GB2136732A - Printhead carriage guide - Google Patents

Abstract

An impact printhead mounted on a carriage that slides along a main support rail parallel to the platen of an impact printer is provided with a guide mechanism engaging a guide rail 26 parallel to the main support rail 22; the guide mechanism comprises first and second half-cylinder guide bearing members 41, 42 molded of a low-friction resin, each mounted on the carriage and engaging the guide rail in diametrically opposed relation, and a W-shaped wire spring 47 mounted on the carriage and urging one bearing member toward the guide rail to afford a low friction zero clearance guide bearing that resists pivotal movement of the carriage responsive to the impact reaction forces generated during printing operations. <IMAGE>

Description

SPECIFICATION Printhead carriage guide Background of the invention The most common general type of printing mechanisms employed in communication printers, in computer output printers, and in typewriters, are the impact printers. In most impact printers, a printhead is mounted on a carriage and is moved along the length of a platen to print a line of text on a paper sheet supported on the platen. Usually, the impact printhead carriage is supported and guided along one or more rails or shafts that extend parallel to the platen. Impact printers of this general type may include column sequential dot matrix printers, character sequential dot matrix printers, daisy wheel printers, and others.

Each time the impact printhead is actuated to print part or all of a character, the force of the print element or elements striking the paper on the platen produces a reaction force on the printhead. Unless the printer has a rigid support rail for the printhead carriage that is aligned directly with the reaction force applied to the printhead, this reaction force tends to cause a pivotal movement of the printhead and its carriage. As a consequence, the printhead and carriage may be displaced pivotally, relative to the platen, with a corresponding displacement of part or all of some of the printed characters relative to others. This can often result in an appreciable degradation of print quality, a degradation that is unacceptable in many applications.The print quality degradation is especially noticeable in dot matrix printers, where the reaction forces are often different for different characters or different portions of characters, but the same effect may also occur in other impact printers such as daisy wheel printers.

It is frequently desirable and even necessary to provide two rails for the printhead carriage; usually one is a main support rail and the other serves primarily as a guide rail. In this construction, the printhead carriage may be pivotally mounted on the main support rail to facilitate adjustment and align ment of the printhead relative to the platen. In a printer mechanism of this kind, the reaction forces acting on the printhead tend to jam the bearing that connects the carriage to the secondary guide rail against that rail, increasing the friction losses during carriage movement and thus undesirably increasing the power requirements for the carriage drive.With a conventional bearing connecting the carriage to the guide rail, any increase in bearing clearance to avoid power losses due to an increase in friction resulting from the impact reaction force on the printhead may result in an increase in degradation of print quality as discussed above.

Summary of the invention It is a general object of the present invention, therefore, to provide a new and improved printhead carriage guide mechanism for an impact printer that effectively minimizes or eliminates the problems of print character degradation and undue friction as discussed above.

A particular object of the invention is to provide a new and improved low friction zero clearance bearing structure in a printhead guide mechanism of an impact printerthat is simple and economical in construction and that minimizes wear and maintenance difficulties.

Accordingly, the invention relates to a printhead guide mechanism for an impact printer of the kind comprising an elongated platen affording a support for a print-receiving record sheet, a printhead carriage, an impact printhead mounted on the carriage, a main support rail extending in parallel spaced relation to the platen, and a main bearing mount for mounting the carriage on the main support rail for sliding movement along the main rail parallel to the platen, with the geometrical relation of the printhead, the carriage, the main support rail and the platen being such that the reaction force from operation of the printhead tends to pivot the printhead and carriage relative to the main rail and thereby produce variations in the alignment of characters or parts of characters as printed.The printhead guide mechanism comprises a guide rail extending in parallel spaced relation to the main support rail and the platen, first and second halfcylinder guide bearing members, each mounted on the printhead carriage engaging the guide rail in diametrically opposed relation, and resilient biasing means, mounted on the carriage and urging one of the bearing members toward the guide rail to afford a low friction bearing with essentially zero clearance relative to the guide rail and to preclude the aforesaid pivotal movement of the printhead and carriage.

Briefdescription ofthe drawings Figure lisa simplifed schematic illustration of an impact printer mechanism; Figure 2 is an elevation view of a printhead carriage guide mechanism according to a preferred embodiment of the invention, taken approximately as indicated by line 2-2 in Figure 1, with a part of the carriage cut away; Figure 3 is a detail sectional view taken approximately as indicated by line 3-3 in Figure 2; Figure 4 is a detail view taken approximately as indicated by line 4-4 in Figure 2, showing only the printhead carriage and the bearing mechanism, with the left-hand portion of the carriage partly in crosssection; and Figure 5 is a plan view of the printhead carriage.

