Printer

Information

  • Patent Grant
  • 6382094
  • Patent Number
    6,382,094
  • Date Filed
    Tuesday, March 21, 2000
    24 years ago
  • Date Issued
    Tuesday, May 7, 2002
    22 years ago
Abstract
A printer capable of printing a multicolor image with a single pass of a paper or similar recording medium includes a plurality of print drums. Drum drive gears each are mounted on a particular print drum such that the print drum is replaceable. The print drums are interlocked to each other by rotatable members including relay gears meshing with the drum drive gears, timing pulleys fixed to the relay gears, a timing belt, and pulleys for adjustment. Each rotatable member has teeth the number of which is selected such that the number of rotations of the rotatable member to occur in a single period of the print drums is an integral multiple of the number of rotations of the print drums.
Description




BACKGROUND OF THE INVENTION




The present invention relates to a stencil printer or similar printer and more particularly to a printer capable of printing a multicolor image by conveying a paper or similar recording medium via consecutive print drums only once.




It is a common practice with a stencil printer to arrange a plurality of print drums each storing ink of particular color in the direction of paper conveyance. While a paper is conveyed from the upstream side toward the downstream side of the printer once, images of different colors are sequentially transferred from the print drums to the paper one above the other. As a result, a multicolor image is printed on the paper by a single pass of the paper. Such a single pass system is far more efficient than a system requiring a print drum to be replaced color by color and requiring a paper to be repeatedly fed. However, the single pass system has an offset ghost problem ascribable to a short distance between consecutive print positions.




Specifically, in the single pass system, a paper carrying an image transferred from an upstream print drum, e.g., a first-color print drum is brought to a downstream print drum, e.g., a second-color print drum without ink forming the image being dried. As a result, the ink is transferred from the paper to a master wrapped around the downstream drum and therefore from the master to the next paper.




The transfer of wet ink from the paper to the master wrapped around the downstream print drum is not critical. However, the ink transferred from the paper to the above master is again transferred to the next paper carrying an image of a first color transferred from the upstream print drum (so-called retransfer). The retransfer does not degrade image quality if the ink can be retransferred to the next paper in accurate register with the image of the first color printed on the paper. The retransferred ink, however, forms an offset ghost and critically degrades image quality if deviated from the image carried on the next paper. For example, for a given deviation, the offset ghost renders thick lines blurred and thin lines doubled.




While the retransfer cannot be obviated in the single pass, multicolor printer, the offset ghost ascribable to the deviation of a retransfer position can be highly accurately controlled if the upstream and downstream print drums rotate in accurate synchronism with each other and if papers are conveyed with high accuracy.




To control the offset ghost, it has been customary to drive the upstream and downstream print drums by inter locking them to each other. Japanese Patent Laid-Open Publication No. 4-329175, for example, teaches an interlocked drive system in which the shafts of the print drums are interconnected by a plurality of gears. Japanese Patent Laid-Open Publication No. 7-17121 proposes another interlocked drive system using timing pulleys and a timing belt.




However, the conventional interlocked drive system, whether it be the gear scheme or the timing belt scheme, has a problem that the gears, timing belt and other rotatable members inter locking the print drums are more or less eccentric for machining reasons and therefore vary their speeds during one rotation. As for the gear scheme, high rigidity available with a gear train can reduce the deviation of the offset ghost if high precision gears are used. However, a plurality of high precision gears increase the production cost of the printer. The printer with the timing belt scheme is low cost because use can be made of inexpensive timing pulleys that can be produced by, e.g., injection molding on a quantity basis. However, the timing belt and timing pulleys involve eccentricity and aggravate the deviation of the offset ghost.




Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 11-129600.




SUMMARY OF THE INVENTION




It is therefore an object of the present invention to provide a printer capable of reducing the deviation of an offset ghost without increasing the cost.




In accordance with the present invention, a printer includes a plurality of print drums spaced from each other in a direction in which a recording medium is conveyed. A plurality of rotatable members interlock the print drums with respect to drive. The print drums and rotatable members are so arranged as to prevent an upstream and a downstream print drum rotating synchronously to each other from being brought out of synchronism when the recording medium arrives at the downstream print drum.




Also, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby interlock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys meshing with the timing belt, and displaces the adjustment pulleys for adjusting a phase between the print drums. The adjustment pulleys each have a number of teeth which is 1/integer of the number of teeth of each drum drive pulley.




Further, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby interlock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys meshing with the timing belt, and displaces the adjustment pulleys for adjusting a phase between the print drums. Steer pulleys are fixed in place between the drum drive pulleys and the adjustment pulleys and contact the rear of the timing belt for steering it. The steer pulleys each have a pitch circle diameter which is 1/integer of the pitch circle diameter of each drive pulley.




Moreover, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby interlock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys contacting the rear of the timing belt between the print drums, and displaces the adjustment pulleys for adjusting a phase between the print drums. The adjustment pulleys each have a pitch circle diameter which is 1/integer of the pitch circle diameter of each drum drive pulley.




In addition, in accordance with the present invention, a printer includes a plurality of print drums spaced in a direction in which a recording medium is conveyed. A plurality of toothed drum drive pulleys each are mounted on a particular print drum. A timing belt is passed over the drum drive pulleys to thereby interlock the print drums with respect to drive. A phase adjusting device includes adjustment pulleys contacting the rear of the timing belt between the print drums, and displaces the adjustment pulleys for adjusting a phase between the print drums. Steer pulleys are fixed in place between the drum drive pulleys and the adjustment pulleys and meshing with the timing belt for steering it. The steer pulleys each have a number of teeth which is 1/integer of the number of teeth of each drive pulley.











