Hanging rack with cantilevered support hooks
FIELD OF THE INVENTION 
     The present invention pertains to support racks used for supporting workpieces in electrically conductive contact during transport through an electrostatic finishing station, and more particularly, to such support racks having removable, interchangeable support hooks. 
     BACKGROUND OF THE INVENTION 
     Workpieces are supported on support racks and carried along by a conveyor through a finishing station which may include the steps of cleaning, rinsing, drying, coating and baking. Usually finishing includes travel through an electrostatic spray booth wherein electrically grounded workpieces are sprayed and coated with electrically charged paint particulate. Workers then removed the finished, coated workpieces from the racks and reuse the racks for subsequent operations. Since the racks are constantly recycled through the finishing system, they become encrusted with multiple layers of the coating material after several runs. 
     It is important in electrostatic coating operations that the support rack be electrically conductive, whereby workpieces can be maintained in a grounded electrical state. The workpieces are electrically connected to ground potential through the conductive support rack so that the electrically charged coating particulate is attracted by an electric field to the workpieces. Thus, it is desirable to provide a support rack wherein the electrical contact between each support hook and the frame of the support rack is maintained as a good, low resistance electrical contact so that the suspended workpieces will remain well grounded. An example of one type of support rack is disclosed in U.S. Pat. No. 5,147,050 entitled &#34;Custom Hanging Rack With Interchangeable Support Hooks and Method Therefore&#34;. 
     Each painting or coating operation applies a coating layer to the exposed, workpiece-engaging portion of the support hooks which coating entirely covers the hooks except for the small area of contact with the workpiece. Unless a very similar workpiece is to be run in a subsequent operation, whereby the workpiece would be in electrical contact with the hook at an identical part, it is necessary to provide a clean support hook to assure the requisite electrical contact between the workpiece and the support hook. Accordingly, it is desirable that the support hooks be detachably engaged to the frame of the rack so that the hooks can be periodically removed and replaced with new or cleaned hooks. Simple and rapid interchangeability of the support hooks is also desirable so that different sizes and shapes of support hooks can be interchanged as desired to allow for customization of the rack to suit the requirements of a wide variety of runs. It is desirable to provide this interchangeability while still maintaining good electrical contact between each support hook and its respective support rack during electrostatic painting operations or the like. 
     It is desirable that the support rack lend itself to simple and inexpensive manufacture while still providing for flexibility and also good electrical contact of the workpieces supported. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an inexpensive support rack for supporting workpieces during transport through a finishing station while maintaining positive electrical contact of the workpieces. The rack comprises a support frame including a tube having an interior surface defining an elongated, substantially enclosed hollow interior. A hook-receiving aperture is formed in the tube, extending into the hollow interior of the tube. A support hook is provided having a tube-engaging portion receivable into the hook-receiving aperture of the tube and engageable with the tube for cantilevered engagement of the support hook with the tube. The support hook also has a workpiece-engaging portion for supporting the workpieces in spaced relation from the tube. During the cantilevered engagement of the support hook with the tube, the tube-engaging portion received in the hook-receiving aperture contacts the interior surface of the tube. The weight of the workpieces engaged with the workpiece-engaging portion of the support hook biases and maintains the tube-engaging portion of the support hook in positive electrical contact with the interior surface of the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, wherein like elements are referenced alike: 
     FIG. 1 is a perspective view of a hanging rack with cantilevered support hooks embodying various features of the present invention; 
     FIG. 2 is a sectional view of the hanging rack of FIG. 1, showing the contact of the hooks with the interior surface of the tube; 
     FIG. 3 is a perspective view of a support hook embodying various features of the present invention; 
     FIG. 4 is a perspective view of another support hook embodying various features of the present invention; 
     FIG. 5 is a perspective view of another support hook embodying various features of the present invention; 
     FIG. 6 is a perspective view of yet another support hook embodying various features of the present invention; and 
     FIG. 7 is a perspective view of another hanging rack with cantilevered support hooks embodying various features of the present invention, in which hook-receiving apertures are provided on either side of the support tubes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A hanging rack embodying various features of the present invention is illustrated in FIG. 1 and referred to generally by reference numeral 20. The hanging rack 20 is shown depending from a chain conveyor drive 22 which transports a plurality of hanging racks 20, with respective workpieces 24 thereon, through electrostatic coating operations and the like. Workpieces 24 are hung from support hooks, which are referred to generally by reference numeral 26, and which are detachably engageable with one or more tubes 28 of a support frame 30. The tubes 28 have hook-receiving apertures 30 which receive tube-engaging portions 32 of the support hooks 26 for cantilevered engagement of the support hooks 26 with respective tubes 28. 
