Printing machine

Abstract: Known printing machines have been found to have poor printing quality if the direction of the printing sheet is reversed through the sheet becoming slack. The present invention overcomes this by ensuring that the amount of play b2 of a first power transmission system (202, 203, 204, 208, 209) between said drive motor and said sheet discharging means is larger than the amount of play b1 of a second power transmission system (202, 203, 204, 205; 202, 203, 204, 205, 206, 207; 202, 203, 210, 211; 202, 401, 402, 403; 202, 401, 404, 405) between said drive motor and said sheet conveying means. The difference in play may be achieved by dimensioning the backlash or the gear train used or by a rotation transmission gap in one of the shafts in the associated gear.

Description: The present invention relates to a printing machine, and more particularly to, but not exclusively to, a sheet conveying unit in the printing machine. An example of a conventional printing machine is as disclosed by Japanese Patent Application (OPI) no. 257871/1987 (the term "OPI" as used herein means an "unexamined published application"). A sheet conveying operation in the printing machine will be described with reference to Fig. 10. In the printing machine, a sheet conveying means comprising a platen 301 and a sheet conveying roller 303 operates to convey a printing sheet 100 in the direction of the arrow into the gap between the platen 301 and a printing head 302 to a printing position (a), and the printing head operates to print data on the printing sheet thus conveyed. The sheet 100 thus printed is conveyed and discharged by sheet discharging means comprising sheet discharging rollers 307 and 308. The sheet discharging means is so designed as to discharge the printing sheet 100 out of the printing machine when the rear edge of the printing sheet 100 has passed through the contact line of the platen 301 and the sheet conveying roller 303. That is to say, when the application of a sheet conveying force from the sheet conveying means to the printing sheet 100 has been suspended. Furthermore, the sheet conveying speed of the sheet discharging means is set higher than that of the sheet conveying means. Also the sheet conveying force of the sheet discharging means is set smaller than that of the sheet conveying so that the printing sheet 100 is allowed to slip with respect to the sheet discharging means. Thus the printing sheet 100 is stretched tight when located between the sheet conveying means and the sheet discharging means and so prevented from being slack. Such an arrangement is also disclosed in EP-A- 0 297 486. In some case it is desirable to provide a printing machine at low cost which can perform a graphic printing operation using bit images and a character printing operation of printing characters including enlarged characters in combination. However, in this case an ordinary printing method cannot be employed because the capacity of a memory (buffer) in the printing machine, or soft control is limited. In this case, the following method should be employed. First, a plurality of lines are printed. Thereafter, the printing sheet is returned to the original position for another kind of printing operation. Thus, the printing of the sheet is accomplished as a whole operation. However, it is essential to accurately convey the printing sheet by about 1cm or ½ inch in the opposite direction. The printing machine of Fig. 10 disclosed by the aforementioned Japanese Patent Application (OPI) No. 257871/1987 has been proposed on the premise that the printing sheet 100 is conveyed in the forward direction only (hereinafter referred to as "a sheet forward-conveying operation", when applicable). No consideration is made for the case where a printing sheet is conveyed in the reverse direction (hereinafter referred to as "a sheet reverse-conveying operation", when applicable). Fig. 11 shows the relationship between the amount of displacement of the printing sheet and the amount of rotation of the drive motor in the case where the sheet forward-conveying operation and the sheet reverse-conveying operation are carried out by the printing machine disclosed in the aforementioned Japanese Patent Application (OPI) NO. 257871/1987. When the printing sheet 10 is conveyed as much as D1, as shown in the first quadrant, the drive motor should be rotated as much as d1 in the sheet forward-conveying direction. Since the sheet conveying speed of the sheet discharging means is set higher than that of the sheet conveying means, the sheet discharging means tends to convey the printing sheet 100 by a greater amount and as indicated by the broken line. However, since the sheet conveying force of the sheet discharging means is less than that of the sheet conveying means, the printing sheet 100 is allowed to slip as much as S1 with respect to the sheet discharging means. Hence, the printing sheet 100 is conveyed to the point P1 from the origin while being stretched tight and so moved as much as D1. When the printing sheet 100 is conveyed only in the forward direction, even if there is an amount of play between the drive motor and the sheet conveying means and the sheet discharging means due to the backlash of the gear train, the amount of play is absorbed in one direction, and therefore the amount of displacement of the printing sheet 100 is proportional to the rotation rate of the drive motor. When the drive motor is turned as much as