Abstract:
A method and apparatus for maintaining a substantially constant belt tension under changing temperature conditions is provided by a geometric layout of a motor, belt and pulley system and a selection of manufacturing materials for component parts having a predetermined relationship of thermal expansion coefficients between themselves.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to belt drives, more specifically to hard copy apparatus having a scanning carriage for translating writing instruments across print media and, more particularly, to a thermal compensation belt drive tensioner for a scanning ink-jet printer carriage. 
     2. Description of Related Art 
     The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed for example, in various articles in the assignee&#39;s  Hewlett - Packard Journal,  Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in  Output Hardcopy [sic] Devices,  chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press. San Diego, 1988). 
     FIG. 1 depicts a hard copy apparatus, in this exemplary embodiment a computer peripheral, ink-jet printer,  101 . A housing  103  encloses the electrical and mechanical operating mechanisms of the printer  101 . Operation is administrated by an electronic controller  102  (usually a microprocessor or application specific integrated circuit (“ASIC”) controlled printed circuit board) connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC&#39;s. Cut-sheet print media  105 , loaded by the end-user onto an input tray  120 , is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station, or “print zone,”  123  where graphical images or alphanumeric text is created. A carriage  109 , selectively positionable on a rod  111 , scans the print medium. An encoder subsystem  113  is provided for keeping track of the position of the carriage  109  at any given time. A set of individual ink-jet pens, or print cartridges,  115 X is mounted in the carriage  109  (generally, in a full color system, inks for the subtractive primary colors, cyan (X=C), yellow (X=Y), magenta (X=M) and true black (X=K) are provided; in some implementations an ink-fixer chemical (X=F) is also used). An associated set of replaceable or refillable ink reservoirs  117 X is coupled to the pen set by ink conduits  119 . Once a printed page is completed, the print medium is ejected onto an output tray  121 . The carriage scanning axis is conventionally designated the x-axis, the print media transit axis is designated the y-axis, and the printhead firing direction is designated the z-axis. For convenience in describing the art and the present invention, all types of ink-jet hard copy apparatus are sometimes hereinafter referred to as “printers;” all types, sizes, and compositions of print media are also referred to as “paper;” all compositions of colorants are sometimes referred to as “ink;” and all embodiments of an ink-jet writing instruments are sometimes hereinafter simply referred to as a “open;” no limitation on the scope of the invention is intended nor should any be implied. 
     Generally, a belt drive subsystem can be used to mount and selectively move the carriage  109  for scanning bidirectionally across the print zone  123 . Instantaneous positioning of the printhead to the print medium is critical to prevent a printing error and ensure throughput performance. Belt tension is an important parameter and is varied dependent primarily on the drive loads required, margin desired, and belt-pulley design. 
     FIG. 2 (PRIOR ART) illustrates a belt  100  connected to a reversible motor  300  and a pulley  500 , both of which are usually affixed to a printer housing framework (not shown). A pulley axle  700  fits slidingly in slots  900  in a mounting frame  110 . Assembled, the belt  100  extends through an aperture  130  in the frame  110 . The pen carriage  109  (not seen in this view) rides on a guide rod  111  as in FIG.  1  and is attached to the belt  100 . Since the pulley axle  700  makes a sliding fit and the belt must be long enough to reach beyond the end  170  of the mount  110  to encircle the pulley  500  before it is fit into the slots  900 , a spring loaded tensioner  190  is used to achieve the proper post-assembly tension. After the pulley  500  is fit into the slots  900 , the tensioner  190  is inserted such that the axle  700  will ride on a tensioner surface  210 . An extension table  230 , with a tensioning spring  250  surrounding it, is rotatingly slipped into slot  270  of the mount  110 . By designing to a close tolerance, the tensioner surface  210  will pull the pulley outward along the slots  900  just until the belt  100  is properly tensioned. Another spring loaded tensioner is shown in U.S. Pat. No. 4,761,154, filed by Beauchamp et al. for a BELT TENSIONER (assigned to the common assignee herein and incorporated herein by reference). 
     Another prior art design, that has a lower belt tension requirement over the spring loaded design, is the standard fixed center tensioner that includes a spring loaded belt tensioner that provides initial belt tension and a locking mechanism to fix the tensioner to a predetermined setting. As the belt stretches under load and during thermal and humidity excursions, the hardware expands and contracts, resulting in changing pulley center-to-center spacing. The result is a change in belt tension from the initial setting. 
     In such solutions, as the belt stretches under load and during thermal and humidity excursions, the hardware can also expand and contract, resulting in a spring deflection and, therefore, a belt tension change. Such systems are designed with the lowest possible spring rate so that such a tension change is minimized. However, at higher acceleration and deceleration loading and higher scanning speeds which increase printing throughput, belt tensions must be increased to prevent belt slip at the motor and pulley interfaces. Moreover, for an ink-jet implementation, due to the advancement in pen design and increasing the number of printheads on-axis (or, for disposable or refillable print cartridges having a self-contained, on-axis, ink supply where the cartridge size is increasing to meet the demand for full bleed printing (e.g., photographic reproductions)), the total carriage weight increases. Increasing belt tension increases loading on the motor axle bearings. One solution is to add precision bearings to the motor shaft, but only at a significant cost impact on manufacturing. During temperature and humidity excursions, the material will experience an expansion or contraction (“EC”) proportional to the respective material coefficient of thermal expansion (“CTE”). Note that CTE can be a positive or negative value. Depending upon the materials used in fabricating the mechanism, differing material CTE may generate a belt tension increase or decrease during temperature excursions. 
     There is a need for a method and apparatus to maintain a substantially constant belt tension during temperature changes, There is a need for a method and apparatus to run relatively low belt tensions in high speed printers to increase motor life. There is also a need for a low cost solution. 