BRIEF DESCRIPTION OF THE DRAWINGS




The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:





FIG. 1

is a front view showing a conventional stencil printer;





FIG. 2

is a waveform diagram showing speeds varying due to the eccentricity of, e.g., adjustment pulleys included in the conventional printer;





FIG. 3

is a waveform diagram showing the combined variations of the speeds shown in

FIG. 2

;





FIG. 4

is a waveform diagram showing speeds varying due to the eccentricity of drum drive pulleys and a timing belt also included in the conventional printer;





FIG. 5

is a waveform diagram showing the combined variations of the speeds shown in

FIG. 4

;





FIG. 6

is a plot showing the deviations of rotation of a print drum also included in the conventional printer and determined by calculating period-by-period areas based on the waveforms of

FIG. 5

;





FIG. 7

is a front view showing a first embodiment of the printer in accordance with the present invention;





FIG. 8

is a front view of phase adjusting means included in the first embodiment;





FIG. 9

is a waveform diagram showing speeds varying due to the eccentricity of, e.g., adjustment pulleys included in the first embodiment;





FIG. 10

is a waveform diagram showing the combined variations of the speeds shown in

FIG. 9

;





FIG. 11

is a front view showing a second embodiment of the present invention;





FIG. 12

is a front view showing a third embodiment of the present invention;





FIG. 13

is an isometric view showing phase adjusting means included in the third embodiment;





FIG. 14

is a waveform diagram showing speeds varying due to the eccentricity of, e.g., adjustment pulleys included in the third embodiment;





FIG. 15

is a waveform diagram showing the combined variations of the speeds shown in

FIG. 14

;





FIG. 16

is a fragmentary view showing the pitch circle diameter of a steer pulley included in the third embodiment;





FIG. 17

is a front view showing a fourth embodiment of the present invention; and





FIG. 18

is a fragmentary view showing the pitch circle diameter of an adjustment pulley included in the fourth embodiment.











DESCRIPTION OF THE PREFERRED EMBODIMENTS




To better understand the present invention, reference will be made to a conventional single pass, multicolor printer of the type connecting the shafts of a plurality of print drums with timing pulleys and a timing belt, shown in FIG.


1


. As shown, the printer includes two print drums


100


and


102


respectively located at the upstream side and downstream side in a direction in which a paper or similar recording medium P is conveyed (direction of paper conveyance hereinafter). Toothed drum drive pulleys or timing pulleys


104


and


106


are respectively mounted on the print drums


100


and


102


. A timing belt


108


is passed over the drum drive pulleys


104


and


106


. In this condition, the print drums


100


and


102


are driven in interlocked relation to each other.




Phase adjusting means


110


intervenes between the print drums


100


and


102


for adjusting a phase between the print drums


100


and


102


, i.e., a deviation between the first and second colors in the direction of paper conveyance or to-and-bottom direction. The phase adjusting means


110


includes a frame


112


movable up and down by being driven by drive means not shown. Toothed adjustment pulleys


114




a


and


114




b


are respectively rotatably mounted on the upper and lower end portions of the frame


112


. A timing belt


108


is passed over the adjustment pulleys


114




a


and


114




b


. Four steer pulleys


116


are fixed in place between the adjustment pulleys


114




a


and


114




b


and the print drums


100


and


102


, as illustrated, so as to steer the timing belt


108


. The displacement of the phase adjusting means


110


in the up-and-down direction implements efficient phase adjustment within a short distance. The steer pulleys


116


that contact the rear of the timing belt


108


are implemented by plain pulleys. There are also shown in

FIG. 1

press rollers


100


and


102


movable into and out of contact with the print drums


100


and


102


, respectively.




When the frame


112


and therefore the adjustment pulleys


114




a


and


114




b


move upward, the print drums


100


and


102


are respectively caused to rotate in directions a and b, i.e., the phases of the print drums


100


and


102


are varied. When the frame


112


is moved downward, the phases of the print drums


100


and


102


are varied in the opposite directions. In this manner, the phase adjusting means is capable of correcting the deviation of an image ascribable to a change in printing speed and is essential with a printer of the type described.




The drum drive pulleys


104


and


106


and adjustment pulleys


114


, all of which mesh with the timing belt


108


, are more or less eccentric due to limited machining and assembling accuracy. Also, the timing belt


108


itself involves unnegligible eccentricity due to the limited accuracy of its core line. Moreover, considering the presence of the phase adjusting means


110


, the irregular thickness of the timing belt


108


over the entire circumference is another eccentricity component due to the steer pulleys


116


contacting the rear of the belt


108


.




An offset ghost occurs once for a single rotation of the print drums


100


and


102


, i.e., the drum drive pulleys


104


and


106


. In this respect, the eccentricity of the drum drive pulleys


104


and


106


, if any, does not disturb the synchronous rotation of the print drums


100


and


102


. However, as for the adjustment pulleys


114


, any eccentricity shifts the phases of the print drums


100


and


102


every time the pulleys


114


rotate or shifts them every time the drum drive pulleys


104


and


106


rotate when combined with the eccentricity of the timing belt


108


.




Why the eccentricity of the adjustment pulleys


114




a


and


114




b


bring about an offset ghost will be described with reference to

FIGS. 2 and 3

. Assume that the ratio of the number of teeth of the drum drive pulleys


104


and


106


to that of the adjustment pulleys


114




a


and


114




b


is 4.3:1, i.e., the former is not an integral multiple of the latter, and that the pulleys


104


and


106


and pulleys


114


are eccentric. Then, the speed of the drum drive pulley


104


and that of the adjustment pulley


114




a


vary, as represented by waveforms in FIG.


2


. The other drum drive pulley


106


and the other adjustment pulley


114




b


vary in speed in the same manner as the pulley


104


and pulley


114




a


although not shown specifically.




In

FIG. 2

, a solid waveform S


1


indicates the speed variation of the drum drive pulley


104


. A solid waveform S


2


indicates the speed variation of the adjustment pulley


114




a


; the origin of the waveform is shown as coinciding with the origin of the waveform representative of the speed variation of the drum drive pulley


104


for the sake of illustration. Further, a dashed waveform S


3


indicates the speed variation of the adjustment pulley


114




a


occurring when the eccentric position of the pulleys


104


and


114




a


are different from each other. As the waveform S


3


indicates, the origin of the waveform of the drum drive pulley


104


and that of the waveform of the adjustment pulley


114




a


are, in many cases, not coincident with each other.





