Abstract:
A drum assembly comprises a replaceable sleeve of resilient material supported by a rigid cylindrical mandrel having an air bearing at one end to facilitate loading and removal of the resilient sleeve. The air bearing is provided by a pair of cooperating plates one of which is scored with equally spaced and radially extending slots. When urged together, the plates define a central air chamber and a plurality of radially-extending passages serving to direct pressurized air radially from one end of the mandrel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is made to the commonly assigned U.S. Patent Applications, the respective disclosures of which being incorporated herein by reference: 
     (1) U.S. application Ser. No. 09/574,425, filed concurrently herewith and entitled “Document Printer/Copier with Decoupleable Drum-Support Member”. 
     (2) U.S. application Ser. No. 09/574,275, filed concurrently herewith and entitled “Cantilever Drum Mount for Document Printer/Copier”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to field of printing and copying. More particularly, it relates to improvements in the structure of printing or image-transfer drums of the type having a resilient outer sleeve that are supported by an underlying mandrel. Such drums are used, for example, in electrostatic document printers and copiers for temporarily receiving a toner image from an image-recording element before there is re-transferred to an image-receiver sheet or the like. 
     BACKGROUND OF THE INVENTION 
     In printing machines, copiers and the like, images are often formed on or transferred to a drum having a flexible or resilient outer sleeve that, from time to time, requires replacement. Typically, the sleeve is operatively supported by a metal cylinder or mandrel. In loading the sleeve onto the mandrel, it is common to inject air into the sleeve, thereby slightly expanding the sleeve diameter, while sliding the sleeve axially onto the mandrel&#39;s supporting surface. Usually, the nominal diameter of the resilient sleeve is slightly less than the mandrel diameter. Thus, upon discontinuing the air flow, the sleeve contracts onto the mandrel and forms a tight, interference fit. 
     In drum assemblies of the type described above, it is known to machine air ports or holes in the sleeve-loading end of the mandrel. See, e.g., the disclosure of U.S. Pat. No. 4,119,032. Air supplied through the mandrel interior emerges through these air ports and provides an air bearing for expanding the sleeve. A uniform air bearing that is sufficient to expand a sleeve of relatively non-elastic material often necessitates a close and uniform spacing of the air ports in the mandrel surface. This need can be problematic from a manufacturing cost standpoint when the mandrel is of relatively large diameter and/or is made of a relatively hard substance. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing discussion, an object of this invention is to provide a low-cost mandrel assembly for supporting a flexible sleeve of the type described. 
     According to one aspect of the invention, a mandrel for supporting a removable outer sleeve comprises: (a) a cylindrical tube of predetermined outer diameter; and (b) a circular end plate positioned at one end of the tube and operatively connected thereto. The end plate has a diameter substantially equal to the predetermined outer diameter of the cylindrical tube and has a plurality of radially-extending slots formed in a planar surface thereof that faces inwardly, toward the center of said tube. The slots are sufficiently long to extend from a centrally-located annular recess formed in the planar surface to the perimeter of the end plate. A pressure plate is operatively connected to the end plate and has a surface that cooperates with the slotted and recessed planar surface of the end plate to define a centrally-located air chamber and a plurality of radially-extending air passageways connecting the air chamber and the perimeter of the end plate. Upon connecting the air chamber to a source of pressurized air, a plurality of radially extending air streams are created at the periphery of the end plate. According to a preferred embodiment, both the end plate and said pressure plate are made of plastic, whereby the cylindrical tube and a supporting axle, both of which are made of metal, can be electrically isolated. This is particularly useful when the drum surface is to be electrically biased relative to a machine frame that supports the axle. Also preferred is that the peripheral edge of the end plate is tapered to facilitate the loading of the removable sleeve onto said mandrel. 
     By virtue of having the air ports of the air bearing defined by a pair of confronting plates, each preferably being molded from plastic, the above-noted machining process of prior art mandrel is obviated. Thus, the manufacturing costs are significantly reduced. 
     The invention and its advantages will be better understood from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings in which like reference characters denote like parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional illustration of a drum assembly embodying the invention; 
     FIG. 2 is an exploded view of a preferred embodiment of the invention; 
     FIG. 3 is an enlarged perspective view of the free end portion of the drum assembly shown in FIG. 1; 
     FIGS. 4 and 5 are enlarged perspective views of two components of the drum assembly shown in FIG. 1; and 
     FIG. 6 is a front plan view of the drum mandrel shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 shows a drum assembly DA of the type used as an intermediate image-transfer drum used in an electrophotographic document printer/copier. An example of such an apparatus is disclosed in the above-referenced Applications filed concurrently herewith. The drum assembly is supported for rotation by rear and front stub axles A 1  and A 2 , respectively. Rear axle A 1  carries a double ring ball bearing B 1  which is mounted in a movable carriage  10  contained in a drum-support member  12 . The latter serves to support the drum assembly in a cantilever fashion when a front drum-support member (not shown) has been decoupled from the drum assembly&#39;s front axle bearing B 2  and moved to a standby position where it does not interfere with servicing of the drum assembly, as described below. Details of the drum assembly support are disclosed in the above-referenced U.S. application Ser. No. 09/574,425. Details of the movable front drum support are disclosed in the above-referenced U.S. application Ser. No. 09/574,275. 