     SUMMARY OF THE INVENTION 
     In its basic aspects, the present invention provides a belt tensioner device, including: an adjustably mountable frame for mounting to a chassis; a pulley fork biasingly mounted to the frame; and a belt pulley mounted between the frame and the fork wherein the frame and fork are co-associated and fabricated of materials each having a complementary CTE to compensate for temperature excursions affecting belt tension. 
     In another basic aspect, the present invention provides a belt tensioner for an apparatus having a chassis with a belt drive motor affixed thereon for providing translational motion to a belt coupled thereto, wherein the chassis formed of a material having a first CTE, including: first mechanisms for holding a belt pulley axle at a first end, the first mechanisms including mechanisms for fixedly attaching the belt tensioner to the chassis such that the belt is in tension between the belt drive motor and the belt pulley and such that material EC of the first means due to thermal expansion and contraction is diametrically opposed to material EC of the chassis, wherein the first mechanisms are formed of a material having a second CTE and wherein the first CTE and the second CTE are related in proportion to a ratio of a linear distance between the motor and the mechanisms for fixedly attaching the belt tensioner and the distance between the pulley axle and the mechanisms for fixedly attaching the belt tensioner such that EC of each is in substantially identical proportion to the ratio; and a second mechanism for holding the belt pulley axle at a second end, wherein the second mechanism is captured by the first mechanisms and the second mechanism is formed of a material having a third CTE wherein the third CTE is substantially lower than the first CTE and the second CTE such that the second mechanism is substantially unaffected during material EC of the chassis and the second mechanism. 
     In another basic aspect, the present invention provides an ink-jet printhead scanning carriage drive belt tensioner subsystem for a drive motor and carriage drive belt system, wherein the drive motor is mounted in a first position on a chassis, including: a belt pulley having an axis of rotation; a frame for positioning the belt pulley with respect to the motor, including fastening mechanisms for affixing the frame to the chassis at a second position; and a fork for positioning the belt pulley on the frame such that the belt pulley is between the first position and the second position and the belt is tensioned between the motor and the pulley; the chassis, the frame, and the fork each being fabricated of a material having a differing, compensating, CTE characteristic such that material EC of the chassis and the frame is balanced wherein the axis of rotation remains in a substantially constant position with respect to the first position. 
     In another basic aspect, the present invention provides a method for maintaining a predetermined tension of a drive belt between a belt drive motor and a belt pulley, including the steps of: affixing the motor to a chassis at a first predetermined position, the mount having a known mount material CTE; affixing the pulley on a frame at a predetermined frame position, the frame having a known frame material CTE; affixing the frame to the chassis at a second predetermined position such that the predetermined tension is established and the pulley is located along a plane between the first predetermined position and the second predetermined position, wherein the length L 2  of the frame and the length L 3  of the fork is related to the frame material CTE and the fork material CTE, the chassis material CTE and length L 1 , and the belt CTE and length L Belt , wherein means for affixing the pulley on the frame comprises a material having an affixing means CTE substantially less than the frame material CTE such that substantially no material EC is experienced by the means for affixing during proportional material EC of the mount and the frame. 
     In another basic aspect, the present invention provides an ink-jet hard copy apparatus, having a scanning carriage for translating at least one ink-jet writing instrument mounted thereon across adjacently positioned print media, including: an apparatus chassis, fabricated of a material having a CTE chassis ; a drive motor mounted to the chassis at a first position; a belt pulley mounted on a pulley frame by a pulley fork, wherein the pulley frame is fabricated of a frame material having a CTE frame  which is greater than CTE chassis  and wherein the pulley fork is fabricated as a fork material having a third CTE fork  which is substantially less than the CTE chassis  such that there is substantially no center-center change between the motor pulley axis and the turn around pulley axis during thermal excursions wherein the belt material CTE is non-affective as relative to the CTE fork , CTE chassis , CTE frame ; and a belt tensioned between the motor and the pulley and having the carriage mounted thereon wherein the pulley frame is affixed to the chassis at a second position with the pulley positioned between the first position and the second position such that a predetermined belt tension is established and maintained regardless of material EC of the chassis and the frame. 
     In another basic aspect, the present invention provides an ink-jet hard copy apparatus, having a beltriven scanning carriage for translating at least one ink-jet writing instrument mounted thereon across adjacently positioned print media, including: a belt tensioner device, including an adjustably mountable frame for mounting to a chassis, a pulley fork biasingly mounted to the frame, and a belt pulley mounted between the frame and the fork, wherein the frame and fork are co-associated and fabricated of materials each having a complementary CTE to compensate for temperature excursions affecting belt tension. 
     Some of the advantages of the present invention are: 
     It maintains a relatively constant belt tension during environmental temperature excursions; 
     it allows for lower belt tension in a belt drive system, allowing the use of lower cost bush bearings and increasing motor life; and 
     it provides for increased printer performance at a lower cost. 
     The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exemplary embodiment of an ink-jet printer which can employ the present invention (not shown in this view). 
     FIG. 2 (PRIOR ART) is a known manner spring loaded belt tensioner system. 
     FIG. 3 is a perspective view schematic drawing of a belt tensioner subsystem in accordance with the present invention. 
     FIG. 3A is an exploded perspective view schematic drawing of a belt tensioner subsystem as shown in FIG.  3 . 
     FIG. 4 is a schematic illustration of the belt tensioner subsystem as shown in FIGS. 3 and 3A to demonstrate operation thereof. 
    
