Sheet transporting device

Abstract: A sheet transporting device includes a drive roller (10) including a high rigidity roller (11) having a surface on which ceramic powdery particles (13) are stuck, so that a surface of the drive roller (10) is irregular. The sheet transporting device further includes a transporting roller (20) including an elastic roller (22) having a surface on which a coating layer (23) made of low friction material is formed. A sheet of printing medium is nipped and fed by the drive roller (10) and the transporting roller (20). The ceramic powdery particles may be made of alumina or silicon carbide. An average diameter of the ceramic powdery particles is 20µm to 70µm. A distribution density of the ceramic powdery particles (13) on the surface (12) is 20% to 80%.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a sheet transporting device for transporting a sheet of printing medium, such as regular paper, coat paper, OHP (overhead projector) sheet, glossy paper, and glossy film. For example, the sheet transporting device is used for a printer. Such a device is known from JP-A-60 258 050. Background An example of the generally known sheet transporting device is a sheet transporting device by use of a pair of rubber rollers. Usually, one of the paired rollers is a drive roller and the other is a follower roller. In the case of the rubber roller, it is difficult to increase an accuracy on an outside diameter variation of the roller. Furthermore, the rubber roller is easy to be worn and a thermal expansion of the roller is large as disadvantages of the rubber roller. In addition, a coefficient of friction of the roller may be remarkably reduced by the paper powder, dust and chemicals of coat paper which attach to the surface of the roller. If the drive roller is formed with the rubber roller, it is very difficult to increase the sheet transporting accuracy. A sheet transporting device that solved the above problems is known as disclosed in Unexamined Japanese Patent Publication No. JP-A. 4-140247. In this sheet transporting device, a drive roller is a metal roller of which the surface is subjected to a blasting process and has a high outside diameter accuracy, and a follower roller is an elastic roller of which the surface is coated with silicon. According to such sheet transporting device, since the friction coefficient of the elastic roller is small, the roller pair exhibits a low resistance to the paper transportation, and further, the use of the rigid roller blasting processed ensures a stable transportation of the printing medium or a sheet of paper. However, the sheet transporting device disclosed in the publication may suffer from the following disadvantages. During the blasting process, the raised edges or peaks of the irregular surface of the drive roller are easy to be plastically deformed. When the printing medium is a film, a coefficient of friction of the paired rollers to the film is insufficient, so that the sheet transporting accuracy is decreased. Additionally, the paired rollers are easy to be worn and hence their endurance is low. SUMMARY OF THE INVENTION In order to solve the aforementioned problem, an object of the present invention is to provide a sheet transporting device which can transport a printing medium, even if it is a film, accurately and is excellent in its endurance. The sheet transporting device according to the invention comprises the features set out in Claim 1. According to the sheet transporting device, even if the printing medium transported by the paired rollers is a film, a sufficient friction coefficient of the roller pair to the film is secured because of the irregular surface of the roller formed by the ceramic powdery particles. Therefore, the sheet transporting device is able to highly accurately transport the printing medium, even if it is a film. Since the peaks of the irregular surface by the ceramic powdery particles is little plastically deformed, the roller pair is durable. In the sheet transporting device, the drive roller is formed in a manner that coating containing the ceramic powdery particles mixed thereinto is directly sprayed on the surface of the high rigidity roller and the sprayed coating is dried. According to the sheet transporting device, the drive roller can be easily manufactured. In the sheet transporting device, the ceramic powdery particles are made of alumina or silicon carbide. According to the sheet transporting device, the cost to manufacture the drive roller may be low. In the sheet transporting device, the average diameter of the ceramic powdery particles is 20µm to 70µm. According to the sheet transporting device, the following useful effect is