Abstract:
A cushioning structure for placement between an impacting surface and a surface of an object to be cushioned against damage caused by impact during transport, storage, or usage, comprising a spring member having a load bearing portion and spring lead portions, and a restraining member adapted for engaging with the spring leads, to restrain the movement of the spring leads while the cushioning structure is subjected to loading or accelerating or decelerating forces.

Description:
TECHNICAL FIELD 
     This invention relates to cushioning structures such as those used for transportation of articles, e.g., computers, fragile mechanical and optical devices, etc., which protect such articles during such transport and handling associated therewith, and cushioning structures such as those used within an electronic assembly, e.g., liquid crystal display, lap top computer, disk drive, etc., which protect such articles during operation and usage associated therewith. 
     BACKGROUND OF THE INVENTION 
     Various methods have been employed to cushion fragile articles, e.g., electronic, optical, electro-mechanical components and equipment, etc., during shipping, storage, and operation. Foamed plastic and various shaped paper fiber spacers and corner elements have been used for shipping fragile articles in containers. 
     Plastic-based expanded foam, while often serving as an effective cushioning and packing material is, nevertheless, expensive and not environmentally friendly, requiring special disposal after the structure use cycle is completed. Various types and styles of paper cushions are often employed such as padded papers and flat papers that are shaped, corrugated fiber board, and molded paper pulp. These solutions are often low cost and more environmentally friendly. These do not, however provide sufficient resiliency or offer complete protection to relatively delicate articles after repeated and multiple drops, a common situation in the shipping and distribution arenas. These paper cushions often flatten out after multiple drops. 
     If an article is dropped, it decelerates over a relatively short distance upon impact, resulting in very high forces sufficient possibly to damage a contained article. The purpose of the cushion is, to obviously prevent any damage to the article. Repeated impacts require that the cushioning itself be able to withstand multiple cycles of impact and be able to recover sufficient resiliency to provide continued protection to the article. 
     Various cushioning members are used to protect comers and surfaces of articles from damage encountered during shipping and handling. Examples of various cushions are described below. 
     In U.S. Pat. No. 5,826,726, to Yang, there is shown a molded pulp structure for positioning and cushioning an article, comprising a plurality of mold strips having a cushion section formed by a number of successive molded pulp units with each unit defining a ridged surface on the molded pulp. 
     In U.S. Pat. No. 5,339,958, to Taravella et al., there is shown a two-piece dunnage device which includes a cushioning piece made of plastic foam material and a supporting piece to which the cushioning piece is mechanically attached without adhesives. The cushioning piece has a plurality of cushioning elements that extend from a connecting web. The supporting piece keeps the cushioning piece away from the sides of a shipping container. 
     In U.S. Pat. No. 5,069,359, to Liebel, there is described an example of packing a round body within a cylindrical paper tube, using triangular shaped corner posts disposed between the outer corrugated box and the cylindrical paper tube. 
     Another example is in U.S. Pat. No. 4,317,517, to Tisdale, where a load spacer or support is constructed of laminated paper having two trapezoidal shaped hollow load cells, one being “W” shaped, and interconnected by a top sheet or panel having depending flanges on both sides. 
     U.S. Pat. No. 3,951,730, to Wennberg et al., describes an isolation or packing material structure comprising bellow-like compressible layers interconnected by a zigzag folded strip which is alternately connected to the layers. Additional short projecting parts are positioned between the zigzag strip and the layers to provide support. 
     In U.S. Pat. No. 3,752,384, to Siburn, there is disclosed a resilient packaging spacer with a plurality of flexible joined triangular shaped elements having a flexible insert for attaching the spacer through a slit in a carton to contain cylindrical or rectangular shaped articles. 
     Still another example is in U.S. Pat. 5,062,751, to Liebel, where a filler assembly is formed by alternately stacking and laminating “V” shaped and “W” shaped sheets which are glued together along the sides and in the center. 
     U.S. Pat. No. 3,559,866, to Olson, describes a carton liner fabricated with a flat strip and a triangle shaped paperboard strip having triangular projections. 
     In yet another example, a plastic foam cushioning element is described in U.S. Pat. No. 4,851,286, to Maurice, wherein a cushion is provided by adhering together two layers of different density foam. 
     Sonopost (TM) Design Chart product literature from Sonoco Products Company, Hartsville, SC, describes various corner post styles used for protecting the corners or edges of an article. 
     A cushioning structure which assures maximum protection to the article during shipping, handling, or usage, by lowering the acceleration level when an article is dropped, providing multiple incident cushioning protection, as defined herein below, has hitherto not been provided. To solve this problem, a new and unique cushioning structure, utilizing few parts, has been developed. It is believed that such a cushioning structure will constitute a significant and important advancement in the art. 
