Sheet conveying apparatus

Abstract: Hitherto, paper has been conveyed in apparatus such as printers by means of rollers or rollers and a push type pin tractor. A primary roller (11,21) has a slightly higher peripheral speed so as to impart tension on the paper (13). When reversing the direction of the conveyance, however, this tension can lead to paper jamming and a deterioration of printing accuracy. The present invention employs rollers with resilient projections which, depending on the direction of rotation, alter the effective radius and hence the effective peripheral speed. This enables the primary roller (11,21) to maintain a slightly higher peripheral speed irrespective of the direction of conveyance.

Description: The present invention relates to a sheet conveying apparatus and in particular to, but not exclusively to, a printer and a printer including such sheet conveying apparatus. Fig. 1 shows an example of a conventional printer in which a piece of paper 13 is conveyed by means of a pair of sheet conveying rollers 14, a platen 12, and another pair of sheet discharging rollers 11. In the printer, the sheet discharging rollers 11 are rotated at a peripheral speed higher than the sheet conveying rollers 14 so that tension is given to the printing sheet. That is to say, during conveyance, the printing sheet is stretched tight. This obviates the problems encountered if the printing sheet is slackened which can result in the sheet being caught in the printer or the printing accuracy being lowered. Fig. 2 shows another example of the conventional printer in which, in order to assist the conveyance of a printing sheet, a pair of sheet conveying rollers 25 are provided between a push type pin tractor 24 (hereinafter referred to merely as "a tractor 24", when applicable) and a platen 22. The sheet conveying rollers 25 are rotated at a peripheral speed which is equivalent to that provided by the tractor 24. Again the pair of sheet discharging rollers 21 are rotated at a peripheral speed higher than the sheet conveying rollers 25 so that the printing sheet is stretched tight between the sheet discharging rollers 21 and the sheet conveying rollers 25 but also between the tractor 24 and the sheet conveying rollers 25. The above-described conventional conveyors both suffer from the following difficulties when the printing sheet is conveyed in the reverse direction: When the paper 13 being conveyed by means of the sheet discharging rollers 11, the platen 12 and the sheet conveying rollers 14 (Fig. 1) in the reverse direction, the tension on the paper slackens between the sheet discharging rollers 11 and the sheet conveying rollers 14 because the peripheral speed of the former 11 is higher than that of the latter 14, and therefore the back-feed or reverse printing accuracy is lowered by as much. In the case where the tractor 24, the platen 22, the sheet conveying rollers 25 and the sheet discharging rollers 21 are used to convey the printing sheet 23 in the printer (Fig. 2), similarly as in the above-described case, the tension of the printing sheet slackens between the sheet discharging rollers 21 and the sheet conveying rollers 25, and it may be caught in the printer. Furthermore, since the peripheral speed of the sheet conveying rollers 21 is higher than that of the sheet conveying rollers 25, the slacking of the paper can cause the perforations in the paper to be detached from the tractor pins leading to paper jamming and the reverse printing capacity being lowered. These problems are also encountered when such sheet conveying apparatus are applied in other situations where both forward and reverse directions are desirable without losing tension on the sheet. US-A-4,838,719 discloses a thermal color printer including a reversible platen roller 9 and associated pinch rollers 11, 10 on the paper feed and discharge sides. The discharge side pinch roller 10 is coupled to the platen roller by "unequal" spur gear 31, 32 and one-way clutches 33 such that it is frictionally driven by the platen roller in the sheet advance direction and gear driven by and at a slower speed than the platen roller in the reverse or sheet return direction. Accordingly, an object of the present invention is to eliminate the above described difficulties accompanying a conventional sheet conveying apparatus particularly in a printer. More specifically, an object of the invention is to provide a sheet conveying apparatus which is stable in operation and high in accuracy for a printer in which tension is given to a printing sheet not only when it is conveyed in the forward direction but also when it is conveyed in the reverse direction. According to a first aspect of the present invention there is provided a sheet conveying apparatus comprising