Abstract:
A seal for a rotary shaft extending from a container containing agitated abrasive particulates through a wall of the container. The seal includes a first polymeric member around the shaft and in engagement with the wall of the container with the shaft rotating inside the first member. The first member is in engagement with a second polymeric member positioned around the shaft for rotation with the shaft and relative to the first member. A third compressible member is positioned around the shaft for rotation with the shaft and the second member. A retaining ring maintains the first, second and third members in compression between the retaining ring and the wall.

Description:
FIELD OF THE INVENTION 
     This invention relates to an improved shaft seal for a rotary shaft extending from a container containing agitated abrasive particulate solids through a wall of the container. 
     BACKGROUND OF THE INVENTION 
     In the operation of copier/duplicator machines, a conductor film is used to transfer an image from an original to a copy. The photo-conductor film is initially charged at a corona charger or the like and then passed to an imaging section where a charged image is placed on the conductor film. The film then passes to a section where a developer and toner are used to deposit a the toner on the charged image. In this section the developer, which is a finely divided magnetic metallic oxide such as iron oxide, becomes associated with toner which is then passed into close proximity to the conductor film which attracts the toner to the charged image. The developer and toner are retained in a developer sump, which includes a rotary blender, optionally a rotary bucket shell and a rotary toner shell or the like, which move the blend of toner and developer into proximity to the photo-conductor film. These components are powered by shafts extending through the wall of the container of developer and toner. The developer is a particularly abrasive material and it has been difficult to develop seals which will provide extended life to bearings and other components used to provide power to the blender, bucket shell, toner shell or the like inside the developer/toner sump. 
     The failure of these bearings is particularly serious since they are not readily replaced in the field by a service representative. Since these bearings cannot be replaced in the field, they require replacement of the entire sump when bearing failures occur. Further the escape of developer and toner can result in the presence of undesired contaminants in the copier/duplicator machine. 
     Accordingly a continuing search has been directed to the development of improved seals for such shafts. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an improved seal for a shaft extending from a container containing agitated abrasive particulates through a wall of the container is provided by a seal comprising: a polymeric first washer member having a first side and a second side and sealingly positioned at its first side against the wall of the container and sealingly positioned around the shaft for rotation of the shaft inside the first washer member; a polymeric second washer member having a first side and a second side and sealingly positioned around the shaft with its first side in contact with the second side of the first member for rotation of the second member with the shaft and for rotation of the first side of the second member relative to the second side of the first member; a polymeric compressible foam third washer member having a first side and a second side and sealingly positioned around the shaft with its first side in contact with the second side of the second member for rotation with the shaft and the second member; and, a retaining member having a first side and a second side and positioned at least partially around the shaft in a groove in the shaft with its first side in engagement with the second side of the third member for rotation with the shaft and the third member, the groove being positioned to retain the first member, the second member and the third member in compression between the retaining ring and the wall of the container. 
    
    
     DESCRIPTION OF THE FIGURE 
     The FIGURE is a schematic diagram of a shaft extending through a wall of a container containing abrasive solids and including a seal according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a shaft seal  10  is shown around a shaft  12  extending through a wall  14  which may be a developer sump wall of a copier/duplicator machine. Gears, bearings and the like required to operate equipment inside the sump are not shown but are positioned to engage shaft  12  outside wall  14 . Shaft  12  as shown extends through wall  14 , which is typically a cast metal, such as aluminum, wall. Accordingly a space  15  exists around shaft  12  and between shaft  12  and a passageway  17  through wall  14 . If developer is allowed to move into and along space  15  between the outside of shaft  12  and the inside of passageway  17  it is extremely detrimental to shaft bearings, gears and the like and has resulted in a very short life for these components. Various attempts have been made to sealingly contain the developer inside the developer sump but in all instances the seals have exhibited a relatively short life. 
     According to the present invention, a first member  18  is positioned around shaft  12  with its first side  20  against inside wall  16  of the sump. Typically, inside wall  16  is not machined and is a relatively rough surface with a relatively high coefficient of friction. First member  18  is typically formed as a washer, which is sealingly positioned around shaft  12 . Desirably, member  18  is formed of a polymeric material, having a high abrasion resistance. While first member  18  is designed to remain stationary relative to wall  16  because of the high coefficient of friction with wall  16 , it may encounter occasional slight movement against wall  16 . Substantially any suitable polymeric material, such as tetrafluoroethylene, TEFLON (trademark of Du Pont, 1007 Market Street, Delaware 19898), may be used. A suitable material for first member  18  is silicone impregnated tetrafluoroethylene. A preferred silicone impregnated tetrafluoroethylene washer is available from Waldes Kohinoor Inc. 41-16 Austel Place, Long Island City, N.Y., 11101 under the trademark GLYCON, Style 3500′. These washers are effective in the present invention. Other polymeric materials could also be used so long as they provide sufficient abrasion resistance and a sufficiently low friction with a second member  24 . 