Description of the preferred embodiment Figure 1 is a simplified schematic plan view of an impact printer 10 in which the printhead guide mechanism of the present invention may be incorporated. Printer 10 includes a frame comprising a base plate 11 secured to vertical side frame members 12 and 13. The frame of printer 10 may also include additional transverse bracing members (not shown) between the side frame members 12 and 13.

A platen 14 is mounted on a shaft 15 that extends across printer 10 and isjournalled in suitable bearings in the vertical side frame members 12 and 13. A knob 16 for manual adjustment of platen 14 may be mounted on one end of shaft 15. Platen 14 provides a support for a print-receiving record sheet 17. Sheet 17 may comprise an impact-sensitive paper or may constitute ordinary paper if printer 10 is provided with a print ribbon.

Printer 10 further comprises a printhead carriage 20 upon which an impact printhead 21 is mounted.

Carriage 20 is slidably mounted upon a main support rail 22 that extends across printer 10 in parallel relation to platen 14, the main support rail 22 being affixed to and supported by the side frame members 12 and 13. A carriage drive belt 23 is connected to carriage 20 and extends around two pulleys 24 and 25 located at opposite sides of printer 10. One of the pulleys is a drive pulley and the other is an idler pulley. The drive pulley is connected to a suitable drive mechanism Inot shown) so that belt 23 can be utilized to drive carriage 20 back and forth across the printer, parallel to platen 14, as indicated generally by arrows A.

In the simplified form shown in Figure 1, printer 10 further comprises a second guide rail 26 that is supported by the vertical side frame members 12 and 13 and extends across the printer in parallel alignment with platen 14. Carriage 20 includes a bearing, described more fully below, that engages guide rail 26. Carriage 20 may also be equipped with appropriate ribbon guides 27 for guiding a ribbon (not shown) through the printing gap between the impact printhead 21 and the paper 17 supported upon platen 14. Guides 27 may be part of a ribbon mechanism that is mounted on and carried by carriage 20, or may be guides for a stationary ribbon mechanism. Of course, in a printer using impact sensitive paperforthe record sheet 17, the ribbon guides 27 are not required.

The geometrical relation of printhead 21, carriage 20, main support rail 22, and guide rail 26 relative to platen 14 is further shown in Figure 2. Printhead 21, in the illustrated printer 10, is of the column sequential dot matrix type, including a plurality of impact print elements (print rods) 28. Only two print rods are shown in Figure 2 but there would usually be seven or more such print rods. To print a portion of a character on sheet 17, print rods 28 are driven into impact with sheet 17 in varying combinations at a plurality of closely spaced column positions as printhead 21 moves across the record sheet as indicated by arrows A (Figure 1).

Each time one or more print rods 28 are actuated to print portions of characters on sheet 17, a reaction force is generated as indicated by arrow F in Figure 2. If the main support rail 22 for printhead carriage 20 were located in centered alignment behind printhead 21, along the impact center line C through platen shaft 15, the reaction force Fwould have no appreciable effect on the quality of the printing.

Usually, however, positioning of main support rail 22 in alignment with center line C is impractical and undesirable. In a common arrangement, as shown in Figure 2, rail 22 is positioned well below center line C, with a spacing L between center line C and the axis of shaft 22. Accordingly, the reaction force F produces a printing force moment M, such that M =Fx L, tending to pivot printhead 21 in carriage 20 upwardly as indicated by arrow M in Figure 2.

Any pivotal movement of carriage 20 and print head 21 in the direction indicated by arrow M may produce appreciable degradation in the quality of the printing on sheet 17. Thus, in printing a single character in a column sequential dot matrix printer of the kind shown in Figure 2, successive columns of the character may be misaligned vertically unless the tendency of carriage 20 and printhead 21 to be displaced vertically as indicated by arrow M is effectively counteracted. In this type of printer, the result may be internal distortion within a given character or misalignment of adjacent characters or both. In a character sequential dot matrix printer, where an entire character is printed all at one time, the distortion takes the form of vertical misalignment of complete characters.This is also the adverse effect that occurs in other types of impact printers, such as daisy wheel printers. For any of these different kinds of printers, the resulting misalignment is unacceptable if the printer is expected to produce a high quality output.

An improved printhead guide mechanism 30 of the present invention is illustrated in Figures 2-4.

That mechanism comprises a guide extension 31 formed as an integral part of printhead carriage 20.

Guide extension 31 is of generally U-shaped configuration, extending in spaced relation around guide rail 26 (Figure 2) and including an upper leg 32 above rail 26, a vertically extending bight portion 33, and a lower leg 34 positioned below the guide rail.

There is a centrally located slot 36 in the upper leg 32 of guide extension 31, a central alignment opening 37 in bight portion 33, and a centrally located slot 38 in leg 34.

Two bearing members 41 and 42 are mounted in guide extension 31 in engagement with guide rail 26.

Bearing members 41 and 42 are essentially identical in size and configuration. Each constitutes a halfcylinder guide bearing member, preferably molded of a relatively dense resin having lowfriction characteristics. An acetal resin such as that commercially available under the designation Delrin 500 AF has been found to be satisfactory, but other bearing materials can be used.

The lower bearing member 42 is seated directly on the lower leg 34 of carriage guide extension 31, with a mounting lug 43 that is an integral part of bearing member 42 projecting into slot 38 in guide extension leg 34. The lower bearing member 42 also includes a pair of retainer elements 44 that extend through the alignment opening 37 in the bight portion 33 of carriage guide extension 31. The upper bearing member 41 has a mounting lug 45, like lug 43, that projects upwardly into slot 36 in the upper leg 32 of carriage guide extension 31. The upper bearing 41 is oriented so that its retainer elements 46 extend outwardly through alignment opening 37 in the bight portion 33 of the carriage guide extension. The lugs at the tips of retainer elements 44 and 46 afford a snap fit within opening 37 that, with slots 36 and 38, allows for movement of carriage 20 relative to guide rail 26, for alignment purposes. The mounting arrangement for bearing members 41 and 42 within the carriage guide extension 31 can be seen from Figures 2-5.

The printhead guide mechanism 30 further comprises resilient biasing means, mounted on carriage 20, that urges the upper bearing members 41 toward guide rail 26 in a construction that, with bearing member 42, affords a low friction bearing having essentially zero clearance relative to the guide rail in an arrangement that precludes pivotal movement of carriage 20 and printhead 21 resulting from the reaction force F that occurs during impact printing.

This resilient biasing means comprises a wire spring 47 of essentially W-shaped configuration (Figure 3).

The ends 48 and 49 of spring 47 are bent upwardly and extend through two small holes 52 and 53, respectively, in the upper leg 32 of the U-shaped carriage extension 31. Thus, the two end portions 48 and 49 of spring 47, in their engagement in apertures 52 and 53 of guide extension leg 32, effectively anchor the spring to the printhead carriage. The center portion of spring 47 extends downwardly at an acute angle (see Figure 2) so that the spring continuously biases the upper bearing 41 downwardly toward guide shaft 26. The other bearing member 42, on the other hand, is supported directly upon guide extension leg 34; thus, it is firmly seated on carriage extension 31. The manner in which the lower bearing member 42 is mounted on carriage extension leg 34 is best seen from Figures 2 and 4.

Figure 5 illustrates a typical arrangement for mounting carriage 20 on the main support rail 22. As shown therein, carriage 20 includes two integral legs 54 and 55 in which sleeve bearings 56 and 57 are mounted, the sleeve bearings each being disposed in encompassing relation to shaft 22. Bearing 57 comprises an outer metal shell 58 with an inner sleeve 59 fitted into that shell. Sleeve 59 preferably comprises an outer cylinder of metal with an internal surface of low-friction resin. Bearing 56 is of similar construction. A spring 61 extends between bearings 56 and 57. This particular construction is not critical to the operation of guide mechanism 30 and other arrangements can be utilized for mounting carriage 20 on shaft 22.However, it it usually desirable to allow for pivotal movement of carriage 20 about shaft 22 to facilitate precision alignment of printhead 21 relative to platen 14 (Figure 2).

In operation, whenever printhead elements 28 are actuated for impact printing on the record sheet 17 supported upon platen 14, a reaction force F is exerted on printhead 21 as indicated in Figure 2. This results in the application of a turning moment M (M=FxL) to carriage 20 and printhead 21 as described above. This turning moment M is transferred to guide rail 26 rigidly through the lower bearing member 42. However, it cannot pivot carriage 20 or printhead 21 about the axis of shaft 22, due to the rigid engagement between carriage extension leg 34 and the lower bearing member 42.