obtained. If the diameter of the ceramic powdery particles is too large (their average diameter is 70µm or larger), the sheet is easy to be scratched. Contrary, if it is too small (their average diameter is 20µm or shorter), the irregular surface of the drive roller is easy to be clogged with paper particles attached to the drive roller. A sufficient friction coefficient cannot be obtained. Thus, according to the sheet transporting device of the invention, since the average diameter of the ceramic powdery particles is 20µm to 70µm as referred to the above, the sheet is little scratched and the necessary friction coefficient is secured. In the sheet transporting device, a distribution density of the ceramic powdery particles on the surface of the high rigidity roller is 20% to 80%. According to the sheet transporting device, the following useful effect is obtained. If the distribution density of the ceramic powdery particles is too large (80% or larger), the ceramic powdery particles are stratified (conglomerated) and the resultant friction coefficient has an insufficient value. Conversely, if it is too small (20% or smaller), the number of contacts of the raised edges or peaks of the irregular surface by the powdery particles with the sheet is reduced. The result is an instable transportation of the sheet. Thus, according to the sheet transporting device of the invention, since the distribution density of the ceramic powdery particles on the surface of the high rigidity roller is 20% to 80%, the ceramic powdery particles are not stratified, and the necessary friction coefficient is secured. Furthermore, a sufficient number of contacts of the peaks of the surface irregularity by the powdery particles with the sheet is secured. Accordingly, the transportation of the sheet is stable. In the sheet transporting device, the transporting roller is a follower roller, and includes a shaft arranged in parallel with the axial line of the drive roller and a pair of rollers, which are mounted on the shaft while being disposed symmetrically with respect to a central part of the shaft (when viewed in the axial direction of the axis) and located on both sides of the central part, and both ends of the shaft are movable only toward the drive roller, and only the central part of the shaft is urged toward the drive roller. Further, in the sheet transporting device, an obverse side of the printing medium is brought into contact with the transporting roller, and a reverse side of the printing medium is brought into contact with the drive roller, whereby the printing medium is nipped and transported. According to the sheet transporting device, the roller pair, or the transporting roller, are uniformly pressed against the drive roller uniformly or by a uniform load, whereby the sheet is transported straightforward. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a sectional view showing a model for explaining an embodiment of a sheet transporting device according to the present invention; Fig. 2 is an enlarged view showing a portion of the sheet transporting device of Fig. 1; Fig. 3 is a side view of an ink jet printer using the sheet transporting device constructed mentioned above; Fig. 4 is a side view showing a portion of the sheet transporting device of Fig. 3; Fig. 5 is a perspective view showing an example of a supporting structure for supporting the transporting roller; and Figs. 6(a) and 6(b) show explanatory diagrams useful in explaining the sheet transporting operations of the conventional sheet transporting device and the sheet transporting device of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 shows a sectional view showing a model for explaining an embodiment of a sheet transporting device according to the present invention. Fig. 2 is an enlarged view showing a portion of the sheet transporting device of Fig. 1. In these figures, reference numeral 10 designates a drive roller; 20 designates a transporting roller; and S designates a sheet of printing medium, e.g., paper. The transporting roller 20 is driven, by a drive means (not shown), to turn in the direction of an arrow, for example, shown in Fig. 1. The transporting roller 20 is urged toward the drive roller 10 in a manner that its shaft 21 is urged in the same direction by an urging means (not shown). By the urging force, the transporting roller 20 is turned in accordance with the turn of the drive roller 10 while being pressed against the drive roller. Therefore, when the sheet S is fed to between the drive roller 10 and the transporting roller 20, the sheet S is transported while being nipped between those rollers, as shown in Fig. 1. The drive roller 10 is formed such that ceramic powdery