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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Provisional Patent Application Ser. No. 61/131,136, filed Jun. 6, 2008. 
    
    
     FIELD OF INVENTION 
     This invention relates to plumbing devices and is particularly directed to improved means for blocking drain openings to prevent passage of large objects, while allowing free flow of fluid therethrough. 
     PRIOR ART 
     As is well known, food preparation kitchens are usually provided with a floor sink having a drain opening which connects to a grease trap or sewer to allow disposal of indirect waste water and the like. Unfortunately, rags, napkins, silverware and other large objects are often washed into the floor sink along with the floor washing water and these objects often get carried into the drain and cause blockage, flooding and other problems. Moreover, the loss of napkins, silverware and the like add significant expense to the operation of the restaurant. Unfortunately, most floor sinks have open drains which are subject to the problems noted above. Some prior art drain plugs or screens have been provided which are permanently installed in the drain opening. However, these often become clogged and simply add to the flooding problem. Thus, none of the prior art sink drain plugs have been entirely satisfactory. 
     These disadvantages of the prior art are overcome with the present invention and an improved floor sink drain plug is provided which positively precludes passage of large objects, while permitting free passage of fluid and which can quickly and easily be removed for cleaning, when desired by means of a special locking key. 
     These advantages of the present invention are preferably attained by providing an improved floor sink drain screen lock having expandable means for sealingly engaging the walls of a floor sink drain, yet being readily collapsible for quick and easy removal when desired. 
     Accordingly, it is an object of the present invention to provide an improved floor sink drain screen locking apparatus. 
     Another object of the present invention is to provide an improved floor sink drain screen lock which positively precludes passage of large objects. 
     A further object of the present invention is to provide an improved floor sink drain screen lock which positively precludes passage of large objects while permitting free passage of liquids. 
     An additional object of the present invention is to provide an improved floor sink drain screen lock which positively precludes passage of large objects while permitting free passage of liquids and which can quickly and easily be removed for cleaning, when desired. 
     A specific object of the present invention is to provide improved floor sink drain screen locks having expandable means for sealingly engaging the walls of a floor sink drain, yet being readily collapsible for quick and easy removal when desired. 
     These and other objects and features of the present invention will be apparent from the following detailed description, taken with reference to the figures of the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an exploded isometric view of a floor sink drain screen lock embodying the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In that form of the present invention chosen for purposes of illustration,  FIG. 1  shows a floor sink drain screen lock, indicated generally at  10  comprising a metal upper disc  12  formed with a central opening  14  surrounded by a plurality of additional openings  16  and having a tapered underside  18 , together with a flexible split ring  20 , a resilient ring  22  encircling said split ring  20 , and a metal lower disc  24  having a tapered upper surface  26  and an internally threaded central opening  28 , surrounded by additional openings  30 . A bolt  32  is inserted through the central opening  14  of the upper disc  12  and through split ring  20  and resilient ring  22  and threadedly engages the opening  28  in the lower disc  24 . Preferably the resilient ring  22  is square in cross section as this permits greater frictional engagement with the walls of the drain opening. 
     In use, the floor sink drain screen lock  10  is inserted into the mouth of a floor sink drain so that the tapered lower surface  18  of the upper disc  12  rests on the periphery of the floor sink drain opening. The bolt  32  is then tightened which serves to draw the lower disc  24  toward the upper disc  12 , which compresses the split ring  20  and resilient ring  22  causing them to expand laterally to frictionally and sealingly engage the wall of the floor sink drain opening. This ensures that the drain screen lock  10  will not be displaced during use. When the floor is washed, water can flow through openings  16  in the upper disc  12  and openings  30  in the lower disc  24 . However, any large objects will be blocked by the remaining structure of the upper disc  12 . If the openings  16  or  30  become clogged over time, bolt  32  can be loosened, allowing split ring  20  and resilient ring  22  to contract and, hence, allowing the drain screen lock  10  to be removed for cleaning. The resilient ring  22  serves to retain the split ring  20  in position between the upper disc  12  and lower disc  24  when the drain screen lock  10  is not inserted into a drain opening. Subsequently, the drain screen lock  10  can be reinserted in the floor sink drain opening in the manner described above for further use. 
     Obviously, numerous variations and modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention described above and shown in the accompanying drawing are illustrative only and are not intended to limit the scope of the present invention.

Summary:
Improved floor sink drain screen locks having expandable means for sealingly engaging the walls of a floor sink drain, yet being readily collapsible for quick and easy removal when desired.