    
     The drawings referred to in this specification should be understood as not being drawn to scale except if specifically noted. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative embodiments are also briefly described as applicable. 
     FIG. 3 is a thermally compensating, fixed center, belt tensioner subsystem in accordance with the present invention. FIG. 3A shows elements of the subsystem to in an exploded view. In FIG. 3, the subsystem is shown attached to a chassis  301 . It will be recognized by those skilled in the art that this pen carriage drive subsystem may also be affixed to a platen, vacuum box, or other printer framework or subchassis as appropriate to a specific implementation. A separate tensioner frame  303  is provided. The frame  303  bears a centrally mounted pulley fork  304  for holding a belt pulley  500  in a general alignment position. A spring  311  (FIG. 4) is attached between the chassis  301  and a spring clip  306  provided on the tensioner frame  303 , providing an initial belt  100  tension bias. Note that the belt pulley  500  is mounted for rotation to the pulley fork  304  and supported by the tensioner frame  303  and affixed with a fastener, such as a screw or bolt,  309  through adjustment aperture  307  to the chassis threaded hole  310 . This approach is different from fixing the fork  304  directly to the chassis  301  as in a conventional fixed center tensioner manner. The fastener hole  307  and fastener  309  are provided in any known or proprietary manner for securing the tensioner frame  303  to the chassis  301 , establishing a modified fixed center tensioner construct. The belt  100  load is now supported by two parts, the tensioner frame  303  and the pulley fork  304 . 
     Usually a printer chassis  301  is fabricated of metal or plastic for stability, having a relatively high CTE It has been common to use a flexible but stiff belt  100 , such as is known to be made of Kevlar™ material. Kevlar type materials have a relatively low CTE, negative in value. 
     In accordance with the present invention, the tensioner frame  303  is fabricated of a material having a relatively high CTE, the pulley fork  304  is fabricated of a material having a relatively low CTE, the chassis has a relatively high CTE, and the belt having a low and negative CTE. The total net extension between the motor pulley shaft center  300  and the turn pulley center  500  is thus represented by the equation: 
     
