document: BACKGROUND 
       The invention relates to pallet systems, and in particular, pallet systems that facilitate loading and unloading, and limit the shifting of loaded items. 
       Pallets are used to ship loads of one or more items that are placed and secured onto pallets. Smaller items may be shipped by packaging the items into larger packaging units, which are then loaded onto the pallets. For example, to ship a large quantity of loose or fragile items such as eggs, the items may be arranged in stackable trays, and the stacked trays are loaded onto the pallet. 
       A pallet loaded with items is often wrapped for shipment to secure the load, sometimes using rigid end boards. 
       SUMMARY 
       In general, in one aspect, the invention features a pallet system for supporting a load of at least one tray having side supports. The pallet system includes a pallet having at least one pair of parallel guide rails raised above a top surface of the pallet, each pair of guide rails configured to be straddled by side supports of a tray. 
       Advantageous embodiments of the invention include one or more of the following features. The space between each pair of guide rails is substantially open. The guide rails are raised above the level of a weight bearing area of the top surface of the pallet. 
       The pallet system includes at least one end stop on the top surface of the pallet, each end stop positioned to limit longitudinal movement of a tray loaded on the pallet. The pallet system includes two end plates, configured to be vertically positioned at edges of the pallet at ends of the guide rails. Each end plate has an inside surface shaped to complement contours of the end surfaces of the tray. The pallet system includes end plate holders for securing the first and second end plates. In one example, the end plate holders are notches in the guide rails. 
       The pallet system includes a pallet base comprising a conventional pallet and a pallet cap securely fitting onto the pallet base to form the pallet, the top surface of the pallet cap forming the top surface of the pallet. 
       In general, in another aspect, the invention features a stacked pallet, including a pallet, having at least one pair of parallel guide rails raised above its top surface, and trays stacked in layers on the pallet, wherein each tray of the first layer has side supports straddling a pair of guide rails on the pallet. 
       Advantageous embodiments of the invention include one or more of the following features. The stacked pallet further includes end plates secured to the stacked trays. For example, wrapping secures the end plates to the stacked trays. Adjacent trays in the first layer of the stacked trays are laterally interlocked by the guide rails straddled by the adjacent trays. Within each layer of trays, the trays laterally interlock. 
       In general, in another aspect, the invention features a method of loading a pallet by providing at least one pair of raised parallel guide rails on a top surface of a pallet and forming a first layer of trays by sliding each tray onto the pallet along a pair of guide rails. 
       Advantageous embodiments of the invention include one or more of the following features. The method provides at least one end stop on the top surface of the pallet, wherein a tray is slid along a pair of guide rails until its front surface contacts an end stop. 
       The method forms a stack of trays by repeatedly sliding trays over trays already on the pallet. Trays of the stack are laterally interlocked. The method secures end plates to the stack of trays. When secured, an end plate has an inside surface facing an outside surface of the stack of trays. This inside surface is contoured to complement the outside surface of the stack of trays. Securing end plates to the stack of trays is achieved in one example by wrapping the end plates to the stack of trays. 
       Among the advantages of the invention are one or more of the following. 
       The pallet system restricts both the lateral and longitudinal movement of items loaded onto the pallet. 
       Vertically positioned end plates at both pallet edges at ends of the guide rails further restrict the longitudinal movement of items on the pallet. The pallet system provides enhanced stability when loaded and wrapped. 
       The pallet allows trays to be slid on and off, and is suitable for automated loading and unloading of the trays. 
       Other features and advantages of the invention will become apparent from the following description and from the claims. 
      
      
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1. is a perspective view of a pallet system in accordance with the invention. 
       FIG. 2 is a perspective view of a pallet cap in accordance with the invention. 
       FIG. 3 is a cross-sectional view of a loaded pallet system in accordance with the invention. 
       FIG. 4 is a top detail view of a pallet cap in accordance with the invention. 
       FIG. 5 is a top view of a loaded pallet in accordance with the invention. 
