Reproduction machine developer apparatus
United States Patent n 1 Dennie 1 1 Apr. 29, 1975 REPRODUCTION MACHINE DEVELOPER APPARATUS Primary Examiner-Richard L. Moses [75] Inventor: Richard H. Dennie, Walworth, N.Y.  
 [57] ABSTRACT A floating platen is provided in a reproduction apparatus employing an endless belt-type photoconductor.  
 [22] Filed: June 21, 1974 the platen being provided opposite the apparatus developing mechanism, and having means to dispose the [21] Appl&#39; 48l60l belt into a substantially flat condition. The platen is supported for limited floating movement between pre- [73] Assignee: Xerox Corporation, Stamford,  
 Conn.  
 [52] US. Cl 355/16; 118/637; 355/3 R set limits. and the developing mechanism itself, which [51} Int. Cl G03g 15/00 is also capable of limited displacement, is located off [58] Field of Search 355/16, 3 R. 3 DD; of the platen. In this way, unwarranted displacement 118/637 of either platen or the developing mechanism is accommodated without affecting or changing the critical [56] References Cited operating spacing therebetween.  
  UNITED STATES PATENTS As a further arrangement, automatic signal means are 3,435,693 4/1969 Wright et a1 355/16 X provided to indicate the positional relationship 3.694.068 9/1972 Jordan H 355/3 R between the developing mechanism and the platen.  
 3.800.743 4/1974 Egnaczak 118/637 X 1227.776 10/1973 Turner 355/3 DD 3 0mm 9 Drawmg &#34;gmimazm 3.880.517  
 SHEET 4 OF 7 REPRODUCTION MACHINE DEVELOPER APPARATUS This invention relates to reproduction machines, and more particularly to an improved developing apparatus for xerographic reproduction machines.  
  In order to obtain the very high copying speeds deemed necessary for future generation copiers and reproduction machines while providing enhanced copy quality, the use of a belt type photoconductor is being seriously considered. However, in contemplating this type of photoconductor, and bearing in mind the relatively high processing speeds that must be sustained &#39;if truly high speed copying is to be achieved, it appears necessary to utilize multiple developing units such as magnetic brushes to develop the latent images produced on the belt. The use ofa battery of such developing brushes appears to be facilitated if carried out against a substantially flattened segment of the belt inasmuch as this allows a magnetic developing blanket to be established and nourished from one brush to another. In addition, working against a flat belt surface enables the critical spacing necessary between the belt surface and the magnetic brushes to be obtained.  
  One way of obtaining the desired flattened segment in the belt is to utilize a platen support with vacuum drawing means to draw the surface of the belt opposite thereto into contact therewith. In that circumstance, the developing apparatus is disposed opposite the platen to take advantage of the flattened belt run, with structure being provided to establish the correct spacing between the developing units and the surface of the belt required for proper development.  
  However, as will be envisioned, the position of both platen and developing apparatus, relative to one another and to the other component parts of the reproduction machine. is dependent upon the rigidity of the supporting structures. If for any reason, the supporting structure should be displaced, even in the smallest degree, the critical spacing between the belt and developing units may be affected with resulting improper and undesirable image development.  
  It is therefore a principal object of the present invention to provide a new and improved reproduction machine.  
  It is a further object of the present invention to provide, in a high speed reproduction apparatus, an improved developing mechanism.  
  It is an object of the present invention to provide, in conjunction with a belt type xerographic machine a floating support platen for the belt.  
  It is an object of the present invention to provide a floating platen with signalling means to indicate proper spacing between the platen and the adjoining developing apparatus.  
  It is an object of the present invention to provide a conjointly movable photoconductive belt support platen and developer housing with means to indicate loss of operative spacing between the support platen and developer housing.  
  It is an object of the present invention to provide, in a reproduction machine of the high speed type and incorporating an endless photoconductive belt, a floating belt run delineating platen with cooperating image developing means supported from the platen and movable conjointly therewith in the event of unwarranted displacement, and signal means to reflect relative dispo sition of platen and developing means.  
  This invention relates to a reproduction machine. the improvement comprising a movable, flexible belt having a photoconductive surface on which a latent electrostatic image of the original being copied is formed and developed, support means for the belt including an internal frame member, plural support rollers for the belt rotatably mounted on the frame member for supporting and guiding the belt in an endless path of movement, a movable platen for the belt disposed between a pair of the rollers, the surface of the platen facing the belt being substantially flat, means to tension the belt whereby to draw the belt against the support platen flat surface to provide a substantially flat belt run, means cooperable with the platen to develop the image on the belt, means to locate the developing means in predetermined spaced relationship to the platen, and means to signal when the platen and developing means are in predetermined spaced relationship.  
