Abstract:
A printer and components of the printer are described, along with methods of adjusting and using the printer. The printer includes a carriage that supports at least one printhead. The at least one printhead ejects ink onto a print media that is fed though a space between the printhead and a platen as the carriage moves laterally along slider rods supported by a beam. The space between the at least one printhead and platen is adjusted along the length of the printing path by applying a bending force to the underside of the beam that raises or lowers the beam relative to the platen at selected points. The adjustment is effected by one or more adjustment assemblies that abut the lower surface of the beam. In one embodiment, the first adjustment is an acorn nut and the second adjustment a differential screw that is engaged with the acorn nut. Rotation of the acorn nut effects a rough adjustment of the printhead to platen spacing, and rotation of the differential screw effects a fine adjustment of the printhead to platen spacing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a printer, and in particular to an ink jet printer typically used in association with a computer. 
     BACKGROUND OF THE INVENTION 
     A parameter of ink jet printers is the spacing (i.e., distance) between the printhead and the platen upon which the print media advances. A typical spacing between the printhead and the platen is 1.20 mm. This spacing must be constant, within a tolerance of, for example, +/−0.15 mm, along the entire printing path of the printhead to obtain uniform printing. For large format printers, it is a challenge to maintain the spacing between the printhead and the platen within a tolerance of +/−0.15 mm or less because a typical length of the printing path is 1.5 m or larger. 
     A conventional printer includes a carriage that supports the printhead above the platen. The carriage moves laterally along one or two steel rods. In the past, practitioners formed the rods to be as straight as possible in order to maintain the amount of variation in the printhead to platen spacing within a specified range along the printing path. 
     Another solution is described in U.S. Pat. No. 5,195,836. The &#39;836 patent discloses a guideway and support structure for a carriage. The guideway includes two rods. The rods are not necessarily pre-straightened, but rather are bent straight to the required tolerance as the rods are attached to and held by the support structure. 
     An ENDCAD printer employs a different approach. The ENDCAD printer comprises carriage that moves on a single rod. Equally spaced along the rod are two adjustment screws that are perpendicular to the rod. The opposing ends of each screw are threaded. The two threads of each screw are the same pitch. The upper end of each screw engages a hole in the lower part of the rod. The opposite lower end of each screw engages a hole in an chassis member located beneath the rod. Rotating a screw moves the rod a distance equal to two times the thread pitch, since there are two equal threads on the screw. Neither of the two screws are differential screws. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention allow for greatly reducing the amount of variation in the spacing between a printhead and a platen along the length of a printing path of a printer. In particular, the present invention allows very precise adjustments of the printhead to platen spacing. Prior art solutions, including the ENDCAD system described above, do not allow the easy and fine adjustments enabled herein. 
     One embodiment of the present invention includes a printer having a printhead, a platen spaced from the printhead, and a carriage that moves the printhead laterally adjacent to the platen. The carriage travels on a pair of rods supported by a beam. One or more adjustment assemblies are located beneath and along the length of the beam. Each adjustment assembly applies selected amounts of bending force to the lower side of the beam, and thereby raises or lowers the beam and the rods relative to the platen at that point. Accordingly, there is a corresponding change in the printhead to platen spacing at that point. The printhead to platen spacing along the length of the printing path may be maintained within a specified range by appropriate adjustment of each of the adjustment assemblies. 
     In one embodiment, each adjustment assembly includes a U-shaped member having a central plate that abuts the lower surface of the beam. The adjustment assembly also includes a first adjustment co-located with a second adjustment. The first adjustment is an acorn nut having a rounded surface that abuts the central plate of the U-shaped member, and the second adjustment is a differential screw that is engaged with the acorn nut. The differential screw has two threads of a different pitch. One thread is engaged with the acorn nut, and the other thread is engaged with a base plate of the adjustment assembly. The amount of adjustment caused by rotation of the differential screw is the difference between the pitch of the two threads. The acorn nut and the differential screw have a common vertical axis that is aligned with a centroid of the beam, which allows the application of vertical force to the beam without the introduction of rotational forces. 
     These and other objects, features and advantages of the present invention will be more readily apparent from the figures and the detailed description of the exemplary embodiments set forth below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a printer  1 . 
     FIG. 2 is a perspective view of an internal portion  20  of the chassis of printer  1 . 