at least one primary roller arranged downstream in a sheet conveying direction and at least one secondary roller arranged upstream in the sheet conveying direction, characterised in that both the primary and the secondary roller have resilient burrs on their surface and arranged such that the apexes of the burrs on the primary roller point in the same direction as the sheet conveying direction and the apexes of the burrs on the secondary roller point in the opposite direction to the sheet conveying direction. In the sheet conveying apparatus of the present invention, not only when the printing sheet is conveyed in the forward direction (sheet forward-conveying direction) but also when it is conveyed in the reverse direction (sheet reverse conveying direction), the primary rollers have a higher speed than the secondary rollers. This is achieved by at least one of the primary rollers being ground in the same direction as the sheet forward conveying direction. Similarly, at least one of the secondary rollers, have been ground in the opposite direction to the sheet forwarding conveying direction. Thus, when the printing sheet is conveyed the diameter of the primary roller substantially increases due to the burrs on it's surface being raised. Conversely, the burrs on the secondary roller are flattened thereby substantially reducing its diameter. The total difference in diameters of the primary and secondary rollers results in the peripheral speed of the primary roller increasing. Tension can thus be applied to the printing sheet at all times, whether it is being conveyed in the forward or reverse direction. Thus, with the sheet conveying apparatus, the sheet can be conveyed with the high stability and with high accuracy. In a second aspect of the present invention there is provided a sheet conveying system in a printer in which at least one sheet conveying roller is arranged upstream in a sheet conveying direction (A), and at least one sheet discharging roller is arranged downstream in the sheet conveying direction (A), and a printing section is arranged between said sheet conveying roller and said sheet discharging roller, characterised in that the surface portions of said sheet conveying roller and sheet discharging roller are made of a rubber material, the surface portion of said sheet conveying roller has been treated with a grinding stone to form projections, the apexes of the projections on the sheet conveying discharge roller pointing in the direction which is opposite to said sheet conveying direction, while the surface portion of said sheet discharging roller has been treated with a grinding stone to form projections, the apexes of the projection on the sheet conveying discharge roller pointing in the direction which is the same as said sheet conveying direction. Preferred embodiments of the present invention will be described with reference to a sheet conveying path in a friction mode, and a sheet conveying path in a tractor mode in an impact dot printer or the like and with reference to the accompanying drawings, of which: Fig. 1 is an explanatory diagram showing a sheet conveying path in a friction mode in which a cut printing sheet (cut paper) is conveyed; Fig. 2 is an explanatory diagram showing a sheet conveying path in a tractor mode using a tractor for conveying a continuous printing sheet (continuous form); Fig. 3(a) and (b) is a plan view and a sectional view respectively illustrating the direction of grinding a rubber roller; Fig. 4 is a sectional view showing the burrs on the roller when the roller acts as a primary roller so that the direction of grinding the roller is the same as the sheet conveying direction; and Fig. 5 is a sectional view showing the burrs on the roller when the roller acts as a secondary roller so that the direction of grinding the roller is opposite to the sheet conveying direction. Fig. 1 shows a first embodiment of the present invention. More specifically, Fig. 1 is an explanatory diagram showing a sheet conveying path in a friction mode in which a pair of sheet conveying rollers 14 and a pair of sheet discharging rollers 11 are arranged before and after a platen 12, respectively, and a cut printing sheet 13 is conveyed by means of the platen 12, the sheet conveying rollers 14 and the sheet discharging rollers 11. In Fig. 1, the printing sheet 13 is conveyed in the direction of the arrow A, i.e., in the forward direction (hereinafter referred to as "a sheet forward-conveying direction", when applicable). The sheet discharging or primary rollers 11 are rotated at a peripheral speed higher than the sheet conveying or secondary rollers 14. The peripheral speed of the sheet conveying rollers 14 is substantially equal to a predetermined sheet conveying speed. Hence, the