d1 in the reverse direction to convey the printing sheet in the reverse direction, then the printing sheet is not moved even when the drive motor is turned in the reverse direction as much as a small amount of play (b). This is due to the backlash of the gear train between the drive motor and the sheet conveying means and the sheet discharging means. In other words, the amount of play between the drive motor and the sheet conveying means is different in value from that between the drive motor and the sheet discharging means. However, they are denoted by the same value (b) since they are generally designed to be small values. In Fig. 11, for convenience of the description, (b) is about half of d1 but in practice, it is much smaller. After the play has been absorbed, the sheet discharging means displaces the printing sheet 100 at a speed higher than the sheet conveying means. As a result, the sheet conveying means displaces the printing sheet as much as D2 in the reverse direction; that is the sheet is moved through the origin and the point P2 to the point P3. However, the sheet discharging means displaces the printing sheet 100 in the reverse direction; that is, the sheet is moved through the origin and the point P2 to the point P4. Thus, the printing sheet 100 is slackened by as much as the difference S2 between D2 and D3. In Fig. 11, D1 > D3 > D2, and so the displacement of the printing sheet 100 is different with the amount of rotation (d1) of the drive motor maintained constant. Thereafter, when the drive motor is turned as much as d1, the play (b) is absorbed in the opposite direction and so the sheet is returned to the origin. That is, the printing sheet 100 is moved from the point P3 through the point P5 to the origin by the sheet conveying means, whereas it is moved from the point P4 through the point P6 to the origin by the sheet discharging means. As is apparent from the above description, when the printing sheet is conveyed in the reverse direction in the printing machine disclosed by the aforementioned Japanese Patent Application (OPI) No. 257871/1987, it is slackened, as a result of which the resultant print may be shifted in position, or the printing sheet may be stained by ink, or caught in the machine (known as jamming). In the case where the printing head 302 is of a wire dot type, the printing sheet 100 caught in the printing machine pushes the ink ribbon (not shown) located between the printing head 302 and the platen 301 through the ribbon mask (not shown) positioned between the printing head 302 and the platen 301 against the printing head 302. The wires of the printing head 302 are liable to be caught by the ink ribbon and at worst, they may be broken. In addition, the printing wires strike the printing sheet raised from the platen 301, thus making the printing noise louder. Accordingly, an object of the present invention is to provide a printing machine obviating the printing sheet being raised from the platen and so slackened during conveyance resulting in the print being shifted in position, and the printing sheet being stained by ink, or caught in the printing machine, and the wires of the printing head being broken. The foregoing object of the invention may be achieved by the provision of a printing machine capable of conveying a printing sheet in both forward and reverse feed directions and comprising: sheet conveying means provided upstream of a printing position with respect to the forward feed direction; sheet discharging means provided downstream of the printing position with respect to the forward feed direction; a drive motor for driving said sheet conveying means and said sheet discharging means to convey the printing sheet in the forward feed direction with the speed of said sheet discharging means higher than that of said sheet conveying means; characterised in that: the drive motor is capable of driving said sheet conveying means to move the printing sheet in the reverse feed direction by a predetermined distance for overprinting a printed area just printed thereon by the printing machine, the printing sheet being driven in the reverse direction while gripped by both the sheet conveying means and the sheet discharging means; the amount of play of a first power transmission system for transmitting power between said drive motor and said sheet discharging means is larger than the amount of play of a second power transmission system for transmitting power between said drive motor and said sheet conveying means; and driving of said sheet conveying means to move the printing sheet in the reverse feed direction by the predetermined distance is less than that required to take up the difference in play between the two power transmission systems. Conveniently, means is provided for rotating said drive motor by an amount equal to the sum of the amount of play of the second transmission system, an amount of rotation of said drive motor which, when no play is provided, is required for said sheet conveyance means to displace a printing sheet by a desired amount of conveyance and the amount of additional rotation in a sheet reverse-conveying direction, and thereafter for rotating said drive motor by an amount equal to the sum of the amount of play of the second power transmission system and the amount of additional rotation in a sheet forward-conveying direction so that the printing sheet is conveyed ion the sheet reverse-conveying direction by a desired amount of sheet reverse