Abstract:
A printer including drive pulleys each being mounted on a particular print drum. Each of the print drums has a pitch circle diameter and a number of teeth related to each other as: 
     
       
           d/z &lt;1 
       
     
     where d denotes the pitch circle diameter (mm) and z denotes the number of teeth.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a printer including a plurality of print drums arranged to produce a color print by passing a paper sheet or similar recording medium only once, and a device for causing a plurality of rotary members to rotate in synchronism with each other. 
     Today, a stencil printer capable of producing a great number of prints at low cost is extensively used. The stencil printer includes a plurality of print drums arranged side by side in a direction in which a paper sheet or similar recording medium is conveyed. The print drums each are assigned to a particular color. While a paper sheet is passed only once, an image of the first color to an image of the last color are sequentially transferred from the print drums to the paper sheet one above the other, completing a color image. While such a single pass system is more efficient than a system of the type replacing a print drum color by color, it has problems ascribable to a short distance between the print drums. 
     Specifically, an ink image transferred from an upstream print drum assigned to, e.g., a first color reaches the nip of a downstream print drum assigned to, e.g., a second color in a wet state. As a result, the ink image is transferred to a master or perforated stencil wrapped around the downstream print drum and then to the next paper sheet. 
     More specifically, the transfer of the wet ink of the first color to the master wrapped around the downstream print drum does not matter for the first paper sheet. As for the second paper sheet, however, the ink of the first color is transferred from the above master to an image of the first color transferred from the upstream print drum to the paper sheet (so-called retransfer). Retransfer, i.e., the overlap of ink of the same color is not critical in the aspect of image quality if free from positional deviation. However, if the retransferred image is deviated from the original image, an offset ghost appears on the paper sheet. For a given amount of deviation, an offset ghost causes a thick line to appear blurred and causes a thin line to appear doubled, lowering image quality to a critical degree. 
     Retransfer stated above is not avoidable with a single pass type of color printer. An offset ghost is, however, ascribable to the positional deviation of transfer and can therefore be accurately reduced if the upstream and downstream print drums accurately rotate in synchronism with each other for thereby conveying a paper sheet with accuracy. 
     To reduce an offset ghost, it has been customary to connect the upstream and downstream print drums as to drive. Japanese Patent Laid-Open Publication No. 4-329175, for example, teaches a system that connects the shafts of the print drums by using a plurality of gears. Japanese Patent Laid-Open Publication No. 7-17121, for example, proposes a system that connects the print drums by using timing pulleys and a timing belt. 
     The gear scheme is capable of reducing the deviation of an offset ghost. This scheme, however, uses a plurality of precision gears and therefore increases the production cost. The timing belt scheme produces an offset ghost and, moreover, aggravates deviation thereof, as will be described specifically later with reference to the accompanying drawings. 
     Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-62737. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a printer capable of reducing an offset ghost while using a timing belt. 
     It is another object of the present invention to provide a printer capable of reducing an offset ghost at low cost while using a timing belt. 
     In a synchronous driving device including a drive member and a driven member, drive pulleys each being mounted on one of the drive member and driven member, and a timing belt passed over the drive pulleys for causing the drive member and driven member to rotate in synchronism with each other, the drive pulleys each has a pitch circle diameter and a number of teeth related as: 
     
       
           d/z&lt; 1 
       
     
     where d denotes a pitch circle diameter and z denotes a number of teeth. 
     In the above configuration, the drive pulleys each may have a pitch of 3 mm or less. 
     Also, in accordance with the present invention, in a printer including a plurality of print drums spaced from each other in the direction of paper conveyance, toothed drive pulleys each being mounted on one of the print drums, and a timing belt passed over the toothed drive pulleys for allowing the print drums to rotate in synchronism with each other, the toothed drive pulleys each has a pitch circle diameter and a number of teeth related as: 
     
       
           d/z&lt; 1 
       
     
     where d denotes a pitch circle diameter and z denotes a number of teeth. 
     In the above configuration, the drive pulleys each may have a pitch of 3 mm or less. 