       
           E   T   =ΣE   i   i= 1,2,3,4  (Equation 1), 
       
     
     where E i =CTE i * L i * ΔT, where i=1,2,3, 4, and L i  are shown in FIG. 4, and where delta-T is the change in temperature in consistent units. Given a Kevlar fiber belt  100  material and a sheet metal chassis  301  material, it has been found that unfilled or low-content glass-filled polymers (such as of polycarbonate or Noryl™), thus having a relatively high CTE, are suitable for use as the tensioner frame  303  in the present invention. 
     Conversely, the pulley fork  304  is fabricated from a relatively low CTE material compared to the chassis  301  and frame  303  materials. It has been found that high glass-filled polymers are suitable for the pulley fork  304  material in accordance with the present invention. The design can be optimized for a specific implementation around the nominal dimensions of the parts and properties of the materials to ensure E T  is theoretically zero during temperature excursions. 
     The dynamics of the construct is illustrated schematically in FIG.  4 . As a manufacturing, post-assembly process, the design specified belt tension is set by affixing the frame  303  to the chassis with a screw  309 , or another attachment method, against the bias of the chassis spring  311 . As ambient temperature rises: 
     1) the chassis  301  expands proportional to CTE 1 , L 1  increases in length, 
     2) the frame  303  expands proportional to CTE 2 , L 2  increases in length, 
     3) the turn fork  304  expands proportional to CTE 3 , L 3  increases in length, and 
     4) the belt  100  expands proportional to CTE 4 , L 4  increases in length, for positive CTE and decreases in length for negative CTE. 
     Hence, the net extension of the motor pulley rotational axis  300  to the idler pulley rotational axis  500  is described by the equation: 
     
       
           E   NET ≡[( CTE   1 *L 1 )−( CTE   2   *L   2 )+( CTE   3   *L   3 )+( CTE   4   *L   4 )]Δ T   (Equation2). 
       
     
     Because of the constructs use of different CTE material and the different length elements, as the chassis  301  and frame  303  expand, increasing L 1  and L 2 , the substantially unaffected pulley fork  304  moves the pulley  500  inwardly, maintaining L BELT  within a predetermined design tolerance such that the design target belt tension as initially set against the spring bias remains substantially constant. 
     Thus, the present invention provides a thermally compensating belt drive tensioner useful in the construction of an ink-jet printer having at least one belt-driven writing instrument which scans the hard copy apparatus printing zone. Exemplary embodiment characteristics illustrating the concept of the present invention is represented in Table 1 below with geometry and properties for a hypothetical design. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Vari- 
                   
                   
                   
               
               
                   
                 Description 
                 able 
                 CTE 
                 L1 
                 Material  
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Chassis 
                 L1 
                 6.30E-06 /F 
                 400 
                 mm 
                 Steel 
               
               
                 2 
                 Frame 
                 L2 
                 3.70E-05 /F 
                 95 
                 mm 
                 Polycarbonate - 
               
               
                   
                   
                   
                   
                   
                   
                 unfilled 
               
               
                 3 
                 Fork 
                 L3 
                 1.30E-05 /F 
                 88 
                 mm 
                 Polycarbonate - 
               
               
                   
                   
                   
                   
                   
                   
                 40% glass 
               
               
                   
                   
                   
                   
                   
                   
                 filled 
               
               
                 4 
                 Belt 
                 L4 
                 −1.00E-06 /C   
                 390 
                 mm 
                 Kevlar fiber  
               
               
                   
               
             
          
         
       
     
     In general, this can be expressed for designing a specific implementation as using materials having a CTE ratio of approximately 1/3 for fork material to frame material and a CTE ratio of approximately 2/1 for fork materials to chassis material. Given Equation 2: 
     
       
           E   NET =[( CTE   1   *L   1 )−( CTF   2   *L   2 )+( CTE   3   *L   3 )+( CTE   4   *L   4 )]Δ T,   
       
     
     and solving for the fork dimension L 3 :            L   3     =         E   NET     -       CTE   1          L   1        Δ                 T     +       CTE   2          L   2        Δ                 T     -       CTE   4          L   4        Δ                 T           CTE   3        Δ                 T         ,                          
     to obtain a solution with no net extension, E NET ˜0, and given temperature excursions of 35° C. and 95° F.:          L   3     =       0   -   0.239   +   0.334   -     (     -   0.014     )       0.001235                            L   3 ≡0. 
     Hence, if the design ensures the “effective net belt extension” is eliminated during thermal excursion, the belt tension change will be unchanged. 
     The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result, The embodiment was chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather means “one or more.” Moreover, no element, component, nor method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the following claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ”