FIELD OF THE INVENTION The invention relates to an ink ribbon cartridge for a printer and, particularly, to apparatus and methods for advancing the ink ribbon in the cartridge.

BACKGROUND

Ink ribbon cartridges for printers are well known and rely on a substantially longitudinal advance of the ribbon in the course of a printing operation. Such longi¬ tudinal advance is uneconomical taking into account the unutilized area of the ribbon in the printing operation.

Accordingly, efforts have been made to maximize the area of the ribbon utilized in the printing operation. These include a feed of the ribbon such that the characters are overstruck thereon. Conventionally, the ribbon is shifted approximately 1/5 of the width of a conventional character to achieve a so called 4/5 over-strike. This is generally acceptable on the finished product .for all characters except those in which there are a succession of horizontal lines such as for the letters E and F and for underlying. Under such circumstances, the horizontal stroke produced on the paper is formed at its left end on a part of the ribbon which is struck fiv times, whereas, at the right end, it is formed on a part of the ribbon struck once. As a consequence, the resulting stroke is uneven in appearance.

In order to avoid this, it is known to vertically displace or dither the ribbon so that the horizontal strokes will be displaced on the ribbon to avoid striking the same region of the ribbon.

This is conventionally achieved by means external of the cartridge and generally, by displacing the entire cartridge and the ribbon therewith. Such conventional construction is complex, costly and generally requires a power drive.

SUMMARY OF THE INVENTION

An object of the invention is to provide an ink ribbon cartridge for a printer and a method of drive of the ribbon by which the ribbon undergoes a dithering displacement without the use of means external of the cartridge.

A further object of the invention is to provide an ink ribbon cartridge in which the advance of the ribbon is effected along an undulating path.

In a particular embodiment of the invention, the undulating path is of saw-tooth shape.

In order to satisfy the above and further objects of the invention, the longitudinally advancing ribbon is subjected to a transverse displacement from within the cartridge.

According to a feature of the invention, the trans¬ verse displacement is effected by engaging the ribbon in the cartridge with a rotatable member such as a pulley which rides on a fixed cam member which produces transverse displacement of the pulley as it rotates under the action of the advance of the ribbon. As a consequence of the arrangement, the pulley rides up and down on the fixed cam member as it rotates to produce the transverse displacement of the ribbon and confer the undulating path of travel to the ribbon.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

Figure 1 is a top plan view of an ink ribbon cart¬ ridge with the top cover broken away and partially removed to show the interior details.

Figure 2 is a view taken along lines 2-2 in- Fig. 1 partly broken away and in section.

Figure 3 is a top plan view of a fixed cam member of a portion of the cartridge.

Figure 4 is a sectional view taken on line 4-4 in Fig. 3.

Figure 5 is a sectional view taken on line 5-5 in Fig. 2.

Figure 6 is a graphical illustration of the path of travel of the ribbon in the cartridge.

DETAILED DESCRIPTION Referring to Fig. 1 therein is shown an ink ribbon cartridge 1, the basic construction of which is conventional in the art. The cartridge is adapted for use with a printer by being mounted on a carrier (not shown) so as to be trans¬ ported past a platen 2 to effect printing on a sheet- of paper 3 by a printer 4 which strikes an ink ribbon 5 carried by the cartridge 1.

In particular, the cartridge includes a supply or storage reel 6 for the ribbon and a take-up reel 7 on which the ribbon is wound after it has completed a printing operation,

The ribbon 5 is driven by a drive wheel 8 which is rotatably mounted in the cartridge. A counter wheel 9 is supported by an arm 10 on a pivot 11 and is biassed to be pressed against wheel 8. The wheels 8 and 9 are knurled in order to frictionally grip the ribbon to pull the ribbon from the supply reel 6 towards the take-up reel 7. A belt 12 is externally mounted on the cartridge to connect the drive wheel with the take-up reel 7 so that as the ribbon is driven by the drive wheel 8, the take-up reel 7 will be rotated to take-up the advancing ribbon and wind it on the take-up reel. The drive of the drive wheel 8 is effected from the carrier by the engagement of a drive member (not shown) on the carrier in a slot 13 at the underside of the wheel 8.