conveyance. In the printing machine, the amount of play of the first power transmission system may be due to the backlash of the gear train. In the printing machine, the amount of play of the first power transmission system may be due to a rotation transmission gap corresponding to the difference in sectional configuration between a hole formed in a gear and a shaft engaged with the hole. In the printing machine, the amount of sheet reverse conveyance may be 1.27cm (½ inch) or more. Preferred embodiments of the present invention will be described with reference to the accompanying drawings, of which: Fig. 1 is an explanatory diagram illustrating a sheet conveying section in a first embodiment of the present invention; Fig. 2 is a perspective view illustrating an external appearance of a printing machine to which the technical concept of the invention may be applied; Fig. 3 is an explanatory diagram illustrating the gear train of a sheet conveying mechanism in the first embodiment of the present invention; Fig. 4 is a diagram illustrating the relationship between the amount of displacement of a printing sheet and amount of rotation of a drive motor in the first embodiment; Figs. 5(a) and 5(b) are explanatory diagrams for a description of the backlash of the sheet conveying mechanism in the first embodiment; Fig. 6 is an explanatory diagram illustrating a sheet conveying mechanism in a second embodiment of the present invention; Figs. 7 and 8 are explanatory diagrams for a description of the operation of a third embodiment of the present invention; Fig. 9 is a diagram outlining a sheet conveying section in a fourth embodiment of the present invention; Fig. 10 is an explanatory diagram for a description of a conventional printing machine; and Fig. 11 is a diagram showing the relationship between the amount of displacement of a printing sheet and the amount of rotation of a drive motor in the conventional printing machine. A printing machine to which the technical concept of the present invention may be applied is shown in figure 2. The printing machine has a sheet path A in which a printing sheet is inserted from above and behind, and a sheet path B in which a printing sheet is inserted from below and in front. Fig. 1 is a diagram showing a sheet conveying section of a first embodiment of the invention. As shown in Fig. 1, the printing center (or printing position (a)) of a printing head 102 is positioned with respect to the normal of a cylindrical platen 101 made of rubber or the like. The printing head 102 and the platen 101 are so positioned that they form a small gap therebetween. In the sheet path A upstream of the printing position (a) on the sheet supplying side, first and second sheet conveying rollers 103 and 104 are provided in such a manner that they are pushed against the platen 101. Similarly, in the sheet path B upstream of the printing position (a) on the sheet supplying side, a sheet conveying drive roller 105 and a third sheet conveying roller 106 are provided in such a manner that they are pushed against each other. In addition, in the sheet path A, a tractor 109 is provided for conveying a continuous printing sheet (continuous form). The platen 101, the first sheet conveying roller 103, and the second sheet conveying roller 104 form first sheet conveying means; the sheet conveying drive roller 105 and the third sheet conveying roller 106 form second sheet conveying means; and the tractor forms third sheet conveying means. A sheet discharging roller 107 is provided in the sheet path downstream of the printing position (a). The sheet discharging roller 107 thus provided pushes the printing sheet 100 so that a predetermined sheet discharging force is obtained with a predetemined gap between the sheet discharging roller 107 and a sheet discharging cover 108. The sheet discharging roller 107 and a sheet discharging cover 108 form sheet discharging means. Thus, the sheet discharging means functions to discharge the printing sheet downstream of it after the rear edge of the printing sheet has passed through the sheet conveying means. In order to eliminate the difficulty that a printing sheet is slack between the sheet conveying means and the sheet discharging means, the sheet conveying speed of the sheet discharging means is set higher than that of anyone of the first, second and third sheet conveying means. Furthermore, in order to allow the printing sheet 100 to slip with respect to the sheet discharging means thereby to control the amount of conveyance of the printing sheet, the sheet conveying force of the sheet discharging means is set smaller than that of any one of the first, second and third sheet conveying means. A sheet guide 110 is provided so that a printing sheet is led to the printing position (a) through any one of the sheet paths A and B. In the first sheet conveying means, the first and second sheet conveying rollers 103 and 104 are provided with a mechanism for spacing them from the platen 101 when necessary. Similarly, in the second sheet conveying means, the third sheet conveying roller 106 is provided with a mechanism for spacing it from the sheet conveying driver roller 105 when required. In conveying a continuous printing sheet, the mechanism is operated so that the first and second sheet conveying rollers 103 and 104 are set apart from the platen 101, and then the tractor 109 is operated. Fig. 3 is a diagram illustrating a sheet conveying drive gear train in the first