sheet conveying rollers 14 give tension to the printing sheet 13 while slipping on the former 11. At least one of the sheet conveying rollers 14 and at least one of the sheet discharging rollers 11 are made of a rubber material such as a mixture of natural rubber and synthetic resin, and are manufactured as follows: First, the rubber material is poured into a predetermined mold, to form a hollow cylinder. The hollow cylinders thus formed are fixedly mounted on a rotary shaft, which is set on a grinding machine. Under this condition, the surfaces of the cylinders are ground or polished with a rotating cylindrical grinding stone, so that the burrs are formed on the surfaces as shown in Fig. 3. Although a rubber material is preferred, the present invention may employ other types of material so long as resilient burrs can be formed on the periphery of the rollers. Other materials include plastics, malleable but hardened resins and the like. When the rubber roller grinding direction is the same as the sheet forward-conveying direction as shown in Fig. 4, i.e., on primary rollers, the burrs are raised by the force of rotation of the roller, and therefore the effective radius Ra is larger than the real radius Rb. Hence, when the roller is rotated, the effective peripheral speed is slightly higher than the real peripheral speed. On the other hand, when the rubber roller grinding direction is opposite to the sheet forward conveying direction as shown in Fig. 5, i.e., on secondary rollers, the burrs are pushed radially inwardly of the roller, so that the effective radius Ra is smaller than the real radius Rb. Hence, when the roller is rotated, the effective peripheral speed is slightly lower than the real peripheral speed. If the diameter of the rubber roller is 16.17mm, then the chosen length of the burrs (dimension A as shown in Fig. 3a) is in the order of 0.5mm. The distance between adjacent burrs is the range 0.8mm to 1.5mm (dimension B as shown in Fig. 3b). When the rubber roller grinding direction is opposite to the sheet forward-conveying direction i.e., on secondary rollers, the effective radius is smaller by approximately 0.3% than the real radius due to the burrs being pushed inwardly as discussed above. Conversely, when the grinding direction is the same as the sheet forward-conveying direction i.e., on primary rollers, the effective radius is large by approximately 0.3% than the real radius because the burrs are raised. Hence, when the printing sheet is conveyed in the forward direction with at least one of the sheet conveying or secondary rollers 14 ground in the direction which is opposite to the sheet forward-conveying direction and with at least one of the sheet discharging or primary rollers 11 ground in the direction which is the sheet forward-conveying direction, the effective peripheral speed of the sheet conveying or secondary rollers 14 is slightly lower than the effective peripheral speed of the sheet discharging or primary rollers 11. Hence, greater tension acts on the printing sheet, thus preventing the printing sheet from slackening. This will improve the printing accuracy, decrease the printing errors, and prevent the printing sheet from being caught in the printer. When, on the other hand, the printing sheet is conveyed in the reverse direction (hereinafter referred to as "a sheet reverse-conveying direction", when applicable,) the effective peripheral speed of the sheet conveying rollers 14, now the primary rollers, is higher, while the effective peripheral speed of the sheet discharging rollers 11, now the secondary rollers, is lower. Hence the tension on the printing sheet does not slacken, which is effective in improving the back feed or reverse printing accuracy. Fig. 2 shows a second embodiment of the present invention. More specifically, Fig. 2 is an explanatory diagram showing a sheet conveying path in a tractor mode in which a pair of sheet conveying rollers 25 are provided between a tractor 24 and a platen 22, and a pair of sheet discharging rollers 21 are arranged in a sheet discharging section, and a continuous printing sheet (continuous form) 23 is conveyed by means of the tractor 24, the platen 22, the sheet conveying rollers 25 and the sheet discharging rollers 21. In Fig. 2, the printing sheet 23 is conveyed in the directions of the arrows A or B. The sheet conveying rollers 25 are arranged between the tractor 24 and the platen 22, and are rotated at a peripheral speed which is equivalent to that provided by the tractor 24. The sheet discharging rollers 21 are positioned on the sheet discharging side of the platen 22, and usually are rotated at a peripheral speed higher than that of the sheet conveying rollers 25. Similarly as in the above-described first embodiment, at least