     The support hooks 26 have a contact portion 32 which resides interiorly of the tube 28 upon engagement of the support hooks 26 with the tubes 28. As best seen in FIG. 2, the contact portion 32 of the support hook 26 is biased into abutment with the interior surface 34 of the tube 28 under the influence of the weight of the workpieces 24 hung from workpiece-engaging portions 38 of the support hooks 26. In addition to serving as a supporting and support hook engaging component, the tube 28 serves as a shield to the particulate 36 and prevents the particulate from contaminating the interior of the tube 28. Since neither the internal surface 34 of the tube 28 nor the contact portion 32 of the support hooks 26 disposed within the tube 28 are exposed to particulate associated with electrostatic painting or the like, a good electrically conductive contact is maintained between the support hooks 26 and the tube 28 despite the build-up of particulate 36 on the exterior surface 40 of the tube 28. Thereby, good electrostatic attraction of the charged particulate to the workpieces 24 supported on the support hooks 26 is maintained throughout electrostatic painting operations. 
     More specifically, as shown in FIG. 1 a plurality of support frames 30 depend from a chain conveyor drive 22 in spaced relation to one another. The support frames 30 each comprise a generally horizontal crossbar 42 with tubes 28 depending downwardly from either end of the crossbar 42 and extending generally vertically. The illustrated tubes 28 are rectangularly cylindrical, having four sides 44, 46, 48 and 50, and having a closed upper end 52, with the lower end 54 of the tubes preferably left open, to allow drainage of water from the tubes. The four sides 44, 46, 48 and 50 and the closed upper end 52 of the tube 28 defining a hollow interior 56. A plurality of vertically spaced apertures 58 are formed in the front side 50 of the tubes 28 extending into the hollow interior 56 of the tubes 28. As discussed in detail below, the apertures 58 receive the contact portions 32 of the support hooks 26 therethrough for simple manual cantilevered engagement of the support hooks 26 with the tubes 28. Also, for reasons which will become clear as the description of the invention proceeds, the apertures 58 are preferably formed by a punching operation to leave short integral projections 60 extending inwardly from the front surface 50 immediately beneath each of the apertures 58. 
     The preferred support hook 26b illustrated in FIGS. 1 and 4, has a workpiece engaging portion 38b from which one or more workpieces 24 may be hung, offset portions 64b for spacing the workpiece-engaging portion 38b from the tubes 28, and contact portions 32b. Each of the workpiece engaging portion 38b, offset portions 64b, and contact portions 32b of the support hook 26b extend mutually perpendicularly with respect to one another, as best seen in FIG. 4. 