     OBJECTS AND SUMMARIES OF THE INVENTION 
     It is therefore, and object of the present invention to enhance the art of cushioning structures particularly in the shipping, transporting and operation of articles. 
     It is another object of the invention to provide a cushioning structure which is integrated within the design of articles. 
     It is yet another object of the invention to provide a cushioning structure which provides effective and continued shock and vibration protection to an article. 
     It is still another object of the invention to provide such a cushioning structure which can be produced using fewer parts thereby representing a cost advantage to the ultimate consumer of the package as well as to those who produce it. 
     According to one aspect of the present invention, there is provided a cushioning structure comprising: a spring member including a load bearing portion having a load bearing surface and a plurality of spring lead portions, each of the spring lead portions including a retaining feature and extending from the load bearing portion and adapted for engaging an external surface; and a restraining member adapted for engaging the retaining feature of each of the spring lead portions, the restraining member flexibly restraining the spring lead portions when a load is applied to the load bearing surface of the load bearing portion of the spring member, the spring lead portions moving in a direction substantially away from the load bearing portion of the spring member when the load is applied, while the spring lead portions engage the external surface. 
     According to another aspect of the present invention, there is provided an electronic assembly comprising: an electronic device; a base member; and, a cushioning structure, positioned between the electronic device and the base member, the cushioning structure comprising a spring member including a load bearing portion having a load bearing surface and a plurality of spring lead portions, each of the spring lead portions including a retaining feature and extending from the load bearing portion of the spring member and adapted for engaging the base member; and a restraining member adapted for engaging the retaining feature of each of the spring lead portions, the restraining member flexibly restraining the spring lead portions when a load applied by the electronic device is transmitted to the load bearing surface of the load bearing portion of the spring member, the spring lead portions moving in a direction substantially away from the load bearing portion of the spring member when the load is applied, while the spring lead portions engage the base member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments of the invention will be described in detail by way of examples, with reference to the accompanying figures, where: 
     FIG. 1 is a perspective view of the two parts of a cushioning structure of one embodiment of the invention which include a spring member with a load bearing portion and spring lead portions, and a boxlike restraining member which includes a load transmitting wall; 
     FIG. 2 is a perspective view of the cushioning structure of FIG. 1 with the boxlike restraining member engaged with the spring member; 
     FIG. 3A shows a cross section view of the cushioning structure shown in FIG. 2; 
     FIG. 3B shows a cross section detail of a spring member which is substantially hollow. 
     FIG. 4 is a view of the cushioning structure in a state with a load placed on top of the restraining member; 
     FIG. 5 shows a view of the loaded cushioning structure in a state under application of an impact force; 
     FIG. 6 shows the cushioning structure in a state, after application of the impact force, where the plurality of spring lead portions and restraining member return the cushioning structure to nearly pre-impact state; 
     FIG. 7 is a perspective view of a cushioning structure similar to that in FIG. 1 except that the restraining member has no load transmitting wall; 
     FIGS. 8A-8E show some examples of retaining features on a spring lead portion of a spring member. 
     FIG. 9 is a perspective view of another embodiment of the invention wherein the spring lead portions are restrained by a restraining member in the form of a continuous band, engaged with projecting upstanding elements on the spring lead portions of the spring member; 
     FIG. 10 is yet another embodiment of the invention, similar to FIG. 9 except that the restraining member is a set of spring clips; 
     FIG. 11 shows an example where the retaining feature on a spring lead portion is a notch and a spring clip is used as the restraining member; 
     FIG. 12 shows notch retaining features with a continuous band restraining member; 
     FIG. 13A is a cross sectional view of a cushioning structure with radially positioned spring lead portions and an annular restraining member; 
     FIG. 13B shows a top view of the cushioning structure of FIG. 13A with the spring member having a load bearing portion which is substantially cylindrically shaped and radially positioned spring lead portions; 
     FIG. 14A is a cross sectional view of a cushioning structure having a spring member with a substantially cylindrical load bearing portion and radially positioned spring lead portions engaging with an annular shaped restraining member having a circular load transmitting wall; 
     FIG. 14B is a top view of the cushioning structure of FIG. 14A; 
     FIG. 15A is a cross sectional view of a cushioning structure having a spring member with a substantially cylindrical load bearing portion, radially positioned spring lead portions, and an annular shaped restraining member engaging with retaining features which are projecting upstanding elements on the spring lead portions; 