The cartridge is provided with a number of guide pins 14 for guiding the ribbon in its travel.

The supply reel 6 is provided with a spring 15 which acts to apply a tension to the ribbon as it is wound off the supply reel. The spring 15 comprises an arm 16 which abuts against the cartridge and a guide 17 which engages the ribbon as it comes off the supply reel to apply a tensile force to the ribbon. The ribbon travels in its longitudinal path from the ^" supply reel 6 through an exit slot 18 where it undergoes passage over pins 19 on the carrier which position the ribbon in front of the print head 4 in suitable position for a printing operation. The ribbon then travels back into the cartridge through a slot 20 for engagement with the knurled wheels 8 and 9 and take-up by the reel 7.

Up to this point the construction of the cartridge is

entirely conventional. -

The invention provides a means 30 positioned within the cartridge to produce transverse or dithering displacement of the ribbon in the course of its longitudinal advance to offset the area of the ribbon utilized in printing.

As seen in Fig. 2, the means 30 for transversely displacing the ribbon comprises a fixed member 31 which is secured to a bottom wall 32 of the cartridge and a moveable member 33 which is rotatable on the fixed member 31 and also axially displaceable thereon. The rotatable member 33 is in the form of a pulley which includes upper and lower flanges 34 and 35 between which the ribbon 5 is embraced. As the ribbon is longitudinally advanced under the drive of the wheels 8 and 9 the ribbon produces rotation of the pulley 33. If necessary, the surface of the pulley can be treated to ensure such rotation by the ribbon 5 and also to ensure its rotation on the fixed member 31. This might take the form of surface treatment of the pulley with high or low friction material as the case may be.

The upper end of the fixed member 31 is formed as a cam 35 and the pulley* 33 is formed as a cam 36 of mirror image to cam 35. The cam 36 rides on the cam 35 to impart the transverse movement, up and down, of the pulley 33 on the fixed member 31.

The cams 35 and 36 are each provided with a plurality of peaks 37 (three as shown in Figs. 3 and 4). The peaks 37 are arranged at equal spacing around the periphery of fixed member 31 and, hence, are spaced at 120° from one another. Each peak 37 ends a cam segment 38 which cooperates successively with the corresponding protuberances 36 as the pulley 33 rotates on member 31. Each cam segment 38 has an upper surface 39 on which the protuberances ride and the surface 39 extends .from the peak 37 in uniformly diminishing height towards the following peak of the next cam segment.

The path of travel of the ribbon 5 is shown in Fig. 6 and therein it can be seen that a transverse displacement component is added to the longitudinal component of advance to produce an undulating path. Because the cam surface 39 uniformly increases from one peak to the next, the undulating

path will be saw-tooth in shape as illustrated in Fig. 6. However, the undulating path could be composed of curved portions by suitable alteration of the shape of cam surfaces 39.

The tight embrace of the ribbon 5 between the flanges 34 and 35 of the pulley will generally be sufficient to move the pulley up and down on the fixed member 31 due to the tension in the ribbon 5. However, to supplement the descent of the pulley 33 on the member 31, a light spring 40 can be secured to the upper wall 41 of the cartridge to apply a downwards bias force to the pulley 33.

As is evident from the above, the provision of the simple means 30 in the cartridge serves to move the ribbon reciprocally up and down in the course of its longitudinal advance which serves to offset the area of the ribbon utilized in the printing operation.

While numerous modifications and variations of .the invention will become evident to those skilled in the art, these are deemed to be within the scope and spirit of the invention if defined in the attached claims.

Thus, for example, the means 30 which is shown within the cartridge could also be mounted on the outside of the cartridge.

WHAT IS CLAIMED IS: