Technical Field

The present invention relates to a printing apparatus and more particularly, but not exclusively, to a dot matrix printing apparatus including a carriage having mounted thereon a coupling device co¬ operating with a toothed belt means for moving the carriage in a reciprocating manner.

Background Art

The printhead and carriage of a dot matrix printing apparatus may be moved on rails by any of several arrangements, such as a cable and pulley combination, a head screw, a cam drive or an endless toothed drive belt. In this manner, a line of printed characters is completed upon travel of the printhead in one direction across record media.

In the arrangement which utilizes an endless toothed drive belt, it is desirable to provide a design of coupling device whereby the endless belt can be readily connected to the carriage, and subsequently readily disconnected from the carriage, in a manner which is reliable and which does not require cutting of the belt. A prior art arrangement of this type has been proposed in an IBM Technical Disclosure Bulletin entitled "Improved Method of Belt Attachment in a Belt Escapement System" Vol. 28, No. 9, February 1986, pp. 4021-22. This prior art arrangement describes a manner in which a toothed timing belt can be attached to a carrier by interlacing the belt between a curved rack and two side tabs. The timing belt can be laterally installed between the rack and tabs, and then tension applied to the timing belt.

One of the drawbacks of the prior art arrangement is that co-operative engagement between the timing belt and the rack is achieved solely by

maintaining a sufficient tension in the timing belt. If during a period of operation the timing belt suddenly becomes untensioned, or gradually loses its tension, the timing belt may disengage from the rack and move away from the carrier. Such an occurrence may result in the timing belt wrapping itself around, or becoming snagged in, nearby components of the printing apparatus possibly causing damage thereto and to the belt itself. Furthermore, the disentangling of the belt after such an occurrence increases the period during which the printing apparatus is inoperative.

Disclosure of the Invention

It is one object of the present invention to alleviate the problems associated with the prior art arrangement.

Accordingly the present invention provides a printing apparatus including a carriage having mounted thereon a coupling device co-operating with a toothed belt means for moving said carriage in a reciprocating manner, drive means for driving said toothed belt means , said coupling device including an arcuate member having a surface complementary to the teeth on said toothed belt means for engagement therewith and a pair of guide members extending from said carriage, said guide members being spaced from one another and being disposed adjacent to and spaced from said surface to define therebetween a guide path for said toothed belt means, characterized in that each of said guide members has a respective projection which is spaced from said arcuate member to define a gap therebetween through which said toothed belt means may be passed when in its untensioned state, said projection extending in a direction under said guide path to provide a support for said toothed belt means whereby said toothed belt means is retained in position along said guide path and, when in operative

connection with said drive means, is retained in co¬ operative engagement with the teeth of said surface.

The provision of said projections provides a means of restraining said toothed belt means against downward movement away from said guide path. Thus the toothed belt means is supported by said projections when in its untensioned state prior to assembly around, or after disassembly from, said drive means. Furthermore, if during operation said toothed belt means should suddenly become untensioned, or gradually lose tension to become only partially tensioned, then said projections restrain it from moving out of said guide path and thereby avoid the danger of said toothed belt means becoming entangled around parts of the printing apparatus.

Brief Description of the Drawings

An embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is an elevation view, partially in section, showing the printhead carriage, the timing belt, and the structure for coupling the two in accordance with one embodiment of the present invention.

Fig. 2 is a sectional view, in a reduced scale, taken along line 2-2 of Fig. 1.

Fig. 3 is a sectional view taken along line 3-3 of Fig. 1.

Fig. 4 is a sectional view, taken along line 4-4 of Fig. 1.

Fig. 5 is a bottom view of the printhead carriage, taken along line 5-5 of Fig. 1, showing the timing belt operatively coupled to the carriage for driving it.

Fig. 6 is a bottom view of the printhead carriage, similar to Fig. 5, showing the timing -belt removed from the coupling structure of the carriage.

Best Mode for Carrying Out the Invention

Referring now to the drawings, a printhead carriage 20 is mounted for sliding movement, in a direction parallel to the line of printing of indicia on record media, on a rail 22. A pair of bearings 24, 26 placed in apertures in the carriage 20 provide the engagement with the rail 22. A printhead 28 is mounted on the carriage 20 and contains one or more solenoid-operated print wires or other suitable printing devices for accomplishing the desired printing on the record media. The printhead 28 is shown only in phantom lines, and will not be further described. The carriage 20 also supports a heat sink (not shown) which partially surrounds the printhead 28 and provides a means for dissipating the heat generated by said printhead during operation. Latching elements 30 and 32 are provided on the carriage 20 for retaining the heat sink in position. The carriage 20 also carries a printed circuit board 34 which contains circuitry utilized in operation of the printhead 28 and which is electrically coupled to a computer or other device which controls character selection and other signals used in printer control.

As shown in Fig. 2, the carriage 20 is driven for reciprocating transverse movement on the rail 22 by a toothed timing belt 36 which in turn is driven by a toothed gear 38, driven by a motor (not shown) . The belt 36 is preferably of the endless type, and its path of movement is also defined by an idler 40, with which it engages at a distance from the gear 38, said idler gear 40 being positioned so that the major paths of movement of the belt 36 are parallel to the rail 22.

The device for coupling the belt 36 to the carriage 20 is designated generally by the reference character 42 and includes a pair of posts 44 and a central arcuate member 46. The posts 44 and the

member 46 are attached to a lower surface of the carriage 20 and may be molded integrally therewith. The member 46, which may also be termed a curved toothed profile, resembles a portion of an annulus in configuration. It includes a convex surface 48 which is provided with a toothed profile that is complementary to the toothed profile of the timing belt 36, and also includes a concave surface 50 on the side of the member which faces away from the belt 36.

As may best be seen in Fig. 2, the posts 44 are of generally rectangular cross-section, with each having one corner of the rectangle being cut away to provide an angled surface 51 which is generally parallel to a portion of the convex surface 48 of member 46 to define a passageway between each post 44 and the member 46, through which the belt 36 may extend. The passageway is of slightly greater width than the width of the belt. At their bottoms, the posts 44 are provided with integral enlarged pads or ledges 52. These ledges 52 have corners 54 which extend under the passageway defined by the posts 44 and the member 46. The ledges 52 perform a retaining function to hold the belt 36 against slipping out of the coupling device 42 when the belt is loose.

The manner in which the belt 36 and the coupling device 42 are assembled can best be seen in Figs. 5 and 6. In Fig. 6, the loose belt 36, not engaged with the gears 38 and 40, is shown as being below and separated from the coupling device 42. The teeth of the belt 36 are then aligned with the complementary recesses in the convex surface 48 of the arcuate member 46. The belt 36 is then moved upwardly, past the protruding corners 54 of the ledges 52, and into the passageways between the convex surface 48 and the surfaces 51 on the posts 44, until it is completely past and above the ledges 52. Though loose, the belt is held against falling out of the

coupling device 42 by the ledges 52. When the belt is tensioned by being placed around the gears 38 and 40, it will be in the position shown in Fig. 5, and will be held in that position by its tension. Driving of the carriage 20 can then be accomplished by operating the motor (not shown) to which the gear 38 is coupled. Rotation of the gear 38 is converted by the belt 36 into linear movement of the carriage 20 for driving it in first one and then the opposite direction.

When disassembly of the belt 36 from the coupling device 42 is desired, as for example, for replacement of a worn belt, the belt is disengaged from the gears 38 and 40 and, being loose, can then be maneuvered past the ledges 52 and their corners 54, and out of the passageways formed by the convex surface 48 of the arcuate member 46 and the angled surfaces 51 of the posts 44.

It will thus be seen that the present invention provides a simple and effective means for coupling an endless belt to the carriage of a printer which does not require separate clamps or fasteners, and in which the coupling and uncoupling can be quickly and efficiently performed.