     FIG. 15B is a top view of the cushioning structure of FIG. 15A; and 
     FIG. 16 shows a cross sectional detail of an electronic assembly having a cushioning structure positioned between an electronic device and a base member. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings. 
     Referring to FIG. 1, there is shown a two-part cushioning structure  100 , having a spring member  15 , and a restraining member  13 . The restraining member is shown here as a five (5) sided box with the lower bottom side of the box open (not shown). The five sided box comprises a load transmitting wall  31 , engaging walls  27 , and connecting walls  29 . The orthogonal boxlike restraining member shown here is a preferred embodiment of the invent ion, but it is understood that other geometries such as, cylindrical, triangular, and irregular shapes are possible variations. The spring member has a load bearing portion  23  having a load bearing surface  25 , and spring lead portions  17  which extend outwardly from the load bearing portion  23 . Elongated ribs  19 , are disposed near the ends of the spring lead portions  17 . The length of the restraining member is slightly larger than the length of the spring member so that the bottom opening of the restraining member has sufficient clearance to fit over the load bearing portion  23  of the spring member to allow the engaging wall edges  33  of the engaging walls  27  to engage with the spring lead portions, along the retaining features  21 , formed along the elongated ribs  19  situated at the ends of the spring lead portions. Another variation of cushioning structure  100  is a spring member which is equal to or longer than the length of the restraining member. In this case connecting wall  29  could be eliminated, leaving at least two engaging walls  27  and one load transmitting wall  31  to form the restraining member. 
     The load bearing portion  23  of spring member  15  is shown here as an elongated rib with a domed shape. The load bearing can be hollow or solid and the shape can be adjusted to meet the demands of the application requirements providing additional cushioning to the overall cushioning structure when a load is applied to the load bearing surface. The load transmitting wall is shown as a flat surface connecting to the engaging walls  27 , but it is understood that the transmitting wall may have a contoured shape depending on the needs of the application. 
     In FIG. 2, the cushioning structure  100  is shown with restraining member  13  positioned over and engaged with the spring member  15 . 
     A cross section taken through line  3 — 3  in FIG. 2 is viewed in FIG.  3 A. The spring lead portions  17  of the spring member contact an external surface  37  at contact points  35 . One example of an external surface is the inside surface of a card board box used for shipping an article. 
     The restraining member has engaging walls  27  and a load transmitting wall  31 . The engaging wall edges  33 , of engaging walls  27 , engage or seat at retaining features  21  formed along the edge of elongated ribs  19 , near the end of spring lead portions  17 . According to a preferred embodiment of the present invention, the underside surface  39  of load transmitting wall  31  is shown in contact with load bearing surface  25 , of load bearing portion  23  of spring member  15 . In this case restraining member  13  makes three (3) point contact with spring member  15  at the two retaining features  21 , and load bearing surface  25 . A gap may be present (not shown) between the spring member&#39;s load bearing surface  25  and the underside surface  39  of the restraining member&#39;s load transmitting wall  31 . Formation of a gap depends on the length of the restraining member&#39;s engaging walls  27 , the length of spring lead portions  17 , the angle of the spring lead portion relative to the load bearing portion  23 , and the angle of the restraining member&#39;s engaging walls  27  relative to load transmitting wall  31 . In such situations, where a gap is present, contact points would initially be made between restraining member  13  and spring member  15  at the retaining features  21 . 
     The restraining member restrains the movement of the spring lead portions  17  of spring member  15 . The restraining member prevents the spring lead portions from spreading apart too far and extending beyond the. yield point of the spring member upon the application of a load. A load may be applied directly to restraining member  13  or to spring member  15 . The load transmitting wall  31  has a surface which distributes a load over the restraining member. The restraining member engaging walls  27  absorb shock and provide vibration dampening. The spring member provides a spring response to an application of a load to load transmitting wall  31 , external surface  37 , or some combination of both. For purposes of illustration, FIG. 3A shows the spring member positioned below the restraining member. It is understood by those skilled in the art that the invention can be positioned in any orientation. For example, the cushioning structure can be inverted so that the load transmitting wall is in contact with an external surface  37 , e.g., the interior of a shipping container, and the spring lead portions of the spring member are in contact with an article requiring cushioning protection. 
     The spring member and restraining member can be adjusted or tuned to the demands of the application requirements. Various characteristics can be changed, e.g., materials, “box geometry”, wall thickness, spring lead length, spring lead portion shape, spring lead bend angle, load bearing portion thickness, load bearing portion geometry, hollow spring member, solid spring member, etc. Examples of some materials which the spring member and restraining member can be made from are: shaped fibrous materials such as paper pulp fiber and paper fiber board, plastic, rubber, elastomer, and metals. The preferred embodiment for shipping and transport applications is shaped paper pulp fiber. 