FIG. 3

shows a solid waveform C


1


representative of the combined speed variation of the waveforms S


1


and S


2


of

FIG. 2

, and a dashed waveform C


2


representative of the combined speed variation of the waveforms S


1


and S


3


of FIG.


2


. As shown, wherever a drum period may begin, the waveforms S


1


and S


3


vary every drum period. As a result, the print drums


100


and


102


are deviated from each other in a particular manner in each period, causing an offset ghost to appear.




Reference will be made to

FIGS. 4 through 6

for describing an offset ghost ascribable to the eccentricity of the timing belt


108


. Assume the ratio of the number of teeth of the drum drive pulleys


104


and


106


to that of the timing belt


108


is 1:2.5, i.e., the latter is not an integral multiple of the former, and that the drum drive pulleys


104


and


106


and timing belt


108


are eccentric. Then, the speed of the drum drive pulley


106


and that of the timing belt


108


vary, as represented by waveforms in FIG.


4


. The other drum drive pulley


104


varies in speed in the same manner as the drum drive pulley


106


although not shown specifically.




In

FIG. 4

, a solid waveform S


4


indicates the speed variation of the drum drive pulley


106


. A solid waveform S


5


indicates the speed variation of the timing belt


108


; the origin of the waveform is shown as coinciding with the origin of the waveform representative of the speed variation of the drum drive pulley


106


for the sake of illustration. Further, a dashed waveform S


6


indicates the speed variation of the drum drive pulley


106


occurring when the eccentric position of the pulley


106


and that of the timing belt


108


are different from each other. As the waveform S


6


indicates, the origin of the waveform of the drum drive pulley


106


and that of the waveform of the timing belt


108


are, in many cases, not coincident with each other.





FIG. 5

shows a solid waveform C


3


representative of the combined speed variation of the waveforms S


4


and S


5


of

FIG. 4

, and a dashed waveform C


4


representative of the combined speed variation of the waveforms S


5


and S


6


of FIG.


4


. As shown, wherever a drum period may begin, the waveforms S


3


and S


4


vary in a particular manner in each drum period. In this case, however, the drum drive pulley


106


and timing belt


108


having the gear ratio of 1:2.5 constantly have five periods and two periods, respectively. That is, the identical waveform C


3


appears every five periods of the drive pulley


106


.





FIG. 6

plots the sums of the areas of the waveform C


3


,

FIG. 5

, indicated by hatching for every period of the drum drive pulley


106


; the sizes indicate how much the synchronization of the drum drive pulley


106


is deviated. As

FIG. 6

indicates, the same deviation of the drum drive pulley


106


occurs every other period of the timing belt


108


.




Preferred embodiments of the printer in accordance with the present invention will be described hereinafter.




1st Embodiment




Referring to

FIG. 7

, a printer embodying the present invention is shown and implemented as a bicolor stencil printer by way of example. As shown, the printer, generally


2


, includes paper feeding means


4


for feeding papers or similar recording media P to a registration roller pair


6


one by one. Two print drums


8


and


10


are spaced from each other in the direction in which the paper P fed from the paper feeding means


4


is conveyed (direction of paper conveyance hereinafter). A press roller or pressing member


12


is movable into and out of contact with the upstream print drum


8


by being driven by a moving mechanism not shown. Separating means


13


separates the paper P carrying an image of a first color from the print drum


8


by sending air. Suction belt type intermediate conveying means


14


conveys the paper P between the print drums


8


and


10


. Another press roller or pressing member


16


is movable into and out of contact with the downstream print drum


10


by being driven by a moving mechanism not shown. Separating means


17


separates the paper P carrying an image of a second color transferred from the print drum


8


over the image of the first color by sending air. Outlet conveying means


18


conveys the paper P separated from the print drum


10


to a print tray


19


. Phase adjusting means


20


adjusts a phase between the print drums


8


and


10


.




Drum drive gears


220


and


222


are respectively mounted on the print drums


8


and


10


such that the print drums


8


and


10


each are replaceable. A relay gear


226


, which has a timing pulley


224


integrally therewith, is fixed in place and held in mesh with the drum drive gear


220


. Likewise, a relay gear


230


having a timing pulley


228


integrally therewith is fixed in place and held in mesh with the drum drive gear


222


of the print drum


10


.




A timing belt


232


is passed over the timing pulleys


224


and


228


with the intermediary of the phase adjusting means


20


, so that the print drums


8


and


10


can be synchronously driven in interlocked relation to each other. Specifically, a main motor


25


is drivably connected to the print drum


8


via a main drive belt


23


. The rotation of the main motor


25


is transferred to the print drum


10


via the relay gear


226


, timing belt


232


, and so forth. A pulley


27


applies a preselected degree of tension to the main drive belt


23


.




In the paper feeding means


4


, a tray


24


is loaded with a stack of papers P and intermittently raised by a motor not shown. A pickup roller


26


, a separator roller


28


and separator pad


30


cooperate to feed the top paper P from the tray


24


toward the registration roller pair


6


while separating it from the underlying papers. The registration roller pair


6


corrects, e.g., the skew of the paper P and conveys it toward the print drum


8


at such a timing that the leading edge of the paper P meets the leading edge of an image formed on the print drum


8


.




At the above timing, the press roller


12


is pressed against the print drum


8


. Ink feeding means arranged within the print drum


8


feeds ink of the first color to the inner periphery of the print drum


8


. The press roller


12


therefore causes the ink to penetrate through the print drum


8


and the perforations of a master, not shown, wrapped around the drum


8


to the paper P. As a result, an image of the first color is printed on the paper P. It is to be noted that the press roller


12


is intermittently pressed against the print drum


8


so as not to interfere with a damper


32


mounted on the outer circumference of the drum


8


.




The separating means


13


separates the paper P carrying the image of the first color thereon from the print drum


8


. The intermediate conveying means


14


conveys the separated paper P while a suction fan, not shown, retains the paper P on the conveying means


14


by suction. The linear velocity of the conveying means


14


is selected to be higher than the linear velocity of the paper P by preselected times. The conveying means


14


conveys the paper P to a nip between the downstream print drum


10


and the press roller


16


.




Ink feeding means, not shown, is also arranged within the downstream print drum


10


and feeds ink of a second color to the inner periphery of the drum


10


. Therefore, when the press roller


16


is pressed against the print drum


10


with the intermediary of the paper P, it causes the above ink to penetrate through the print drum


10


and a master, not shown, wrapped around the drum


10


to the paper P. Consequently, an image of the second color is printed on the paper P over the image of the first color existing on the paper P. The press roller


16


is intermittently pressed against the print drum


10


so as not to interfere with a damper


34


mounted on the outer circumference of the drum


10


.




The separating means


17


separates the paper P carrying the composite image of the first and second colors thereon from the print drum


10


. The outlet conveying means


18


conveys the separated paper P while a suction fan, not shown, retains the paper P on the conveying means


18


by suction. Finally, the paper or print P is driven out to the print tray


19


. At this instant, a jump board


180


provides the paper P with an adequate degree of stiffness.




The phase adjusting means


20


includes two adjustment pulleys or timing pulleys


40


and


42


. Four steer pulleys


44


are fixed in place between the adjustment pulleys


40


and


42


and the relay gears


226


and


230


. The steer pulleys


44


allow phase adjustment based on the up-and-down movement of the phase adjusting means


20


to be efficiently effected within a short distance. In the illustrative embodiment, the steer pulleys


44


serve as tension pulleys as the same time. The drum drive gears


220


and


222


, timing pulleys


224


and


228


, relay gears


226


and


230


, timing belt


232


, adjustment pulleys


40


and


42


and steer pulleys


44


are rotatable members interlocking the two print drums


8


and


10


with respect to drive.




As best shown in

FIG. 8

, the phase adjusting means


20


includes a frame


54


elongate in the up-and-down direction. The adjustment pulleys


40


and


42


are respectively rotatably mounted on the upper and lower end portions of the frame


54


. A stationary screw shaft


58


extends upward from the top of the frame


54


. A nut gear


60


is held in mesh with the screw shaft


58


and fixed in place by a bracket not shown. A motor


62


has an output shaft on which a drive gear


64


is mounted. The drive gear


64


is held in mesh with the nut gear


60


. In this configuration, the motor


62


selectively causes the frame


54


to move upward or downward while being guided by guides, not shown, supported by the side walls of the printer body.




The steer pulleys


44


, implemented as plain pulleys, each are rotatably mounted on a respective shaft


66


fixed to the sidewalls of the printer body. The steer pulleys


44


are positioned between the pulleys


40


and


42


and the relay gears


226


and


230


in such a manner as to squeeze the timing belt


232


and held in contact with the rear of the timing belt


232


.




When the motor


62


is driven to move the frame


54


upward, as indicated by an arrow X, the frame


54


raises the pulleys


40


and


42


and thereby causes the print drums


8


and


10


to respectively rotate in directions c and d shown in FIG.


7


. As a result, the phases of the print drums


8


and


10


are varied to correct color deviation. The motor


62


may be driven in the opposite direction to move the frame


54


downward, as indicated by an arrow Y, thereby adjusting the above phases in the opposite direction.




In the illustrative embodiment, a single period of each of the above rotatable members is selected to be equal to or shorter than a single period of each print drum


8


or


10


. In addition, during a single period of the print drum


8


or


10


, each rotatable member is caused to make a number of rotations which is an integral multiple of the number of rotations of the drum


8


or


10


. For example, the ratio of the number of teeth of each drum drive gear


220


or


222


to that of each relay gear


226


or


230


is 4:1 while the ratio of the number of teeth of each timing pulley


224


or


228


to that of each adjustment pulley


40


or


42


is 1:1. In addition, the ratio of the number of teeth of the timing pulley


224


or


228


to that of the timing belt


232


is 1:4. These gear ratios allow the timing belt


232


to make one rotation during one rotation of the print drum


8


or


10


. The ratio of the pitch circle diameter of the timing pulley


224


or


228


to that of each steer pulley


44


is 1:1.




If all the various ratios including the gear ratios are integral, the number of rotations of each rotatable member is an integral multiple of the number of rotations of the print drum


8


or


10


for a single period of the drum


8


or


10


. This is successful to obviate a phase difference (deviation in synchronism) between the print drums


8


and


10


and therefore an offset ghost. This will be described more specifically with reference to

FIGS. 9 and 10

.




Assume that the drum drive gears


220


and


222


and pulleys


40


and


42


are eccentric, and that the ratio of the number of teeth of each drum drive gear


220


or


222


to that of each pulley


40


or


42


is 4:1. Then, the speed of the drum drive gear


220


and that of the pulley


40


vary, as shown in FIG.


9


. The other drive pulley


222


and the other pulley


42


, respectively vary in speed in the same manner as the drum drive gear


220


and pulley


40


although not shown specifically.




In

FIG. 9

, a solid waveform S


7


indicates the speed variation of the drum drive gear


220


. A solid waveform S


8


indicates the speed variation of the adjustment pulley


40


; the origin of the speed variation is shown as coinciding with the origin of the speed variation of the drum drive gear


220


. Further, a dashed waveform S


9


indicates the speed variation of the adjustment pulley


40


occurring when the eccentric position of the drum drive gear


220


and that of the adjustment pulley


40


are different from each other. As shown, so long as the eccentric positions are coincident, just four periods of the adjustment pulley


40


or


42


occur in a single period of the drum drive gear


220


or


222


.