     The support hook 26b is easily manually engageable with the tube 28. To engage the support hook 26b with the tube 28, the support hook 26b is manually lifted and oriented such that its contact portions 32b extend substantially horizontally and its workpiece-engaging portion 32b extends horizontally and above the contact portions 32. The pair of contact portions 32b are inserted through respective apertures 58 of the tubes 28 and the support hook 26b is then released whereupon the workpiece-engaging portion 32b of the support hook 26b falls downwardly under the influence of gravity to the position shown in FIGS. 1 and 2. Downward movement of the support hook 26a when released is limited by abutment of the contact portions 32a of the support hook 26a with the interior surfaces 34 of the front side 50 of the tubes. More specifically, upon release of the support hook 26b following insertion of its contact portions 32b into respective apertures 58, the offset portions 64b each bear against a corner 68 at the lower end of the aperture 58. The corners 68 serve as fulcrums against which, and about which, the support hook 26b pivots under the influence of gravity. The support hook 26b pivots about the corner 68 until is limited by contact of the ends 66b of the contact portion 32b of the support hook 26b abut the interior surface 34 of the tube 28. The support hook 26b remains engaged with the tube 28 and supported by the tube 28 in cantilevered engagement therewith as shown in FIGS. 1 and 2. That is, the peripheries of the ends 66b of the support hook 26b remain bearing against the inner surfaces 34 of respective tubes 28, and the offset portions 64b of the support hook 28 remain bearing against respective corners 68, to maintain the support hook 26b in cantilevered engagement with the tubes 28. 
     As seen in FIGS. 1 and 2, with the support hook 26b in cantilevered engagement with the tubes 28, the workpiece-engaging portion 28b of the support hook 26b is laterally offset from the corners 68. Hence, hanging of workpieces 24 from the workpiece-engaging portion 38b of the support hook 26b exerts a downward force on the workpiece-engaging portion 38b which forces or biases the end 66b of the contact portion 32b of the support hook 26b  into firm abutment with the interior surface 34 of the tube 28. The heavier the workpieces, the greater the force biasing the contact portions 32b into abutment with the interior surface 34 of the tube 28. More specifically, the offset portion 64b is preferably made sufficiently long that the distance between the workpiece-engaging portion 38b and the fulcrum corner 68 is greater than the distance between the end 66b and the fulcrum corner 68. Hence, there is a greater moment arm between the workpiece-engaging portion 38b of the support hook 26b and the fulcrum corner 68 of the tube 28 than there is between the end 66b and the fulcrum corner 68, so that the downward force exerted by the workpieces 24 on the workpiece-engaging portion 38b of the support hook 26a results in an even greater force urging the end 66b against the interior surface 34. 
     As discussed above, and as illustrated in FIG. 2, electrostatic painting results in the build-up of particulate 36 over the exterior of the tube 28, the workpiece-engaging portion 38b of the support hook 26b, and the surfaces of the offset portion 64b of the support hook 26b protruding outwardly of the tube 28. The tube 28 acts as a shield or barrier, so that the internal surface 34 of the tube 28 and the surfaces of the offset portion 64b disposed within the interior 56 of the tube 28 are not exposed to the particulate. 
     Though the tubes 28 are not fully enclosed, i.e. the tubes each have a plurality of apertures 58 extending into the tube interior 56, electrically charged particulate does not pass through the apertures of the grounded tube 28 since the electrostatic phenomenon thereat approximates a Faraday Cage, and the faraday cage effect resists passage of particulate through the apertures. Also, the edges of the tube 28 surrounding the apertures 58 produce a high density electrostatic charge thereat which further deflects the charged particulate and prevents it from passing through the apertures 58 into the tube interior 56. 
     Since both the interior surface 34 of the tube 28 and the contact portions 32b of the support hook 26b are substantially free of particulate, and the contact portions 32b of the support hook 28 are maintained biased against the interior surface 34 of the tube 28, good electrical contact is maintained between the tube 28 and the support hook 26b, despite the build-up of particulate 36 over the portions of the tubes 28 and support hooks 26 disposed externally of the tube interior 56. The maintenance of good electrical contact between the tube 28 and the support hook 26b from which workpieces 24 are hung is essential in carrying out successful electrostatic coating operations. 