     FIG. 3 is a perspective rear view of chassis portion  20  of FIG. 2, which depicts three adjustment assemblies  37  located beneath beam  21 . 
     FIG. 4 is a side view of the connections of beams  21 ,  22 , and  23  to side plates  25  of chassis portion  20 . 
     FIG. 5 is a cross-sectional side view of an adjustment assembly  37 . 
     FIG. 6 is a cross-sectional side view of an upper portion of adjustment assembly  37  of FIG.  5 . 
     FIG. 7 is an exploded view of adjustment assembly  37 . 
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     FIG. 1 shows a printer  1  having an internal chassis (not shown) that is supported by a pair of spaced legs  2 . Printer  1  includes a plurality of printheads  4  mounted on a transversely moveable carriage  3 . In one embodiment, carriage  3  supports six ink jet printheads. The number of printheads can vary. An internal portion  20  of the chassis of printer  1  is shown in FIGS. 2 and 3. 
     Chassis portion  20  of FIGS. 2 and 3 includes three support beams  21 ,  22 , and  23 . Beams  21 ,  22 , and  23  are supported by and extend between two side plates  25 . FIG. 4 shows the connections between beams  21 ,  22  and  23  and one of the side plates  25 . Beams  21 ,  22 , and  23  are formed of extruded aluminum, and side plates  25  are formed of stamped aluminum plate. 
     Beam  21  is the printer carriage beam. Beam  21  has two sets of bushing supports  24 . Each set of bushing supports  24  supports one of the two parallel steel rods  27  upon which carriage  3  travels. Each bushing support  24  has a generally V-shaped cross-section that serves to precisely locate the respective rod  27  relative to beam  21 . 
     Rods  27  are fixed to beam  21  in a manner that allows thermal expansion and contraction of the rods in the lateral direction, without allowing orthogonal movement of rods  27  relative to beam  21 . To satisfy this requirement, each rod  27  is attached at spaced intervals to its respective bushing supports  24  by means of a shoulder screw that enters into a threaded hole in the lower portion of the rod through a corresponding hole in the respective support  24 . Springs are provided on the shafts of the screws. A total of eight such screws are engaged with each rod  27 . Two of the screws are located near the lateral center of the rod. The six remaining screws are spaced along the length of the rod. For those six screws, the screw hole in the respective support  24  is elongated to allow the above-described lateral expansion and contraction of the rod. Fingers  26  (FIG. 4) of side plates  25  also support rods  27 . 
     As shown in FIG. 5, beam  21  has a cross-section with three main horizontal limbs  29 ,  30 , and  31 , and a side portion  34  adjacent to limb  31 . Limb  31  and side portion  34  are omitted beyond side plates  25 . Screws (not shown) attach side plates  25  to screw holes  75  in limb  31  and side portion  34  of beam  21  (FIG.  4 ). 
     In one embodiment of a printer  1 , beam  21  has a length of 1.595 m between side plates  25  and a total length of 2.028 m. Such a printer can print on paper having widths of up to 1.54 m. 
     Referring to FIGS. 2-5, beam  22  is a printing beam that also is connected between side plates  25 . Beam  22  supports a grooved platen  35  (shown in part in FIG. 2) over which the paper  36  (FIG. 5) or other print media travels during printing. The grooves allow vacuum to be applied to paper  36  so as to hold the paper against platen  35 . The main paper drive roller (not shown) is arranged to be located to the left of beam  22 . 
     Beam  23  is the lower beam of the chassis, and also is supported between side plates  25 . As shown in FIG. 5, beam  23  has a relatively large cross section, and thus provides strength and stiffness. 
     Referring to FIG. 3, three adjustment assemblies  37  are connected between beams  21  and  23 . Adjustment assemblies  37  are located beneath beam  21  and behind beam  23 . Adjustment assemblies  37  are evenly or almost evenly spaced along the length of beam  21  between side plates  25 . Each adjustment assembly  37  is connected to beam  21  by two screws  47  (FIG.  2 ). 
     The number and location of adjustment assemblies  37  can vary. The minimum is one adjustment assembly  37 . Alternative embodiments may include two or four adjustment assemblies  37 . Adjustment assembly  37  is made of steel, except for guides  72  (FIG.  7 ). Guides  72  are molded polycarbonate. 