     Further, in accordance with the present invention, a device for driving a drive side and a driven side in synchronism includes a rotary member with a toothed drive pulley positioned at the drive side, a rotary member with a toothed drive pulley positioned at the driven side, and a timing belt passed over the toothed drive pulleys for causing the rotary members to rotate in synchronism with each other. The toothed drive pulleys each has a pitch circle diameter and a number of teeth related as: 
     
       
           d/z&lt; 1 
       
     
     where d denotes a pitch circle diameter and z denoted a number of teeth. 
     In the above construction, the drive pulleys each may have a pitch of 3 mm or less. 
    
    
     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 connecting system using a timing belt included in a conventional stencil printer; 
     FIG. 2 is a fragmentary view showing a relation between the positional deviation of a core wire included in the timing belt and the axis of a drive pulley mounted on a print drum; 
     FIG. 3 is a fragmentary view of the timing belt showing a relation between a pitch and the size of a tooth; 
     FIG. 4 is a graph showing velocity variations to occur when a pulley for adjustment, for example, is eccentric; 
     FIG. 5 is a graph showing combined waveforms derived from the waveforms of FIG. 4; 
     FIG. 6 is a graph showing velocity variations to occur when the drive pulley and a timing belt are eccentric; 
     FIG. 7 is a graph showing combined waveforms derived from the waveforms of FIG. 6; 
     FIG. 8 is a graph plotting the deviations of rotation of a print drum in terms of the sum of areas derived from the waveforms of FIG. 7; 
     FIG. 9 is a front view showing a printer or a synchronous driving device embodying the present invention and implemented as a stencil printer by way of example; 
     FIG. 10 is an isometric view showing a phase adjusting device included in the illustrative embodiment; 
     FIG. 11 is a view showing a relation between the pitch circle diameter of a drive pulley included in the illustrative embodiment and the deviation of an image on a print drum; 
     FIG. 12 is a graph showing velocity variations to occur when a pulley for adjustment, for example, is eccentric; 
     FIG. 13 is a graph showing combined waveforms derived from waveforms of FIG. 12; and 
     FIG. 14 is a view showing the concept of the pitch circle diameter of a pulley for deflection. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To better understand the present invention, reference will be made to a conventional single path, color stencil printer constructed to obviate offset ghosts, shown in FIG.  1 . As shown, the stencil printer includes two print drums  100  and  102  spaced from each other in a direction in which a paper sheet or similar recording medium P is conveyed. The print drums  100  and  102  are respectively located at the upstream side and downstream side in the above direction. Toothed drive pulleys  104  and  106  are respectively mounted on the print drums  100  and  102 , serving as timing pulleys. 
     A timing belt  108  is passed over the drive pulleys  104  and  106 . In this configuration, the print drums  100  and  102  are driven while being connected together by the timing belt  108 . A phase adjusting device  110  is positioned between the print drums  100  and  102 . The phase adjusting device  110  adjusts a relative phase between the print drums  100  and  102 , i.e., corrects a positional deviation between a first and a second color in the direction of paper conveyance or top-and-bottom direction. 
     Specifically, the phase adjusting device  110  includes a frame  112  movable up and down by being driven by drive means not shown. Toothed pulleys  114   a  and  114   b  for adjustment are rotatably mounted on the upper end and lower end of the frame  112 , respectively, and held in mesh with the timing belt  108 . Two pulleys  116  are fixed in place between the pulleys  114   a  and  114   b  and the print drum  100  while other two pulleys  116  are fixed in place between the pulleys  114   a  and  114   b  and the print drum  102 . These pulleys  116  deflect the timing belt  108  and allow phase adjustment to be efficiently effected on the basis of the displacement of the phase adjusting means  110  in the up-and-down direction. The pulleys  116 , which contact the rear surface of the timing belt  108 , are implemented by spur pulleys. Press rollers  118  and  120  are movable into and out of contact with the print drums  100  and  102 , respectively. 
     When the frame  112  and therefore the pulleys  114   a  and  114   b  for adjustment are moved upward, the print drums  100  and  102  are caused to rotate in directions a and b, respectively, and vary their phases. When the frame  112  is moved downward, the phases of the print drums  100  and  102  are varied in the opposite direction. The phase adjusting device  110  is capable of correcting a positional deviation between images to be printed on the paper sheet P in the direction of paper conveyance and is essential with a color stencil printer. The deviation is ascribable to a change in print speed by way of example. 
     If an upstream and a downstream print drum are connected together by a timing belt and accurately rotated in synchronism with each other, as stated above, then no offset ghosts appear on a paper sheet. In practice, however, offset ghosts appear due to various causes, as will be described hereinafter. 
     At the moment when the teeth of the timing belt  108  and those of the drive pulley  104  or  106  mesh with each other or when they leave each other, the angular velocity of the drive pulley  104  or  106  varies, resulting in so-called jitter. Jitter occurs once for a single tooth during rotation. Jitter increases with an increase in period with the result that the deviation of the rotation of the downstream print drum  102  from the rotation of the upstream drum  100  increases. Therefore, to reduce the deviation of the downstream print drum  102 , it is necessary to provide the drive pulleys  104  and  106  with as many teeth as possible for increasing jitter frequency for a single rotation of the drive pulleys  104  and  106 . 
     Another cause of an offset ghost is irregularity in the position of a core wire included in the timing belt  108  and defining a pitch circle. The core wire guarantees drive transmission from the timing belt  108  to the drive pulleys  104  and  106 . Specifically, as shown in FIG. 2, the timing belt  108  is generally made up of tooth portions  108   a , a back portion  108   b , and a core wire  108   c  sandwiched between the tooth portions  108   a  and the back portion  108   b . If the core wire  108   c  is not accurately positioned, the distance between the axis of the drum pulley  104  or  106  and the core wire  108   c  varies (r 1 ≠r 2 ). Consequently, the rotation angle of the drive pulley  104  or  106  varies. 
     Generally, as shown in FIG. 3, the timing belt  108  has a pitch n that is a distance between nearby tooth portions  108   a . The thickness m of the timing belt  108  increases with an increase in pitch n and makes it difficult to accurately position the core wire  108   c . It follows that to reduce the deviation of rotation of the downstream print drum  102 , a timing belt having as small a pitch as possible must be selected. 
     Further, in the phase adjusting device  110 , the pulleys  116  contact the rear surface of the timing belt  108 . Consequently, irregularities in the thickness of the timing belt  108  over the entire length of the belt  108  aggravate eccentricity. 
     Moreover, the drive pulleys  104  and  106  and pulleys  114   a  and  114   b  each involves some eccentricity due to limited machining accuracy and assembling accuracy. As for the drive pulleys  104  and  106 , eccentricity does not disturb the synchronous rotation of the print drums  100  and  102  because an offset ghost appears only once for a single rotation of the print drums  100  and  102 , i.e., a single rotation of the drive pulleys  104  and  106 . However, the eccentricity of the pulleys  104   a  and  104   b  disturbs the relative phase between the print drums  100  and  102  every time the pulleys  114   a  and  114   b  rotate or, when the timing belt  108  involves an eccentricity component, every time the drive pulleys  104  and  106  rotate. Why an offset ghost appears when the pulleys  114   a  and  114   b  are eccentric will be described hereinafter with reference to FIGS. 4 and 5. 
     Assume that the ratio of the number of teeth of the drive pulley  104  or  106  to that of the pulley  114   a  or  114   b  is 4.3:1, i.e., the former is a non-integral multiple of the latter. Also, assume that the drive pulley  104  or  106  and pulley  114   a  or  114   b  are eccentric. FIG. 4 shows waveforms representative of the velocity variations of only the drive pulley  104  and pulley  114   a  by way of example measured under the above conditions. Specifically, a solid waveform S 1  shows the velocity variation of the drive pulley  104 . A solid waveform S 2  shows the velocity variation of the pulley  114   a ; the origin of the waveform S 2  is shown as being coincident with the origin of the waveform S 1  for better understanding the relation. A phantom waveform S 3  shows the velocity variation of the pulley  114   a  occurred when the drive pulley  104  and pulley  104   a  were different from each other in the position of eccentricity. As the waveform S 3  indicates, the waveform of the pulley  114   a  has an origin that is, in many cases, not coincident with the original of the waveform of the drive pulley  104 . 
     FIG. 5 shows a solid waveform C 1 , which is a combined form of the waveforms S 1  and S 2  of FIG. 4, and a phantom waveform C 2 , which is a combined form of the waveforms S 1  and S 3  of FIG.  4 . As shown, wherever one drum period may begin, the velocity varies in a different manner every period. Consequently, the deviation between the print drums  100  and  102  varies in a different manner every period, resulting in an offset ghost. 
     Reference will be made to FIGS. 6 through 8 for describing an offset ghost ascribable to the eccentricity of the timing belt  108 . Assume that the ratio of the number of teeth of the drive pulley  104  or  106  to that of the timing belt  108  is 1:2.5, i.e., the latter is a non-integral multiple of the former. Also, assume that the drive pulley  104  or  106  and timing belt  108  are eccentric. FIG. 6 shows waveforms representative of the velocity variations of only the drive pulley  106  and timing belt  108  by way of example measured under the above conditions. 
     In FIG. 6, a solid waveform S 4  shows the velocity variation of the drive pulley  106 . A solid waveform S 5  shows the velocity variation of the timing belt  108 ; the origin of the waveform S 5  is shown as being coincident with the origin of the waveform S 4  for better understanding the relation. A phantom waveform S 6  shows the velocity variation of the drive pulley  106  occurred when the drive pulley  106  and timing belt  108  were different from each other in the position of eccentricity. As the waveform S 6  indicates, the waveform of the drive timing belt  108  has an origin that is, in many cases, not coincident with the original of the waveform of the drive pulley  106 . 
     FIG. 7 shows a solid waveform C 3 , which is a combined form of the waveforms S 4  and S 5  of FIG. 6, and a phantom waveform C 4 , which is a combined form of the waveforms S 5  and S 6  of FIG.  6 . As shown, wherever one period begins, the velocity varies in a different manner every period. However, the timing belt  108  and drive pulley  106  respectively have two periods and five periods because of the preselected relation in the number of teeth. The waveform C 3  therefore has the same pattern repeating every five periods of the drive pulley  106 . 
     FIG. 8 plots the sums of the areas of hatched portions shown in FIG. 7 that occur during every period of the drive pulley  106 . Each sum indicates a particular deviation of the synchronism of the drive pulley  106 . It will be seen that the synchronism of the drive pulley  106  repeatedly deviates by the same amount every two periods of the timing belt  108 . 
     Generally, gears, a timing belt and so forth that connect print drums involve some eccentricity do to limited machining accuracy, so that velocity unavoidably varies during one rotation. A gear train connecting print drums is highly rigid and allows the deviation of an offset ghost to be reduced if the accuracy of the individual gear is increased. However, using a plurality of precision gears is undesirable from the cost standpoint. 
     On the other hand, a timing belt connecting print drums reduces the overall cost because timing pulleys or similar low-cost parts, which can be produced by injection molding or similar technology on a quantity basis, suffice. This, however, brings about the previously discussed problem that the eccentricity of the timing belt and timing pulleys aggravates the deviation of an offset ghost. 
     The problems stated above are not particular to a printer, but also occur in any other synchronous drive system in which a timing belt connects a drive member and a driven member. 
     As stated above, to reduce the deviation of rotation of a downstream print drum, it is necessary to provide each drive pulley with as many teeth as possible and to raise jitter frequency during one rotation of the drive pulley. To enhance the positional accuracy of a core wire, a timing belt whose pitch is as small as possible is required. These are the rough measures against an offset ghost ascribable to a connecting system using a timing belt. However, when it comes to a thin timing belt having a small pitch, another problem is brought about as to, e.g., durability in the event of heavy-load drive. The reduction of an offset ghost from this standpoint has not been addressed to yet. 
     Referring to FIG. 9, a stencil printer embodying the present invention and representative of a printer or a synchronous driving device will be described. As shown, the stencil printer, generally  302 , includes two print drums  308  and  310 . Paper feeding means  304  feeds a paper sheet P toward the print drums  308  and  310  via a registration roller pair  306 . The print drums  308  and  310  are spaced from each other in the direction in which the paper sheet P is conveyed. A moving mechanism, not shown, moves a press roller  312  into and out of contact with the upstream print drum  308 . Intermediate conveying means  314  is positioned between the print drums  308  and  310  for conveying the paper sheet P and includes an endless belt. A moving mechanism, not shown, moves a press roller  316  into and out of contact with the downstream print drum  310 . Outlet conveying means  318  conveys the paper sheet P peeled off from the print drum  310  to a print tray not shown. A timing belt  320  connects the print drums  308  and  310 . Phase adjusting means adjusts a relative phase between the print drums  308  and  310 . 
     A main motor  325  causes the upstream print drum, or drive member,  308  via a main drive belt  323 . The rotation of the print drum  308  is transmitted to the downstream print drum, or driven member,  310  via the timing belt  320 . A pulley  327  applies tension to the main drive belt  323 . 
     The paper feeding means  304  includes a tray  324  loaded with a stack of paper sheets P and intermittently movable upward. A pickup roller  326 , a separator roller  328  and a separator pad  330  cooperate to pay out the top paper sheet P from the tray  324  toward the registration roller pair  306 . 
     The registration roller pair  306  corrects, e.g., the skew of the paper sheet P. The roller pair  306  then drives the paper sheet P toward the print drum  308  at such a timing that the leading edge of the paper sheet P meets the leading edge of an image formed on the print drum  308 . 
     Ink feeding means, not shown, is arranged within the print drum  308  and feeds ink of a first color to the inner periphery of the drum  308 . The press roller  312  presses the paper sheet P arrived at the print drum  308  against the drum  308  via a master, which is wrapped around the drum  308 . As a result, the ink is transferred to the paper sheet P via the porous portion of the print drum  308  and perforations formed in the master, printing an image on the paper sheet P in the first color. The press roller  312  is intermittently pressed against the print drum  308  so as not to interfere with a master damper  332  mounted on the drum  308 . 
     Peeling means peels off the paper sheet P carrying the image thereon from the print drum  308 . Subsequently, the previously mentioned belt included in the intermediate conveying means  314  conveys the paper sheet. At this instant, a fan also included in the conveying means  314  sucks the paper sheet P to thereby retain the paper sheet P on the belt. The conveying means  314  conveys the paper sheet P at a linear velocity preselected times higher than the linear velocity of the paper sheet P. 
     Ink feeding means, not shown, is also arranged within the print drum  310  and feeds ink of a second color to the inner periphery of the drum  310 . As the paper sheet P with the image of the first color arrives at a nip between the downstream print drum  310  and the press roller  316 , the roller  316  presses the paper sheet P against the drum  310 . As a result, the ink is transferred to the paper sheet P via the porous portion of the print drum  308  and perforations formed in the master, printing an image on the paper sheet P in the second color over the image of the first color. The press roller  316  is intermittently pressed against the print drum  310  so as not to interfere with a master damper  334  mounted on the drum  310 . 
     Peeling means, not shown, peels off the paper sheet or bicolor print P from the print drum  310 . Subsequently, a belt included in the outlet conveying means  318  conveys the bicolor print P to the print tray not shown. At this instant, a fan also included in the conveying means  318  sucks the print P to thereby retain it on the belt. 
     As shown in FIG. 10, the print drums  308  and  310  are mounted on shafts  350  and  352 , respectively. Toothed drive pulleys, or timing pulleys,  336  and  338  are respectively mounted on the rear ends of shafts  350  and  352  (front ends as viewed in FIG. 10) such that the print drums  308  and  310  are replaceable. A timing belt  320  is passed over the drive pulleys  336  and  338 . 
     The phase adjusting means  322  includes a frame  354  elongate in the up-and-down direction. An upper pulley  340  and a lower pulley  342  for adjustment are respectively mounted on the upper end and lower end of the frame  354 , playing the role of timing pulleys. Four pulleys  344  are fixed in place between the pulleys  340  and  342  and the drive pulleys  336  and  338 , as illustrated. The pulleys  344  allow the relative phase to be efficiently adjusted by a small displacement of the frame  354 . The pulleys  344  play the role of tension pulleys at the same time. The phase adjusting means  322  additionally includes a rack  354   a  formed in the frame  354 , a pinion, not shown, meshing with the rack  354   a , and a motor, not shown, for driving the pinion. 
     As shown in FIG. 10, elongate slots  354   b  and  354   c  are respectively formed in the upper portion and lower portion of the frame  354 , and each extends in the up-and-down direction. Guide pins  356  and  358  are studded on a sidewall, not shown, included in the printer body. The guide pins  356  and  358  are received in the slots  354   b  and  354   c , respectively. The frame  354  is movable up and down while being guided by the guide pins  356  and  358  and guide members, not shown, affixed to the sidewall of the apparatus body. 
     The pulleys, or spur pulleys,  344  each are rotatably mounted on a respective shaft  360  affixed to the sidewall of the printer body. The pulleys  344  contact the rear surface of the timing belt  320  while squeezing the belt  320 , as illustrated. 
     Assume that the pinions, not shown, are rotated to cause the frame  354  to move upward in a direction X. Then, the pulleys  340  and  342  are moved upward together with the frame  354 , causing the print drums  308  and  310  to rotate in directions a and b, respectively. As a result, a relative phase between the print drums  208  and  310  varies so as to correct a positional deviation between the first and second colors. When the pinion is rotated in the opposite direction, the frame  354  is moved downward in a direction Y and effects phase adjustment in the opposite direction. 
     The drive pulleys  336  and  338  have the same number of teeth, which is greater than the number of teeth of the pulleys  340  and  342  included in the phase adjusting means  322 . The pulleys  340  and  342  have the same number of teeth. 
     In the illustrative embodiment, the drive pulleys  336  and  338  each has a pitch circle whose diameter is smaller than 1 when divided by the number of teeth, i.e., smaller than the number of teeth. The timing belt  320  has a pitch of 3 mm or less. These conditions were derived from the following experiments. 
     In a first experiment, the print drums  308  and  310  each had a diameter of 180 mm while the drive pulleys  336  and  338  each had a pitch circle diameter of 152.79 mm and ninety-six teeth (pitch of 5 mm). The maximum deviation of rotation of the downstream drive pulley  338  was 0.17 mm, as measured at the pitch circle diameter portion of the pulley  338 . In a second experiment, the print drums  308  and  310  each had a diameter of 180 mm while the drive pulleys  336  and  338  each had a pitch circle diameter of 137.51 mm and 144 teeth (pitch of 3 mm). The maximum deviation of rotation of the downstream drive pulley  338  was 0.1 mm, as measured at the pitch circle diameter portion of the pulley  338 . 
     The results of the first and second experiments indicate that what is important for the reduction of an offset ghost is to provide the drive pulleys  336  and  338  or the timing belt  320  with a small pitch. 
     How the pitch circle diameter of the drive pulleys  336  and  338  and the pitch of the timing belt  320  should be selected will be described hereinafter. Assume that a plurality of print drums are interconnected by a timing belt, and that the upstream and downstream drums should be accurately synchronized to each other. Then, the greater the pitch circle diameter of the drive pulleys, the smaller the influence of the deviation ascribable to the timing belt on an image. This will be described with reference to FIG.  11 . 