particles 13 are stuck onto the surface 12 of a high rigidity roller 11 (e.g., a metal roller). Therefore, the surface of the drive roller 10 is irregular. As shown in Fig. 2, the raised edges 14 of the powdery particles on the irregular surface of the drive roller 10 are relatively acute. For example, the ceramic powdery particles 13 may be powdery particles of alumina (AL 2 O 3 ) or silicon carbide (SiC). The average diameter of the ceramic powdery particles 13 is 20µm to 70µm. More preferably, the average diameter of the ceramic powdery particles 13 is 25µm to 70µm. A distribution density of the ceramic powdery particles 13 on the area of the surface 12 is 20% to 80%. The drive roller 10 in this embodiment is manufactured in a manner that coating 15 containing the ceramic powdery particles 13 mixed thereinto is directly sprayed on the surface 12 of the high rigidity roller 11, and then, the sprayed coating is dried, The coating may contain acrylic resin as a major component. The transporting roller 20 is formed such that a coating layer 23 made of low friction material is formed over the surface of an elastic roller 22 (e.g., a rubber roller). In this embodiment, the coating layer 23 is a fluorine coating. The hardness of the elastic roller 22 is 60° to 95° in rubber hardness. If the hardness of the elastic roller 22 is high (in excess of 95°), when the transporting roller 20 is pressed against the drive roller 10, the width (length extending in the sheet transporting direction) of a support part N (so called a nip) of the transporting roller 20 for the sheet S is insufficient, and the sheet transportation is instable. If the hardness of the roller is low (60° or less), the transporting roller 20 is excessively deformed, and a contact of the sheet S with the nip is instable. The coating layer 23 (fluorine coating layer) is 50µm to 20µm in thickness, in this embodiment. If the coating layer 23 is too thick (in excess of 20µm), a smooth elastic deformation of the elastic roller 22, per se, is hindered. Conversely, if it is too thin (shorter than 5µm), it is impossible to reduce a friction coefficient µ2 of the transporting roller 20 against the sheet S. If the friction coefficient µ2 is large, the following problems arise. For example, when the leading edge of the sheet S enters the nip N, the leading edge will be turned up. In the skewing operation of the sheet, the leading edge of the sheet S sometimes will be folded. More specifically, in the skewing operation, the leading edge of the sheet S passes through the nip N, and the drive roller 10 and the transporting roller 20 are reversely turned till the leading edge of the sheet S is backwardly moved and passes through the nip N, and then those rollers are turned forward. At this time, the leading edge of the sheet S will be turned up and will be folded. A proper value of the friction coefficient µ2 is 0.30 or less. To attain this value, the thickness of the coating layer 23 must be set at 5µm or thicker. In the case of the elastic roller 22 formed with a rubber roller, rubber of the rubber roller contains plasticizer. If this plasticizer is eluted on the surface of the transporting roller 20, the eluted plasticizer may attach to the surface of the sheet S (e.g., coat sheet). Accordingly, the diameter of the printed ink dot may be reduced. Further, a roller trace scar may appear on the sheet S due to the boundary between a portion having the eluted elasticizer and a portion not having the eluted elasticizer. Furthermore, if the plasticizer is attached to the coating 15 of the drive roller 10, the coating 15 may be solved due to a reaction with the acrylic resin of the coating 15. These problems, however, can be solved in a manner that the thickness of the coating layer 23 is selected to be 5µm or thicker to thereby prevent the plasticizer from being eluted. Fig. 3 is a side view showing an ink jet printer using the sheet transporting device constructed mentioned above. This ink jet printer includes the above-mentioned sheet transporting device 1, a sheet supplying device 30 supplying a sheet S to the sheet transporting device 1, a printing head 40 jetting ink droplets onto the surface of the sheet S being transported by the sheet transporting device 1 to thereby form an image (including characters), and an exit roller pair 50 discharging the printed shoot S. The printer further include. a main frame 60 to which those device and components are mounted, a first sub-frame 61, a second sub-frame 62, a pair of side frames (not shown), and the like. The sheet transporting device 1 is constructed such that its drive roller 10 is supported by a side frame (not shown) and driven by a proper drive means. The transporting roller 20 is supported by a support structure (described later) and is turnable in accordance with the turn of the drive roller 10. A sheet supplying device 30 includes a supplying roller 31, a hopper (not Shown) urging a sheet S toward the supplying roller 31, and a separation pad 32 which associates with the supplying roller 31 to nip the sheet S therebetween and separates the sheet S from a stack of sheets S in the hopper. A stack of sheets S is set in the hopper. In supplying sheets, the hopper presses stacked sheets against the supplying roller 31 turning by one turn, the separation pad 32 separates a sheet S from the stacked sheets, and the sheet S is fed to the sheet transporting device 1. The sheet S is then guided to the sheet transporting device 1 by a lower guide 70 mounted on the first sub-frame 61 and an upper guide 80 mounted on the main frame 60. The printing head 40 is mounted on a carriage 41. The carriage 41 is movable in the direction orthogonal to the paper surface of the drawing, with the aid of the top end 60a of the main frame 60 and a carriage guide 42. An ink tank 43 is carried on the carriage 41. In printing operation, the printing head 40 jets ink droplets onto the sheet S of printing medium to print one line while the carriage 41 is moved in the direction orthogonal to the drawing paper surface. Per one line being printed, the sheet S is fed at a preset pitch (usually a space between the adjacent lines) by the sheet transporting device 1. The sequence of operations is repeated to print on the full page of the sheet. Reference numeral 44 designates a guide/distance defining member for guiding the sheet while supporting the underside of the sheet S and defining a space between the sheet and the printing head 40. The exit roller pair 50 includes a drive roller 51 and a follower star wheel 52 being urged toward the drive roller 51, and the exit roller pair 50 discharges the sheet S to the exterior. The follower star wheel 52 is mounted on the second sub-frame 62. The transporting roller 20 in the sheet transporting device 1 will be described with reference to Figs. 3 to 5. As seen from those figures, the transporting roller 20 is rotatably mounted on the upper guide 80. The upper guide 80 is shaped like a plate-lime member as a whole. A base 81 of the upper guide is rotatably mounted on a support shaft 90. The support shaft 90, as shown in Fig. 4, is supported by hook portions 63 and 64, which are bent at the lower end of the main frame 60. Also, as shown in Fig. 4, the ends of the support shaft 90 are in contact with a rear side 65 (the right-side surface in Fig. 4) of the main frame 60. Therefore, the support shaft 90 is arranged in parallel with the axial line of the drive roller 10 of the sheet transporting device 1. As shown in Fig. 5, the transporting roller 20 includes a single shaft 21 and a couple of rollers 20' and 20', which are mounted on the shaft while being arranged symmetrically with respect to the central part 21a of the shaft 21. Furthermore, the rollers 20' and 20' are arranged on the shaft 21 to bypass the central part 21a. Elongated holes 82 and 82 elongated in the vertical direction (toward the drive roller 10), which support both ends 21b and 21b of the shaft 21, and a pushing part 83 which comes in contact with the central part 21a of the shaft 21 are formed at the distal end of the upper guide 80. Those elongated holes 82 and 82 are equally distanced from the base 81 or the support shaft 90. A torsion spring 100 is applied to the support shaft 90. One end 101 of the torsion spring 100, as shown in Fig. 3, is hooked at a hook portion 66 of the main frame 60. The other end of the spring 100 is brought into contact with a pushing part 83 of the upper guide 80 to urge it toward the drive roller 10. The ends 21b and 21b of the transporting roller 20 are supported so as to allow both the ends 21b and 21b of the transporting roller to be movable only toward the drive roller 10. Only the central part 21a of the shaft 21 is urged toward the drive roller 10. Therefore, the shaft 21 is swingably movable about its central part 21a (when viewed from the front) independently of the support shaft 90, and it is pressed against and along the drive roller 10. The elongated holes 82 and 82, which support respectively the ends 21b and 21b of the shaft 21, are located at equal distance apart from the support shaft 90. Therefore, the shaft 21 and the shaft 21 are parallel to each other. The support shaft 90 is pressed against the rear side 65 of the main frame 60 by means of the torsion spring 100. A parallelism between the support shaft 90 and the drive roller 10 is kept accurately, so that a parallelism between the shaft 21 of the transporting roller 20 and the axial line of the drive roller 10 is kept accurately. In particular, the shaft 21 of the transporting roller 20 is swingably movable about the central part 21a (when viewed from the front side) independently of the support shaft 90. Because of this, its parallelism when viewed from the front side is kept extremely accurately. The ends 21b and 21b of the transporting roller 20 are supported so as to allow both the ends 21b and 21b of the transporting roller to be movable only toward the drive roller 10. Only the central part 21a of the shaft 21 is urged toward the drive roller 10. Therefore, the shaft 21 is uniformly pressed against and along the drive roller 10. In the printer, the transporting rollers 20, or the plurality of the transporting rollers 20, thus supported are provided with respect to the drive roller 10 in its axial direction. The sheet transporting device thus constructed has the following useful effects. a) The drive roller 10 includes a high rigidity roller formed such that the surface 12 of the roller is coated with ceramic powdery particles 13 and hence irregular. The transporting roller 20, which associates with the drive roller 10 to nip and transport the sheet S, includes the elastic roller 22 formed such that the coating layer 23 of low friction material is layered over the surface of the transporting roller. Therefore, even if the sheet S to be transported is a film, a sufficient friction coefficient µ1 (Fig. 1) of the roller pair against the film is secured because of the presence of the irregularity of the ceramic powdery particles 13. The sheet transporting device of the invention can highly accurately transport a sheet S of printing medium, even if it is a film. Further, the irregularity of the ceramic powdery particles 13 is little plastically deformed. In this respect, the drive roller has a high durability. b) The drive roller 10 is manufactured in a manner that coating 15 containing the ceramic powdery particles 13 mixed thereinto is directly sprayed on the surface 12 of the high rigidity roller 11 and the sprayed coating is dried. This manufacturing method is simple. c) The ceramic powdery particles 13 may be powdery particles of alumina or silicon carbide. The cost to manufacture is low. d) The diameter of the ceramic powdery particles 13 is 20µm to 70µm. Therefore, the drive roller has the following useful effect. If the diameter of the ceramic powdery particles is too large (their average diameter is 70µm or larger), the sheet S is easy to be scratched. If it is too small (their average diameter is 20µm or shorter), the irregular surface of the drive roller 10 is easy to be clogged with paper particles attached to the drive roller. A sufficient friction coefficient µ1 cannot be obtained.On the other hand, in the sheet transporting device of the invention, the average diameter of the ceramic powdery particles 13 is 20µm to 70µm as referred to above. Therefore, the sheet S is little scratched and the necessary friction coefficient µ1 is secured. e) A distribution density of the ceramic powdery particles 13 on the surface 12 of the high rigidity roller is 20% to 80%. The following useful effect is obtained. If the distribution density of the ceramic powdery particles is too large (80% or larger), the ceramic powdery particles are stratified (conglomerated) and the resultant friction coefficient µ1 has an insufficient value. Conversely, if it is too small (20% or smaller), the number of contacts of the raised edges or peaks (indicated by numeral 14 in Fig. 2) of the irregular surface by the powdery particles with the sheet S is reduced. The result is an instable transportation of the sheet S. In the sheet transporting device of the invention, the distribution density of the ceramic powdery particles 13 on the surface 12 of the high rigidity roller is 20% to 80%. Therefore, the ceramic powdery particles are not stratified, and the necessary friction coefficient µ1 is secured. Further, a sufficient number of contacts of the peaks of the surface irregularity by the powdery particles with the sheet S is secured. Accordingly, the transportation of the sheet S is stable. f) The transporting roller 20 is a follower roller. Further, it includes the shaft 21 arranged in parallel with the axial line of the drive roller 10 and the roller pairs 20' and 20', which are mounted on the shaft 21 while being located on both sides of the central part 21a of the shaft. Both ends 21b and 21b of the shaft 21 are movable only toward the drive roller 10. Only the central part 21a of the shaft 21 is urged toward the drive roller 10. Therefore, the roller pair 20' and 