one of the sheet conveying rollers 25 are ground or polished in the opposite direction to the sheet forward-conveying direction, whereas at least one of the sheet discharging rollers 21 are ground or polished in the sheet forward conveying direction. Hence, similarly as in the first embodiment, when the printing sheet is conveyed in the forward direction, the sheet discharge rollers 21 act as primary rollers having a higher peripheral speed and so greater tension acts on the printing sheet, thus preventing the latter from slackening. This will effectively improve the printing accuracy, decrease the printing errors, and prevent the printing sheet from becoming slack. In the case when the printing sheet is conveyed in the reverse direction the sheet conveying rollers 25 now act as primary rollers and so the printing sheet is also prevented from slackening leading to improvements in the back-feed or reverse capacity. It will be appreciated that the aforegoing embodiments of the present invention have been described with reference to an impact dot printer, but it should be noted that the invention is not limited thereto or thereby. That is, the technical concept of the invention can be widely applied to printing machines such as page printers, thermal printers and ink jet printers in which sheet conveying rollers or tractors are arranged upstream and downstream of the printing mechanism, and a printing operation is carried out by conveying the printing sheet both in the forward direction and in the reverse, direction. Similarly, the conveying apparatus is also applicable to other sheet conveying applications. According to the present invention, the sheet discharging rollers of rubber are ground in the direction which is the same as the sheet forward-conveying direction, while the sheet conveying rollers of rubber are ground in the opposite direction. Hence, when the printing sheet is conveyed in the forward direction, it is prevented from becoming slack. This will effectively improve the printing accuracy, decrease the printing noise, and prevent the printing sheet from being caught in the printer. When the printing sheet is conveyed in the reverse direction, it is also prevented from slackening, which is effective in improving the back-feed capacity and the back-feed printing accuracy.

Claims: A sheet conveying apparatus comprising at least one primary roller (11, 21) arranged downstream in a sheet conveying direction (A) and at least one secondary roller (14, 25) arranged upstream in the sheet conveying direction (A), characterised in that both the primary (11, 21) and the secondary (14, 25) roller have resilient burrs on their surface and arranged such that the apexes of the burrs on the primary (11, 21) roller point in the same direction as the sheet conveying direction (A) and the apexes of the burrs on the secondary(14, 24) roller point in the opposite direction to the sheet conveying direction (A). A sheet conveying apparatus as claimed in claim 1, in which the surface of both the primary (11, 21) and secondary (14, 25) rollers are fabricated from a rubber material. A sheet conveying apparatus as claimed in claim 2, in which said burrs are formed by a grinding stone. A sheet conveying apparatus as claimed in any one of claims 1 to 3, further comprising a tractor means arranged in the path of the sheet conveying direction (A). A printer comprising a sheet conveying apparatus as claimed in any one of the preceding claims, further comprising a print means (1, 12; 22) disposed between the primary and the secondary rollers. A sheet conveying system in a printer in which at least one sheet conveying roller (14, 25) is arranged upstream in a sheet conveying direction (A) and at least one sheet discharging roller (11, 21) is arranged downstream in the sheet conveying direction (A), and a printing section (1, 12, 22) is arranged between said sheet conveying roller (14, 25) and said sheet discharging roller (11, 21), characterised in that the surface portions of said sheet conveying roller and sheet discharging roller are made of a rubber material, the surface portion of said sheet conveying roller has been treated with a grinding stone to form projections, the apexes of the projections on the sheet conveying roller pointing in the direction which is opposite to said sheet conveying direction, while the surface portion of said sheet discharging roller has been treated with a grinding stone to form projections, the apexes of the projections on the sheet discharging roller pointing in the direction which is the same as said sheet conveying direction.

Inventors: KASHIWABARA KAZUTOSHI, KASHIWABARA, KAZUTOSHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1992
Main classification: B41J13
IPC codes: B41J15, B41J13