US 3901186 A
A transfer roller assembly having a transfer roller, is mounted adjacent a curved photoconductive surface of an electrostatographic machine so that the transfer roller is biased into contact with the surface with a minimal force sufficient only to maintain uniform contact therewith during machine operation; the roller is mounted so that it may move in a direction which is at least generally perpendicular to a tangent at the point of contact. The ratio of the radius of the photoconductive surface to that of the roller at the point of contact is not less than 3, and preferably in the range of 3 to 3.5.
Description (OCR text may contain errors)
United States Patent [191 Hoffman et al.
[ Aug. 26, 1975 TRANSFER ROLLER ASSEMBLY  Assignee: Xerox Corporation, Stamford,
 Filed: July 2, 1973 [211 App]. No.1 376,086
3,751,156 8/1973 Szostak et a1 355/3 Primary Examiner-Mervin Stein Assistant ExaminerDouglas Salser 57 ABSTRACT A transfer roller assembly having a transfer roller, is mounted adjacent a curved photoconductive surface of an electrostatographic machine so that the transfer roller is biased into contact with the surface with a minimal force sufficient only to maintain uniform contact therewith during machine operation; the roller is mounted so that it may move in a direction which is at least generally perpendicular to a tangent at the point of contact. The ratio of the radius of the photoconductive surface to that of the roller at the point of contact is not less than 3, and preferably in the range of 3 to 3.5.
2 'Claims, 4 Drawing Figures PATENTED AUG 2 6 I975 SIIZU 3 n5 3 BACKGROUND OF THE INVENTION This invention relates generally to electrostatography, but more particularly to a novel transfer roller assembly for an electrostatographic machine.
In the practice of xerography as described in US. Pat. No. 2,297,691 to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.
The latent electrostatic image may then be developed by contacting it with a finely divided electrostatically attractable material, such as a resinous powder. The powder is held in the image areas by the electrostatic fields on the layer. Where the field is greatest, the greatest amount of material is deposited; and where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other transfer surface, and suitably affixed thereto to thereby form a permanent print.
The toner may be fixed to the paper by passing the latter between a heated roller and a second roller in pressure contact therewith, whereby the toner becomes fused to the sheet of paper.
The image is generally transferred to the image receiving member (e.g. paper) by passing such member between the photoconductive surface including the developed image and a transfer roller in contact therewith.
Roller electrode transfer systems employ DC electric fields to move charged particles such as xerographic toner from first to second supporting surfaces, e.g., from a photoconductive surface to a sheet of paper. The purpose of this is to exert an electrical force on the charged particles that moves them from the first to the second surface. Examples of bias roller transfer systems are described in US. Pat. Nos. 2,807,233; 2,068,555; 3,043,684; 3,267,840; 3,598,580; 3,625,146; 3,630,591; 3,691,993; 3,702,482; and 3,684,364, French Pat. No. 2,065,390, German Application OLS 2,102,634 and British Pat. Nos. 2,310,666 and 1,302,922.
Among the problems that arise in some bias transfer systems is one of the defects in copy quality, e.g., hollow characters which may result from a sheet of paper being subjected to an excessive pressure as it passes between the transfer roller and the photoconductive surface during the transfer step or process.
Another copy quality problem which sometimes arises in bias transfer systems relates to undesired nonuniform densities in the transferred image. This results from variations in pressure caused by excessive shift of the transfer nip during the transfer step.
Dimensional variations in the machine and/or the paper affect the dynamic stability of the machine during operation, which in turn affects the operation of the transfer system, thus often resulting in one or more of j the above problems.
Also, there is occasionally a problem with the paper wrapping itself around the transfer roller during the transfer step or operation.
BRIEF SUMMARY OF THE INVENTION It is also an object of the present invention to provide I a transfer roller assembly which minimizes or eliminates wrapping of a support member (e.g., a sheet of paper) around the transfer roller during operation.
These and other objects are obtained by providing a transfer roller assembly which is supported within an electrostatographic machine adjacent a photoconductive surface. The transfer roller assembly is comprised of two rigid frame assemblies rotatably supporting spring loaded pivoting arms in which the transfer roller is journalled for rotation; this structure provides the pressure contact between the transfer roller and the photoconductive surface. The force biasing the transfer roller toward the photoconductive surface is that minimal force sufficient only to maintain uniform contact during operation of the machine. The transfer roller is mounted so that any shift of the transfer roll during operation of the machine is in a direction which is at least generally perpendicular to a tangent at the point of contact. To eliminate or minimize the problem of paper wrapping itself around the transfer roller, the ratio of the radius of the curved photoconductive surface to the radius of the transfer roller at the point of contact is not less than 3, and preferably in the range of 3 to 3.5. Electrically, for example, the transfer roller assembly of the present invention may operate as disclosed in US. patent application Ser. No. 309,562 filed on Nov. 24, 1972, or in other suitable conventional manners.
BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention as well as other objects and further features thereof will become apparent upon consideration of the following detailed disclosure thereof, especially when taken with the accompanying drawings wherein like numerals designate like parts throughout.
FIG. 1 is a schematic sectional view of an electrostatic reproduction machine embodying the principles of the present invention.
FIG. 2 is an elevational view of ,a transfer roller assembly embodying the principles of the present inventlon.
FIG. 3 is a sectional view of the transfer roller assembly of FIG. 2 taken along the line 3-3 of FIG. 2.
FIG. 4 is a side view of the transfer roller assembly taken along the line 4-4 thereof of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION For a general understanding of the illustrated copier/reproduction machine in which the present invention may be incorporated, reference is had to FIG.
1 in which the various system componentsfor the machine are schematically illustrated.
A document D to be copied is placed upon a transparent support platen P fixedly arranged in an illumination assembly, indicated generally by the reference numeral l and positioned at the left end of the machine. Light rays from an illumination system are flashed upon the document to produce image rays corresponding to the informational areas. The image rays are projected by means of an optical system onto the photosensitive surface of a xerographic plate in the form of a flexible photoconductive belt 12 arranged on a belt assembly, generally indicated by the reference numeral 14.
The belt 12 comprises a photoconductive layer of selenium which is the light receiving surface and imaging medium for the apparatus, on a conductive backing. The surface of the photoconductive belt is made photosensitive by a previous step of uniformly charging the same by means of a corona generating device 13.
The belt isjournalled for continuous movement upon three rollers 20, 21, and 22 positioned with their axes in parallel relationship. The photoconductive belt assembly 14 is slidably mounted upon two support shafts 23 and 24 secured to the frame of the apparatus with the roller 22 rotatably supported on the shaft 23, and is rotatably driven by a suitable motor and drive assembly (not shown) in the direction of the arrow at a constant rate. During exposure of the belt 12, the portion exposed is that portion of the belt running between rollers and 21. During such movement of the belt 12, the reflected light images of such original document positioned on the platen is flashed on the surface of the belt to produce an electrostatic latent image thereon at exposure station A.
As the belt surface continues its movement, the electrostatic image passes through a developing station B in which there is positioned a developer assembly, generally indicated by the reference numeral 15, and which provides development of the electrostatic image by means of multiple brushes 16 as the same moves through the development zone.
The developed electrostatic image is transported by the belt to a transfer station C wherein the transfer roller assembly 26 of the present invention is positioned and where a sheet of copy paper is moved between the nip formed by the assembly 26 and the belt 12. The paper is moved at a speed in synchronism with the moving belt, and the transfer of the developed image is accomplished solely by an electrical bias on the transfer roller. There is also provided at station C a sheet transport mechanism generally indicated at 28 adapted to transport sheets of paper from a paper handling mechanism generally indicated by the reference numeral 30 to the developed image on the belt at the station C.
After the sheet is stripped from the belt 12, it is conveyed into a fuser assembly, generally indicated by the reference numeral 31 wherein the developed and transferred xerographic powder image on the sheet material is permanently affixed thereto. After fusing, the finished copy is discharged from the apparatus at a suitable point for collection externally of the apparatus.
Further details regarding the structure of the belt assembly l4 and its relationship with the machine and support therefor may be found in the copending application Ser. No. 102,312 assigned to the same assignee.
Referring now to FIGs. 2, 3, and 4, the transfer roller assembly 26 is illustrated, the assembly being comprised of a transfer roller 32, pivoting arms 34a and 34b, and rigid frame assemblies 36a and 36b. The transfer roller 32 is mounted for rotation about shafts 38 disposed in arms 34a and 3411, the latter being joumalled for rotation about shafts 40 positioned within the assemblies 36a and 36b, respectively, such that any perturbations of the transfer roller 32 against the photoconductive belt 12 due to dimensional variations will not be deleterious to the image transfer process. A support shaft 42 is mounted for rotation in the assemblies 36a and 36b above and in parallel alignment with the transfer roller 32, and a gear 44 is affixed to an end of the shaft 42 for driving a cleaning brush member 48 through a suitable drive means in contact with the transfer roller 32. Bearings 50 are disposed on the shaft 42 outwardly of the assemblies 36a and 36b, and are positioned in support assemblies of the machine indicated generally as 52 and 54.
Shafts 38 are journalled for rotation in the pivoting arms 34a and 34b as are the shafts 40, thereby permitting rotation of the transfer roller 32 with respect to the arms 34a and 34b, and also permitting the transfer roller 32, and assemblies 34a and 34b to rotate as a unit about the shafts 40.
A shaft disposed above and in parallel relationship to the shafts 40, is joumalled for rotation in the assemblies 36a and 36b and has the portions thereof extending beyond these assemblies provided with earns 62 affixed thereto. A threaded rod member 64 is rotatably mounted by a pin 66 to each of the assemblies 36a and 36b. The end of the threaded rod member 64 opposite the pin mount is provided with a centrally formed orifice (not shown) into which a rod member 'is positioned in sliding relationship. A spring 68 is positioned about the rod member 65 between each of the arms 34a and 34b and a nut 70 threaded on the rod 64. A lever 72 is connected to the shaft 60 to rotate the latter and is held in operative relationship by a locking arm 74 extending from the assembly 36b.
When it is necessary to move the transfer roller 32 into a non-operational mode, i. e., where the roller is not in contact with the photoconductive belt 12, the lever 72 is moved clockwise as viewed in FIG. 3 (counterclockwise as viewed in FIG. 4). This would be necessary, for example, when it is desired to change the photoconductive belt 12. When the lever is so moved, the cams 62 on each side of the assembly 26 are moved against the arms 34a and 34b. For example, as can be seen in FIG. 4, cam 62 would rotate counterclockwise against the surface 74 of arm 34b. A corresponding action would take place with respect to arm 34a. Cams 62 would thus rotate arms 34a and 34b about the shafts 40. It should be noted at this point, that because of the location of shafts 40, the transfer roller 32 moves in a direction which is at least generally perpendicular to a tangent at the point of contactbetween the transfer roller and the photoconductive belt 12. Thus, even during the transfer process, any movement of the transfer roller 32 would be in this direction. Heretofore, transfer rollers have been mounted so that any movement of the roller during the transfer process was in a direction generally parallel to a tangent at the point of contact, which resulted in poor copy quality as discussed above. It should also be noted at this point, that the springs 68 bias the transfer roller 32 toward the photoconductive belt 12 with a minimal force which is sufficient only to maintain uniform contact with the belt and the paper passing between the belt and transfer roller during machine operation.
It will be understood that the transfer roller 32 will contact the photoconductive surface in a manner to provide even contact therebetween similar to the shock absorbing action of the front end of a vehicle and that the compressional forces of the springs 68 are adjustable by rotation of the respective nuts 78.
Another aspect of the present invention relates to the ratio of A, the radius of photoconductive belt 12 at the point of contact with the transfer roller 32 to B, the radius of the transfer roller at that point. This ratio should not be less than 3, and preferably is in the range of 3 to 3.5.
Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure which modifications are intended to be included within the scope of this invention.
What is claimed is:
1. Apparatus comprising:
a. means defining a surface adapted to support a developed electrostatic image, the surface being mounted for movement around a closed path;
b. a transfer roller assembly having a transfer roller in contact with the surface, wherein the ratio of (i) the distance from the surface at the point of contact to the center of curvature of the surface at that point to (ii) the radial dimension of the roller, is not less than 3;
0. means for mounting the transfer roller so that it may move in a direction which is at least generally perpendicular to a tangent at the point of contact between the surface and the roller;
d. means for feeding a support sheet between the surface and the roller; and
e. means for biasing the roller against the support sheet with a minimal force which is sufficient only to keep the roller in contact with the support sheet and the support sheet in contact with the surface during movement of the support sheet between the surface and the rollers during operation of the apparatus so that transfer can be effected solely by an electrical bias applied to the transfer roller.
2. Apparatus as set forth in claim 1 wherein the ratio of (i) to (ii) is in the range 0f3 to 3.5.