embodiment. In the embodiment, drive gear trains are employed for a power transmission system between the sheet conveying means and the drive motor, and for a power transmission system between the sheet discharging means and the drive motor. A drive motor 201 is provided for driving the sheet conveying means and the sheet discharging means. A motor pinion 202 is fixedly mounted on the output shaft of the drive motor 201. The motor pinion 202 is engaged with the large gear of a reduction gear 203, the small gear of which is engaged with a platen gear 204 and a fourth transmission gear 210. The platen gear 204 is fixedly secured to the platen 101 and engaged with a first transmission gear 205 and a third transmission gear 208. A sheet conveying drive roller gear 207 is engaged through a second transmission gear 206 with the first transmission gear 205. The sheet conveying drive roller gear 207 and the sheet conveying drive roller 105 are fixedly mounted on one and the same shaft. A tractor gear 211 is engaged with the fourth transmission gear 210, and is fixedly mounted on a shaft adapted to drive the tractor 109. A sheet discharging roller gear 209 in the sheet discharging means which is provided downstream of the printing position (a) is engaged with the third transmission gear 208. The sheet discharging roller gear 209 and the sheet discharging roller 107 are fixedly mounted on one and the same shaft. The motor pinion 202, the reduction gear 203, the platen gear 204, the first transmission gear 205, the second transmission gear 206, the fourth transmission gear 210, the sheet conveying drive roller gear 207, and the tractor gear 211 are shifted in gear configuration in a positive direction, and are smaller in backlash than standard gears. Hence, an amount of play b1 due to the backlash of the power transmission systems between the drive motor 201 and the first, second and third sheet conveying means is small. Similarly in the case of the amount of play b1 due to the backlash between the drive motor and the sheet conveying means and the sheet discharging means in the prior art described with reference to Fig. 11. Since those gears are the ones shifted in a positive direction as was described above, the distance between the axes of adjacent gears (hereinafter referred to as "an inter-axes distance", when applicable) is set larger than the sum of the pitch diameters thereof. On the other hand, the amount of play between the sheet discharging means and the drive motor is set as follows (sic): The inter-axes distances of the platen gear 204, the third transmission gear 208 and the sheet discharging roller gear 209 are increased, and the third transmission gear 208 and the sheet discharging roller gear 209 are shifted in a negative direction, thus increasing the backlash. Thus, the amount of play b2 is much larger than the amount of play b1 of the power transmission system between the driver motor 201 and the sheet conveying means. An accurate method of setting the amount of play b2 will be described with reference to Fig. 4 later. In the first sheet conveying means, the rotation of the drive motor 201 which is forward with respect to a sheet conveying direction when driven for conveyance of printing sheets will be referred to as "forward rotation", or "clockwise rotation " in the drawing, and the rotation of the drive motor which is reverse will be referred to as "reverse rotation", or "counterclockwise rotation". The relationship between the rotation of the drive motor 201 and the displacement of the printing sheet 100 will be described with reference to Figs. 1, 3, 4 and 5. The torque produced when the drive motor 201 is rotated as much as d1 clockwise, is applied through the motor pinion 202 and the reduction gear 203 to the platen gear 204 to turn the platen 101 clockwise. The torque is further applied through the third transmission gear 208 to the sheet discharging roller gear 209 to turn the sheet discharging roller 107 clockwise. In this operation, a cut printing sheet inserted through the sheet path A is displaced as much as D1 in the forward direction by the first sheet conveying means comprising the platen 101 and the first and second sheet conveying rollers 103 and 104, thus being supplied along the sheet guide 110. That is to say, the printing sheet is passed through the first and second sheet conveying rollers 103 and 104 pushed against the platen 101, and through the gap between the platen 101 and the printing head 102 and the gap between the sheet discharging roller 107 and the sheet discharging cover 108, thus being discharged into the sheet path C. In the case where the cut printing sheet is laid over both the sheet conveying means and the sheet discharging means, the sheet discharging means tends to convey the sheet as much as the amount of displacement indicated by the broken line in the first quadrant of Fig. 4 because the sheet conveying speed of the sheet discharging means is set higher than that of the sheet conveying means. However, since the sheet conveying force of the sheet discharging means is smaller than that of the sheet conveying means, the sheet is allowed to slip, as much as S1, with respect to the sheet discharging means. Thus, the sheet is moved from the original to the point P1 along the solid line, while being stretched tight and is displaced as much as D1. In the case where the cut printing sheet is conveyed in the forward direction as described above, the play due to