  For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in conjunction with the accompanying drawings wherein:  
  FIG. I is a schematic sectional view of an exemplary electrostatic reproduction machine showing the various processing stations and incorporating the present invention;  
  FIG. 2 is a partial cross-sectional view of the photoconductor belt module for the reproduction machine shown in FIG. 1;  
  FIG. 3 is a partial sectional view of the belt module shown in FIG. 2 showing details of the module mounting arrangement and the belt tensionning means;  
  FIG. 4 is a partial sectional view of the belt module of FIG. 2 showing the electrical and vacuum connecting arrangements between the module and the repro duction machine;  
  FIG. 5 is a plan view, partly broken away, of a vac uum holddown platen;  
  FIG. 6 is a sectional view taken along lines 66 in FIG. 5 showing the holddown platen in an inoperative condition;  
  FIG. 7 is a view similar to FIG. 6 showing the hold down platen in operative condition;  
  FIG. 8 is a view partly in section showing details of the holddown platen and developer assembly mounting arrangement; and  
 FIG. 9 is an electrical schematic of the signal circuit.  
  For a general understanding of the invention, there is illustrated an exemplary copier/reproduction machine, designated generally by the numeral 10. As in all electrostatic systems such as a xerographic machine of the type illustrated, a light image of a document to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material to form a xerographic powder image, corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be fused by a fusing device whereby the powder image is caused permanently to adhere to the support surface.  
  In the illustrated machine 10, an original l2 to be copied is placed upon a transparent support platen 14 fixedly arranged in an illumination assembly generally indicated by the reference numeral 15, arranged at the left end ofthe machine as viewed in FIG. 1. While upon platen 14. an illumination system. which includes flash lamps l7 and reflectors l8, flashes light rays upon the original 12 thereby producing image rays corresponding to the informational areas on the original. The image rays, projected by means of an optical system [9, expose the photosensitive surface of a xerographic plate in the form of a flexible photoconductive web or belt 20 arranged on a belt assembly generally indicated by the reference numeral 21.  
  As will be described below, the photoconductive belt assembly 2] is slidably supported upon two shafts 23, 24, shaft 23 of which is secured to frame 9 of the machine 10 and is adapted to drive a belt in the direction of the arrow at a constant rate. During movement of belt 20, the reflected light image of the original 12 on the platen 14 is flashed upon the xerographic surface of the belt to produce electrostatic latent images thereon at exposure station 25.  
  As the belt surface continues its movement, the elec trostatic image produced at exposure station passes through a developing station 26 where there is positioned a developer assembly generally indicated by the reference numeral 27 and where belt 20 is maintained in a flat condition as will appear. The developer assembly 27 provides development of the electrostatic image by means of multiple magnetic brushes 29 as the image moves through the development station 26.  
  The developed electrostatic image is transported by the belt 20 to a transfer station 30 whereat a sheet of copy paper 32 is brought between transfer roller 33 and belt 20 at a speed in synchronism with the moving belt in order to accomplish transfer of the developed image. A sheet transport mechanism generally indicated at 35 serves to bring forward sheets of paper 32 from a paper handling mechanism generally indicated by the reference numeral 36 to transfer station 30.  
  After transfer, the sheet is stripped from the belt 20, and conveyed into a fuser assembly generally indicated by the reference numeral 37 wherein the xerographic powder image on the sheet 32 is permanently affixed thereto. After fusing, the finished copy is discharged from the apparatus at a suitable point for collection externally of the apparatus.  
  Drive means is arranged to drive the belt 20 in conjunction with the timed flash exposure of an original 12 to be copied, to operate developer assembly 27, and operate the paper transport and handling mechanisms 35, 36, respectively, in timed sequence to produce copies of the original.  
  The belt 20 comprises a photoconductive layer of selenium alloy. which is the light receiving surface and imaging medium for the machine 10, on a conductive backing. Belt 20 is journaled for continuous movement upon three rollers 40, 4], and 42 located with parallel axes at approximately the apex ofa triangle. During exposure of the belt 20, the portion of belt 20 being exposed comprises the belt run between the roller 40 and the roller 41. As shown in FIG. 3, the photoconductive belt assembly 2] is illustrated with the photoreceptor belt 20 partially broken away removed in order to illustrate the assembly mechanisms located adjacent the belt.  