     Drum assembly DA basically comprises a cylindrical mandrel  20  that supports a flexible sleeve  22 . In the case where the drum assembly constitutes an image-transfer drum in an electrophotographic printer/copier or the like, sleeve  22  may comprise a thin, flexible band of nickel or the like having a relatively compliant outer layer of urethane. Sleeve  22  is sized to slide axially, as indicated by the arrows, onto the outer surface of the mandrel and be supported thereby with an interference fit. To facilitate loading of sleeve  22  onto the mandrel, an air bearing is established (as described below) around the entire circumference of the mandrel in the vicinity of region C, i.e., in the vicinity of the free end of the mandrel. Upon fitting a portion of the end of sleeve  22  over this air bearing, a sufficient volume of air will be trapped between the mandrel and the inside surface of the sleeve to cause the sleeve to expand slightly. At this point, the remainder of the sleeve can be easily slid over the entire surface of the mandrel to the position shown in FIG.  1 . When air is no longer applied to region C, the sleeve contracts and seats snugly on the mandrel surface. 
     Referring now to the exploded view of PIG.  2 , mandrel  20  is shown to comprise a cylindrical metal tube  25  having a plurality of internal struts  26  that extend radially, from the inside surface of tube towards the tube&#39;s central (longitudinal) axis. The respective free ends of the struts are connected to, and thereby support, a second tube  28  that is concentrically arranged with respect to tube  25  and is co-extensive therewith. Thus, tube  28  provides an enclosed cylindrical air passage  29  from one end of tube  25  to the other. Preferably tube  25  and its internal structure is made of extruded aluminum. 
     In addition to providing an air passage through tube  25 , the internal second tube  28  serves to support the stub axles Al and A 2  which, as noted above, extend outwardly from opposite ends of the mandrel. Each axle is preferably made of stainless steel. To electrically isolate the stub axles from tube  25  so that an electrical bias voltage may be applied to the drum independent of the electrical potential of the drum support, it is preferred that a pair of non-conductive, preferably plastic, bushings  36 ,  38  be interposed between the inside surface of tube  28  and the axles. 
     Referring to FIG. 1, each of the stub axles A 1  and A 2  has an internal, axially extending internal passage,  40 ,  42 , respectively, which communicates with air passage  29  defined by tube  28 . Note, axle passages  40  and  42  extend only partially through their respective axle, each axle being closed at its outside end. Passage  40  in axle A 1  communicates with a radially-extending airport AP 1  to which air may be selectively applied from an external source to introduce air to the interior of the mandrel, more specifically, into passages  40 , 42  and  29 . Passage  42  in axle A 2  communicates with a pair of radially-extending air ports AP 2  through which pressurized air can exit from passage  42  and enter an air chamber defined by plated  52  and  60 , discussed below. As shown in FIG. 1, the free end of axle A 1  supports a pair of seals  44  which cooperate with walls of the drum support carriage  10  to define a sealed region surrounding axle A 1  to which pressurized air can be applied, through air port AP, and thus introduce air into the aforementioned passages even as the drum axle rotates in bearing B 1 . The rear end of drum  25  is capped by an end plate  50  which is attached to the radial struts  26  by bolts or other suitable means. Note, end plate  50  has a central opening  50 A that is sufficiently large as not to contact the circular flange  51  on axle A 1  and thereby electrically short the axle and tube  25 . 
     Referring now to FIGS. 3-5, the front end  25 A of tube  25  is capped by a circular end plate  52  that is connected to the internal struts  26  of tube  25  by a set of bolts  55  and pins  56 . End plate  52  has a diameter substantially equal to the outer diameter of tube  25  and has a circular opening  53  through which the free end  54  of stub axle A 2  projects. A flange  52 B of reduced thickness surrounds opening  53  and defines an annular recess  52 C in the planar surface  52 A of the plate. When plate  52  is bolted to tube  25 , a portion of flange  52 B engages a flange  54 A on axle A 2  and urges the axle flange inwardly, toward engagement with a pressure plate  60 , discussed below. Plate  52  has a plurality of radially-extending slots  58  formed in a planar surface  52 A thereof that faces inwardly toward the center of said tube. The slots extend from the centrally-located annular recess  52 C formed in the planar surface to the circular perimeter of the end plate. Preferably, each of the slots has a square cross-section, about 0.5 to 2 mm. on a side. End plate  52  is preferably made plastic by an injection molding process. 
     Positioned against the slotted surface of end plate  52  and attached thereto by the same bolts used to connect plate  52  to tube  25  is a pressure plate  60 . Plate  60  is also made of injection-molded plastic and has a planar surface  60 A that cooperates with the slotted planar surface  52 A of end plate  52  to define a plurality of radially-extending air passageways  66  connecting the annular recess  52 C and the perimeter of said end plate. Radially inward from surface  60 A is an annular ring  68  connected to the outer annulus  69  by a plurality of spoke-like members  70 . Between the spoke members are openings  72 . When bolted together, ring  68  rests upon an outer rim portion  36 A of bushing. Air emerging through airports AP 2  is thus confined to openings  72  which, in turn, communicate with the annular recess  52 C to define an internal air chamber for supplying the radially-extending passageways  66  with pressurized air. Thus, when air is applied to the mandrel through airport AP 1 , a plurality of radially extending air streams are created at the periphery of the end plate, thereby producing an air bearing at the interface of tube  25  and end plate  52 , as shown in FIG.  3 . Preferably, the peripheral edge  52 D of said end plate  52  is tapered at about 5 degrees (as shown in FIG. 3) to facilitate the loading of the removable resilient sleeve  22  onto the mandrel. 
     As a result of the above-described structure, the need for a costly machining process for producing air ports in solid surface is avoided. The air bearing effect is achieved with greater precision through the use of low cost, injection moldable components. 
     The invention has been described with reference to a particularly preferred embodiment. It will be apparent, however, that certain modifications can be made without departing from the spirit of the invention, and such modifications are intended to be protected by the following claims.