the backlash of the gear trains between the drive motor and the sheet discharging means is absorbed in one direction as shown in Fig. 5(a). Therefore the amount of rotation of the drive motor and the amount of displacement of the printing sheet 100 are in proportion to each other. When, with the cut printing sheet in contact with both of the first sheet conveying means and the sheet discharging means, a signal for conveying a printing sheet in the opposite direction (hereinafter referred to as "a sheet reverse-conveying signal", when applicable) is applied to the drive motor 201, the latter is turned counterclockwise. The torque produced by the drive motor 201 is transmitted through the motor pinion 202 and the reduction gear 203 to the platen gear 204 to turn the latter 204 counterclockwise. The torque is further applied through the third transmission gear 208 to the sheet discharging roller gear 209 to turn the sheet discharging roller 107 counterclockwise. When it is required to displace the cut printing sheet as much as a desired amount of conveyance D4 in the opposite direction (hereinafter referred to as "an amount of sheet reverse conveyance D4", when applicable) the following steps occur. First the drive motor 201 is turned in the reverse direction as much as [the amount of play b1 due to the backlash or the like of the power transmission system between the first sheet conveying means and the drive motor 201] + [the amount of rotation d2 of the drive motor 201 with which, when no play is provided, the sheet conveying means conveys the printing sheet as much as the amount of sheet reverse conveyance D4] + [a small amount of additional rotation γ]. As a result, the cut printing sheet is moved from the origin to the point P8 via P7 in Fig. 4. While the printing sheet is moved from the origin to the point P7, the backlash of the gear train between the drive motor 201 and the first sheet conveying means is changed from the state shown in Fig. 5(a) to that shown in Fig. 5(b). If, in this case, the amount of play b2 of the power transmission system between the sheet discharging means and the drive motor is set larger than the following amount then the sheet discharging means is not driven during the operation, and therefore the cut printing sheet will not be slackened between the first sheet conveying means and the sheet discharging means. That is to say, larger than: [b1] + [the amount of rotation Max (d2) of the drive motor with which, when no play is provided, the sheet conveying means displaces the printing sheet as much as a desired maximum amount of sheet reverse conveyance Max(D4)] + [γ]. In other words, when the sheet discharging means starts with the play b2 of the power transmission system between the drive motor and the sheet discharging means absorbed at the point P11 which is located on the left side of the point P8 in Fig. 4, then the printing sheet will not be slackened while being conveyed in the reverse direction. In the embodiment, the maximum amount of sheet reverse conveyance Max(D4) is set to 1.27cm (½ inch) in anticipation of the combination of graphic printing operations using bit images and character printing operations of characters including enlarged characters of, 32 points. Naturally, when it is required to print characters larger than 32 points, the value b2 should be increased accordingly. Thereafter, the drive motor 201 is rotated as much as [b1 + γ] in the forward direction, so that the sheet is moved from the point P8 through the point P9 to the point P10. That is to say, the sheet is moved as much as the desired amount of sheet reverse conveyance D4. When the drive motor 201 is further rotated as much as d2, the sheet is moved from the point P10 to the origin and so set at the initial position. In the above-described operation, in the interval of P9 -- P10 -- Origin, the sheet conveying means is driven in the forward direction, and the sheet discharging means is not driven. In this connection, it should be noted that, in order to make the sheet conveying force of the sheet discharging means smaller than that of the sheet conveying means, the force of contact of the sheet conveying means with a printing sheet is made relatively small, and the sheet discharging roller 107 is allowed to race. Hence, the cut printing sheet conveyed by the sheet conveying means in the forward direction is passed through the gap between the sheet discharging roller 107 and the sheet discharging cover 108 without being slackened between the platen 101 and the sheet discharging means owing to its stiffness (against buckling), and finally the printing sheet is discharged out of the printing machine. In the case of conveying a cut printing sheet such as an air mail letter which is extremely thin and relatively unstiff, a mechanism is provided which causes the sheet discharging roller 107 to move away from the sheet discharging cover 108 when necessary. Hence, in the interval of P9 -- P10 -- Origin, similarly as in the case of the ordinary cut printing sheet, the extremely thin cut printing sheet can be discharged without being slackened by operating the mechanism to move the sheet discharging roller 107 away from the sheet discharging cover 108. The second sheet conveying means provided for the sheet path B is different from the above-described first sheet conveying means in that it comprises the sheet conveying drive roller 105 and the third sheet conveying roller 106. The printing sheet 100 is