     As shown in FIG. 11, assume that the print drums  308  and  310  each has a diameter D, that the drive pulleys  336  and  338  each has a pitch circle diameter d, and that a deviation a has occurred on the pitch circle of the timing belt  320 . Then, the deviation of an image on the print drum  308  or  310  is expressed as (a×D/d) and inversely proportional to d. The drive pulleys  336  and  338  may therefore be provided with a pitch circle diameter d equal to or greater than the diameter D of the print drums  308  and  310 . This, however, increases the cost and thereby impairs or practically cancels the merit of the connecting system using a timing belt. Moreover, if the pitch circle diameter d is increased, then the distance between the print drums  308  and  310  must be increased in order to insure the effective function of the phase adjusting means  322 , scaling up the layout. 
     For the reasons described above, the pitch circle diameter d should lie in a range that is not excessively large or excessively small. For example, assume a printer capable of printing images of size A3 or below. Then, the drive pulleys  336  and  338  each has a diameter of 160 mm to 190 mm. It follows that if D/d=about 1.3, then the pitch circle diameter d of the drive pulleys  336  and  338  is 120 mm to 150 mm. 
     As for the second experiment in which the print drums  308  and  310  have a diameter of 180 mm, the maximum deviation of an image on the drum  308  or  310  ascribable to the drive of the timing belt  320  is 0.1×180/137.51=0.13 mm in width. 
     In the first experiment not improving an offset ghost, the ratio of the pitch circle diameter of the drive pulleys  336  and  338  to the number of teeth of the same is 152.79/96=1.59, which is greater than 1. By contrast, in the second experiment improving an offset ghost, the above ratio is 137.51/144=0.95, which is smaller than 1. 
     To reduce the deviation of rotation of a downstream drum connected to an upstream drum by a timing belt, not only high accuracy but also rigidity great enough to withstand heavy loads are essential. Generally, to cope with heavy loads, it is a common practice to use a thick timing belt having a great pitch or a wide belt having a small pitch. 
     The illustrative embodiment satisfies the above-stated ratio of the pitch circle diameter to the number of teeth that is smaller than 1. The pitch circle diameter of the drive pulleys  336  and  338  is therefore as great as 120 mm to 150 mm as a relative condition (in the case of a printer assigned to size A3 or below). The illustrative embodiment therefore achieves a sufficient transmission ability even if the timing belt  320  has a small pitch, i.e., even if use is made of a commercially available timing belt whose pitch is 3 mm or less for accurate transmission. Such a timing belt not only increases jitter frequency, but also insures highly accurate position of the core wire of the timing belt  320 , thereby accurately reducing an offset ghost ascribable to the timing belt  320 . 
     How the illustrative embodiment reduces an offset ghost ascribable to the eccentricity of rotary members other than the timing belt  320  will be described hereinafter. In the illustrative embodiment, the pulleys  340  and  342  for adjustment each has a number of teeth that is 1/integer of the number of teeth of the drive pulley  336  or  338 . Stated another way, the drive pulleys  336  and  338  each has a number of teeth that is an integral multiple of the number of teeth of the pulley  340  or  342 . For example, when the drive pulley  336  or  338  has 144 teeth, the pulley  340  or  342  has thirty-six teeth. In this condition, even if the pulleys  340  and  342  are eccentric, no deviation in Aid phase or synchronous rotation occurs between the print drums  308  and  310  because of the relation in the number of teeth. Consequently, an offset ghost is successfully reduced. This will be described more specifically hereinafter with reference to FIGS. 12 and 13. 
     FIG. 12 shows velocity variations ascribable to the eccentricity of the drive pulleys  336  and  338  and that of pulleys  340  and  342  and determined when the ratio of the number of teeth of the pulleys  336  and  338  to that of the pulleys  340  and  342  was 4:1. It is to be noted that FIG. 12 concentrates on the drive pulley  336  and pulley  340  by way of example. 