     FIG. 5 is a cross-sectional view of chassis portion  20  of FIG.  2 . Carriage  3  travels on rods  27 . Carriage  3  supports printheads  4  adjacent to and above platen  35 . Platen  35  is supported by beam  22 . A small vertical space  38  is between printheads  4  and platen  35 . Paper  36  passes through space  38  on platen  35 . 
     Adjustment assembly  37  of FIG. 5 is connected between a planar lower surface  39  of beam  21  and beam  23 . A lower portion of adjustment assembly  37  includes a bracket  40 . Bracket  40  is connected to beam  23 . 
     Adjustment assemblies  37  are used to apply a point force that bends beam  21  and rods  27  relative to platen  35  at the location of the particular adjustment assembly  37 . Beam  21  and rods  27  are thereby raised or lowered relative to platen  35 . By iteratively adjusting each adjustment assembly  37 , the height of space  38  between printheads  4  and platen  35  can be adjusted so as to set the height of space  38  at a relatively constant value (e.g., +/−0.15 mm or less) along the entire length of the printing path to achieve uniform printing. 
     FIG. 6 is a cross-sectional side view of an upper portion of adjustment assembly  37  between bracket  40  and lower surface  39  of beam  21 . As shown, adjustment assembly  37  includes two rotatable adjustments having a common vertical axis: (1) an acorn nut  41  and (2) a differential screw  42 . Rotation of acorn nut  41  and/or differential screw  42  causes a selected amount of orthogonal force to be applied to beam  21 , thereby raising or lowering beam  21  and rods  27  relative to platen  35  at that point. 
     Acorn nut  41  has a rounded surface  43  that abuts a planar central portion of a U-shaped member  44  (see FIG. 7) opposite beam  21 . The common central vertical axis of acorn nut  41  and differential screw  42  is aligned with a centroid (i.e., center of gravity) of beam  21 . The rounded surface  43  on acorn nut  41  allows the alignment and application of force to beam  21  and U-shaped member  44  without the introduction of moments that would produce an unwanted twist or bend of beam  21  as acorn nut  41  or differential screw  42  are rotated. 
     FIG. 7 is an exploded view of an adjustment assembly  37 . Beginning at the top of FIG. 7, adjustment assembly  37  includes a U-shaped member  44 . U-shaped member  44  includes a planar central plate  45  and two opposing tongues  46  that are perpendicular to central plate  45 . 
     Referring to FIGS. 5-7, an upper surface of central plate  45  of U-shaped member  44  abuts a planar lower surface  39  of beam  21 . Central plate  45  includes two holes  48  (FIG.  7 ). Screws  47  (FIG. 2) pass through holes  48  and connect central plate  45  to beam  21 . Rounded surface  43  of acorn nut  41  abuts a lower surface of central plate  45  opposite beam  21 . 
     Referring to FIGS. 6 and 7, acorn nut  41  includes a shaft  49  that is opposite rounded surface  43 . Shaft  49  of acorn nut  41  is hollow, and includes a central opening  50  (FIG.  6 ). Inner threads  51  are on the walls of central opening  50 . In the present embodiment, inner threads  51  are M 4 , right hand. The pitch of inner threads  51  is 0.7 mm. 
     Shaft  49  of acorn nut  41  extends through spring  52 . Spring  52  rests on a support member  53 . Spring  52  provides compliance, allows for tolerance stacks, and is useful to withstand shock and vibration loads. Acorn nut  41  is supported from below by differential screw  42 . 
     Referring to FIG. 7, support member  53  includes a planar first surface  54  upon which spring  52  rests, and an opposite planar second surface  55  that faces base plate  67 . Differential screw  42  extends through a central hole  56  (FIG. 6) in support member  53 . Support member  53  also includes two orthogonal upward extensions  57  that provide rigidity and prevent spring  52  from slipping during assembly. Support member  53  also includes two orthogonal downward extensions  58  and a hole  59  opposite extensions  58 . Extensions  58  extend through rectangular holes  60  in left tongue  46  of U-shaped member  44 . A screw (not shown) extends through hole  59  and attaches support member  53  to a corresponding hole in a horizontal flap  61  extending from right tongue  46  of U-shaped member  44 . 