inserted in the different direction. Thus, the drive roller 105 is opposite in the direction of rotation to the drive motor 201 (as the drive motor 201 rotates clockwise [in the forward direction], the sheet conveying drive roller 105 is rotated counterclockwise). However, in the second sheet conveying means, as the drive motor 201 is rotated in the forward direction, the printing sheet is conveyed downstream of the sheet path. Thus, the function and control of the second sheet conveying means are the same as those of the first sheet conveying means. The third sheet conveying means provided for the sheet path A is substantially equal in function to the above-described first sheet conveying means. That is, in the third sheet conveying means, the tractor 109 is employed instead of the platen 101, the first sheet conveying roller 103 and the second sheet conveying roller 104 of the first sheet conveying means, and a continuous printing sheet suitable for the tractor 109 is employed instead of the cut printing sheet. The tractor 109 is different in the direction of rotation from the drive motor 201 (as the drive motor 201 rotates clockwise in the forward direction, the tractor gear 211 is rotated counterclockwise. However, as the drive motor 201 is rotated in the forward direction, the printing sheet is conveyed downstream of the sheet path. The function and control of the third sheet conveying means are the same as those of the first sheet conveying means. It should be noted that, before the third sheet conveying means is operated, the first and second sheet conveying rollers 103 and 104 are spaced apart from the platen 101. The above-described embodiment has three sheet conveying means and two sheet paths but the present invention is not limited thereto or thereby. That is to say, the printing machine merely needs to have at least one sheet conveying means and one sheet path. The sheet conveying means can be provided at any position upstream of the printing position (a). Fig. 6 is a diagram showing a sheet conveying mechanism in a second embodiment of the invention. In the second embodiment, a first endless belt 402 and a second endless belt 404 are employed instead of the first, second and fourth transmission gears 205, 206 and 210 in the first embodiment. That is, the second embodiment is equal in function and in control to the first embodiment. The employment of the endless belts greatly reduces the amount of play b1 of the power transmission systems between the drive motor and the second and third sheet conveying means, and decreases the fluctuation in the amount of play b1. Figs. 7 and 8 are diagrams for a description of the operation of a third embodiment of the present invention. In the embodiment, the sheet discharging means has a sheet discharging roller gear 209 with a sector-shaped hole; and a sheet discharging roller shaft 501 which is in the form of the character "D" in section which is formed by extending one of the sides of the sector. The sheet discharging roller shaft 501 is engaged with the sector-shaped hole of the sheet discharging roller gear 209. Since the sheet discharging roller shaft 501 is D-shaped in section as was described above, there is a gap corresponding to an angle of rotation β between the roller shaft 501 and the sector-shaped hole of the roller gear 209. In the first embodiment, the amount of play b1 of the power transmission system between the drive motor and the sheet conveying means is decreased, and the backlash of the gear train between the sheet discharging means and the drive motor 201 is increased, so that the following relation is established: [b1] + [Max(d2)] + [γ] < [b2] On the other hand, in the third embodiment, the sector-shaped hole is formed in the sheet discharging roller gear 209 while the sheet discharging roller shaft 501 is made D-shaped in section so that the angle of rotation β is formed therebetween, and the same relation is established. In the third embodiment, the sheet discharging roller gear 209 has the sector-shaped hole, and the sheet discharging roller shaft 501 is D-shaped in section but the present invention is not limited thereto or thereby. That is, the hole of the roller gear 209 and the section of the roller shaft 501 may be different in configuration from those which have been described above, if the angle of rotation β is obtained and the torque applied to the sheet discharging roller gear 209 is transmitted to the sheet discharging roller shaft 501. When the drive motor 201 is rotated clockwise (or in the forward direction, the sheet discharging roller gear 209 is also rotated clockwise, and therefore the sheet discharging roller gear 209 is engaged with the sheet discharging roller shaft 501 as shown in Fig. 7. When the sheet reverse-conveying signal is given, the drive motor 201 is rotated counter clockwise, the roller of the sheet conveying means is rotated with the amount of play b1 absorbed immediately, so that the sheet discharging roller gear 209 and the sheet discharging roller shaft 501 are engaged with each other as shown in Fig. 8. As is apparent from the above description, the third embodiment is different from the first embodiment in the production of play. However, the former is the same as the latter in function and in control. In the third embodiment, in controlling the amount of play, it is unnecessary to perform intricate component control as in controlling the amounts of shift of gears. Fig. 9 is a diagram