  The upper roller 42 is rotatably supported on a hollow shaft 43 journaled in side plates 44 and 45. each having the general configuration of a triangle. The upper apex of the side plate 44 is formed with an opening for containing and supporting a bearing 46 which rotatably supports one end of the shaft 43. At the other end. the shaft 43 is journaled in a bearing 47 supported at the upper apex for the side plate 45 in the same manner. Positioned within and along the hollow shaft 43 and projecting through a machine support frame plate 48 there is a drive shaft 23 formed with a conical element 51 and a reduced diameter extension 52. Drive shaft 23 is supported on the frame plate 48 by a bearing 53 mounted in the frame and is connected to a drive mechanism (not shown). The conical element 51 is axially fixed and extends outwardly from the frame plate 48 so as to be mated with an inwardly tapered chamfeted end 54 of the hollow shaft 43 when the belt assembly is mounted on the frame. At the other end of the hollow shaft 43, the drive shaft extension 52 terminates in a threaded shank 55 which cooperates with a locking member 56 formed with a conical portion 57 adapted to mate with an inwardly chamfered surface 58 of the outer end of the hollow shaft 43. The locking member 56 is also formed with a rotary hand-operated knob 59 for facilitating easy removal of the locking member 56 when it is desired to release the belt assembly 21.  
  Extension 52 of drive shaft 23 serves to aid in supporting the belt assembly 21 and for rotating the roller 42 and, consequently the belt 20 when the belt assembly 21 is in operating position; that is, when the side plate 45 is positioned against the main frame plate 48. This arrangement also permits the easy removal of the belt assembly 21 from the machine frame 9.  
  The side plates 44, 45 are maintained in parallel planes and rigidly supported in spaced relation for supporting the rollers 40, 41, and 42 and all of the other structures that comprise the belt assembly 21 by internal structural plates 62 and by the shaft 43, the end of shaft 43 being fixed to the inner races of the bearing 46, 47. The plates 44, 45 are also spaced and held in position by shaft 63 which is identical to the rotatable hol low shaft 43 except that shaft 63 is permanently secured to side plates 44, 45, and does not rotate. At the end adjacent the frame plate 45, the shaft 63 is chamfered at 64 and arranged for cooperation with the conical portion 65 of a support shaft 24 positioned axially in the hollow shaft 63. The shaft 24 is secured to the frame plate 48 and has its outer end formed as a threaded portion 67 which cooperates with a locking member 68. Member 68 is provided with an inwardly tapering or conical portion 69 which is cooperable with an inwardly chamfered surface 70 formed at the outer end of the hollow shaft 63. A knob 71 on the member 68 allows manual locking of the inner shaft 24 relative to and within the hollow shaft 63. From the foregoing. it will he understood that the belt assembly 21 is mounted in cantilever fashion on the frame support plate 48 by means of the hollow shafts 43 and 63, and that by virtue of the use of tapered or conical portions 5], 57 in the case of the shaft 43 and the conical portions 65, 69 in the case of the shaft 63, the belt assembly 2] is adapted to be accurately positioned in its required location after movement of the assembly from the frame plate 48. It will be appreciated that this structure allows quick and easy removal and replacement of the belt assembly upon the frame while still providing means for driving the belt and permit removal of the belt from its supporting assembly 21.  
  The belt assembly 21 is provided with an arrangement for tracking the belt 20 in the event it deviates laterally in either direction during its movement around the rollers 40, 41, 42. The arrangement includes the mounting of the roller 41 for rocking movement and an edge guiding device associated with the roller 40 for producing the rocking movement of the roller 41 in response to lateral deviation of the belt as more fully described in U.S. application Ser. No. 102.312, filed Dec. 27, 1970 in the name of William E. Jordan and assigned to the present assignee.  