     In FIG. 12, a solid waveform S 7  shows the velocity variation of the drive pulley  336 . A solid waveform S 8  shows the velocity variation of the pulley  340 ; the origin of the waveform S 8  is shown as being coincident with the origin of the waveform S 7  for better understanding the relation. A phantom waveform S 9  shows the velocity variation of the pulley  340  occurred when the drive pulley  336  and pulley  340  were different from each other in the position of eccentricity. If positions of eccentricity are coincident, just four periods of the pulley  340  ( 342 ) occur during one period of the drive pulley  336  ( 338 ). 
     FIG. 13 shows a solid waveform C 5 , which is a combined form of the waveforms S 7  and S 8  of FIG. 12, and a phantom waveform C 6 , which is a combined form of the waveforms S 8  and S 9  of FIG.  12 . As shown, wherever one period begins, the velocity varies in the same manner every period, i.e., the velocity varies in the same pattern on both of the waveforms C 5  and C 6 . It follows that the print drums  308  and  310  deviate in the same manner every period, obviating an offset ghost. 
     The illustrative embodiment includes the pulleys  344  for deflecting the timing belt  320 . Even if the pulleys  344  are absent, the previously stated condition of 1/integer successfully reduces an offset ghost for the reasons described above. 
     When the pulleys  344  are present, the pitch circle diameter of the pulleys  344  may be selected to be 1/integer of the pitch circle diameter of the drive pulleys  336  and  338  in addition to the previous condition of 1/interger relating to the number of teeth. Stated another way, the drive pulleys  336  and  338  each has a pitch circle diameter that is an integral multiple of the pitch circle diameter of the pulleys  344 . For example, when the ratio of the pitch circle diameter of the drive pulleys  336  and  338  to that of the pulleys  344  may be selected to be 5:1. The pulleys  344  have the same pitch circle diameter. In this case, as shown in FIG. 14, the pulleys  344  each has a pitch circle diameter d 1  extending to the pitch line (core wire) t of the timing belt  320 . 
     Assume that the pulleys  340  and  342  for adjustment are free from eccentricity, but the pulleys  344  for deflection are eccentric. Then, an offset ghost can be reduced only if the pitch circle diameter of the pulleys  344  are selected to be 1/integer of the pitch circle diameter of the drive pulleys  336  and  338 . 
     144 teeth assigned to the drive pulleys  336  and  338  and  36  teeth assigned to the pulleys  340  and  342  are a preferred example of the ratio of 4:1. If the ratio of 4:1 using other numbers of teeth or another integral ratio of 3:1 or 5:1 is selected in consideration of balance between accuracy and cost, then the number of teeth of the drive pulleys  336  and  38  should be between 108 and 180. 
     As shown in FIG. 9, the illustrative embodiment connects the print drums  308  and  310  simply with the timing belt  320  passed over the drive pulleys  336  and  338  and rotary members including pulleys  340  and  342  for adjustment and pulleys  344  for deflection. This obviates the need for precision gears. Therefore, even if any one of the above rotary members is eccentric, the ratio of the pitch circle diameter of the individual rotary member to that of the drive pulley  336  and  338  remains to be 1/integer, obviating a phase difference between the print drums  308  and  310 . However, the ratio of the number of teeth of the timing belt  320  to that of the drive pulleys  336  and  338  cannot be 1:1 due to the extremely simple connecting scheme. As a result, the eccentricity of the timing belt  320  itself is the only possible cause of phase deviation. 
     Nevertheless, the timing belt  320  can be implemented by one having a pitch as small as 3 mm or less and a core wire highly accurately positioned. Therefore, an offset ghost ascribable to the timing belt  320  can be reduced. This, coupled with the reduction of an offset ghost ascribable to the other rotary members, promotes accurate reduction of an offset ghost of the entire printer. 
     While the illustrative embodiment has concentrated on a synchronous driving device arranged between nearby print drums of a printer, it is similarly applicable to any other synchronous drive arrangement between a drive member and a driven member. 
     In summary, it will be seen that the present invention provides a printer capable of increasing jitter frequency as to the meshing of a timing belt and timing pulleys and insuring highly accurate position of a core wire included in the belt. The printer therefore accurately reduces an offset ghost and other troubles ascribable to the timing belt. 
     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.