showing a sheet conveying section in a fourth embodiment of the invention. In the fourth embodiment there is arranged linearly, first sheet conveying means comprising a sheet conveying drive roller 701 and a sheet conveying roller 702; second sheet conveying means comprising a tractor 109; and sheet discharging means comprising a sheet discharging drive roller 703 and a sheet discharging roller 704. They are arranged linearly to improve the conveyance of the printing sheet 100. The fourth embodiment is the same as the first embodiment both in function and in control. In the printing machine according to the present invention, the amount of play b2 of the power transmission system between the drive motor and the sheet discharging means is set so as to meet the following relation: [the amount of play b1 of the power transmission system between the sheet conveying means and the drive motor] + [the amount of rotations Max (d2) of the drive motor with which, when no play is provided, the sheet conveying means displaces the printing sheet as much as a desired maximum amount of sheet reverse conveyance Max(D4)] + [the small amount of additional rotation γ] < [b2]. When it is required to displace the printing sheet as much as a desired amount of sheet reverse conveyance D4 in the reverse direction, the drive motor is rotated as much as [b1 + d2 + γ] in the reverse direction, and then rotated as much as [b1 + γ] in the forward direction so that the printing sheet is prevented from becoming slack between the sheet conveying means and sheet discharging means. Accordingly, the printing machine is free from the difficulty encountered when the printing sheet is raised between the sheet conveying means and the sheet discharging means, and accordingly obviates the resultant print being shifted in position, and the printing sheet being stained by ink, or caught in the printing machine. Furthermore, in the case where the printing head is of a wire dot type, the printing wire is prevented from being caught by the ink ribbon, and accordingly from being broken. In addition, the printing machine of the present invention is relatively quiet by obviating the large printing noise which is produced by the printing wire striking on the printing sheet when raised from the platen.

Claims: A printing machine capable of conveying a printing sheet in both forward and reverse feed directions and comprising: sheet conveying means (101, 103, 104; 105, 106; 109) provided upstream of a printing position (a) with respect to the forward feed direction; sheet discharging means (107, 108) provided downstream of the printing position (a) with respect to the forward feed direction; a drive motor (201) for driving said sheet conveying means and said sheet discharging means to convey the printing sheet in the forward feed direction with the speed of said sheet discharging means higher than that of said sheet conveying means; characterised in that : the drive motor (201) is capable of driving said sheet conveying means to move the printing sheet in the reverse feed direction by a predetermined distance (D4) for overprinting a printed area just printed thereon by the printing machine, the printing sheet being driven in the reverse direction while gripped by both the sheet conveying means and the sheet discharging means; the amount of play (b2) of a first power transmission system (202, 203, 204, 208, 209) for transmitting power between said drive motor and said sheet discharging means is larger than the amount of play (b1) of a second power transmission system (202, 203, 204, 205; 202, 203, 204, 205, 206, 207; 202, 203, 210, 211; 202, 401, 402, 403; 202, 401, 404, 405) for transmitting power between said drive motor and said sheet conveying means; and driving of said sheet conveying means to move the printing sheet in the reverse feed direction by the predetermined distance (D4) is less than that required to take up the difference in play between the two power transmission systems. A printing machine as claimed in claim 1, in which means is provided for rotating said drive motor by an amount equal to the sum of the amount of play (b1) of said second transmission system, an amount of rotation (d2) of said drive motor which, when no play is provided, is required for said sheet conveying means to displace a printing sheet by the predetermined distance (D4) and an amount of additional rotation (γ) in the reverse direction, and thereafter for rotating said drive motor by an amount equal to the sum of said amount of play (b1) of said second power transmission system and said amount of additional rotation (γ) in the forward direction. A printing machine as claimed in claim 1 or claim 2, in which said amount of play (b2) of said first power transmission system is provided by the backlash of a gear train. A printing machine as claimed in claim 1 or claim 2, in which said amount of play (b2) of said first power transmission system is provided by a rotation transmission gap corresponding to the difference in sectional configuration between a hole formed in a gear (209) and a shaft (501) engaged with said hole. A printing machine as claimed in claim 1, in which said predetermined distance (D4) is 1.27cm (½ inch) or more.

Inventors: YAMAGUCHI NAOTO, YAMAGUCHI, NAOTO, Yamaguchi, Naoto, c/o Seiko Epson Corporation
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1992
Main classification: B41J13
IPC codes: B41J15, B65H5, B65H23, B41J11, B41J25, B41J13, B65H29