  As shown in FIGS. 2 and 3, the roller 41 is secured to a shaft 75 journaled in bearings secured to the ends of the parallel legs 76, 77 ofa yoke member 78. At the mid-point of the yoke member and extending in an opposite direction relative to the legs 76, 77, there is mounted a shaft 80 which serves to support the yoke member 78 for limited rocking movement about the axis of this shaft and to permit slight retraction. The shaft 80 is preferably in the form ofa cylinder 81 of relatively large diameter and being rotatably mounted in spaced bearings 82 mounted on the frame 62 and having a coaxial reduced portion 83 secured to the lower cylinder 81 and secured within a suitable opening formed in the bight portion of the yoke member 78. A relatively heavy coil spring 84 encircles the portion of shaft 80 between the yoke member 78 and the lower surface of the lower bearing 82. Rocking movement of the yoke member 78 about the axis of the cylinder shaft 80 will impart rotation of the cylinder 81 within the bearings 82. The spring 84 imparts a continuous outward force upon the yoke 78 and consequently the roller 41. when the belt 20 is mounted on its supporting rollers thereby placing the belt under slight tension during operation.  
  Means are provided for retracting the roller 41 inwardly of the belt assembly 21 to permit the easy removal and mounting of a belt 20 upon the assembly. To this end, the inner end of the cylinder 81 is connected to a quickacting toggle arrangement which will draw the cylinder inwardly so as to carry the roller 41 therewith to a retracted position. The toggle arrangement also serves to retain the yoke member 78 and roller 41 in the belt assembly 14.  
  The upper part of the cylinder shaft 81 is formed with a tubular extension 91 axially aligned therewith. Suitable openings are formed in the sleeve in diametrically opposed positions for supporting a pin 92 which pivotally connects one end ofa link 93 to this end of the cylinder 81. The link 93 is pivotally connected at its other end to one end of another link 94 having its opposite end pivotally connected to the frame structure 62. 1t will be apparent that the links 93, 94 and their respective connections form a toggle device which when actuated into alignment will cause the cylinder 71 to move outwardly and when actuated into a buckled condition will cause the cylinder to be retracted inwardly. These actuations of the toggle links are produced by a drive link 95 which is pivotally connected at one end to the pivot connection between the links 93, 94 and extends transverse of the belt assembly. through the outer wall 44 therefor and ending in a pivotal handle 96 for actuating the link 95 axially for causing actuation of the toggle 93, 94 in either of its two opearing conditions. ln order to remove or to replace a belt 20, the handle 96 is suitably manipulated to cause buckling of the toggle links 93, 94 which action draws the cylinder 61 in wardly to retract the roller 41. This forms sufficient slack in the belt as to permit an operator to move a belt easily relative to the rollers 40, 41, and 42. After a belt is placed on these rollers for machine operation. the handle 96 is manipulated in the opposite direction in order to place the links 93, 94 in their aligned condition. as shown in FIG. 3, for forcing the roller 41 against the belt 20 and locking the roller in this position. In this operation, the spring 84 serves as an over drive and, as previously mentioned, as a tensioning means for the belt.  
  Mounted within the belt assembly module 21 is an electrode device for providing an electrical connection of the photoconductive belt 20 and wiring associated with the machine electrical power and control circuits. Each of the belt supporting rollers 40, 41, and 42 are coated with a thin material such as rubber to facilitate driving of belt 20 and to prevent electrical leakage or dissipation of voltage levels on the electrostatic latent images on the belt. The electrode device includes a brush 100 having metallic bristles preferably made of stainless steel in brushing contact relative to the conductive substrate of the belt 20. The brush 100 is insulatingly secured by a bracket 101 to the frame structure and is electically connected by a wire 102 to an insulated binding post 103 secured to the outside surface of the side support plate 45 of the belt module. The side plate 45 is formed with an opening for allowing the exterior protrusion of the wire 102 and its mounting exterior of the belt module.  
  The binding post 103 is electrically connected to an elongated conductive bar 105 secured on insulating material 106 which. in turn, is secured to the side support 45. Cooperating with the bar 105 in an electrical relationship is a flexure electrode element 107 secured by means of an electrical insulating block 108 to a support plate 110 mounted on the machine frame 9. The flexure element 107 is connected by a conductor wire 111 to the electrical lead 112 which forms part of the machine control circuits and logic. Since machine circuits and logic assume such diverse forms and complexity, the, are not illustrated in the drawings. The purpose of the metallic brush 100, wire 102, bar 105, flexure element 107, wire 111 and lead 112 is to electri cally connect the conductive substrate which forms an integral part of the photoconductor belt 20, to the control circuits for the machine.  
  in addition to electrical power being supplied to the dismountable belt assembly, there is also supplied, so to speak, air under reduced pressure, or a vacuum via hose 114.  