     Load bearing portion  23  could include a hollow opening  24  (phantom) extending the entire length of the rib or partially along the length. It is also possible to provide similar openings, e.g.,  24 ′ and  24 ″ in one or both of the elongated ribs  19 . These openings are also shown in phantom in FIG.  3 A. Openings  24 ′ and  24 ″ can also extend through or partially within the rib. Elongated ribs  19  may be of various shapes other than that shown in FIG. 3A, e.g., orthogonal, triangular, oval, irregular, etc. The shape would depend on the application. For example, if the surface  37  is very smooth and slippery producing a low coefficient of kinetic friction, it may necessitate increasing the surface area of contact point  35  by changing the shape of the elongated rib  19 . Similarly, the load bearing portion  23  although shown in the preferred embodiment as a domed shaped elongated rib, can be made into other shapes, e.g., flat, contoured, triangular, oval, irregular, etc., depending on the application. For example, the shape of the load bearing surface may be made flat to enable a stable engagement with underside surface  39  of load transmitting wall  31 . This would be advantageous particularly if engaging walls  27  are shorter than those shown in FIG. 3A, and do not initially make contact with retaining feature  21  prior to imparting a load onto cushioning structure,  13 . 
     FIG. 3B is a cross section of a spring member  15 ′ of a preferred embodiment of the spring member, shown with a hollow interior and formed by a continuous layer of material. This can be made by extrusion processing of fibrous pulp paper or gluing the ends of a pre-formed layer of material to create a continuous loop. An example of such material is available from Sonopost (TM) Protective Corner Board manufactured by Sonoco, Inc., Hartsville, S.C. Spring lead portions  17 ′ provide a spring response to an application of a force to spring member  15 ′, as these move outwardly away from each other. Additionally, the various folds and bends in the continuous loop, hollow spring members provide an internal spring response to a loading condition. Depending on the stiffness of the material, upper segment  45  may not move in unison with lower segment  47  upon initial loading of spring member  15 ′. Upon loading, upper segment  45  moves downwards toward lower segment  47 . Some initial spring response is provided by the material stiffness of these segments as these absorb some of the forces applied to the spring member. Before or when upper segment  45  contacts lower segment  47 , the spring lead portions  17 ′ as a whole, will begin to move apart from each other. In situations where the acceleration forces are large, the spring action provided by these individual segments will have minor contribution. Cushioning would depend primarily on the spring action provided by the spring lead portions and stiffness of engaging walls  27  and connecting walls  29  (shown in FIG. 1.) of restraining member  13 . In situations where acceleration forces are small or sizes are miniature, the spring action provided by these individual segments could provide the more significant cushioning response. 
     FIG. 4 shows cushioning structure  100 , having the restraining member  13  at “three point contact” with spring member  15 . A load  41 , is positioned on load transmitting wall  31 . Spring lead portions  17  contact external surface  37 , which in this example is the inner surface of a transport container. For simplification, external surface  37  and load transmitting wall  31  are shown as flat planes but these surfaces can be of any shape. The load  41 , eg., article for transport, electronic assembly, is shown supported by cushioning structure  100 , at initial rest position  43 . 
     FIG. 5 illustrates the condition where a deceleration force is applied to the load and cushioning structure, such as when the transport container is dropped to a floor. Load  41  moves toward external surface  37 , compressing the cushioning structure  100 . Upon impact, spring lead portions  17  spread apart, absorbing some of the forces of deceleration. Engaging walls  27  of restraining member  13  move outwardly, along with the spring lead portions, providing additional force absorption. Engaging walls restrain the movement of the spring lead portions preventing the spring lead portions from exceeding the yield point. 
     After absorbing the deceleration force, engaging walls  27  of restraining member  13 , and spring lead portions  17  of the spring member, attempt to recover their original shape and position, as FIG. 6 illustrates. Depending on the forces applied, some permanent deformation may be experienced by the respective members resulting in a final rest position  44 , below that of initial rest position  43 . 
     Shock tests were performed on a set of cushioning structures as described in FIG. 1 made of paper fiber board. The cushioning structures were put into a card board shipping box with a 30 pound steel plate placed on top of the cushioning structures. An accelerometer was attached to the steel plate. The dimensions of each cushioning structure were approximately 2¾″ high×19″ long×5″ wide. The card board shipping box was repeatedly dropped from a height of 36 inches at approximately 5 minute intervals. The time domain acceleration level readings on the plate for each drop is recorded in TABLE I. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Drop Number 
                 Paper Cushioning Structure (gs) 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 57.74 
               
