Abstract:
A positioning apparatus for an optical head includes a cylindrical photoconductive drum, an optical head, and at least one spacer. The cylindrical photoconductive drum extends in a direction of a longitudinal axis thereof. The optical head extends parallel to the longitudinal axis. The spacer is disposed to abut the photoconductive drum, limiting a distance between the optical head and the photoconductive drum. The photoconductive drum has a photoconductor and the spacer is in contact with the photoconductor through sliding friction. The spacer has a first surface in contact with the photoconductor. The photoconductor has a second surface in contact with the first surface. The first surface has a first curvature and the second surface has a second curvature. When the first surface is pressed against the second surface, the spacer may deform resiliently so that the first curvature becomes substantially equal to the second curvature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical head positioning apparatus. 
     2. Description of the Related Art 
     An electrophotographic printer that employs an LED head incorporates a photoconductive drum, which is positioned so that a charged surface of the photoconductive drum is at a focal point of a convergent lens such as a SELFOC lens array (SLA). During an exposure process of the electrophotographic printer, light emitted from LED array chips illuminates the surface of the photoconductive drum through the SLA to form an electrostatic latent image on the surface. 
     FIG. 35 is a cross-sectional view of a conventional LED head. 
     An LED array chip  1  is mounted on a printed circuit board  3 . An SLA holder  4  holds an SLA  2  thereon. A base  5  holds the printed circuit board  3 , SLA holder  4 , and SLA  2  thereon and accurately positions them relative to the surface of the photoconductive drum  6 . In order to focus an image on the photoconductive drum  6 , the LED head requires to be accurately positioned with respect to the photoconductive drum  6 . Thus, the LED head is positioned so that a distance Lo from the LED chip  1  to a light-entering surface of the SLA  2  is equal to a distance Li from the light-exiting surface of the SLA  2  to a focal point on the photoconductive drum  6 . The SLA  2  is the distance Lo away from the LED array chip  1  and is fixed to the SLA holder  4  by an adhesive. In other words, the distance Lo cannot be adjusted once the SLA  2  has been mounted on the SLA holder  4 . Thus, the photoconductive drum  6  should be positioned accurately relative to the LED head so that the distance Lo is equal to the distance Li. 
     FIG. 36 is a front view of the conventional LED head. 
     The positional relation between the conventional photoconductive drum  6  and the LED head will be described with reference to FIG.  36 . The photoconductive drum  6  has one axial end to which a gear  7  is mounted and the other axial end to which a flange  11  is mounted. The gear  7  and flange  11  are formed with a hole  9  and a hole  10  therein, respectively, through which a shaft  8  of the photoconductive drum  6  extends. The gear  7  and flange  11  rotate on the shaft  8 . The gear  7  is driven in rotation by a drive source, not shown, thereby driving the photoconductive drum  6  to rotate. 
     The photoconductive drum  6  is disposed in an ID unit, not shown, and is covered with an upper frame  16  such that the photoconductive drum  6  is shielded from light except a surface area that opposes the light-exiting end of the SLA  2 . The shaft  8  is rotatably supported at its longitudinal end portions by side frames  12   a  and  12   b  of the ID unit. Adjusting mechanisms  13   a  and  13   b  are disposed under both end of the SLA holder  4  and operated to adjust the distance Li between the light-exiting end of the SLA  2  and the surface of the photoconductive drum  6 . 
     The adjusting mechanisms are fixed permanently after adjusting the distances Lo and Li. The LED head is urged toward the shaft  8  of the photoconductive drum  6  by springs  14   a  and  14   b , which are mounted on an upper portion of the both end portions of the LED head. The adjusting mechanisms  13   a  and  13   b  abut abutting surfaces  15   a  and  15   b  formed on the side frames  12   a  and  12   b . The adjusting mechanisms  13   a  and  13   b  maintain the distance Li at a fixed value so that light is focused on the surface of the photoconductive drum  6 . 
     The conventional apparatus of the aforementioned construction suffers from the following drawbacks. The distance Li is adjusted with the LED head mounted on a jig. When the thus adjusted LED head is assembled to a printer, the adjusting mechanisms  13   a  and  13   b  abut the abutting surfaces  15   a  and  15   b  of the side frames  12   a  and  12   b  in the ID unit. At this moment, the distance Li changes slightly so that a focal position deviates somewhat from its correct position, preventing formation of well focused images. 
     This is due to the fact that the distance of the photoconductive drum  6  from the SLA  2  deviates from a designed value Li. The deviation of the distance is within ±100 μm of the designed Li. The factors that cause the manufacturing variations of Li primarily include tolerances of the shaft  8 , holes  9  and  10 , the height of the abutting surfaces  15   a  and  15   b , and the wear of the photoconductive drum  6 . For this reason, the adjusting mechanisms  13   a  and  13   b  of each ID unit are adjusted to a corresponding ID unit when the IED head is assembled to the ID unit. However, ID unit is a consumable item. When the ID unit reaches the end of its useful life, the user replaces the ID unit by a new, unused one. Thus, after the ID unit is replaced, the distance Li between the SLA  2  and the surface of the photoconductive drum  6  may be different from that before the ID unit is replaced. 