FIG. 10

shows a solid waveform C


5


representative of the combined speed variation of the waveforms S


7


and S


8


of

FIG. 9

, and a dashed waveform C


6


representative of the combined speed variation of the waveforms


75


and S


9


of FIG.


9


. As shown, wherever a drum period may begin, the waveforms C


5


and C


6


each vary in an identical manner in all drum periods. That is, the print drums


8


and


10


deviate from each other in the same manner in all periods and prevent an offset ghost from appearing. More specifically, even when the timing belt


232


involves an eccentric component, an offset ghost does not occur so long as the ratio of the period of the timing belt


232


to that of the print drum


8


or


10


is 1:1. This is because all the other rotatable members have integral ratios to the print drums


8


and


10


; the integral multiples cancel the eccentric components of the rotatable members in a single period of the print drums


8


and


10


.




An offset ghost occurs once for a single rotation of the print drums


8


and


10


. While a change in speed may occur after the leading edge of the paper P has moved away from a print position assigned to the second color, the change is absorbed by the warp of the paper P being conveyed. It follows that if the print drum


10


is accurately synchronous to the print drum


8


when the paper P enters a nip between the drum


10


and the press roller


16


, an offset ghost does not appear.




Assume that while the paper P is being conveyed over the nip between the print drum


8


and the press roller


12


and the nip between the print drum


10


and the press roller


16


, the print drums


8


and


10


are brought out of synchronism. Then, the warp successfully absorbs the resulting phase difference. After the trailing edge of the paper P has moved away from the nip between the print drum


10


and the press roller


16


, the above phase difference does not matter at all.




The above relation also holds with a tricolor or a tetracolor printer. The crux is that an upstream and a downstream print drums be accurately synchronized to each other when a paper arrives at the downstream drum. The numbers of rotations which are the integral multiples of the number of rotations the print drums


8


and


10


are a specific example capable of maintaining the drums


8


and


10


in the above relation.




2nd Embodiment





FIG. 11

shows an alternative embodiment of the printer in accordance with the present invention. In

FIG. 11

, structural elements identical with the structural elements of the first embodiment are designated by identical reference numerals and will not be described specifically in order to avoid redundancy. As shown, the printer includes phase adjusting means


70


.




The phase adjusting means


70


includes a gear


72


meshing with a drum drive gear


222


. The gear


72


has a shaft on which a sector gear


74


is rotatably mounted. A motor


76


has an output shaft on which a drive gear


76


is mounted. The drive gear


76


is held in mesh with a gear portion


74




a


included in the sector gear


74


. A small diameter gear


80


is supported by the major part of the sector gear


74


and held in mesh with the gear


72


. A drum drive gear


220


has a shaft supporting one end of an arm


82


such that the arm


82


is angularly movable. A small diameter gear


84


is rotatably supported by the other end of the arm


82


and held in mesh with the drum drive gear


220


and small diameter gear


80


. An arm


86


connects the small diameter gears


80


and


84


. The motor


78


causes the sector gear


74


to move in either one of directions indicates by arrows R and L, thereby correcting color deviation between the print drums


8


and


10


.




In the illustrative embodiment, the drum drive gears


220


and


222


, gear


72


and small diameter gears


80


and


84


are rotatable members for causing the two print drums


8


and


10


to rotate in synchronism with each other.




A single period of each of the above rotatable members is selected to be equal to or shorter than a single period of the print drums


8


and


10


, as in the previous embodiment. In addition, during a single period of the print drums


8


and


10


, each rotatable member is caused to make rotations the number of which is an integral multiple of the number of rotations of the drums


8


and


10


. For example, the ratio of the number of teeth of the drum drive gears


220


and


222


to that of the gear


72


is 1:1 while the ratio of the number of teeth of the drum drive gear


220


or


222


to that of the small diameter gears


80


and


82


is 4:1. Such integral ratios or integral multiples are successful to cancel the eccentricity component of each rotatable member, thereby obviating an offset ghost.




3rd Embodiment




Referring to

FIGS. 12 through 16

, another alternative embodiment of the printer in accordance with the present invention is shown. As shown, the printer, generally


302


, includes paper feeding means


304


for feeding the papers P to a registration roller pair


306


one by one. Two print drums


308


and


310


are spaced from each other in the direction of paper conveyance. A press roller or pressing member


312


is movable into and out of contact with the upstream print drum


308


by being driven by a moving mechanism not shown. Suction belt type intermediate conveying means


314


conveys the paper P between the print drums


308


and


310


. Another press roller or pressing member


316


is movable into and out of contact with the downstream print drum


310


by being driven by a moving mechanism not shown. Out let conveying means


318


conveys the paper P separated from the print drum


310


to a print tray not shown. A timing belt


320


allows the print drums


308


and


310


to be driven in synchronism with each other. Phase adjusting means


322


adjusts the phases of the print drums


308


and


310


.




A main motor


325


causes the upstream print drum


308


to rotate via a main drive belt


323


. The rotation of the upstream print drum


308


is transferred to the downstream print drum


310


by the timing belt


320


. A pulley


327


applies an adequate degree of tension to the main drive belt


323


.




In the paper feeding means


304


, a tray


324


is loaded with a stack of papers P and intermittently raised by a motor not shown. A pickup roller


326


, a separator roller


328


and separator pad


330


cooperate to feed the top paper P from the tray


324


toward the registration roller pair


306


while separating it from the underlying papers. The registration roller pair


306


corrects, e.g., the skew of the paper P and conveys it toward the print drum


308


at such a timing that the leading edge of the paper P meets the leading edge of an image formed on the print drum


308


.




At the above timing, the press roller


312


is pressed against the print drum


308


. Ink feeding means arranged within the print drum


308


feeds ink of the first color to the inner periphery of the print drum


308


. The press roller


312


therefore causes the ink to penetrate through the print drum


308


and the perforations of a master, not shown, wrapped around the drum


308


to the paper P. As a result, an image of a first color is printed on the paper P. It is to be noted that the press roller


312


is intermittently pressed against the print drum


308


so as not to interfere with a clamper


332


mounted on the outer circumference of the drum


308


.




Separating means, not shown, separates the paper P carrying the image of the first color thereon from the print drum


308


. The intermediate conveying means


314


conveys the separated paper P while a suction fan, not shown, retains the paper P on the conveying means


314


by suction. The linear velocity of the conveying means


314


is selected to be higher than the linear velocity of the paper P by preselected times. The conveying means


314


conveys the paper P to a nip between the downstream print drum


310


and the press roller


316


.




Ink feeding means, not shown, is also arranged within the downstream print drum


310


and feeds ink of a second color to the inner periphery of the drum


310


. Therefore, when the press roller


316


is pressed against the print drum


310


with the intermediary of the paper P, it causes the above ink to penetrate through the print drum


310


and a master, not shown, wrapped around the drum


310


to the paper P. Consequently, an image of the second color is printed on the paper P over the image of the first color existing on the paper P. The press roller


316


is intermittently pressed against the print drum


310


so as not to interfere with a damper


334


mounted on the outer circumference of the drum


310


.




Separating means, not shown, separates the paper P carrying the composite image of the first and second colors thereon from the print drum


310


. The outlet conveying means


318


conveys the separated paper P while a suction fan, not shown, retains the paper P on the conveying means


318


by suction. Finally, the paper or print P is driven out to a print tray not shown.




Two toothed drum drive pulleys or timing pulleys


336


and


338


are respectively mounted on the shafts


350


and


352


of the print drums


308


and


310


such that the print drums


308


and


310


are replaceable. A timing belt


320


is passed over the drum drive pulleys


336


and


338


. The phase adjusting means


322


includes two adjustment pulleys or timing pulleys


340


and


342


. Four steer pulleys


344


are fixed in place between the adjustment pulleys


340


and


342


and the drum drive pulleys


336


and


338


. The steer pulleys


344


allow phase adjustment based on the up-and-down movement of the phase adjusting means


322


to be efficiently effected within a short distance. In the illustrative embodiment, too, the steer pulleys


344


serve as tension pulleys as the same time.




As shown in

FIG. 13

, the phase adjusting means


322


includes a frame


354


elongate in the up-and-down direction. The adjustment pulleys


340


and


342


are respectively rotatably mounted on the upper and lower end portions of the frame


354


. A pinion, not shown, is held in mesh with a rack


354




a


forming part of the frame


354


and is driven by a motor not shown. Elongate slots


354




b


and


354




c


are respectively formed in the upper half and lower half of the frame


354


, and each extends in the up-and-down direction. Guide pins


356


and


358


are affixed to the side walls of the printer body and received in the slots


354




a


and


354




b


, respectively. The frame


354


is movable up and down while being guided by the guide pins


356


and


358


and guides, not shown, also fixed to the above side walls.




The steer pulleys


344


, implemented as plain pulleys, each are rotatably mounted on a respective shaft


360


fixed to the side walls of the printer body. The steer pulleys


344


are positioned between the adjustment pulleys


340


and


342


and the drum drive pulleys


336


and


338


in such a manner as to squeeze the timing belt


320


and held in contact with the rear of the timing belt


320


.




When the motor drives the pinion in order to move the frame


354


upward, as indicated by an arrow X, the frame


354


raises the pulleys adjustment


340


and


342


and thereby causes the print drums


308


and


310


to respectively rotate in directions a and b. As a result, the phases of the print drums


308


and


310


are varied to correct color deviation. The motor may be driven in the opposite direction to move the frame


354


downward, as indicated by an arrow Y, thereby adjusting the above phases in the opposite direction.




The drum drive pulleys


336


and


338


have the same number of teeth which is greater than the number of teeth of the adjustment pulleys


340


and


342


of the phase adjusting means


322


. The adjustment pulleys


340


and


342


have the same number of teeth.




In the illustrative embodiment, the adjustment pulleys


340


and


342


each have a number of teeth which is 1/integer of the number of teeth of the drum drive pulleys


336


and


338


. Stated another way, the number of teeth of the drum drive pulleys


336


and


338


is an integral multiple of the number of teeth of the adjustment pulleys


340


and


342


. For example, the drum drive pulleys


336


and


338


each have


144


teeth while the adjustment pulleys


340


and


342


each have thirty-six teeth. With this relation, it is possible to obviate a phase difference (deviation in synchronism) between the print drums


308


and


310


and therefore an offset ghost even if the adjustment pulleys


340


and


342


are eccentric. This will be described more specifically with reference to

FIGS. 14 and 15

.




Assume that the drive pulleys


336


and


338


and adjustment pulleys


340


and


342


are eccentric, and that the ratio of the number of teeth of each drum drive pulley


336


or


338


to that of each adjustment pulley


340


or


342


is 4:1. Then, the speed of the drum drive pulley


336


and that of the adjustment pulley


340


vary, as shown in FIG.


14


. The other drum drive pulley


338


and the other adjustment pulley


342


respectively vary in speed in the same manner as the pulleys


336


and


340


although not shown specifically.




In

FIG. 14

, a solid waveform S


10


indicates the speed variation of the drum drive pulley


336


. A solid waveform S


11


indicates the speed variation of the adjustment pulley


340


; the origin of the speed variation is shown as coinciding with the origin of the speed variation of the drum drive pulley


336


for the sake of illustration. Further, a dashed waveform S


12


indicates the speed variation of the adjustment pulley


340


occurring when the eccentric position of the drum drive pulley


336


and that of the pulley


340


are different from each other. As shown, so long as the eccentric positions are coincident, just four periods of the adjustment pulley


340


or


342


occur in a single period of the drum drive pulley


336


or


338


.

FIG. 15

shows a solid waveform C


7


representative of the combined speed variation of the waveforms S


10


and S


11


of

FIG. 14

, and a dashed waveform C


8


representative of the combined speed variation of the waveforms


710


and S


12


of FIG.


14


. As shown, wherever a drum period may begin, the same waveform C


5


and C


6


each vary in the same manner in all drum periods. That is, the print drums


8


and


10


deviate from each other in the same manner in all drum periods and prevent an offset ghost from appearing.




While the illustrative embodiment includes the steer pulleys


344


, it is also capable of obviating an offset ghost with the above 1/integer configuration even if the steer pulleys


344


are absent.




When the steer pulleys


344


are present, the pitch circle diameter of each steer pulley


344


may also be selected to be 1/integer of the pitch circle diameter of each drum drive pulley


336


or


338


. Stated another way, the pitch circle diameter of each drum drive pulley


336


or


338


may be an integral multiple of the pitch circle diameter of each steer pulley


344


. For example, the ratio of the pitch circle diameter of each drum drive pulley


336


or


338


to that of each steer pulley


344


may be 5:1. The steer pulleys


344


have the same pitch circle diameter. In this case, as shown in

FIG. 16

, the steer pulleys


344


each have a pitch circle diameter d


1


extending to the pitch line, or core line position, t of the timing belt


320


.




The illustrative embodiment is a solution to the problem that the eccentricity of the steer pulleys


44


is also causative of a phase difference between the print drums


308


and


310


. Experiments showed that this embodiment could cope with an offset ghost at a higher level.




Assume that the adjustment pulleys


340


and


342


of the phase adjusting means


322


are not eccentric, but the steer pulleys


344


are eccentric. Then, an offset ghost can be control led only if the pitch circle diameter of the steer pulleys


344


is selected to be 1/integer of the pitch circle diameter of the drum drive pulleys


336


and


338


.




It is to be noted that


144


teeth and thirty-six teeth respectively assigned to the drum drive pulleys


336


and


338


and adjustment pulleys


340


and


342


, as mentioned earlier, are a preferable example of the ratio of 4:1. When the integral ratio of 4:1, 3:1 or 5:1 is selected in consideration of balance between accuracy and cost, the drum drive pulleys


336


and


338


should preferably have 108 to 180 teeth.




As shown in

FIG. 13

, the print drums


308


and


310


are connected to each other by an extremely simple mechanism not using precision gears. Specifically, the timing belt


320


is passed over the rotatable members implemented as the drum drive pulleys


336


and


338


, adjustment pulleys


340


and


342


, and steer pulleys


344


. Therefore, even if the rotatable members are eccentric, a phase difference between the print drums


308


and


310


does not occur so long as the pitch circle diameters of the rotatable members and that of the drum drive pulleys


336


and


338


are held in the 1/integer relation. However, the ratio of the number of teeth of the timing belt


320


to that of the drum drive pulleys


336


and


338


cannot be 1:1 due to the extremely simple configuration, so that only the eccentricity of the timing belt


320


itself may bring about a phase difference.




The above phase difference ascribable to the timing belt


320


effects the pitch circle diameter of the downstream drum drive pulley


338


. A deviation on the print drum


310


expected to form an image thereon is increased by the ratio of the diameter of the print drum


310


to the pitch circle diameter of the drum drive pulley


338


. It follows that an offset ghost can be reduced more positively as the pitch circle diameter of the drum drive pulleys


336


and


338


increases. However, because pulleys as large as the print drums


308


and


310


increase the cost, the pitch circle diameter of the drum drive pulleys


336


and


338


must be selected in consideration of balance between accuracy and cost.




Further, the accuracy of the timing belt


320


is the potential cause of an offset ghost, as stated above. The timing belt


320


should therefore be as accurate as possible and should consequently be provided with a belt pitch of 3 mm or less. On the other hand, considering the fact that the timing belt


320


should be rigid enough to withstand heavy loads in order to implement highly accurate drive transmission, the belt pitch should not be 2 mm or less. Consequently, the optimal belt pitch is 3 mm. It follows that when the ratio of the number of teeth of the drum drive pulleys


336


and


338


to that of the adjustment pulleys


340


and


342


is selected to be 4:1, 3:1 or 5:1, the timing belt


320


should preferably have a pitch of 3 mm while the drum drive pulleys


336


and


338


should preferably have


108


to


180


teeth each.




4th Embodiment





FIGS. 17 and 18

show still another alternative embodiment of the printer in accordance with the present invention. In

FIGS. 17 and 18

, structural elements identical with the structural elements shown in

FIGS. 12 through 16

are designated by identical reference numerals and will not be described specifically in order to avoid redundancy. As shown, the phase adjusting means


322


includes a frame


354


on which adjustment pulleys or plain pulleys


362


and


364


are mounted. The adjustment pulleys


362


and


364


are positioned close to each other and contact the rear of the timing belt


320


. Four toothed steer pulleys


366


are fixed in place between the adjustment pulleys


362


and


364


and the drum drive pulleys


336


and


338


and held in mesh with the timing belt


320


.




In the illustrative embodiment, to obviate an offset ghost, the adjustment pulleys


362


and


364


are provided with a pitch circle diameter that is 1/integer of the pitch circle diameter of the drum drive pulleys


336


and


338


. For example, the ratio of the pitch circle diameter of the drum drive pulleys


336


and


338


to that of the adjustment pulleys


362


and


364


is selected to be 4:1. The drum drive pulleys


336


and


338


have the same pitch circle diameter which is greater than the pitch circle diameter of the adjustment pulleys


362


and


364


. The adjustment pulleys


362


and


364


have the same pitch circle diameter. As shown in

FIG. 18

, the pulleys


362


and


364


each have a pitch circle diameter d


2


extending to the pitch line t of the timing belt


320


.




Again, even if the adjustment pulleys


362


and


364


are eccentric, a phase difference between the print drums


308


and


310


does not occur because of the 1/integer relation between the pitch circle diameters. This prevents an offset ghost from appearing.