     Referring to FIGS. 6 and 7, differential screw  42  is engaged with threads  51  of acorn nut  41 . Differential screw  42  includes a shaft  62  having an upper first end  63  and an opposite lower second end  64 . Shaft  62  includes first threads  65  (FIG. 7) beginning at first end  63 . First threads  65  are engaged with inner threads  51  (FIG. 6) of acorn nut  41 , and accordingly have the same pitch. Shaft  62  is hexagonal-shaped at second end  64  to accommodate a wrench. An intermediate portion of shaft  62  of differential screw  42  includes second threads  66  (FIG.  7 ). In the present embodiment, second threads  66  of shaft  62  are M 5 , right hand. The pitch of second threads  66  is 0.8 mm. Second threads  66  are engaged with complimentary threads on the walls of a hole  68  (FIG. 7) in the center of base plate  67 . Second end  64  of shaft  62  extends through a central hole  73  (FIG. 6) in planar portion  69  of bracket  40 , and thus is accessible for rotation. 
     Base plate  67  of FIGS. 6 and 7 is positioned between support member  53  and a planar upper portion  69  of bracket  40 . Base plate  67  is connected to planar portion  69  of bracket  40  by three screws  70 . Below base plate  67 , the shaft of each screw  70  is within a spring  71 . Springs  71  provide compliance, allow for tolerance stacks, and are useful to withstand shock and vibration loads. 
     Referring to FIGS. 6 and 7, tongues  46  of U-shaped member  44  slide vertically within polycarbonate guides  72  (FIG.  7 ). Guides  72  are connected to opposite sides of bracket  40 . Guides  72  engage tongues  46  and prevent rotation of U-shaped member  44  and beam  21 . The slideable engagement of tongues  46  within guides  72  prevents rotation of beam  21 , while allowing vertical motion to absorb shocks. 
     As previously mentioned, inner threads  51  (FIG. 6) of acorn nut  41  and upper first threads  65  of differential screw  42  (FIG. 7) are M 4 , and second threads  66  of differential screw  42  are M 5  in this embodiment. Because the pitches of these threads differ (0.7 mm verses 0.8 mm), a rough adjustment of the height of beam  21  and rods  25  relative to platen  35  can be made by rotating acorn nut  41 , and a fine adjustment can be made by rotating differential screw  42 . One rotation of acorn nut  41  will raise or lower beam  21  a distance of 0.7 mm relative to fixed bracket  40  and platen  35 . One rotation of differential screw  42  will raise or lower beam  21  a distance of 0.1 mm (0.8 mm minus 0.7 mm) relative to bracket  40  and platen  35 . In alternative embodiments, the pitch of inner threads  51  of acorn nut  41  and/or second threads  66  of differential screw  42  can be varied so that different amounts of change can be made in the height of beam  21  and space  38 . 
     Making a rough adjustment to the height of beam  21  at the point where an adjustment assembly  37  is located involves rotation of acorn nut  41 . While ensuring that differential screw  42  does not rotate, acorn nut  41  is rotated counter clockwise to raise beam  21  relative to bracket  40  and platen  35 , and rotated clockwise to lower beam  21 . This method can be performed for each adjustment assembly  37  of printer  1  or for a selected number of adjustment assemblies  37 , depending on whether an adjustment is necessary at that point along the length of beam  21 . A laser gauge or other gauge is used to measure the height of space  38 . 
     Making a fine adjustment to the height of beam  21  at the point where an adjustment assembly  37  is located involves rotation of differential screw  42 . While ensuring that acorn nut  41  does not rotate, differential screw  42  is rotated counter clockwise (viewed from below) to lower beam  21  relative to bracket  40  and platen  35 , and rotated clockwise to raise beam  21 . This method can be performed for each adjustment assembly  37  that supports beam  21  or for a selected number of adjustment assemblies  37 , depending on whether an adjustment is necessary at that point along the length of beam  21 . A typical situation may require both a rough and a fine adjustment of the height of beam  21  at the location of each adjustment assembly  37 . 
     After the space  38  is set, a printing method includes feeding a print media, such as paper  36  of FIG. 5, through space  38  between printheads  4  and platen  35 . Ink is ejected from printheads  4  onto the print media as carriage  3  moves printheads  4  along rods  27  according to information input from, for example, a computer connected to printer  1 . 
     The embodiments described herein are merely examples of the present invention. Artisans will appreciate that variations are possible within the scope of the claims.