     FIG. 37 illustrates the relationship between ΔLi and MTF (Modulation Transfer Function). The closer to 100% the MTF is, the more faithful to an original image the printed image is. From FIG. 37, it can be seen that a deviation of Li of 50 μm causes a decrease of MTF of more than 10%. For a printer having a resolution of 1200 DPI (about 24 line pairs/mm), a decrease of MTF in excess of 10% impairs the resolution of a printed image. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve the aforementioned problem. 
     Another object of the invention is to improve the accuracy of positioning of an LED head with respect to the surface of a photoconductive drum so that an image is focused accurately on the surface of the photoconductive drum. 
     A positioning apparatus for an optical head includes a cylindrical photoconductive drum, an optical head, and at least one spacer. The cylindrical photoconductive drum extends in a direction of a longitudinal axis. The optical head extends parallel to the photoconductive drum. The at least one spacer is disposed to abut the photoconductive drum, the spacer limiting a distance between the optical head and a surface of the photoconductive drum. 
     The photoconductive drum has a photoconductor and the spacer is contact with the surface of the photoconductor through sliding friction. 
     The spacer has a first surface in contact with the surface of the photoconductor. The first surface has a groove formed therein. 
     The photoconductor has a second surface in contact with the first surface. The first surface has a first curvature and the second surface has a second curvature. When the first surface is pressed against the second surface, the spacer deforms resiliently so that the first curvature becomes substantially equal to the second curvature. 
     The electrophotographic printer further includes a charging roller that extends in a direction in which the photoconductive drum extends, the charging roller being in contact with the photoconductor. The spacer is located outside of an area in which the charging roller is in contact with the photoconductor. 
     The photoconductive drum has a member coaxial with the photoconductive drum and rotates in contact with the first surface together with the photoconductive drum. 
     The second surface of the spacer is on an opposite side from the first surface. The electrophotographic printer further includes an adjusting mechanism that is held sandwiched between the optical head, and the second surface having the second curvature. The adjusting mechanism is operated to adjust the position of the optical head relative to the photoconductive drum. The adjusting mechanism may be an eccentric cam mechanism. 
     The first surface and the second surface have a curvature and are concentric to each other. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein: 
     FIG. 1 is a front-view of a first embodiment; 
     FIG. 2 is a cross-sectional side view taken along the line A—A of FIG. 1; 
     FIG. 3 illustrates the relationship between a curvature of the abutting surface of the spacer and a curvature of the surface of the photoconductive drum; 
     FIG. 4 illustrates the adjusting mechanisms in detail; 
     FIG. 5 compares the curvatures of the abutting surface of the spacer and the surface of the photoconductive drum; 
     FIG. 6 compares the curvature of the spacer with that of the photoconductive drum; 
     FIG. 7 is a side view of the spacer according to a second embodiment; 
     FIG. 8 illustrates details of the spacer according to the second embodiment; 
     FIG. 9 illustrates the results of an experiment in which investigation was made to determine amounts of wear of spacers when the spacers of different materials are used for a predetermined time period; 
     FIG. 10 is a side view of the pertinent portion of a third embodiment; 
     FIG. 11 illustrates an ID unit of the third embodiment; 
     FIG. 12 is a graph of the accumulated number of printed pages versus the change in height of the spacers; 
     FIG. 13 is a front view of a fourth embodiment; 
     FIG. 14 illustrates the arrangement of the respective rollers in the ID unit; 
     FIG. 15 is a front view of an apparatus according to a fifth embodiment; 
     FIG. 16 illustrates a problem that a sixth embodiment is to solve; 
     FIG. 17 is a side view illustrating the sixth embodiment; 
     FIG. 18 is a front view of a structure according to the sixth embodiment; 
     FIG. 19 is a cross-sectional view taken along the line C—C of FIG. 18; 
     FIG. 20 is a cross-sectional view taken along the line B—B of FIG. 18; 
     FIG. 21 is a top view of the adjusting mechanisms when the adjusting mechanisms are disposed on the longitudinal end portions of the SLA holder and abut the spacers; 
     FIG. 22 is a front view of a pertinent portion of a seventh embodiment; 
     FIG. 23 is a perspective view of an eccentric cam mechanism, looking upward from the photoconductive drum; 
     FIG. 24 illustrates a cam portion of the eccentric cam mechanism; 
     FIG. 25 is a perspective view of the eccentric cam mechanism, looking upward from the photoconductive drum; 
     FIG. 26 illustrates a cam portion of the eccentric cam mechanism; 
     FIG. 27 is a cross-sectional side view taken along the line D—D of FIG. 22; 
     FIG. 28 is a cross-sectional side view taken along the line E—E of FIG. 22; 
     FIG. 29 is a top view of the pertinent portion; 
     FIG. 30 illustrates the spacer when the spacer is not in a horizontal plane; 
     FIG. 31 illustrates sink marks developed in a spacer during the manufacture of the spacer; 
     FIG. 32 is a perspective view of the spacer according to an eighth embodiment; 
     FIG. 33 is a cross-sectional view of the spacer of FIG. 32; 
     FIG. 34 is a side view of the spacer of FIG. 32; 
     FIG. 35 is a cross-sectional view of a conventional LED head; 
     FIG. 36 is a front view of the conventional LED head; and 
     FIG. 37 illustrates the relationship between ΔLi and MTF. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     FIG. 1 is a front view of a first embodiment. 