Spring 47 continuously urges the upper bearing member 41 downwardly against guide rail 26, tending to tilt the font portion of carrige 20 and printhead 21 upwardly. The force exerted by spring 47 is quite limited and does not result in the creation of undue friction between bearing 41,42 and guide rail 26. Thus, guide mechanism 30 effectively affords a low friction zero clearance guide bearing for printhead carriage 20 in which spring 47 minimizes any adverse effect of the reaction force F and assures high quality printing without undue loading of the drive mechanism for printhead carriage 20. Rapid assembly and alignment are facilitated by the mounting arrangements employed for bearing members 41 and 42 and for spring 47.

Claims (11)

1. A printhead guide mechanism for an impact printer comprising: an elongated platen affording a support for a print-receiving record sheet; a printhead carriage; an impact printhead mounted on the carriage; an main support rail extending in parallel spaced relation to the platen; and a main bearing mount for mounting the carriage on the main support rail for sliding movement along the main rail parallel to the platen; the geometrical relation of the printhead, the carriage, the main support rail and the platen being such that the reaction force from operation of the printhead tends to pivot the printhead and carriage relative to the main rail and thereby produce variations in the alignment of characters or parts of characters as printed; the printhead guide mechanism comprising:: a guide rail extending in parallel spaced relation to the main support rail and the platen; first and second half-cylinder guide bearing members, each mounted on the printhead carriage engaging the guide rail in diametrically opposed relation; and resilient biasing means, mounted on the carriage and urging one of the bearing members toward the guide rail to afford a low friction bearing with essentially zero clearance relative to the guide rail and to preclude the aforesaid pivotal movement of the printhead and carriage.

2. A printhead guide mechanism for an impact printer according to Claim 1 in which the resilient biasing means comprises a spring interposed between the carriage and the first guide bearing member, and the second guide bearing member is firmly seated on the carriage.

3. A printhead guide mechanism for an impact printer according to Claim 2 in which the spring is a wire spring of generally W-shaped configuration, with the outer legs of the spring anchored to the carriage and the central portion of the spring engaging the first guide bearing.

4. A printhead guide mechanism for an impact printer according to Claim 2 in which the carriage comprises a guide extension of generally U-shaped configuration extending in spaced relation around the guide rail, with the legs of the U parallel to the direction of impact reaction force, in which the second guide bearing member is seated on one leg of the carriage guide extension, and in which the biasing spring is anchored to the other leg of the carriage guide extension, engaging and urging the first guide bearing member toward the rail and toward the second guide bearing.

5. A printhead guide mechanism for an impact printer according to Claim 4 in which the main support rail is located below the platen and generally beneath the impact mechanism of the printhead, in which the guide rail is positioned intermediate the main support rail and the platen, and in which the one leg of the carriage guide extension is located below the guide rail, with the spring acting generally downwardly on the first bearing member.

6. A printhead guide mechanism for an impact printer according to Claim 5 in which the spring is a wire spring of generally W-shaped configuration, with the outer legs of the spring anchored to the carriage and the central portion of the spring engaging the first guide bearing.

7. A printhead guide mechanism for an impact printer according to Claim 4 in which each bearing member is a unitary molded member of low-friction resin.

8. A printhead guide mechanism for an impact printer according to Claim 7 in which each bearing member includes at least one integral retainer element projecting from one edge of the bearing member into engagement with the bight portion of the carriage guide extension.

9. A printhead guide mechanism for an impact printer according to Claim 8 in which each bearing member includes two parallel spaced retainer elements, each including an integral retention lug at the outer end thereof, and in which the retainer elements on the bearing members are mounted in a snap fit engagement in an alignment aperture in the bight portion of the carriage guide extension.

10. A printhead guide mechanism for an impact printer according to Claim 9 in which the main support rail is located below the platen and generally beneath the impact mechanism of the printhead, in which the guide rail is positioned intermediate the main support rail and the platen, and in which the one leg of the carriage guide extension is located below the guide rail, with the spring acting generally downwardly on the first bearing member.

11. A printhead guide mechanism for an impact printer according to Claim 10 in which the spring is a wire spring of generally W-shaped configuration, with the outer legs of the spring anchored to the carriage and the central portion of the spring engaging the first guide bearing.