               
                   
                 2 
                 73.01 
               
               
                   
                 3 
                 83.64 
               
               
                   
                 4 
                 86.81 
               
               
                   
                 5 
                 73.21 
               
               
                   
                   
               
             
          
         
       
     
     The data shows that after the second drop, the acceleration levels for the paper board cushioning structure remain relatively consistent, demonstrating very good resiliency. 
     FIG. 7 is another embodiment of the invention which is very similar to the cushioning structure shown in FIG. 1 except that the load transmitting wall  31  of restraining member  13  is not included. If the article that needs to be protected is larger than the periphery of the restraining member  13 ′, and the underside of the article is sufficiently strong to support the body of the article, then a load transmitting wall, as in FIG. 1, may be eliminated. If the article to be protected is smaller than the periphery of the restraining member, then a load transmitting wall would be needed. Similarly, if the article is larger than the span between either engaging walls  27 ′ or connecting walls  29 ′ and if the underside of the article is sufficiently strong to support the body of the article, then a transmitting wall may be eliminated as well. 
     FIGS. 8A-8E show some examples of retaining features which may be employed to engage with various restraining members. FIG. 8A shows a spring member  15  with elongated ribs  19  on the spring lead portions  17 . Cusp  21  is an example of a retaining feature which is formed by the geometry of the elongated rib  19 . FIG. 8B is an example of retaining feature which is a notch  18  cut out of spring lead portion  17 . FIG. 8C shows a hole  10  made in the elongated rib  19  of spring lead portion  17 , with the end of a restraining member, a spring clip  22  inserted into the hole. FIG. 8D is yet another example of a retaining feature, in this case, a channel  24 , formed in spring lead portion  17 . The engaging wall edges of engaging walls and continuous bands are examples of restraining members which would engage with the channel restraining feature. FIG. 8E is an example of a retaining feature which is a projecting upstanding element  26  on spring lead portion  17 . 
     Other examples of retaining features which can be employed to flexibly retain engaging walls of a restraining member, particularly in applications where the direction of loading or force application is cyclical and alternates direction, include, engaging walls with push through snap pegs which pass through holes in the spring lead portions, vertical oriented flexible “C-clamps” which snap over the engaging walls, ball and socket snap fit, adhesives which maintain elasticity, etc. 
     FIG. 9 is another embodiment of the invention wherein cushioning structure  120  has a restraining member  49  which is a continuous band engaging with projecting upstanding element  51 , in this case an elongated retaining rib, on spring lead portions  17  of spring member  16 . An example of how this embodiment would be used is where two cushioning structures  120  are placed under the base of an article which needs cushioning protection. The continuous band restraining member can be made of a material which is resilient enough to restrain the movement of the spring member preventing the spring member from exceeding the yield point, e.g., elastomers, metals, plastics, rubber, paper pulp fiber, paper fiber board, etc. 
     FIG. 10 is yet another embodiment of the invention where cushioning structure  130  has spring clip restraining members  53 , engaging with projecting upstanding elements  51 , e.g., elongated ribs, nubs, etc., on the spring lead portions  17 . 
     FIG. 11 shows a cushioning structure  140  having an alternative to the projecting upstanding elements of FIG.  10 . In this embodiment, notches  18  are cut into the spring lead portions  17 , to enable spring clip  53  (restraining member) to engage with the spring lead portions. 