     A photoconductive drum  6  is generally in the shape of a hollow cylinder. Each of spacers  51   a  and  51   b  has a recessed abutting surface  52 , preferably, a curved surface having a curvature that describes an arc. The abutting surface  52  abuts a surface of the photoconductive drum  6 . The recessed abutting surface  52  need not have a curvature but may be V-shaped, for example. Adjusting mechanism  13   a  ( 13   b ) is disposed on the opposite side from the recessed abutting surface  52 . Springs  14   a  and  14   b  are mounted on opposed longitudinal end portions of an SLA holder  4  and urge the SLA holder  4  toward the photoconductive drum  6  through the spacers  15   a  and  15   b . The spacers  51   a  and  51   b  may be secured to the adjusting mechanisms  13   a  and  13   b  or may simply abut the adjusting mechanisms  13  and  13   b . If the spacers  51   a  and  51   b  simply are to abut the adjusting mechanisms  13  and  13   b , the spacers  51   a  and  51   b  may be manufactured as separate structural members or may be loosely fitted into holes formed in a chassis, not shown, of the SLA holder  4 . Still alternatively, only one spacer may be provided in a longitudinal direction substantially at a midpoint of the photoconductive drum  6 . 
     FIG. 2 is a cross-sectional side view taken along the line A—A of FIG. 1. A small amount of toner may be left on the photoconductive drum  6  after transferring and unwanted toner may adhere to the photoconductive drum  6  during a developing operation. When unwanted toner particles and foreign matters reach the spacers  51   a  and  51   b  as the photoconductive drum  6  rotates in a direction shown by arrow A, edge portions  53  of the spacers  51   a  and  51   b  scratch the unwanted toner particles and foreign matters from the photoconductive drum  6 . Thus, the toner is prevented from entering between the spacers  51   a  and  51   b  and the photoconductive drum  6 . In addition, the abutting surfaces  52  of the spacers  51   a  and  51   b  rub the surface of the photoconductive drum  6  to rake away the toner particles from a gap between the spacers  51   a  and  51   b  and the photoconductive drum  6 . Thus, a film of toner, which will otherwise build up on the photoconductive drum  6 , will not push up the spacers  51   a  and  51   b . Thus, the relation that Li=Lo can be maintained. 
     FIG. 3 illustrates the relationship between a curvature rs of the abutting surface  52  of the spacer and a curvature rd of the surface of the photoconductive drum. 
     As shown in FIG. 3, the curvature rs of the abutting surface  52  is slightly smaller than, preferably equal to, that rd of the surface of the photoconductive drum  6 . 
     FIG. 4 illustrates the adjusting mechanisms  13   a  and  13   b  in detail. 
     Each of the adjusting mechanisms  13   a  and  13   b  is generally in the shape of a wedge and has a resilient member  17  with a hook  17   a.  The SLA holder  4  has a rack  4   a  having a plurality of grooves  4   b  formed in its surface. The adjusting mechanisms  13   a  and  13   b  are assembled to the SLA holder in such a way that the hook  17   b  engages the groove  4   b . The adjusting mechanisms  13   a  and  13   b  each have an inclined surface that slides on an inclined surface of the rack  4   a . The distance Li can be adjusted by incrementally moving the adjusting mechanism  13   a  ( 13   b ) in a direction shown by arrow C. In other words, operating the adjusting mechanisms  13   a  and  13   b  allows adjustment of the distance Li such that Li=Lo. 
     Second Embodiment 
     In the first embodiment, the abutting surface  52  of the spacer  51   a  ( 51   b ) that abuts the photoconductive drum  6  has preferably the same curvature as the photoconductive drum  6 . However, the curvatures of the abutting surface  52  and the photoconductive drum  6  may differ slightly due to manufacturing variations. For this reason, when the spacers  51   a  and  51   b  abut the photoconductive drum  6 , only a part of the abutting surface can be brought into contact with the surface of the photoconductive drum  6 . A second embodiment is directed to the shape of the spacer  51   a  ( 51   b ) to ensure that the abutting surface  52  of the spacer  51   a  ( 51   b ) is brought in its entirety into intimate contact with the photoconductive drum  6 . 
     FIG. 5 compares the curvatures of the abutting surface of the spacer and the surface of the photoconductive drum. 
     Referring to FIG. 5, if the curvature rs=r3 of the spacer  51   a  is smaller than that rd=r2 of the photoconductive drum  6 , then the edge portion  53  abuts the surface of the photoconductive drum  6 . Therefore, the spacers are substantially in line contact with the photoconductive drum  6 . As a result, the pressure per unit area (N/cm 2 ) of an area in contact with the photoconductive drum  6  is higher when only a part of the abutting surface contacts the photoconductive drum  6  than when the whole abutting surface contacts the photoconductive drum  6 . Thus, when only a part of the abutting surface contacts the photoconductive drum  6 , a large frictional force is developed between the spacer  51   a  ( 51   b ) and the surface of the photoconductive drum  6 . The wear of the spacers  51   a  and  51   b  causes the distance Li to decrease with the result that the condition of Li=Lo cannot be maintained. 
     FIG. 6 compares the curvature of the spacer with that of the photoconductive drum. 
     If the curvature rs=r4 of the abutting surface  52  is larger than that rd=r2 of the photoconductive drum  6 , the edge portions  53  of the spacers  51   a  ( 51   b ) cannot rake the toner  55  from the photoconductive drum  6 . Instead, the toner  55  enters a gap between the spacers  51   a  and  51   b  and the photoconductive drum  6 . This construction has a sliding friction through which the toner in the gap is raked from the gap. However, if the toner  55  entering the gap exceeds an amount that can be raked from the gap, the toner  55  will be trapped in the gap. The toner trapped in the gap causes the distance Li to change so that Li is no longer equal to Lo, resulting in poorly focussed images. 
     FIG. 7 is a side view of the spacer according to the second embodiment. 
     FIG. 8 illustrates details of the spacer according to the second embodiment. 
     In the second embodiment, the curvature rs of the abutting surface  52  of the spacer  51   a  ( 51   b ) is smaller than that rd of the photoconductive drum  6 . The spacer  51   a  ( 51   b ) is thinner in a circumferential direction at a midpoint of the abutting surface  52  than at circumferentially end portions of the abutting surface  52 . Specifically, the curvature rs of the spacers  51   a  and  51   b  are selected to be rs=r1, and the curvature rd of the photoconductive drum  6  is selected to be rd=r2. The r1 is selected to be about 1% smaller than r2. The thickness t at the middle portion of the spacer is, for example, 1 mm. The spacer  51   a  ( 51   b ) may be shaped as shown in FIG. 5 by using a resilient material. In the second embodiment, the spacer is of thick construction but may be of thin construction if sufficient resiliency is ensured. 
     When a resulting urging force F of the springs  14   a  ( 14   b ) urges the spacers  51   a  and  51   b , the resilient portion  56  is deformed such that the abutting surface  52  is in intimate contact with the photoconductive drum  6 . The intimate contact of the abutting surface  52  makes the curvature rd of the spacer substantially equal to that of the photoconductive drum  6 . In other words, the spacers  51   a  and  51   b  are brought into area-contact with the photoconductive drum  6  rather than into line-contact with photoconductive drum  6 . Thus, the pressure per unit area (N/cm 2 ) of the surface  52  can be decreased to reduce wear of the spacers  51   a  and  51   b  and the photoconductive drum  6 . As the photoconductive drum  6  rotates, the toner  55  deposited on the photoconductive drum  6  reaches the spacers  51   a  and  51   b  and the edge portions  53  of the spacers  51   a  and  51   b  scrape the toner  55  off the photoconductive drum  6 . 
     FIG. 9 illustrates the results of an experiment in which investigation was made to determine amounts of wear of spacers when spacers of different materials are used for a predetermined time period. 
     There are two types of materials for the spacers  51   a  and  51   b : polyacetal that is general purpose engineering plastics and PTFE resin that is a special purpose engineering plastics. The surface material of the photoconductive drum  6  is formed of a layer of plolycarbonate resin. As shown in FIG. 9, the spacers  51   a  and  51   b  formed of polyacetal (POM) having a modulus of elasticity of 4×10 3  kg/cm 2  showed a change of about 10 μm in height after 40,000 pages have been printed. The spacers  51   a  and  51   b  formed of PTFE resin having a modulus of elasticity of 3.5×10 3  kg/cm 2  showed changes in a wide range, i.e., 10 to 120 μm. The photoconductive drum  6  was also damaged noticeably. The tolerance of the distance Li between the light exiting end of the SLA  2  and the surface of the photoconductive drum  6  is Lo±50 μm. The wear of the spacers  51   a  and  51   b  formed of PTFE resin is out of this tolerance. The wear of the spacers  51   a  and  51   b  formed of polyacetal resin is about 10 μm at the most, which is sufficiently practical taking into account manufacturing variations and expansion and contraction of the material due to environmental changes. While practical results were obtained from the spacers  51   a  and  51   b  formed of polyacetal resin, the materials and shapes (curvature of the surface in contact with the photoconductive drum, and thickness at the middle of the spacer) are arbitrary conditions selected in this experiment. These conditions are only exemplary and may change depending on the urging force of the spring that urges the LED head, the width of the surface in contact with the photoconductive drum  6 , and the material with which the spacers  51   a  and  51   b  are brought into contact. 
     In the second embodiment, the spacers  51   a  and  51   b  have resiliency in their middle portions, the surfaces of the spacers can be in intimate contact with the surface of the photoconductive drum  6 . Thus, wear of the surfaces of the spacers  51   a  and  51   b  and photoconductive drum  6  is minimized and the toner is prevented from entering the gap between the spacers and the photoconductive drum  6 . Thus, the structure provides an apparatus in which when the LED head is assembled to the apparatus, the distance Li between the light exiting end of the SLA  2  and the surface of the photoconductive drum  6  do not vary over a wide range. 
     Third Embodiment 
     A third embodiment is characterized in that spacers, which determine the distance Li between the light exiting end of the SLA  2  and the photoconductive drum  6 , is not provided on the LED head side but on the ID unit side to which the photoconductive drum  6  is mounted. 
     FIG. 10 is a side view of the pertinent portion of the third embodiment. 
     FIG. 11 illustrates an ID unit of the third embodiment. 
     The spacers  51   a  and  51   b  have abutting surfaces  52  having substantially the same curvature as the cylindrical photoconductive drum  6 . The abutting surface  52  abuts the surface of the photoconductive material of the photoconductive drum  6 . The adjusting mechanisms  13   a  and  13   b  are provided to abut the surface  54  on the side opposite from the surface  52 . The adjusting mechanisms  13   a  and  13   b  serve to precisely adjust the distance Li so that Li=Lo. The springs  14   a  and  14   b  are mounted on longitudinal top end portions and urge the SLA holder  4  toward the photoconductive drum  6 . The spacers  51   a  and  51   b  have short projections  56 . The short projections  56  engage the upper frame  16 , thereby positioning the spacers  51   a  and  51   b  with respect to the photoconductive drum  6  to prevent the spacers from swinging above the photoconductive drum  6  or coming off due to vibration exerted thereon during transportation. 
     FIG. 12 is a graph of the accumulated number of printed pages versus the change in height of the spacers  51   a  and  51   b.    
     The surfaces  52  of the spacers  51   a  and  51   b  have substantially the same curvature as the photoconductive drum  6 . During printing, the surfaces of the spacers  51   a  and  51   b  and photoconductive drum  6  are in area contact with each other through sliding friction. The degree of wear of a member is usually considered proportional to the distance over which the member slides on other member, provided that the member slides on the other member with a constant pressure (N/cm2) and at a constant speed (mm/s). As shown in FIG. 12, the accumulated number of printed pages increases 10,000, 20,000, and then 40,000 pages, the rate of change of height of the spacer increases in proportion to the accumulated number of printed pages. The usable lifetime of the apparatus was 1,000,000 pages and the designed lifetime of the LED head is substantially the same as that of the apparatus. This implies that the spacers on the LED head side should have as long a lifetime as 1,000,000 pages. However, as is clear form FIG. 12, if the apparatus operates up to 1,000,000 pages, the heights of the spacers will have changed by several hundred microns. The tolerance of the distance Li is Lo=±50 μm. Therefore, it is apparent that if the apparatus is operated until it reaches the end of its lifetime, the distance Li cannot be maintained within the tolerance. Usually, an ID unit in which a photoconductive drum is incorporated is a consumable item that is replaced by a new, unused one at regular intervals. The ID unit used in the experiment is of design in which the ID unit is replaced after the accumulated number of printed pages reach 40,000 pages. FIG. 12 shows that when an accumulated number of printed pages reaches 40,000 pages, the amount of wear of the spacer is less than 10 μm. The amount of wear is sufficiently practical within a period between replacement. 
     In the third embodiment, the spacers are provided on the ID unit side. This implies that replacement of the ID automatically replaces the spacers by new, unused ones. Thus, the distance Li between the light exiting end of the SLA and the surface of the photoconductive drum  6  can be maintained constant until the apparatus reaches the end of its lifetime, thereby providing stable print quality. 
     Fourth Embodiment 
     A fourth embodiment is characterized in that the spacers provided on longitudinal ends of the photoconductive drum  6  are outside of a surface area of the photoconductive drum  6  that is in contact with a charging roller. 
     Just as in the first and second embodiments, the spacers may be provided on the LED head side. Alternatively, the spacers may be provided on the ID unit side just as in the third embodiment. When the spacers are provided on the LED head side, the spacers may be secured to the LED head just as in the first embodiment, or may simply abut the LED head. 
     FIG. 13 is a front view of the fourth embodiment. 
     The surfaces  52  of the spacers  51   a  and  51   b  have substantially the same curvature as the surface of the photoconductive drum  6 . The surfaces  52  abut the surface of the photoconductive drum  6 . The adjusting mechanism  13   a  ( 13   b ) is secured to or simply abuts the surface of the spacer  51   a  ( 51   b ) on the opposite side of the recessed abutting surface  52 . The adjusting mechanisms  13   a  and  13   b  are adjusted so that Lo=Li. The springs  14   a  and  14   b  are mounted on opposed longitudinal end portions of the SLA holder  4  and urge the SLA holder  4  toward the photoconductive drum  6  through spacers  15   a    15   b.    
     FIG. 14 illustrates the arrangement of the respective rollers in the ID unit. 