     Though the accumulation of particulate on the exterior of the tube 28 does not affect the conductivity between the tube 28 and the support hooks 24, accumulation of particulate on the workpiece-engaging portion 38b of the support hooks 24 does present potential conductivity problems. That is, during transport through an electrostatic coating operation, the entire surface of the exposed workpiece engaging portion 38b of the support hook 26b will accumulate particulate thereon except on the areas at which the workpieces 24 are hung. The workpieces 24 may be hung either directly from the workpiece engaging portions 38 of the support hooks 26 (see workpiece 24a hung directly on support hook 26a, which is discussed further below) or may be hung indirectly from the workpiece engaging portions 38 through hangers 62 (see workpieces 24b and 24c). 
     The workpiece-engaging portion 38b of the support hook 26b is shielded by the workpieces 24, or its supporting hanger 62 if employed, at the points of connection of the workpieces 24 and/or hangers 62 thereto, thus preventing particulate from accumulating directly at the point of connection. Thus, good electrically conductive contact is maintained between the support hook 26b and the workpiece supported thereby throughout transport despite the accumulation of particulate on the support hook 26b everywhere except the point of connection of the workpieces to the support hook 26b. 
     However, when a different workpiece is run which contacts the workpiece-engaging portion 38b of the support hook 26b at a different location than the previously run workpiece, the point of contact between the workpiece 24 and the support hook 26b may not include the previously shielded, and thus uncoated, portion of the support hook 26b. Thus, unless the support hook is cleaned to remove the coated particulate therefrom, there will not be good electrical contact between the workpieces 24 and the support hook 26b. 
     The support hooks 26b of the present invention are easily manually attachable and detachable from the tubes 28 of the support frame to allow for removal of coated support hooks 26b and insertion of clean support hooks 26 following a coating run. Thus, by maintaining a supply of extra support hooks 26, clean support hooks can be employed at all times, while particulate-coated support hooks 26 are being cleaned for later use. Continual employment of clean support hooks 26 assures good electrical contact between the support hooks 26 and their associated workpieces 24 depending therefrom. The construction of the hanging rack 20 of the present invention allows easy manual removal of coated support hooks 26, and easy manual replacement of clean support hooks 26 into cantilevered engagement with the tubes. 
     The support hooks 26 are easily manually removed from engagement with their respective tubes 28 by simply raising the workpiece engaging portion 38 upward to pivot the contact portions 32b substantially back to horizontal, and thereafter sliding the support hook 26b away from the bars 28 to slide the contact portions 32b out from their respective apertures 58. 
     The formation of the apertures 58 in the tube 28 by punching or the like to leave an inwardly extending projection 60 beneath each of the apertures 58, as discussed above, eliminates operator error in attempting to engage the hooks upside-down. That is, if an operator were to attempt to engage a support hook 24 with its contact portion 32b extending downwardly (opposite to the orientation shown in FIGS. 1 and 2), the projections 60 would interfere with the contact portions 32b, giving an indication to the operator that the engagement is incorrect. The inwardly extending projections 60 beneath each of the apertures 58 also provide a smooth and relatively large fulcrum surface against which the offset portions 64b bear, as compared with the drilled-out holes or the like not having projections. The projections 60 thus reduce wear to the offset portions of the support hook 64b. The punching out of the apertures 58 to leave inwardly extending projections 60 is also less expensive than other manufacturing methods such as drilling or the like, so that even if operator error and wearing of support hook 64b are not of concern, the formation of the apertures 58 by punching or the like to leave inwardly extending projections 60 is preferred. 
     While the preferred embodiment discussed above was with regard to support hook 26b which engages with a pair of tubes 28, the hanging rack 20 of the present invention lends itself to use with an infinite variety of different support hooks 26. An alternative embodiment support hook 26a is illustrated in perspective view in FIG. 3, and shown engaged with a tube 28 in FIGS. 1 and 2. The support hook 26a engages with a single tube 28, as compared with the support hook 26b discussed above which engages with a pair of tubes 28. 