     Second member  24  is also sealingly positioned around shaft  12  and has its first side  26  positioned against second side  22  of the first member. Second member  24  is desirably of a polymeric material (such as tetrafluoroethylene), which exhibits a low coefficient of friction with first member  18 . Second member  24  is sealingly positioned on shaft  12  and is designed to rotate with shaft  12 . Accordingly, the rotation of the shaft is accommodated in the seal by the rotation of second member  24  against the low friction surfaces of first member  18 . Preferably the coefficient of friction between first member  18  and second member  24  is from about 0.15 to about 0.35. 
     A particularly suitable material for use in the fabrication of second member  24 , which is also typically fabricated as a washer adapted to sealingly fit over shaft  12 , is a graphite—filled tetrafluoroethylene available from Waldes Kohinoor Inc, 47-16 Austel Place, Log Island City, N.Y., 11101 under the trademark GLYCON, Style 3530. The materials described above are particularly suitable for second member  24 . Substantially any polymeric material having a suitable coefficient of friction with first member  18  at the conditions in the developer sump can be used for either first member  18  or second member  24 . A second side  28  of second member  24  is positioned in contact with a first side  32  of a third member  30 . 
     Third member  30  is desirably a polymeric foam having a suitable density and a suitable compressibility and is configured as a washer—like member around shaft  12 . Desirably third member  30  has a compressibility of from about 1.9 to about 3.6 pounds per inch. Any suitable polymeric foam, may be used, although cellular urethane polymers are preferred. A suitable polymeric foam washer is available from Rogers Corporation, 245 Woodstock Road, Connecticut 06281-1815 under the trademark PORON. Washers of a very low modulus to washers of a very high modulus are available. While these materials are suitable, any foam of a density sufficient to maintain pressure between a retaining ring  36  and first member  18  and second member  24  at the temperature in the developer sump is suitable. Third member  30  is designed to rotate with shaft  12  and is in compression with its first side  32  against a second side  28  of second member  24 . 
     As shown, retaining ring  36  is positioned with its first side  38  against a second side  34  of third member  30 . Retaining ring  36  maintains compression on third member  30 , which in turn maintains pressure on first member  18  and second member  24 . The entire seal assembly is thus maintained in compression. Retaining ring  36  is retained in position relative to shaft  12  by a groove  42  which includes a shoulder  44  which axially restrains retaining ring  36 . Groove  42  has a length along the length of shaft  12  greater than the width of retaining ring  36 . This permits the assembly of the seal components on shaft  12  in an uncompressed position with the shaft then being passed through opening  17  and pushed into compression by the interaction of shoulder  44  on shaft  12  with retaining ring  36  and the remaining components of the seal. The shaft is then retained in position so that shaft seal  10  remains in compression. 
     Retaining ring  36  may be of any suitable material having sufficient strength to retain first, second and third members in compression. Retaining ring  36  may be a spring ring which extends around about 270° of the outside of shaft  12 . A wide variety of materials such as steel, stainless steel, beryllium copper and the like may be used to fabricate retaining ring  36 . 
     Third member  30  as shown includes a section  46  which is in contact with shaft  12  and centers member  30  around shaft  12 . A section  48  of third member  30  opposite groove  42  is free to radially expand as member  30  is compressed. 
     By the combination of components and features of the present invention, the motion between seal components is limited to the rotary interaction between first member  18  and second member  24 . Both these elements are polymeric materials, which have relatively low coefficients of friction so that they freely rotate relative to each other. First member  18  is retained in position relative to wall  14  by the high coefficient of friction between the first side of first member  18  and the inside  16  of wall  14 . This results in the interaction of the sealing elements being limited to movement between two very low coefficient of friction surfaces. These surfaces are maintained in compressive engagement by retaining ring  36  and third member  30  which remains in compression and exerts compression on first member  18  and second member  24 . This arrangement is very effective to prevent the escape of developer and toner from the developer sump through opening  17  and the like. The use of these components provides longer service life and greater reliability then previously available materials. Further this design is effective over a wide range of tolerances and since it retains the developer and toner in the sump, it greatly improves the life of the bearings and related materials which are used to operate the blender, bucket shell, toner shell and the like inside the sump. The net result is much longer service life for the developer/toner sump and greatly reduced risk of the escape of contaminants into the copier/duplicator machine. Further since the seal is retained in compression against wall  14  by shaft retainers outside wall  14 , less shaft motion is experienced in the bearings and gears outside wall  14 . The net result is a substantial improvement in a sump life. 
     Having thus described by reference to certain of its preferred embodiments, it is respectfully pointed out that the embodiments described are illustrative rather than limiting in nature and that many variations and modifications are possible within the scope of the present invention. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon the foregoing description of preferred embodiments.