     The photographic process of the photographic printer will be described briefly with reference to FIG.  14 . The photographic process includes charging, exposing, developing, and transferring. These steps are sequentially carried out to print on the print paper. During charging, the charging roller  21  receives a high voltage so that the charging roller  21  uniformly charges the photoconductive drum  6  with negative charges. During exposing, the LED head  23  illuminates the charged surface of the photoconductive drum  6  to selectively dissipate the charges in accordance with print data. The potential of illuminated areas decreases while that of non-illuminated areas remains negatively high. Therefore, the illuminated areas and non-illuminated areas form an electrostatic latent image as a whole. This electrostatic latent image advances to the developing roller  24  as the photoconductive drum  6  rotates. During developing, toner is deposited on the electrostatic latent image to develop the electrostatic latent image into a toner image. The toner is charged due to the friction between the developing roller  24  and a developing blade, not shown. The charged toner migrates by the Coulomb force to the photoconductive drum  6  in the electric field developed due to the potential difference between the developing roller  24  and the photoconductive drum  6 , so that the toner particles are deposited on the electrostatic latent image to form a toner image. During transferring, the transfer roller  25  receives a positive voltage to negatively charge the back side of the print paper  20 , so that the negatively charged toner  55  on the photoconductive drum  6  is transferred by the Coulomb force to the print paper  20 . 
     If, for some reason, hard foreign matters enter the ID unit from outside and penetrates the photoconductive material to reach the core tube of the photoconductive drum  6 , leakage may occur across the photoconductive drum  6  and the charging roller  21  that receives a high voltage. Even if leakage does not occur, a damage deep in the photoconductive material on the photoconductive drum  6  prevents the drum surface from being properly charged so that the toner charged on the developing roller  24  migrates from the developing roller  24  to the photoconductive drum  6 . The toner  65  migrated to the surface of the photoconductive drum  6  falls between the charging roller  21  and the photoconductive drum  6  to be rubbed therebetween, and the rubbed toner migrates to the charging roller  21  as well. If excessive toner is deposited on the charging roller  21 , the surface of the charging roller  21  becomes away from the surface of the photoconductive drum  6  by the thickness of the deposited toner layer. As a result, the toner is deposited over a wide area of the surface of the photoconductive drum  6  so that the toner  55  causes soiling of the surface of the photoconductive drum  6 . 
     In the invention, the spacers  51   a  and  51   b  are disposed outside of an area W in which the charging roller  21  rotates in contact with the photoconductive drum  6 . Therefore, even if the spacers  51   a  and  51   b  cause a scratch deep in the surface of the photoconductive drum  6 , the scratch does not cause the toner  55  to migrate to the charging roller  21  and will not affect print quality. 
     Fifth Embodiment 
     The apparatus according to the fourth embodiment tends to be of large size because the spacers are disposed outside of the area in which the charging roller  24  rotates in contact with the photoconductive drum  6 . A fifth embodiment provides a structure that offers the same advantages as the fourth embodiment while also maintaining the same overall size of the apparatus. 
     FIG. 15 is a front view of an apparatus according to the fifth embodiment. 
     The photoconductive drum  6  is generally cylindrical. The spacers  51   a  and  51   b  have recessed abutting surfaces  52 . The abutting surface  52  of the spacer  51   a  is in sliding contact with a gear  7  provided at one longitudinal end portion of the photoconductive drum  6 . The abutting surface  52  of the spacer  51   b  is in sliding contact with a flange  11  provided at the other longitudinal end portion of the photoconductive drum  6 . The spacers  51   a  and  51   b  may be disposed on the LED head side just as in the first embodiment or on the ID unit side to which the photoconductive drum  6  is attached just as in the third embodiment. If the spacers  51   a  and  51   b  are disposed on the LED side, they may be secured to the SLA holder  4  just as in the first embodiment or may simply abut the LED head. The gear  7  takes the form of a bevel gear and is driven in rotation by a drive source, not shown, thereby driving the photoconductive drum  6  in rotation. The gear  7  and flange  11  have holes in their centers through which the rotational shaft of the photoconductive drum  6  extends. The flange  11  is in the shape of a disk and is in line with the photoconductive drum  6 . Adjusting mechanisms  13   a  and  13   b  simply abut or are secured on the surfaces of the spacers  51   a  and  51   b  opposite from the abutting surfaces  52 . The adjusting mechanisms  13   a  and  13   b  are operated such that Li=Lo. The springs  14   a  and  14   b  mounted on longitudinal ends of the SLA holder  4  urge the SLA holder  4  toward the photoconductive drum  6 . The aforementioned structure does not cause the spacers  51   a  and  51   b  to scratch the surface of the photoconductive drum  6 , thereby preventing the soiling of the print paper just as in the fourth embodiment. 
     Sixth Embodiment 
     FIG. 16 illustrates a problem addressed by a sixth embodiment. If the spacers  51   a  and  51   b  have flat surfaces that abut the adjusting mechanisms  13   a  and  13   b , then the frame  16  is formed with holes  16   a  into which upper projected portions of the spacers enter. The hole  16   a  is somewhat larger than the upper projected portions such that when the upper projected portion enters the hole  16   a , there is a gap  61  between the upper projected portion and the frame  16  that defines the hole  16   a . The holes  16   a  allow the spacers  51   a  and  51   b  to smoothly displace relative to the upper frame  16  when the springs  14   a  and  14   b  urge the spacers  51   a  and  51   b . However, the holes  16   a  may also cause the spacers  51   a  and  51   b  to be oriented at an angle ±β with the vertical line passing through a center O of the photoconductive drum  6 . This angular deviation also causes the abutting surfaces  54  of the spacers  51   a  and  51   b  to be at an angle with a horizontal plane, resulting in a change in the height of the adjusting mechanisms  13   a  and  13   b . Thus, the inclined orientation of the spacers  51   a  and  51   b  at an angle with the vertical line passing through a center O of the photoconductive drum  6  causes the distance Li to change, so that Li is no longer equal to Lo. A further problem is the inclination of the LED head. 
     FIG. 17 is a side view illustrating the sixth embodiment. 
     The spacers according to the sixth embodiment are not mounted on the LED head side but on the ID side. The sixth embodiment is characterized in that the abutting surface  54  of the spacer  51   a  ( 51   b ) that abuts the adjusting mechanism  13   a  ( 13   b ) is a curved surface having a curvature r5. In other words, the curved surface is concentric to the photoconductive drum  6 . The same elements as the first to fifth embodiments have been given the same reference numerals and only a portion different from the first to fifth embodiments will be described. 
     The spacers  51   a  and  51   b  have short projections  56 . The projections  56  engage the upper frame  16 , thereby positioning the spacers  51   a  and  51   b  with respect to the photoconductive drum  6  so that the spacers  51   a  and  51   b  are prevented from swinging above the photoconductive drum  6  or coming off the photoconductive drum  6  during transportation. The height  57  of the spacers  51   a  and  51   b  with respect to the surface of the photoconductive drum  6  will not change even if the spacers  51   a  and  51   b  are urged in a direction at an angle β with a vertical line passing through the center O of the photoconductive drum  6 , or the height of any part of the spacers  51   a  and  51   b  with respect to the photoconductive drum  6  remain unchanged. Thus, the distance Li between the light exiting end of the SLA  2  and the surface of the photoconductive drum  6  is maintained constant reliably. 
     Seventh Embodiment 
     FIG. 18 is a front view of a structure according to the sixth embodiment. The spacers  51   a  and  51   b  are disposed at longitudinal end portions of the photoconductive drum  6  and slides in contact with the surface of the photoconductive drum  6 . The adjusting mechanisms  13   a  and  13   b  are disposed on the spacers  51   a  and  51   b.  The SLA holder  4  is disposed on the adjusting mechanisms  13   a  and  13   b . The springs  14   a  and  14   b  urge the SLA holder  4  toward the photoconductive drum  6  in such a way that the adjusting mechanisms  13   a  and  13   b  abut at a total of four locations to firmly position the LED head with respect to the photoconductive drum  6 . By the use of the adjusting mechanisms  13   a  and  13   b , the distance Li between the light exiting surface of the LED head and the photoconductive drum  6  can be accurately set. 
     FIG. 19 is a cross-sectional view taken along the line C—C of FIG.  18 . 
     FIG. 20 is a cross-sectional view taken along the line B—B of FIG.  18 . 
     A structure where the SLA holder  4  is supported at four locations suffers from the following problem. For example, while the adjusting mechanisms  13   a  and  13   b  are designed to be of the same length or height, they cannot be exactly the same in reality. In other words, the four bottom portions are of slightly different height due to manufacturing error as shown in FIGS. 19 and 20. 
     FIG. 21 is a top view when the adjusting mechanisms disposed on the longitudinal end portions of the SLA holder abut the spacers, respectively. 
     As shown in FIGS. 19-21, the spacers  51   a  and  51   b  abut three bottoms (e.g., locations C, D, and E) of the adjusting mechanisms  13   a  and  13   b  whose lengths match one another. One remaining bottom does not abut the spacer. Actually, the two springs  14   a  and  14   b , are disposed at longitudinal end portions of the SLA holder  4 , and urge the SLA holder  4  in slightly different directions from each other. Therefore, the spacers  51   a  and  51   b  do not necessarily receive the same three bottoms of the adjusting mechanisms  13   a  and  13   b . This implies that the LED head cannot be held reliably relative to the photoconductive drum  6  to ensure that Li=Lo at all times. 
     Thus, the seventh embodiment solves the aforementioned problem, thereby holding the LED head with respect to the photoconductive drum  6  such that Li=Lo at all times. The seventh embodiment uses eccentric cam mechanisms  60  and  61  in place of the adjusting mechanisms  13   a  and  13   b.    
     FIG. 22 is a front view of a pertinent portion of the seventh embodiment. 
     FIG. 23 is a perspective view of the eccentric cam mechanism  60 , looking upward from the photoconductive drum  6 . 
     FIG. 24 illustrates a cam portion  60   a  of the eccentric cam mechanism  60 . 
     FIG. 25 is a perspective view of the eccentric cam mechanism  61 , looking upward from the photoconductive drum  6 . 
     FIG. 26 illustrates a cam portion  61   a  of the eccentric cam mechanism  61 . 