20', or the transporting roller 20, are uniformly pushed against the drive roller 10 uniformly or by a uniform load, whereby the sheet S is transported straightforward. This will be described with reference to Figs. 6(a) and 6(b) in which models of the load exertion are illustrated. Fig. 6(a) is a plan view showing a nip N' of the paired rubber rollers in a general sheet transporting device. In the figure, an arrow a indicates a transportation direction of the sheet. In this sheet transporting device, the press contact of the paired rollers one with the other are not always uniform. If the contact is not uniform, a contact portion, or a nip N', of them is somewhat deformed from its rectangular shape. Under this condition, the vectors F1, F2 and F3 of the transporting force exerting on the sheet are not parallel to one another. However, a slipping phenomenon occurs between the sheet and the roller pair. Therefore, it never happens that the sheet is wrinkled or skewed. Where the friction coefficient is increased to be large between the drive roller 10 and the sheet, if the parallelism of the vectors F1 to F3 of the transporting force are lost in its degree, a slip is hard to occur between the drive roller 10 and the sheet S. The result is that the sheet is wrinkled and skewed. In this connection, it is noted that, the sheet transporting device of the invention, the transporting roller 20 is parallel to the drive roller 10, and the transporting roller is pressed against the drive roller by a uniform load. Under this condition, the nip N is formed to be rectangular in shape and the parallelism of the vectors F1 to F3 of the transporting force is kept at a high degree. The sheet S is not wrinkled and fed straightforward. While the present invention has bee described in a specific form, it should be understood that the invention is not limited to the above-mentioned embodiment, but may variously be modified, altered and changed. The transporting roller 20 may be the drive roller, although it is the follower roller in the above-mentioned embodiment. Besides, it is evident that the sheet transporting device of the invention is applicable to any of other suitable machines than the printer (for example, copying machines and facsimiles).

Claims: A sheet transporting apparatus device, comprising a rigid drive roller (10) and a transporting follower roller (20) associating with the drive roller (10) to feed a sheet (S) of printing medium while the sheet (S) is held between the transporting roller (20) and the drive roller (10), said transporting roller (20) being a follower roller including an elastic roller portion (22) having a surface on which a coating layer made of low friction material (23) is formed, characterized in that: the drive roller (10) includes a high rigidity roller portion (11) having a surface (12) on which ceramic powdery particles (13) are stuck, so that the outer surface of the drive roller (10) is irregular, the transporting roller (20) includes a shaft (21) arranged in parallel with the axial line of the drive roller (10) and a pair of rollers (20'), which are mounted on the shaft (21) while being disposed symmetrically with respect to a central part (21a) of the shaft (21), said rollers (20') being arranged on the shaft (21) to bypass said central part (21a), both ends (21b) of said shaft (21) being movably supported only toward the drive roller (10) and the central part (21a) of the shaft (21) being urged toward the drive roller (10), whereby said shaft (21) is swingably movable about its central part (21a). The sheet transporting device of claim 1, wherein the drive roller (10) is formed in a manner that coating containing the ceramic powdery particles mixed thereinto is directly sprayed on the surface of the high rigidity roller portion (II) and the sprayed coating is dried. The sheet transporting device of claim 1, wherein the ceramic powdery particles are made of alumina or silicon carbide. The sheet transporting device of claim 1, wherein an average diameter of the ceramic powdery particles is 20µm to 70µm. The sheet transporting device of claim 1, wherein a distribution density of the ceramic powdery particles on the surface of the high rigidity roller portion (11) is 20% to 80%. The sheet transporting device of claim 1, wherein an observe side of the printing medium is brought into contact with the transporting roller (20), and the reverse side of the printing medium is brought into contact with the drive roller (10), so that the printing medium is nipped and transported.

Inventors: OHSHIMA KEIICHI, OHSHIMA, KEIICHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1997
Main classification: B65H5
IPC codes: B65H27, B65H5