     FIG. 12 is another embodiment of a cushioning structure  150  employing the use of notches  18  to engage with a continuous band restraining member  50 . Various materials can be used for the continuous band, e.g., rubber, plastic, paper fiber board, paper pulp fiber, metal, elastomer, etc. 
     FIGS. 13A and 13B show a cushioning structure  160 , having a spring member  66  with radially positioned spring lead portions  59  around a substantially cylindrical load bearing portion  57 . The spring lead portions  59  have separations  67  which allow the spring lead portions  59  to move outwardly upon application of a load or accelerating force. Load bearing portion  57 , has a circular shaped load bearing surface  55 . Each spring lead portion  59 , is shown with a curvilinear rib  65 . The curvilinear rib  65  is an example of a retaining feature which can be used to engage with an annular restraining member  63  along the cusp  61  at each curvilinear rib. This would be the annular version of the orthogonal version of restraining member  13 ′ in FIG.  7 . The spring member may be made by various molding or press forming processes depending on material. For example, for a pulp paper fiber spring member, a press forming process could be employed. Because the radial shape is somewhat complicated, the structure could be divided up into pie-like segments for forming and then glued together to complete the radial cushioning structure. An injection molding process could be used for plastic materials. Additional materials can be used to fabricate this radial cushion, e.g., elastomers, metals, plastics, rubber, paper fiber board, etc. The materials may be used in combination or singularly. 
     FIGS. 14A and 14B are cross sectional and top views of a cushioning structure  170  having a restraining member  71  with a substantially circular shaped load transmitting wall  73 . The restraining member is the cylindrical version of the orthogonally shaped restraining member  13  in FIG.  1 . 
     FIGS. 15A and 15B are cross sectional and top views of a cushioning structure  180  with projecting upstanding elements  63  used to retain restraining member  75 . The projecting upstanding elements shown here are curvilinear ribs. The restraining member is shown as a continuous band which is retained by the curvilinear ribs on spring lead portions  59 . The projecting upstanding elements could be of various configurations or combinations of configurations, e.g., nubs, buttons, pins, ribs, etc. As seen in  15 B, the spring lead portions  59  have separations  67  which allow the spring lead portions to move outwardly. The cushioning structure  180  is the radial version of cushioning structure  120  shown in FIG.  9 . Projecting upstanding elements  63  retain a restraining member  75  which in this case is a continuous band of substantially elastic material. 
     FIG. 16 is a cross sectional view of an electronic assembly  190 , having an electronic device  81 , a base member  83  and a cushioning structure  85 . The cushioning structure  85  is shown here as an integral part of the electronic assembly having a spring member  89  and restraining member  87 . Cushioning structure  85  is positioned between the underside  95  of the electronic device  81  and the surface  97  of base member  83 . Restraining member  87  has a load transmitting wall  91  and engaging walls  93  to engage with spring member  89 . Spring lead portions  99  contact surface  97  of base member  83 . 
     The afore described cushioning structures can be used in protecting an article in a packaging or shipping container. It can also be integrated within the designs of various electronic assemblies, e.g., computers, data storage units, testers, etc., and to isolate fragile parts within the assembly, e.g., lap top computer, liquid crystal display, disk drive, test heads, transducers, etc. The cushioning structure can be built into the base member of an electronic assembly, e.g., housing, protective case, etc., to isolate the electronic device from potentially damaging vibration or impact shock imparted to the base member. The restraining member could be molded into a base member to simplify electronic assembly construction. 
     While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various modifications can be made therein without departing from the scope of the invention as defined by the appended claims.