  As shown in FIG. 2, the belt assembly is in a form which provides three belt runs; that is, sections of the belt that lie in flat planes. In order to provide runs which are truly flat and which insure the positioning of a belt run in a precisely located plane. free of vibration. inadvertent deflection and which will assume the working position at all times, the belt assembly is provided with a flat holddown device for each of the runs. To this end. the exposure belt run, that is, the run between the rollers 40, 41 has associated therewith a holddown device 125, the development run between the rollers 41, 42 has the holddown device 126 and the cleaning run between the rollers 42, 40 has the holddown device 127. Since each of the devices are the same, except for size. only one. the development holddown device 126 will be described below.  
  As shown in FIG. 5. the vacuum holddown device 126 comprises a vacuum plenum 130 having a flat surface plate 131 formed therethrough with many openings 132 upon which the photoconductive belt 12 is stretched across during movement thereof. On the other side of the plate away from the belt 20, the plenum includes a plurality of ribs 133 made integral with plate and projecting from the surface thereof. Along with the ribs. there is also integrally formed on the same side of the plate 131, a circumferential guard wall 134 which is connected to the exterior ends of all the ribs 133 and also which surrounds all of the openings 132.  
  The spaces between the ribs 133 and the circumfer ential wall 134 are closed off on that side of the plate 131 by a manifold 135 made of flexible plastic material and having a relatively flat web portion 136 positioned to span across all of the ribs and the wall 134. The outer circumferential edge of the manifold 135 is formed with a wall 137 which extends toward the plate 131 and completely surrounds the wall 134. The wall 137 terminates in a flange 138 which is secured to the adjacent surface of the plate 131 by suitable lock washers 140. In order to insure a vacuum seal within the interior chamber defined by the flat portion 136 of the manifold 135 and the plate 131, a circumferential gasket 14] or sealing device is positioned between the flange 138 and the plate. The manifold 135 also includes a chamber 142 formed with material from the web portion 136 and which terminates in a hose connector 143 to which a hose of an air evacuation system may be se cured. The chamber walls 142 extend beyond the plane of the web portion 136 approximately centrally of the plate 131 in order to provide access to all of the spaces between the ribs 133. As shown in FIG. 6, the chamber is in communication with all of the spaces between all of the ribs 133. some of which terminate short of the central core section 144 of the vertical and horizontal ribs as viewed in this Figure.  
  In order to extend the vacuum effectiveness of the holddown device 126, the plate 131 on the side sup porting the belt 12 is formed with grooves 146 of semicircular cross-section connected to some of the outermost series of openings 132 and terminating adjacent the outer opposed edges of the plate 131. Since air is evacuated out ofthe space defined by the circumferen tial wall 134 within which the outermost series of open-- ings 132 are confined, the effect of the vacuum produced by this evacuation of air will be felt at the extreme outer limits of the grooves 146 and along the same. With the belt applied to the outer surface of the plenum plate 131, as shown in FIG. 5, and with the plenum subjected to a vacuum producing system, the belt will be forced against the plate by the resultant pressure differential effected between the spaces on ei ther side ofthe plate 131. The effect ofthis force on the belt will extend between the outer ends ofthe series of grooves 146 on one edge of the plate 131 to the outer ends of the series of grooves at the other edge of this plate.  
  This force on the belt will be continuous as the same moves on the belt assembly 14 and will produce flattened. unwavering portion of the belt for whate\ er processing is to occur opposite the holddown device. As will appear. holddown device 126, which is opposite the developer apparatus 27 and in the development zone 26 will be maintained in a predetermined fixed spaced relationship to the developer apparatus and magnetic brushes 29 thereof despite any unwarranted displacement in either the belt assembly 21 or devel oper housing 27.  
  The vacuum holddown device 126 also is provided with a flexible web material 147 stretched across the plenum plate 131 for various purposes. The web material 147 is electrically insulating since the surface of the belt 12 which slides across the material and which is under considerable force thereon, comprises the con ductive layer of the photoconductive belt. The material 147 then electrically insulates the belt 20 relative to the holddown device. In addition. the material. preferably made of soft fabric or paper. protects the conductive layer of the belt against abrasions and wear. as well as to reduce the friction between the moving belt and the plate 131 and to prevent the plugging or restricting of the openings 132 by dirt, etc.  
  The web 147 is maintained on the outer surface of the plenum plate 131 by means of a pair of parallel arranged rods 150, 151 secured to each end of the mate rial and held in fixed positions under the outer edges of the plenum plate. As shown in H6. 6, the web 142 turns around both outer edges 152, 153 of the plate 131 with one end of the material wrapped around the rod 151 for a few turns and the other end of the material wrapped around the rod 152 for a few turns.  
  The rod adjacent the plate edge 152 and below the same is journaled for rotation in a pair of aligned bosses 154 (one of which is shown in FIG. 5) located at opposite ends of the plenum 131. Since both of the rods 150, 151 and their associated structure are identical, only the rod 150 and attendant structure will be described in detail. The rod 150 extends through and is slidably retained by the boss 154 and has one of its outer ends secured to a knob 155 which is formed on one side with serrations 156. These serrations are engageable with corresponding serrations formed in the outer surface of the boss 154 and when engaged therewith. prevents the knob and the rod 150 from rotation. The knob is forced against the boss so that the cooperating serrations are interlocked by means of a coil spring 157 which encircles the rod 150 and held in light compression between the inside surface of the boss and a pin 158 secured to the rod. With this arrangement, in order to place the web material 147 under tension, which is desirable. an operator need only to pull the knob 155 against the bias of the spring 157 so as to disengage the coating serrations, and to turn the knob in a direction which will wind the material around the rod 150. Similarly, the rod 151 is also provided with a web tightening arrangement associated with the rod 151. To remove the web material, the rod 150 need only be rotated to unravel the material carried thereby. Suitable material gripping devices may be provided on each of the rods 150. 151 for securing ends of the material and permit winding upon each of the rods.  
  Vacuum holddown device 126 is resiliently supported on belt assembly frame 62 for limited movement in a direction generally perpendicular to belt 20. As will appear. the ability of holddown device 126 to move, albeit to a limited degree, provides automatic compensation should belt module 21 and/or developer apparatus 27 shift unexpectedly.  
  Referring to FlGS. and 8, base 131 of holddown device 126 is provided with a mounting opening 200 adjacent each corner, and outside the track of belt 20. A corresponding opening 201, which is internally threaded, is provided in the portion of the belt module frame member 62. Mounting bolts 204, the lower portion 205 of which is externally threaded for cooperation with threaded openings in frame member 62. serve to releasably retain the vacuum holddown device 126 in place on belt module 21. It is understood that the stop surface provided by the relatively larger diameter bolt heads 206 serve to limit movement of hold down device 126 outward, i.e., away from belt module 21, while contact of the lower surface 13] of base l3l with the adjoining surface of belt module frame member 62 serves to limit movement of holddown device 126 inwardly, i.e., toward belt module 21.  
  To maintain the vacuum holddown device in desired operative position and against the underside of belt 20, springs 208 are provided between base 131 of vacuum holddown device 126 and frame member 62 of belt module 2l.  
  Developing apparatus 27 includes a suitable exterior housing 220 within which the several magnetic brushes 29 are rotatably journaled. In addition, the lower portion 221 of housing 220 forms a sump for the developer mix. A developer inlet housing 222, supported by developer housing 220 adjacent the upper portion thereof, communicates with the developer housing interior to enable fresh developer material to be supplied. Details ofthe developing apparatus may be seen in US. Pat. No. 3,967,050, issued Oct. l2, l972, in the name of Michael A. Stanley.  
  Housing 220 of developing apparatus 21 is pivotally supported along the lower portion thereof by means of journal shaft 230. Shaft 230 is in turn rotatably sup ported on the base pan section 9&#39; of the machine frame by upstanding straps 23]. By this arrangement, the entire developing apparatus 21 may be swung downwardly about the axis of journal shaft 230 to separate the developer assembly including magnetic brushes 29 from belt 20. This facilitates servicing of belt module 21 as well as servicing and reloading of developing apparatus 27.  
  To locate the developing apparatus 27 in operative position opposite belt 20 and vacuum holddown device 126, a manually operated cam 235 is supported on the base pan 9&#39; of the machine frame, cam 235 riding against the exterior of flange 220 of housing 220 and cooperable therewith to force housing 220 into juxtaposition with belt 20 upon rotation thereof. On release of cam 235, the developing apparatus 27 separates from belt 20 and belt module 21 under the influence of gravity.  