While the illustrative embodiment also includes the steer pulleys


366


, it is also capable of obviating an offset ghost with the above 1/integer configuration even if the steer pulleys


366


are absent.




When the steer pulleys


366


are present, the number of teeth of the steer pulleys


366


may also be selected to be 1/integer of the number of teeth of the drum drive pulleys


336


and


338


. For example, the ratio of the number of teeth of the drum drive pulleys


336


and


338


to that of the steer pulleys


366


may be 4:1. In this case, the drum drive pulleys


336


and


338


have the same number of teeth which is greater than the number of teeth of the steer pulleys


366


. The steer pulleys


366


have the same number of teeth.




The illustrative embodiment is a solution to the problem that the eccentricity of the steer pulleys


366


meshing with the timing belt


320


is also causative of a phase difference between the print drums


308


and


310


. Experiments showed that this embodiment could cope with an offset ghost at a higher level.




Assume that the adjustment pulleys


362


and


364


of the phase adjusting means


322


are not eccentric, but the steer pulleys


366


are eccentric. Then, an offset ghost can be control led only if the number of teeth of the steer pulleys


366


is selected to 1/integer of the number of teeth of the drum drive pulleys


336


and


338


.




While the third and fourth embodiments each move the frame


354


of the phase adjusting means


322


up and down with a rack and pinion scheme, the rack and pinion scheme may be replaced with a screw shaft and nut scheme.




In summary, it will be seen that the present invention provides a printer having various unprecedented advantages, as enumerated below.




(1) Rotatable members for interlocked drive are so arranged as to insure the synchronous rotation of an upstream and a downstream print drum when a paper arrives at the downstream print drum. This is successful to obviate an offset ghost even if the rotatable members are eccentric. The printer can therefore obviate an offset ghost despite the interlocked drive system without increasing the cost.




(2) Adjustment pulleys included in phase adjusting means each have teeth the number of which is 1/integer of the number of teeth of each drum drive pulley. Therefore, a phase difference between the print drums ascribable to the eccentricity of the adjustment pulleys is obviated. This allows the printer to reduce offset ghosts with a minimum of cost particular to an interlocked drive system using a timing belt.




(3) Steer pulleys each have a pitch circle diameter which is 1/integer of the pitch circle diameter of each drum drive pulley. This obviates a phase difference between the print drums ascribable to the eccentricity of the steer pulleys and thereby reduces offset ghosts at a high level. In addition, the low cost configuration of an interlocked drive system using a timing belt is also available.




(4) The adjustment pulleys (plain pulleys) each have a pitch circle diameter which is /1 integer of the pitch circle diameter of each drum drive pulley. Therefore, a phase difference between the print drums ascribable to the eccentricity of the adjustment pulleys is obviated. This also allows the printer to reduce offset ghosts with a minimum of cost particular to an inter locked drive system using a timing belt.




(5) The steer pulleys (toothed pulleys) each have teeth the number of which is 1/integer of the number of teeth of each drum drive pulley. The printer therefore obviates a phase difference between the print drums ascribable to the eccentricity of the steer pulleys and thereby reduces offset ghosts at a high level. In addition, the low cost configuration of an interlocked drive system using a timing belt is also available.




Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.



Claims
  • 1. A printer comprising:a plurality of print drums spaced from each other in a direction in which a recording medium is conveyed; and a plurality of rotatable members associated with said plurality of print drums, and configured to drive the plurality of print drums in an interlocked fashion, wherein a rotating period of each of the plurality of rotatable members is less than or equal to a rotating period of the plurality of print drums, and is preselected so each rotatable member makes, in a single period of the print drums, a number of rotations that is an integral multiple of a number of rotations of the print drums so as to prevent an upstream and a downstream print drum rotating synchronously to each other from being brought out of synchronism when the recording medium arrives at said downstream print drum.
  • 2. A printer as claimed in claim 1, wherein one of said plurality of rotatable members comprises a timing belt passed over said plurality of print drums.
  • 3. A printer as claimed in claim 2, wherein said timing belt has a single period equal to the single period of the plurality of print drums.
  • 4. A printer as claimed in claim 1, wherein the plurality of print drums comprise at least first and second print drums, andwherein the plurality of rotatable members comprises: first and second rotatable drive members respectively associated with the first and second print drums and configured to rotate the print drums; first and second relay gears fixed in place and held in mesh with the first and second print drums, respectively, each of the first and second relay gears respectively including first and second timing pulleys; first and second rotatable adjustment members associated with and disposed between the first and second print drums, and configured to adjust a rotational phase between the first and second print drums; a timing belt passing over the first and second relay gears, and the first and second rotatable adjustment members so as to synchronously rotate the first and second print drums; and four steer pulleys fixed in place between the first and second rotatable adjustment members with a backside of the timing belt pass thereover, and configured to steer the timing belt.
  • 5. A printer as claimed in claim 4, wherein:a number of teeth of each of the first and second rotatable drive members is an integral multiple of a number of teeth of each of the first and second relay gears; a number of teeth of each of the first and second timing pulleys is the same as a number of teeth of each of the first and second rotatable adjustment members; and a number of grooves of the timing belt is an integral multiple of the number of teeth of each of the timing pulleys.
  • 6. A printer as claimed in claim 5, wherein a pitch circle diameter of each of the first and second timing pulleys is the same as a pitch circle diameter of each of the four steer pulleys.
Priority Claims (3)
Number Date Country Kind
11-185285 Jun 1999 JP
11-185435 Jun 1999 JP
2000-022335 Jan 2000 JP
US Referenced Citations (3)
Number Name Date Kind
6038968 Hara et al. Mar 2000 A
6109176 Fujio et al. Aug 2000 A
6314877 Takasawa Nov 2001 B1
Foreign Referenced Citations (3)
Number Date Country
4-329175 Nov 1992 JP
7-017121 Jan 1995 JP
11-129600 May 1999 JP