     The eccentric cam mechanisms  60  and  61  are disposed at longitudinal end portions of the LED holder  4 . The cam portion  60   a  is firmly rotatably held against the SLA holder  4  by two fingers  60   e  of a retainer  60   a . The cam portion  60   a  is formed with a cross-shaped groove  60   c  into which a Phillips screwdriver is inserted. Driving the groove  60   c  with the screwdriver causes the cam portion  60   d  to rotate about an axis H. The cam portion  61   a  is firmly rotatably held against the SLA holder  4  by two fingers  61   e  of a retainer  61   a . The cam portion  61   a  is formed with a cross-shaped groove  61   c  into which a Phillips screwdriver is inserted. Driving the groove  61   c  with the screwdriver causes the cam portion  61   d  to rotate about an axis I. As mentioned above, driving the grooves  60   c  and  61   c  with a screwdriver allows adjustment of the height of the SLA holder  4  with respect to the spacers. Because the fingers  60   e  and  61   e  firmly hold the eccentric cam mechanisms, the cam portions  60   a  and  61   a  stay where they are adjusted. 
     FIG. 27 is a cross-sectional side view taken along the line D—D of FIG.  22 . 
     FIG. 28 is a cross-sectional side view taken along the line E—E of FIG.  22 . 
     FIG. 29 is a top view of the pertinent portion. 
     FIG. 30 illustrates the spacer when the spacer is not in a horizontal plane. 
     The spacers  51   a  and  51   b  slide over the surface of the photoconductive drum  6  at longitudinal end portions of the photoconductive drum  6 . The SLA holder  4  abuts the top surfaces  54  of the spacers  51   a  and  51   b . The springs  14   a  and  14   b  are disposed on the SLA holder  4  and urge the SLA holder  4  toward the photoconductive drum  6 . The eccentric cam mechanisms  60  and  61  are sandwiched between the spacers  51   a  and  51   b  and the SLA holder  4 . Operating the eccentric cam mechanisms  60  and  61  allows adjustment of the distance Li between the light exiting end of the SLA and the photoconductive drum  6  such Li=Lo. 
     The eccentric cam mechanism  60  abuts two locations J and K on the flat surface  54  of the spacer  51   a  while the eccentric cam mechanism  61  abuts a location L on a curved surface of the spacer  51   b  eccentric to the surface of the photoconductive drum  6 . This ensures that the spacer  51   b  is always urged in a direction passing through a rotational axis O of the photoconductive drum  6 . Thus, even if the spacer  51   b  is urged in a direction at an angle with the vertical axis passing through the rotational axis O of the photoconductive drum  6 , the height of the eccentric cam mechanism  61  with respect to the photoconductive drum  6  will not change. 
     Therefore, the LED head is held at three locations. As shown in FIG. 30, when the spacer  51   a  is angularly displaced somewhat from its designed position, the flat surface  54  of the spacer makes an angle with a horizontal lane. However, the friction between the spacer  51   a  and the photoconductive drum  6  is very small. Therefore, when the SLA holder  4  descends in a direction shown by arrow D toward the photoconductive drum  6 , the bottom portions of the eccentric cam mechanism  60  first abuts a higher portion of the spacer  51   a . Then, the eccentric cam mechanism  60  pushes down the higher portion, causing the spacer  51   a  to rotate in a direction shown by arrow E such that the flat surface lies in a horizontal plane. As a result, the eccentric cam mechanism  60  abuts two locations on the surface  54  of the spacer  51   a  as shown in FIG.  23 . 
     The seventh embodiment uses the eccentric cam mechanisms  60  and  61  as a mechanism for adjusting the distance Li. The mechanism can be of any type, provided that the height of the SLA holder  4  can be properly adjusted with respect to the photoconductive drum  6 . 
     Eighth Embodiment 
     An eighth embodiment is characterized in that the spacer has a circumferentially extending groove that is formed in the surface in contact with the photoconductive drum. 
     FIG. 31 illustrates sink marks developed in a spacer during the manufacture of the spacer. 
     FIG. 32 is a perspective view of the spacer according to the eighth embodiment. 
     FIG. 33 is a cross-sectional view of the spacer of FIG.  32 . 
     FIG. 34 is a side view of the spacer of FIG.  32 . 
     As shown in FIG. 31, the spacers  51   a  and  51   b  have grooves  63  formed in the abutting surface  52  that slides in contact with the photoconductive drum  6 . As described in the second embodiment, the spacers  51   a  and  51   b  are formed of general purpose engineering plastics and are usually formed by injection molding for low manufacturing cost. Due to differences in thickness of various portions, a molded part often suffers from differences in shrinkage during manufacture. The differences in shrinkage cause sink marks  58 , i.e., dimple-like recesses in the surface of the molded part. Just like ordinary molded parts, the spacers  51   a  and  51   b  actually had sink marks  58  developed in the abutting surface  52  in contact with the photoconductive drum  6 . As shown in FIG. 31, the sink marks  58  in the abutting surface  52  reduces an area in contact with the photoconductive drum  6 , increasing a pressure per unit area (N/mm 2 ) so that the surfaces of the photoconductive drum  6  and the spacers  51   a  and  51   b  wear out quickly. Forming the grooves  63  in the abutting surfaces  52  of the spacers  51   a  and  51   b  as shown in FIGS. 32-34 reduces differences in thickness among various portions of the spacers, thereby minimizing chances of sink marks  58  developing and thus reducing the frictional wear of the photoconductive drum and spacers. Thus, the distance Li between the light exiting end of the SLA and the photoconductive drum  6  can be maintained in the relation of Li=Lo for a long period of time. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art intended to be included within the scope of the following claims.