoplastics, by means of co-extrusion or other commercial process. The plastic interior 296 of comprised of a first inferior property material, and the exterior is another property material. For example, the interior or core 296 may comprise low cost and low property material, such as recycled carpet, while the exterior or sheath 295 may comprise a material which has better strength and impact properties, such as high density polyethylene or polypropylene. In the material combination example just given, the interior will tend to have lower impact resistance, lower environmental stress crack resistance, and less ductility than the material on the exterior, but the resultant leader board will have lower cost than a leader board made entirely of the better material.

The width a replacement the leader board should be minimized, so as to not substantially change the combustion characteristics of the pallet and minimize cost. Thus a wood slat may be replaced by a leader which has an outer edge portion comprised of plastic and the invention features, and an inner portion which is wood. The leaders can be used for repair of damaged pallets or construction of new pallets. The invention may be applied to pallets having plywood or other continuous surface decks by shortening the deck from the original design, so that a leader of the invention can be placed at one or both ends.

Although this invention has been shown and described with respect to one or more preferred embodiments, it will be understood by those skilled in this art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.

Claims

1. A pallet comprised of: a rectangular base; a plurality of spaced apart outer columns extending upwardly from the periphery of the base; a rectangular top, mounted on and attached to the outer columns, the top comprising a rectangular frame having inner edges and outer edges, the frame comprising bridges which span the spaces between the outer columns; a deck, having edges which are inset from the outside edges of the frame, for receiving goods placed on the pallet; and, deck support means, for attaching the deck to the top; wherein each bridge has at least one impact absorber comprising a zone running lengthwise along the bridge in proximity to the outer edge thereof, for absorbing horizontal impacts on the edge.

2. The pallet of claim 1 wherein the impact absorber zone extends across substantially the whole width of each bridge.

3. The pallet of claim 1 wherein each bridge has a multiplicity of parallel lengthwise impact absorbing zones, each zone having a different compliance to a horizontal impact force.

4. The pallet of claim 3 wherein the compliances of the multiplicity of impact absorber zones change progressively and step-wise across the width of the bridge.

5. The pallet of claim 1 wherein the unit section modulus of the impact absorber zone about its horizontal midpoint axis is several times greater than the unit section modulus of the impact absorber zone about its vertical midpoint axis.

6. The pallet of claim 1 further comprising: a metal stay extending lengthwise in proximity to the outer edge of the bridge, the stay positioned between the impact absorber zone and the outer edge.

7. The pallet of claim 1 wherein the impact absorber zone is comprised of a multiplicity of open cells bounded by a combination of straight and arcuate ribs.

8. The pallet of claim 1 wherein said impact absorber zone is comprised of a multiplicity of closed cells.

9. The pallet of claim 8 wherein said closed cells are bounded by vertical ribs and top and bottom plates

10. The pallet of claim 9 wherein each bridge has a multiplicity of parallel lengthwise impact absorbing zones, each zone having a different compliance to a horizontal impact force; and, wherein the cell shapes in a zone are different from the cell shapes of an adjacent zone.

11. The pallet of claim 10 further comprising: a multiplicity of vertically open cells spaced apart amongst said closed cells.

12. The pallet of claim 8 wherein each bridge has a multiplicity of parallel lengthwise impact absorbing zones; wherein said zones are comprised of a multiplicity of essentially rectangular closed cells bounded by vertical ribs running both lengthwise and transversely; and wherein the cells of the impact absorber zone proximate the outer edge are smaller and have greater vertical corner radii than do the cells of an impact absorber zone further from the outer edge.

13. The pallet of claim 1 wherein there each side of the pallet has a column at the midpoint of the side; wherein each side of the frame comprises two adjacent bridges, each bridge extending from the midpoint column to an outer column at a corner of the pallet; and wherein there is no continuity between the impact absorber zones of the two adjacent bridges where the frame overlies the midpoint column.

14. The pallet of claim 1 wherein the impact absorber zone is proximate the outer edge of each bridge; wherein the lengthwise portion of each bridge which proximate the inner edge of the bridge is free of impact absorber; and wherein the deck support means comprises attachment of the deck said lengthwise portion of the bridge which is proximate the inner edge.

15. The pallet of claim 14 wherein each bridge comprises at least a second parallel lengthwise impact absorber zone located intermediate said outer edge zone and said portion where the deck is attached; the second zone having a different compliance from the compliance of the first zone.

16. The pallet of claim 1 wherein the top of the pallet further comprises a subframe spaced apart from the inner edge of the frame; wherein the deck support means comprises attachment of the deck to the subframe.

17. The pallet of claim 16 wherein each bridge comprises at least a second parallel lengthwise impact absorber zone located intermediate said outer edge zone and the inner edge the second zone having a different compliance from the compliance of the first zone.

18. The pallet of claim 17 wherein the compliance of the outer edge zone is lower than the compliance of the second zone.

19. The pallet of claim 1 wherein the base comprises outer rails connected in the form of a rectangle, cross rails connecting opposing side center points of the outer rails, wherein eight outer columns; and, wherein there is one center column to which the deck is attached.

20. The pallet of claim 1 wherein the deck has less compliance to horizontal impact load than does the impact absorber.

21. The pallet of claim 20 wherein the deck is made of corrugated metal.

22. The pallet of claim 21 wherein the deck is made of thermoplastic material.

23. In a pallet of the type used with fork lifting devices, having a rectangular top comprised of a deck for receiving goods placed on the pallet, wherein the top is supported on eight spaced apart columns extending upwardly from the periphery of a rectangular base, the improvement which comprises: a top comprised of a thermoplastic frame which forms the outermost portion of the top, the frame comprising bridges spanning spaces between the columns, having inner edges and outer edges; and a lengthwise impact absorber zone proximate the outer edge of each bridge.

24. The pallet of claim 23 wherein the exterior dimensions of the pallet are about 40 inches×80 inches; and, wherein and the pallet is a GMA type pallet.

25. The pallet of claim 24 wherein each bridge comprises a multiplicity of parallel lengthwise impact absorber zones, each zone having a different compliance to a horizontal impact load.

26. In a predominately thermoplastic pallet of the type used with fork lifting devices, wherein the pallet has a rectangular top comprised of a deck for receiving goods placed on the pallet, wherein the top is supported on eight spaced apart columns extending upwardly from the periphery of a rectangular base, wherein bridge portions of the top span adjacent columns, the improvement which comprises: a rectangular deck having edges which are spaced apart from the outside edges of the top; and, bridges which comprise impact absorbers, for absorbing the impact of horizontal loads on the top.