     The support hook 26a has a workpiece engaging portion 38a from which workpieces 24 may be hung, an offset portion 64a for spacing the workpiece-engaging portion 38a from the tube 28, and a contact portion 32a for contacting the interior surface 34 of the tube 28 to which the support hook 26a is engaged. The support hook 26a engages with the tube 28 in essentially the same manner as the support hook 26b engages with the pair of tubes 28a (discussed above), except that the support hook 26a only engages with a single tube 28. As discussed above with regard to the support hook 26b, the support hook 26a engages with the tube 28, for cantilevered engagement therewith, by raising the workpiece-engaging portion 38a of the support hook 26a to move the contact portion 32a to a substantially horizontal orientation and then inserting the contact portion 32a through an aperture 58 in the tube 28. Upon release of the support hook 26a, the workpiece-engaging portion 38a falls downwardly under the influence of gravity. Downward movement of the workpiece-engaging portion 38a of the support hook 26a is limited by abutment of the end 66a of the contact portion 32a with the interior surface 34 of the tube 28. The corner 68 serves as a fulcrum and the offset portion 64a bears against the corner 68, with the end 66 a of the contact portion 32a bearing against the interior surface 34 of the tube 28, to support and maintain the support hook 26a in cantilevered engagement with the tube 28. The weight of workpieces 24 suspended from the workpiece-engaging portion 38a of the support hook 26a forces the end 66a of the contact portion 32a of the support hook 26a into firm abutment with the interior surface 34 of the tube 28 to assure that good electrical contact is maintained between the support hook 26a and the tube 28. The workpiece engaging portion 38a of the support hook 26a is V-shaped to prevent workpieces 24 from falling off the support hook 26a and to maintain a consistent point of contact of workpieces 24 engaged with the workpiece-engaging portion 38a of the support hook 26a, to help in maintaining good electrical contact between the support hook 26a and the workpieces 24 suspended therefrom. The support hook 26a also has an integral U-shaped portion 70 which is disposed adjacent each of the front, inner and outer sides 50, 48, and 44 respectively, to prevent the support hook 26a from rotating and possibly becoming disengaged from the tube 28, and to reduce lateral swinging of the support hook 26a with respect to the tube 28. 
     An alternative embodiment hanging rack 120 is illustrated in FIG. 7. In this embodiment of the invention, hook-receiving apertures 158 are provided in both the front and rear walls 146 and 150 of the tube 128, with the apertures 158 in the front wall 150 being offset from the apertures 158 in the rear wall 148. The offsetting of the apertures prevents interference between parts hung from hooks engaged in the front wall apertures and parts hung from hooks engaged in the rear wall apertures. FIG. 7 shows hooks 126 engaged in some of the apertures in both the front and rear walls of the tubes. With reference to FIGS. 5 and 6, further alternative embodiments of support hooks are illustrated. In the embodiment of FIG. 5, a hook 226 is shown. A bar 231, such as the hook 26a of FIG. 4, is provided which has a plurality of short hooks 135 welded thereto and also welded to a rod 137 disposed substantially parallel to the bar 231. The welding of the hooks 135 to both the bar 231 and rod 137 has been found to allow the composite hook 226 to withstand the high temperatures associated with burnoff ovens in which the temperature may be in the range of approximately 800° F. Stainless steel hooks 135 having a diameter of approximately 0.091 inch welded to a bar 231 and rod 137 each of approximately 3/16 inch diameter, and approximately 25 foot length, has been found to provide good results. 
     In the embodiment of FIG. 6, another hook 326 is illustrated. A bar 231, such as the hook 26a of FIG. 4, is provided which has its hook-engaging portion 338 disposed in a tube 331. Short hooks 335 have an end 337 wrapped about and welded to the tube 331. 
     While the invention has been described with reference to a preferred embodiment and alternative embodiments, it will be understood to those skilled in the art that various changes and modifications may by made, and equivalents may be substituted for elements thereof, without departing from the scope of the invention. In addition, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.