       FIG. 6 is a cross-sectional view of a stack of end plates in accordance with the invention. 
       FIG. 7 is a cross-sectional view of the pallet cap of FIG. 2. 
       FIG. 8 is a cross-sectional view of a stack of pallet caps in accordance with the invention. 
      
      
       DETAILED DESCRIPTION 
       Referring to FIG. 1, a pallet system based on a conventional pallet, referred to here as the pallet base 10, is fitted with a pallet cap 20 to provide a pallet in accordance with the invention. Alternatively, the pallet may be a single integrated unit. The pallet system can be used, for example, to ship trays 30 such as those described in pending U.S. Pat. No. 5,816,406, which is incorporated by reference. 
       FIG. 1 shows in an exploded view the relationship of elements of the pallet system. Several layers of trays are loaded onto the pallet, and the end plates 40a, 40b are respectively positioned at front and back ends of the load to help secure the load of trays 30 to each other and to the pallet, as well as to protect the ends of the trays and provide rigidity to the stack of trays. Typically, though not shown in FIG. 1, the entire load including the end plates 40a, 40b is wrapped with a film, either manually or using a machine and methods such as those described in U.S. Pat. Nos. 5,423,163 and 5,531,327. 
       The pallet base 10 is a conventional plastic pallet. As shown, a conventional plastic pallet typically has pockets 11 that elevate the top surface of the pallet and provide recesses 12 which allow the pallet to be handled by standard equipment such as forklifts. Because the exact shape and placement of the pallet pockets 11 may vary among pallet models and manufacturers, the pallet cap 20 has pockets 21a, 21b, 21c customized to fit within the pockets 11 of a particular pallet 10. 
       The pallet cap pockets 21a, 21b, 21c position the pallet cap 20 on the pallet base 10 and provide support for the weight of loads borne by the pallet cap 20. In the configurations shown in FIGS. 1 and 2, the pallet cap pocket 21a extends to the base of the pallet base pocket 11 to provide support and has ribbed walls to provide structural strength. Pallet cap pockets 21b and 21c are provided primarily for positioning the pallet cap 20 on the pallet base 10. 
       Referring to FIG. 2, the pallet cap 20 has raised, parallel guide rails 22, which are labelled in pairs 22a, 22b, 22c. The number of guide rails 22 may vary, depending on the size and shape of the trays to be loaded. For example, FIG. 2 shows a pallet cap 20 having three pairs of guide rails 22a, 22b, 22c, designed for a load three trays wide, as is illustrated in FIG. 1. 
       Referring to FIG. 3, each tray 30 has side supports 31. The pairs of guide rails, such as pair 22c, are sized and spaced so that when such a tray is loaded directly on the pallet cap 20, the tray side supports 31 closely straddle a pair of guide rails, thus constraining the lateral movement of the tray 30. The guide rails 22c help to position the tray 30, and as shown, also help to interlock adjacent trays 30. In FIG. 3, the top surface of pallet cap 20 is substantially open between the pair of guide rails 22c, which enables the open space to be used for packing items, in this case, eggs. The guide rails 22c avoid contact with the tray 30 or its load, and are raised above the level of the weight bearing surface of the pallet cap 20. 
       The arrangement of parallel guide rails allows trays to be loaded onto the pallet by sliding the tray over a pair of guide rails from a front end of the pallet towards a back end, along the direction of the arrow 25 of FIG. 2. The opposite action may be used to unload trays from the pallet. The motion of a tray may be stopped by optional end stops 23, which are positioned at the outer edges of each pair of guide rails 22, towards the back end of the pallet. These end stops 23 limit the longitudinal movement of the trays parallel to the guide rails 22. FIG. 4 provides a more detailed illustration of the positions of the end stops 23. 