  As will be understood by those skilled in the art, the spacing between magnetic brushes 29 and the surface of belt 20 is critical if uniform and optimum development of the images on belt 20 is to be effected. To effectuate this, the lower face 240 of developer housing 220 at the corners thereof is provided with projecting locating stops 24! adapted, on movement of the developing apparatus 27 into operative position opposite belt 20 and vacuum holddown device 126, to abut base 13] thereof. Stops 241 are dimensioned to provide the necessary and critical spacing between the developing apparatus magnetic brushes 29 and belt 20. Stops 241 are preferably adjustable and may for this purpose be arranged to threadingly engage developer housing 220 to permit displacement thereof relative to the devel oper housing.  
  As will be apparent from the foregoing description, the vacuum holddown device 126 is capable of limited displacement against the bias imposed by springs 208 in accommodation of any unwarranted movement or displacement of belt module 21, as may sometimes occur during machine installation and/or use. And since the developing apparatus 27 is itself located against the floating holddown device 126, any untowed displacement of the developing apparatus, or the structure supporting the developing apparatus, may be accommodated through displacement of the holddown device 126. At the same time, the critical spacing between the magnetic brushes 29 and belt 20 is maintained despite such displacement.  
  Referring now to FIGS. 1, 8, and 9, there is provided an arrangement to permit the reproduction machine service rep or operator to determine when developing apparatus 27 is in proper operative relation with belt 20 and holddown device 126. According to this embodi ment, locating stops 241 effectively serve as switch contacts such that on engagement of each locating stop 24] with base 131 of holddown device 126, a signal light 250 associated therewith is illuminated. Lights 250 may be conveniently supported within the reproduction machine housing on panel 251. As seen schematically in FIG. 9, a suitable source of electrical energy represented by battery 253 is provided. the individual lights 250 and their respective switches, namely, stops 241 and base 131, being connected thereacross in parallel circuits 254. In this case, individual leads are run to each locating stop 241 while base 131 of holddown device 126 is electrically connected via bolts 204, belt module frame 62, brush 100, and wire to the ground side of battery 253.  
  In this manner, proper positioning of the developing apparatus 27 is obtained when all of the lamps 250 light. Failure of one or more of the lamps 250 to light indicates that developer housing 220 is not located properly with respect to holddown device 126 creating the possibility of improper developing action by magnetic brushes 29.  
  While the invention has been described with reference to the structure disclosed, it is not confined to the details set forth, but is intended to cover such modifications or changes as may come within the scope of the following claims.  
 What is claimed is:  
  1. In a reproduction machine of the type incorporating a movable, flexible belt having a photoconductive surface on which a latent electrostatic image of the original being copied is formed and developed,  
 support means for said belt including an internal frame member, plural support rollers for said belt rotatably mounted on said frame member for supporting and guiding said belt in an endless path of movement, a platen for said belt disposed between &#39;a pair of said rollers, the surface of said platen facing said belt being substantially flat. and means to tension said belt whereby to draw said belt against said support platen flat surface to provide a substantially flat belt run whereat the image on said belt may be developed,  
 mounting means for supporting said platen on said frame member, said mounting means being adapted to permit displacement of said platen rela tive to said frame member in accommodation of unwarranted movement of said frame member.  
 developing apparatus opposite said platen for developing images on said belt flat surface, said developing apparatus being supported for movement relative to said platen;  
 means for locating said developing apparatus relative to said platen to provide predetermined spacing therebetween; displacement of said platen in accommodation of frame member movement being accompanied by conjoint movement of said developing apparatus to maintain said predetermined spacing therebetween the improvement comprising:  
 means responsive to establishment of said predetermined spacing between said platen and said devel&#39; oping apparatus to generate a signal.  
 2. The reproduction machine according to claim 1 in which said developing apparatus includes a developer housing, said locating means comprising plural locating abutments on said developer housing and engageable with said platen at discrete points to provide said predetermined spacing therebetween; and  
 means to establish an electrical circuit through each of said locating abutments adapted for completion upon contact of the abutment with said platen. each of said circuits including a signal element adapted on completion of the circuit to provide a signal indicating contact of the locating abutment associated therewith with said platen and establishment of said predetermined spacing.  
  3. The reproduction machine according to claim 2 in which said platen is of substantially rectangular configuration.  
 said locating abutments being provided adjacent each corner of said platen whereby to enable said signal elements to provide data reflecting uniform spacing between said platen and said developing apparatus.