       When a pallet is loaded and ready to be prepared for shipping, the end plates 40a, 40b are vertically positioned at the front and back ends of the pallet as shown in FIG. 1. End plates are vertically positioned in one or more of the notches 24 found at both ends of each guide rail 22 (FIGS. 2 and 4). The notches 24 act as end plate holders for holding the end plates in position. 
       End plates may provide greater stability to a stack of trays by being shaped to complement the shapes of the trays loaded onto the pallet. For example, FIG. 5 shows a top view of a pallet loaded with trays 30 shaped like those described in U.S. Ser. No. 08/673,698. The end plates 40a, 40b are shaped to complement the contour of the tray edges. 
       Referring to FIGS. 1 and 5, the end plates 40a, 40b at both ends of the pallet may be substantially identical or interchangeable. The end plates may be narrower than the pallet or load, as shown, and advantageously may be approximately 22 inches wide which allows them to be used with pallets and loads of different widths. For example, a pallet having an industry standard size of 40 inches by 48 inches can support layers having two rows of three trays, where the trays are egg trays of a standard size, such as is disclosed in U.S. Ser. No. 08/673,698. However, pallets of other sizes can be used, including 24 inches by 48 inches (supporting two rows of two trays) and 36 inches by 24 inches (supporting one row of three trays). The end plates 40a, 40b shown in FIGS. 1 and 5 may be used for pallets having any of these dimensions. FIG. 6 illustrates a cross-section of a stack of end plates 40. As shown, the end plates may be shaped such that they nest within one another, which saves space when they are not being used. 
       The components of the pallet system may be made of various materials. For example, the pallet cap and end plates may be comprised of a plastic such as polypropylene or ABS plastic. 
       Each component can be manufactured by a variety of methods. For example, FIG. 7 illustrates a cross-section of the pallet cap 20 shown in FIG. 2, created from a sheet of plastic having a thickness approximately in the range of 0.08-0.125 inches. The plastic sheet can be formed by methods such as thermo forming, rotomolding, and injection molding. The same thickness and methods apply to the end plates as well. 
       Various features of the pallet system may be customized for its intended load. For example, the pallet system exemplified in the figures is customized for supporting egg trays described in U.S. Ser. No. 08/673,698. As shown in FIG. 3, because the trays 30 carry eggs, which are fragile and have rounded bottoms, the side of the guide rails 22 likely to contact the portion of the tray 30 holding an egg have a slanted edge. At their widest point, the guide rails 22 have a width of approximately 0.5 inches, and at their highest point, have a height of approximately 0.25 inches. As shown, the guide rails 22 do not support the weight of the trays and their load. Because the trays 30 are designed to be slid into place on the pallet, end stops 23 are provided at only one end of the guide rails 22, as shown in FIG. 2. Because the trays 30 have honeycomb-shaped edges, the inside surface of the end plate is shaped to complement this surface, as shown in FIG. 5. The raised portions of the end plate have a height of approximately 0.5 inches and a width of approximately 1 inch. As shown in FIG. 6, the end plates 40 are shaped such that they are interchangeable and nest within one another, which saves space when they are not being used. FIG. 8 illustrates that the pallet cap 20 is also shaped to allow several pallet caps to be nested within one another. 
       Other embodiments are within the scope of the following claims. For example, the pallet may be a single integrated unit rather than a pallet cap fitting onto a conventional pallet. The sizes and positions of pallet cap pockets may vary. The length and height of guide rails, as well as the number of guide rails on a pallet may vary. The shapes and positions of the end stops may vary. For example, end stops may be implemented as a continuous rail across the pallet surface. End stops may be provided at both ends of the pallet if trays are not slid onto the pallet. End plates may have a different width, such as the full width of a pallet.
summary: A pallet system for supporting a load of at least one tray having side supports includes a pallet, having at least one pair of raised parallel guide rails on its top surface, each pair of guide rails configured to be straddled by side supports of a tray. The pallet may be loaded by sliding trays onto the pallet, and lateral movement of a tray loaded onto the pallet is limited by the pair of guide rails that it straddles.